Teo Milanez
email: milanez@dcc.ufmg.br

Caroline Collange
email: sylvain.collange@dcc.ufmg.br

Fernando Magno
email: fernando@dcc.ufmg.br

Quintão Pereira

Wagner Meira
email: meira@dcc.ufmg.br

A Renato
email: renato@dcc.ufmg.br

Ferreira

Data and Instruction Uniformity in Minimal Multi-Threading *

Simultaneous Multi-Threading (SMT) is a hardware model in which different threads share the same instruction fetching unit. This model is a compromise between high parallelism and low hardware cost. Minimal Multi-Threading (MMT) is a technique recently proposed to share instructions and execution between threads in a SMT machine. In this paper we propose new ways to explore redundancies in the MMT execution model. First, we propose and evaluate a new thread reconvergence heuristics that handles function calls better than previous approaches. Second, we demonstrate the existence of substantial regularity in inter-thread memory access patterns. We validate our results on the four data-parallel applications present in the PARSEC benchmark suite. The new thread reconvergence heuristics is, on the average, 82% more efficient than MMT's original reconvergence method. Furthermore, about 69% to 87% of all the memory addresses are either the same for all the threads, or are affine expressions of the thread identifier. This observation motivates the design of newly proposed hardware that benefits from regularity in inter-thread memory accesses.

Introduction

Resource sharing at the hardware level is an alternative that computer architects have been adopting to decrease the costs of highly parallel processors. One form of resource sharing is known as minimal multithreading [START_REF] Long | Minimal multi-threading: Finding and removing redundant instructions in multi-threaded processors[END_REF], or MMT for short. An MMT-based architecture organizes threads into groups that share the instruction fetch logic, and might share execution units. Each thread keeps its own program counter (PC). At fetch time, the hardware chooses heuristically the next PC to serve. If the chosen PC is the same across several threads, then all of them receive an instruction to execute. If this instruction has the same input values, then the computations are combined as well, so the instruction is issued once on behalf of all participating threads.

Minimal Multi-Threading, being a recent notion, still offers room for improvements. In particular, Long et al.'s original formulation uses an intricate reconvergence heuristics, which, in the words of the authors themselves, has impact on the hardware's performance [START_REF] Long | Minimal multi-threading: Finding and removing redundant instructions in multi-threaded processors[END_REF]. This heuristics is expensive because it looks up the program counter's history every execution cycle. On the other hand, the previous alternative, Quinn's algorithm, which exists since the late 80's [START_REF] Quinn | Compiling C* programs for a hypercube multicomputer[END_REF], cannot couple well with function calls, as we show in this paper. In addition to these shortcomings, MMT, in its original conception, does not explore any form of redundancies in memory access patterns. The reason for this limitation is simply the fact that researchers have not yet demonstrated that such redundancies are common in the Single Program, Multiple Data (SPMD) scenario. Nevertheless, this type of redundancy has been already acknowledged, in the GPU world, as a promising way to save hardware space and to reduce energy consumption [START_REF] Coon | System and method for managing divergent threads in SIMD architecture[END_REF].

The objective of this paper is to advance the research on minimal multi-threading, a task that we accomplish in two ways. First, we propose a new heuristics to keep threads synchronized. Second, we provide an analysis of memory access patterns of typical applications to motivate new designs of data fetching units. We draw the conclusions that we present in this paper from the simulation of the four dataparallel applications found in the PARSEC benchmark suite [START_REF] Bienia | The PARSEC benchmark suite: characterization and architectural implications[END_REF]. We analyze only the data-parallel ap-plications because in this case threads execute the same program, following Darema's SPMD model [START_REF] Darema | A single-program-multiple-data computational model for epex/fortran[END_REF]. Hence, we have more opportunities to share instructions among threads. These experiments are meaningful because the PARSEC programs are remarkably large and complex, and they give us execution traces with billions of x86 instructions.

Our first contribution is a new thread reconvergence heuristics that improves on Quinn's algorithm [START_REF] Quinn | Compiling C* programs for a hypercube multicomputer[END_REF]. Keeping threads as much synchronized as possible is important because if two separate threads read different program counters, then they will compete for the shared pipeline front-end, causing pipeline stalls and/or increased energy consumption. Quinn's reconvergence criterion gives priority to the thread with the minimum program counter. This strategy was proposed to reconverge SIMD programs on distributed computers, and today it is used to synchronize threads in GPU-like architectures [6,[START_REF] Lee | Exploring the tradeoffs between programmability and efficiency in data-parallel accelerators[END_REF]. However, it requires the compiler and linker to statically lay out the binary code of each function according to the function call graph, complicating the build process and preventing its use on existing applications. Our new heuristics gives priority to the thread with the minimum stack-pointer. In case of ties, it then uses the minimum PC criterion. We call it min-SP/PC. Our experiments show that the min-SP/PC heuristics is 82% more effective than min-PC.

Our second contribution is an analysis of the memory access patterns in the MMT setting. Such patterns describe the relative arrangement of addresses in the load and store instructions used by each thread. We have identified three different access patterns: uniform, affine and scattered, which we shall define in Section 4. As we show in that section, we have observed substantial regularity in inter-thread access patterns. These forms of regularity have not been previously noticed because Long et al. [START_REF] Long | Minimal multi-threading: Finding and removing redundant instructions in multi-threaded processors[END_REF] considered multi-process workloads, whereas we are analyzing multi-thread programs sharing a single address space. This fact motivates the adoption, in the MMT world, of recent memory coalescing hardware mechanisms that have been proposed for GPUs [START_REF] Coon | System and method for managing divergent threads in SIMD architecture[END_REF]. For instance, if all the threads read data from the same location, or from regularly spaced locations, then the hardware can bring all this data to registers with only one cache access. Patterns in this fashion happen in over 70% of all the memory accesses. On the other hand, if simultaneous memory accesses are randomly scattered, then we lose inter-thread locality, and access fragmentation puts an increased pressure on caches.

MMT in Perspective: Instruction and data sharing are not new ideas in the computer architecture world. In the mid nineties Tullsen et al. introduced the notion of Simultaneous Multi-Threading (SMT) [START_REF] Tullsen | Simultaneous multithreading: maximizing on-chip parallelism[END_REF][START_REF] Tullsen | Exploiting choice: instruction fetch and issue on an implementable simultaneous multithreading processor[END_REF]. In the SMT execution model, several threads share the same superscalar pipeline, including the front-end fetching and decoding instructions. In this way, the hardware is better equipped to avoid control and data hazards; hence, keeping the many stages of its pipeline always in use. In 2008 Gonzalez et al. brought in the concept of Thread Fusion, as a way to decrease the energy consumption of SMT machines [START_REF] González | Thread fusion[END_REF]. Thread fusion consists in giving the same instruction to different threads, whenever they have the same program counter. In order to reconverge threads, Gonzalez et al. would require the compiler to insert barriers at control independent program points. Long et al. [START_REF] Long | Minimal multi-threading: Finding and removing redundant instructions in multi-threaded processors[END_REF] extended Gonzalez's work by introducing the idea of minimal multi-threading. They have designed a hardware mechanism that reconverges threads without the intervention of a compiler. Notice that whereas SMT implies resource multiplexing, MMT strives for resource sharing. That is, in the former case, only one thread can use a given resource at a given time. In the latter, several threads cooperatively use a resource to perform the same action, promising higher energy reductions than what could be achieved with independent thread execution.

Methodology

The Benchmarks: In this paper we chose to analyze the four data-parallel applications present in the PARSEC benchmark suite:

• Blackscholes:

option pricing with Black-Scholes Partial Differential Equation (PDE).

• Bodytrack: computer vision application that tracks a human body with multiple cameras through an image sequence.

• Fluidanimate: fluid dynamics for animation purposes with Smoothed Particle Hydrodynamics (SPH) method.

• Swaptions: application that uses the Heath-Jarrow-Morton (HJM) framework to price a portfolio of swaptions.

These four programs are data-parallel applications implemented in C, on top of the pthreads library. Figure 1 shows some characteristics of these applications. We have compiled them to a 64-bit x86 architecture; hence, by "number of instructions" we mean "number of x86 instructions". We obtain the dynamic traces, i.e., the number of executed instructions, by feeding each application with its simsmall input [4, p.73].

Simulation: We have instrumented the binary programs using the Pin framework 1 . All the traces that we produce in this paper are obtained from the execution of these instrumented programs.

The Available Thread-Level Parallelism (TLP). The more opportunities for parallel execution an application presents, the more effectively it can be handled by an MMT-based hardware. The four applications that we have chosen are highly parallel. To estimate TLP, we consider an ideal multi-threaded machine executing the instructions of each thread in sequence with a throughput and latency of 1 cycle, and with unlimited TLP. Figure 2 plots how often we had n, 1 ≤ n ≤ 16 threads active at each execution cycle. Critical sections prevent us from having all the threads always active. Swaptions is the most "parallel" application. Its regularity makes it possible to have all the 16 available threads simultaneously active in 99.08% of all the execution cycles of the application. Bodytrack and Fluidanimate are less parallel, as we could expect from the number of instructions in critical sections shown in Figure 1. In Bodytrack, for instance, only the main thread is active in 44% of all the execution cycles.

Instruction Sharing

We are interested in maximizing the amount of common instructions executed by independent threads. In other words, we want to keep these threads as

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Definition 3.1 Maximal Sharing Instance: an alphabet of opcodes Σ = {σ 0 , . . . , σ m }, plus a set {t 0 , t 1 , . . . , t n } of finite strings ranging on Σ, e.g., t i [j] = σ k .

Problem: find the shortest string T ranging on Σ, with the following properties:

1. totality: for any i, j, there exists x such that

t i [j] = T [x].
2. ordering: for any i, u, v, there exists x, y, such

that if T [x] = t i [u], T [y] = t i [v]
, and v > u, then y > x.

Each t i represents the sequence of instructions executed by a thread; hence we call it an execution trace. We consider two versions of maximal sharing: offline and on-line. The off-line version of this problem is equivalent to the shortest common supersequence problem, and it is NP-complete [START_REF] Barone | An approximation algorithm for the shortest common supersequence problem: an experimental analysis[END_REF]. However, we are more interested in the on-line version of maximal sharing. In this case, each trace t is seen as a stack, with top at t[0]. We can only perform one of two operations on each t i : "inspect top t i " and "pop t i ". The first operation lets us see the opcode at the top of t i . The second removes the opcode from the top of t i , and places it at the top of T . Notice that we are not allowed to store opcodes before inserting them into T . Due to this last restriction, plus the fact that traces might contain random sequences of opcodes, there is no algorithm that solves on-line maximal sharing optimally [START_REF] Borodin | Online computation and competitive analysis[END_REF]. Therefore, this problem must be solved by heuristics.

Heuristics for Instruction Sharing

While simulating the PARSEC programs, we have experimented with two different instruction schedulers, which were based on the the min-PC and the min-SP/PC heuristics. Before presenting numbers, it is interesting to discuss why threads diverge, and how each heuristics reconverges them. Figure 3 illustrates the min-PC heuristics. We assume that the hardware supports two threads executing simultaneously the same instruction. The program in Figure 3(a) is written in a C-like language with a special keyword, fork, that passes a function to a freshly created thread. Figure 3(b) provides a static view of the code that will be executed by each thread. In this rather artificial example thread zero will follow the "then" path after the branch, whereas thread one will follow the "else" path. Thus, the execution diverges after the test at instruction 3. In face of a divergence, the min-PC heuristics keeps feeding the thread that is reading instructions from the lowest program counter. The rational behind this heuristics is that the next instruction that is not controlled by a branch is normally located below that branch in the program code. In this example shared instruction fetching resumes at instruction six.

Unfortunately, the min-PC heuristics might take too long to reconverge threads in code that contains function calls. Figure 4 illustrates this phenomenon.

In this new example function bar has been laid out before function foo in the program text. The call to foo happens inside a conditional when only one thread is active. Because foo is located after bar, thread one will finish bar before thread zero has a chance to execute foo. In this case, the resulting execution trace is two instructions longer than that sequence seen in Figure 3(c).

Computer architectures usually provide a stack pointer (SP) register to track the data manipulated by the current active function. We propose to use this value, combined with the program counter, to reconverge threads. Priority is always given to the thread with the lowest string "SP:PC" in lexicographic order. Assuming a conventional downwardgrowing stack, this policy gives priority to the most inward call nesting level. In this way, if a function f calls a function g, threads that must execute code in g receive priority over the threads still executing f . This heuristics provides optimal reconvergence in the example of Figure 4. We show empirically that this heuristics fits well the existing SPMD applications expected in the minimal multi-threading scenario.

Figure 5 compares these two heuristics. The measure of efficiency, in this case, is the total number of instructions executed by an MMT-enabled hardware. The shorter the length of the instruction trace, the more efficient is the heuristics. For instance, in Figure 4(c), the min-PC heuristics produces a trace with ten instructions. On the other hand, the min-SP/PC heuristics would produce a trace with eight instructions, because it would be able to share program counters 4 and 5. As we can see in the charts of Figure 5, the min-SP/PC heuristics is more efficient. On the average, it produces traces 33.3% shorter for blackscholes, 33.8% shorter for bodytrack, 33.1% shorter for fluidanimate and 46.7% shorter for swaptions, given two threads in flight. With 16 threads the difference is even larger: 74.1%, 74.4%, 75.3% and 74.4% respectively. The min-SP/PC heuristics produces remarkably good results for very regular applications. For example, for swaptions, over 98% of all the instructions issued with 16 threads are shared among all the threads.

Memory Access Patterns

Data locality is an important player in the development of high-performance programs. The literature traditionally considers two types of locality in sequential applications: spatial and temporal [START_REF] Hennessy | Computer Architecture: A Quantitative Approach[END_REF]. Data in close memory locations have good spatial locality. And data that is likely to be accessed often within short periods of time have good temporal locality. Recently, Meng et al. have introduced the notion of inter-thread locality in the context of the Single Instruction, Multiple Data (SIMD) execution model [START_REF] Meng | Dynamic warp subdivision for integrated branch and memory divergence tolerance[END_REF]. If two separate threads simultaneously read data from nearby memory cells, then these accesses are said to have good inter-thread locality. In this case, a single memory access might provide data to several different threads. The importance of interthread locality is clear in the realm of graphics processing units, given that memory access coalescing is, according to many authors, the most important optimization in this environment [START_REF] Ryoo | Optimization principles and application performance evaluation of a multithreaded gpu using cuda[END_REF][START_REF] Yang | A GPGPU compiler for memory optimization and parallelism management[END_REF][START_REF] Lashgar | Performance in GPU architectures: Potentials and distances[END_REF].

In this paper we look into the potential of interthread locality in the context of minimal multithreading. With this objective, we define three types of memory access patterns: uniform, affine and scattered. If we have n active threads executing a memory access instruction such as a load, e.g., v = * a, or a store, e.g., * a = v, then we assume that each thread i reads from, or writes to an address a i . Given this assumption, we say that a memory access is uniform if a 0 = a 1 = . . . = a n . If T id is an integer that uniquely identifies a thread, then we say that the address is affine if a i = c 1 × T id + c 2 , for any two integer constants c 1 and c 2 . Finally, if none of these patterns applies, then we say that the access is scattered.

Current multi-threaded hardware has not been de-

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-./01234.52" -46789/01" :;<6/=<>/85" 2?/@A4=2" %"B395/62" '"B395/62" +"B395/62" $)"B395/62" signed to benefit from uniform and affine memory patterns: independent on the target address, n simultaneous threads require n accesses to memory ports. However, there exist proposals for new hardware designs that proceed differently [START_REF] Coon | System and method for managing divergent threads in SIMD architecture[END_REF]. In these processors a uniform address causes only one access to the data cache. Likewise, if n threads execute an affine access, e.g., a load v = * (c 1 × T id + c 2 ), then a set of n memory cells, spaced by c 1 words, and starting at base address c 2 is accessed at once. In case c 1 is equal to the word size, accesses are contiguous and can be combined into a single memory transaction.

Counting Access Patterns. Figure 6 shows how often each pattern is found in our simulation of the PARSEC data-parallel applications. The large quantity of affine accesses that we have observed is mostly due to the fact that we use a customized loader to allocate stack frames. Local variables created during function calls, such as t in Fig- ure 3, are placed in allocation units called activation records. These records are stored in a space called the stack frame. The core idea of the loader is very simple: we allocate the same amount of space for each thread to create its stack frame, and we make sure that the stack of activation records always starts at the beginning of this region. In this way, the local variables accessed by each thread are equally spaced.

Access Distance. If the data simultaneously accessed by active threads is within a short distance of each other, then it may fit into the same cache line. In this case, if the data is already cached, then every thread scores a hit. Otherwise, it can be brought to the cache with just one trip to a lower level in the memory hierarchy. Figure 7 shows the average maximum distance between the data accessed by all the threads, considering the setting with 16 threads in flight. Each number, e.g., 0 to 63, is the base-2 logarithm of the maximum distance between any two addresses given sixteen simultaneous memory accesses.

In general we have observed very long distances between the addresses used by threads when simultaneously processing load and store instructions. Figure 7(Left) shows the maximum distances between affine addresses. The longest distance that we have observed in this case is 2 30 . Long spaces between affine accesses are common because each thread receives 2 26 bytes of memory to allocate their stack frames. Thus, local variables stored in the stack are likely to be spaced 2 26 bytes. Closer addresses are found in affine accesses of data stored either in static memory, or, more usually, in the memory heap. Blackscholes and bodytrack use more these memory regions. 27.98% of the memory accesses that happen during the execution of blackscholes are within a memory block less than 2 15 bytes long. The proportion of these accesses in bodytrack is even larger: 63.92%. We have observed very close addresses only in fluidanimate. 2.75% of the memory accesses performed by this benchmark are within blocks less than 2 7 bytes long. Figure 7(Right) shows the maximum distance among scattered accesses. In this case, we have observed accesses distant as much as 2 40 bytes away. Furthermore, we have not observed very close accesses, e.g., within blocks up to 2 7 bytes long.

We speculate that these large spaces between addresses are common because the target benchmarks have not being coded with memory coalescing in mind. The PARSEC benchmarks are meant to run in traditional CPUs, and in this case inter-thread local-ity is not at a premium. On the contrary, close interthread locality would be harmful in the context of multi-core platforms with coherent private caches, by causing false sharing of cache lines. Collange has observed a substantially different behavior in GPGPU applications [START_REF] Collange | Dynamic detection of uniform and affine vectors in GPGPU computations[END_REF]. In that case, inter-thread proximity is much more common, as this type of locality contributes notoriously to performance improvements.

Conclusion

In this paper we have advanced the research on minimal multi-threaded hardware in two different directions. First, we have proposed a new thread synchronization heuristics: min-SP/PC, and compared it with min-PC, a heuristics originally proposed for SIMD languages. Our empirical evaluation has demonstrated that min-SP/PC is remarkably more efficient to keep threads synchronized. We have also studied the memory access patterns typical of dataparallel multi-threaded applications, and have found a substantial amount of regularity between concurrent threads. This regularity is a further motivation for new hardware designs that have been proposed in the literature, but are yet to be manufactured. The paper brings in one negative result: data accessed by different threads tend to be distant in memory. This distance makes it difficult to take benefit from spatial locality in inter-thread memory accesses. It suggests the data layout of call stack memory should be reconsidered in the context of inter-thread locality. We leave this new study as future work.

Figure 1 :

 1 Figure 1: Characteristics of the benchmarks. (S) Number of x86 instructions in the program text. (C) Instructions in critical section executed by a single thread. (D) Number of instructions executed in a single-threaded processor with simsmall input.

Figure 2 :

 2 Figure 2: Histogram showing the number of cycles in which n threads were active, where 1 ≤ n ≤ 16.

Figure 3 :goto 4 3: a = i+1 4 :

 344 Figure 3: A example where the min-PC heuristics is able to reconverge optimally divergent threads.

Figure 4 :

 4 Figure 4: Example where the min-PC criterion fails to reconverge divergence threads, but the min-SP/PC does it optimally. In part (c) we prefix each instruction with its stack pointer plus program counter.

Figure 5 :

 5 Figure 5: (Left) Instruction trace reduction produced by the min-PC heuristics. (Right) Instruction trace reduction produced by the min-SP/PC heuristics.

Figure 7 :

 7 Figure 7: (Left) Maximum access distance among affine addresses. (Right) Maximum access distance among scattered addresses.

This work was partially supported by CNPq, CAPES, FAPEMIG and InWEB.


M Beccuti
email: beccuti@mfn.unipmn.it

G Franceschinis

M Kaaniche
email: mohamed.kaaniche@laas.fr

K Kanoun

Multi

Keywords: Marco Beccuti, Giuliana Franceschinis, Mohamed Kaâniche, Karama

Introduction

There is a wide consensus that developing modeling frameworks for understanding interdependencies among critical infrastructures and analyzing their impact is a necessary step for building interconnected infrastructures on which a justified level of confidence can be placed with respect to their robustness to potential vulnerabilities and disruptions. Modeling can provide useful insights into how component failures might propagate and lead to cascading, or escalating failures in interdependent infrastructures, and assess the impact of these failures on the service delivered to the users. In the context of CRUTIAL, we focus on two interdependent infrastructures: the electric power infrastructure (EI) and the information infrastructure (II) supporting management, business, control and maintenance functionality.

As discussed in [START_REF] Kaaniche | Methodologies Synthesis[END_REF], there has been extensive work on the modeling of individual infrastructures and various methods and tools have been developed to predict the consequences of potential disruptions within an individual infrastructure. However, the modeling and evaluation of interdependent infrastructures is still at an exploratory stage. The modeling activities carried out in CRUTIAL aim at contributing to fill this gap taking into account in particular: a) the three types of failures that are characteristic of interdependent infrastructures [START_REF] Rinaldi | Identifying, understanding, and analyzing critical infrastructure interdependencies[END_REF] (cascading 3 , escalating 4 , and common-cause failures), b) various classes of faults that can occur, including accidental as well as malicious threats, c) the temporal and structural characteristics of the power and information infrastructures investigated. A major challenge lies in the complexity of the modeled infrastructures in terms of largeness, multiplicity of interactions and types of interdependencies involved. To address this problem, a number of abstractions and appropriate approaches for composition of models are necessary. In CRUTIAL, the interdependencies have been analyzed at different levels: from a very abstract view expressing the essence of the typical phenomena due to the presence of interdependencies, to an intermediate detail level representing in a rather abstract way the structure of the system (in some scenarios of interest), to a quite detailed level where the system components and their interaction are modeled in a fairly realistic way and simulation is used to derive interesting reliability measures. In this paper a two-level modeling approach is proposed and illustrated through a simple scenario inspired by the CRUTIAL project. This is part of a multi-level and multi-formalism approach to the qualitative and quantitative study of the interdependencies between the EI and the II controlling and managing it.

In Section 2 the highest abstraction level is considered, showing the sequences of (abstract) events leading to typical interdependency phenomena such as cascading and escalation. In Section 3 a (simple) scenario is used to illustrate a more refined, second level representation, from which quantitative information can be provided to enable performance/reliability analysis. We will show how the higher level models can be composed with the more refined one and used to highlight possible instantiations of the abstract interdependencies phenomena. Section 4 concludes the paper.

High-level abstract models of interdependencies

This section summarizes the high-level abstract models presented in [START_REF] Laprie | Modelling interdependencies between the electricity and information infrastructures[END_REF]. We model the EI and II behavior globally, taking into account the impact of failures in the infrastructures, and their effects on both infrastructures, without taking into account explicitly their underlying implementation structure. For sake of clarity, events and states of the II are prefixed by i-while those of the EI are prefixed by e-. We first address accidental failures in II, then malicious attacks.

Accidental failure model

The model, given in Fig. 1, is based on assumptions related to the behavior of the infrastructures as resulting from their failures and mutual interdependencies. These assumptions are summarized, before commenting the model. Impact of i-failures. Accidental i-failures, affecting the II can be either masked (unsignaled) i-failures, leading to latent errors, or signaled. Latent errors can be either passive (i.e., without any action on the EI, but keeping the operators uninformed of possible disruptions occurring in the EI) or active (provoking undue configuration changes in the EI). After signaled i-failures, the II is in a partial i-outage state. Latent errors can accumulate. Signaled i-failures may take place when the II is in latent error states. When the II is in a partial ioutage state, i-restoration is necessary to bring it back to an i-working state. We assume that an i-failure puts some constraints on the EI (i.e., cascading failure), leading to a weakened EI (e.g., with a lower performance, unduly isolations, or unnecessary off-line trips of production plants or of transmission lines). From an e-weakened state after a signaled i-failure, a configuration restoration leads EI back into a working state, because no e-failures occurred in the EI. Accumulation of untimely configuration changes, may lead to e-lost state (i.e., a blackout state), from which an e-restoration is required to bring back the EI into an e-working state. The above events and the resulting states are recapitulated in Table 1.

Impact of e-failures. We consider that the occurrence of e-failures leads the EI to be in a partial e-outage state, unless propagation within the infrastructure leads to losing its control (e.g., a blackout of the power grid) because of an i-failure (this latter case corresponds to escalating events). Also e-failures may lead the II to an i-weakened state in which parts of the II can no longer implement their functions, although they are not failed, due to constraints originating from the failure of the EI. The above events and the states are recapitulated in Table 2.

Malicious attacks model.

Attacks fall into two classes: deceptive attacks provoking unperceived malfunctions, thus similar to the latent errors previously considered, and perceptible attacks creating detected damages. Deceptive attacks can be passive (i.e., without any direct action of the II on the EI) or active, provoking configuration changes in the EI, by the II.

Fig. 2 gives the state machine model of the infrastructures. Due to the very nature of attacks, a distinction has to be performed for both infrastructures between their real status and their apparent status. For the EI, the apparent status is as reported by the II. Models of Figs. 1 and 2 are very similar: they differ by the semantics of the states and of the inter-state transitions.

In state 2, the effects of the passive deceptive attack are: i) the II looks like working while it is in a partial i-outage state due to the attack, ii) it does not perform any action on the EI, but informs wrongly the operator that the EI is in partial i-outage, and as consequence iii) the operator performs some configuration changes in the EI leading it to a i-weakened state. Accumulation of configuration changes by the operator may lead the EI into e-lost state.

In state 3, the effects of the active deceptive attack are: i) the II looks like working while it is in a partial i-outage state due to the attack, ii)it performs some configuration changes in the EI leading it to a weakened state without informing

Events

Signaled i-failure Detected i-failure. Masked i-failure Undetected i-failure.

i-restoration

Action for bringing back the II in its normal functioning after ifailure(s).

States i-working

The II ensures normal control of the EI. Passive latent error Parts of the II have an i-failure, which prevents monitoring of the EI: e-failures may remain unnoticed. Active latent error Parts of the II have an i-failure, that may lead to unnecessary, and unnoticed configuration changes. Partial i-outage Parts of the II have knowingly an i-failure. Partial i-outage is assumed: the variety of functions and of the components of the infrastructure, and its essential character of large network make unlikely total outage.

i-weakened

Parts of the II can no longer implement their functions, although they do not have an i-failure, due to constraints originating from e-failures (e.g., shortage of electricity supply of unprotected parts).

Table 1. States and events of the information infrastructure (II).

Events e-failure

Malfunctioning of elements of the power grid: production plants, transformers, transmission lines, breakers, etc.

e-restoration

Actions for bringing back the EI in its normal functioning after e-failure(s) occurred. Typically, e-restoration is a sequence of configuration change(s), repair(s), configuration restoration(s). e-configuration change Change of configuration of the power grid that are not immediate consequences of e-failures, e.g., off-line trips of production plants or of transmission lines. e-configuration restoration Act of bringing back the EI in its initial configuration, when configuration changes have taken place.

States e-working

Electricity production, transmission and distribution are ensured in normal conditions.

Partial e-outage

Due to e-failure(s), electricity production, transmission and distribution are no longer ensured in normal conditions, they are however somehow ensured, in degraded conditions.

e-lost

Propagation of e-failures within the EI led to losing its control, i.e., a blackout occurred. e-weakened Electricity production, transmission and distribution are no longer ensured in normal conditions, due to i-failure(s) of the II that constrain the functioning of the EI, although no e-failure occurred in the latter. The capability of the EI is degraded: lower performance, configuration changes, possible manual control, etc.

Table 2. States and events of the electricity infrastructure (EI).

the operator, for whom the EI appears as if it were working. Accumulation of configuration changes by the II may lead the EI into a e-lost state. The difference between states 2 and 3 is that in state 2 the operator has made some actions on the EI and is aware of the e-weakened state, while in state 3 the operator is not aware of the actions performed by the II on the EI. After detection of the attack, the apparent states of the infrastructures become identical to the real ones (state 4), in which i-restoration and e-configuration restoration are necessary to bring back the infrastructures to their working states. States 5, 6 and 7 are very similar respectively to states 5, 6 and 7 of Fig. 1, except that in state 6 the II is in a partial i-outage state following a perceptible attack in Fig. 2 and following a signaled i-failure in Fig. 1. State 8 corresponds to e-lost state but the operator is not aware, he/she has been informed wrongly by the partial i-outage of II that it is in a partial e-outage state.

Global conceptual model

The global abstract model, taking into both account accidental failures and malicious attacks, results from the superposition of the two models. In [START_REF] Kaaniche | Preliminary modelling framework[END_REF], a unified model is presented. In this paper we have presented the separate models for sake of simplicity. Our aim is to illustrate how to join the abstract modeling level to detailed models allowing dependability quantification.

Detailed models of scenarios

The high level abstract models show typical failure scenarios and the combined states of the infrastructures as resulting from their interdependencies. The evaluation of quantitative dependability measures based on these models requires the specification of the probability distributions associated with the transitions of the abstract models. As these transitions result from the occurrence of several elementary events affecting the components of the infrastructures, the development of more detailed models highlighting these events and taking into account the internal behavior of the infrastructures should help to identify representative probability distributions. States in Fig. 1 and 2 are in reality macro states gathering a set of elementary states of the infrastructures in which the service delivered is equivalent. Let us for example consider the transition from state 1 to state 4 in Fig. 1. This transition takes place only when the accumulation of elementary events result in a significant degradation of the service delivered by EI, leading it to an e-weakened state" Quantification of dependability measures requires to model the underlying systems behavior. A measure of dependability could be for example the distribution of the time to reach state4 from state 1, either directly or through states 2 and 3, i.e. the distribution of the time to a signaled failure (Fig. 1), or the distribution of the time to a perceptible attack (Fig. 2). In this section, we show a simple example of a detailed model allowing the evaluation of this distribution. We will describe the underlying system and its associated models and show the relationship between the detailed and the highlevel abstract model.

A more detailed model of a simple scenario

The system considered is described in [START_REF] Garrone | Analysis of new control applications[END_REF] and it is illustrated in Fig. 3. It represents the teleoperation function performed between a Control Centre (CC) and a SubStation (SS), by means of a communication network. We suppose that the communication between the sites is performed in the following way: the CC sends requests to the SS to obtain the execution of a command by the SS (e.g. arming, etc.), or to retrieve data from the SS (signals, measures, etc.). The SS replies to the CC by acknowledging the command execution, or by sending the required data. Each communication needs a minimum level of available bandwidth to be completed. In this context we consider two types of i-failures, bringing the system from state 1 to state 4 of Figs 1 and 2 models respectively. 1. A signaled i-failure of the CC that can occur in the two following cases: the TELECONTROL devices (ATS or ATS Web Server) are not available or the communication inside the CC is not available due to the failure of the LAN, or the Firewall, or the Router. 2. A perceptible denial of service (DoS) attack to the communication network. Such attack consists of sending a high number of packets on the communication network, with the effect of reducing the available bandwidth and causing excessive delay or loss of the packages between CC and SS. A DoS attack may last for a random period of time, and it may be blocked by the success of a countermeasure (firewalling, traffic monitoring, etc.).

Description of the model

For modeling the above events, scenario we use a multi-formalism combining the Stochastic Well-formed Net (SWN) [START_REF] Chiola | Stochastic well-formed coloured nets for symmetric modelling applications[END_REF] and Fault Tree (FT) [START_REF] Schneeweiss | The Fault Tree Method[END_REF] formalisms. In particular, the multi-formalism model is composed by two submodels: an SWN model and an FT model. The first is an SWN model (Fig. 4), which represents the exchange of requests and replies between the CC and the SS by means of the communication network, and the possibility of the occurrence of a DoS attack on the same network. Instead the second one, a FT model(Fig. 5), represents the failure mode of the CC.

The SWN formalism has been chosen to this aim, because it allows to model in compact way the exchange of several requests and replies at the same time, and gives also the possibility to extend easily the model for two or more SSs. In fact the SWN formalism extending the PN formalism with "colour" allows to have distinguished tokens, which can be graphically represented as dots of different colours, so that the colour attached to a token carries some kind of information. Thanks to this feature it is possible to have a compact and parametric description of the system. Moreover this formalism provides also efficient solution techniques exploiting automatically the system symmetries. SWN model description. The SWN model is shown in Fig. 4 where the dashed boxes represent the CC, the SS and the attacker respectively. The transition CC send models the generation of a request to be sent to the SS, by putting a colored token inside the place CC buf f er out and inside the place Commands describing the requests to be sent on the network, and the requests waiting for a reply, respectively. The bandwidth is modeled by a set of tokens inside the place Bandwidth; each time a request has to be sent (a colored token is present in CC buf f er out), the marking of Bandwidth is reduced by one for modeling the reduction of bandwidth due to the transmission (transition CC transmit).

When the transition CC transmit fires the colored token representing the request is moved from the place CC out to the place SS buf f er in, in order to model the receipt of the request by the SSs. Moreover, the firing of CC transmit determines the increase of the marking of the place Bandwidth, in order to model that more bandwidth is now available.

The requests to be processed by the SS are represented by colored tokens inside the place SS buf f er in. The processing is modeled by the transition process. The replies are represented by colored tokens put inside the place SS buf f er out; their transmission is represented by the transition SS transmit; as in the case of the requests, the transmission of replies determines a temporary decrease of the marking of the place Bandwidth. Once the reply is received by the CC (colored token inside the place CC buf f er in), the corresponding pending request is removed from the place Commands.

The failure event of the CC is modeled by the transition CC f ail whose firing rate 5 is computed by solving FT model in Fig. 5. The firing of such transition determines the marking of the place CC f ailed modeling the state of failure. The marking of CC f ailed causes the inhibition of the transition CC send generating the requests.

The attacker state is modeled by the places Idle and Active; the initial state is idle, but it can turn to active after the firing of the transition Begin attack. In the active state, the attacker generates packets (transition AT send whose firing is controlled by the FT) to be transmitted on the communication network (transition AT transmit). As in the case of the transmission of requests and replies, the transmission of the attacker packets determines the reduction of the marking of the place Bandwidth. The complete unavailability of the bandwidth (the success of the DoS attack) is modeled by the place DoS becoming marked when no tokens are present in the place Bandwidth. The state of the attacker can turn back to idle if the transition End attack fires representing that the countermeasure discovers the attack.

The loss of replies is modeled by the timed transition Delay: if a colored token (pending request) stays inside the place Commands for a long time (the corresponding reply has not been received during that time), the transition Delay may fire leading to the marking of the place P acket loss modeling the loss of a reply. Moreover the transition Reset removes a token by the place P acket loss.

Finally the transitions perceptible attack, signaled failure, recovery and recov-ery1, and the places af terP A and af terSF are used for mapping this model on the abstract model (sect. 2), as we will describe in details in the next section. FT model description. Fig. 5 shows the FT model representing the failure mode of the CC. Such failure is represented by the top event called CONTROL CENTRE. Such event is the output of an OR gate whose inputs are the event TELECON-TROL and NETWORK ; therefore, the top event (the CC failure) occurs if the telecontrol function or CC network fails. The event T ELECON T ROL represents the failure of the telecontrol devices; such event is the output of an OR gate having AT S and AT S W EB SERV ER as input events. Therefore the event T ELECON T ROL is caused by the failure of the AT S4 or by the failure of ATS WEB SERVER. Finally the N ET W ORK fails if the ROU T ER, the FIREWALL or the LAN fails.

Interpretation of the model measures w.r.t. the abstract model

The abstract model introduced in Sec. 2 allows capturing at a high abstraction level the interesting interdependency phenomena. The example introduced in this section can be mapped on the abstract model as follows:

1. The signaled i-failure in the CC is triggered by the firing of the transition CC f ailed whose firing time is controlled by the FT model. 2.The perceptible attack corresponds to a loss of responsiveness due to a DoS attack and is modeled by a transition firing activated when n commands are lost in a short period. Observe that in the model command messages (and the corresponding acknowledgments) are never actually lost, however if the acknowledgment of a transmitted command arrives later than a specified maximum amount of time, this is interpreted as a command loss. This mechanism is implemented by introducing a Delay transition, activated when a command has been sent from the CC, and working as a timeout used to record an excessive delay of the command acknowledge. When Delay fires, another timeout starts to count, which is used to forget about command/acknowledge losses after a certain time since their occurrence. If the model manages to accumulate enough (n) command losses before they expire, this is interpreted as an indication that some misbehavior is happening which should be signaled.

The connection between the detailed and abstract models can be performed in different ways: the first option is to define a correspondence among states: so for example we could say that all states with at least one token in place af terP A or in place af terSF correspond to state 4 while all states where these two places are not marked correspond to state 1. So to compute the distribution of the time required to reach abstract state 4 from abstract state 1 can be performed on the detailed model by simply making the states with m(af terP A)+m(af terSF ) > 0 as absorbing and computing on the model the distribution of the time to absorption. If we consider also the possibility of restoration (which for the moment is represented in the detailed model as two simple "reset" transitions, called recovery and recovery1, which bring the whole net back to the initial state), then we can also compute steady state behavior measures, e.g. the probability of being in state 1 or 4.

The alternative way to couple the two models is by making a correspondence between the transitions: in this example this is particularly simple because transitions "Signaled failure" and "Perceived attack" (as well as "recovery" and"recovery1") can be directly put in correspondence with the homonymous transitions in the abstract model: in this case the mapping among the states is indirect (but can be made explicit by adding some "implicit places" in the detailed model representing the abstract model states and connect them to the matching transitions).

Finally, in order to compute performance measures it is necessary to associate a firing delay probability distribution with every timed transition in the detailed model. These firing delay probability distributions can be deduced from experimental data obtained both by real system behavior observation and by testbed simulation. After that, if all these distributions can be expressed by negative exponential distributions then the system performance measures can be computed by numerical analysis, else by simulation.

Conclusion and perspective

This paper presented a dependability modeling approach that takes into account interdependencies related failures affecting electrical infrastructures and associated information infrastructures supporting e.g., management, control and monitoring activities. Two abstraction levels are considered. At the highest level, each infrastructure is modeled globally as a black box and the proposed models identify cascading and escalating related failure scenarios and corresponding service restoration actions resulting from accidental failures or malicious attacks.

The failure scenarios highlighted at this abstraction level result from the occurrence and propagation of elementary events originating from the subsystems and components of the infrastructures.

The development of detailed models taking into account the structure and the internal behaviour of the infrastructures is useful to link the elementary failure events to the high level scenarios of cascading and escalating failures. Also, the detailed models can contribute to the definition of the probability distributions to be associated to the transitions in the high level abstract model to evaluate quantitative measures charac-terizing the impact of interdependencies with regards to the occurrence of blackouts. One of the critical issues that need to be addressed in this context is the mapping of the detailed models to the high-level abstract models. The example presented in this paper, inspired from a case study investigated in CRUTIAL, is aimed at illustrating how this mapping can be achieved and how the effects of accidental and malicious failures can be analyzed together.

So far we have considered simple scenarios. More complex detailed models are currently investigated in the context of the project taking into account the main subsystems and components of both the electrical and the information infrastructures.

Future work also includes the development of a general compositional approach based on stochastic Petri nets to facilitate the construction and processing of the abstract models and their mapping to lower level more models.

Fig. 1 .

 1 Fig. 1. Model of the two infrastructures when considering accidental failures.

Fig. 2 .

 2 Fig. 2. Model of the two infrastructures when considering malicious attacks.

Fig. 3 .

 3 Fig. 3. Architecture of the EI and II considered for the example

5Fig. 4 .Fig. 5 .

 45 Fig. 4. SWN model representing the exchange of requests and replies between the CC and the SS by means of the communication network.

Cascading failures occur when a failure in one infrastructure causes the failure of one or more component(s) in a second infrastructure

Escalating failures occur when an existing failure in one infrastructure exacerbates an independent failure in another infrastructure, increasing its severity or the time for recovery and restoration from this failure

Acknowledgment. The work presented in this paper is partially financed by CRitical Utility InfrastructurAL Resilience (CRUTIAL). Project co-funded by the European Commission within the Sixth Framework Programme 2002-2006 (http://crutial.cesiricerca.it).


Katarzyna Krupa
email: katarzyna.krupa@xlim.fr

Alessandro Tonello

Victor V Kozlov

Vincent Couderc

Philippe Di Bin

Stefan Wabnitz

Alain Barthélémy

Laurent Labonté

Sébastien Tanzilli

Bragg-Scattering conversion at telecom wavelengths towards the photon counting regime

Keywords:

We experimentally study Bragg-scattering four-wave mixing in a highly nonlinear fiber at telecom wavelengths using photon counters. We explore the polarization dependence of this process with a continuous wave signal in the macroscopic and attenuated regime, with a wavelength shift of 23 nm. Our measurements of mean photon numbers per second under various pump polarization configurations agree well with the theoretical and numerical predictions based on classical models. We discuss the impact of noise under these different polarization configurations.

Introduction

Current studies of four-wave mixing (FWM) in third-order nonlinear media, such as hot atomic vapours, silicon waveguides or silica fibers, aim to develop high-performance devices for ultrafast, all optical, signal data processing [START_REF] Ding | Image transfer through two sequential four wavemixing processes in hot atomic vapor[END_REF][START_REF] Alic | Two-pump parametric optical delays[END_REF][START_REF] Uesaka | Wavelength exchange in a highly nonlinear dispersion-shifted fiber: theory and experiments[END_REF]. In the context of quantum computing and communication, Bragg Scattering type FWM (BS-FWM) or frequency exchange [START_REF] Uesaka | Wavelength exchange in a highly nonlinear dispersion-shifted fiber: theory and experiments[END_REF] has a high potential for achieving coherent single photon manipulation and entanglement-preserving frequency conversion, as it has already been developed in second-order nonlinear crystals [START_REF] Langford | Efficient quantum computing using coherent photon conversion[END_REF][START_REF] Tanzilli | A photonic quantum information interface[END_REF][START_REF] Ramelow | Polarization-entanglement-conserving frequency conversion of photons[END_REF][START_REF] Curtz | Coherent frequency-down-conversion interface for quantum repeaters[END_REF][START_REF] Takesue | Single-photon frequency down-conversion experiment[END_REF]. The BS-FWM can actually provide a simple, efficient, and noiseless means for photon-photon optical frequency conversion. This has been recently demonstrated both theoretically [START_REF] Mckinstrie | Translations of quantum states by four-wave mixing in fibers[END_REF] and experimentally [START_REF] Mcguinness | Quantum frequency translation of singlephoton states in a photonic crystal fiber[END_REF] taking advantage of special photonic crystal fibers operating in the visible range of wavelengths. A very recent work shows low-noise BS-FWM in SiN x waveguides, with pumps at 1550 nm and signals and idler at 980 nm [START_REF] Agha | Low-noise chip-based frequency conversion by fourwave-mixing Bragg scattering in SiN x waveguides[END_REF]. In view of possible integration in single photon based quantum communication fiber networks, extending BS-FWM based frequency converters to the telecom range for signal and idler is of high interest. In another recent work coherent FWM has been proposed for shifting on-demand the carrier frequency of single photons at telecom wavelengths in the pulsed regime [START_REF] Clemmen | Towards frequency-coded q-dit manipulation using coherent four-wave mixing[END_REF].

In this work, for the first time to the best of our knowledge, we investigate experimentally the polarization properties of frequency conversion of an attenuated continuous-wave (CW) telecom laser using vector BS-FWM and CW pumps. That is, a signal field at 1549.2 nm is converted into an idler field at 1526.43 nm. For the first time we also discuss some of the limiting factors on exploitability of BS-FWM coming from noise. In our setup it turns out that the noise follows the spectral distribution of Raman gain: we study its polarization dependence as this represents one of the crucial issues towards the implementation at the single photon regime. Because of the anisotropic nature of the Raman gain, we show that both parallel and perpendicular contributions to the Raman noise are significant for relatively small frequency detunings [START_REF] Li | Measurements of co-and cross-polarized Raman spectra in silica fiber for small detunings[END_REF].

Experimental setup

Our all-fiber experimental setup is shown in Fig. 1. To mitigate Brillouin scattering in the fiber, we took advantage of two independent linearly polarized intra cavity frequency-shifted feedback laser (IFSFL) pumps [START_REF] Krupa | Four-wave mixing in nonlinear fiber with two intracavity frequency-shifted laser pumps[END_REF] operating in the CW regime at 1540.7 nm (P1) and 1563.9 nm (P2). The two IFSFLs were amplified separately using two erbium-doped fiber amplifiers (EDFA), which were filtered and isolated from unwanted back reflections. The ASE noise from the amplifiers was reduced thanks to inline thin-film bandpass filters (BPF). Next, to further reduce the background spectral level, each pump was filtered using a 100 GHz wideband filter (nBF) having a noise rejection of about 60 dB (AOS GmbH). The signal (S) at 1549.2 nm was provided by a 5 mW extended cavity laser (ECL) attenuated by 25.6 dB when working with photon counters. We controlled the polarization state of the two pumps and the signal by means of fiber polarization controllers (PC). The two pumps were combined at a 50/50 fiber coupler, while the signal was added through a 5/95 tap coupler, which provided a further insertion loss of 13 dB for the signal channel. We used a L = 450m long highly nonlinear fiber (HNLF) whose zero dispersion wavelength (ZDW) was at 1545 nm. The fiber dispersion slope was 0.018ps/(km • nm 2 ) and the fiber nonlinear coefficient was of γ = 10W -1 km -1 (see also Ref. [START_REF] Méchin | 180-nm wavelength conversion based on Bragg scattering in an optical fiber[END_REF]. At the HNLF input the average powers of pumps were 22 mW (P1) and 15 mW (P2). The spectral bandwidth for pumps were of 44 GHz for P1 and 50 GHz for P2 (see Ref. [START_REF] Krupa | Four-wave mixing in nonlinear fiber with two intracavity frequency-shifted laser pumps[END_REF] for details). The ECL laser bandwidth was of 100kHz. At the fiber output, we placed a second 50/50 coupler to analyze two spectral regions simultaneously: one at 1549.2 nm (the residual signal) and one at 1526.43 nm (the idler, whose frequency is the sum of the signal frequency and the frequency difference between the two pumps). We isolate signal and idler from the two intense pumps by an appropriate narrow filtering stage. Therefore, at each output arm of the coupler, we cascaded two filters (nBF filter followed by a Newport tunable filter) with a total noise rejection of about 100 dB. To explore different pump and signal polarization arrangements, we combined, at the output, the PC with a fiber polarizer (Pol). Depending on the intensity of the signal channel, we employed two different detection schemes. With a non attenuated signal, we directly measured the fiber output spectrum with an optical spectrum analyzer. On the contrary, when the signal was attenuated, we used two photon counters (IDQ 201) operating at 100 kHz trigger rate and 10% detection efficiency.We used a gate duration of 2.5 ns, and under these conditions the photon counters have a dark-count probability of 2.7 × 10 -6 ns -1 .

Frequency conversion with non attenuated signal: numerical simulations and experimental results

Following Ref. [START_REF] Mckinstrie | Translations of quantum states by four-wave mixing in fibers[END_REF], we show first in the left panel of Fig. 2 the results of numerical simulations based on two incoherently coupled nonlinear Schrödinger equations (Manakov system) with parameters close to our experimental setup. We considered four different input conditions representing different combinations of polarization states for the pumps and the signal. The pump powers were set to a fixed values when comparing the effect of the different input polarization states in the numerical simulations and in the experiments. To account for the broadband nature of the pumps, each plot represents the average spectral power taken over an ensemble of 20 numerical runs [START_REF] Krupa | Four-wave mixing in nonlinear fiber with two intracavity frequency-shifted laser pumps[END_REF]. In case A, pumps and signal are all co-polarized, leading to the strongest signal-idler coupling, hence the highest BS FWM conversion efficiency [START_REF] Mckinstrie | Translations of quantum states by four-wave mixing in fibers[END_REF]. In our implementation the low value of dispersion limits the conversion efficiency of BS-FWM (from 1549.2 nm to 1526.43 nm); moreover, other nonlinear wave mixing effects are also active. For example, degenerate four-wave mixing (DFWM) involving P1 converts the same signal into an idler at 1532.3 nm. The relatively large intensity of the DFWM-induced idler is due to the proximity of the signal wavelength to the fiber ZDW. Additionally, spontaneous FWM induced by the two pumps only generates the two external sidebands in Fig. 2 left. In cases B and D, the two pumps are cross-polarized, so that only one of them is co-polarized with the signal. Instead in case C, the two pumps are co-polarized, while being cross-polarized with the signal and the BS-idler. Note that in cases B and C the conversion efficiency is 6 dB lower than in case A [START_REF] Mckinstrie | Translations of quantum states by four-wave mixing in fibers[END_REF]. However, DFWM is switched off due to the lack of parity in the number of photons per polarization state. The opposite happens in case D: here the signal is co polarized with P1, such that DFWM is active and BS is switched off, as predicted in Ref. [START_REF] Mckinstrie | Translations of quantum states by four-wave mixing in fibers[END_REF]. These numerical studies are in excellent agreement with the obtained experimental spectra shown in the right panel of the same Fig. 2.

More specifically, we observe the cases where both BS-FWM and DFWM are present (case A of Fig. 2, right panel), and where they are mutually exclusive (cases B, C and D in Fig. 2, right panel), in full agreement with the relevant polarization arrangements. Case C is of particular interest for quantum frequency conversion due to a reduced Raman noise [START_REF] Langford | Efficient quantum computing using coherent photon conversion[END_REF][START_REF] Mckinstrie | Translations of quantum states by four-wave mixing in fibers[END_REF].

Experimental results with an attenuated signal

Let us now consider the problems arising when aiming at operating in the photon counting regime. Because of the low conversion efficiency of our experimental configuration, the simplest way to distinguish clearly the idler from noise, in all polarization configurations, was to provide a large enough signal power. For this reason we measured the residual signal after a 18dB attenuator (see Fig. 1). The 3dB coupler and a chain of filters at the fiber output gave an insertion loss of 9dB. At the fiber output we estimated then 600 signal photons/gate. We underline that our signal remains an attenuated laser and not a single photon source. Note however that no attenuation was employed for the idler detection for which we estimate 5 photons/gate (the idler optical bandwidth at macroscopic signal was 75 GHz thanks to the IFSFLs). The experimental results are illustrated in the left panel of Fig. 3. The histograms provide the mean number of detected photons per second, either at 1526.43 nm (BS-idler channel, see top left and bottom left and right panels), or at 1549.2 nm (signal channel, top right panel). We observe that the ratio between signal and idler counts has the same order of magnitude which was previously observed at the macroscopic level, when we consider the effect of the 18dB attenuator for the signal channel (see Fig. 2). For each case, the counts were recorded for four different input conditions. That is, when all P1, P2 and S are present (blue bars), when only P1 and S are present (green bars), when only S is present (red bars), and finally when only P2 and S are present (black bars). Dark counts were negligible being in the order of 1.2 counts/s. By looking at the top right panel, which corresponds to case A, we observe that the signal is slightly amplified by P1 (compare red and green bars) and slightly attenuated by P2 (compare red and black bars). Indeed, since the signal wavelength is in-between those of P1 and P2, the observed signal gain/loss is likely due to stimulated Raman scattering. When both pumps and the signal are present, a further signal depletion was measured (compare red and blue bars). Correspondingly, we observed a clear count enhancement at the BS-idler wavelength (see the blue bar of the top left histogram). Nevertheless, nonzero counts were observed at 1526.43 nm even when one of the pumps was switched off. We believe that this noise contribution is mainly induced by spontaneous Raman emission from the pumps, as will be discussed later on. Case A maximizes the idler counts, thanks to the associated larger BS-FWM conversion. This case also leads to the largest (parallel) Raman noise. By suitably choosing the polarization states of pumps and signal, we could vary the number of counts for both the BS idler (blue bars) and the Raman noise (green and black bars) in the 1526 nm channel. For instance, in case D, we could reduce the BS counts down to the noise level, despite the presence of both pumps and the signal. This is due to the switching off of the BS-FWM (see Fig. 2). Here, the blue bar combines the Raman noise carried by the two pumps, and its level is equal to the sum of the green and black bars. Finally, in case C we could reach an intermediate level of conversion: in this situation the sidebands experience perpendicular Raman gain from the pumps. Note that the overall performances of cases A and C are similar, namely in terms of nearly equal signal-to-noise ratio for the BS idler. With respect to case A, in case C we observe a reduction of the Raman noise (which still remains nonzero due to the perpendicular Raman contribution, see the right panel of Fig. 3) simultaneously with a nearly equal reduction of the BS-FWM conversion efficiency, due to the halving of the coupling coefficient for the fields [START_REF] Mckinstrie | Translations of quantum states by four-wave mixing in fibers[END_REF]. As far as the dominant physical mechanism which is responsible for noise in our experiments, we observe first that for each pump k the product γP k L ∼ 0.15. Following Ref. [START_REF] Lin | Photon-pair generation in optical fibers through four-wave mixing: role of Raman scattering and pump polarization[END_REF], and for the range of frequency detunings of our experiments, we can expect that at room temperature the Raman noise is significantly larger than the noise originating from spontaneous FWM, which moreover requires phase matching. Since the frequency conversion processes take place for relatively small detunings with respect to the Raman gain bandwidth, two important issues have to be considered when studying the noise figure [START_REF] Mckinstrie | Translations of quantum states by four-wave mixing in fibers[END_REF]. On one hand, the Raman gain in silica glass fibers has a relevant anisotropic contribution [START_REF] Lin | Raman response function for silica fibers[END_REF], and on the other hand, spontaneous Raman noise at both Stokes and anti Stokes wavelengths exists at room temperatures [START_REF] Brainis | Spontaneous growth of Raman Stokes and anti-Stokes waves in fibers[END_REF]. In particular, anti Stokes emission is possible due to distributed coupling with phonons. Indeed, for small sideband detunings, the phonons energy is comparable to k B T , where k B is the Boltzmann constant and T is the temperature. Following Refs. [START_REF] Li | Measurements of co-and cross-polarized Raman spectra in silica fiber for small detunings[END_REF][START_REF] Brainis | Spontaneous growth of Raman Stokes and anti-Stokes waves in fibers[END_REF], we analysed experimentally the spontaneous Raman emission which is separately induced by each of the two pumps in the absence of the signal. We then calculated the parallel (R g, ) and perpendicular (R g,⊥ ) frequency-dependent Raman gain taking into account the input photon counts. In the right panel of Fig. 3 we present our experimental results obtained with the ECL laser at 1540.7nm (as P1) with 1.1 mW at the HNLF input. Similar results were obtained when using the IFSFLs pumps, but thanks to the narrow spectral width of the ECL, we could explore smaller frequency detunings. For each measurement, we averaged the number of detected photons over 60 seconds. The solid curves in Fig. 3 (right panel) provide the Raman model depicted in Ref. [START_REF] Lin | Raman response function for silica fibers[END_REF] for describing the parallel and perpendicular components of the Raman gain. That model includes both the isotropic (R g,a ) and anisotropic (R g,b ) contributions to the Raman gain. We found that this model fits our measurements very well, except for its perpendicular component on the Stokes side, where we observed a slightly larger orthogonal gain. Fig. 3 (right panel) shows that the anisotropic Raman contribution is relevant in our experiments, since for small detunings from the pumps this contribution is higher than the isotropic one [START_REF] Lin | Raman response function for silica fibers[END_REF]. As we have obviously R g, = R g,a + R g,b , while R g,⊥ = R g,b /2, the gain depolarization factor is therefore as high as R g,⊥ /R g, ∼ 0.3. This is the reason why Raman noise was observed even in case C. Such a factor is expected to drop significantly when the signal detuning grows larger, owing to the predominance of the isotropic contribution. Spontaneous FWM may explain some residual disagreements between the experimental results and the pure Raman noise assumption.

Conclusion

To conclude, we explored the BS-FWM process and related polarization dependence in both strong signal and photon-counting regimes for small (20 nm) detunings from the pumps at telecom wavelengths. We also analysed its competition with noise, mainly of Raman origin. We observed a clear enhancement of the mean number of photons per second at the idler channel (1526 nm) when BS FWM was active, and the corresponding depletion of signal. We reported a significant difference of BS-idler counts between case A, where signal and pumps are all copolarized and case C, where signal is cross polarized with co-polarized pumps (or case B, where signal is cross-polarized with pump 1 and cross-polarized with pump 2), which is in reasonable agreement with the expected 6 dB difference of the conversion efficiency. One may improve the conversion efficiency with a better suited fiber dispersion values and set of wavelengths; the idler bandwidth can be reduced by using an appropriate phase modulation for pumps [START_REF] Méchin | 180-nm wavelength conversion based on Bragg scattering in an optical fiber[END_REF]. Then, to go towards the single photon regime, smarter solution of detection based on time and spectral filtering should be implemented. Differently from high index contrast waveguides, HNLF fibers have naturally a ZDW close to telecom bandwidth but their full exploitation will require new generations of HNLF with specific control of the dispersion fluctuations along the fiber length as described in Ref. [START_REF] Kuo | Dispersion-stabilized highly-nonlinear fiber for wideband parametric mixer synthesis[END_REF].

  Photon Counter 2Photon Counter 1

Fig. 1 .

 1 Fig. 1. Experimental setup: See the text for details.

Fig. 2 .

 2 Fig. 2. Left. Numerical simulations of dual pump FWM. Red: signal at 1549.2 nm; Blue: BS idler at 1526.43 nm; Green: DFWM at 1532.3 nm. Cases A -D are described in the text. Right. Experimental spectra from an optical spectrum analyzer in the strong signal regime. Cases A-D correspond to the numerical simulations shown of the left panel.

Fig. 3 .

 3 Fig. 3. Left. Experimental results with photon counters (see text for details). Right. Raman gain calculated from noise measurements of an extended cavity laser (ECL) at 1540.7 nm.

Acknowledgments

We acknowledge the financial support from the Region Limousin and from the French National Research Agency, under grant ANR 08-JCJC-0122 PARADHOQS.


Gerard Huyghebaert

Philippe Delahaut

Els Daeseleire

V Vandenberge
email: valerie.vandenberge@ilvo.vlaanderen.be

E Delezie

S Croubels

and muscle tissues due to cross-contamination in the feed

Keywords: Veterinary drug residues -sulphonamides, Veterinary drug residuestetracycline, Veterinary drug residues -transmission Animal feed, Animal products -meat Residue, sulfadiazine, doxycycline, muscle, liver, feed, cross-contamination, 26 depletion 27 28 29 30

published or not. The documents may come

Residues of sulfadiazine and doxycycline in broiler liver

and muscle tissues due to cross-contamination in the feed

Introduction 31

The veterinary drugs used therapeutically in poultry industry are generally given via mass 32 application in feed or drinking water (Kan and Petz 2000). Antibiotics are used to raise 33 production efficiency , as they enhance growth and feed efficiency and reduce disease 34 [START_REF] Donoghue | Antibiotic residues in poultry tissues and eggs: Human health concerns? 355[END_REF]. Antibiotics have been used as growth promoters in the member states of 35 the EU but concerns about development of antibacterial resistance led to the withdrawal of 36 these agents as growth promoters in the EU since January 1, 2006 [START_REF] Castanon | History of the use of antibiotic as growth promoters in European poultry 344 feeds[END_REF]. Some 37 antimicrobial drugs, such as sulfonamides are used to treat coccidiosis (Roudaut and Garnier 38 2002; Sentepe and Eraslan 2010). In veterinary medicine the sulfonamides are effective 39 chemotherapeutics for bacterial or protozoal diseases [START_REF] Emea | Committee for veterinary medicinal products sulfonamides (2) summary report 364[END_REF][START_REF] Bcfi | Gecommentarieerd Geneesmiddelenrepertorium voor Diergeneeskundig 342 gebruik[END_REF]. Doxycycline 40 belongs to the group of tetracycline antibiotics, which have a broad spectrum activity (EMEA 41 1996). These drugs are widely used as mass medication in both pig and poultry industry.

42

Concerning feed medication, in Belgium, sulfadiazine in combination with trimethoprim is 43 available as pre-mix for pigs and non-laying hens and as oral powder to be mixed in the feed 44 of pigs. Doxycycline is registered as a premix only for pigs and also an oral powder to be weighed and 1 ml of the internal standard demethylchlortetracycline (20 µg mL -1 ) was 153 added. After the addition of 25 mL of methanol, the samples were placed on a rotary shaker 154 for 20 minutes and centrifuged for 10 min at 4000 rpm. Two hundred µL of the supernatant 155 was transferred in a vial, 800 µL of HPLC water was added and the vial was vortex mixed.

156

Extraction and clean-up of broiler samples

157 For sulfadiazine analysis in muscle and liver, we used the method described by Mortier et al.

158 (2005). Two grams of minced muscle or liver were weighed and the internal standard (20 µl 159 of 10 µg mL -1 ) was added. The samples were vortex mixed, were allowed to stand for 10 min 160 and 6 g (muscle) or 8 g (liver) of anhydrous sodium sulphate was added. The tissue was 161 carefully mixed with a spatula until a powdery mixture was obtained and 10 mL (muscle) or 162 20 mL (liver) of acetonitrile was added. After vortex mixing, the tubes were placed on a 163 horizontal shaker for 30 min. The tubes were centrifuged (15 min, 4000 rpm), 5 mL (muscle) 164 or 10 mL (liver) of the supernatant was transferred into a tube and was evaporated to 165 dryness under nitrogen in a water bath of 45°C. Then, the sample was redissolved in 1 mL of 166 an acetonitrile/water mixture (50:50,v:v) containing 0.1 % formic acid, vortex mixed, 167 sonicated for 5 min and filtered through a 0.22µm filter into a HPLC vial.

168 For doxycycline analysis in tissues, 2 g of minced tissue were weighed and extracted 169 as described by [START_REF] Cherlet | Quantitative multi-residue analysis 346 of tetracyclines and their 4-epimers in pig tissues by high-performance liquid 347 chromatography combined with positive-ion eclectrospray ionization mass spectrometry[END_REF]. After the addition of the internal standard 170 demethylchlortetracycline (50 µl of 10 µg mL -1 ), the samples were vortex mixed an were 171 allowed to stand for 5 min. Ten mL of a 0.1 M sodium succinate buffer was added, the 172 samples were placed on a rotary shaker for 20 minutes and centrifuged at 4000 rpm for 10 173 min. The supernatant was added to a centrifuge tube containing 1 mL of a 20 % 174 trichloroacetic acid, vortex mixed and centrifuged at 4000 rpm for 10 min. The supernatant 175 was filtered through a Whatman® filter paper and was ready for a further solid-phase clean-176 up step. In the solid-phase clean-up, the Oasis® HLB (hydrophilic lipophilic balance) column 177 was preconditioned consecutively with 3 mL methanol, 3 mL of a 1N HCl solution and 3 mL 178 of water and the extract was allowed to pass slowly through the HLB column. The HLB 179 column was washed with 1 mL of water and dried. The analytes were eluted with 3 mL of > 107.95, 250.94 -> 92.18, 194 250.94 -> 156.06; doxycycline: m/z 462.45 -> 428.1). In this study, the limit of quantification 195 (LOQ) of the method was 5 mg kg -1 and 2.5 mg kg -1 for sulfadiazine and doxycycline 196 respectively for feed. For muscle and liver, the method LOQ was 2 and 10 µg kg -1 for 197 sulfadiazine and doxycycline. For both sulfadiazine and doxycycline the maximum authorized dose is 250 mg kg -1 218 (instruction leaflets). Therefore a value of 25, 12.5 and 6.25 mg kg -1 was calculated for the 10 219 %, 5 % and 2.5 % carry-over groups, respectively. Mean concentrations ± standard deviation 220 and recovery for the 10 %, 5 % and 2.5 % groups were 21.2 ± 1.8 (79 %), 12.7 ± 0.9 (93 %) 221 and 5.5 ± 0.4 (88 %)mg kg -1 for sulfadiazine and 24.7 ± 4.6 (99 %), 13.2 ± 4.1 (106 %), 5.5 ± [START_REF] Baert | Pharmacokinetics and oral bioavailability 337 of sulfadiazine and thrimetoprim in broiler chickens[END_REF]. Sulphonamides may be metabolized in various tissues, However, once the "cross-contaminated feed" was no longer administered, we observed a 305 fast decline in measured concentrations.

306

The use of cross-contaminated feed may have several consequences for animal as 307 well as human health. Besides the possible adverse effect on the health of non-target 308 species and the indirect toxic effect by the promotion of resistant strains of bacteria, the 309 MRL may also be exceeded in food matrices of animal origin, and consequently food safety 310 may not be guaranteed [START_REF] Donoghue | 352 Modeling drug residue uptake by eggs: Yolks contain ampicillin residues even after drug 353 withdrawal and nondetectability in the plasma[END_REF]Segato et al. 2011;Vincent et al. 2011)

  , however exceeded the corresponding Maximum Residue Limits (MRLs).

  feeding stuffs are prepared in multi-product plants following good Acros (Geel, Belgium), respectively. The internal standards, namely 93 sulfachloropyridazine and demethylchlortetracycline were purchased from Sigma. 94 Acetonitrile, distilled water and methanol came from Biosolve (Valkenswaard, The 95 Netherlands). Water was HPLC grade and generated by a Q Gard 2 system (Millipore, 96 Billerica, MA, USA). The anhydrous sodium sulphate came from Merck (Darmstadt, 97 Germany). Filters for filtration of the sulfadiazine extract, i.e. 0.22 µm Millex® GV were from 98 Millipore (Billerica, MA, USA). The solid-phase extraction (SPE) columns used for the clean-99 up of doxycycline containing samples were Oasis® columns (HLB SPE column 60 mg/3 ml, 100 Waters, Milford, MA, USA). The Robot Coupe 2, used to mince muscle and liver samples, 101 came from C-Tech Systems (Lille, Belgium). 102 Preparation of the experimental feed and animal experiments 103 The feed preparation and the animal experiments were carried out at ILVO's Animals 104 Sciences Unit with approval from the ILVO ethical committee (EC no. 2008/89, 2009/108). 105 The experimental feed was mixed with sulfadiazine or doxycycline at cross-106 contamination levels of 10 %, 5 % and 2.5 % of the maximum allowed dose of 250 mg kg -1 107 feed for both drugs. The premix resp. oral powder was first mixed with a small amount of 108 blank feed in the Varimixer Bear (Dehaeck construct, Ghent, Belgium), which was then 109 added to the remaining blank feed in the feed mixer constructed by Silobouw (Zulte, 110 Belgium). The final experimental feed was mixed at 35 rpm during 30 minutes with a switch 111 of direction every five minutes. Before and between the preparations of each experimental 112 feed, rinsing batches were carried out. Ten samples were taken from the moving stream of 113 experimental feed and from the top (n=3), the middle (n=4) and the bottom (n=3) of each 114 experimental feed to determine the concentration achieved and two samples of each rinsing 115 batch were analyzed to exclude cross-contamination. Because of the large amount needed, 116 the preparation of each experimental feed was performed in two phases. 117 A growth trial was set-up with 575 one-day-old broilers, male Ross 308, which were 118 housed per concentration in a concrete floor pen under conventional conditions of 119 ventilation, temperature and lighting (22 hours light/day). The animals were randomly 120 assigned to three treatments, distributed among three pens and separated from each other 121 by a narrow walkway. Each study group counted 188 chickens, of which 102 were 122 small group of 11 broilers was used as control group. Each study consisted of a

  180 methanol and the eluate was evaporated to dryness at 40 °C under nitrogen. After 181 redissolvation with 250 µL of a 0.5 % formic acid solution and vortex mixing, the sample was 182 transferred to an autosampler vial. was performed on a Waters Alliance 2695 Separation Module 185 combined with a Waters Micromass Quattro MS instrument. An X-terra® column (C 18 5 µm, 186 150 x 2.1 mm) was used and the analysis was performed with a gradient of 0.1 % formic acid 187 (FA) in H 2 O (solvent A) and 0.1 % FA in ACN (solvent B). The analysis of doxycycline was 188 performed on a Waters Alliance 2695 Separation Module combined with a Waters 189 Micromass Quattro Ultima MS instrument. Separation was performed on a PLRP-S polymer 190 column (5 µm, 150 x 2.1 mm). An isocratic mobile phase consisting of 70 % solvent A (HPLC 191 H 2 O+0.5% formic acid and 0.001 M oxalic acid; 0.5 % tetrahydrofurane) and 30 % of solvent 192 B (ACN) was used. The analysis of sulfadiazine and doxycycline was performed in MRM mode 193 and electrospray positive ion mode (sulfadiazine: m/z 250.94 -

198

  Pharmacokinetic and statistical analysis199 These studies were not repeated, therefore, no statistical analysis of the effect of the 200 experimental feed on the zootechnical parameters was performed. Due to the limited 201 dataset of the experimental feed, no statistical analysis to determine the homogeneity of the 202 experimental feeds could be carried out. The terminal elimination half-life and elimination 203 rate constant in tissues were calculated using pharmacokinetic (PK) functions for Excel and 204 were based on the terminal slope of the tissue concentration-time curve after withdrawal of 205 the experimental feed (Usansky et al, Allergan, Irvine, USA). Statistical analysis was 206 performed with Statistica 9.0 (StatSoft. Inc., Tulsa, OK, USA). Significance level α was set at 207 0.05. Statistical analysis was performed on the mean of the residue concentrations of the 208 individual chickens and the pooled sample for each of the different matrices. The 209 comparison between measured residue concentrations in thigh muscle and in breast muscle 210 was performed with a t-test for dependent samples. The pooled sample on day 13 of 211 treatment period was analysed six times. The mean of these results was compared with the 212 mean of the individual chickens by an independent t-test. experimental feed on the performance parameter, broiler weight, were 216 studied. The mean broiler weights are presented for sulfadiazine and doxycycline in Figure 1.

  217

  222 1.2 (88 %) mg kg -1 for doxycycline. 223 Residue concentrations in breast muscle, thigh muscle and liver 224 The residue concentrations for respectively sulfadiazine and doxycycline for all groups in 225 breast muscle are shown in Figure 2. The plateau phase was reached at day 3 for all dietary 226 treatments of the doxycycline and the 2.5 and 5 % carry over group of sulfadiazine. A rapid 227 depletion phase was noted after withdrawal of the experimental feed. The breast muscle 228 tissue elimination half-life was calculated for both molecules but due to the rather fast 229 decline below the LOQ, elimination half-life could only be calculated for the 5 and 10 % 230 group of doxycycline, namely 3.1 and 3.5 days, respectively. The concentrations in the three 231 tissue matrices for the 10 % group during plateau phase are presented in Figure 3. Small 232 differences in concentrations were noted between the different types of muscles for 233 sulfadiazine as well as for doxycycline but these were not statistically significant. Measured 234 residue concentrations in liver were similar from the concentrations in muscle for 235 sulfadiazine although higher concentrations were measured in liver compared to muscle for 236 doxycycline. The ratio liver/breast muscle was 1.2 and 1.8; the ratio liver/thigh muscle was 237 1.3 and 1.9 for sulfadiazine and doxycycline respectively. Transfer factors, 238 ౙౙ౪౨౪ ౪ ౣ౪౨౮ ౚ౯ భయ ౪౨౪ౣ౪ ౦౨ౚ ౨ ౪ భబ % ౨౫౦ ౣ౩౫౨ౚ ౙౙ౪౨౪ ౪ ౚ * ଵ 239 are presented for liver, breast muscle and thigh muscle for sulfadiazine and doxycycline.

  287mainly the liver at varying degrees and through varying mechanisms such as acetylation and 288 oxidation and are mainly excreted via the kidneys (Sentepe and Eraslan 2010).289In this study, besides individual tissue analysis we also analyzed a pooled sample. We 290 may conclude that pooled tissue samples are a good reference for the mean of the 6 291 individual chickens for the studied matrices. An important remark is the animal dependent 292 variability, noted for each of the different matrices. This needs to be kept in mind while 293 sampling and interpreting the results in case of a cross contamination problem occurs on MRL's have been set at 100 µg kg -1 for sulfadiazine as well as for 296 doxycycline (BCFI 2010). The 10 % group of sulfadiazine and the 5 and 10 % group of 297 doxycycline generated concentrations above the MRL, whereas intake of 5 % carry-over of 298 sulfadiazine and the 2.5 % of doxycycline resulted in concentrations around the MRL. Only 299 the 2.5 % group of sulfadiazine generated concentrations that were below the MRL. For 300 liver, MRL'-s have been set at 100 µg kg -1 and 300 µg kg -1 for sulphonamides and doxycycline 301 resp.[START_REF] Bcfi | Gecommentarieerd Geneesmiddelenrepertorium voor Diergeneeskundig 342 gebruik[END_REF]. Only liver tissue from the animals receiving the 10% concentration in feed 302 was analyzed and resulted in concentrations exceeding the MRLs. It may be concluded that 303 may lead to the presence of residual concentrations above MRL. 304

  The measured residue concentrations of the tissues from the 6 individual chickens 243 and the six repetitive analyses' of the pooled sample were compared. The calculated means 244 and standard deviations are presented in Table1. For both molecules no significant 245 difference between the 2 means was noted for all matrices (P>0.05).

	273	be interesting to determine the edible tissue with the highest concentration if the target
	274	tissue is muscle (Reyes-Herrera et al. 2005). Elimination half-lives were calculated for breast
	275	muscle. Atef et al. (2002) found an elimination half-life of 1.8 days for breast muscle in
	246 276	contrast to our calculated value of 3.1 to 3.5 days. This difference may be due to the
	247 277	Discussion different ways of oral administration and the duration of the administration, a dose of 15
	278	mg/kg was provided orally twice daily during 5 successive days. Doxycycline had higher
	248 279	For the experimental feed of sulfadiazine and doxycycline, the target concentrations of 2.5, 5 residue concentrations in the liver compared to muscle as reflected by the feed/tissue
	249 250 280 281	F o and 10 % of the maximum allowed dose were reached (79 -106 %) but no statements can be made about the homogeneity of the experimental feeds. The achievement of adequate transfer ratio's, which has also been described by other researchers (Atef et al. 2002; Ismail F o and El-Kattan 2004). Doxycycline is known to be well absorbed from the gastro-intestinal
		r mixing of an active ingredient in final feed depends on the composition of the final feed, the r tract (Yoshimura et al. 1991), to have a high volume of distribution with the highest detected
		P P
		e e r e e r
		R e v i e w R e v i e w
	263 264 265 266	O n l y concentrations in breast and thigh muscle. Most of the sampling occasions, the residue concentrations in breast muscle were slightly higher compared to the concentrations in thigh O n muscle. For doxycycline, this was confirmed by Atef et al. (2002) while Reyes-Herrera et al. l y
	267	(2005, 2008) found significantly higher concentrations of enrofloxacin in breast muscle
	268	compared to thigh muscle but no difference in concentrations between different breast
	269	sections (Reyes-Herrera et al, 2005; Reyes-Herrera et al, 2008). Other authors reported
		differences between residual concentrations in thigh and breast muscle, which may be
	242 272

240

Transfer factors of breast muscle, thigh muscle and liver were 1.04, 0.95 and 1.37 % for 241 sulfadiazine and 1.27, 1.62 and 2.90 % for doxycycline. 251 precision and the size of samples taken for analysis and other factors such as particle size, 252 electrostatic properties, types of mixing machinery and mixing in stages or trituration

(EMEA 253 1996)

. The ideal way to mix a drug is to add it at the beginning of the pre-mix process just 254 prior to pelleting

[START_REF] Daniel | Drugs and chemicals in aquafeeds: The problems and solutions[END_REF]

. Good sampling requires sufficient samples of adequate size 255 and sampling from a moving stream is better than static sampling

[START_REF] Borras | Analysis of antimicrobial agents in animal feed[END_REF]

. Since 256 very small volumes of pre-mixes were added to large amounts blank finished feed and 257 because the experimental feed differs in composition from the feed used for target species 258 for which the medicated pre-mix is intended, it is not easy to guarantee a homogeneous 259 feed. For each experimental feed, ten dynamic samples were taken after 30 min of mixing at 260 the top (n = 3), the middle (n= 4) and the bottom (n=3) of the final feed in order to provide 261 as good as possible sampling procedure.

262

For both compounds studied, no significant differences were found between the 270 explained by a different amount of intramuscular fat content. According to Reyes-Herrera, 271 different antibiotics may have different sites of preferential deposition and therefore it may 282 levels in liver and kidney

(EMEA 1996)

and to have a high tissue binding

(Santos et al. 1997)

. 283 Doxycycline has an absolute oral bioavailability of 73.4 % and may be metabolised up to 40 284 % by the liver and is largely excreted in faeces, via bile (EMEA 1996; Laczay et al. 2001). After 285 oral dosing, a good bioavailability (approximately 80 %) of sulfadiazine is obtained in non-286 fasted chickens

Table 1 :

 1 European Commission. 2002. Regulation (EC) No 178/2002 of the European parliament and 360 of the Council laying down the general principles and requirements of food law, establishing 361 the European Food Safety Authority and laying down procedures in matters of food safety. Mean and standard deviation (µg kg -1 ) of the pooled sample and the six individual

	. These

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Acknowledgements

319

Special thanks go to our lab personnel; namely P. De Neve, P. Van Herreweeghe, E. Verween,