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Simple Network Management Protocol

Simple Network Management Protocol (SNMP) is an Internet Standard protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behavior. Devices that typically support SNMP include cable modems, routers, switches, servers, workstations, printers, and more.

SNMP is widely used in network management for network monitoring. SNMP exposes management data in the form of variables on the managed systems organized in a management information base (MIB), which describes the system status and configuration. These variables can then be remotely queried (and, in some circumstances, manipulated) by managing applications.

Three significant versions of SNMP have been developed and deployed. SNMPv1 is the original version of the protocol. More recent versions, SNMPv2c and SNMPv3, feature improvements in performance, flexibility and security.

SNMP is a component of the Internet Protocol Suite as defined by the Internet Engineering Task Force (IETF). It consists of a set of standards for network management, including an application layer protocol, a database schema, and a set of data objects.

Overview and basic concepts
In typical uses of SNMP, one or more administrative computers called managers have the task of monitoring or managing a group of hosts or devices on a computer network. Each managed system executes a software component called an agent which reports information via SNMP to the manager.

An SNMP-managed network consists of three key components:
 * Managed devices
 * Agent – software which runs on managed devices
 * Network management station (NMS) – software which runs on the manager

A managed device is a network node that implements an SNMP interface that allows unidirectional (read-only) or bidirectional (read and write) access to node-specific information. Managed devices exchange node-specific information with the NMSs. Sometimes called network elements, the managed devices can be any type of device, including, but not limited to, routers, access servers, switches, cable modems, bridges, hubs, IP telephones, IP video cameras, computer hosts, and printers.

An agent is a network-management software module that resides on a managed device. An agent has local knowledge of management information and translates that information to or from an SNMP-specific form.

A network management station executes applications that monitor and control managed devices. NMSs provide the bulk of the processing and memory resources required for network management. One or more NMSs may exist on any managed network.

Management information base
SNMP agents expose management data on the managed systems as variables. The protocol also permits active management tasks, such as configuration changes, through remote modification of these variables. The variables accessible via SNMP are organized in hierarchies. SNMP itself does not define which variables a managed system should offer. Rather, SNMP uses an extensible design that allows applications to define their own hierarchies. These hierarchies are described as a management information base (MIB). MIBs describe the structure of the management data of a device subsystem; they use a hierarchical namespace containing object identifiers (OID). Each OID identifies a variable that can be read or set via SNMP. MIBs use the notation defined by Structure of Management Information Version 2.0 (SMIv2, ), a subset of ASN.1.

Protocol details
SNMP operates in the application layer of the Internet protocol suite. All SNMP messages are transported via User Datagram Protocol (UDP). The SNMP agent receives requests on UDP port 161. The manager may send requests from any available source port to port 161 in the agent. The agent response is sent back to the source port on the manager. The manager receives notifications (Traps and InformRequests) on port 162. The agent may generate notifications from any available port. When used with Transport Layer Security or Datagram Transport Layer Security, requests are received on port 10161 and notifications are sent to port 10162.

SNMPv1 specifies five core protocol data units (PDUs). Two other PDUs, GetBulkRequest and InformRequest were added in SNMPv2 and the Report PDU was added in SNMPv3. All SNMP PDUs are constructed as follows:

The seven SNMP PDU types as identified by the PDU-type field are as follows:

 * GetRequest: A manager-to-agent request to retrieve the value of a variable or list of variables. Desired variables are specified in variable bindings (the value field is not used). Retrieval of the specified variable values is to be done as an atomic operation by the agent. A Response with current values is returned.
 * SetRequest: A manager-to-agent request to change the value of a variable or list of variables. Variable bindings are specified in the body of the request. Changes to all specified variables are to be made as an atomic operation by the agent. A Response with (current) new values for the variables is returned.
 * GetNextRequest: A manager-to-agent request to discover available variables and their values. Returns a Response with variable binding for the lexicographically next variable in the MIB. The entire MIB of an agent can be walked by iterative application of GetNextRequest starting at OID 0. Rows of a table can be read by specifying column OIDs in the variable bindings of the request.
 * GetBulkRequest: A manager-to-agent request for multiple iterations of GetNextRequest. An optimized version of GetNextRequest. Returns a Response with multiple variable bindings walked from the variable binding or bindings in the request. PDU specific non-repeaters and max-repetitions fields are used to control response behavior. GetBulkRequest was introduced in SNMPv2.
 * Response: Returns variable bindings and acknowledgement from agent to manager for GetRequest, SetRequest, GetNextRequest, GetBulkRequest and InformRequest. Error reporting is provided by error-status and error-index fields. Although it was used as a response to both gets and sets, this PDU was called GetResponse in SNMPv1.
 * : Asynchronous notification from agent to manager. While in other SNMP communication, the manager actively requests information from the agent, these are PDUs that are sent from the agent to the manager without being explicitly requested. SNMP traps enable an agent to notify the management station of significant events by way of an unsolicited SNMP message. Trap PDUs include current sysUpTime value, an OID identifying the type of trap and optional variable bindings. Destination addressing for traps is determined in an application-specific manner typically through trap configuration variables in the MIB. The format of the trap message was changed in SNMPv2 and the PDU was renamed SNMPv2-Trap.
 * : Acknowledged asynchronous notification. This PDU was introduced in SNMPv2 and was originally defined as manager to manager communication. Later implementations have loosened the original definition to allow agent to manager communications. Manager-to-manager notifications were already possible in SNMPv1 using a Trap, but as SNMP commonly runs over UDP where delivery is not assured and dropped packets are not reported, delivery of a Trap was not guaranteed. InformRequest fixes this as an acknowledgement is returned on receipt.

specifies that an SNMP implementation must accept a message of at least 484 bytes in length. In practice, SNMP implementations accept longer messages. If implemented correctly, an SNMP message is discarded if the decoding of the message fails and thus malformed SNMP requests are ignored. A successfully decoded SNMP request is then authenticated using the community string. If the authentication fails, a trap is generated indicating an authentication failure and the message is dropped.

SNMPv1 and SNMPv2c use communities to establish trust between managers and agents. Most agents support three community names, one each for read-only, read-write and trap. These three community strings control different types of activities. The read-only community applies to get requests. The read-write community string applies to set requests. The trap community string applies to receipt of traps. SNMPv3 also uses community strings, but allows for secure authentication and communication between SNMP manager and agent.

Protocol versions
In practice, SNMP implementations often support multiple versions: typically SNMPv1, SNMPv2c, and SNMPv3.

Version 1
SNMP version 1 (SNMPv1) is the initial implementation of the SNMP protocol. The design of SNMPv1 was done in the 1980s by a group of collaborators who viewed the officially sponsored OSI/IETF/NSF (National Science Foundation) effort (HEMS/CMIS/CMIP) as both unimplementable in the computing platforms of the time as well as potentially unworkable. SNMP was approved based on a belief that it was an interim protocol needed for taking steps towards large-scale deployment of the Internet and its commercialization.

The first Request for Comments (RFCs) for SNMP, now known as SNMPv1, appeared in 1988:
 * — Structure and identification of management information for TCP/IP-based internets
 * — Management information base for network management of TCP/IP-based internets
 * — A simple network management protocol

In 1990, these documents were superseded by:
 * — Structure and identification of management information for TCP/IP-based internets
 * — Management information base for network management of TCP/IP-based internets
 * — A simple network management protocol

In 1991, (MIB-1) was replaced by the more often used:
 * — Version 2 of management information base (MIB-2) for network management of TCP/IP-based internets

SNMPv1 is widely used and is the de facto network management protocol in the Internet community.

SNMPv1 may be carried by transport layer protocols such as User Datagram Protocol (UDP), OSI Connectionless-mode Network Service (CLNS), AppleTalk Datagram Delivery Protocol (DDP), and Novell Internetwork Packet Exchange (IPX).

Version 1 has been criticized for its poor security. The specification does, in fact, allow room for custom authentication to be used, but widely used implementations "support only a trivial authentication service that identifies all SNMP messages as authentic SNMP messages." The security of the messages, therefore, becomes dependent on the security of the channels over which the messages are sent. For example, an organization may consider their internal network to be sufficiently secure that no encryption is necessary for its SNMP messages. In such cases, the community name, which is transmitted in cleartext, tends to be viewed as a de facto password, in spite of the original specification.

Version 2
SNMPv2, defined by and, revises version 1 and includes improvements in the areas of performance, security and manager-to-manager communications. It introduced GetBulkRequest, an alternative to iterative GetNextRequests for retrieving large amounts of management data in a single request. The new party-based security system introduced in SNMPv2, viewed by many as overly complex, was not widely adopted. This version of SNMP reached the Proposed Standard level of maturity, but was deemed obsolete by later versions.

Community-Based Simple Network Management Protocol version 2, or SNMPv2c, is defined in –. SNMPv2c comprises SNMPv2 without the controversial new SNMP v2 security model, using instead the simple community-based security scheme of SNMPv1. This version is one of relatively few standards to meet the IETF's Draft Standard maturity level, and was widely considered the de facto SNMPv2 standard. It was later restated as part of SNMPv3.

User-Based Simple Network Management Protocol version 2, or SNMPv2u, is defined in –. This is a compromise that attempts to offer greater security than SNMPv1, but without incurring the high complexity of SNMPv2. A variant of this was commercialized as SNMP v2*, and the mechanism was eventually adopted as one of two security frameworks in SNMP v3.

64-bit counters
SNMP version 2 introduces the option for 64-bit data counters. Version 1 was designed only with 32-bit counters which can store integer values from zero to 4.29 billion (precisely 4,294,967,295). A 32-bit version 1 counter cannot store the maximum speed of a 10 gigabit or larger interface, expressed in bits per second. Similarly, a 32-bit counter tracking statistics for a 10 gigabit or larger interface can roll over back to zero again in less than one minute, which may be a shorter time interval than a counter is polled to read its current state. This would result in lost or invalid data due to the undetected value rollover, and corruption of trend-tracking data.

The 64-bit version 2 counter can store values from zero to 18.4 quintillion (precisely 18,446,744,073,709,551,615) and so is currently unlikely to experience a counter rollover between polling events. For example, 1.6 terabit Ethernet is predicted to become available by 2025. A 64-bit counter incrementing at a rate of 1.6 trillion bits per second would be able to retain information for such an interface without rolling over for 133 days.

SNMPv1 and SNMPv2c interoperability
SNMPv2c is incompatible with SNMPv1 in two key areas: message formats and protocol operations. SNMPv2c messages use different header and protocol data unit (PDU) formats than SNMPv1 messages. SNMPv2c also uses two protocol operations that are not specified in SNMPv1. To overcome incompatibility, defines two SNMPv1/v2c coexistence strategies: proxy agents and bilingual network-management systems.

Proxy agents
An SNMPv2 agent can act as a proxy agent on behalf of SNMPv1-managed devices. When an SNMPv2 NMS issues a command intended for an SNMPv1 agent it sends it to the SNMPv2 proxy agent instead. The proxy agent forwards,  , and   messages to the SNMPv1 agent unchanged. GetBulk messages are converted by the proxy agent to  messages and then are forwarded to the SNMPv1 agent. Additionally, the proxy agent receives and maps SNMPv1 trap messages to SNMPv2 trap messages and then forwards them to the NMS.

Bilingual network-management system
Bilingual SNMPv2 network-management systems support both SNMPv1 and SNMPv2. To support this dual-management environment, a management application examines information stored in a local database to determine whether the agent supports SNMPv1 or SNMPv2. Based on the information in the database, the NMS communicates with the agent using the appropriate version of SNMP.

Version 3
Although SNMPv3 makes no changes to the protocol aside from the addition of cryptographic security, it looks very different due to new textual conventions, concepts, and terminology. The most visible change was to define a secure version of SNMP, by adding security and remote configuration enhancements to SNMP. The security aspect is addressed by offering both strong authentication and data encryption for privacy. For the administration aspect, SNMPv3 focuses on two parts, namely notification originators and proxy forwarders. The changes also facilitate remote configuration and administration of the SNMP entities, as well as addressing issues related to the large-scale deployment, accounting, and fault management.

Features and enhancements included:
 * Identification of SNMP entities to facilitate communication only between known SNMP entities – Each SNMP entity has an identifier called the SNMPEngineID, and SNMP communication is possible only if an SNMP entity knows the identity of its peer. Traps and Notifications are exceptions to this rule.
 * Support for security models – A security model may define the security policy within an administrative domain or an intranet. SNMPv3 contains the specifications for a user-based security model (USM).
 * Definition of security goals where the goals of message authentication service include protection against the following:
 * Modification of Information – Protection against some unauthorized SNMP entity altering in-transit messages generated by an authorized principal.
 * Masquerade – Protection against attempting management operations not authorized for some principal by assuming the identity of another principal that has the appropriate authorizations.
 * Message stream modification – Protection against messages getting maliciously re-ordered, delayed, or replayed to affect unauthorized management operations.
 * Disclosure – Protection against eavesdropping on the exchanges between SNMP engines.
 * Specification for USM – USM consists of the general definition of the following communication mechanisms available:
 * Communication without authentication and privacy (NoAuthNoPriv).
 * Communication with authentication and without privacy (AuthNoPriv).
 * Communication with authentication and privacy (AuthPriv).
 * Definition of different authentication and privacy protocols – MD5, SHA and HMAC-SHA-2 authentication protocols and the CBC_DES and CFB_AES_128 privacy protocols are supported in the USM.
 * Definition of a discovery procedure – To find the SNMPEngineID of an SNMP entity for a given transport address and transport endpoint address.
 * Definition of the time synchronization procedure – To facilitate authenticated communication between the SNMP entities.
 * Definition of the SNMP framework MIB – To facilitate remote configuration and administration of the SNMP entity.
 * Definition of the USM MIBs – To facilitate remote configuration and administration of the security module.
 * Definition of the view-based access control model (VACM) MIBs – To facilitate remote configuration and administration of the access control module.

Security was one of the biggest weaknesses of SNMP until v3. Authentication in SNMP Versions 1 and 2 amounts to nothing more than a password (community string) sent in clear text between a manager and agent. Each SNMPv3 message contains security parameters which are encoded as an octet string. The meaning of these security parameters depends on the security model being used. The security approach in v3 targets:
 * Confidentiality – Encryption of packets to prevent snooping by an unauthorized source.
 * Integrity – Message integrity to ensure that a packet has not been tampered while in transit including an optional packet replay protection mechanism.
 * Authentication – to verify that the message is from a valid source.

v3 also defines the USM and VACM, which were later followed by a transport security model (TSM) that provided support for SNMPv3 over SSH and SNMPv3 over TLS and DTLS.
 * USM (User-based Security Model) provides authentication and privacy (encryption) functions and operates at the message level.
 * VACM (View-based Access Control Model) determines whether a given principal is allowed access to a particular MIB object to perform specific functions and operates at the PDU level.
 * TSM (Transport Security Model) provides a method for authenticating and encrypting messages over external security channels. Two transports, SSH and TLS/DTLS, have been defined that make use of the TSM specification.

the IETF recognizes Simple Network Management Protocol version 3 as defined by – (also known as STD0062) as the current standard version of SNMP. The IETF has designated SNMPv3 a full Internet standard, the highest maturity level for an RFC. It considers earlier versions to be obsolete (designating them variously Historic or Obsolete).

Implementation issues
SNMP's powerful write capabilities, which would allow the configuration of network devices, are not being fully utilized by many vendors, partly because of a lack of security in SNMP versions before SNMPv3, and partly because many devices simply are not capable of being configured via individual MIB object changes.

Some SNMP values (especially tabular values) require specific knowledge of table indexing schemes, and these index values are not necessarily consistent across platforms. This can cause correlation issues when fetching information from multiple devices that may not employ the same table indexing scheme (for example fetching disk utilization metrics, where a specific disk identifier is different across platforms.)

Some major equipment vendors tend to over-extend their proprietary command line interface (CLI) centric configuration and control systems.

In February 2002 the Carnegie Mellon Software Engineering Institute (CM-SEI) Computer Emergency Response Team Coordination Center (CERT-CC) issued an Advisory on SNMPv1, after the Oulu University Secure Programming Group conducted a thorough analysis of SNMP message handling. Most SNMP implementations, regardless of which version of the protocol they support, use the same program code for decoding protocol data units (PDU) and problems were identified in this code. Other problems were found with decoding SNMP trap messages received by the SNMP management station or requests received by the SNMP agent on the network device. Many vendors had to issue patches for their SNMP implementations.

Using SNMP to attack a network
Because SNMP is designed to allow administrators to monitor and configure network devices remotely it can also be used to penetrate a network. A significant number of software tools can scan the entire network using SNMP, therefore mistakes in the configuration of the read-write mode can make a network susceptible to attacks.

In 2001, Cisco released information that indicated that, even in read-only mode, the SNMP implementation of Cisco IOS is vulnerable to certain denial of service attacks. These security issues can be fixed through an IOS upgrade.

If SNMP is not used in a network it should be disabled in network devices. When configuring SNMP read-only mode, close attention should be paid to the configuration of the access control and from which IP addresses SNMP messages are accepted. If the SNMP servers are identified by their IP, SNMP is only allowed to respond to these IPs and SNMP messages from other IP addresses would be denied. However, IP address spoofing remains a security concern.

Authentication
SNMP is available in different versions, and each version has its own security issues. SNMP v1 sends passwords in plaintext over the network. Therefore, passwords can be read with packet sniffing. SNMP v2 allows password hashing with MD5, but this has to be configured. Virtually all network management software support SNMP v1, but not necessarily SNMP v2 or v3. SNMP v2 was specifically developed to provide data security, that is authentication, privacy and authorization, but only SNMP version 2c gained the endorsement of the Internet Engineering Task Force (IETF), while versions 2u and 2* failed to gain IETF approval due to security issues. SNMP v3 uses MD5, Secure Hash Algorithm (SHA) and keyed algorithms to offer protection against unauthorized data modification and spoofing attacks. If a higher level of security is needed the Data Encryption Standard (DES) can be optionally used in the cipher block chaining mode. SNMP v3 is implemented on Cisco IOS since release 12.0(3)T.

SNMPv3 may be subject to brute force and dictionary attacks for guessing the authentication keys, or encryption keys, if these keys are generated from short (weak) passwords or passwords that can be found in a dictionary. SNMPv3 allows both providing random uniformly distributed cryptographic keys and generating cryptographic keys from a password supplied by the user. The risk of guessing authentication strings from hash values transmitted over the network depends on the cryptographic hash function used and the length of the hash value. SNMPv3 uses the HMAC-SHA-2 authentication protocol for the User-based Security Model (USM). SNMP does not use a more secure challenge-handshake authentication protocol. SNMPv3 (like other SNMP protocol versions) is a stateless protocol, and it has been designed with a minimal amount of interactions between the agent and the manager. Thus introducing a challenge-response handshake for each command would impose a burden on the agent (and possibly on the network itself) that the protocol designers deemed excessive and unacceptable.

The security deficiencies of all SNMP versions can be mitigated by IPsec authentication and confidentiality mechanisms. SNMP also may be carried securely over Datagram Transport Layer Security (DTLS).

Many SNMP implementations include a type of automatic discovery where a new network component, such as a switch or router, is discovered and polled automatically. In SNMPv1 and SNMPv2c this is done through a community string that is transmitted in clear-text to other devices. Clear-text passwords are a significant security risk. Once the community string is known outside the organization it could become the target for an attack. To alert administrators of other attempts to glean community strings, SNMP can be configured to pass community-name authentication failure traps. If SNMPv2 is used, the issue can be avoided by enabling password encryption on the SNMP agents of network devices.

The common default configuration for community strings are "public" for read-only access and "private" for read-write. Because of the well-known defaults, SNMP topped the list of the SANS Institute's Common Default Configuration Issues and was number ten on the SANS Top 10 Most Critical Internet Security Threats for the year 2000. System and network administrators frequently do not change these configurations.

Whether it runs over TCP or UDP, SNMPv1 and v2 are vulnerable to IP spoofing attacks. With spoofing, attackers may bypass device access lists in agents that are implemented to restrict SNMP access. SNMPv3 security mechanisms such as USM or TSM can prevent spoofing attacks.

RFC references

 * (STD 16) — Structure and Identification of Management Information for the TCP/IP-based Internets
 * (Historic) — Management Information Base for Network Management of TCP/IP-based internets
 * (Historic) — A Simple Network Management Protocol (SNMP)
 * (STD 17) — Management Information Base for Network Management of TCP/IP-based internets: MIB-II
 * (Informational) — Coexistence between version 1 and version 2 of the Internet-standard Network Management Framework (Obsoleted by )
 * (Experimental) — Introduction to Community-based SNMPv2
 * (Draft Standard) — Structure of Management Information for SNMPv2 (Obsoleted by )
 * (Standards Track) — Coexistence between Version 1 and Version 2 of the Internet-standard Network Management Framework
 * (Informational) — Introduction to Version 3 of the Internet-standard Network Management Framework  (Obsoleted by )
 * (STD 58) — Structure of Management Information Version 2 (SMIv2)
 * (Informational) — Introduction and Applicability Statements for Internet Standard Management Framework
 * STD 62 contains the following RFCs:
 * — An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks
 * — Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)
 * — Simple Network Management Protocol (SNMP) Applications
 * — User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)
 * — View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP)
 * — Version 2 of the Protocol Operations for the Simple Network Management Protocol (SNMP)
 * — Transport Mappings for the Simple Network Management Protocol (SNMP)
 * — Management Information Base (MIB) for the Simple Network Management Protocol (SNMP)
 * (Experimental) — Simple Network Management Protocol (SNMP) over Transmission Control Protocol (TCP) Transport Mapping
 * (BCP 74) — Coexistence between Version 1, Version 2, and Version 3 of the Internet-standard Network Management Framework
 * (Proposed) — The Advanced Encryption Standard (AES) Cipher Algorithm in the SNMP User-based Security Model
 * (Proposed) — Simple Network Management Protocol (SNMP) over IEEE 802 Networks
 * (STD 78) — Simple Network Management Protocol (SNMP) Context EngineID Discovery
 * (STD 78) — Transport Subsystem for the Simple Network Management Protocol (SNMP)
 * (STD 78) — Transport Security Model for the Simple Network Management Protocol (SNMP)
 * (Proposed) — Secure Shell Transport Model for the Simple Network Management Protocol (SNMP)
 * (Proposed) —  Remote Authentication Dial-In User Service (RADIUS) Usage for Simple Network Management Protocol (SNMP) Transport Models.
 * (STD 78) — Transport Layer Security (TLS) Transport Model for the Simple Network Management Protocol (SNMP)
 * (Proposed|Historic) — HMAC-SHA-2 Authentication Protocols in the User-based Security Model (USM) for SNMPv3
 * (Proposed) — HMAC-SHA-2 Authentication Protocols in User-Based Security Model (USM) for SNMPv3

title: Simple Network Management Protocol


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Simplex signaling

Simplex signaling (SX) is signaling in which two conductors are used for a single telecommunication circuit, and a center-tapped coil, or its equivalent, is used to split the signaling current equally between the two conductors. The return path for the current is through ground. It is distinct from a phantom circuit in which the return current path for power or signaling is provided through different signal conductors. SX signaling may be one-way, for intra-central-office use, or the simplex legs may be connected to form full duplex signaling circuits that function like composite (CX) signaling circuits with E&M lead control.

Simplex is also used to describe a powering method where one or more signal conductors carries direct current to power a remote device, which sends its output signal back on the same conductor. Phantom powering as used in audio is a form of simplex powering, as the return current flows through the ground or shield conductor.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Simplex signaling


text:
SINAD

The signal-to-noise and distortion ratio (SINAD) is a measure of the quality of a signal from a communications device, often defined as

\mathrm{SINAD} = 10\log_{10} \frac{P_\text{signal} + P_\text{noise} + P_\text{distortion}}{P_\text{noise} + P_\text{distortion}}, $$ where $$P$$ is the average power of the signal, noise and distortion components. SINAD is usually expressed in dB and is quoted alongside the receiver RF sensitivity, to give a quantitative evaluation of the receiver sensitivity. Note that with this definition, unlike SNR, a SINAD reading can never be less than 1 (i.e. it is always positive when quoted in dB).

When calculating the distortion, it is common to exclude the DC components.

Due to widespread use, SINAD has collected several different definitions. SINAD is commonly defined as: Information on the relations between SINAD, ENOB, SNR, THD and SFDR can be found in the footnotes of this article.
 * 1) The ratio of (a) total received power, i.e., the signal to (b) the noise-plus-distortion power. This is modeled by the equation above.
 * 2) The ratio of (a) the power of a test signal, i.e. a sine wave, to (b) the residual received power, i.e. noise-plus-distortion power. With this definition, it is possible to have a SINAD level less than one. This definition is used in the calculation of ENOB for DACs and ADCs.

Commercial radio specifications
A typical example, quoted from a commercial hand held VHF or UHF radio, might be:

 * Receiver sensitivity: 0.25 μV at 12 dB SINAD.

This is stating that the receiver will produce intelligible speech with a signal at its input as low as 0.25 μV. Radio receiver designers will test the product in a laboratory using a procedure, which is typically as follows:
 * With no signal present on the input, the noise and distortion of the receiver are measured at a convenient level.
 * A signal is applied to the input such that the output increases by 12 dB.
 * The level of the signal needed to produce this is noted. In this case, it was found to be 0.25 microvolts.

According to the radio designer, intelligible speech can be detected 12 dB above the receiver's noise floor (noise and distortion). Regardless of the accuracy of this output power in regards to intelligible speech, having a standard output SINAD allows easy comparison between radio receiver input sensitivities. This 0.25 μV value is typical for VHF commercial radio, while 0.35 μV is probably more typical for UHF. In the real world, lower SINAD values (more noise) can still result in intelligible speech, but it is tiresome work to listen to a voice in that much noise.

title: SINAD


text:
Single-frequency signaling

In telephony, single-frequency signaling (SF) is line signaling in which dial pulses or supervisory signals are conveyed by a single-frequency tone in each direction in the voice-band. SF and similar systems were used in 20th-century carrier systems.

An SF signaling unit converts DC signaling (usually, at least in long-distance circuits, E&M signaling) to a format (characterized by the presence or absence of a single voice-frequency tone), which is suitable for transmission over an AC path, e.g., a carrier system. The SF tone is present in the on-hook or idle state and absent during the seized state. In the seized state, dial pulses are conveyed by bursts of SF tone, corresponding to the interruptions in dc continuity created by a rotary dial or other DC dialing mechanism.

The SF tone may occupy a small portion of the user data channel spectrum, e.g., 1600 Hz or 2600 Hz (SF "in-band signaling)". There may be a notch filter at the precise SF frequency, either filtering the circuit at all times or only when the circuit is off-hook, to prevent the user from inadvertently disconnecting a call if the users voice has a sufficiently strong spectral content at the SF frequency, a falsing condition known as talk-off. Notoriously, this property was exploited by blue boxers and other toll fraudsters. The SF tone may also be just outside the user voice band, e.g., 3600 Hz.

The Defense Data Network (DDN) transmitted DC line signaling pulses or supervisory signals, or both, over carrier channels or cable pairs on a four-wire circuit basis using a 2600 Hz signal tone. The conversion into tones, or vice versa, is done by SF signal units.

SF was developed in the early 20th century and standardized in middle century. It declined in the 1970s due to the adoption of T-carrier, and was largely abandoned late in the century in favor of common-channel signaling.

title: Single-frequency signaling


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Single-mode optical fiber

[[Image:Singlemode fibre structure.svg|thumb|right|The structure of a typical single-mode fiber. 1. Core 8–9 μm diameter

2. Cladding 125 μm diameter

3. Buffer 250 μm diameter

4. Jacket 900 μm diameter]]

In fiber-optic communication, a single-mode optical fiber (SMF), also known as fundamental- or mono-mode, is an optical fiber designed to carry only a single mode of light - the transverse mode. Modes are the possible solutions of the Helmholtz equation for waves, which is obtained by combining Maxwell's equations and the boundary conditions. These modes define the way the wave travels through space, i.e. how the wave is distributed in space. Waves can have the same mode but have different frequencies. This is the case in single-mode fibers, where we can have waves with different frequencies, but of the same mode, which means that they are distributed in space in the same way, and that gives us a single ray of light. Although the ray travels parallel to the length of the fiber, it is often called transverse mode since its electromagnetic oscillations occur perpendicular (transverse) to the length of the fiber. The 2009 Nobel Prize in Physics was awarded to Charles K. Kao for his theoretical work on the single-mode optical fiber. The standards G.652 and G.657 define the most widely used forms of single-mode optical fiber.

History
In 1961, Elias Snitzer while working at American Optical published a comprehensive theoretical description of single mode fibers in the Journal of the Optical Society of America.

At the Corning Glass Works (now Corning Inc.), Robert Maurer, Donald Keck and Peter Schultz started with fused silica, a material that can be made extremely pure, but has a high melting point and a low refractive index. They made cylindrical preforms by depositing purified materials from the vapor phase, adding carefully controlled levels of dopants to make the refractive index of the core slightly higher than that of the cladding, without raising attenuation dramatically. In September 1970, they announced they had made single-mode fibers with attenuation at the 633-nanometer helium-neon line below 20 dB/km.

Characteristics
Unlike multi-mode optical fiber, single-mode fiber does not exhibit modal dispersion. This is due to the fiber having such a small cross section that only the first mode is transported. Single-mode fibers are therefore better at retaining the fidelity of each light pulse over longer distances than multi-mode fibers. For these reasons, single-mode fibers can have a higher bandwidth than multi-mode fibers. Equipment for single-mode fiber is more expensive than equipment for multi-mode optical fiber, but the single-mode fiber itself is usually cheaper in bulk. A typical single-mode optical fiber has a core diameter between 8 and 10.5 μm and a cladding diameter of 125 μm. There are a number of special types of single-mode optical fiber which have been chemically or physically altered to give special properties, such as dispersion-shifted fiber and nonzero dispersion-shifted fiber. Data rates are limited by polarization mode dispersion and chromatic dispersion. , data rates of up to 10 gigabits per second were possible at distances of over 80 km with commercially available transceivers (Xenpak). By using optical amplifiers and dispersion-compensating devices, state-of-the-art DWDM optical systems can span thousands of kilometers at 10 Gbit/s, and several hundred kilometers at 40 Gbit/s.

The lowest-order bounds mode is ascertained for the wavelength of interest by solving Maxwell's equations for the boundary conditions imposed by the fiber, which are determined by the core diameter and the refractive indices of the core and cladding. The solution of Maxwell's equations for the lowest order bound mode will permit a pair of orthogonally polarized fields in the fiber, and this is the usual case in a communication fiber.

In step-index guides, single-mode operation occurs when the normalized frequency, V, is less than or equal to 2.405. For power-law profiles, single-mode operation occurs for a normalized frequency, V, less than approximately
 * $$2.405 \sqrt{\frac{g+2}{g}}$$,

where g is the profile parameter.

In practice, the orthogonal polarizations may not be associated with degenerate modes.

OS1 and OS2 are standard single-mode optical fiber used with wavelengths 1310 nm and 1550 nm (size 9/125 μm) with a maximum attenuation of 1 dB/km (OS1) and 0.4 dB/km (OS2). OS1 is defined in ISO/IEC 11801, and OS2 is defined in ISO/IEC 24702.

Connectors
Optical fiber connectors are used to join optical fibers where a connect/disconnect capability is required. The basic connector unit is a connector assembly. A connector assembly consists of an adapter and two connector plugs. Due to the sophisticated polishing and tuning procedures that may be incorporated into optical connector manufacturing, connectors are generally assembled onto optical fiber in a supplier's manufacturing facility. However, the assembly and polishing operations involved can be performed in the field, for example to make cross-connect jumpers to size.

Optical fiber connectors are used in telephone company central offices, at installations on customer premises, and in outside plant applications. Their uses include:
 * Making the connection between equipment and the telephone plant in the central office
 * Connecting fibers to remote and outside plant electronics such as optical network units (ONUs) and digital loop carrier (DLC) systems
 * Optical cross connects in the central office
 * Patching panels in the outside plant to provide architectural flexibility and to interconnect fibers belonging to different service providers
 * Connecting couplers, splitters, and wavelength-division multiplexers (WDMs) to optical fibers
 * Connecting optical test equipment to fibers for testing and maintenance.

Outside plant applications may involve locating connectors underground in subsurface enclosures that may be subject to flooding, on outdoor walls, or on utility poles. The closures that enclose them may be hermetic, or may be “free-breathing.” Hermetic closures will prevent the connectors within being subjected to temperature swings unless they are breached. Free-breathing enclosures will subject them to temperature and humidity swings, and possibly to condensation and biological action from airborne bacteria, insects, etc. Connectors in the underground plant may be subjected to groundwater immersion if the closures containing them are breached or improperly assembled.

The latest industry requirements for optical fiber connectors are in Telcordia GR-326, Generic Requirements for Single-Mode Optical Connectors and Jumper Assemblies.

A multi-fiber optical connector is designed to simultaneously join multiple optical fibers together, with each optical fiber being joined to only one other optical fiber.

The last part of the definition is included so as not to confuse multi-fiber connectors with a branching component, such as a coupler. The latter joins one optical fiber to two or more other optical fibers.

Multi-fiber optical connectors are designed to be used wherever quick and/or repetitive connects and disconnects of a group of fibers are needed. Applications include telecommunications companies’ central offices (COs), installations on customer premises, and outside plant (OSP) applications.

The multi-fiber optical connector can be used in the creation of a low-cost switch for use in fiber optical testing. Another application is in cables delivered to a user with pre-terminated multi-fiber jumpers. This would reduce the need for field splicing, which could greatly reduce the number of hours necessary for placing an optical fiber cable in a telecommunications network. This, in turn, would result in savings for the installer of such cable.

Industry requirements for multi-fiber optical connectors are covered in GR-1435, Generic Requirements for Multi-Fiber Optical Connectors.

Fiber optic switches
An optical switch is a component with two or more ports that selectively transmits, redirects, or blocks an optical signal in a transmission medium. According to Telcordia GR-1073, an optical switch must be actuated to select or change between states. The actuating signal (also referred to as the control signal) is usually electrical, but in principle, could be optical or mechanical. (The control signal format may be Boolean and may be an independent signal; or, in the case of optical actuation, the control signal may be encoded in the input data signal. Switch performance is generally intended to be independent of wavelength within the component passband.)

Quadruply clad fiber
In fiber optics, a quadruply clad fiber is a single-mode optical fiber that has four claddings. Each cladding has a refractive index lower than that of the core. With respect to one another, their relative refractive indices are, in order of distance from the core: lowest, highest, lower, higher.

A quadruply clad fiber has the advantage of very low macrobending losses. It also has two zero-dispersion points, and moderately low dispersion over a wider wavelength range than a singly clad fiber or a doubly clad fiber.

Advantages

 * No degradation of signal
 * Low dispersion
 * Good suitability for long distance communication

Disadvantages

 * More difficult manufacturing and handling
 * Higher price
 * Difficult coupling of light into the fiber

title: Single-mode optical fiber


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S interface

The S interface or S reference point, also known as S0, is a user–network interface reference point in an ISDN BRI environment, characterized by a four-wire circuit using  144kbit/s (2 bearer and 1 signaling channel; 2B+D) user rate.

The S interface is the connection between ISDN terminal equipment (TE) or terminal adapters (TAs) and an NT1 (network terminator, type 1.) Not all TE or TAs connect externally to an S interface, but instead integrate an NT1 so they can connect directly to a U interface (local loop from central office.)

Contrast to the T interface, which connects between an NT2 (PBX or other local switching device) and NT1. However, the S interface is electrically equivalent to the T interface, and the two are jointly referred to as the S/T interface.

The S interface operates at 4000 48-bit frames per second; i.e., 192kbit/s, with a user portion of 36bits per frame; i.e., 144kbit/s.

title: S interface


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Skew

Skew may refer to:

In mathematics

 * Skew lines, neither parallel nor intersecting.
 * Skew normal distribution, a probability distribution
 * Skew field or division ring
 * Skew-Hermitian matrix
 * Skew lattice
 * Skew polygon, whose vertices do not lie on a plane
 * Infinite skew polyhedron
 * Skew-symmetric graph
 * Skew-symmetric matrix
 * Skew tableau, a generalization of Young tableaux
 * Skewness, a measure of the asymmetry of a probability distribution
 * Shear mapping

In science and technology

 * Skew, also synclinal or gauche in alkane stereochemistry
 * Skew ray (optics), an optical path not in a plane of symmetry
 * Skew arch, not at a right angle

In computing

 * Clock skew
 * Transitive data skew, an issue of data synchronization

In telecommunications

 * Skew (fax), unstraightness
 * Skew (antenna) a method to improve the horizontal radiation pattern

Other uses

 * Volatility skew, in finance, a downward-sloping volatility smile
 * SKEW, the ticker symbol for the CBOE Skew Index

title: Skew


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Skip zone

A skip zone, also called a silent zone or zone of silence, is a region where a radio transmission can not be received. The zone is located between regions both closer and farther from the transmitter where reception is possible.

Cause
When using medium to high frequency radio telecommunication, there are radio waves which travel both parallel to the ground, and towards the ionosphere, referred to as a ground wave and sky wave, respectively. A skip zone is an annular region between the farthest points at which the ground wave can be received and the nearest point at which the refracted sky waves can be received. Within this region, no signal can be received because, due to the conditions of the local ionosphere, the relevant sky waves are not reflected but penetrate the ionosphere.

The skip zone is a natural phenomenon that cannot be influenced by technical means. Its width depends on the height and shape of the ionosphere and, particularly, on the local ionospheric maximum electron density characterized by critical frequency foF2. It varies mainly with this parameter, being larger for low foF2. With a fixed working frequency it is large by night and may even disappear by day. Transmitting at night is most effective for long-distance communication but the skip zone becomes significantly larger. Very high frequency waves and higher normally travel through the ionosphere wherefore communication via skywave is exceptional. A highly ionized Es-Layer that occasionally may appear in summer may produce such Sporadic E propagation.

Avoidance
A method of decreasing the skip zone is by decreasing the frequency of the radio waves. Decreasing the frequency is akin to increasing the ionospheric width. A point is eventually reached when decreasing the frequency results in a zero distance skip zone. In other words, a frequency exists for which vertically incident radio waves will always be refracted back to the Earth. This frequency is equivalent to the ionospheric plasma frequency and is also known as the ionospheric critical frequency, or foF2.

Other
Skip zone is the subject of a film 'SKIPZONE' made in 1992 by UK artist, Peter Lee-Jones. It refers to areas in Scottish Highlands where it is difficult to obtain radio and TV reception.

In the episode "Short Wave" of Father Knows Best, the family hears a distress call from a small boat at sea. Jim explains that the reason they, and not the Coast Guard, can hear the transmission is because of a "skip".

title: Skip zone


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Slant range

In radio electronics, especially radar terminology, slant range or slant distance is the distance along the relative direction between two points. If the two points are at the same level (relative to a specific datum), the slant distance equals the horizontal distance.

An example of slant range is the distance to an aircraft flying at high altitude with respect to that of the radar antenna. The slant range (1) is the hypotenuse of the triangle represented by the altitude of the aircraft and the distance between the radar antenna and the aircraft's ground track (point (3) on the earth directly below the aircraft). In the absence of altitude information, for example from a height finder, the aircraft location would be plotted farther (2) from the antenna than its actual ground track.

title: Slant range


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Slave clock

In telecommunication and horology, a slave clock is a clock that depends on another clock, the master clock. Modern clocks are synchronized through the Internet or by radio time signals, to Coordinated Universal Time. UTC is based on a network of atomic clocks in many countries. For scientific purposes, precision clocks can be synchronized to within nanoseconds by dedicated satellite channels. Slave clock synchronization is usually achieved by phase-locking the slave clock signal to a signal received from the master clock. To adjust for the transit time of the signal from the master clock to the slave clock, the phase of the slave clocks are adjusted so that both clocks are in phase. Thus, the time markers of both clocks, at the output of the clocks, occur simultaneously.

The predecessors of atomic clocks, computer clocks, and digital clocks, these electric clocks were synchronized by an electrical pulse, wired to their master clock in the same facility. Thus the terms "master" and "slave." From the late 19th to the mid 20th centuries, electrical master/slave clock systems were installed, all clocks in a building or facility synchronized through electric wires to a central master clock. Slave clocks either kept time by themselves, and were periodically corrected by the master clock, or required impulses from the master clock. Many slave clocks of these types were in operation, most commonly in schools, offices, military bases, hospitals, railway networks, telephone exchanges and factories the world over. School bells of elementary schools, high schools, and others were able to be synchronized across an entire campus, connected to the system. In schools, the master clock was in the principal's office, with slave units in classrooms which were in other buildings on campus. In factories, a system with a bell or horn could signal the end of a shift, lunchtime or break time. Very few relics of this electrical, analogue system operate in the 21st century. Most 21st century systems of the type are digital.

Pictures
Mechanical slave clocks from the 1950s and 1960s era.

title: Slave clock


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Slave station

Slave station may refer to:

 * A collection and transfer location utilized in the slave trade
 * A station having a clock synchronized by a remote master station in a navigation system
 * In a data network a station that is selected and controlled by a master station

title: Slave station


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Slip

Slip or SLIP may refer to:

 * Slip (clothing), an underdress or underskirt

Music

 * The Slip (band), a rock band
 * Slip (album), a 1993 album by the band Quicksand
 * The Slip (album) (2008), a.k.a. Halo 27, the seventh studio album by Nine Inch Nails
 * "Slip" (song), a 2013 song by Stooshe
 * "Slip", a song by Linkin Park from LP Underground 11
 * "Slip", a song by Shawn Austin from the 2022 extended play Planes Don't Wait
 * "Slip", an instrumental by Deadmau5 from his 2008 album Random Album Title

Nickname

 * Slip Carr (1899–1971), Australian rugby union player and Olympic sprinter
 * Slip Madigan (1896–1966), American college football player and multi-sport college coach

Biology

 * Slip (fish), also known as Black Sole
 * Slip (horticulture), a small cutting of a plant as a specimen or for grafting
 * Muscle slip, a branching of a muscle, in anatomy

Computing and telecommunications

 * SLIP (programming language), (Symmetric LIst Processing language)
 * Slip (telecommunication), a positional displacement in a sequence of transmitted symbols
 * Serial Line Internet Protocol, a mostly obsolete encapsulation of the Internet Protocol

Earth science

 * Silicic-dominated Large Igneous Province (SLIP), a geological feature consisting of a large area of igneous rocks of a certain type
 * Slip, the relative movement of geological features present on either side of a fault plane
 * Land slip or landslide, commonly called a slip in New Zealand

Materials

 * Slip (ceramics), an aqueous suspension of minerals
 * Slip (materials science), the process by which a dislocation motion produces plastic deformation

Mechanical systems

 * Slip (vehicle dynamics), the relative motion between a tire and the road surface
 * Slip (electrical motor), the difference in speed between the frequency supplied to an induction motor and rotor shaft speed
 * Slip, a type of rail switch
 * Slip gauge or gauge block, a system for producing precision lengths

Psychology

 * Slip (treatment), a temporary return to a negative behavior after therapy intended to correct that behavior
 * Freudian slip, an error in speech, memory or physical action that arises from the unconscious mind

Other uses in science and technology

 * Slip (aerodynamics), an aerodynamic state in which an aircraft is moving sideways as well as forward relative to the oncoming airflow
 * Slipway, a ramp on the shore by which ships or boats can be moved to and from the water

Television

 * "Slip" (Better Call Saul), an episode of Better Call Saul
 * Slip (TV series), an American comedy television series
 * "The Slip" (The Amazing World of Gumball), an episode of The Amazing World of Gumball

Other uses

 * Slip (cricket), a fielding position in cricket
 * Slip (needlework), an embroidered or appliquéd image of a plant-cutting
 * Slip opinion, a kind of judicial opinion, published on the day of the decision and subject to later revision
 * Ferry slip, a specialized docking facility that receives a ferryboat
 * Packing slip, a shipping document that accompanies delivery packages
 * The Slip, a section of Swansea Beach

title: Slip


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Spatial application

A spatial application is a technological application (such as video) requiring high spatial resolution, possibly at the expense of reduced temporal positioning accuracy, such as increased jerkiness.

Examples of spatial applications include the requirement to display small characters and to resolve fine detail in still video, or in motion video that contains very limited motion.

title: Spatial application


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Specific detectivity

Specific detectivity, or D*, for a photodetector is a figure of merit used to characterize performance, equal to the reciprocal of noise-equivalent power (NEP), normalized per square root of the sensor's area and frequency bandwidth (reciprocal of twice the integration time).

Specific detectivity is given by $$D^*=\frac{\sqrt{A \Delta f}}{NEP}$$, where $$A$$ is the area of the photosensitive region of the detector, $$\Delta f$$ is the bandwidth, and NEP the noise equivalent power in units [W]. It is commonly expressed in Jones units ($$cm \cdot \sqrt{Hz}/ W$$) in honor of Robert Clark Jones who originally defined it.

Given that noise-equivalent power can be expressed as a function of the responsivity $$\mathfrak{R}$$ (in units of $$A/W$$ or $$V/W$$) and the noise spectral density $$S_n$$ (in units of $$A/Hz^{1/2}$$ or $$V/Hz^{1/2}$$) as $$NEP=\frac{S_n}{\mathfrak{R}}$$, it is common to see the specific detectivity expressed as $$D^*=\frac{\mathfrak{R}\cdot\sqrt{A}}{S_n}$$.

It is often useful to express the specific detectivity in terms of relative noise levels present in the device. A common expression is given below.

 * $$D^* = \frac{q\lambda \eta}{hc} \left[\frac{4kT}{R_0 A}+2q^2 \eta \Phi_b\right]^{-1/2}$$

With q as the electronic charge, $$\lambda$$ is the wavelength of interest, h is Planck's constant, c is the speed of light, k is Boltzmann's constant, T is the temperature of the detector, $$R_0A$$ is the zero-bias dynamic resistance area product (often measured experimentally, but also expressible in noise level assumptions), $$\eta$$ is the quantum efficiency of the device, and $$\Phi_b$$ is the total flux of the source (often a blackbody) in photons/sec/cm2.

Detectivity measurement
Detectivity can be measured from a suitable optical setup using known parameters. You will need a known light source with known irradiance at a given standoff distance. The incoming light source will be chopped at a certain frequency, and then each wavelength will be integrated over a given time constant over a given number of frames.

In detail, we compute the bandwidth $$\Delta f$$ directly from the integration time constant $$t_c$$.

 * $$ \Delta f = \frac{1}{2 t_c} $$

Next, an average signal and rms noise needs to be measured from a set of $$N$$ frames. This is done either directly by the instrument, or done as post-processing.

 * $$ \text{Signal}_{\text{avg}} = \frac{1}{N}\big( \sum_i^{N} \text{Signal}_i \big) $$

 * $$ \text{Noise}_{\text{rms}} = \sqrt{\frac{1}{N}\sum_i^N (\text{Signal}_i - \text{Signal}_{\text{avg}})^2} $$

Now, the computation of the radiance $$H$$ in W/sr/cm2 must be computed where cm2 is the emitting area. Next, emitting area must be converted into a projected area and the solid angle; this product is often called the etendue. This step can be obviated by the use of a calibrated source, where the exact number of photons/s/cm2 is known at the detector. If this is unknown, it can be estimated using the black-body radiation equation, detector active area $$A_d$$ and the etendue. This ultimately converts the outgoing radiance of the black body in W/sr/cm2 of emitting area into one of W observed on the detector.

The broad-band responsivity, is then just the signal weighted by this wattage.

 * $$R = \frac{\text{Signal}_{\text{avg}}}{H G} = \frac{\text{Signal}_{\text{avg}}}{\int dH dA_d d\Omega_{BB}}$$

Where,
 * $$R$$ is the responsivity in units of Signal / W, (or sometimes V/W or A/W)
 * $$H$$ is the outgoing radiance from the black body (or light source) in W/sr/cm2 of emitting area
 * $$G$$ is the total integrated etendue between the emitting source and detector surface
 * $$A_d$$ is the detector area
 * $$\Omega_{BB}$$ is the solid angle of the source projected along the line connecting it to the detector surface.

From this metric noise-equivalent power can be computed by taking the noise level over the responsivity.

 * $$ \text{NEP} = \frac{\text{Noise}_{\text{rms}}}{R} = \frac{\text{Noise}_{\text{rms}}}{\text{Signal}_{\text{avg}}}H G $$

Similarly, noise-equivalent irradiance can be computed using the responsivity in units of photons/s/W instead of in units of the signal. Now, the detectivity is simply the noise-equivalent power normalized to the bandwidth and detector area.

 * $$ D^* = \frac{\sqrt{\Delta f A_d}}{\text{NEP}} = \frac{\sqrt{\Delta f A_d}}{H G} \frac{\text{Signal}_{\text{avg}}}{\text{Noise}_{\text{rms}}} $$

title: Specific detectivity


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Spectral width

In telecommunications, spectral width is the width of a spectral band, i.e., the range of wavelengths or frequencies over which the magnitude of all spectral components is significant, i.e., equal to or greater than a specified fraction of the largest magnitude.

In optical communications applications, the usual method of specifying spectral width is the full width at half maximum (FWHM). This is the same convention used in bandwidth, defined as the frequency range where power drops by less than half (at most −3 dB).

The FWHM method may be difficult to apply when the spectrum has a complex shape. Another method of specifying spectral width is a special case of root-mean-square deviation where the independent variable is wavelength, λ, and f (λ) is a suitable radiometric quantity.

The relative spectral width, Δλ/λ, is frequently used where Δλ is obtained according to note 1, and λ is the center wavelength.

title: Spectral width


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Speed of service

In telecommunication, speed of service is the time for a message to be received. For example:
 * 1) The time between release of a message by the originator to receipt of the message by the addressee, as perceived by the end user. (originator-to-recipient speed of service)
 * 2) The time between entry of a message into a communications system and receipt of the message at the terminating communications facility, i.e., the communications facility serving the addressee, as measured by the system.

title: Speed of service


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Spill-forward feature

In telecommunication, a spill-forward feature is a service feature, in the operation of an intermediate office, that, acting on incoming trunk service treatment indications, assumes routing control of the call from the originating office. This increases the chances of completion by offering the call to more trunk groups than are available in the originating office.

title: Spill-forward feature


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Spread spectrum

In telecommunication, especially radio communication, spread spectrum are techniques by which a signal (e.g., an electrical, electromagnetic, or acoustic) generated with a particular bandwidth is deliberately spread in the frequency domain over a wider frequency band. Spread-spectrum techniques are used for the establishment of secure communications, increasing resistance to natural interference, noise, and jamming, to prevent detection, to limit power flux density (e.g., in satellite downlinks), and to enable multiple-access communications.

Telecommunications
Spread spectrum generally makes use of a sequential noise-like signal structure to spread the normally narrowband information signal over a relatively wideband (radio) band of frequencies. The receiver correlates the received signals to retrieve the original information signal. Originally there were two motivations: either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to hide the fact that communication was even taking place, sometimes called low probability of intercept (LPI).

Frequency-hopping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum (CSS), and combinations of these techniques are forms of spread spectrum. The first two of these techniques employ pseudorandom number sequences—created using pseudorandom number generators—to determine and control the spreading pattern of the signal across the allocated bandwidth. Wireless standard IEEE 802.11 uses either FHSS or DSSS in its radio interface.

 * Techniques known since the 1940s and used in military communication systems since the 1950s "spread" a radio signal over a wide frequency range several magnitudes higher than minimum requirement. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to-point communication at the same data rate.
 * Resistance to jamming (interference). Direct sequence (DS) is good at resisting continuous-time narrowband jamming, while frequency hopping (FH) is better at resisting pulse jamming. In DS systems, narrowband jamming affects detection performance about as much as if the amount of jamming power is spread over the whole signal bandwidth, where it will often not be much stronger than background noise. By contrast, in narrowband systems where the signal bandwidth is low, the received signal quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth.
 * Resistance to eavesdropping. The spreading sequence (in DS systems) or the frequency-hopping pattern (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it obscures the signal and reduces the chance of an adversary making sense of it. Moreover, for a given noise power spectral density (PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower — often significantly lower than the noise PSD — so that the adversary may be unable to determine whether the signal exists at all. However, for mission-critical applications, particularly those employing commercially available radios, spread-spectrum radios do not provide adequate security unless, at a minimum, long nonlinear spreading sequences are used and the messages are encrypted.
 * Resistance to fading. The high bandwidth occupied by spread-spectrum signals offer some frequency diversity; i.e., it is unlikely that the signal will encounter severe multipath fading over its whole bandwidth. In direct-sequence systems, the signal can be detected by using a rake receiver.
 * Multiple access capability, known as code-division multiple access (CDMA) or code-division multiplexing (CDM). Multiple users can transmit simultaneously in the same frequency band as long as they use different spreading sequences.

Invention of frequency hopping
The idea of trying to protect and avoid interference in radio transmissions dates back to the beginning of radio wave signaling. In 1899, Guglielmo Marconi experimented with frequency-selective reception in an attempt to minimize interference. The concept of Frequency-hopping was adopted by the German radio company Telefunken and also described in part of a 1903 US patent by Nikola Tesla. Radio pioneer Jonathan Zenneck's 1908 German book Wireless Telegraphy describes the process and notes that Telefunken was using it previously. It saw limited use by the German military in World War I, was put forward by Polish engineer Leonard Danilewicz in 1929, showed up in a patent in the 1930s by Willem Broertjes ( issued Aug. 2, 1932), and in the top-secret US Army Signal Corps World War II communications system named SIGSALY.

During World War II, Golden Age of Hollywood actress Hedy Lamarr and avant-garde composer George Antheil developed an intended jamming-resistant radio guidance system for use in Allied torpedoes, patenting the device under "Secret Communications System" on August 11, 1942. Their approach was unique in that frequency coordination was done with paper player piano rolls, a novel approach which was never put into practice.

Clock signal generation
Spread-spectrum clock generation (SSCG) is used in some synchronous digital systems, especially those containing microprocessors, to reduce the spectral density of the electromagnetic interference (EMI) that these systems generate. A synchronous digital system is one that is driven by a clock signal and, because of its periodic nature, has an unavoidably narrow frequency spectrum. In fact, a perfect clock signal would have all its energy concentrated at a single frequency (the desired clock frequency) and its harmonics.

Background
Practical synchronous digital systems radiate electromagnetic energy on a number of narrow bands spread on the clock frequency and its harmonics, resulting in a frequency spectrum that, at certain frequencies, can exceed the regulatory limits for electromagnetic interference (e.g. those of the FCC in the United States, JEITA in Japan and the IEC in Europe).

Spread-spectrum clocking avoids this problem by reducing the peak radiated energy and, therefore, its electromagnetic emissions and so comply with electromagnetic compatibility (EMC) regulations. It has become a popular technique to gain regulatory approval because it requires only simple equipment modification. It is even more popular in portable electronics devices because of faster clock speeds and increasing integration of high-resolution LCD displays into ever smaller devices. As these devices are designed to be lightweight and inexpensive, traditional passive, electronic measures to reduce EMI, such as capacitors or metal shielding, are not viable. Active EMI reduction techniques such as spread-spectrum clocking are needed in these cases.

Method
In PCIe, USB 3.0, and SATA systems, the most common technique is downspreading, via frequency modulation with a lower-frequency source. Spread-spectrum clocking, like other kinds of dynamic frequency change, can also create challenges for designers. Principal among these is clock/data misalignment, or clock skew. A phase-locked loop on the receiving side needs a high enough bandwidth to correctly track a spread-spectrum clock.

Even though SSC compatibility is mandatory on SATA receivers, it is not uncommon to find expander chips having problems dealing with such a clock. Consequently, an ability to disable spread-spectrum clocking in computer systems is considered useful.

Effect
Note that this method does not reduce total radiated energy, and therefore systems are not necessarily less likely to cause interference. Spreading energy over a larger bandwidth effectively reduces electrical and magnetic readings within narrow bandwidths. Typical measuring receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. If the system under test were to radiate all its energy in a narrow bandwidth, it would register a large peak. Distributing this same energy into a larger bandwidth prevents systems from putting enough energy into any one narrowband to exceed the statutory limits. The usefulness of this method as a means to reduce real-life interference problems is often debated, as it is perceived that spread-spectrum clocking hides rather than resolves higher radiated energy issues by simple exploitation of loopholes in EMC legislation or certification procedures. This situation results in electronic equipment sensitive to narrow bandwidth(s) experiencing much less interference, while those with broadband sensitivity, or even operated at other higher frequencies (such as a radio receiver tuned to a different station), will experience more interference.

FCC certification testing is often completed with the spread-spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits. However, the spread-spectrum functionality may be disabled by the user in some cases. As an example, in the area of personal computers, some BIOS writers include the ability to disable spread-spectrum clock generation as a user setting, thereby defeating the object of the EMI regulations. This might be considered a loophole, but is generally overlooked as long as spread-spectrum is enabled by default.

title: Spread spectrum


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Squelch

In telecommunications, squelch is a circuit function that acts to suppress the audio (or video) output of a receiver in the absence of a strong input signal. Essentially, squelch is a specialized type of noise gate designed to suppress weak signals. Squelch is used in two-way radios and VHF/UHF radio scanners to eliminate the sound of noise when the radio is not receiving a desired transmission.

Squelch
In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset. Receivers in base stations, or repeaters at remote mountain top sites, are usually not adjustable remotely from the control point.

In two-way radios (also known as radiotelephones), the received signal level required to unsquelch (un-mute) the receiver may be fixed or adjustable with a knob or a sequence of button presses. Typically the operator will adjust the control until noise is heard, and then adjust in the opposite direction until the noise is squelched. At this point, a weak signal will unsquelch the receiver and be heard by the operator. Further adjustment will increase the level of signal required to unsquelch the receiver. Some applications have the receiver tied to other equipment that uses the audio muting control voltage, as a "signal present" indication; for example, in a repeater the act of the receiver unmuting will switch on the transmitter. Squelch can be opened (turned off), which allows all signals to be heard, including radio frequency noise on the receiving frequency. This can be useful when trying to hear distant or otherwise weak signals, for example in DXing.

Carrier squelch is the most simple variant of all. It functions strictly on the signal strength, such as when a television mutes the audio or blanks the video on "empty" channels, or when a walkie-talkie mutes the audio when no signal is present. Carrier squelch uses receiver Automatic gain control (AGC) to determine the squelch threshold. Single-sideband modulation (SSB) typically uses carrier squelch.

Noise squelch is more reliable than carrier squelch. A noise squelch circuit is noise-operated and can be used in AM or FM receivers, and relies on the receiver quieting in the presence of an AM or FM carrier. To minimize the effects of voice audio on squelch operation, the audio from the receiver's detector is passed through a high-pass filter, typically passing 4,000 Hz (4kHz) and above, leaving only high frequency noise. The squelch control adjusts the gain of an amplifier which varies the level of the noise coming out of the filter. This noise is rectified, producing a DC voltage when noise is present. The presence of continuous noise on an idle channel creates a DC voltage which turns the receiver audio off. When a signal with little or no noise is received, the noise-derived voltage is reduced and the receiver audio is unmuted.

Noise squelch can be defeated by intermodulation present in the high-pass band. For this reason, many receivers with noise squelch will also use a carrier squelch set at a higher threshold than the noise squelch.

Tone squelch and selective calling
Tone squelch, or another form of selective calling, is sometimes used to solve interference problems. Where more than one user is on the same channel (co-channel users), selective calling addresses a subset of all receivers. Instead of turning on the receiver audio for any signal, the audio turns on only in the presence of the correct selective calling code. This is akin to the use of a lock on a door. A carrier squelch is unlocked and will let any signal in. Selective calling locks out all signals except ones with the correct key to the lock (the correct code).

In non-critical uses, selective calling can also be used to hide the presence of interfering signals such as receiver-produced intermodulation. Receivers with poor specifications—such as inexpensive police scanners or low-cost mobile radios—cannot reject the strong signals present in urban environments. The interference will still be present, and will still degrade system performance, but by using selective calling the user will not have to hear the noises produced by receiving the interference.

Four different techniques are commonly used. Selective calling can be regarded as a form of in-band signaling.

CTCSS
CTCSS (Continuous Tone-Coded Squelch System) continuously superimposes any one of about 50 low-pitch audio tones on the transmitted signal, ranging from 67 to 254 Hz. The original tone set was 10, then 32 tones, and has been expanded even further over the years. CTCSS is often called PL tone (for Private Line, a trademark of Motorola), or simply tone squelch. General Electric's implementation of CTCSS is called Channel Guard (or CG). RCA Corporation used the name Quiet Channel, or QC. There are many other company-specific names used by radio vendors to describe compatible options. Any CTCSS system that has compatible tones is interchangeable. Old and new radios with CTCSS and radios across manufacturers are compatible. For those PMR446 radios with 38 codes, the codes 0 to 38 are CTCSS Tones:

SelCall
Selcall (Selective Calling) transmits a burst of up to five in-band audio tones at the beginning of each transmission. This feature (sometimes called "tone burst") is common in European systems. Early systems used one tone (commonly called "Tone Burst"). Several tones were used, the most common being 1,750 Hz, which is still used in European amateur radio repeater systems. The addressing scheme provided by one tone was not enough, so a two-tone system was devised—one tone followed by a second tone (sometimes called a "1+1" system). Motorola later marketed a system called "Quik-Call" that used two simultaneous tones followed by two more simultaneous tones (sometimes called a "2+2" system) that was heavily used by fire department dispatch systems in the US. Later selective call systems used paging system technology that made use of a burst of five sequential tones.

DCS
DCS (Digital-Coded Squelch), generically known as CDCSS (Continuous Digital-Coded Squelch System), was designed as the digital replacement for CTCSS. In the same way that a single CTCSS tone would be used on an entire group of radios, the same DCS code is used in a group of radios. DCS is also referred to as Digital Private Line (or DPL), another trademark of Motorola, and likewise, General Electric's implementation of DCS is referred to as Digital Channel Guard (or DCG). Despite the fact that it is not a tone, DCS is also called DTCS (Digital Tone Code Squelch) by Icom, and other names by other manufacturers. Radios with DCS options are generally compatible, provided the radio's encoder-decoder will use the same code as radios in the existing system.

DCS adds a 134.4 bit/s (sub-audible) bitstream to the transmitted audio. The code word is a 23-bit Golay (23,12) code which has the ability to detect and correct errors of 3 or fewer bits. The word consists of 12 data bits followed by 11 check bits. The last 3 data bits are a fixed '001', this leaves 9 code bits (512 possibilities) which are conventionally represented as a 3-digit octal number. Note that the first bit transmitted is the LSB, so the code is "backwards" from the transmitted bit order. Only 83 of the 512 possible codes are available, to prevent falsing due to alignment collisions.

DCS codes are standardized by the Telecommunications Industry Association with the following 83 codes being found in their most recent standard, however, some systems use non-standard codes. For those PMR446 radios with 121 codes, the codes 39 to 121 are DCS codes:

XTCSS
XTCSS is the newest signalling technique, and provides 99 codes with the added advantage of "silent operation". XTCSS-fitted radios are purposed to enjoy more privacy and flexibility of operation. XTCSS is implemented as a combination of CTCSS and in-band signalling.

Uses
Squelch was invented first and is still in wide use in two-way radio. Squelch of any kind is used to indicate loss of signal, which is used to keep commercial and amateur radio repeaters from continually transmitting. Since a carrier squelch receiver cannot tell a valid carrier from a spurious signal (noise, etc.), CTCSS is often used as well, as it avoids false keyups. Use of CTCSS is especially helpful on congested frequencies or on frequency bands prone to skip and during band openings.

Professional wireless microphones use squelch to avoid reproducing noise when the receiver does not receive enough signal from the microphone. Most professional models have adjustable squelch, usually set with a screwdriver adjustment or front-panel control on the receiver.

title: Squelch


text:
Standard test signal

In telecommunication, a standard test signal is a single-frequency signal with standardized level used for testing the peak power transmission capability and for measuring the total harmonic distortion of circuits or parts of an electric circuit.

Standardized test signal levels and frequencies are listed in MIL-STD-188-100 and in the Code of Federal Regulations Title 47, part 68.

title: Standard test signal


text:
Standby

Standby may refer to:
 * Standby (air travel), a list in which passengers may request to be placed on to request an earlier or more convenient flight
 * Standby (theater), an actor or performer who will appear in a particular role if the regular performer is not present
 * Sleep mode (in electronics), also known as standby mode—a mode in which electronic appliances are turned off but still under power and ready to activate on command
 * Standby (or ACPI S3), an ACPI mode of a computer
 * Modern Standby (or InstantGo), a computer power management system
 * Standby power, energy consumed by an electronic device while it is turned off or in sleep mode
 * Standby (TV series), a South Korean sitcom
 * Standby Records, an American independent record label
 * "Standby", a song by Macintosh Plus from Floral Shoppe

title: Standby