Service Access Code (SAC) refers to a code of the form "NO/lX" that takes the place of an NPA in the dialing sequence in order to access a particular service provided by an IC, BOC, or independent company. SACs currently in use include 700 (IC Services), 800 (800 Service), and 900 (National DIAL-IT* network communications service). SACs are associated with supplementary features that may be ordered by an IC as part of the access charge tariff. A BOC may screen calls and route to an IC-specified point of presence based upon a SAC dialed. There is no requirement that SACs be of the Form N00 nor that all N00 codes be SACs.
SAM: Subsequent Address Message
Subsequent Address Message (SAM) refers to a SS7 (Signaling System No. 7) signaling message which carried further dial numbers after the initial address message has been sent.
Switched Access Service (SAS) refers to the services provided by telecommunication carriers to their customers for their use in furnishing their services to end users. It provides a two-point electrical communications path to a customer's facilities from an end user's premises. It provides for the use of common terminating switching and transport facilities and common subscriber plant of the Telephone Company. Switched Access Service provides for the ability to originate calls from an end user's premises to a customer's facilities, and to terminate calls from a customer's facilities to an end user's premises in the LATA where it is provided. There are 4 types of Switched Access Services: (1) Feature Group A: Switched Access provides line-side interconnection to Verizon's end office switches through an end user seven-digit access code (NXX-XXXX) for the long distance customer's use in originating and terminating calls to end users. (2) Feature Group B: Switched Access provides trunk-side interconnection to Verizon's end office switches through a uniform seven digit access code (950-XXXX) for the long distance customer's use in originating and terminating calls to end users. (3) Feature Group C: Switched Access provides trunk-side interconnection to Verizon's end office switches for providers of MTS and WATS for originating and terminating communications. (4) Feature Group D: Switched Access provides trunk-side interconnection to Verizon's end office switches through either a 101XXXX arrangement or on a pre-subscribed basis for the long distance customer's use in originating and terminating calls to end users.
Satellite Operated PBX is a PBX system which is operating at a remote location and is tied into the primary PBX by means of tie lines. The satellite does not have its own unique incoming central office trunks. All incoming calls are extended to the stations by means of the tie lines.
SCA: Selective Call Acceptance
Selective Call Acceptance (SCA) is a telecommunication feature that allows customers to create a list of phone numbers from which they are willing to accept calls. Callers with phone numbers not on your list hear an announcement informing them that you are not receiving calls at this time.
Scanner, in a telecommunication network, refers to a device in an electronic switching office that scans various parts of the switching network to determine when some change in operations is occurring. Information about the change is passed from the scanner to temporary memory in the controlling computer.
SCF: Selective Call Forwarding
Selective Call Forwarding (SCF) is a feature in telecommunications system. Not every call is important. With Selective Call Forwarding, customers can forward callers from a select group of numbers to another number. Customer's list of acceptable numbers can be changed at any time from your phone. Selective Call Forwarding reserves customer's time for only the most important calls.
Service Control Node (SCN) is an intelligent network function in a communication network which collects traffic data for call services without requiring alteration of, or addition to, programs in all of the service switching nodes in the communication network even if a new call service is introduced as an intelligent network service. A service control node (SCN) has a unit assigning a call service identification (ID) to a new call service and the service switching node provides a registration table receiving the identification (ID) and registering the same in it. If a call service requested by the calling party is in the registration table, the traffic data is collected for the call service.
Selective Call Rejection (SCR) is a feature in telecommunication system that enable customer to reject incoming calls they don't want to take. Selective Call Rejection screens every call against a "do not accept" list of numbers customers create and rejects those calls on the list. Selective Call Rejection assures you of avoiding calls, freeing customer's line for the calls you want to make and receive!
Screen phone is a device that looks similar to a standard desk telephone, but is equipped with a screen (LCD screen) to display communication information such as caller ID. It may also include a small keyboard for the Internet access.
SDA: Selective Distinct Alerting
Selective Distinct Alerting (SDA) is a feature in telecommunication system that enables customer's telephone to identify special or important calls. Selective Distinctive Alert easily allows customer to create and update a list of telephone numbers from which calls will ring differently than other calls. The service alerts customer to calls from numbers on your list by providing a distinctive ringing pattern or a distinctive Call Waiting tone.
SDH: Synchronous Data Link Hierarchy
Synchronous Data Link Hierarchy (SDH) is an European standard for data transmission over optical fiber network equivalent to SONET of North America. SDH's basic unit, the STM-1 (Synchronous Transport Module-level 1), operates at 155.52 Mbit/s. Transmission rates of up to 10 Gbit/s can be achieved in today's SDH systems and the 40 Gbit/s systems are possible. SDH systems are fully compatiable to SONET systems.
SDMT: Synchronized Discrete Multitone
Synchronized Discrete Multitone is a modulation technique for point-to-point transmission on a channel whose SNR (Signal to Noise Ratio) is highly frequency-dependent, and it has been adopted by ANSI as the standard for ADSL [up to 7 Mbit/s on unshielded twisted-pair (UTP)]. Synchronized DMT (SDMT), a variation of DMT for multiple point-to-point transmission, will be proposed to the ATIS committee T1E1.4 for VDSL (up to 51.8 Mbit/s on hybrid fiber/UTP), and to the IEEE committee 802.14 for the upstream channel of an HFC system.
Software Defined Network (SDN) is a network that provides customers with the ability to achieve a corporate virtual private network (VPN) while using the facilities of the switched Network. The former AT&T's SDN, for example, is a customer's VPN that resides in the 4ESS-based switched Worldwide Intelligent Network and provides networked-based features and management capabilities that are usually not found in private networks. Some of the networked-based features are customized routing, advance numbering plans, call screening, authorization codes, remote access, security codes and customized billing. SDN is compatible with most private networks and PBXs and, as such, protects these existing investments. Since SDN does not require a sophisticated PBX base, businesses can choose tariffed access or dial-up. The service fully supports analog data transmission at up to 28.8 Kbps and end-to-end digital data transmission at 56/64 Kbps.
Switched Digital Service (SDS) is a data application service providing a digital access line, by means of digital central offices, where facilities permit, to a customer's premises enabling the customer to transmit simultaneously two-way digital signals at a speed of 56 Kbps on a dial-up basis where both the originating and terminating stations are suitably equipped for SDS.
SDSL: Single-line Digital Subscriber Line
Single-line Digital Subscriber Line, also known as Symmetric Digital Subscriber Line (SDSL), is one variaton of DSL technologies that provides equal bandwidth for both uploads and downloads. SDSL delivers 1.544 Mbps both downstream and upstream over a single copper twisted pair. The use of a single twisted pair limits the operating range of SDSL to 10,000 feet (3048.8 meters).
SMDS DSU is a type of Data Service Unit (DSU) for access to Switched Multimegabit Data Service (SMDS) via High-Speed Serial Interfaces (HSSIs) and other serial interfaces.
Secure telephone is a telephone that provides voice security in the form of end-to-end encryption for the telephone call, and in some cases also the mutual authentication of the call parties, protecting them against a man in the middle attack.
Seize is an action of a switching system in selecting an outgoing trunk or other components for a particular call.
Selector Switch is the intermediate distributing switch in a step-by-step system. It is directly controlled by the customer dial in its vertical motion and hunts for an idle path in its angular motion.
Sender refers to an equipment in a switching system used to transmit and/or receive the called number to or from a distant office; a sender is usually arranged to transmit on a multi-frequency or dial-pulse basis. Under certain conditions of trouble, a sender may remain connected to a trunk without performing its intended function; this is know as a stuck sender.
Serial Transmission is a method of data transmission in which the bits of a data character are transmitted sequentially over a single channel.
Service Indicator is part of the information contained in a signaling message that is used to identify the user to whom the message belongs.
Service Observing Tone is a steady 135 Hz audible tone. It indicates that the trunk to which it is applied is being service-observed. .
Super Frame (SF), also called D4 framing, is a common framing type used on T1 circuits. SF consists of 12 frames of 192 bits each, with the 193rd bit providing error checking and other functions. SF is superseded by Extended Super Frame (ESF) but is still widely used.
SHDSL: Single-Pair High-Speed Digital Subscriber Line
Single-pair high-speed digital subscriber line (SHDSL) is a form of Digital Subscriber Line similar to HDSL but providing T1 or E1 connections over a single twisted-pair copper line.
SIF: Signaling Information Field
Signaling Information Field (SIF) is the bits of a message signal unit that carry information particular to a certain user transaction and always contain a label.
Signal Path is the route of a signal channel that carries signaling data.
Signal Unit refers to the group of bit streams of a signaling message that functions as a transferable entities used to convey information over a signaling link.
Signalling refers to the process of setting-up communications links between two systems such as between a user system and a service provider system. Signaling is used in both traditional telephony and in packet-based communications. SS7/C7 is the most famous signaling system in telephony, while in data communication, the various VOIP signaling methods such as SIP and H.323 are introduced..
Signaling Data Link refers to a type of signaling link that consists of two data channels that operate in opposite directions at the same data rate, and function as a single signaling system.
Signaling Group is several signaling links that directly connect two signaling points together and have the same physical characteristics (e.g., bit rate and propagation delay).
Signaling Link refers to: (1) a transmission facility in signaling network that consists of a signaling data link and its transfer control functions, used for reliable transfer of a signaling message between signaling points as an entity. (2) an assembly of signaling information that pertains to a call management transaction, and the like and that is transferred as an entity.
Signaling Message refers to an assembly of signaling information pertaining to a call that includes, but not limited to, management transactions, that is transferred as an entity.
Signaling Message Route refers to a signaling link (or consecutive signaling links connected in tandem) that is used to convey a signaling message from an originating point to its destination.
Signaling Point is a node in a signaling network (such as SS7/C7 signaling network) that either originates and receives signaling messages, or transfers signaling messages from one signaling point to another.
Signaling Point Code (SPC) is a binary code uniquely identifying a signaling point in a signaling network. This code is used, according to its position in the label, either as destination point code or as originating point code.
Signaling Point Numbering Plan
Signaling Point Numbering Plan is a formal (standardized) description of the method (numbering plan) for converting "end-user-provided" address information into an address understood by the signaling network.
Signaling Relation refers to a relation formed by two signaling points that involve the possibility of information interchange between corresponding user part functions.
Signaling Route is predetermined path, described by a succession of signaling points, that can be traversed by signaling messages (directed by a signaling point) toward a specific destination point.
Signaling System is a language or protocol that enables two switching equipment (element) to converse for the purpose of setting up calls. It possesses a vocabulary of varying size and varying precision, i.e., a list of signals which may also vary in size and a syntax in the form of a more or less complex set of rules governing the assembly of these signals.
Signaling Time Slot refers to a time slot starting in a particular phase of each frame that is allocated for transmitting signaling messages.
Signaling Traffic Management Functions
Signaling Traffic Management Functions is a control function of routing information used by message routing function. These functions insure the transfer of signaling traffic is performed in a manner that prevents irregularities in message flow.
Signaling Transport (SIGTRAN) refers to a protocol stack for the transport of Switched Circuit Network (SCN) signaling protocols (such as SS7/C7 an Q.931) over an IP network. SIGTRAN, an evolution of the PSTN signaling, defines adaptors and a core transport capabilities that blend SS7 and packet protocols to provide users with the best both technologies have to offer. Applications of SIGTRAN include: Internet dial-up remote access, IP telephony interworking with PSTN and other services as identified.
Simplex refers to the capability for transmission in only one direction between a sending station and a receiving station. Broadcast television is an example of a simplex technology.
Simplex Circuit is a transmission path which is capable of transmitting in only one direction. The ITU definition differs from this more common definition. ITU simplex is a path which can operate in either direction, but only one direction at a time. This is commonly called half-duplex.
Single Frequency Signaling is a method of conveying dial-pulse and supervisory signals from one end of a trunk or line to the other, using the presence or absence of a single specified frequency.
Single Order Tone is one of the Order Tones. This is a relatively long (0.5 second) signal which means that the originating operator should pass the office name and number. Single-order tone is one 0.5 spurt of High Tone.
Sink refers to the end point in telecommunications model which is also called receiver. In this model, a message flows from the "Source" via media (transmission path) to a "Sink".
SIP: Session Initiation Protocol
Session Initiation Protocol (SIP) is an application layer control protocol that can establish, modify and terminate multimedia sessions such as Internet telephony calls. SIP can also invite participants to already existing sessions, such as multicast conferences. Media can be added to (and removed from) an existing session. SIP transparently supports name mapping and redirection services, which supports personal mobility; users can maintain a single externally visible identifier regardless of their network location.
SMDS Interface Protocol(SIP) is used for communications between CPE and SMDS carrier equipment. SIP provides connectionless service across the subscriber network interface (SNI), allowing the CPE to access the SMDS network. SIP is based on the IEEE 802.6 Distributed Queue Dual Bus (DQDB) standard for cell relay across metropolitan-area networks (MANs). SIP consists of three levels. SIP Level 3 operates at the Media Access Control (MAC) sublayer of the data link layer of the OSI reference model. SIP Level 2 operates at the MAC sublayer of the data link layer. SIP Level 1 operates at the physical layer of the OSI reference model.
Session Initiation Protocol (SIP) for Telephones (SIP-T), previously known as SIP-BCP-T, is a mechanism that uses SIP to facilitate the interconnection of the PSTN with IP. SIP-T allows traditional IN-type services to be seamlessly handled in the Internet environment. It is essential that SS7 information be available at the points of PSTN interconnection to ensure transparency of features not otherwise supported in SIP. SS7 information should be available in its entirety and without any loss to the SIP network across the PSTN-IP interface. SIP-T defines SIP functions that map to ISUP interconnection requirements.
Skip Route is a control application that causes traffic to bypass a specific route and advance to the following rate selection.
Slave, in a telecommunication network, refers to the end or side of communications which responds to the master's commands. Master refers to the side in telecommunications that initiates and controls the session.
Signaling Link Code (SLC) is a field with certain signaling network management messages regarding T1 connections. SLC indicates the particular signaling link to which the message refers among those interconnecting the two involved signaling points.
Switching Module (SM) is a component of 5ESS switch of Lucent Technologies which terminates subscriber loops (lines) and inter-exchange trunks interfacing the 5ESS switch. The SM is the basic "growth module" of the 5ESS switch, which service providers can add more when subscriber increases.
Sneak Current is an unwanted but steady current which seeps into a communication circuit. It is usually too weak to cause immediate damage, but if unchecked could potentially create harmful heating effects.
SNI: Subscriber Network Interface
Subscriber Network Interface (SNI) refers to the TDM access links such as DS0, DS1/T1, DS3/T3, E1, E3 that connects CPE and an SMDS switch of the PSTN network.
Softswitch is the next generation voice and multimedia switch based on the IP technologies. It is design to replace the Class 5 and Class 4 switches based on the circuit switching technologies. Softswitch gets its name because typically it is a software based solution implemented on general purpose computers/servers, while the traditional Class 5 and Class 4 switches are rely on dedicated facilities for inter-connection and are designed primarily for voice communications. Sometimes Call Agent or Media Gateway Controller, a key component in the VOIP solution, is also called Softswith, though the definition of Softswith is often extend to the whole solution.
SONET Line Terminating Equipment
SONET Line Terminating Equipment refers to the element that terminates a SONET line.
SONET multiplexing is the process of transfering low speed traffic to high speed traffic in a SONET network. It involves generation of the STS-1 signal for each tributary. The STS-1 signal contains the payload traffic plus transport overhead allowing for a variety of tributary types to be accommodated. An STS-n signal is created by interleaving bytes from n STS-1 signals that are mutually synchronized, so there is no timing adjustment required when going to higher level multiplex formats. Drop Module Interfaces (DMI) provide SONET/SDH formatting and multiplexing/demultiplexing of the STS-1 signal; ports for low-speed DS1/E1, DS3/E3, STS-1, OC-1 lines; and termination and framing services for tributaries.
SONET/SDH: Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH)
The Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH), often combined as SONET/SDH, are a set of related standards for synchronous data transmission over fiber optic networks that are often used for framing and synchronization at the physical layer. SONET is the United States version of the standard published by the American National Standards Institutue (ANSI). SDH is the international version of the standard published by the International Telecommunications Union (ITU). Transmission rates of up to 10 Gbit/s can be achieved in today's SONET/SDH systems and the 40 Gbit/s systems are possible. SONET/SDH systems are fully compatiable with each other.
SONET: Synchronous Optical Network
The Synchronous Optical Network (SONET), the US version of the standard published by the American National Standards Institutue (ANSI), is a set of standards for synchronous data transmission over fiber optic networks that are often used for framing and synchronization at the physical layer. SONET is based on transmission at speeds of multiples of 51.840 Mbps, or STS-1. SDH is the international version of the standard published by the International Telecommunications Union (ITU). Transmission rates of up to 10 Gbit/s can be achieved in today's SONET systems and the 40 Gbit/s systems are possible. SONET systems are fully compatiable to SDH systems.
Source refers to the starting point in telecommunications model which is also called sender. In this model, a message flows from the "Source" via media (transmission path) to a "Sink".
Source User refers to the user who provides the information to be transferred to a destination user during a particular information transfer transaction.
Space Division Switching is a switching in which single transmission-path routing determination is accomplished in a switch by using a physically separated set of matrix contacts or cross-points.
Signaling Point Code is a binary code uniquely identifying a signaling point in a signaling network. This code is used, according to its position in the label, either as destination point code or as originating point code.
Special Dial Tone is used with Three-Way Calling, Centrex station dial transfer, and Centrex conference (station or attendant) services. The user on an existing connection flashes the switch-hook, receives special dial tone, and dials number of the third party to be added to the connection. Special Dial Tone is 350 Hz and 440 Hz at -13 dBm0/frequency for 0.1 second on, 0.1 second off, 0.1 second on, 0.1 second off, 0.1 second on, 0.1 second off, and then on steady.
Special LATA Access refers to a BOC-tariffed service offering that provides for a non-switched communications path (access link) between an IC Point of Presence (POP) and the premises of its end users, or to a BOC central office for Centrex services. It includes all LATA access services that do not use a BOC switching system.
Speech Digit Signaling is a type of channel-associated signaling where time slots (primarily used to transport of encoded speech) are periodically used to carry signaling messages.
Speed Calling is an optional feature of a telephone system which allows the placing of calls to frequently called numbers through the use of an abbreviated number of dial digits. It is also called abbreviated dialing.
Spelled fiber is a transparent thin fiber, usually made of glass or plastic, for transmitting light. Optical fiber is a cylindrical structure that transmits light along its axis. The fiber consists of a core surrounded by a cladding layer.
SPID: Service Profile Identifier
Service Profile Identifier(SPID) is a number that some service providers use to define the services to which an ISDN device subscribes. The ISDN device uses the SPID when accessing the switch that initializes the connection to a service provider.
Spill-forward feature is a service feature in telecommunications that is 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.
Splitter is an electronic low pass filter that separates the analogue voice or ISDN signal from ADSL data frequencies when they get to the subscriber premises. For outgoing traffic, when they are transmitted from the subscriber premises, it combines the voice and the data frequencies onto one line. This allows a POTS phone connection to operate at the same time as ADSL digital data is transmitted or received on the same line. One splitter is located at the central office and another at the subscriber premises. The splitter at the central office can be separate device or may be incorporated into the DSLAM.
Splitting refers to a switching function used for several purposes including: (1) disconnection (isolation) of a channel that precedes the point where signaling frequencies are injected, and (2) disconnection (isolation) of a channel that succeeds the point where the signal receiver is connected. Splitting can be used to prevent false operation of signaling equipment (caused by signal reflections and spillover) when signaling information is being received. Splitting can also prevent interference from a preceding circuit or nearby equipment (terminal, or device) when a signaling point is receiving a signaling message.
SPM: Subscriber Pulse Metering
Subscriber Pulse Metering (SPM) is a set of functions that are employed by an analogue telephone system primarily to enable call-charge recording and billing. In order to achieve this, the relevant telephone equipment must be able to accurately determine several factors, amongst which is the ability to measure the duration of the call.
Subsystem Number (SSN), a term in the SS7 network, is the identification number that is used by the SCCP (Signalling Connection Control Part) to determine the local subsystem.
Service Switching Point(SSP) is an element of a SS7-based Intelligent Network that performs call origination, termination, or tandem switching. A SSP can be a combination voice switch and SS7 switch, or an adjunct computer connected to the local exchange's voice switch. The SSP provides the functionality of communicating with the voice switch via the use of primitives and creating the packets, or signal units, needed for transmission in the SS7 network. The Service Switching Point (SSP) must convert signaling from the voice switch into SS7 signaling messages, which can then be sent to other exchanges through the SS7 network. The exchange will typically send messages related to its voice circuits to the exchanges with a direct connection to it. In the case of database access, the SSP will be sending database queries through the SS7 network to computer systems located centrally to the network. This was the first usage of the SS7 network, as the need for 800 number lookup became necessary.
Start Dial Supervision is the line protocol that defines how the equipment seizes the E&M trunk and passes the address signaling information (sends dual tone multifrequency (DTMF) digits). The three main start dial supervision protocols used on E&M circuits are Immediate Start, Wink Start, and Delay Dial.
Station Code is the final four digits of a standard seven- or ten-digit address. These digits define a connection to a specific customer's telephone(s) within the larger context of an NPA and central office code. The term "main station code" is an equivalent expression. In the past, a line number and a party letter often were combined to provide station identification. With the discontinuance of party letters, the four numerics have assumed the role of station identification.
Step-by-Step Switch (SXS) Switching Equipment
Step-by-Step Switch (SXS) Switching Equipment is a two-wire or four-wire progressive control switching system with a space division network. The two-wire version is used for local, local tandem, toll tandem, and private line switching. It can have ANI, CAMA, CENTREX, and private line switching features. The four-wire version is used as a local tandem or toll tandem.
STM: Synchronous Transmission Module
Synchronous Transmission Module (STM), also known as Synchronous Transfer Mode, is the basic rate of transmission of the SDH ITU-T fiber optic network transmission standard. It has a bit rate of 155.52 Mbit/s and is the SDH equivalent of an OC-3 (SONET).
STM-0: Synchronous Transmission Module level zero
Synchronous Transmission Module level zero (STM-0) is the SDH ITU-T fiber optic network transmission standard. It has a bit rate of 51.84 Mbit/s and is the SDH equivalent of an OC-1 (SONET).
STM-1: Synchronous Transmission Module level one
Synchronous Transmission Module level one (STM-1) is the basic rate of transmission of the SDH ITU-T fiber optic network transmission standard. It has a bit rate of 155.52 Mbit/s and is the SDH equivalent of an OC-3 (SONET).
STM-4: Synchronous Transmission Module level 4
Synchronous Transmission Module level 4 (STM-4) is one of the transmission mode of the SDH ITU-T fiber optic network transmission standard. It has a bit rate of 622.08 Mbit/s and is the SDH equivalent of an OC-12 (SONET).
STM-n: Synchronous Transmission Module level n
Synchronous Transmission Module n (STM-n), also known as Synchronous Transfer Mode n, refers to a group of the transmission modes of the SDH ITU-T fiber optic network transmission standard, which is the multiplexing "n" of the STM-1 frames.
Stored Program Control refers to a control of a telephone switch network by means of programmable operating instructions, resident in a semipermanent memory. The main purpose for introducing stored program control is to provide flexible systems where additions and changes can be introduced primarily through program modification rather than through changes in hardware.
Signal Transfer Point(STP) is an element of a SS7-based Intelligent Network that performs routing of the SS7 signaling.
Stratum is a hierarchical clock reference in the PSTN network, where 1 represents the highest possible quality of clocking.
Stratum 3 is a precision timing reference that provides a free-run accuracy of plus or minus 4.6 parts per million (PPM), pull-in capability of 4.6 PPM, and holdover stability of fewer than 255 slips during the first day.
STS-1: Synchronous Transport Signal level 1
Synchronous Transport Signal level 1(STS-1) is the basic building block signal of SONET, operating at 51.84 Mbps. Faster SONET rates are defined as STS-n, where n is a multiple of 51.84 Mbps.
STS-3c: Synchronous Transport Signal level 3, concatenated
Synchronous Transport Signal level 3, concatenated(STS-3c) is the SONET format that specifies the frame structure for the 155.52-Mbps lines used to carry ATM cells.
STS-N: Synchronous Transport Signal Level N
Synchronous Transport Signal Level N (STS-N) refers the basic logical building block signal of synchronous optical networks (SONET) data rate. There are 5 different data rates currently available and denoted as STS-N where N = 1,2,12,48, and 192). The STS-1 signal is known as the SONET basic rate (data rate of 51.84 Mbps) and is designed to carry one DS-3 Signal (the most popular long-haul digital signal in North America).
Signal Unit(SU) is an element in the SS7 network in which signaling information is passed over the signaling links in messages. Signal units are continuously transmitted in both directions on any link that is in service. SS7 uses three different types of signal units: Message Signal Units (MSUs); Link Status Signal Units (LSSUs); and Fill-In Signal Units (FISUs).
Submarine Communications Cable
Submarine Communications Cable is a cable laid beneath the sea to carry telecommunications between countries. The first submarine communications cables carried telegraphy traffic. Subsequent generations of cables carried first telephony traffic, then data communications traffic. All modern cables use fiber optic technology to carry digital payloads, which are then used to carry telephone traffic as well as Internet and private data traffic.
Subscriber Loop is a communication channel between a subscriber station and a carrier's central office. Usually this is a pair of wires.
Subscriber Loop Carrier is a system that allows one pair of wires, that would normally provide one phone line, to carry multiple conversations. Various models are available, with a capacity ranging from 2 to 96 lines. It is used between phone company central offices and areas where there are too many customers for the cable that is in place.
Subsequent Address Message is a SS7 (Signalling System No. 7) signalling message which carries further dial numbers after the initial address message has been sent.
Sub-voice Grade is a communications channel with a bandwidth less than that of a voice grade channel. Usually it is derived from a voice grade channel by some form of multiplexing.
Supervisory Signals refer to Signals used to indicate or control the states of circuits involved in a particular connection. A supervisory signal indicates to equipment, to an operator, or to a customer that a particular state in a call has been reached and may signify the need for action to be taken.
A switch is a networking device that connects network segments. There are two categories of switches: packet switch and circuit switch. Packet switch is for data networking operating typically at the layer 2 (Data Link Layer) of the OSI model. It is capable of inspecting the data packets as they are received, determining the source and destination device of that packet, and forwarding that packet appropriately. On the other hand, the circuit switch that establishes connections on demand and permits exclusive use of those connections until they are released. A circuit-switched network is a type of network in which a physical path is obtained for, and dedicated to, a single connection between two end-points in the network during the connection. The traditional voice phone service using PSTN (not the voice over IP) is circuit-switched.
Switch Hook, also called hook switch, is a plunger or switch where a telephone handset sits when the telephone is on hook, or hung up. When the handset is lifted, the switch hook goes up and the telephone is off hook.
Switch Matrices are the ratio of the number of input terminals to the number of output terminals of a switch.
Switch Train refers to the sequence of switches which are involved in the completion of a circuit path from an originating station to a destination station.
Switched Access is a service or feature that provides the transport of switched traffic between end-user and carriers. Switched Access connects end-users to Interexchange Carriers (IXCs), wireless carriers, and competitive local exchange Carriers (CLECs). Long distance calls and calls to cellular phones both rely upon switched access services to connect end-users to the networks of the called carrier. Switched Access is composed of the following fundamental elements: Physical Transport, Switching and Associated Options, and Carrier Common Line (Local Loop).
Switched Calls refer to the traditional telephone calls in which a user picks up a phone, hears a dial tone, and enters the destination phone number to reach the other phone. Switched calls also can be private line auto-ringdown (PLAR) calls, or tie-line calls for fixed point-to-point connections.
Switched-1536, also known as H11 data service, is a data service consisting of a single 1536 kbps circuit, called an H11 channel. The H11 channel is comprised of all 24 channels on the line. Two T1 PRI lines are required to access switched-1536. One line carries the user data, and the other line contains the D-channel. NFAS is required for this data service because the D channel must be on a separate line. This service is available over T1 PRI lines only.
Switched-384, also known as H0 data service, is a data service consisting of a single 384 kbps circuit, called a H0 channel. The H0 channel is comprised of 6 B channels. This service is available over T1 PRI lines only.
Switched-56 is a data service consisting of a single 56 kbps channel. This service is available over any type of line. It is the only service available to T1 access lines and Switched-56 lines. Because Switched-56 was the first available data service, both the service itself and the lines that accessed it were called Switched-56. However, any type of line can now access Switched-56 data service, and there are other new services in addition to Switched-56.
Switchhook is a switch on a telephone set, associated with the structure supporting the receiver or handset. It is operated by the removal or replacement of the receiver or handset on the support.
Switchhook Flash refers to a signaling technique whereby the signal is originated by momentarily depressing the switchhook. The technique is sensitive to variations in the time of depression. Too short a signal will not be recognized and too long a signal will be interpreted as a disconnected signal.
Switching Fee is per-line fee imposed by the LEC (Local Exchange Carrier) to reprogram their switching system to change customer's default long distance carrier. Some long distance carrier will reimburse the subscribers for this fee.
Switching Matrices are the mechanism that provides signal paths between its input and output terminations. Modern matrices are electronic and involve either time or space division switching. A time division switch employs a TDM process, in a time-slot interchange (TSI) arrangement. In space division, a physical, electrical, spatial link is established through the switch matrix. Whereas older space division switches used electro-mechanical mechanisms with metallic contacts, modern space-division switches are implemented electronically using integrated circuits.
Switchless Reseller refers to a reseller who owns no communications facilities and buys long distance service in bulk from a long distance company and resells that service to smaller users.
Step-by-Step Switch(SXS) refers a bidirectional, biselective switch (vertical and angular) in which the moving contact selects from among 100 fixed contacts arranged in a 10 x 10 array.
Symmetric Digital Subscriber Line
Symmetric Digital Subscriber Line (SDSL), also known as Single-line Digital Subscriber Line, is one variaton of DSL technologies that provides equal bandwidth for both uploads and downloads. SDSL delivers 1.544 Mbps both downstream and upstream over a single copper twisted pair. The use of a single twisted pair limits the operating range of SDSL to 10,000 feet (3048.8 meters).
Synchronous network is a telecommunication network in which clocks are controlled to run, ideally, at identical rates, or at the same mean rate with a fixed relative phase displacement, within specified limited range. Ideally, the clock is synchronous, but in practice, they may be mesochronous.
System X is the old name of the UK's first national digital telephone exchange system.