websters new world telecom dictionary phần 9 doc

support it. See also all of the features listed above. See also CCS, CLASS, CO, ILEC, IXC, and signaling and control. SSA (Serial Storage Architecture) An IBM interface specification for a serial transport protocol based on a ring topology and operating in full duplex (FDX) at a maximum of 20 MBps per channel, with as many as two channels per cable. SSA maps into the pre-existing Small Computer System Interface (SCSI) and SSA devices are SCSI devices.The Transport Layer protocol is non-return-to-zero (NRZ) and utilizes 8B/10B encoding. See also 8B/10B, FDX, NRZ, protocol, ring topology, SCSI, serial, and Transport Layer. SSB (Single SideBand) The process of amplitude modulation (AM) results in the creation of two side- bands. An upper sideband is above the carrier frequency and a lower sideband is below the carrier fre- quency. SSB transmission suppresses one of the sidebands. See also AM, amplitude, carrier, DSB, frequency, modulation, sideband, and VSB. SSID (Service Set IDentifier) In IEEE 802.11b wireless LAN (WLAN) specifications, a security mechanism in the form of an authorization code established by the system administrator.A device seek- ing to gain access must be in possession of the SSID. See also 802.11b, authorization, and WLAN. SSL (Secure Sockets Layer) A security protocol developed by Netscape Communications Corpora- tion, SSL includes authentication and negotiates point-to-point security between client and server, includ- ing type of encryption scheme and exchange of encryption keys. SSL sends messages over a socket, which is a secure channel at the connection layer and existing in virtually every TCP/IP application. Although SSL can accommodate a number of encryption algorithms, Netscape has licensed RSA end-to-end pub- lic key encryption, as well as key creation and certification. Unlike S-HTTP,SSL is application independ- ent and works with all Internet tools, not just the World Wide Web (WWW). SSL has emerged as a de facto standard. See also authentication, client, de facto, encryption, Internet, protocol, public key encryption, RSA, server, S-HTTP, socket, standard, TCP/IP, and WWW. SSP (Service Switching Point) In the advanced intelligent network (AIN) architecture, a public switched telephone network (PSTN) switch that acts on the instructions dictated by centralized AIN databases.An SSP can be an end office or tandem switch. See also AIN, database,end office, PSTN, and tandem switch. Standard Wire Gauge (SWG) Synonymous with British Standard Gauge (BSG). See also BSG and gauge. standard A rule, principle, or measure established as a model or example by authority, custom, or gen- eral consent. Standards generally are in the form of baseline specifications according to which manufac- turers can develop products with the assurance that they will interconnect and interoperate with those of other manufacturers, at least at a fundamental level. Standards typically allow for options that manufacturers can exercise in various fashions peculiar to their own product development philosophies, strategies, and so on, thereby distinguishing those products from others.Although standards have been criticized as common denominator or consensus solutions that stifle creativity, they in fact provide a common framework of technical specifications within which manufacturers can exercise a considerable level of creativity. Stan- dards serve to create the technical basis for a competitive market that offers buyers a choice of products, while ensuring interconnectivity and interoperability at a fundamental level. Standards take several forms. • De jure: From Latin, literally meaning from the law. Formal specifications that do not have the force of law, but often have considerable weight as they are set by formal standards bodies that generally are established by governmental or regulatory bodies, or at least by industry consensus. Such formal bodies include the American National Standards Institute (ANSI), the European Telecommunications Standards Institute (ETSI), and the International Telecommunications Union (ITU). Governments sometimes give these standards the force of law, as in requiring new buildings to comply with the National Electrical Code (NEC). SS7 (Signaling System 7) 458 74570c19.qxd 9/12/07 12:41 AM Page 458 • De facto: From Latin, literally meaning from what is done, that is, in fact. Standards not established by such formally constituted bodies, that may even be established by a dominant vendor in its own self- interest and often for its own internal use in the context of an ad hoc solution. De facto standards take on the effect of formal standards simply because they become so widely accepted. Hayes, IBM, and Microsoft, for example, have developed numerous specifications that have become de facto indus- try standards. • Du jour: From French, meaning of the day.The popular standard of the day. One day 10 years ago, ATM was really hot and a lot of people made a lot of money talking about ATM and selling products based on ATM. It seemed like only the next day that IP was really cool. (I made this one up.) standard definition television (SDTV) See SDTV. Standard Generalized Markup Language (SGML) A language used by Web developers and design- ers for creating declarative markup languages like Hypertext Markup Language (HTML). Extensible Markup Language (XML) is a condensed form of SGML that is published and maintained by the World Wide Web Consortium (W3C). See HTML, W3C, WWW, and XML. start bit In asynchronous transmission, a bit that alerts the receiving computer of the arrival of a char- acter.A stop bit, or sometime two stop bits, signals the end of the character. See also asynchronous transmis- sion and bit. star topology A network structure comprising a central node to which all other devices attached directly and through which all other devices intercommunicate. As illustrated in Figure S-9, the central node is in the form of a hub, switch, or router with multiple ports to which devices connect, usually through unshielded twisted pair (UTP) or shielded twisted pair (STP). In the public switched telephone network (PSTN), each carrier serving area (CSA) is a star, with local loops radiating from the central office (CO). Star configurations include 100Base-T and 1000Base-T local area networks (LANs). See also 100Base-T, 1000Base-T, CO, CSA, hub, node, PSTN, router, STP, switch, Token Ring, topology, and UTP. Figure S-9 start-stop transmission See asynchronous transmission. stateful Referring to a system or process that is aware of the status or condition, perhaps in detail, of an activity in which it participates. stateful autoconfiguration An IPv6 address assignment approach in which the configuration servers dynamically assign unique addresses to devices as they require them, drawing from a pool of such addresses. 459 stateful autoconfiguration 74570c19.qxd 9/12/07 12:41 AM Page 459 This approach resembles Dynamic Host Configuration Protocol (DHCP) used in IPv4. See also DHCP, IPv4, IPv6 address, and stateless autoconfiguration. stateful inspection firewall A security firewall that examines packets, notes the port numbers that they use for each connection, and shuts down those ports once the connection is terminated. See also authen- tication, authorization, firewall, and security. stateless Referring to a system or process that is not aware of the status or condition of an activity in which it participates. stateless autoconfiguration An IPv6 address assignment approach that employs two IP addresses, one that is assigned permanently to the mobile device and the other that is used to route data to the network to which the mobile device is connected at the time.This stateless approach is much like sending a data- gram to a device in care of a network and is useful in the context of mobile devices that move among pager, cellular,packet radio,wireless LAN (WLAN),and other wireless networks. See also cellular radio,data- gram, IPv6, IPv6 address, pager, stateless autoconfiguration, and WLAN. static address Referring to an Internet Protocol (IP) address permanently or semi-permanently assigned to a specific host. See also dynamic address, host, IP, and IP address. static bend The long term bend in a cable at rest, i.e., after installation. See also bend diameter. static load The long term load, i.e., force or weight,placed on a cable, such as a riser cable, which hangs vertically. See also load. station A terminal or endpoint on a network, such as a telephone set or data terminal. station message detail record (SMDR) See SMDR. statistical time division multiplexer (STDM mux or stat mux) A device that performs statistical time division multiplexing (STDM), an STDM MUX is commonly known as a stat mux. See also mux and STDM. statistical time division multiplexing (STDM) See STDM. stat mux (statistical time division multiplexer) A device that performs statistical time division multiplexing (STDM). See also mux and STDM. STDM (Statistical Time Division Multiplexing) An improved TDM method that makes use of intelligent muxes, or stat muxes, that can dynamically adapt to the changing nature and associated require- ments of the load placed on it in consideration of the available capacity of the circuit. STDM muxes can allocate bandwidth in consideration of the device and application priorities.An STDM can oversubscribe a trunk, supporting aggregate port speeds that can be multiples of the trunk speed, exercising flow control by buffering data during periods of high activity, restraining low-priority transmissions in favor of those of higher priority. STDM muxes may perform data compression, error detection and correction, and report- ing of traffic statistics. As shown in Figure S-10, STDMs typically divide a high-speed, four-wire digital circuit into multiple time slots to carry multiple voice conversations or data transmissions. Channelized T1 (North America), for example, commonly provides 24 time slots of 64 kbps. Channelized E-1 (European) commonly pro- vides 30 time slots.Additionally, the individual channels can be grouped to yield higher transmission rates (superrate) for an individual, bandwidth-intensive communication such as a videoconference.The individ- ual channels also can be subdivided into lower-speed (subrate) channels to accommodate many more, less bandwidth-intensive communications, such as low speed data. Also, many muxes allocate bandwidth on a priority basis, providing delay-sensitive traffic, such as real-time voice or video, with top priority. See also buffer, channel, FDM, flow control, oversubscribe, and TDM. stateful autoconfiguration 460 74570c19.qxd 9/12/07 12:41 AM Page 460 Figure S-10 STE (Section Terminating Equipment) In an SDH or SONET network, a repeater, add/drop multi- plexer (ADM), or anything else that attaches to either end of an optical fiber link. See also ADM, link, repeater, SDH, and SONET. steer A restoral mechanism employed in Resilient Packet Ring (RPR). In the event of a node or link failure, the steer option calls for the originating station to exercise sufficient intelligence to avoid the failed ring and place the traffic on the ring that retains continuity. See also dual counter-rotating ring, fiber optics, link, node, RPR, and wrap. steganography From Greek and translating as covered writing or hidden writing, and dating to 440 B.C., steganography is the art or science, or system, of hiding the existence of a message. In The Histories of Herodotus, the Greek historian Herodotus mentions several examples. Into the wood backing of a wax tablet, Demeratus carved a message warning his countrymen of an impending attack. He then applied the wax, which hid the message from view until it was removed by the intended recipient.Another method involved shaving the head of a slave and tattooing a message on his scalp.After the hair grew back enough to cover the message, the slave could be sent through enemy lines, and his head could be shaved again to read the message. More recently, microfilm dots have been hidden under postage stamps, or disguised as punctuation marks in typewritten letters. Contemporary stenography takes more technologically sophisti- cated forms, such as a message hidden in a data file, for example, in an HTML file, a JPEG file, or an MP3 file. Such a hidden file also is typically encrypted for additional security. See also encryption and watermark. step-by-step (SxS) See SxS. step-index fiber A type of glass optical fiber (GOF) characterized by a sharp difference, or step, in the index of refraction (IOR) at the interface between the core and the cladding.The layer of cladding has a uniform IOR that is sharply lower (typically one percent or more), which causes errant light rays striking the interface to reflect back into the core, which is the primary light-conducting medium. Light rays strik- ing the interface at extreme angles less than the critical angle can be lost in the cladding, as illustrated in Figure S-11. Multi-step fibers comprise multiple layers of cladding with sharp steps in IOR to compound the effect. See also critical angle, GOF, graded-index fiber, IOR, reflection, and total internal reflection. 1 3 46 47 48 1 2 2 3 46 47 48 Frame Frame 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 461 step-index fiber 74570c19.qxd 9/12/07 12:41 AM Page 461 Figure S-11 STM (Synchronous Transport Module) The electrical equivalent of the Synchronous Digital Hier- archy (SDH) optical signal, according to ITU-T international standards.The STM is known as Synchro- nous Transport Signal (STS) in Synchronous Optical Network (SONET) terminology, according to North American standards.The signal begins in electrical format as three T3 signals plus SDH signaling and con- trol overhead and converts to optical format for transmission over the SDH optical fiber facilities. Each STM-1 frame is transmitted in 125µs, yielding raw bandwidth of 155.52 Mbps.The STS frame includes five elements: Synchronous Payload Envelope (SPE), Section Overhead (SOH), Line Overhead (LOH), Path Overhead (POH), and Payload. See also bandwidth, frame, ITU-T, LOH, overhead, payload, POH, SDH, signaling and control, SOH, SONET, SPE, and T3. stop bit In asynchronous transmission, a bit, or sometimes two bits, that signals the end of a character. A start bit alerts the receiving computer of the arrival of a character. See also asynchronous transmission, bit, and start bit. Storage Area Network (SAN) See SAN. store-and-forward A transmission method by which a device receives a complete message or protocol data unit (PDU) and temporarily stores it in a buffer before forwarding it toward the destination. Having the whole message allows the device to check for errors and discard an errored frame or packet before for- warding it and wasting bandwidth on the next hop. A switch or router, for example, may have buffers to store incoming frames or packets of data until internal computational resources are available to process them and buffers to store outgoing frames or packets until bandwidth is available on a circuit in the for- ward direction.That way the device can mitigate issues of switch and circuit congestion. Messaging sys- tems add significant value by storing voice, e-mail, and image (e.g., fax) messages when the intended recipient is unavailable and forwarding them on demand when the recipient is available. Facsimile systems also may store international fax messages until off-peak hours, when calling rates are lowest. stored program control (SPC) See SPC. store locator service Also known as single number dialing, a service of the advanced intelligent network (AIN) that provides the ability to advertise a single number.The network routes calls to the closest store location in terms of either geography or time zone, based on the originating address (i.e., telephone number) of the caller. See also AIN. STP 1. Shielded Twisted Pair. Synonymous with Shielded Foil Twisted Pair (SFTP), STP is a copper cable configuration comprising a metallic foil shield that surrounds each insulated pair, of which there may be several. An uninsulated steel or tinned copper conductor in contact with each inner shield serves as a drain wire, ensuring that the continuity of the shield remains intact in the event that the foil is broken or cracked.The core of shielded pairs is then surrounded by an overall metallic shield of metallic tape or braid, or both, which is encased in a thermoplastic cable jacket, as illustrated in Figure S-12.The outer shield typically consists of helically or longitudinally applied plastic and aluminum laminated solid tape, although it may comprise a woven mesh, and steel or copper may also be used. Each shield absorbs ambient energy and conducts it to ground through the drain wire, thereby protecting the signal transmitted through the center conductors.The shield also serves to confine the electromagnetic field associated with the transmitted Cladding Light Source Cladding Core STM (Synchronous Transport Module) 462 74570c19.qxd 9/12/07 12:41 AM Page 462 signal within the core conductors, thereby reducing signal loss and maintaining signal strength over a longer distance. Screened twisted pair (ScTP) is a simpler version with only an outer shield. See also Cat 6, Cat 7, ScTP, and UTP. 2. Signal Transfer Point. In the advanced intelligent network architecture (AIN) architecture, a packet switch that routes signaling and control messages between a service switching point (SSP) and a service control point (SCP), and between STPs. See also AIN, packet, SCP, signaling and con- trol, SSP, and switch. 3. Spanning Tree Protocol.A bridge protocol for learning bridges, as defined in IEEE 802.1D standards. Spanning tree bridges are self-learning, filtering bridges for use in connecting LANs or LAN segments on a point-to-point basis.The bridge can be programmed or can teach itself the addresses of all devices on the network; subsequently, the network tree of the bridge provides only one span con- nection. Some spanning tree bridges also have the capability to provide security by denying access to certain resources based on user and terminal ID. Bridges that support the spanning tree algorithm have the ability to automatically reconfigure themselves for alternate physical paths if a network segment fails, thereby improving overall reliability. Radia Perlman invented STP while working for Digital Equipment Corporation (DEC). See also algorithm, bridge, filtering bridge, LAN, path, protocol, segmentation, and self-learning bridge. Figure S-12 streaming mode service In asynchronous transfer mode (ATM),a type of service used for framed data in which multiple interface data units (IDUs) are passed in a data stream. Streaming mode service is sup- ported by ATM Adaptation Layer 3/4 (AAL3/4). AAL3/4 also supports message mode service, a type of service for framed data in which only one IDU is passed. See also AAL3/4, ATM, IDU, and message mode service. stream-oriented Referring to an application that generates a continuous flow of data,rather than bursts of intense data activity interspersed with periods of inactivity.Real-time voice and video are stream-oriented. See also application, bursty transmission, data, flow, real-time, video, and voice. strength Toughness or durability.The ability to withstand force, pressure, strain, or stress. See bend radius, break strength, flex strength, mechanical strength, strength member, and tensile strength. strength member A load-bearing component of cables, particularly aerial and riser cables, designed to increase the overall tensile strength of the cable and to relieve the wires and fibers from bearing the load directly.As optical fiber, in particular, stretches very little before breaking, the strength members also must Conductor Jacket Shields Insulation 463 strength member 74570c19.qxd 9/12/07 12:41 AM Page 463 have very low elongation at the expected tensile loads.The strength members in optical fibers commonly are of aramid fiber yarn, invented and marketed by Dupont as Kevlar®, while those in copper cables com- monly are rods of fiberglass epoxy (FGE). As both aramid and fiberglass are dielectrics, they provide strength without contributing to issues of electromagnetic interference (EMI). Steel strength members were used historically, but are electrically conductive, which presents problems not only in terms of EMI, but also with respect to power surges such as those caused by lightening strikes.As steel and fiberglass,how- ever, are more thermally stable than aramid fiber, they are preferred when extreme cold temperature performance is required. See also aramid, dielectric, EMI, FGE, and fiberglass. string A linear series of things, such as bits or characters. string coding A compression technique that replaces long strings of redundant data with code words of much shorter fixed length. See also compression and run-length encoding. Strowger, Almon B. A Kansas City undertaker who invented the step-by-step (SxS) switch, also known as the Strowger switch, which was the first automatic circuit switching system. See also SxS. Strowger switch Step-by-step (SxS) switch. See SxS. structured wiring plan A comprehensive, documented plan for inside wire and cable systems in new building construction, incorporating voice, data, audio, video, security, and any other applications, from switch to jack. Such a plan considers placement of repeaters, hubs, switches, routers, and other network devices, as well as terminal equipment. A structured wiring plan must also consider power requirements and should address intermediate-term and even relatively long-term requirements. STS 1. Speech-To-Speech. A Telecommunications Relay Service (TRS) offering that enables a person with a speech disability to use his or her own voice to speak to the called party through a call administra- tor (CA) specially trained to understand speech affected by a variety of disorders.The CA acts as a facili- tator, repeating the spoken words in a clear and understandable manner. See also TRS. 2. Synchronous Transport Signal.The electrical equivalent of the Synchronous Optical Network (SONET) optical signal, according to North American standards.The STS is known as Synchronous Transport Module (STM) in Synchronous Digital Hierarchy (SDH) terminology, according to ITU-T international standards.The sig- nal begins in electrical format as a T3 signal plus SONET/SDH signaling and control overhead, and con- verts to optical format for transmission over the SONET optical fiber facilities. Each STS-1 frame is transmitted in 125µs, yielding raw bandwidth of 51.84 Mbps.The STS frame includes five elements: Syn- chronous Payload Envelope (SPE), Section Overhead (SOH), Line Overhead (LOH), Path Overhead (POH), and Payload. See also bandwidth, frame, ITU-T,LOH, overhead, payload, POH, SDH,signaling and con- trol, SOH, SONET, SPE, and T3. stuff bit A bit added into a bit stream during a process known as bit stuffing, in order to 1) ensure syn- chronization technique used in time division multiplexing (TDM) by avoiding long streams of 0 bits, 2) adjust for slight timing discrepancies between incoming bit streams when being multiplexed into faster links (e.g., multiplexing T1s into a T3 using an M13 multiplexer), and 3) prevent the appearance of the 0x7e flag character within an HDLC frame (4-zero suppression). See also bit, bit stream, bit stuffing, flag, HDLC, M13, multiplexer, synchronize, T1, T3, and TDM. stuttered dial tone See dial tone. subcarrier A fr equency channel that occupies only a portion of RF bandwidth allocated to the carrier and, therefore, has a smaller information capacity.A subcarrier sometimes is used for signaling between sta- tions on a network. See also bandwidth, carrier, channel, frequency, RF, and signaling. submarine cable Cable designed to be placed underwater. Such cables must be specially protected against moisture.At shallow depths on continental shelves, submarine cables commonly are plowed in and strength member 464 74570c19.qxd 9/12/07 12:41 AM Page 464 armored to protect them against ship anchors, trawler nets, and sharks, which are attracted to the electro- magnetic fields and like to gnaw on the cables and repeaters. See also cable. subnet (subnetwork) A network, either physical or logical, that operates as part of a larger network. In a local area network (LAN), for example, there may be many virtual LANs (VLANs), each of which may comprise many users on separate physical segments.The users are grouped in VLAN domains by physical port number,Transmission Control Protocol (TCP) port address, medium access control (MAC) address, or Internet Protocol (IP) address. Each VLAN operates as a subnet. See also LAN and VLAN. subnetting A technique that enables a network administrator to divide a single private Internet Protocol (IP) network into multiple smaller logical subnetworks by subdividing the host address into a subnetwork address and host address. Routers establish borders between subnets. See also IP, router, and subnet mask. subnet mask In Internet Protocol version 4 (IPv4), an address mask, i.e., address filter, that selectively includes or excludes certain values to distinguish between the subnetwork address and the host address in order to enable a router to forward packets correctly in a network that has been subnetted. See also IPv4, router, subnet, and subnetting. subnetwork (subnet) See subnet. subrate A rate lower than the normal rate. A channel bank typically derives multiple 64-kbps voice- grade channels from a circuit. If multiple low-speed data applications require less bandwidth, a sufficiently sophisticated time division multiplexer (TDM mux) can subdivide a channel into multiple subrate data channels. See also bandwidth, channel, channel bank, superrate, TDM, and voice grade. Subrate Digital Loop (SRDL) See SRDL. subscriber In telecommunications, an entity (individual, company, or other organization) that leases a circuit or contracts to use a public telecommunications service. subscriber identification Module (SIM) See SIM. Subscriber Line Carrier-96 (SLC-96) See SLC-96. subscriber line charge (SLC) See SLC. suit A mildly derisive term for an anonymous business executive or bureaucrat, referring to the fact that such people typically wear suits of clothes and may lack individuality.A suit, especially an empty suit, is in sharp contrast to a techie. See also empty suit and techie. superframe In the T-carrier D2, D3, and D4 framing conventions, a 12-frame sequence. Extended superframe (ESF) defines a 24-frame sequence. See also D2, D3, D4, ESF, frame, and T-carrier. supernetting The aggregation of multiple Internet Protocol version 4 (IPv4) address blocks. See CIDR, IPv4, and IPv4 address. surface wave An electromagnetic wave that propagates close to the surface of the Earth. See also ground wave and skywave. surge A strong, sudden, and transient spike in voltage or current. See also current and voltage. surge protector See protector. Super High Frequency (SHF) See SHF. superrate A rate higher than the normal rate.A time division multiplexer typically derives multiple 64- kbps voice grade channels from a circuit. If a data application requires more bandwidth, a sufficiently sophisticated mux can group multiple channels into a superrate channel. See also subrate and TDM. 465 superrate 74570c19.qxd 9/12/07 12:41 AM Page 465 supervision A basic signaling function that indicates the status of a component, such as trunk idle or busy, telephone on-hook or off-hook. In early switchboard operation, a human operator put a receiver across a line to monitor the status of a call, to determine if the call was in progress or had been terminated. See also call, signaling and control, switchboard, and trunk. suppression Forceful constraint, prevention, or subduing. In electronics, the elimination or intentional attenuation of an unwanted oscillation, such as a sideband, a carrier, or an echo. In some voice encoding mechanisms, silence suppression senses periods of inactivity in a voice conversation and simply ceases send- ing data associated with that conversation. See also attenuation, carrier, echo,encode, oscillate, sideband,and silence suppression. surge An elevated voltage level lasting longer that a spike. See also ground loop, spike, and voltage. sustainable cell rate (SCR) See SCR. SVC (Switched Virtual Circuit) A shared path established between two hosts through a packet net- work on command, i.e., via signaling as the call is placed. Once the path is selected, all packets in a given session travel the same path, which is selected in consideration of both the condition of the network and the load on it at the instant the connection is required. Thereby, an SVC bypasses failed and congested switches and circuits and improves overall performance through automatic load balancing.Although SVCs are defined in frame relay specifications, they are unusual in public networks due to the carrier’s fear that frame relay SVCs would cannibalize more expensive services like ISDN and long distance voice. Rather, frame relay networks employ permanent virtual circuits (PVC), which are predetermined, prepro- grammed paths.Globally,X.25 networks largely are based on SVCs.See also channel, circuit, frame, frame relay, load balancing, packet, path, PVC, virtual circuit, and X.25. SW (Short Wavelength) Referring to fiber optic systems operating in the 850 nm range, with the IEEE 802.3ae specification for 10GBase-SR, SW being one example. See also 10GBase-SR, SW,and LW. SWG (Standard Wire Gauge) Synonymous with British Standard Gauge (BSG).See also BSG and gauge. switch 1. A mechanical, electromechanical, or electronic device that opens, closes, or changes the con- nections in an electrical circuit. 2. A device that establishes, maintains, and changes logical connections over physical circuits. Switches flexibly connect transmitters and receivers across networks of intercon- nected links, thereby allowing network resources to be shared by large numbers of end users. Without switches, each transmitter/receiver pair would require a dedicated circuit in order to transfer data.There are a number of types of switches. In terms of switching technology, there are circuit switches and packet switches. a. Circuit switches establish connections between circuits, on demand and as available. Those connections are temporary, continuous, and exclusive in nature. Circuit switches were developed for voice communications, but will support any type of information transfer. Common examples of circuit switches include private branch exchanges (PBXs) and central office exchanges (COs or COEs). b. Packet switches switch data organized into packets, discrete sets of data that may take the specific form of packets, frames, or cells depending on the network technology specifics. For example, packet switches switch packets in networks based on the Internet Protocol (IP), frames in networks based on the frame relay or Ethernet protocols, and cells in those based on the asynchronous transfer mode (ATM) protocol. Packet switches were initially developed for data networking, but can support other forms of data, as well, although with varying degrees of success. With respect to physical placement, there are edge switches and core switches. c. Edge switches are positioned at the physical edge of a public network.The user organization gains access to an edge switch via an access link, or local loop.A central office (CO) is an example of an edge switch in the context of the circuit-switched public switched telephone network (PSTN). In a Local Area Network (LAN), a workgroup switch is the equivalent of an edge switch in a public network. d. Core switches, also known as tandem switches and backbone switches, are high-capacity switches positioned in the physical core, or back- bone, of a network and serving to interconnect edge switches. supervision 466 74570c19.qxd 9/12/07 12:41 AM Page 466 Although switches can be very intelligent in many respects, they operate only at the Layer 2, the Data Link Layer of the OSI Reference Model.That is to say that they operate link-by-link, or hop-by-hop, gen- erally under the control of a centralized set of logic that can coordinate their activities in order to estab- lish end-to-end connectivity across a multi-link circuit.A switch has no concept of the network as a whole, from end-to-end. See also ATM, backbone switch, cell, CO, core switch, Data Link Layer, edge switch, Ethernet, frame, frame relay, IP, LAN switch, OSI Reference Model, PSTN, router, and tandem switch. switchboard The first switching device, the switchboard literally was a series of small, mechanical switches mounted on a board.The operator manually switched the wires from one contact to another to establish a unique physical and electrical path or circuit to connect two parties in order that they might engage in a voice conversation.As all of the links, contacts, and switches are physically separated, a switch- board is a type of space division switch. Although the switchboard was superseded by the cordboard, the term remains widely used to refer to an operator console. See also cordboard and switch. Switched 56 (Switched 56 kbps Service) More formally known as Digital Switched Access (DSA).A switched digital data service that operates much like the public switched telephone network (PSTN) oper- ates for voice calls. Switched 56 service operates over a public data network (PDN) that actually is a phys- ical and logical partition of the PSTN.Where the PDN supports out-of-band signaling and control, the service sometimes is known as Switched 64, as the full 64 kbps bandwidth of a DS-0 channel is available to support end user data transmission. Switched-1536 service supports a full ISDN PRI of 24 channels, each of which provides the full 64 kbps of DS-0 bandwidth, with all signaling and control taking place out-of-band on another PRI circuit through a technique known as non-facility associated signaling (NFAS). See also DS-0, ISDN, NFAS, out-of-band signaling and control, PDN, PRI, and PSTN. Switched 64 See Switched 56. switched circuit A circuit established through one or more intermediate switching devices, such as cir- cuit switches or packet switches.A typical switched circuit can comprise a dedicated circuit from an orig- inating device to an ingress switch port, or point of interface, a switch matrix through which a path is established to an egress port, and a dedicated circuit to a destination device.There may be many interme- diate switches in a more complex scenario. Switched networks are highly shared, as a number of users con- tend for access to limited network resources through switches, which serve as points of contention, with connectivity between transmitters and receivers provided through the network on demand and as avail- able.This sharing of limited network resources clearly allows the network providers to realize significant operational efficiencies, which are reflected in lower overall network costs.The end users realize the addi- tional advantages of flexibility and resiliency, as the network generally can provide connectivity between any two physical locations through multiple alternate transmission paths.A switched circuit is in marked contrast to a dedicated circuit, which is dedicated to connecting two or more physical locations. Such a dedicated circuit is highly available, offers reliable levels of performance, and provides guaranteed band- width, but is inflexible and susceptible to catastrophic failure. See also circuit, circuit switch, dedicated circuit, packet switch, and switch. switched circuit network (SCN) See SCN. Switched Multimegabit Data Service (SMDS) See SMDS. switched virtual circuit (SVC) See SVC. switch hook An early telephone handset hung from a hook that activated a switch.When the telephone was not in use, the handset hung on the hook, or was on-hook.When the telephone was in use, the hand- set was off of the hook, or off-hook. When the user lifted the handset off the hook, a spring lifted the hook, which closed a switch and closed a circuit, drawing current from the central office (CO).The term now refers to the mechanical buttons or plungers that are mounted in the cradle of a telephone set, but the process remains essentially the same. Synonymous with hook switch. See also off-hook and on-hook. 467 switch hook 74570c19.qxd 9/12/07 12:41 AM Page 467 [...]... spectrum, UNE, UNE-P, and USF Telecommunications Device for the Deaf (TDD) See TDD Telecommunications Industry Association (TIA) See TIA Telecommunications Network (TELNET) See TELNET Telecommunications Relay Service (TRS) See TRS telecommunications organization (TO) See TO telecommunications service As defined in the U.S.Telecommunications Act of 199 6,“the offering of telecommunications for a fee directly... (GTE), but was sold to AT&T in 198 9 to form part of AT&T Technologies In 199 6, AT&T spun that company off to form Lucent, which was sold to Alcatel (France) in 2006 GTE is now part of Verizon SxS technology was considered state of the art until the appearance of the crossbar (Xbar) switch in 193 8 Large numbers of SxS switches remained in service into the 197 0s and even 198 0s, and some likely remain in... rather than telecommunications services See also broadband, cable modem, DSL, FCC, information services, Telecommunications Act of 199 6, and wireline telecommuter A businessperson who works and communicates largely from home through the use of information and communications technologies, rather than traveling to a company office.The terms telecommuting and telework were coined in the early 197 0s by Jack... SMR 74570c20.qxd 9/ 12/07 12:44 AM Page 490 TO (Telecommunications Organization) 490 TO (Telecommunications Organization) A generic term referring to the government agency formed in many countries to assume responsibility for telegraph and telecommunications services from the Post, Telegraph, and Telephone (PTT) agency Most developed nations have since fully or partially privatized telecommunications... machines published by the ITU-T in 198 0 Machines conforming to T.4 later became known as Group III See also facsimile, Group III, ITU-T, T.2, T.3, and T.6 T.434 An ITU-T specification, published in 199 9, for binary file transfer (BFT) that permits compliant facsimile devices to send any image file type (e.g., eps, pcx, and bmp) in the form of an editable file that 74570c20.qxd 9/ 12/07 12:44 AM Page 474 T.434... T1-to-T2) terminals that multiplex four T1 signals, which yields 96 DS-0 channels at 64 kbps per channel.T2 is unusual, although some does remain in place in the local loop, where Bell System companies used it in digital loop carrier (DLC) applications The Subscriber Line Carrier -96 (SLC -96 ) system introduced by Western Electric (now Lucent) in 197 9, for example, essentially is a remote line shelf and time... received symbol is assumed to 74570c20.qxd 9/ 12/07 12:44 AM Page 477 477 TCP (Transmission Control Protocol) have been errored in transit.TCM is specified by the ITU-T Recommendations for modems at speeds of 19. 2 kbps and higher ITU-T Recommendations for dial-up modems (and maximum speeds) specifying TCM currently include V.32 (96 00 bps), V.32bis (14.4 kbps), V.32ter ( 19. 2 kbps), V.34 (28.8 kbps), and V.34bis... Presentation 5 Session 1 Physical SMTP Simple Mail Transfer Protocol Transmission Media SNMP Simple Network Management Protocol TELNET Telecommunications Network UDP User Datagram Protocol ICMP Internet Control Message Protocol IP Internet Protocol 74570c20.qxd 9/ 12/07 12:44 AM Page 4 79 4 79 TDM (Time-Division Multiplexing) See also ARP, ARPA, FTP, ICMP, Internet, IP, LAN, MAN, NIC, OSI Reference Model, protocol,... four-wire circuit with a TDM multiplexer, or mux, placed on each end of the circuit, as illustrated in Figure T-3 1 Frame 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2 2 3 3 22 22 23 23 Frame 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 24 24 Figure T-3 At the transmitting end of the circuit, the mux scans the buffers associated with the ports to which individual... Technologies Originally Bell Communications Research (Bellcore), the research and development arm of the Regional Bell Operating Companies (RBOCs) SAIC acquired the company 198 8 and changed the name to Telcordia Technologies in April 199 9, with the stated focus of emerging technologies.Telcordia is now a private, standalone organization involved in the development of OSSs and network management software, . 2 3 46 47 48 Frame Frame 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 461 step-index fiber 74570c 19. qxd 9/ 12/07 12:41 AM Page 461 Figure. as they require them, drawing from a pool of such addresses. 4 59 stateful autoconfiguration 74570c 19. qxd 9/ 12/07 12:41 AM Page 4 59 This approach resembles Dynamic Host Configuration Protocol (DHCP). inventing an automated system that served 99 subscribers.The tele- phones that worked with that first automatic switch had two buttons. In order to reach subscriber 99 , for example, the caller slowly