AAL --ATM adaptat ion layer. Service-dependent sublayer of the data link layer. The AAL accepts data from different applications and presents it to the ATM layer in the form of 48-byte ATM payload segments. AALs consist of two sublayers: CS and SAR. AALs differ on the basis of the source-destination timing used, whether they use CBR or VBR, and whether they are used for connection-oriented or connectionless mode data transfer. At present, the four types of AAL recommended by the ITU-T are AAL1, AAL2, AAL3/4, and AAL5.
AAL2 --ATM adap tation layer 2. One of four AALs recommended by the ITU-T. AAL2 is used for connection-oriented services that support a variable bit rate, such as some isochronous video and voice traffic.
ATM-- Asynchronous Transfer Mode. International standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media such as E3, SONET, and T3.
Backhaul --A scheme where telephony signaling is reliably transported from a gateway to a Media Gateway Controller across a packet switched network.
BGW --see Business Gateway
Business Gateway --An xGCP media gateway which is a business customer premises equipment that has connection(s) to the VoIP network as well as connection(s) to the user's telephony equipment (typically a PBX, a corporate LAN or WAN). Such gateways are used to eliminate or reduce the need for individual medium (voice, data, and so forth) connectivity.
CA --see Call Agent
Call Agent --An intelligent entity in an IP telephony network which handles call control in an MGCP model Voice over IP network.
CAS --channel associated signaling. A form of signaling that the circuit state is indicated by one or more bits of signaling status sent repetitively and associated with that specific circuit. CAS is used on a T1 line. With CAS, a signaling element is dedicated to each channel in the T1 frame. This type of signaling is sometimes called Robbed Bit Signaling (RBS) because a bit is taken out (or robbed) from the user's data stream to provide signaling information to and from the switch.
CBR--constant bit rate. QoS class defined by the ATM Forum for ATM networks. CBR is used for connections that depend on precise clocking to ensure undistorted delivery.
CCS --common channel signaling. A signaling system used in telephone networks that separates signaling information from user data. A specified channel is exclusively designated to carry signaling information for all other channels in the system.
CID --channel identifier
CLASS --Custom Local Area Subscriber Services, usually referred to as "Custom Calling" features
Codec --Coder-decoder. Device that typically uses pulse code modulation to transform analog signals into a digital bit stream and digital signals back into analog signals. In Voice over ATM, it specifies the voice coder rate of speech for a dial peer.
Dial peer --An addressable call endpoint. In Voice over ATM, there are two kinds of dial peers: POTS and VoATM.
DS-0 --digital signal level 0. Framing specification used in transmitting digital signals over a single channel at 64-kbps on a T1 facility. A 64-K B-channel on an E1 or T1 WAN interface.
DTMF --dual tone multifrequency. A type of signaling that combines two distinct frequencies to generate a tone for each digit or character dialed. Sometimes referred to as Touchtone. This analog dial signaling uses two distinct tones to represent dialing digits.
E&M --Stands for 2-wire or 4-wire interfaces with separate signaling paths (from "Ear and Mouth", also "recEive and transMit"). E&M is a trunking arrangement generally used for two-way switch-to-switch or switch-to-network connections. Cisco's analog E&M interface is an RJ-48 connector that allows connections to PBX trunk lines (tie lines). E&M connections are also available on E1 and T1 digital interfaces.
Fault Tolerance --The level of ability within a system to operate properly even if errors occur.
FXO --Foreign Exchange Office. An FXO interface connects to the PSTN's central office and is the interface offered on a standard telephone. Cisco's FXO interface is an RJ-11 connector that allows an analog connection to be directed at the PSTN's central office. This interface is of value for off-premise extension applications.
FXS --Foreign Exchange Station. An FXS interface connects directly to a standard telephone and supplies ring, voltage, and dial tone. Cisco's FXS interface is an RJ-11 connector that allows connections to basic telephone service equipment, keysets, and PBXs.
Layer 1 --This describes the Physical Layer of the OSI Reference Model defined in ITU X.200. It is responsible for the electric signal being sent and received. This can be viewed as a bit stream coming in, and going out, of the system. Scope must be considered when using this term. For example, Layer 1 on a T1 is 1.544 Mbps but Layer 1 on a DS-0 timeslot in the T1 is 64 kbps.
Layer 2 --This describes the Datalink Layer of the OSI Reference Model defined in ITU X.200. It is responsible for point-to-point delivery of a PDU. Layer 2 protocols have two basic classes: reliable (meaning delivery is guaranteed or an error is reported) and unreliable (meaning delivery may not occur with no indication to the upper layers).
Layer 3 --This describes the Network Layer of the OSI Reference Model defined in ITU X.200. It is responsible for the network routing and delivery of a message. Examples of Layer 3 protocols include X.25 Packet Layer Protocol and the Internet Protocol. Q.931 is not considered a Layer 3 protocol because it is not concerned with routing and delivery of a message but rather the message body itself.
Media Gateway --Equipment that connects the PSTN or a PBX with the VoIP network. It is controlled by a Call Agent via MGCP. A Media Gateway terminates facilities (trunks), packetizes the PCM stream into IP/ATM and/or forwards packets into the IP/ATM network. It performs these functions in reverse order for media streams flowing from the packet network to the PSTN.
MG --see Media Gateway.
MGC --Media Gateway Controller. A Media Gateway Controller provides call control capability to handle signaling traffic from a variety of sources. It also manages connections and resources of its Media Gateways. Can also be called a Call Agent.
MGCP --Media Gateway Control Protocol
Package --A set of parameter values that define a type of voice endpoint or connection. Examples include line-package, trunk-package, dtmf-package, and atm-package.
PBX --private branch exchange. Privately owned central switching office.
PCM--pulse code modulation. Technique of encoding analog voice into a 64-kbit data stream by sampling with eight-bit resolution at a rate of 8000 times per second.
PDU --protocol data unit. OSI term for packet.
PLAR --private line, automatic ringdown. Leased voice circuit that connects two single endpoints together. When either telephone handset is taken off-hook, the remote telephone automatically rings.
POTS --plain old telephone service. Basic telephone service supplying standard single line telephones, telephone lines, and access to the PSTN.
POTS dial peer --Dial peer connected via a traditional telephony network. POTS peers point to a particular voice port on a voice network device.
PRI --primary rate interface. ISDN interface to primary rate access. Primary rate access consists of a single 64-Kbps D channel plus 23 (T1) or 30 (E1) B channels for voice or data.
PSTN --Public Switched Telephone Network. General term referring to the variety of telephone networks and services in place worldwide. Sometimes called POTS.
RSIP--ReStart In Progress. MGCP command used to indicate that a span (or collection of spans) has come into service, has gone out of service, or is about to go out of service.
SPVC --soft permanent virtual circuit. A generic term for any communications medium that is permanently provisioned at the end points, but switched in the middle. In ATM, there are two kind of SPVCs: smart permanent virtual path connections (SPVPCs) and Smart permanent virtual channel connections (SPVCC).