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Frame Relay is a network interface, or access, standard which was defined in 1988 by the ITU-T in its I.122 recommendation, Framework for Providing Additional Packet Mode Bearer Services. Access to a Frame Relay network is accomplished using the LAP-D (Link Access Protocol-D channel) signaling protocol developed for ISDN, as it originally was intended as an ISDN framing convention for a bearer service. Frame Relay standards say nothing about the manner in which the internal network operates. As is the case with X.25 packet networks, such issues are left to manufacturers that develop proprietary switching technologies [11-2]. Access rates generally range up to T1, although speeds of T3 and above are possible [11-3]. Notably, Frame Relay is backward-compatible, as it considers the characteristics of the embedded networks and the standards on which they are based. Electrical interfaces are not rigidly defined for Frame Relay.
Very much analogous to a streamlined and supercharged form of X.25 packet switching, Frame Relay sets up and tears down a call with control packets, as does X.25. Frame Relay also forwards packets of data, in the form of frames. Similar to X.25, Frame Relay effectively multiplexes frames of data over a shared network of virtual circuits for maximum network efficiency. The interface is in a FRAD (Frame Relay Access Device) which can be implemented on the customer premise, much as is the case with a X.25 PAD (Packet Assembler/Disassembler). As is the case with X.25, Frame Relay is intended for bursty data traffic. While both can support carry voice, video and audio, the presentation can be less than pleasing due to packet delay and loss.
At that point, the two technologies diverge (see Table 11.1 for a comparison between X.25 and Frame Relay). Frame Relay is a connection-oriented service that assumes a high-speed user link, as well as a high-speed transport system that is generally fiber optic in nature. Frame Relay assumes no responsibility for error detection and correction in the user information field as there are assumed to be no errors in transmission; rather, error control is the responsibility of the end user. In other words, Frame Relay operates at Layers 1 and 2 of the OSI Reference Model. This ceding of responsibility to the end user reduces the processing load on the network, reducing latency (delay) significantly and yielding faster transmission of each frame of data. Frame Relay does check for errors in the control field, which is used for routing and other network purposes; errors in this field cause frames to be discarded. Frame Relay is protocol-independent and provides no protocol conversion services.
ATTRIBUTE | X.25 | FRAME RELAY |
---|---|---|
FACILITIES | Analog Assumed | Digital Assumed |
PAYLOAD | 128B/256B Fixed | < 4,096B Variable |
ACCESS SPEED | < 56 Kbps-DS1 | < 56 Kbps-DS1, DS3 |
LINK LAYER PROTOCOL | LAP-B | LAP-D/LAP-F |
LATENCY | High | Moderate |
ORIENTATION | Connection-Oriented | Connection-Oriented |
ERROR CONTROL | Network | User |
PROTOCOL CONVERSION | Yes | No |
PRIMARY APPLICATION | Interactive Data | LAN-to-LAN |
Frame Relay is supported by a wide range of manufacturers and carriers, both domestic and international. Standards bodies include ANSI, ETSI and the ITU-T. The Frame Relay Forum, a voluntary group of manufacturers and other interested parties, develops and promotes Implementation Agreements (IAs), to address manufacturer interoperability issues based on the standards. Select applicable standards are noted in Table 11.2.
SUBJECT AREA | ITU-T | ANSI |
---|---|---|
Architecture and Service Description | I.233 | T1.606 |
Data Link Layer Core Aspects | Q.922 Annex A | T1.618 |
PVC Management | Q.933 Annex A | T1.617 Annex D |
Congestion Management | I.370 | T1.606a |
SVC Signaling | Q.933 | T1.617 |
Source: DigiNet Corp. |
Frame Relay access is provided on the basis of a dedicated digital link into a Frame Relay node. The typical speed of the access link can range up to 44.736 Mbps and can be in the form of DDS (56/64 Kbps), Switched 56/64 Kbps, ISDN BRI (64/128 Kbps) or PRI (1.544 Mbps), FT1 (N x 64 Kbps), T1 (1.544 Mbps) and T3 (44.736). In March 1996, LDDS Worldcom (MS) [11-4], and MCI (DC) announced Frame Relay service at speeds of 6 Mbps, 10 Mbps and 19.8 Mbps. No information is available relative to the success of these offerings, and no other carriers have followed suit at the time of this writing.
Customer DCE is the form of a FRAD (Frame Relay Assembler/Disassembler), which may be standalone, although it generally is embedded under the skin of another device such as a router. From the FRAD, access is gained to the link and, subsequently, to the Frame Relay network node. As the node resides in the FRND (Frame Relay Network Device). The UNI (User Network Interface), as defined by ANSI and ITU-T, defines the nature of this access interface.
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