By Eran Sharabi, Yaniv Roten and Shai Ben-Naim
Frame relay was originally conceived of as a protocol for use over ISDN interfaces , and initial proposals to this effect were submitted (in 1984) to the Consultative Committee for International Telegraph and Telephone (CCITT). Work on frame relay was also undertaken in the American National Standards Institute (ANSI)-accredited T1S1 standards committee in the U.S.
The rapid increase in high bandwidth communication is the main reason for improving and using Frame Relay technology. There are two main factors which influence the rapid demand for high speed networking:
These factors led to new technologies and two principles: Frame Relay and Cell Relay.
Frame Relay technology was intended to be an intermediate solution for the demand for high bandwidth networking. It is a packet switching technology which relies on low error rate digital transmission links and high performance processors. Frame Relay technology was designed to cover the following:
Frame Relay is a standard communication protocol that is specified in CCITT recommendations I.122 and Q.922 which add relay and routing functions to the data link layer (layer 2 of the OSI reference model).
Some of the functions associated with packet transport, such as error correction, flow control, etc., are still formed, but on an end-to-end basis by the end-user devices, instead of by the network. Frames are constructed by encapsulating layer 2 messages (excluding the CRC and flags), with a two byte header, a CRC, and a flag delimiter. The frame relay header consists of a data link connection identifier (DLCI) that allows the network to route each frame on a hop-by-hop basis along a virtual path defined either at call setup or subscription time. The start and end flags, and the CRC are identical to those used by HDLC or SDLC, based interfaced packages.
**As described in the Protocol Architecture Diagram, A initiates the communication process by sending a request for session establishment to the transport layer via the presentation and session layers. The transport layer forwards call control information through the ISDN via the D channel using Q.931 procedures. The signaling message is routed through the network and is used to define the virtual path and calls parameters that will be used during the data transfer stage.
Once the call is established, data is transferred through the network between applications A and B on a hop-by-hop basis by using the DLCI in the frame header and routing information at each node as determined during call setup. One of the characteristic of Frame Relay is that it minimizes the amount of processing performed on each frame by the network and allows for very fast transfer of information.
Frame Relay frame structure is essentially identical to that defined for Lap-D.The frame relay format can be distinguished from Lap-D by its absence of a control field.
Each frame relay PDU consists of the following fields:
- FECN=Forward Explicit Congestion notification bit
- BECN=Backward Explicit Congestion Notification bit
- DE=Discard Eligibility bit
The frame relay network uses a simplified protocol at each switching node. The simplicity is achieved when link-by-link flow control is missing. As a result, the performance of frame relay networks has largely been determined by the offered load. When the offered load is high, due to the bursts in some services, temporary overload at some frame relay nodes causes a collapse in network throughput. Therefore, some effective mechanisms are required to control the congestion.
The congestion control in Frame Relay networks includes the following elements:
Once a connection has been established in the network, the edge node of the frame relay network must monitor the connection's traffic flow to ensure that the actual usage of network resources does not exceed this specification. Frame relay defines some restrictions on the user's information rate. It allows the network to enforce the end user's information rate and discard information when the subscribed access rate is exceeded.
Explicit congestion notification is proposed as the congestion avoidance policy. It tries to keep the network operating at its desired equilibrium point so that a certain QOS for the network can be met. To do so, special congestion control bits have been incorporated into the address field of the frame relay: FECN and BECN. The basic idea is to avoid data accumulation inside the network.
X.25 was designed to provide error-free delivery using high error-rate links. Frame relay takes advantage of the new, lower error rate links, enabling it to eliminate many of the services provided by X.25. The elimination of functions and fields, combined with digital links, enables frame relay to operate at speeds 20 times greater than X.25.
X.25 is defined for layers 1, 2 and 3 of the ISO model, while frame relay is defined for layers 1and 2 only. This means that frame relay has significantly less processing to do at each node, which improves throughput by order of magnitude.
X.25 prepares and sends packets, while frame relay prepares and sends frames. X.25 packets contain several fields used for error and flow control, none of which is needed by frame relay. The frames in frame relay contain an expanded address field that enables frame relay nodes to direct frames to their destinations with minimal processing .
X.25 has a fixed bandwidth available. It uses or wastes portions of its bandwidth as the load dictates. Frame relay can dynamically allocate bandwidth during call setup negotiation at both the physical and logical channel level.
Frame relay limits the functionality inside the network, based on the premise that the probability of frame error is very small. Other functions, such as error recovery, flow control, etc. can be performed at higher layers by users. Frame relay is considered as a cost-effective replacement for leased line services in addition to being technically superior to the X.25 services.