By Eran Sharabi, Yaniv Roten and Shai Ben-Naim

BACKGROUND

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.

PRINCIPLES

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:

• The rapid increase in use of LANs
• The use of fiber optic links

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:

• Low latency and higher throughput
• Bandwidth on demand
• Dynamic sharing of bandwidth
• Backbone network

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 PROTOCOL DATA UNIT

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:

• Flag Field. The flag is used to preform high level data link synchronization which indicates the beginning and end of the frame with the unique pattern 01111110. To ensure that the 01111110 pattern does not appear somewhere inside the frame, bit stuffing and destuffing procedures are used.

• Address Field. Each address field may occupy either octet 2 to 3, octet 2 to 4, or octet 2 to 5, depending on the range of the address in use. A two-octet address field comprising the EA=ADDRESS FIELD EXTENSION BITS and the C/R=COMMAND/RESPONSE BIT.

• DLCI-Data Link Connection Identifier Bits. The DLCI is used to identify the virtual connection so that the receiving end knows which information connection this frame belongs to. It should be pointed out that this DLCI has only local significance. Several virtual connections can be multiplexed over the same physical channel.

• FECN, BECN, DE BITS. These are congestion reporting capability bits:

- FECN=Forward Explicit Congestion notification bit
- BECN=Backward Explicit Congestion Notification bit
- DE=Discard Eligibility bit

• Information Field. The maximum number of data bytes that may be put in a frame is a system parameter. The actual maximum frame length may be negotiated at call set-up time. The standard specifies that the maximum information field size (to be supported by any network) is at least 262 octets. Since end-to-end protocols typically operate on the basis of larger information units, it is recommended that the network support the maximum value of at least 1600 octets, to avoid the need for segmentation and reassembling by end users.

• Frame Check Sequence (FCS) Field. Since the bit error rate of the medium is not completely negligible, it is necessary to implement error detection at each switching node to avoid wasting bandwidth due to the transmission of erred frames. The error detection mechanism used in frame relay is based on the Cyclic Redundancy Check (CRC).

CONGESTION CONTROL IN FRAME RELAY NETWORK

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:

• Admission Control. This is the principle mechanism used in frame relay to ensure that resource requirement, once accepted, can be guaranteed. It is also generally used to achieve high network performance. The network decides whether to accept a new connection request based on the relation of the requested traffic descriptor and the network's residual capacity. The traffic descriptor is defined as a set of parameters communicated to the switching nodes at call set-up time or service subscription time, which characterizes the connection's statistical properties. The traffic descriptor consists of three elements:
• Committed Information Rate (CIR). The average rate (in bit/s) at which the network guarantees to transfer information units over a measurement interval T. This T interval is defined as: T = Bc/CIR .
• Committed Burst Size (BC). The maximum number of information units that can be transmitted during the interval T.
• Excess Burst Size (BE). The maximum number of uncommitted information units (in bits) that the network will attempt to carry during the interval T.
  

TRAFFIC ENFORCEMENT

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.

CONGESTION NOTIFICATION

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.

FRAME RELAY VS. X.25

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.

SUMMARY

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.

References

• The Frame Relay Explosion, Data Communications, February 1995.
• Computer Communications, July, 1993.
• Internetworking Interview -- Cisco.
• Frame Relay -- A New Realism, Feature 1993.
• Evaluating Frame Relay, Telecommunications, 1992.
• Frame Relay, Computer Communications, 1992.
• Frame Relay, Technology, Feature, 1990.