Frame Relay, A Simple Discussion

Frame Relay is a simplified form of connection-based, packet-switching service in which synchronous frames of data are routed to destinations indicated on the header information. Frame Relay assumes an error-free physical link and therefore does not guarantee data integrity. Error detection and correction responsibility is left with the end devices. It uses the synchronous High-level Data Link Control (HDLC) frame format up to 4096 octets in length. Each frame contains a start flag, two octets that contain the information required for multiplexing across the link, the data information (payload), two octets generated by a cyclic redundancy check (CRC) of the rest of the octets between the flags, and the end flag.

Frame Relay can be used for various types of connections. It should be seen as a flexible protocol that lies on the Data Level of the Connections between routers. It can currently be used effectively for carrying all sorts of data, up to speeds of about 4 megabits per second.

Frame Relay Networks

In the diagram above, a mainframe communicates with each of the workstations (devices) on the LAN over a separate virtual circuit. The Frame Relay protocol identifies a virtual circuit by a 10-bit address called a Data Link Connection Identifier (DLCI). Each DLCI is unique on its local Frame Relay link. However, DLCIs are NOT unique throughout the network.

Since the DLCI is a 10-bit number, the Frame Relay protocol defines 1024 possible DLCIs. Of these, 2 (0 and 1023) have been reserved for signalling and 30 (1 to 15 and 1008 to 1022) have been reserved for future use.

For instance, networks that have implemented the optional multicasting feature reserve DLCIs 1019 to 1022 for that purpose. The remaining 992 DLCIs, (16 to 1007), are available to subscribers.

A Frame Relay network relies on the higher-layer protocols in its attached devices to recover from errors or congestion. In practice, this means that the higher layers must recognize that the network has discarded one or more frames of data.

Most higher-layer protocols use rotating sequence numbers to recognize frames that have been discarded. When a device receives a sequence number out of order, it requests that its partner retransmit all frames in order since the last frame it received with a correct sequence number.

In a well-tuned network, this typically includes the missing frame and all frames that its originator had transmitted in the time the destination device took to recognize the discard and send a message across the network requesting retransmission. In most cases, the originating device retransmits more data than would have been necessary.

This is a very reliable way to recover data lost through occasional transmission errors. However, when data's been discarded because of traffic congestion, bulk retransmission can only make the problem worse.

Fortunately, most higher-layer protocols use some form of throttling or flow control mechanism to recognize and prevent congestion.

The Frame Relay protocol also provides a way for the network to alert its subscribers when it becomes congested. The header of each Frame Relay frame contains two Explicit Congestion Notification bits that the network can set if it transmits that frame over a congested path. Each of these bits signifies congestion in a specific direction on the virtual route.

   

The Frame Relay frame header is illustrated above. The first octet is a flag field that delimits the frame from another frame or from idle time on the circuit. The second octet contains the first 6 bits of the 10-bit DLCI followed by a Command/Response bit (C/R) and the frame's first Extended Address (EA) bit.

The next octet contains the remaining four bits of the DLCI followed by the FECN and BECN bits described above, a Discard Eligibility (DE) bit, and the frame's second EA bit.

The subscriber or the network may set the value of the DE bit to 1 to indicate that the network may discard this frame in preference to frames in which the value of the DE bit is 0. (This occurs only after it has discarded all frames transmitted in excess of their subscribers' CIR and Bc).

The subscriber's data follows the Frame Relay header in most Frame Relay frames, and the data is followed in turn by the 2- octet Frame Check Sequence (FCS) and a final flag octet. A frame must contain at least one octet of user data for a total of 5 octets between flags.

A frame may not exceed 8192 octets between flags, counting header and FCS. The latest Frame Relay standards recommend a maximum frame size of 1600 octets overall.

Pro's and Cons of Frame Relay.

PRO: In many scenario's involving long haul, high speed connections, it is cheaper than dedicated lines.

PRO: There is a cheap solution to incorporate redundancy in the network.

PRO: Mixed speeds can be converted, traffic bursts can be buffered.

PRO: Less hardware is needed to for the same amount of connections

CON: There may be jams; no guaranteed bandwidth

CON: In a point-to-point scenario it is not economically feasible.

CON: In short haul, it is not economically feasible.

Sources:

http://www.myhome.org/pg/frame.htm

http://www.etisalat.ae/index.jsp?lang=en&type=service&currentid=10d3d1f598540310VgnVCM1000000c24a8c0____&contentid=a5aaa34673d3b010VgnVCM1000000c24a8c0RCRD

http://www.techfest.com/networking/wan/frrel.htm

http://www.dcbnet.com/notes/framerly.html

Broadband Communications Handbook