Frame Relay

       Frame relay was introduced in 1992.  It is a virtual circuit wide area network that was designed to respond to demands for a new type of WAN. Some wide area networks uses virtual circuit switching called X.25. This network carries packets from one place to another. Frame relay falls in to the category of a packet switching family.

         Packet switch is a switching technology that technology that store and forward messages. When messages are broken down into smaller parts it now called packets. Each packet contains a destination address and control information. It uses the time division multiplexing technique in transferring data. Packets are sent from the source to the destination through a shared network. Packet switching has several applications including electronic fund transfer, credit card approvals, point of sale equipment, short files and email.

      Combining packet switching and a fast network using high speed communications and delay networking are called Fast packet switching. It is designed to reduce the delay by using a hold and forward technology. With this technology it reduces overhead and processing, improve speed and reduce cost. It is also designed to run on high speed circuits with low error rates.

Frame Relay Layers

Physical Layer – Frame Relay uses whatever protocol is available. It supports any of the protocol supported by American National Standards Institute (ANSI).

Data Link Layer – In this layer the Frame Relay employs a simplified version of High Level Data Link Control (HDLC). HDLC provides extensive error and flow control fields that are not needed in Frame Relay. 

Here is a figure that show the format of a frame relay frame:

·         Address (DLCI) field – The 1^st part of the DLCI field is 6 bits with the first byte. The second part of the DCLI uses the first 4 bits of the second byte.

·         Command Response (C/R) – It allows the upper layer to identify a frame as either a command or a response.

·         Extended Address (EA) – It indicates whether the current byte is the final byte of the address.

·         Forward Explicit Congestion Notification (FECN) – It is set by any switch to indicate that traffic is congested in the direction in which the frame is traveling.

·         Backward Explicit Congestion Notification (BECN) – it is set to indicate a congestion problem in the direction opposite to the one in which the frame is traveling.

·         Discard Eligibility (DE) – It indicates the priority level of the frame.

·         Extended Address (EA) – The frame relay address has been extended from the original 2-byte address to 3 or 4 byte addresses.

Frame Relay was developed to carry data traffic across the WAN link and Local Area Networks (LANs) to other LANs. Here is a figure of the transmission of data across to the local loop to the local telephone company’s central office that is connected to the interexchange carriers:

Advantages of Frame Relay

•  The main advantage of Frame Relay over point-to-point leased lines is cost. Frame Relay can provide  performance similar to that of a leased line, but with significantly less cost over long distances.

Disadvantages of Frame Relay

• One main disadvantages of Frame Relay are slowdowns due to network congestions. Because all of the company’s costumers on Frame Relay uses common network and there are times that the network exceeds its capacity on data transmission.

• Another main disadvantage is that there are difficulties on ensuring the Quality of Service (QoS). It is due to the fact that Frame Relay uses variable length packets. It is easier to guarantee QoS when using a fixed-length packet.

Sources:

Broadband Telecommunications Handbook by Regis J. “BUD” Bates

Data Communications and Networking third edition by Behrouz A. Forouzan

Wan Design with Frame Delay by David Horton