Frame Relay

Packet Switching was developed as a method of multiplexing packets across the phone systems.

The term Bandwidth on Demand arises when using X.25 and Frame Relay. Both use packet switching which is a fancy way of saying that the packets are multiplexed on a common line. LAN traffic tends to be very bursty - periods of inactivity then large fast bursts of data. This is a very inefficient use of bandwidth if a large proportion of the time the line is idle. By using statistical multiplexing, bandwidth is used only when required hence the term Bandwidth on Demand.

Frame Relay is very similar to X.25. In fact Frame Relay was developed to improve on X.25's shortcomings: (slow speed and large overhead). Frame Relay was improved over X.25 by the following:

  • Decreased Protocol Overhead
  • Digital Switching
  • Increased bandwidth to 2 Mbps+
  • Voice over data

Decreased Protocol Overhead

Frame Relay works on the Data Link and Physical layers:

It uses the LAPD (Link Access Protocol D) for framing the packets. Higher layer protocols such as TCP/IP are encapsulated in the LAPD packet.

Frame Relay Frame


F  - Flag
HDR - Header
DLCI - DL connection
C/R - Command Response
EA - Extended Address
CF - Congestion Forward Notification
CB - Congestion Backward Notification
DE - Discard Eligibility bit
CRC  - Cyclic Redundancy Check

Frame Relay drops error checking at each node and relies on the upper layers such as TCP/IP to perform error checking. If a CRC (bit level error checking) error is found the packet is dropped. It is up to the Network layer to retransmit the packet. The assumption is that the Public Network is reliable enough to allow this.

This means that only the end devices are responsible for recovery from transmission errors. Routers A, B and C do not have to worry about error recovery. It is the responsibility of the end devices: Host A & B to take care of error recovery.

Digital Switching

Frame Relay is made to interface with T1 (1.544 Mbps) and E1 (2.048 Mbps) digital switching lines. This results in a higher transfer speed and a much more reliable connection. This reliability allows Frame Relay to drop some of the error checking from the Data Link and Physical layers and rely on the Network Layer.

Increased Bandwidth

Frame Relay can presently operate up to 2.048 Mbps (E1) but there has been successful testing to 155 Mbps. Due to the low overhead, a Frame Relay network will be more efficient at sending data than an X.25 network. This means that more data will be transferred for the same amount of bytes.

LAPD - Link Access Protocol D channel

LAPD (Link Access Protocol D channel) deals with Logical connections: Logical Channel Numbers. LAPD takes care of the Virtual Connections (dial-ups) and Permanent Virtual Connections (leased lines). Again, you can have 1 physical connection to the Frame Relay network and many virtual connections (logical). The logical channel numbers are referred to as DLCI (Data Link Connection Identifiers) in Frame Relay. Looking at the Frame Relay frame, we can see that there are only 10 bits assigned to the DLCI. This means that there can be a maximum of 2^10 = 1024 logical channels per physical connection.

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Copyright July 2013 Eugene Blanchard