Frame Relay frame format is as shown in the figure. Out of all the fields address field can be varied as 2 bytes, 3 bytes, or 4 bytes depending on the extension required in the network.
- Flag: It is 1-byte field used to mark start and end of Frame Relay frame.
- DLCI: First six bytes of address fields make 6 uppers bits of DLCI while 4 bits from second byte makes lower 4 bit of 10-bit Data Link Connection Identifier defined by the standard. When 10 bits are less for connection identifiers additional bits are added which increases the address width to 3 or 4 bytes instead of 2 bytes.
- C/R: It allows the upper layers of network to identify the frame as a command or a response frame. It is not used by Frame Relay protocol but used for higher level protocols.
- EA: It indicates whether the current byte of the address is final byte of address. When it is set to 0 it means another byte of address is to be followed and when it is 1 it indicates end of address.
- DE: Discard Eligibility bit indicates the priority level of the frame. In emergency situation system may need to discard frame to reduce congestion and prevent network from collapsing due to overload. When set to 1 it allows network to discard bits whenever there is congestion in the network.
- FECN/BECN: These bits are used for congestion control. Detailed explanation is given below under congestion control.
Congestion Control:
- Congestion in Frame Relay decreases throughput and increases delay. A high throughput and low delay is the main goal of Frame Relay protocol. Frame Relay does not have flow control and it allows user to transmit burst data. This means that a Frame Relay network has potential to be really congested with traffic, requiring congestion control.
- Frame Relay uses congestion avoidance by means of two bit fields present in the Frame Relay frame to explicitly warn source and destination of presence of congestion:
- BECN: Backward Explicit Congestion Notification (BECN) warns the sender of congestion present in the network. This is achieved by resending the frame in reverse direction with the help of switches in the network. This warning can be responded by the sender by reducing the transmission data rate, thus reducing congestion effects in the network.
- FECN: Forward Explicit Congestion Notification (FECN) is used to warn the receiver of congestion in the network. It might appear that receiver cannot do anything to relieve the congestion, however the Frame Relay protocol assumes that sender and receiver are communicating with each other and when it receives FECN bit as 1 receiver delays the acknowledgement. This forces sender to slow down and reducing effects of congestion in the network.
Quality of Service:
- mQoS in Frame Relay is achieved by defining four attributes to control traffic in the network:
- Access Rate: It is average rate at which the data is sent over channel. This rate depends on the bandwidth allocated for the channel. User can never exceed this rate.
- Committed Burst Size: For every connection V defines a committed burst size (B_c) this is maximum number of bits in predefined time in a network is committed to transfer without discarding any frame or setting DE bit high.
- Excess Burst Size: This is the maximum number of bits (B_e) in excess of B_c that a user can send over a predefined time. The network is committed to transfer these bits if there is no congestion.
- If the user never exceeds B_c the network is committed to transmit the frames without discarding any. If the used exceeds B_c by less than B_e the network has to transfer all the frames if there is no congestion. And if there is congestion some of the frames will be discarded.
- The first switch that receives data from user has counter which is set and makes DE bit 1 when user data rate exceeds (B_c). Rest of the switches will discard this frames if there is congestion in the network.
- But when data rate exceeds above B_cby (B_e) then all the frames thereafter are discarded.
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