Control fields identify the frame type. This is especially important for higher level applications and in the use of instruments when monitoring circuits. The control field also helps in the establishment and de-establishment of logical connections within switches.
With applications that have flow control and error control such as X25, the control field contains sequence numbers which allow the recipient to control flow and the transmitter to re-transmit should it be determined that a frame is missing or damaged.
| Flag 1 byte | Address 2 bytes | Information variable length | FCS 2 bytes | Flag 1 byte |
The flag field is a unique string of bits which indicates the start and end of of a frame.
The address field is variable in length from 2 - 4 octets. the DLCI is contained in this field. One of the reasons why this field is variable is due to the fact that there is the possibility that 1024 (1022 if LMI (1023) is excluded) DLCI's may not be enough. The information field contains actual data of the user from applications that operate at higher levels of the OSI model than Frame Relay. It can also be used for call controlling.
The frame check sequence (FCS) is a stored calculation inserted when the frame is transmitted. Upon receipt, the FCS is re-calculated and compared to the stored value. If it does not match, it is assumed that the frame is in error and discarded.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|
| DLCI high order | C/R | EA 0 | |||||
| DLCI low order | FECN | BECN | DE | EA 1 | |||
The address field can vary from 2 to 4 octets in size. The primary function of the address field is to carry the DLCI. This allows multiple connections to be carried over a single channel (multiplexed).
The EA bits allow the address to be extended in size from 10 bits to 17 or 24 bits respectively. Thus the address field has built in scalability.
The Command/Response bit (C/R) is used by higher end applications to manage end-end connections. It is not used at OSI level 2 (Frame Relay).
Forward Explicit Congestion Notification (FECN) is activated when the frame is switched onto a link where the Frame Relay interface has become congested. Thus the receiving device knows that the PVC is congested.
Backward explicit Congestion Notification (BECN) is activated to tell the switch to engage in congestion avoidance where traffic is in the opposite direction of the received frame. ie user transmitted frames will encounter congestion. (See Frame Relay Congestion Management for more.)
Discard Eligibility (DE) is set to indicate that a certain frame should be dropped in preference to other frames without the bit set when and if the link becomes congested.
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