written 6.7 years ago by | • modified 2.7 years ago |
Subject: Advanced Network Technologies
Topic: WAN Technologies
Difficulty: Medium
written 6.7 years ago by | • modified 2.7 years ago |
Subject: Advanced Network Technologies
Topic: WAN Technologies
Difficulty: Medium
written 6.6 years ago by |
Fig. Shows the frame relay frame format. This frame is very similar to the HDLC frame except for the missing control field here.
• The control field is not needed because flow and error control are not needed.
• The Flag, FCS and information fields are same as those of HDLC.
• The address field defines the DLCI along with some other bits required for congestion control and traffic control.
• Their description is as follows:
1. DLCI field:
The first part of DLCI is of 6 bits and the second part is of 4 bits. They together form a 10 bit data link connection identifier.
2. Command / Response (C / R):
The C/R bit allows the upper layers to identify a frame as either a command or response. It is not used by the frame relay protocol.
3. Extended Address (EA):
• This bit indicates whether the current byte is the final byte of the address.
• If EA = 1 it indicates that the current byte is the final one but if EA = 0, then it tells that another address byte is going to follow.
4. Forward Explicit Congestion Notification (FECN):
• This bit can be set by any switch to indicate that traffic is congested in the direction of travel of the frame.
• The destination is informed about the congestion via this bit.
5. Backward Explicit Congestion Notification (BECN):
• This bit indicates the congestion in the direction opposite to the direction of frame travel.
• It informs the sender about the congestion.
6. Discard Eligibility (DE):
• The DE bit indicates the priority level of the frame. In the overload situations a frame may have to be discarded.
• If DE = 1 then that frame can be discarded in the event of congestion.
• DE bit can be set by the sender or by any switch in the network.