Frame Relay Frame Format
Frame Relay uses Link Access Procedure for Frame Relay (LAPF) for frame format. LAPF is yet another variation of the HDLC frame format; it supports Frame Relay. There is no control frame, flow control, and error control, and no sequence numbers. Figure 5-7 shows the LAPF frame format.
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Number of bytes: 8 16 |
Variable |
16 |
8 |
||||
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| Flag | Address |
1 Data |
1 |
FCS |
1 Flag 1 |
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Address sub-fields: Bits: 8 7 6 5 4 3 2 |
1 8 7 6 5 |
4 |
3 |
2 |
1 |
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| DLCI | C/R |
1 EA 1 DLCI |
1 FECN 1 |
BECN 1 |
DE |
1 ea i |
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The 10-bit DLCI provides the PVC identifier, which has local significance between the router and the Frame Relay switch. The C/R bit is not used. The EA (extended address) bit is a mechanism that extends the address field an additional 2 bytes. There are two EA bits in a 16-bit address field. The eighth bit of every byte in the address field indicates the extended address. If the bit is set to 0, the containing byte is not the last byte of the address. If the bit is set to 1, the byte is the last byte of the address. Implementations that use the nonextended DLCI addresses set the first EA bit to 0 and the second EA bit to 1.
The control field has the three bits: FECN, BECN, and DE, which were discussed in the previous section.
Table 5-1 displays the Frame Relay LAPF frame address field.
Table 5-1 Frame Relay LAPF Frame Address Field
Table 5-1 displays the Frame Relay LAPF frame address field.
Table 5-1 Frame Relay LAPF Frame Address Field
|
Bit |
Description |
|
DLCI |
10-bit PVC identifier |
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C/R |
Not used |
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EA |
Extended Address; indicates if the containing byte is the last in the addressing field; |
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a mechanism to extend DLCI length beyond 10 bits |
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FECN |
Bit set in frames traveling to the destination, when congestion is on traffic from |
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source to destination |
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BECN |
Bit set in frames traveling to the source, when congestion is on traffic from |
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source to destination |
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DE |
Bit set to indicate that the frame is eligible for discard |
Continue reading here: Frame Relay show Commands
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