The Frame Relay frame format is shown in
the Figure. The
flag fields indicate the beginning and end of the frame. Following the
leading flag field are 2 bytes of address information. 10 bits of
these 2 bytes make up the actual circuit ID (that is, the DLCI).
The following are the Frame Relay frame
fields:
- Flag -- Indicates the beginning and the
end of the Frame Relay frame.
- Address -- Indicates the length of the
Address field. Although Frame Relay addresses are currently all 2
bytes long, the Address bits allow for the possible extension of
Address lengths in the future. The eighth bit of each byte of the
Address field is used to indicate the address. The Address
contains the following information:
- DLCI Value -- Indicates the DLCI
value. Consists of the first 10 bits of the Address field.
- Congestion Control -- The last 3 bits
in the address field, which control the Frame Relay congestion
notification mechanisms. These are the FECN, BECN, and discard
eligible (DE) bits.
- Data -- Variable-length field that
contains encapsulated upper-layer data.
- FCS -- Frame check sequence (FCS), used
to ensure the integrity of transmitted data.
|