| 单词 | Figure L-3 |
| 释义 | Figure L-3 The fields in the LAPF frame are as follows: • Flag: A specific one-octet field that delimits (i.e., establishes the limits or boundaries of) the beginning and end of the frame. This octet is always 01111110 in binary notation (7E in hexadecimal notation), which is prevented from appearing in the payload by bit stuffing. • Address: A two-octet (default) or four-octet (option) field that includes the necessary control information in the Data Link Connection Identifier.The address field also contains a Command/Response field, Address Field Extension, Forward and Backward Explicit Congestion Notification fields, and Discard Eligibility data. • Data Link Connection Identifier (DLCI, pronounced delsey): 10 bits (or 20 in the 4-octet header) that identify the data link, the virtual circuit (VC), and its service parameters, which include frame size, Committed Information Rate (CIR), Committed Burst Size (B ), Burst Excess Size (B ), c e and Committed Rate Measurement Interval (T ). c • Command/Response (C/R): 1 bit reserved for use of the frame relay access device (FRAD) in order to facilitate the transport of polled protocols (e.g., SNA), which require a command/response for signaling and control purposes. • Address Field Extension (EA): 2 bits that signal the extension of the addressing structure beyond the 2-octet default. The use of EA must be negotiated with the carrier when the service is established. • Forward Explicit Congestion Notification (FECN): Pronounced feckon. A 1-bit field available to the network to advise downstream devices that the frame has experienced congestion. Should the receiving FRAD determine that subsequent frames were discarded or corrupted in transmission, it is advised that recovery may be required in the form of requests for retransmission. 74570c12.qxd 9/11/07 12:25 PM Page 280 |
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