| 单词 | Figure L-2 |
| 释义 | Figure L-2 The fields in the LAPD 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. Flags also fill idle time on the line. • Address: A two-octet field known as the Data Link Connection Identifier (DLCI).The first octet is the Service Access Point Identifier (SAPI), which identifies the destination service access point, each of which can support multiple terminal devices.The second octet is the Terminal Endpoint Identifier (TEI), which is the address of the destination terminal device. • Control: A one- or two-octet field that identifies the LAPD frame type. An information (I) frame carries upper-layer information and some control data. A supervisory (S) frame carries control information such as information-frame acknowledgement, request for retransmission, and flow control. An unnumbered (U) frame carries control data such as disconnection request, acknowledgement, and frame rejection. 74570c12.qxd 9/11/07 12:25 PM Page 279 |
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