Multiplexer User's Guide
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Mux_guide_v06 Page 18 of 36 30.06.2004
4.2.1 Flag sequence
A flag sequence is a specific bit pattern (usually 11111001; hexadecimal: 0xF9) used to mark
the beginning and the end of a frame of data.
Each frame begins and ends with a flag sequence. Only one flag sequence occurs between
any two frames. If two successive flag sequences do occur, the frame is regarded as being
empty and is discarded.
The flag sequence is used for the synchronization of frames.
4.2.2 Address field
Data link connection identifier is a frame relay term defining a 10-bit field of the address field.
The DLCI identifies the data link and its service parameters, including the frame size.
The values for the Data Link Connection Identifier (DLCI) are dynamically defined apart from
DLCI = 0.
DLCI
DLCI: Data Link Connection Identifier
C/R: Command / Response
EA: extension bit; EA = 1
EA C/R
Table 3: Assignment of the DLCI
DLCI number (decimal) Priority
Multiplexer control channel
(see chapter 4.3.6)
0 0
highest priority
AT commands, data, fax, GPRS 1 7
AT commands, GPRS 2,3 7
The command/response bit identifies the frame as a command or response. A command
contains the address of the data link connection to which the command is sent. A response
contains the address of the data link connection sending the frame.
Table 4: Use of the command/response bit
Command/Response Direction C/R
Customer µC → GSM engine 1
Command
(SABM, DISC) GSM engine → Customer µC 0
Customer µC → GSM engine 0
Response
(UA, DM) GSM engine → Customer µC 1
Every command expects a response. No provision is made to repeat the command if no
response is received.
Table 2: Address field