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2.4.
X.21
X.21 is a protocol where the data transmission between the DTE and the DCE is synchronous.
The transmission speeds between the DTE and DCE can be assigned between 600 and 64000
bps. The physical interface of X.21 comprises 6 lines: T-transmit data, R-receive data, S-signal
element timing, B-byte timing, C-control, I-indicator.
The control (C) and indicator (I) lines perform the line signalling while the register signals (like
dialled number) are transferred via the transmit (T) and receive (R) lines. The data on the
transmit and receive lines, which is dedicated for the call control, is according to the
International Alphabet No. 5 and has odd parity. For character synchronisation each call
control data block is preceded by at least two SYN characters (Hexadecimal `16').
The connection of X.21 terminals to the SOPHO network is illustrated below.
X.21
LAM
309
DNR
CV
X.21
LAM
309
DNR
CV
Figure 2-6 X.21 Connections.
Two dialling options are offered for the LAM 309. The option is selected by an OM command
parameter when creating the CV.
• Switched line
The destination is `dialled' and a connection is made according to one of the following
methods:
-
The LAM user `dials' the destination using a keyboard dialling command. The LAM
analyses the command and sends a D-channel message.
-
The number that is to be dialled can be stored in the LAM 309. When the DTE initiates
a `direct call', the LAM sends the stored number.
(The direct call address, of up to 30 digits, can be downloaded to the LAM. For this
purpose the DNR of the data port is assigned to a function key menu. In this menu the
direct call address is programmed under function key number 0. After downloading to
the DNR of the data port the direct call address is stored in the LAM.)
• Leased line
2B+D
IMP
64 kbps
64 kbps
DLC
2B+D
IMP
64 kbps
64 kbps
DLC
SN
21
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