Section 15: X.21 Operation
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T E C H N O L O G Y
Page 98
Section 15.
X.21 Operation
X.21 is a protocol that runs over subscriber networks (i.e. leased line connections).
X.21 can operate as a full duplex connection with data rates from between 9,600 bps
to 2 megabits.
X.21 is an ITU-T data interface standard for connecting a DTE to a DCE and
transferring synchronous serial data. The properties of X.21 define the physical,
electrical and call control procedures for interfacing using this standard. The basic
interface signals performed in most X.21 signalling include transmit, receive, control,
indicate and timing. X.21 connections use balanced data, clock and control signal
lines. X.21 connections have been fully integrated into
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codecs.
64 – 384 kbps connections are most common and
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codecs support X.21 leased
line mode connections from 64 – 2,048 kbps.
Ensure that the bit rate setting in your
codec is matched to that of your ISDN TA/X.21 DCE device. If you are unsure of the
bit rate of your ISDN TA/X.21 DCE device, please check its reference manual to
discover the correct setting.
The physical interface to a
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codec is via a DB15 connector (ISO 4903). The
pin-outs for D-15 X.21 module connections are displayed in the Connector Wiring
section of this reference manual. A straight-through cable is required from the male
D15 connector on the X.21 module to a female DB15 connector, which will attach
to an X.21 DCE Device (such as an ISDN Terminal Adaptor/TA). The connection
profiles supported are mono, stereo, dual mono and mono/IFB.
Once a call has been made successfully between two codecs that are programmed to
use the X.21 protocol, any data can be transferred over the connection. In
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codecs, choose from
G.711, G.722,
MPEG 2 Layer 2 (
MP2)
and
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Music
Algorithms (
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Music
is capable of 15 kHz mono or stereo over 64kbps or
128kbps with 70ms delay) as standard.
15.1.
How Do X.21 Modules Connect to the Network?
X.21 modules use a D-15 connector to
connect to leased line networks. The X.21
module has a male connector and it
connects to a female version of the same
connector.
15.2.
How does it Work?
First you will require a
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X.21 module in each of your codecs. Next, you will
need to ensure that you have correctly connected the codecs to your leased line
network. A diagram follows with an example of how this might look.
Pin 1
Pin 15