120
2.048Mbps Wireless Modem
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Appendix C – Fractional E1/T1 Applications
Introduction
This appendix explains under which conditions it is recommended to use the Fractional
E1/T1 LIU. It also describes some of the possible ways to connect a Fractional E1/T1
2.048Mbps Wireless Modem to Fractional E1/T1 end equipment. For each possible
connection, the indications given by the modem unit and the expected end equipment
behaviour are described. The RJ-45 balanced four-wire interface is also discussed.
This appendix also provides information about Fractional E1/T1 LIU board jumper settings.
Some explanations are provided on T1 CSU/DSU applications.
The last part of this appendix provide explanations on echo cancellation and its
use in voice channels.
When to Use the Fractional E1/T1 LIU
The Fractional E1/T1 LIU can only be used in applications in which the end equipment
supports framed data streams in accordance with E1 (CEPT-1) or T1 (DSX-1) standards. The
Fractional E1/T1 LIU cannot be used with unframed E1/T1 data streams (e.g. with routers).
It is recommended to use the Fractional E1/T1 LIU in applications with voice channels and to
take advantage of the built-in echo canceller to achieve an improved voice quality.
The Fractional E1/T1 LIU may also be used in applications without voice channels.
However, the cost of the Fractional E1/T1 LIU is higher than that of the regular E1/T1 LIU.
In some cases, the use of the Fractional E1/T1 LIU in non-voice fractional E1/T1 applications
should be preferred as it may improve the quality of the link and increase the achievable
range.
An E1 link is comprised of 32 channels of 64 Kbps each. A T1 link is comprised of 24
channels. In Fractional E1/T1 applications, any number of links up to the maximum of 32
(E1) or 24 (T1) may be used for transmission of a data, fax or voice channels. The data
stream of a regular fractional E1/T1 link is at the full rate of 2048(E1) or 1544 (T1) Kbps.
When the 2.018Mbps Wireless Modem is used, it transmits, only the channels that are in
actual use, typically resulting in a lower link data rate. For example, in a fractional E1
application with 4 channels in use, the effective data rate of the modem is 320Kbps (4 data
channels plus 1 channel for frame alignment signals TSO), and the default modem rate is just
0.5Mbps vs. modem rate of 3Mbps required for a regular E1 link. The distance achievable
with a modem rate of 0.5Mbps is approximately 4 to 5 times the distance achievable with a
modem rate of 3Mbps. Thus, a significant improvement in achievable distance may be
available through the use of the Fractional E1/T1 LIU compared with performance achievable
with the regular E1/T1 LIU.
