44
ISDN TA/V
AT command string, and sends this to the unit. As a result you, the operator,
do not need to have a great understanding of the particular command
structure, since all you have to do is select the parameter settings you want to
use from the menu options.
When using a dedicated terminal or computer attached to the ISDN T/A’s
CONTROL
port, AT commands can be entered into the unit at any time,
irrespective of the configuration of the DTE interface or LINE interface, by
entering the relevant command on the attached keyboard.
10.1.2 E
RROR
C
ORRECTION
The method of error detection supported by your ISDN TA/V is ITU V.42
error correction based on the LAPM (Link Access Procedure) protocol. This
protocol does not use start and stop bits to group data bytes; instead, your
ISDN TA/V and the remote TA each break down a transmission into blocks
of data (frames) and calculate a checksum for the block—the sum of the data
bytes in the block. The block and its checksum are then transmitted. The
receiving ISDN TA/V then calculates a checksum on the received data block
and compares this with the received checksum. If these numbers differ the
receiving ISDN TA/V requests a resend of that data block from the
transmitting ISDN TA/V.
V.42 error correction is an important feature of your ISDN TA/V in that it
automatically compensates for data corruption that has been caused by
intermittent line quality. However, since the method of correction is to
request a resend it cannot, of course, compensate for a line that is constantly
corrupting data. If you are receiving a lot of errors on a particular line, we
recommend that you contact your line supplier and have them check the
quality of the line.
10.1.3 D
ATA
C
OMPRESSION
Data compression is a technique that is automatically negotiated by your
ISDN TA/V, which significantly improves the throughput of a connection
by reducing the time required to transmit and receive blocks of data.
Data compression is accomplished by the sending ISDN TA/V analysing the
outgoing data for repeated patterns, and assigning a shorter data code to the
pattern. Hence, the more often the same data codes occur in a transmission,
the greater the overall compression will be.
The international data compression standard supported by your ISDN TA/V
is ITU.42bis, which can provide compression capabilities of up to 4:1. This is
especially suited to text file and graphic-file transmissions.
