Determining The Packet Length - NETGEAR XM128 ISDN Reference Manual

Determining the Packet Length

The user's information is sent on a frame-by-frame basis for V.120 and X.75, sometimes called
packetized. The maximum frame length on the sending side should not exceed the maximum
frame length that the receiving side allows. Sometimes this information is exchanged during
handshaking. However, few manufacturers, if any, have implemented this mechanism.
If the sending side sends packets greater than what the receiving side allows, the receiving side
discards the frame and replies with a Frame Reject Frame (FRMR). The FRMR indicates that the
information received is too long. Both sides then reset their link layer negotiation and send the
frame again. Usually this process repeats until the call gets disconnected.
The Model XM128 modem has a fixed maximum receiving frame size of 2048 octets, which is
larger than most devices can support. The default maximum sending frame size is 252 octets for
V.120 and 2048 for X.75. If you need to change the maximum sending frame size, the ATCL
command is used.
To change the maximum sending frame size, use the following commands and follow these steps:
Enter ATCL252 (set the frame size to 252 octets, user value between 1 and 2048).
1.
You should receive this message:
OK
Enter ATCL? (to inquire about the current setting of the packet length).
2.
You should receive this message:
Maximum user data length in a packet (byte) : 252
ISDN Overview
Reference Guide for the Model XM128 ISDN Digital Modem
6-5