Digital Data; Determining The Packet Length - ZyXEL Communications Omni TA128 User Manual

comes in, the TA128 will send the call to the analog port as the
factory default, Phone 1 and then Phone 2.

Digital Data

The TA128 currently supports Circuit Switched Data (CSD) for
ISDN data applications. The CSD protocols supported by the TA128
include: PPP, MPPP, V.120, X.75, and V.110. PPP is the most
popular protocol used in North America; it is used by most Internet
service providers. Once the TA128 answers a call, it will examine the
incoming data to determine which protocol to use, and automatically
switch to this mode. The TA128 is able to auto-switch for PPP,
MPPP, V.120, X.75, V.110, and above protocols over the speech
channel. In most cases, you can rely on the auto-switching feature for
your applications. If you need more specific settings for answering
calls, refer to the section entitled "Answering a Call using MSN"
found later in this chapter.
O
For DTE Port 2, only PPP, V.120, and X.75 are supported.
:
NOTE

Determining the Packet Length

The user's information is sent on a frame-by-frame basis for V.120
and X.75. Sometimes we call it "packetized." The maximum frame
length on the sending side should not exceed the maximum frame
length that the receiving side allows. Sometimes this information will
be 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 will discard the frame and reply with a
Frame Reject frame (FRMR). FRMR indicates that the information
received is too long. Both sides will then reset their link layer
negotiation and re-send the frame again. Usually this will happen
repeatedly until the call gets disconnected.
The TA128 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
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