ZyXEL Communications Omni TA128 User Manual page 29

Chapter 4 - ISDN Communication Basics
When an ISDN data call comes in, the Omni TA128 will try to negotiate a connection
using the proper ISDN protocol. When an analog call comes in, the Omni TA128 will
send the call to the analog port as the factory default, Phone 1 and then Phone 2.
Digital Data
The Omni TA128 currently supports Circuit Switched Data (CSD) for ISDN data
applications. The CSD protocols supported by the Omni 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 of Internet service providers. Once the Omni TA128 answers a call, it will
examine the incoming data to determine which protocol to use, and automatically switch
to this mode. This operates transparently to the user. The Omni TA128 is able to auto-
switch for PPP, MPPP, V.120, X.75, V.110, and above protocols over 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, please refer to the section entitled
"Answering a Call using MSN" found later in this chapter.
Determining the Packet Length
The user's information is sent on a frame-by-frame basis for V.120. 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). The 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 Omni TA128 has a fixed maximum receiving frame size of 2048 bytes which is
larger than most devices can support. The default maximum sending frame size is 252
bytes, which is small enough that it should not create any problems. If you need to
change the maximum sending frame size, the ATCL command should be used.
Type:
ATCL252<Enter>
Omni TA128 responds:
OK
Type:
ATCL?<Enter>
Omni TA128 responds:
Maximum user data length in a packet (byte) : 252
(Set the frame size to 252 octets, user value between 1-2048)
(To inquire about the current setting of the packet length)
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