Technical Reference for ZyXEL omni.net series
10-5
D-channel and DTE-DCE interface. The EPA is designed for hobbyists as well as users with
technical backgrounds. The EPA enables you to examine messages exchanged between your ISDN
TA and the Central Exchange office when making an ISDN call. You can review the packets sent
or received through the B-channel (for X.75 or V.120) to or from the remote site. AT commands
issued from an application software program can also be checked. This will help you understand
their causal relationship with other events.
In addition to its tutorial purpose, the EPA is very useful for diagnostics. If you have compatibility
problems with your Central Exchange or with another ISDN TA device at a remote site, the EPA
will be your first aid resource. According to the EPA analysis, you may decide to fix the problem
yourself (for example, modify the configuration and try again) or log the analyzed results as a file (a
very comprehensive bug report), and then send it to
ZyXEL Technical Support
department.
Capturing the Protocol Data
The data captured by the EPA can be classified into three categories:
!
B-channel user data protocols
!
D-channel signaling protocols
!
DTE-DCE protocols
The B-channel user data protocols include X.75 and V.120. Only the layer-2 header (addresses and
control bytes) and layer-3 header are captured. Since X.75 may be used with various layer-3
protocols (for example, T.70, T.90, and ISO8208), only the first 8 octets of the information field are
recorded as the layer-3 header, then displayed in raw data form. ZyXEL Technical Support
department will carry out the analysis of the protocol data.
The D-channel signaling protocols include layer-2 and layer-3 call control protocols. Frames and
messages exchanged via the D-channel are all recorded for further analysis. These data messages
are essential to understanding interactive operations between an ISDN TA and the ISDN network.
They contain the compatibility information for the ISDN TA and your Central Exchange.
The DTE-DCE protocols (at the R reference point according to the ISDN nomenclature) include
the AT commands/responses as well as the CAPI internal interface. The CAPI internal interface is
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