Northern UTA220/UTA220k - Manual (Page 42)

4-8
UTA220/UTA220k
ISDN
Layer Two
The data link layer ensures error-free transmission of
D-channel messages between the terminal and the network
switch. The link layer in effect provides a virtual error-free
connection for the next higher protocol layer to use. The LAPD
protocol is the error detection and correction mechanism for
layer two. Layer two protocols are based on CCITT
recommendation Q.921, and are almost identical. Minor
differences do exist, however, primarily in how the protocols
respond to certain very unlikely error conditions.
Layer Three
The network layer consists of signaling messages passed
between the terminal and the network, using the data link layer.
These messages communicate call setup (dialing) and release
messages from the TA, call progress information from the
network, and messages concerning special call-related
features. Layer three of the D-channel can also be used to
exchange packetized data using X.25, if the TA and the switch
are set up to allow it.
All layer three protocols are based on CCITT
Recommendation Q.931. However, the protocol used depends
on the brand and version of the network switch. There are
differences in the exact procedures used by each switch to set
up and release calls. In addition, because switches are
manufactured by competing companies, each switch offers a
different set of extra features. These features are called
supplementary services.
Unfortunately, each vendor implements these features
differently. Also, there are different versions of each of the
switches, each of which implements a different set of features.
As a result, you must configure your TA for the particular
switch type and version so that the TA can use the correct
signaling protocol for that switch. The switch type and version
are two of the most important network parameters in the TA.