Northern UTA220/UTA220k manual Layer Two, Layer Three, Isdn

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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.

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