Transmission Mode Changes and Modem Pools

7-8DATA CONNECTIVITY — AN OVERVIEW

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Channel Assignment — Dial-up vs. Dedicated Connections

The assignment of a communications stream to a path or channel through a circuit switch is made dynamically or on a permanent basis.

∙The dynamic assignment of a communications stream to a channel is provided when the user dials the telephone number of the destination. The resulting connection is sometimes called a dial-up connection.

∙When the originating DTE cannot dial a destination, an internal switch channel can be permanently assigned to the connection. The permanent assignment of a communications stream to a channel is achieved through a feature called the dedicated switched connection on the System 85 (R2V3 and R2V4*) and Generic 2 communications system. On the System 75 and Generic 1 communications system, permanent assignments are provided through the permanent switched connection feature. Connections that result from these features are sometimes called nailed-up connections.

Permanent and dedicated switch connections can be used for connections that normally require special wiring. One application is in the connection of packet switches through a dedicated channel on the System 75, System 85, and Generic 1 and 2. See chapter 9 to determine how these connections are conﬁgured.

Channel Information — Bearer vs. Signaling Channels

Channels on a communication link can pass data created at the originating DTE, or signaling data created by the switch or DCE.

On DS1 links administered for 24th-channel signaling, the switch creates and inserts signaling information on the 24th channel. All other channels pass DTE/DCE-created information.

On PRI links, the switch uses the D-channel to pass both signaling and caller information. The D-channel’s ability to carry information is used in telemarketing gateway applications to give a host information on an incoming call. It is also used in the lookahead interﬂow application to tell a switch of an upcoming ACD call. (See chapter 8 for more details on the telemarketing gateway applications.)

Note: Although the Generic 2 communications system provides a BRI port, which interfaces with the ISDN-BRI standard of two B-channels and one D-channel, it cannot handle X.25 packets transported over the D-channel.