J 1 J127

Figure 10.2-1 Data link connections on a D-channel.

at the exchange. Moreover, a frame that originates at a network-layer function at one end of the D-channel is delivered to the peer function at the other end.

There are a number of data link connections on a D-channel. Each connection is identified by a combination of TEI and SAPI values. The connections with TEI = 0-126 are bidirectional point-to-point connections, between a TE on a DSL and its "peer" function at the exchange. For example, the connection (TEI = 2, SAPI = 0) carries call-control frames to and from the terminal identified by TEI = 2.

The connections with TEI = 127 are "point-to-multipoint" in the direction from exchange to the user. All TEs on a DSL examine received frames with this TEI value. An exchange can broadcast a message to all TEs on a DSL, by sending a frame with TEI = 127.

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