AT&T 7100 series manual Tie Trunks, Signaling Links, Switch Physical Links ⎜Logical Channels/Link

DISTRIBUTED COMMUNICATIONS SYSTEM (DCS) 6-3

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Tie Trunks

In ETNs, the tie trunks connecting DCS nodes are intermachine tie trunks. On the System 75 and Generic 1 communications system, they are administered as tandem trunks; while on the DIMENSION, System 85, and Generic 2 communications system, they are type 41 trunks. In MS/T networks, on the System 75 and Generic 1 communications system, the tie trunks are administered as tie; while on the DIMENSION, System 85, and Generic 2 communications system, they are designated with a trunk type of 78. See the Trunking section of chapter 1 for more details on how these links should be administered.

Signaling Links

The signaling links physically connecting DCS nodes are processor interface (PI) links on the System 75 and Generic 1 communications system; on the DIMENSION, System 85, and Generic 2 communications system, they are data communications interface unit (DCIU) links.

Each physical PI/DCIU link contains a set number of logical channels. For features to work transparently, the switches involved in a connection must be provided with a dedicated logical channel over which the DCS signals are transmitted. The number of physical links available on each switch, with the corresponding number of channels per link are given below:

	Table 6-2.Link Capacity
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⎜	Switch	⎜	Physical Links	⎜		⎜
⎜		⎜		⎜Logical Channels/Link		⎜
___________________________________________________________						⎜
⎜		⎜		⎜
⎜		⎜	4	⎜	20	⎜
⎜ DIMENSION		⎜		⎜		⎜
⎜ System 75, R1V2 and V3		⎜	4	⎜	64	⎜
⎜ Generic 1		⎜	2 sets of 4	⎜	64	⎜
⎜ System 85		⎜	8	⎜	64	⎜
⎜ Generic 2		⎜	8	⎜	64	⎜
⎜		⎜		⎜		⎜

Signaling Link Speed

The recommended speed over a DCS signaling channel is 9.6 kbps.

Signaling Link Protocol

At layer 1 (the physical layer), the links are either RS-232C or RS-449. (Although RS-449 is the physical connection on the DCIU, a cable [PEC 65259] can convert the connection to RS-232C.) At layer 2 (the data-link layer), all communications systems use BX.25 protocol over their signaling link connections. (See appendix D for an explanation of these protocols, and appendix E for a the physical layer pinout explanations.)

Over the signaling link, all transmissions are numbered and the DCIU/PI keeps a copy of each packet. As it sends out a packet, the DCIU or PI sets a timer. If it does not receive confirmation of delivery before an