Tie Lines for PBX Interconnection

Figure 2-4. Number Translation Through a PBX

If a business has two sites and they have a large call volume between them, the business usually purchases a tie-line. Recall that a tie-line is simply a permanent circuit between two points (T1, E1, fractional T1/E1, or some other transport). For this scenario to be cost-effective, it must cost less to run a call between site A and site B over the PSTN than it does to send a call over a permanent circuit.

Figure 2-5 shows two sites (one in San Jose, California, and one in Dallas, Texas), with a T1 circuit between them.

Figure 2-5. Tie-Line Between San Jose and Dallas

Figure 2-5. Tie-Line Between San Jose and Dallas

This tie-line still uses the PSTN, but the business pays a flat rate for the dedicated use of the circuit between San Jose and Dallas.

The PBX uses a preprogrammed Automatic Route Selection (ARS) table to determine which trunk should be used. Referring back to Figure 2-5, the PBX is configured to use the tie-line between San Jose and Dallas. If that tie-line becomes full, the PBX uses the Central Office (CO) trunks as overflow to the PSTN.

To determine whether having a tie-line is cost-effective, a careful analysis of the call volume and cost between San Jose and Dallas as compared to the cost of the T1 circuit must be performed. Figure 2-6 shows that the break-even point for a tie-line is reached when there are 30-35 hours worth of calls between San Jose and Dallas each month. (This is sample data, and your experience might differ.) Anything over the 30-35 hours of calls between these two sites becomes additional savings, as long as the traffic is balanced so that it might all traverse the dedicated T1 circuit.

Figure 2-6. Tie-Line Costs Compared to PSTN Costs




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Tie-lines are another way in which ET network designers can route their traffic. The routing of call traffic is a very complex issue that requires a myriad of experience and knowledge. Entire books cover the subject of call-traffic modeling. Chapter 15, "Voice over IP Applications and Services," covers traffic analysis in more detail.

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