Section 13.1. Legacy Trunks

13.1. Legacy Trunks

Legacy trunks are links that connect private voice switches using a traditional technology like FXO/FXS or T1. There may be many reasons why you use legacy trunks, even if your heart's desire is to move totally to VoIP:

• Legacy trunks are the only way to integrate legacy PBXs with VoIP systems

• Existing legacy trunks, especially point-to-point T1s, may be subject to service contracts that require you to maintain them for a whilesometimes as long as seven years from the date of inception

• Legacy trunks are known for quality. Since they derive from the circuit-switched PSTN, legacy trunks have intrinsic bandwidth guarantees . If you can't justify (or convince other decision- makers on) the merits of packet-switched QoS across your private network, then legacy trunks provide a solid connection between voice switches

13.1.1. Private Analog Lines

If two PBXs are in the same building or on the same campus, they can be connected by analog copper and FXO/FXS interfacing. This technique is effectively the same as that in Project 12.2 that linked two softPBXs using a POTS line. The difference in this case is that you aren't using the PSTN; you're just using a standard telephone cable with RJ11 connectors on both ends. One end connects to an FXO/FXS port on each PBX. Then, the PBXs' dial-plans are programmed to route calls appropriately between them.

13.1.1.1 Leased lines

If the two PBXs are not within the same campus or building, then the PSTN should be enlisted to provide analog (or TDM) connectivity between them. If the PSTN connection is going to be utilized very heavily, the phone company can provide a dedicated, monitored connection called a leased line .

Leased lines were very popular for data connections before the advent of cheap T1 circuits. They can provide the same data throughput as a POTS line (around 56 kbps) but with a much higher level of reliability.

13.1.1.2 Dry lines

Almost all connections provided by the phone company cross through its networkthe PSTN. But dry lines don't. They are copper loops that begin at one customer's premises, route through the CO without entering the local exchange switch, and terminate at another customer's premises. Dry lines can be used to link PBXs via FXO/FXS. Also called dry pairs , these lines were once commonly used to connect security system monitoring companies with their customers. Dry lines can be used only to link sites that are served by the same CO. This means the sites must be within a 3- or 4-mile radius in urban areas. In rural areas, dry lines may not be practical because of distance-imposed attenuation problems.

Not all telephone companies offer dry lines because, if you use the right hardware, dry lines can compete directly with their other, more expensive services such as POTS and DSL. Other telephone companies may have a policy of not selling dry lines to customers who plan to use them for voice applications. Check with your telephone company to be sure.

13.1.2. Private Digital Trunks

T1s and ISDN BRI connections are used to connect PBXs that have the appropriate digital interfaces, though BRI's practicality in this role has evaporated. T1s are also used to connect groups of TDM phones to the PBX by way of a device called a channel bank . This permits 24 TDM phones to be used with a single T1 port on the PBX.

In order to connect two PBXs by T1, a DSU/CSU (data and channel service unit) device is required at both ends of the T1. The DSU/CSU's role is to provide low-level signaling and diagnostic feedback for the T1 data link. DSU/CSUs are used in both legacy voice and data encapsulation scenarios on a T1 (such as TCP/IP and IPX/SPX). They are used to configure how the T1's 24 channels are broken into voice and data functions.

If two PBXs are located in the same building, a T1 cable can be used to directly connect them. If not, the LEC provides point-to-point T1 service that uses the PSTN to provide a completely unswitched connection between the two PBXs. This means that no dial-tone services or local access signaling is included with the T1. These services aren't needed for a private trunk, because the PBXs are going to signal directly to each other without using PSTN switching.

Private T1 trunks don't need a local signaling technology like PRI or DID, so you need pay only for the loop, and not for dial-tone.

For trunk connections of hundreds of miles, a recipe of long-haul technologies is required to use T1 for point-to-point private trunking. Many network carriers provide T1 access across large SONET links that encircle entire metropolitan areas. IXC network uplinks haul T1 connections even furtherfrom coast to coast , if necessary. Using VoIP (rather than legacy T1 voice signaling) for trunking is more appealing as the distance increases , because (unlike T1), VoIP trunks aren't paid for by the mile.

If you have T1 links that have very long contract periods remaining, you might have to keep them in place even if your endgame is to replace them with VoIP-based trunks. Some phone companies will assess a hefty termination liability fee if you cancel your T1 lease before it's up!