Trunk

From Hill2dot0

A trunk is a transmission facility deployed between switches. The term is most commonly associated with circuit-switched voice services, specifically the facilities deployed between circuit-based voice switches, but it can also be accurately used to refer to facilities between packet-switched voice switches (e.g., SIP trunks). Less commonly, it is sometimes used to refer to a transmission facility between some forms of data switch (e.g., VLAN trunk. Because the most common use of the term is in relationship to circuit-based voice services, this article will focus on that definition.

Circuit-based Voice Trunks

Typically, trunks in this context are high capacity, digital facilities. They are metallic or optical facilities and are usually time-division multiplexed (TDM). Commonly found technologies include T-1, T-3, and SONET. Trunks can be hundreds or even thousands of miles long, which is often the case for trunks between IXC backbone switches or international circuits.

The capacity of a trunk is shared by voice users. Once a trunk is fully utilized (e.g., all of its channels are being used for calls), no further calls can be supported across that facility until one of the current calls is released. Within the PSTN, trunks are placed between switches that have a high community of interest. If there is an insufficient community of interest between a pair of switches to justify a direct trunk connection, a call can be routed through several intervening switches over trunks that interconnect them.

Because trunks can be quite long and are far fewer in number than local loops, it is necessary (and economically justifiable) to provide much better transmission quality on the trunks than on the local loops. Consequently, trunks are typically implemented using optical fiber. Today most interoffice facilities (IOF) use diversely routed SONET rings for trunks. This approach provides a high degree of reliability.

Three Trunks

The PSTN is a public network, but private voice networks can also be constructed using PBXes. This means that trunks can exist, in general, in three contexts:

• Within the PSTN: Trunks are deployed between the switches within the PSTN. They can extend between Class 5 switches, or connect a Class 5 switch to a Class 4 switch. The extend between the LEC network and the IXC POP. They also extend between IXC switches. The link between two Class 5 switches may be no more than a few T-1s. As you move deeper into the core of the PSTN, trunks historically have higher capacity (e.g., SONET). It should be noted that the core of the PSTN is increasingly packet-based, which means traditional circuit-based trunks are fading from use in this context.

• Between the PSTN and a private voice network: If a voice services subscriber has a PBX, they have a voice switch. To get voice services to their internal users, they need to connect their PBX to the LEC and/or IXC switch. They will require a trunking services to do this. Most carriers offering these services offer T-1 or ISDN PRI trunking services. Some will also support single-channel DS-0 trunking services.

• Within a private voice network: If an organization has multiple locations and PBXs at two or more of these locations, they may wish to implement trunks between their PBXs. These trunks are referred to as tie-lines and are typically implemented using T-1 technology. To the service provider, this circuit is not perceived as a voice circuit at all. Its a simple private line which the customer has elected to use as a tie-line. Today, these circuits are fading from use as organization migrate to IP PBX technology and replace their circuit-based trunks with packet-based technologies.

Some voice switches support the use of remote nodes or remote terminals. These devices aggregate multiple independent phone lines onto a single multiplexed facility for transport back to the voice switch. Trunk technology is often found in these contexts as well. Examples include digital loop carrier (DLC) and subscriber loop carrier (SLC) systems.

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