Digital cross-connect system

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A digital cross-connect system (DCS) is a form of multiplexer. However, where a multiplexer is designed to combine lower-speed circuits onto a higher-speed transport facility, a DCS is primarily designed to transfer lower-speed circuits from one high-speed transport facility to another.

Historical Backdrop

Multiplexing is not a new concept. In telecommunications, electronic multiplexing was done even in the early telegraph network. However, once a signal was multiplexed onto a higher speed facility, it road with the over multiplexed signals to the terminus of the higher-speed circuit where it was demultiplexed.

Consider the situation that occurs when three such multiplexed facilities are being used to connect three branch locations to a central site. Any circuits from any branch to the central location are simply multiplexed onto the high-speed facility, and demultiplexed at the hub location. Consider what must be done, however, to build a circuit from one branch to another. It needs to be multiplexed from the branch to the hub, where it is demultiplexed. The circuit coming off one multiplexer is then cross-connected to another multiplexer to continue on to the other branch location. Every branch-to-branch circuit would need this type of cross-connect. Early in the age of telecommunications, this was done manually with a cross-connect panel and patch cables.

With the advent of digital circuits, however, this function could be performed by a single device. So today, rather than have three multiplexers and a manual cross-connect panel, the three high-speed circuits all terminate on line cards in a DCS. Within the DCS is a switching matrix that makes it possible to take a low-speed circuit from one high-speed facility, and electronically cross-connect it to another.

Benefits of a DCS

Clearly a DCS has the potential to significantly reduce cost. Rather than requiring an entire array of multiplexers and cross-connect panels to support the wide variety of circuit types in use today, a single large DCS can handle all of the work.

The DCS is also highly flexible. Unlike the manual process, the DCS can be configured remotely, and reconfigured without anyone touching a wire or fiber. The equipment primarily needs to be physically altered when a new facility is being connected to it.

Although a DCS can be used in many places in a telecommunications network, they are most commonly found in central offices and other carrier communication hubs (e.g., the main telecommunications center of an MSO).

DCS vs. Switch

A DCS performs a function very similar to a circuit switch. Consider what a Class 4 voice switch (e.g., a tandem switch) or an IXC POP does; it takes a DS-0 circuit carrying a telephone call, which arrives on one digital, multiplexed trunk, and cross connects it to another digital, multiplexed trunk. Based on this, the devices appear to be identical.

In fact, they are very similar. The primary difference between them is that the switch assembles and disassembles circuits dynamically, in response to signaling requests by the end user (e.g., dialing a telephone number). The DCS, on the other hand, is pre-programmed and does not alter its configuration until someone alters its programming. So where a circuit switch would be used to support such things as POTS and ISDN dial services, a DCS would be used to build private lines or leased lines. They can also be used to build the trunk circuits that are used to interconnect the voice switches.

That being said, the industry has done much to muddy the waters. Today, there are more and more DCSs that can be remotely reconfigured by customers on demand, and the industry is moving rapidly to high-speed, dynamically configured circuits. So the line between a circuit switch and a DCS is blurring.

See Also

PodSnacks

<mp3>http://podcast.hill-vt.com/podsnacks/2008q1/dcs.mp3%7Cdownload</mp3> | Digital cross-connect system (DCS)