Basics Of A DS3 Line or T3 Line

The digital telephony took a new approach to carrying voice calls. It introduced the concept of multiplexing where multiple circuits could be aggregated into one link. This way multiple people can share the same resource there by distributing the costs among customers. One of the most popular aggregation links was the T3 line.

The benefits of an aggregation can be best understood with an example. Let us say that there is a cable that can support a data rate of 10Mbps. However without multiplexing, 10 users each would need to get this cable and pay for it, even though they need just 1Mbps each. As a service provider, you need in lay 10 cable pairs to connect the customers in the same building.

With multiplexing you can now combine all 10 user traffic on to one single cable and spend on just a single cable. The concentrator of the 10 signals into one is called the multiplexer. With a multiplexer, the customers now pay just a tenth of the cable cost and a tenth of the multiplexer cost which is a lot cheaper than a full cable.

With a T3 line, the multiplexing is done at a T1 level. 28 T1 lines are multiplexed into a single T3. Given that each T1 itself consists of 28 multiplexed voice channels; a T3 can support 672 voice channels at PCM rates. This high capacity made them ideal choices for trunk circuits between central offices.

A T3 or a DS3 (which is its digital component) can support a link rate of 44.736Mbps which comes from 28 T1 rates and a signaling channel. Even today, this link rate is too high for most business needs. So you would rarely find a T3 being pulled right up to a customer location. In most cases they are built to carry voice channels or to support T1 rate data circuits.

However, there are special customer needs that may require T3 level connectivity. In such cases, the service is delivered to them over SONET. SONET is an optic network standard that supports DS3 signals. Being fibre based it can travel much longer than a T3 on copper, which cannot go beyond 600m.

Most commercial DS3 connections are carried over SONET rings to the nearest Add-Drop-Multiplexers and then extended to the customer site on coaxial cable with BNC connections. This ensures maximum reusability of the fibre network for the service provider and minimum copper on the last mile.