T1 Channels (DS0)

In the old phone system, the caller would make a phone call and either the operator, or a digital device would create a temporary end-to-end connection for the duration of that call. While that call was in progress, nothing else could use the inter-office trunk, and thereby any other home attached to that local office.

The first solution to increasing capacity was to install more trunk lines. However, this proved extremely expensive and time consuming. The phone system was growing too rapidly. Western Electric and Northern Telecom came up with a method of digitizing, multiplexing, packetizing and switching the data between offices. The new trunk lines used the same 4-wire cables but could handle 24 simultaneous calls on trunk lines that previously handled only 1 call. This increased capacity dramatically.

Individual voice circuits used Digital Signalling level zero (DS0). Twenty four DS0's were multiplexed into one T1. As the need for high speed communication grew, the phone company extended T1 service to reach past the local loop to the customer's premise.

TodayT1 circuits are sold in both channelized and non-channelized capacities to anyone who can afford to have them installed. A T1 can be purchased as non-channelized at it's full 1.544 Mbps speed. Channelized T1's are composed of 24 channels (each called a DS0). Each channel is either 56 or 64kbps in speed, depending upon the encoding scheme used, and end users can buy as many channels as they need without buying the full capacity.

T1's are usually multiplexed to their full capacity by the local phone company.