History And Development
In the old analog phone system, the caller would make a phone call and either the operator, or a electromechanical device would create a temporary end-to-end connection for the duration of that call. If the caller wished to reach another subscriber connected to a different central office, the first call would be disconnected and a second call would be built over an inter-office trunk.
Each trunk line could only support one call. So multiple trunk lines had to be run between central offices. During that phone call, the trunk line used was unavailable to be used by any other caller. If more calls were placed than phone lines were available, someone had to wait to make their phone call. As more phones were installed in more locations and more central offices were built, more trunk lines were needed. This system of circuit switching of calls across trunk lines became impractical.
The first solution to increasing capacity in the phone system was to install more trunk lines. However, this proved extremely expensive and time consuming. The phone system was growing too rapidly to keep up with the demand. Western Electric and Northern Telecom came up with a method of digitizing voice signals, breaking the voice data into packets and then multiplexing multiple voice lines into a single communications path over a single trunk between offices. The new digital trunk lines used the same 4-wire cabling but now the trunks could handle 24 calls and could add or drop calls in and out of the multiplexing scheme as needed. This dramatically increased capacity and gave the phone systems a flexibility and redundancy it previously lacked.
Individual voice circuits used Digital Signalling level zero (DS0). Twenty four DS0's were multiplexed into one DS1. Soon came the Internet and with it the World Wide Web. As the need for high speed data communication grew, the phone company extended DS1 service to reach past the local loop to the customer's premise.
TodayDS1 circuits are sold in both channelized and non-channelized capacities to anyone who can afford to have them installed. A DS1 can be purchased as non-channelized at it's full 1.544 Mbps speed. Channelized DS1's are composed of 24 channels (each called a DS0) and sold as Fractional DS1, or Channelized DS1. Each channel is either 56 or 64kbps in speed, depending upon the encoding and framing schemes used and end users can buy as many channels as they need without buying the full capacity.
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