T1 and E1 Circuit

T-1 is a digital circuit that uses the DS-1 (Digital Signalling level 1) signaling format to transmit voice/data over the PSTN network at 1.544 Mbps. T-1 can carry up to 24 uncompressed digital channels of 64 Kbps (DS0) for voice or data. E-1 is the European equivalent of the T-1, except E-1 carries information at the rate of 2.048 Mbps. E-1 is used to transmit 30 64Kbps digital channels (DS0) for voice or data calls, plus a 64Kbps channel for signaling, and a 64Kbps channel for framing and maintenance.

A T1/E1 circuit is a dedicated circuit and is always composed of two parts: the local loop and the carrier circuit. A T1/E1 circuit is the first multiplexed level of the digital signalling multiplexing scheme. T1s use what is called a Stratum 3 clock to maintain what is called clocking on the line.

Within the communications network, copper twisted pairs are used. One pair for transmit, and another for receive making four wires for each T1. This allows T-carrier systems to transmit and receive simultaneously in both directions at full speed (full duplex). T1 trunk cables are made in bundles of 25 pairs of 22 AWG copper wires designed for carrying multiple T1's. A T1 uses a bipolar signalling method where voltage states range between +/- 12 volts. A binary zero is signaled with zero voltage; binary ones are signalled using either positive or negative voltage.

A T1 service is provided by three ways: full T1/Bustable T1 Service, channelized T1 and fractional T1.

• Full T1 Service - A full T1 service provides a complete circuit of up to 1.544Mbps total speed. This communications channel is often referred to as a digital trunk line. The full circuit can be either data or voice, but not both.
• Burstable T1 Service - This is a full T1 with some sort of measuring technology attached at the ISP's end. Customer gets an amount of bandwidth for each month and pays a premium when their T1 exceeds this level of data.
• Channelized T1 - A channelized T1 contains 24 individual DS0 channels, each capable of carrying voice or data. The full set of channels has the same speed as a full T1, but the individual channels may be split into voice lines or data lines using a device called a Channel Service Unit/Data Service Unit or CSU/DSU. The CSU/DSU allows the voice channels to be connected to a phone system or PBX. The data lines are then connected to a router serial interface and often are used to provide Internet connectivity.
• Fractional T1 - A fractional T1 is one or more channels bundled together and sold to a customer as a set. This allows a consumer to purchase less than a full T1's bandwidth at a lower cost. Like the channelized T1, individual channels can be voice or data and a CSU/DSU is used to split the channels, however less than the full set of 24 channels is available to the customer for use.

source: http://www.networkdictionary.com/telecom/T1andE1Circuit.php