T1 Circuit

Description

A T1is sold one of three ways, full T1, channelized T1 and fractional T1. A T1 circuit is a dedicated circuit and is always composed of two parts: the local loop and the carrier circuit. The local loop is provided by the local telephone company because they are the ones that put the wire that is conncted to the site into the ground. If the T1 is for Internet service, the other end of the circuit will be routed to the telecommunications provider of your choice and Intenret service will be turned on. Thus, your Internet T1 contains a local phone company portion of the circuit closest to you and an Internet provider part of the circuit owned by your ISP. Always remember that when troubleshooting an Internet T1, you need to get both the local phone company and the Internet provider on the phone at the same time to do any work on the circuit.

In some places, long distance companies (who are usually the big ISP's) also own the local circuits and are the local telecommunications provider. This is very rare. More often when you buy a T1, you are buying a T1 where the local phone company's part of the circuit is being resold by the ISP to you as a complete T1. This is a better way to purchase it if you can because you can hold the ISP accountable for all problems in the local phone company portion of the circuit. However, it will be more expensive.

Full T1 Service

A full T1 service is usually sold as a complete circuit of up to 1.544Mbps total speed. This communications channel is often referred to as a digital trunk line. A T1 is a dedicated circuit from the telecommunications provider to the customer and is not shared with any other customers. This is why it is often referred to as a 'dedicated' line. The full circuit can be either data or voice, but not both.

Burstable T1 Service

This is a full T1, sold with some sort of measuring technology attached at the ISP's end. You purchase an ammount of bandwidth that you receive each month and pay a premium when your T1 exceeds this level of data.

Channelized T1

A channelized T1 contains 24 individual 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 is used to split off the voice channels from the data channels, allowing 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. The price of T1's has fallen significantly however and fractional T1 service is becoming ever more rare. 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.

PHYSICAL SPECIFICATIONS

GENERAL

A T1 circuit is an always on connection. It in no way resembles a voice circuit which only generates expenses when a call is placed. For this reason, T1's are sometimes referred to as private lines or dedicated data line because they are always in use (whether data is being transmitted or not) and thus, can never be used for carrying traffic from multiple subscribers. It is because T1's are dedicated circuits that they cost more than ISDN or Frame Relay circuits which assume that the customer's connection is shared with other subscribers. T1's are 'always on' and thus must be a dedicated data line.

A T1 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. Devices supporting a T1 estimates its synchronization with the T1 based on the data received over the link. Thus, T1 communication is not always fully synchronous (this is one cause of frame slips). T1's are more properly called pleisiosynchronous connections.

T1s are often referred to as DS1 (Digital Signalling level 1). A T1's speed can reach as high as 1.544 Mbps on the circuit. T1's are the most common high speed circuit provided by telecommunications carriers because a T1 can be regenerated and extended to reach any location. Other solutions such as DSL do not have the same total reach as standard T-carrier services. T1's carry 24 channels of digital information and must maintain a certain level of 1's density in order to assure the clock is maintained at both ends of the circuit.

ELECTRICAL

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. This positive and negative value is refered to as polarity. When a series of 1's are received on a T1 link, the voltage is alternated from positive to negative voltage. From this change in voltage, the receiving equipment can detect and synchronize with the remote device's clocking. Zeroes are indicated by zero voltage on the line. Because clocking is derived by detecting changes in voltage, a long stream of zeroes from an inactive line would cause problems.

CABLING

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. Most carriers today (as of 2004) use fiber optics wherever possible between central offices and have been using T3's for many years. This type of physical plant is slowly fading out of use and will probably be completely gone by the turn of the next century; at least at the rate most carriers currently perform upgrades. The trunk cables carry multiple bundles of 25 pairs and the standard method used for carrying transmission is to separate transmit from receive on each pair into separate bundles to reduce cross talk. Occasionally a few extra pair are added to these bundles for line management functions such as fault location or provisioning.

source: http://www.inetdaemon.com/tutorials/telecom/t-carrier/T1/