The T1 carrier is a digital transmission service commonly used today. It consists of twenty-four separate channels using PCM or pulse code modulation signals with TDM or time-division multiplexing, at a total rate of 1.544 Mbps. Originally, T1 lines used copper wire but today they include wireless and optical media. The T1 outstate system is recently developed for long distance between cities. It is normal for the Internet access provider to get connected to the Internet as a POP or point-of-presence over a T1 line that is owned by a major telephone network. A lot of businesses make use of T1 lines in connecting to the Internet access provider.

T1 can be sold in any of three ways; full T1, channelized T1 and fractional T1. T1 circuit is dedicated and is always made up of two parts: the carrier circuit and the local loop. Local loop is provided by your local telephone company since they are responsible in putting the wire connected to the site in the ground. The T1 is outfitted for Internet service while the other circuit end is routed towards your telecommunications provider and the Internet service is activated. Hence, Internet T1 contains a portion of circuit owned by your local phone company nearest to you and a part of a circuit owned by your Internet Service Provider. Remember that when troubleshooting T1 Internet, you have to get both your local phone company and Internet service provider simultaneously on the phone to do the circuit work. Long distance companies in some places, usually the large ISP's also own local circuits and are also the local telecommunications provider. This is rather uncommon. Oftentimes when buying T1 lines, you are actually buying a T1, by which part of the local phone circuit has been resold by the ISP to you as full T1. But this is better than having to purchase it if because this way you can hold the Internet service provider accountable for any and all problems that occurred in the local phone company circuit portion. However, it is usually more expensive.

Usually, full T1 lines are sold as complete circuit up to 1.544Mbps in total speed. This channel of communications is often referred as the digital trunk line. T1 is a dedicated circuit running from the provider to the customer and not shared with any third party customers. This is the reason it is often referred to as 'dedicated' line. The full circuit can be voice or data, but not both. A fractional T1 is one and more channels packed together and sold as set to customers. This allows the consumer to purchase lesser than full T1 bandwidth at lower cost. The T1 price has fallen significantly and fractional T1 service is becoming more uncommon. Just like channelized T1, each channel can be data or voice and a DSU/CSU is used to split channels, but less than full 24 channels will be available for customer use.



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