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t1 lineA T1 or T-1 is a high-speed data transmission line most commonly used in the United States, Canada, and Japan. It is a widely used standard in telecommunications to transmit voice and data between devices. Bell labs, and American telecommunications company, introduced the first T1 in 1962, which greatly increased the number of telephone calls the telephone network was capable of transmitting at one time.

A T1 line is a dedicated circuit made up of twenty-four 8-bit channels, each channel has a data transmission rate of about 64 Kbps. In total, a T1 can carry 1.544 Mbps (8×64 plus 1 “framing” bit).  This original T1 configuration has been updated to carry Integrated Services for Digital Networks (ISDN) signals. ISDN is designed to handle voice, video, data, and other network traffic rather than just voice.

The T1 line is said to have a signal level of DS1. DS0 is the signal level of one voice channel in digital form which is 64kbps. So a DS1 is the next level with 24 DS0 channels combined (using time division multiplexing or TDM), though the DS1 will carry data as well as voice. The voice bandwidth is just used as a benchmark and heralds from the origins of the T1 in telecommunications systems of Bell Labs.

T1 lines were also widely used for connecting branch offices, but in time many companies have replaced these direct connections with virtual private networks (VPN) over the Internet.

Most Internet Service Providers (ISPs) connect to the Internet using a minimum of T1 lines, usually owned by a major telephone network. This allows the ISP to have a point-of-presence (POP) on the Internet which allows them to provide better services and connection speeds than standard residential connections allow. Numerous companies also use T1 lines to connect to an ISP to ensure they have enough bandwidth to support their business needs at greater speeds and bandwidth than an average home connection.

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