A leased T1 line is the casual, informal name for a connection to the Digital Data Network, the internal digital switching system used by the telephone companies. It provides a permanent virtual circuit from one fixed location to another. It is an interface to the telephone company’s internal bit-synchronous data system.

This class of service requires fairly expensive equipment compared to modem-based connections, but offers far superior performance and lower cost per bit (kilobytes per second, latency). A typical high-performance installation might have your computers at your site, wired up on an Ethernet LAN. Also on your Ethernet is a device called a router, which forwards TCP/IP packets in and out of your Ethernet. The router connects to a device called a CSU/DSU; think of this as a modem for your leased-line. The CSU/DSU is plugged into the leased-line from the phone company. The other end of the leased-line is plugged into another CSU/DSU at your Internet provider, which is plugged into a router, and thus into the rest of the Internet.

The maximum speed for dial-up modems is 56K. “56K” means 56,000 bits per second or 56 kilobits per second (kbps). It’s sometimes called “DS0”, which stands for “Data Service Zero”. The original name from the 1970’s is “Digital Dataphone Service”, or “DDS”. A DS0 is 1/24 of a T1 or 1 channel.

A T1 leased-line is very similar to a 56K leased-line, except for speed: 1.536 million bits per second (Mbps). It operates in a similar manner, but with much higher performance. T1 is sometimes called “DS1”, or “Data Service One”. (When used for voice purposes, a T1 is split up into 24 DS0’s with a device called a “channel bank”. None of this voice-based equipment is used for IP.)

There is also Fractional T1 service, which is just what it sounds like: fractions of T1 speeds, generally in 128K increments: 128K, 256K, 384K, 512K, 768K, etc.

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