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What Is T1?

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Page updated on:
Fri, 06 Aug 2004 09:59:00 GMT
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What is T1?

A T1 is a term for a digital carrier facility used to transmit a DS-1 formatted digital signals at 1.544 megabits per second. This is made up of 24 digital channels. This requires a digital connection device (CSU/DSU {customer switching unit/digital switching unit}) to connect to four wires to carry the information. Most small Internet providers have a T1 (or a fractional T1) line as their connection to the Internet. A Full T1 should accommodate from one to over 200+ users and other services from an Internet provider. Unlike the modem that is in most computers a T1 line requires a CSU/DSU and the connection. The modem that is in your computer is analog. The newer 56K modems are a transition from analog to user affordable digital technology. The newer ISDN modems are digital to allow for the higher speeds.

How it works:

The T1 is like a large water main that serves a city, a large amount of water or traffic flows through it. Unlike the water hose in your front yard (your modem) the T1 is the major carrier of the Internet traffic. The T1 connects the backbone provider to the ISP provider via the telco (telecommunications provider) The signal comes into the CSU/DSU and then goes to the router. From there it goes into the master name server and may be routed to other servers. One of these severs may be a modem or terminal server that allows you to connect to the Internet. You log in and are verified as a user on the local network and then are allowed to proceed to the larger network (Internet).

What may affect You?

Things that may affect you are how busy the site is on the other side of the Internet. If I work from a Unix prompt and move files from one site to another using the FTP protocol. I would expect them to move as fast as my network connection (1.54megabits/sec) would allow, a 1 megabyte file will travel at about 120K/sec or take 8.3 seconds. If you are accessing the same file from a 28.8 modem then it would be expected that (because you have a 28.8 connection) that it would take 347 seconds or 5.8 minutes. This is optimum situation, however because of traffic at the other site it may take longer due to the number of people accessing the site and the files. The kind of connection that particular site has (ie T1). The number of users that are actually accessing the site affects what I would expect to see for a file transfer. If I'm the only one accessing a web site then I would expect the files to travel very quickly. If there are many users all accessing the files then I would expect things to slow down because of the traffic. The more traffic the slower the files travel.

What is a CSU/DSU?

A CSU/DSU [Channel Service Unit / Data Service Unit] is a piece of equipment that connects a leased line from the telephone company to the customer's equipment (such as a router). It performs line encoding and conditioning functions and often has a loopback function for testing. Although CSU/DSU's look similar to modems, they are not modems, and they don't modulate or demodulate between analog and digital. All they really do is interface between a 56K, T1, or T3 line and serial interface (typically a V.35 connector) that connects to the router. Many newer routers have 56K or T1 CSU/DSUs built into them. CSU/DSU's for 56K, T1, and T3 lines are NOT the same and are not interchangable as a general rule. In the case of a T1 CSU/DSU, it passes the data in 64K chunks (time slots) on the 24 different channels (64K x 24 = 1.54MB).

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