What are T1 Lines and T3 Lines?

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T1 and T3 are two common types of digital data transmission systems used in telecommunications. Originally developed by AT&T in the 1960s to support plain old telephone service, T1 lines and T3 lines later became a popular option for supporting business-class Internet service.

T-Carrier and E-Carrier 

AT&T designed its T-carrier system to allow grouping of individual channels together into larger units.

 A T2 line, for example, consists of four T1 lines aggregated together. Similarly a T3 line is comprised of 28 T1 lines. The system defined five levels - T1 through T5 - as shown in the table below.

T-Carrier Signal Levels
NameCapacity (maximum data rate)T1 multiples
T11.544 Mbps1
T26.312 Mbps4
T344.736 Mbps28
T4274.176 Mbps168
T5400.352 Mbps250

Some people use the term "DS1" to refer to T1, "DS2" to refer to T2, and so on. The two kinds of terminology can be used interchangeably in most contexts. Technically, DSx refers to the digital signal running over the corresponding physical Tx lines (which can be copper and/or fiber cabling).  "DS0" refers to the signal on one T-carrier user channel, which supports a maximum data rate of 64 Kbps. (Note that there is no physical T0 line.)

Note that T1 and T3 are the names of certain tram lines in Paris, France. These are unrelated to T1 and T3 telecommunications lines.

While T-carrier communications was deployed throughout North America, Europe adopted a similar standard called E-carrier. An E-carrier system supports the same concept of aggregation but with signal levels called E0 through E5 and different signal levels for each.

Leased Line Internet Service

Some Internet providers offer T-carrier lines for businesses to use as dedicated connections to other geographically separated offices and to the Internet.

Owners of apartment buildings and hotels These leased line Internet services traditionally offer either T1 or T3 (or fractional T3) levels of performance as those are most cost effective.

More About T1 Lines and T3 Lines

Owners of small businesses, apartment buildings, and hotels often relied on T1s as their primary method of Internet access before business class DSL became prevalent. T1 leased lines can cost $1000 USD or more per month. The price is not fixed per customer but rather varies based on coverage distance: It's possible to obtain T1 for under $250 or sometimes even under $100 per month, but only in restricted circumstances. T1 does not have nearly enough capacity to support significant Internet usage nowadays.

Besides being used for long-distance Internet traffic, T3 lines (one or more) were also often used to build the core of a business network at its headquarters. T3 line costs are proportionately higher than those for T1. So-called "fractional T3" lines allow subscribers to pay for a lesser number of channels than a full T3 line, lowering leasing costs somewhat.