What Are T1 Lines and T3 Lines?

These high-speed lines are suitable for business networking uses

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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 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 the grouping of individual channels together into larger units.  A T2 line, for example, consists of four T1 lines aggregated together.

Similarly, a T3 line consists 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 or fiber cabling.  "DS0" refers to the signal on one T-carrier user channel, which supports a maximum data rate of 64 Kbps. There is no physical T0 line.

While T-carrier communications were 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. Businesses use leased line internet services traditionally to offer T1, T3 or fractional T3 levels of performance because those are most cost effective options.

More About T1 Lines and T3 Lines

Owners of small businesses, apartment buildings and hotels once relied on T1 lines as their primary method of internet access before business-class DSL became prevalent. T1 and T3 leased lines are high-priced business solutions that are not suitable for residential users, especially now that so many other high-speed options are available to homeowners. A T1 line does not have nearly enough capacity to support significant demand for internet usage nowadays.

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