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 services, T1 lines and T3 lines later became a popular option for supporting business Internet services.
DS0 – beginning of digital telephony (T1, T3, E1, E3)
AT&T designed its T-carrier system to allow the grouping of individual channels into larger units. For example, a T2 line consists of four T1 lines joined together. A T3 line similarly consists of 28 T1 lines. The system defined five levels — T1 through T5:
Some people use the term "DS1" to refer to T1, "DS2" to refer to T2, and so on. The two types of terminology can be used interchangeably in most contexts. Technically, DSx refers to the digital signal that travels over the corresponding physical Tx lines, which can be copper or fiber optic cables. "DS0" refers to the signal on a single T-carrier user channel, which supports a maximum data rate of 64 Kbps. There is no physical T0 line.
Some ISPs 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 to provide T1, T3, or fractional T3 performance levels, as these are the most cost-effective options.
