T1 and T3 are typically rented by organizations at a monthly or yearly rate to connect geographically separated offices for private voice and/or data networking. The high cost of these lines prevents most individuals from leasing them.
About T1 LinesThe T1 standard for data communication was developed by AT&T in the 1960s. T1 leased lines offer the same data rate as symmetric DSL (1.544 Mbps).
A T1 line typically costs $1000 USD or more per month. So-called fractional T1 lines, starting at 128 Kbps, reduce this cost somewhat and can be found in some apartment buildings and hotels where they provide residential Internet access.
About T3 LinesT3 lines are a common aggregation of 28 T1 circuits that yields 44.736 Mbps total network bandwidth. Besides being used for long-distance traffic, T3 lines are also often used to build the core of a business network at its headquarters. A T3 line typically costs more than $3000 USD per month.
The concept of T2, T4 and T5 aggregated lines also exist in telecommunications, but these are much less commonly used in practice.
Note that T1 and T3 are the names of certain tram lines in Paris, France. These are unrelated to T1 and T3 telecommunications lines.