What is a T1?

T1: Transmission System 1


T-1 is the common name for a circuit used in to deliver voice and data lines to businesses. This is a specific type of “trunk”, or transmission channel between two points.

In the world of telecom and Unified Communications (UC), there are many circuit connection options available. T1, however, is the most commonly used “last mile” connection for businesses to access the PSTN (publicly switched telephone network). A T1 is made up of 24 distinct channels, and each one of those channels can be used separately or bonded together within that channel to provide a faster connection for certain services. Depending on signaling, each channel can support either 56KB or 64KB. Other common circuits are PRI and SIP trunks.

When a T1 is delivered from a phone service provider’s Central Office to a customer, it comes into a box in the building called a CSU/DSU, which stand for Channel Service Unit and Data Service Unit respectively. The CSU/DSU is connected in turn to a T1 supported device.

The T1 might go directly into a PBX (private branch exchange) business phone system, or into a router that connects computers to a network.

Most devices today can support a T1 connection or something similar, although it may sometimes need to be broken down into the 24 separate channels in order to be compatible with an older phone system. .


T1 is very consistent and predictable, and the service level agreement that comes with a T1 is typically the highest available of any service.

When a T1 services goes down, there is generally a 4 hour MTTR (mean time to repair).  Carriers considered a T1 to be one of the highest priority circuits to repair, and therefore may be an excellent choice for a business that relies heavily on consistent call availability.

When predictability, stability, and reliability are more important than price and speed, a T1 may be the connection of choice for many company’s business phone systems.  There are less expensive or potentially faster alternatives, however, depending on your call volume and data transmission needs…. so talk to an experienced telecom consultant to find out what kind of circuits are the best options for your business.