T1 or PRI Service

>Digital signal 1 (DS1, also known as T1, sometimes "DS-1") is a T-carrier signaling scheme devised by Bell Labs. Originally, T1 meant "Transmission - Level 1", and had to do with the media that the signal was passed over. DS-1 meant "Digital Service - Level 1", and had to do with the service to be sent (originally 24 digitized voice channels over the T1). The terms T1 and DS1 have become synonymous and include a many different services from voice to data to clear-channel pipes. The line speed is always consistent at 1.544 Mbit/s, but the payload can vary greatly.

Metropark will discuss T1 in several different ways:

  • Data T1 = 1.544mb of bandwidth
  • Voice T1 = 24 channelized digital voice trunks (T-span)
  • T1 PRI = 23 channels of digital voice and 1 data channel
  • Frac T = less than 24 channelized digital voice trunks
  • Integrated T = less than 24 channelized digital voice trunks and remaining channels bonded as data bandwidth
  • Dynamic T = Voice and Data working un-channelized together over the 1.544mb
  • Point to Point T1 = dedicated 1.544mb of data and voice between two locations

A DS1 circuit is made up of twenty-four 8-bit channels (also known as timeslots and DS0's), each channel being a 64 kbit/s DS0 multiplexed pseudo-circuit. A DS1 is also a full-duplex circuit, meaning, in theory, the circuit can send 1.544 Mbit/s and receive 1.544 Mbit/s concurrently. A total of 1.536 Mbit/s of bandwidth is achieved by sampling each of the twenty-four 8-bit DS0's 8000 times per second. This sampling is referred to as 8-kHz sampling. An additional 8 kbit/s is obtained from the placement of a framing bit, for a total of 1.544 Mbit/s, calculated as follows:

t1 speeds

The primary rate interface (PRI) is a telecommunications standard for carrying multiple DS0 voice and data transmissions between two physical locations. All data and voice channels are ISDN and operate at 64 kbit/s. North America and Japan use a T1 of 23 B channels and one D channel which corresponds to a T1 line. Fewer active B channels (also called user channels) can be used for a fractional T1. More channels can be used with more T1's, or with a fractional or full T3.

Higher Ts consists of bonding T1s to make much larger bandwidth options for the customer:


Level zero (Channel data rate)
64 kbit/s (DS0)
First level
1.544 Mbit/s ( D1 ) (24 user channels) (T1)
Intermediate level
3.152 Mbit/s (DS1C) (48 Ch.)
Second level
6.312 Mbit/s (DS2) (96 Ch.)
Third level
44.736 Mbit/s (DS3) (672 Ch.) (T3)
Fourth level
274.176 Mbit/s (DS4) (4032 Ch.)
Fifth level
400.352 Mbit/s (DS5) (5760 Ch.)