Ref Doc: Introduction to T1 Digital Services
What are T1 and Fractional T1?
- T1 and Fractional or Channelized T1 are digital communication lines.
- Their function is to supply 24 "channels" for voice and data to travel over.
- There are two principle elements that make up the T1 line.
Access: The infrastructure we call T1 is built on two 22-guage twisted wire pairs that connect the customer’s equipment to the telephone company. One pair is for transmitting and the other pair is for receiving.
Span Type: How the T1 is delivered.
Long Haul This portion begins at the Telephone company’s central office and is routed to the customers premise equipment (cpe) via the DS-1 interface.
Short Haul This delivery method is done via the DX-1 interface which "normally" comes from the local PBX.
24 Channels
- Each T1 line consists of 24 channels or Digital Signal level 0 (DS-0) time slots.
- Each DS-0 has the capability to transmitting and receiving data at 64 kbps.
- The Illustration below will depict a full T1 line
- There are 24 DS-0 time slots
- Each DS-0 operate at a bandwidth of 64 kbps for a total bandwith of 24X64,000=1.536 Mbps.
- An additional 8 kbps is included and used for framing and other overhead bits.
- When you add the 8 kbps to the 24 DS-0 time slots, the data rate is now 1.544 Mbps. This is referred to as a DS-1 signal
T1 Bandwidth
- Each data circuit is allotted 64 kbps.
- However, some of today's applications require higher speed channels.
- These high-speed channels are accomplished by grouping together time slots in multiples of 64 kbps.
Fractional T1
- If a full T1 line is not needed, Fractional T1 could be the answer.
- Fractional T1 lines operate the same as full T1 lines except that they offer the allocation of individual DS-0s up to 12 DS-0s.
- Fractional T1 can be acquired in one of two ways: - It may be purchased in 64 kbps increments up to 768 kbps (12 DS-0s) from a telephone company. An in-house T1 line may be subdivided by a multiplexer.
Line Codes and Framing formats of a T1
Alternate Mark Inversion (AMI)
Bipolar 8-Zero Substitution (B8ZS)
Superframe (D4)
Extended Superframe Format (ESF)
What is AMI?
AMI line coding is a bipolar coding scheme in which successive ones must alternate in polarity.
What is B8ZS?
B8ZS is a coding technique that is used to satisfy the ones density requirements of the T1 carrier facilities while allowing clear channel data.
What is D4?
D4 framing format is a 193 bit frame format in which the 193rd bit is used for framing and signaling information. A superframe consists of 12 frames.
What is ESF?
ESF is a T1 framing format that is similar to D4 except the Extended Superframe is made up of 24 frames instead of 12 frames. ESF also provides CRC error detection and a 4 kHz Facility Data Link (FDL).
Digi International DSU/CSU
The Digi DataFire RAS 24/48 PTE board has internal DSU/CSU that support :
- Line codes of B8ZS (preferred) or AMI.
- Framing formats of ESF (preferred) or D4.
Revised 7/21/99: CL
