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Comparing T1 and E1 Line Services
How the popular T1 and E1 line technologies compare for bandwidth service worldwide

By: John Shepler

Two of the most popular digital line services for voice and data are T1 and E1. Depending on where you are located, you are probably familiar with one of these services but not the other. That’s because they are geographical standards. If your business crosses international borders, you’ll want to be familiar with both telecom standards.

Where T1& E1 Lines Are Used
T1 service is found in the United States, Canada, Japan and South Korea. E1 is used in Europe and most other areas of the world. I mention Europe, because that’s where the “E” in E1 comes from. The “T” in T1 means T-Carrier. It is a standard developed by Bell Labs right after WWII and used to provide trunking of multiple telephone calls between switching centers. Later, both T1 and E1 moved beyond their telephony heritage and became popular for digital data service to businesses and other organizations. Both are still highly popular today.

T1 E1 Similarities
The reason to discuss T1 and E1 at the same time is that they are more alike than different. Both were based on digitizing telephone calls and transmitting them in channels of a time division multiplexed bitstream. Each channel is called a DS0 and is 64 Kbps in size. The physical interface is two pair of twisted pair copper telephone lines, the same type of lines that are used for home and business phone service. One pair is used for transmit and the other pair is for the receive channel. By keeping transmit and receive separate, you can talk and listen at the same time - a requirement for phone calls. This is often referred to as full duplex operation.

E1 Is An Extended T1
It’s no accident that T1 and E1 are so similar. The United States developed its standard first. Europe copied most aspects of T1, but added some improvements for their standard. The biggest difference is in the line speed or bandwidth. T1 lines run at 1.544 Mbps. Of that, 1.536 Mbps is used to transmit information called payload. The other 8 Kbps is for synchronization and maintenance functions. T1 lines have 24 time slots that carry 24 DS0 channels. Each channel can be one telephone call or all 24 time slots can be used to carry data at a bandwidth of 1.536 Mbps. This BW is often referred to as 1.5 Mbps.

E1 Characteristics
E1 lines run at 2.048 Mbps. That bandwidth transports 32 time slots. Unlike T1 lines, E1 lines assign one time slot or channel specifically for synchronization and maintenance. Another time slot is generally reserved for signaling. Thus, an E1 line supports a user payload of 1.920 Mbps in 30 time slots or 1.984 Mbps in 31 time slots. This is often referred to as a bandwidth of 2 Mbps.

The Japanese J1 Line
It should also be mentioned that while Japan uses the T1 line standard of 1.544 Mbps, there are differences in the use of the synchronization and maintenance bits. A T1 line in Japan is called a J1 line.

How Line Differences Are Accommodated
While there are numerous subtle differences between T1, E1 and J1, these are accommodated by thespecific circuit design of the appropriate interface cards. At the switching centers, it is relatively simple to interface these lines due to their common heritage. You should have no problem connecting telephone calls or data transfers between any of your worldwide locations. It’s worth noting, however, that E1 lines accommodate more phone lines or data bandwidth per line than T1 lines.

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