How T1 and E1 Lines Compare

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.

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.

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.

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 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.

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.

While there are numerous subtle differences between T1, E1 and J1, these are accommodated by the specific 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.

Does your organization have a need for voice and data connectivity at any line speed required? It’s easy to have our expert Telarus consultants check bandwidth prices and availability for all of your locations worldwide and make recommendations to save you the most money on your telecom needs.