What is a T1 Line, Why is T1 Line required, Architecture Diagram of T1 Line

Architecture Diagram of T1 Line - Telephone / Telecom Trunk Line

What is a T1 Line & Why is a T1 line required?

You should be familiar with trunk lines – The single land-line phone connection provided by your telecom provider at both residential as well as commercial (office) locations. So, you can use these trunk lines to make outgoing calls – Local Call, Outstation Calls, etc. But you can make only one call at a time. So, if two or more users want to call different people, it may not be possible with a single analog trunk line.

If you have an office, you should be having a small EPABX too. You can apply for multiple analog trunk lines and share it with all the users. Having 2/3 analog trunk lines for a small office is ok, but if you have an office with more than 50/100 people it becomes cumbersome to apply for more individual analog trunk lines. Terminating each trunk line requires a separate trunk port in your EPABX & you get separate bills for each line, every month. With more lines, the process just becomes too cumbersome.

That’s when you should look at T1 lines – A T1 line has 24 Digital Voice Channels (trunk lines) that are transmitted using a single circuit. Yes, a single circuit can carry up to 24 calls simultaneously! They use multiplexers (at the public telephone exchange office end) and de-multiplexers (at the customer premises – private EPABX) to achieve this. A T1 line occupies just one T1 port in the PBX but still 24 people can simultaneously make/receive calls through the T1 line terminated on the PBX.

T1 lines are popular in the United States/ Japan / Korea. E1/PRI Lines are popular in Europe/Asia. E1/ PRI lines are quite similar to T1 lines but instead of the 24 channels, they carry 30 channels in a single line.

Salient points / Advantages of T1 lines:

1. T1 lines can be terminated on an EPABX using a T1 card which is either in-built or supplied additionally by the EPABX manufacturer.

2. If you have an open source PBX, you can add a T1 card to the PCI slot of the server (hosting the soft-pbx) and terminate the T1 line on the T1 card.

3. You can terminate multiple T1 lines (using multiple T1 ports) in the same PBX if the PBX has enough empty T1 ports, and can support so many T1 lines.

4. It is possible to carry both voice and data using a T1 line. The total data capacity of a T1 line is 1.544 Mbps when no voice calls are made using it. You might need a data modem to terminate the T1 line and to connect to the Internet using a T1 line.

5. The data capacity of 1.544 Mbps mentioned above is bi-directional. That means, you get 1.544 Mbps for upload as well as the same capacity for download. Generally, only leased lines provide such a facility (not available with broadband). This could be useful for applications like video conferencing, hosting a web-server etc, where a solid uplink connection is a must.

6. Some telecom service providers provide fixed capacity T1 lines for voice and data while some of them provide dynamic capacity T1 lines that can automatically provision the maximum data bandwidth after deducting the bandwidth consumed by active voice calls.

7. For T1 lines, there is a single fixed rental per month (with a certain number of free calls), based on the plan you select. You get a single bill for all the calls made through a T1 line.

8. T1 lines can be provided either using copper circuits or using optical circuits, depending on your location and the service provider. There is not much of a difference in capacity between the two though.

9. There is something called as “Fractional T1 lines” provided by some telecom service providers that allow you to buy a limited capacity T1 line. So, instead of 24 channels, you can get 12 channels and pay the rental/call charges accordingly.

10. T1 lines are very useful if you want to have a DID (Direct Inward Dialing) facility in your office. Generally, telephone companies block a series of land-line numbers and give them to you along with a T1 line (or this needs to be purchased separately). So, instead of someone having to dial through the PBX and typing the extension number every time to reach you, they can just reach you by dialing your own dedicated number from anywhere.

11. T1 lines can be useful at places where you are not able to get an Internet Leased Line / Broadband connection.

12. T1 line can also act as a back-up to your normal data line for Internet connectivity.

T1 lines are digital lines and are different from the VOIP / IP Trunks that you can purchase from ITSP’s. But you can use the data capacity offered by the T1 lines to make VOIP calls.


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