Telecom Trunk Lines – Difference between Analog line, PRI/E1/T1 Digital Line & GSM Gateway

By Rajesh K

In this post, we will see a short overview of the various trunk lines available for connecting your PBX to the external world. We will have a look at analog lines, PRI/E1/T1 Digital lines & GSM Gateway, in particular.  This topic was requested by a reader in the comments section and hence I am writing an article on this topic. If you want to know more about any topic, leave a comment in this blog and I will try to write elaborately on the same.

Difference between analog line, PRI/E1/T1 line and GSM/ cellular gatewayWhy do you need a PBX?

Before we go into the actual post, we need to understand some background about what is a PBX and why we need it. PBX actually refers to Private Branch Exchange. Imagine this situation – You have 100 employees in your organization, and all of them work from their office. Each one of them need to make outgoing calls (to partners, customers, etc) and also need to talk to their colleagues.

If there was no PBX, you have to procure 100 different analog trunk lines (from a Telecom service provider) to give telephone access to all of them. They can call each other, but those calls will also be charged as they are routed from the public telephone exchange. So, obviously this method is inefficient.

So, a small private branch exchange – PBX (Telecom switch) is kept at the organization to achieve two important purposes :

1. To share the incoming (trunk) lines from public telephone exchange to all employees as every employee is not going to be making outgoing calls/ receiving incoming calls all the time. So, instead of buying a trunk line for every employee, you might just buy one line for ten employees (sufficient in most cases) and share them.

2. To make internal calls (based on extension numbers) without any charge. Since the PBX is a mini-switch, it can route calls from one extension to another extension internally.

Analog Trunk Lines:

The phone line (land-line) that you get in your home is an example for an analog trunk line. At homes, one line would suffice. But in organizations, multiple such lines might be required (as the number of telephone users are more). These analog trunk lines carry a single channel of voice call per line (meaning, one person can make or receive a phone call from outside at any given point of time).

For terminating the analog trunk lines in a PBX, special analog trunk cards (12 ports, 24 ports, etc) are used. In an IP PBX, Analog Terminal Adapters (ATA) or PCI based Analog Cards may be used. These devices convert the analog calls into IP so that an IP PBX can understand them and transmit them over the network.

Separate rental / call charges needs to be paid for each analog trunk. Analog trunk lines are sufficient for small organizations which require a few lines.

PRI / E1 / T1 Lines:

For larger organizations that require a lot of trunk lines, procuring additional analog trunks might not be a practical idea as they need to buy separate (perhaps multiple) analog trunk cards to terminate these lines in their PBX (and) they need to pay separate rental for each line where the free call capacity for some individual lines might never be reached!

So, these organizations can procure a digital line called as PRI/ E1 (In Europe/ Asia) or T1 (In America). Each PRI / E1 line can carry up to 30 channels of voice communications simultaneously. They can do this by using multiplexing/ de-multiplexing techniques (at the public telephone exchange end -and- the customer PBX end). A T1 line, similarly can carry up to 24 channels of voice communications simultaneously.

At the customer end, they need to procure a PRI / E1/ T1 Card, to terminate this PRI/ E1/ T1 line in their PBX. Just imagine – One line coming into your organization but thirty people can simultaneously make or receive calls through it! And add to it a single consolidated rental / billing package from the Telecom company. You can read more about PRI lines and its advantages from here.

There is another type of digital line called ISDN line. With ISDN, two simultaneous calls can be made/ received at the same time using a single line.

GSM/ Cellular Gateway:

In some situations, it may not be feasible to lay analog lines (over a hill, rural areas, etc). But those places might be covered by cellular companies/ signal from cell phone towers. In such situations, you can use a GSM / CDMA Cellular gateway device called as Fixed Cellular Terminal and use the Cell phone SIM cards as analog trunks for your PBX. These devices are in fact connected to the analog trunk ports of the PBX.

A Fixed Cellular Terminal is a small device in which you can insert a SIM card and connect the single-pair copper telephone cable coming out of it, directly to a telephone (or) to a PBX. At the other end of the PBX, the extensions (normal phones) are connected. So, when someone makes an outgoing call, the request reaches the PBX and the call is directed to the Cellular Gateway which has the SIM card. Some cellular gateways have multiple SIM cards, enable multiple simultaneous calls to/from outside.

So, even though people might dial a number from their land line extension, the call actually goes through the cellular network using the GSM/ CDMA Cellular Gateway.

VOIP / SIP Trunks:

Of late, VOIP service providers have become very popular. They use the Internet backbone (mostly) to transmit the call as IP packets to the destination. They have gateways (where required) to convert the IP calls to analog calls before passing them on to the local telephone exchanges so that people can reach normal landline/ cell phone numbers using the VOIP Service. You can read more about SIP Trunks from here.

excITingIP.com

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27 Responses to “Telecom Trunk Lines – Difference between Analog line, PRI/E1/T1 Digital Line & GSM Gateway”

  1. Also i would request you to provide us the difference B/w PRI and BRI

    #31252
  2. Ramarao

    Thank you sir, Nice infomation about Trunks.

    #85096
  3. Than

    Many thanks, make me more understanding.

    #93508
  4. Hasnain

    it was great, it is beautifully explained with practical scenario

    #112836
  5. Rakesh

    Very good explanation…..thanks

    #118789
  6. Nalin

    Very Nicely Explained.

    #119744
  7. Ben Glover

    We have a trunk line system in our office, however we would like to add several handheld phones we could walk around with that would function same as our multi-line desk phones do. We would probably use 5-10 additional handheld units.
    Thanks,
    Ben

    #123056
  8. thanks guys it has helped me alot

    #141386
  9. Aravindan

    Hi,

    Great post and very clear.
    Can you please post about Calling search space ,Partition, call routing,
    routelist

    Thanks
    Aravindan

    #159906
  10. Anonymous

    Can some one tell me about how many toll free numbers can be mapped in a single PRI line

    #168720
  11. ahmad

    Many Thanks

    #183680
  12. rani

    Thanks, your explanation really helped. Could you explain PRI over DOCSIS how is it different and its advantages? Many thanks

    #184977
  13. sepo

    I am starting a WISP business here in the US, what is the process if I want to offer my customers T1?

    Thanks

    #199655
  14. RENJI KV

    can you please explain how E1 works for both voice and data.

    #201192
  15. saber

    thenks alot.it was very useful.

    #202653
  16. AZRUL

    Distinguish between Line and Trunk Signaling. Describe the process a telephone system hacker
    would use to obtain free long-distance service using old in-band signaling telephone network. Why
    is the out-band signaling such as SS7 network faster than the old in-band signaling method?

    #204870
  17. Jyoti

    Thanksss a lot it was very helpful….!!!

    #215800
  18. Cliff

    I have a situation where the cable company is bringing in a PRI on their Fiber and expecting to connect to a PRI card in the Key System(KSU). The KSU is not PRI compatible, Is there a device that stands alone that will break out the chanels from the cable Co PRI to analoge for impute into the KSU and separate FAX and Credit card machine lines?

    #227298
  19. Arpit Parekh

    Very useful post.In the morning while studying the document of my company. Compnay’s data center shifting from PRI to VoIP. I just wanted to know that how PRI carries the voice ?

    #245699
  20. Khan

    gud article very useful

    #254556
  21. Can you make SIM card work is a e1 line

    #261467
  22. Likith

    Excellent!!! Nice info.Many thanks to you sir.

    #263663
  23. Mukoko

    Good article

    #265112
  24. chetan

    sir,
    it’s nice and i need information on this also
    please tell me how the data and voice connection is possible with practical example

    #272067
  25. kondal rao

    Good Artile

    #274252
  26. Naren

    Hi ,

    Your Explanation is Excellent am very much Impressed

    Can you pls Explain about RNC’s & EVORNC’s and MGWs and Blade Clusters

    I mean the Overview

    Rgds,
    naren
    9000400302

    #275292

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