sip trunking

What is sip trunking and how does it work?

Sip Trunking example Sip Trunking

With the growth of many business across globe. SIP trunking is the new and convenient method of communication using VoIp.  Almost every business requires large call volume. Sip trunking is the technology which makes VoIP suitable for business. This leads to the need of a PBX and a high capacity connection for voice, fax etc.

SIP trunking have two parts one is SIP and other is trunking. To explain better we will explain each part in detail.

What is SIP ?

The full form of SIP, is session initiation protocol.  It is a signaling protocol. What is signaling protocol ? There are two types of channels/paths are in communication.  One is signaling channel and other is media or voice channel. SIP does signaling for setup a media session. A media session can be for voice, video fax etc.

For setup a session , sip messages carries , media end point information in session description protocol (SDP).

SIP Uses , service of underlying TCP/IP or UDP/IP.

What is trunk ?

What is trunk
What is trunk.

Trunk is a very common term in telecom.  You have heard about in olden days a long distance call was called a trunk call.  A group of communication lines is trunk, lines can be shared by many callers or agents.  For example in call center, all customers dials a same customer support number.  But at the same time multiple customers can be served. Because for each call a line is allocated from the trunk. The telephone line in our home for PSTN  is a trunk with one line only.

Each communication line in the group is called circuit. PSTN uses ISUP protocol for signaling. In ISUP each line is a circuit which identified by the circuit identification code (CIC) , with dedicated bandwidth. When there is an incoming or outgoing call a CIC is set to busy. PSTN uses, SS7 for the transport of ISUP signaling over network.  In SS7 a trunk can have maximum 31 or 24  channels , few are for Signaling and remaining for voice or media.

In SIP trunk , there is no physical limit on the number of lines. But concurrent calls can be limit by the underlying IP network bandwidth or from the SIP trunk provider.  Based on above , SIP trunking is a logical connection provided by a VoIP company for making multiple concurrent calls over IP network.  The example is IP PBX, of a company. It connects to the trunking over SIP and internally to the phones called extensions.

How Sip trunking works :

The working is based on VoIP call concepts. If someone or a company wants to use the SIP Trunk. He has to purchase the account details from a service provider for the SIP trunk parameters.  A company should have a PBX , if  the PBX is not supporting IP calls (not an IP PBX). Then PBX needs to upgrade for voip feature.

Next step is, to configure trunking connection details in the IP PBX.

Now call from an extension to the external phone number. IP-PBX , do the SIP signaling with the trunk provider.  As a result VoIp call is setup with the external phone number.  The external phone number may be a voip number, pstn or a mobile phone number. If not a voip number then there will be conversion from VoIP to PSTN.  You do not need to worry about this conversion. Trunking provider take care of all the necessary conversion.

Voip and Sip Trunking:

From the name it looks that only SIP protocol is sufficient.  But it is not true. SIP alone can do only signaling, for a complete call other VoIP technologies works together.  This includes streaming of media over IP network.

Sip trunking vs pri:

The first question comes here what is pri? We all have seen traditional phone lines coming to our office or home. These lines are called Primary Rate Interface (PRI) lines.  On one end a line connects to the handset and other side it connects a telephone box.  Following is a comparison of Pri lines with the SIP lines in tabular form.

Relatively a new. Old and traditional lines.
No special hardware is required. Ethernet or Wifi connectivity is only required for SIP trunk. Special hardware required. The hardware is a PRI card. The system using card, also needs to install the software or driver for the card.
There is no type all connections uses same IP lines for communications. There is no limitations on channels. A pri line can be of T1 or E1 type. T1 have 24 channels and E1 have 31 channels. Maximum bandwidth for T1 is 1.544 Mbps and for E1 2.048 Mbps.
Same IP network, there is nothing like voice and data channels. Signaling and actual voice data flows on separate channels.
Bandwidth is not guaranteed. Speed depends on underlying IP network speed. Bandwidth is guaranteed.
Low cost and need no dedicated lines. High in cost and need to maintain separate lines.
Easy to scale, when concurrent calls increases. Not easy , as scaling requires to add  more physical lines.
SIP trunk provider can provide redundancy by rerouting traffic in case on failure to another SIP server for VoIP calls. There is only a single switching center which connects to the PRI line and calls can  not reroute , so not redundant.


sip trunking providers:

There are many sip trunking providers.  Which provider fits for you is depends on your requirements.  Here is the list of many sip trunking providers.

sip trunking pricing:

We have discussed about the sip trunking and its benefits for business. Now next thing come , how much does this cost ? The answer is not as simple. The trunking cost depends on many factors. Few are listed here.

Setup Fee: Initially moving to new providers need to setup connections , this may take some significant time. The user might not be expert in technical stuff for setup voip.  This may lead to hire for an expert or the sip trunking company will setup and charge an amount.

Concurrent Calls: Capacity is always a major factor for deciding cost. SIP trunking comes with the advantage , where capacity is not limited and easy to scale.

Current PBX : If a company using IP-PBX, then no cost is required for using SIP trunking.  But if no PBX or not an IP-PXB .  This adds additional software in sip trunking pricing. Either PBX needs to upgrade for VoIP  or purchase a new IP-PBX.  There option may be cloud based hosted  IP PBX or own in house installed solution.

VoIP Numbers:  If business already have a number, then number needs to port on VoIP. Else new number will be purchase. The option chosen will impact the sip trunking pricing.

Sip Trunking for business:

Business needs advantages for flexibility and cost. . With Voip, no limit on channels as PRI lines have.   A business can choose option on usage basis. Can pay minimum for only one channel and on needs on can pay for 1000 channels.


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