Setting up a SIP interconnection

When you run a VoIP wholesale business, one of your regular tasks is to setup interconnections with your partners, be it vendors (providing termination services) or customers (originating the voice calls). These interconnections are used to send and receive voice traffic between VoIP companies. Nowadays the interconnections between VoIP carriers are based on the SIP (Session Initiation Protocol). While I'll not be explaining the SIP internals within this post (I'll do it in a separate post), I'll try to briefly describe what happens in a typical scenario when two VoIP wholesale providers establish a SIP interconnection between their softswitches.

So a typical workflow would be like this (in this example our/your company is establishing interconnection with a vendor so that we can send voice traffic to them):


  1. Our sales rep closes a deal with a partner (let's call him VoipN), they agree the prices and destinations that VoipN will accept voice traffic to (let's keep it simple and say that they agreed that VoipN will terminate traffic only to Nigeria mobile networks at a price of $0.07 per minute).
  2. Our sales rep already has a customer that is willing to pay $0.09 per minute for calls to Nigeria mobile networks, so we have a profitable case.
  3. Since we don't have any interconnection to VoipN, we have to establish one.
  4. Since this is a post about establishing an interconnection we'll leave all the legal stuff out (contracts etc)
  5. Our sales rep and VoipN sales rep exchange a so called Interconnection form.
  6. Both reps forward the forms to their NOC teams and after the forms are filled they exchange the filled forms.
Since the interconnection forms now contain all the relevant information to establish a voice interconnection between us and VoipN, both sides now have all the parameters to establish a interconnection.

Let's look at minimum information such an interconnection form holds:

Field name
Description
Company name Name of the company
NOC e-mail E-mail address of the NOC
NOC phone number Phone number where NOC is accessible
IP address IP address of the softswitch
Protocol Voice protocol to be used (mainly SIP nowadays)
Port Network port where the switch is listening (5060 for SIP)
Supported codecs Supported codecs (G.711, G.729 etc)
Tech prefix Prefix that is expected to be prepended to any number that is to be called over this interconnection
DTMF mode Mode of transferring DTMF tones

So, what this form holds is essentially:

  • Company name of the partner
  • NOC contact information (used to contact in case of technical problems)
  • Interconnection parameters (needed for the two endpoints to establish an interconnection)
As the filled forms are exchanged, both parties (we and the VoipN) have all the relevant information to configure our equipment in a way that they can successfully communicate and transfer voice traffic.
At this point the NOC teams of both parties configure the equipment according to the parameters in the form sent from the remote party.

Since VoipN is about to terminate the traffic to Nigeria mobile networks, their NOC team has to configure their equipment in a way that the traffic coming from our side gets routed to Nigeria (we don't know their routes, that is what they hold as a business secret, because if we knew where they route the traffic to, we could circumvent VoipN altogether leaving them out of business - I'll dig more into operations security in VoIP business in a separate post).

On our side, our NOC team configures our equipment so that all the traffic to Nigeria mobile networks gets routed to the VoipN trunk (the newly established interconnection with VoipN).

As both sides have configured their equipment, we want to test the interconnection. What we do is initiate a call to a number from Nigeria mobile network via VoipN trunk. We can do it manually (which is usually a first step), but we can also do it automatically, using an automated VoIP route testing tool (take a look at RouteInspector for an example of a specialized tool that we provide to VoIP carriers worldwide. It is an essential tool when debugging a SIP interconnection problems - more on this in a separate post).

When we conclude that that the interconnection is working, that the calls are of agreed quality etc. we start sending live voice traffic (originated from our customers) to VoipN.

If everything is fine, there is no need to any actions from NOC teams from any side. However, that is not the case all the time, and as the problems occur, the NOC teams get in contact and try to resolve the problem so the traffic can continue. What can go wrong? Many things actually. I'll cover some of the interconnection problems and debugging processes in a separate post.

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