January 05, 2004

Will there be a “middle” in VoIP

I am sure you have heard that unlike PSTN, in an IP network, everything takes place at the edges. Indeed it has become an old saw. This is true theoretically; but it is seldom true when VoIP is involved. Almost all the providers (whether they use MGCP, SIP or proprietary architectures) place themselves in the middle (of both the signaling and the voice flow) between the two end-points. It is natural that service providers need to process and act on the signaling messages. But why the media flow? MGCP architecture requires the media flow go through the Call Agent to handle supplementary services. Many SIP providers require media flow through their Border Session Controllers to address many issues, NAT traversal being the chief among them. Even Skype, which is billed a peer-to-peer system, uses such an intermediary node. These intermediary nodes may be at the edge from the Network layer point of view; but it is certainly in the “middle” from the application point of view.

So what is wrong with having a “middle”? In the case of PSTN, it is not easy to change the service provider because 1) the point of connection to the “middle” is the network address of the user and 2) it is an elaborate and expensive process for alternate service providers to locate this “middle” node. On the other hand, in an IP network, the second point does not hold. Even though the network address and the “middle” node have no relationship, a service provider can use the application layer address as a vehicle to retain a subscriber. This is happening in the email service domain. Fortunately, there are ways for a user to overcome this and have user specific SIP address and have it mapped to service provider specific address, as long as calls are made in the native mode. For PSTN networking, we must ensure that users can carry their PSTN numbers from one service provider to another – that is Local Number Portability must be applicable to VoIP providers as well.

Posted by aswath at January 5, 2004 12:04 AM
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