Features offered by SIP Service Providers?!
As you probably know that there are many VoIP Service Providers. Whether they offer free service or charge a monthly fee they claim they support a long list of features. As somebody who wants to understand SIP architecture, we must evaluate how these services are offered. Today’s entry is an attempt at analyzing some of the features. (Even if you are a consumer, but otherwise uninterested, you are better off reading the full analysis.)
Call waiting: Since SIP is an “out-of-band” message oriented signaling protocol, it is simpler for the SIP Proxy to deliver the Invite messages without analyzing the state of the terminal. That means, the call waiting service is offered by the user and not the service provider. Many of them claim that call waiting is included in the monthly fee. This is not truth in advertising.Transfer: This is another feature that is implemented in the user terminal; so it is not clear what is the role of the service provider.Caller ID: The SIP Invite message carries the information of the caller; but this field is populated by the user. It is not clear whether the information appearing on the Caller ID display device is based on the original information or whether the Service Provider validates this information. If it is the latter, then the claim is justified; otherwise the claim needs to be diluted.Caller ID Blocking: In PSTN, it is a simple matter to block the delivery of caller ID information. But in VoIP, it is not sufficient to block this information in the signaling domain. The RTP packets carry the source IP address which may need to be masked as well. That means there is a need for anonymizer function.
One can continue this analysis down the list. The service provider is needed for a small set of functions like advertising reach information, NAT traversal, 3-way/multiway conferencing. But in the rest of the cases there role is superfluous; worse the claims may be misleading.
Posted by aswath at January 13, 2004 05:46 PM
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