Session Initiation Protocol, a lightweight and extensible signaling protocol for voice over IP and other applications, is a critical component for voice-over-Wi-Fi deployments because it provides organizations with more hardware alternatives and paves the way for adoption of other technologies for mobile users down the road.
With standard VOIP deployments, companies have ensured interoperability by deploying client devices from vendors whose products are known to work with their central PBX. This strategy doesnt work when it comes to Wi-Fi-enabled phones, however, because currently so few options are available.
As the number of available SIP-enabled devices does increase during the next year, administrators will face another hurdle—user expectations. The wide array of cell phones available today has conditioned users to expect a core set of applications and services from their device of choice, presented with a style reflective of the phones user. Organizations have never had to bridge this kind of gap with standard business-class desktop phones, but voice-over-Wi-Fi adopters may find it difficult to gain user buy-in.
SIP is poised to help companies cure some of these ills, but its not a wonder drug yet.
When purchasing SIP-enabled phones, companies may have to forgo certain advanced telephony features that might be accessible through a proprietary solution.
However, SIP allows companies to investigate products from a variety of vendors, with the expectation that the phone will interoperate with whatever SIP-compliant PBX is in use and will provide a core set of features, including dial tone, send and receive, caller ID, redial, transfer, and conference. Phone vendors provide a range of features above and beyond these basic capabilities, although availability may depend on the customers central voice implementation.
Because SIP is a signaling protocol responsible for call setup and teardown—but not the actual content delivery—SIP registration servers and proxy servers stay out of the content delivery path, as media is delivered in a peer-to-peer fashion. Therefore, SIP can be quite extensible and used for services beyond voice.
RFC 3586 describes technology that would, within the SIP framework, establish user presence capabilities, signifying not only the availability but the willingness of a user to communicate on a particular device across a range of potential applications. Instant messaging, MMS (Multimedia Messaging Service) and push to talk are among a host of applications that can leverage SIP.
As dual-mode phones integrating cellular technologies with Wi-Fi radios become more prevalent, SIP use will also ease roaming between cellular and Wi-Fi networks. This will allow vendors and service providers to concentrate their efforts on improving performance, and mobile users will be able to register with their home SIP server or proxy to announce user presence, no matter which network they connect from.
Gaining buy-in from cellular wireless carriers for dual-mode devices will take some time. Dual-mode phones will gain widest adoption if carriers actively support and sell the devices, but carriers will likely be leery of the prospect of losing airtime minutes on their own networks to Wi-Fi deployments.
Enterprise customers will lead the way because leveraging internal wireless networks for voice communications will allow cellular carriers to provide better coverage indoors for their larger customers. In addition, cellular carriers will be able to leverage the faster connections Wi-Fi confers to provide the other applications and services that SIP will allow, in addition to faster data or video delivery services.