SIP-Based Push-to-Talk Moving Into Enterprise Territory

When Nextel Communications introduced cellular-based, walkie-talkie communications with its Direct Connect service, it revolutionized consumer telephony. Now the Open Mobile Alliance (OMA) is standardizing that same technology for enterprise telephony with PoC (Push-to-Talk over Cellular), a specification providing Direct Connect's always-on service across SIP-based, wireless platforms.

With PoC deployed, field personnel and mobile teams will have a more efficient medium for short interactions. It provides the same sort of benefit beyond conventional voice conversation that IM offers compared with e-mail. However, while iDEN (the Motorola-developed technology on which Direct Connect is based) uses bandwidth normally allocated to telephone calls, causing capacity and cost problems for carriers, PoC will be more efficient by using the data side of the wireless network.

The OMA has the backing of big telecom players, as well as some cell carriers, and it's reasonable to believe that enterprise adoption of PoC isn't too far away. Still, PoC must gain industry traction, which would be accomplished most quickly with greater cooperation from carriers.

Genesis Of PTT

For quick, single-question-and-answer conversations, a conventional phone call takes too long. Playing voice-mail phone tag makes it all the more tedious.

In contrast, PTT functions like a walkie-talkie, with an immediate response. All communication is carried out in a one-at-a-time (half-duplex) manner, in which one person talks while the other listens. Although not as fluid as a full-duplex telephone conversation, this model of communication is fine for a quick dialog. But Nextel's PTT service has some fundamental pitfalls--iDEN's circuit-switched technology requires that PTT conversations share a portion of the cell node's precious telephone bandwidth used for regular phone calls. And enterprise phone system integration is a distant dream; Nextel's service lets customers reach only fellow Nextel customers.

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Enter SIP PTTThe OMA's PoC efforts will enable the use of SIP as the signaling protocol over cellular data networks. The OMA believes that with the increasing bandwidth offered by next-generation cellular, all PTT communication can take place on the data side of the cellular network, without impacting any voice bandwidth. That approach clearly distinguishes PoC from iDEN or Kodiak Networks' Real-Time Exchange, another circuit-switched PTT technology. The OMA explicitly mentions supporting cellular networks developed by the 3GPP and 3GPP2 industry groups, which provide the 3G specs GSM and CDMA, respectively.

Major mobile telecommunications players, including Motorola, Nokia, Siemens and Sony Ericsson, along with a handful of domestic and international cell carriers, such as AT&T Wireless, are backing OMA.

The PoC standard extends the SIP framework to facilitate PTT communication. SIP messages are used end-to-end for session dialog between all devices and servers, with some new procedures added to the SIP stack that are relevant to the handling of wireless SIP UAs (User Agents). Unlike a standard SIP UA, in which other UAs or proxies communicate directly with the end device, PoC introduces a server that caches the status and settings of the wireless device. When a wireless UA joins the network or changes its status, it updates the PoC server using "publish" and "notify" messages. When another device wants to contact this particular UA, it checks with the PoC server to determine if the wireless device is ready to receive PTT requests, then proceeds accordingly. This process effectively prevents unnecessary wireless bandwidth use.

Status, supported natively in SIP as "presence," is important to PTT technology. Through presence information, the phones and network know if a user is available.

Stack IT On Enterprise SIP

PTT technology is not too far from enterprise adoption, though the most obvious and immediate implementers of PoC technology will be wireless carriers with a large subscriber base. As SIP becomes de facto for enterprise voice and IMS (IP Multimedia Subsystems), layering the added service of PTT is the logical next step. Because SIP is an IP-routed protocol, an enterprise needn't develop a 3G cellular network to provide this service. Instead, it can use mobile carrier data infrastructures to route VoIP and SIP traffic to its IMS system. Whether mobile carriers will be agreeable to this remains to be seen.

Nonetheless, even if 3G carriers refuse to open the doors to enterprises wanting to use their networks to carry out PTT capabilities, all hope is not lost. Since one of the primary objectives of the PoC development effort is to be network-agnostic, generally available wireless technologies, such as Wi-Fi, can be used.

Despite the optimistic outlook for PTT, three years after being dubbed the "killer app" within the SIP community, its adoption is running at a slow pace. But it's important to remember that PoC 1.0 was just released last year, and the OMA is working on the second version. The new spec promises improved interoperability not only between enterprises and carriers using the PoC standard, but also with those using other PTT technologies. Nevertheless, keep your speed dials and ringtones fresh--any form of enterprise-ready SIP PTT is still at least a year or two away.

Matt Vlasach is CEO of Pacific Swell Networks. Write to him at [email protected].0