There has been much talk about the quad-play being the most likely candidate for cable’s attention after voice over Internet protocol (VoIP). There’s little doubt that seamless mobility is on cable’s horizon, but there are many factors that influence how far ahead that horizon lies. Seamless mobility is a telephony application that we can do either quickly and easily or elegantly—and, as usual, there are tradeoffs. The point of this month’s column is to get a bird’s-eye view of our possible routes to this application and be able to use that vision to estimate how near we are to what we see. Motivations Let’s first briefly review the market factors that are driving us to this destination. From a competitive standpoint, we are being drawn into this fourth dimension of communications because incumbent carriers, in particular the telcos, are finding ways to offer video and data services to complement already established landline and cellular offerings. If they succeed in developing services that the market perceives as competitive to our video and data offerings, cable would be at a disadvantage without a mobility offering. Apart from the competitive factor, there are a lot of consumer-based reasons to enter this business. Letting your calls find you at the most efficient call completion point has advantages for which consumers will pay. There is the elimination of extra cellular charges if you can conveniently use landline transmission. Then, there’s the convenience of one reach number. (How many of us have the tag line on our voice mail: “If you need to reach me sooner, my cellular number is ….?”) Speaking of voice mail, it would be nice to have only one number to call for messages, wouldn’t it? Finally, I can think of several occasions where I have begun a conference call on my office phone, only to have to re-originate on a cellular connection because I have to leave the office before my virtual meeting has ended. Implementations We have motivation to bring an offering to market. Now let’s look at technical implementations. I am indebted to Jay Strater, Motorola senior systems engineer, for a talk he delivered at the National Show that details much of this technical material. As it stands, there are three possibilities for integrating cellular service with a PacketCable-based telephony offering: call forwarding, unlicensed mobile access (UMA) and IP multimedia subsystem (IMS). All depend upon subscribers having a dual mode (cellular and wireless) session initiation protocol (SIP)-based telephone and some type of customer premises network. For a residence, that usually is called a home network. Call forwarding is the quick and easy approach, but still requires significant development. It is also the least efficient. This technology depends upon a modified call management server (CMS) that includes protocol interfaces for mobile connectivity as well as the usual network call signaling (NCS)-based call processing features. As a seamless mobile customer, you are assigned two phone numbers, one for the landline phone and one for the cellular connection. Only the landline number is published, to ensure all calls will be processed by the CMS. An incoming call generates a sequence that identifies if your phone is within the wireless local area network (WLAN) of your home network and, if so, rings the landline network connection. If the phone is outside the LAN range, the CMS forwards the call to the cellular connection. The obvious inefficiency is that call forwarding inherently requires routing a call via the decision point, in this case the CMS, before establishing the connection. If a cellular network is the destination, CMS resources are committed unnecessarily, routing is inefficient and delays are added to call processing. These disadvantages need to be weighed against a quicker time to market than alternative implementations. You may remember that we used a similar strategy and technology to deliver number portability in early telephony offerings. The UMA and IMS solutions are more elegant, but also more complex. The UMA approach assumes the cable operator has a cellular global system for mobile communications (GSM) infrastructure, either by ownership or partnership. It includes the addition of a UMA network controller (UNC) to the GSM network and depends upon the creation of IPSec tunnels to carry GSM and general packet radio service (GPRS) signaling and real time protocol (RTP) and IP data traffic between a dual mode handset and the UNC when the handset is in a WLAN. The obvious drawback is that operators without a GSM infrastructure would have to commit to a major investment in that technology. Less obvious is the number of protocol issues to be resolved for the tunnel architecture to be workable. The IMS solution is an overlay architecture atop a PacketCable-compliant network. It creates a new third generation partnership project (3GPP) IMS within the MSO network, which includes a service agnostic IP core network. The IMS core would appear to a cellular mobile switching center (MSC) as another MSC. With IMS technology, VoIP is provided via SIP signaling and RTP traffic between a dual-mode handset and call control elements in an IMS core network when the dual-mode handset is in a WLAN. The same applies if a dual-mode handset is in a 3G cellular network that supports VoIP over its data network. When a handset is in a circuit-switched cellular network, circuit-switched signaling and voice are converted to SIP signaling and RTP at an IMS signaling and media gateway, respectively. IMS does not use existing CMSs and media gateways for VoIP-based landline phone service, and the overlay IP core network will be a major change to the operator’s PacketCable infrastructure. Depending on the way PacketCable was implemented, this could range from software redesign to forklift upgrade. Premises issues In addition to the network-based aspects of the technology that we just covered, we also need to consider two vital customer premises issues. Seamless mobility depends upon having a WLAN at the customer premises, which in turn depends upon at least a rudimentary home network. Although cable is beginning to penetrate this market, home networks are by no means ubiquitous (one of my favorite legacy telephony words, meaning “it’s everywhere!”). Furthermore, there are several nontrivial issues to be solved associated with dual-mode handsets. Power consumption is a big issue, especially given that there are two radio receivers in each unit. Large-scale integration that allows those two receivers to be placed on one chip is another. These both tie to unit cost, which will be borne by either the operator or the consumer directly. Dual-mode handset and landline interworking is expected to be the subject for Phase 2 of the PacketCable 2.0 cellular integration initiative. The bottom line for seamless mobility is that we have compelling reasons to create an offering, but are still in the early stages of technology. Most likely, this means we will see markets opening similarly to the way cable telephony was introduced—strategic trials and early offerings where competition is high in the next year or two, but at least three to five years before widespread availability across multiple MSOs Justin J. Junkus is president of KnowledgeLink and telephony editor for Communications Technology. Reach him at email@example.com. Did this article help you? Email comments to firstname.lastname@example.org.