... part II

Network management

This portion of the spec centers upon the SNMP (Simple Network Management Protocol)-based information received at the headend from cable modems in the form of Management Information Base (MIB) data. This data is used to measure the reliability and speed of a network. A draft of the spec is due in July.

Residing "on top" of the core services layer of the PacketCable specification architecture is an application layer, comprised of:

  • Audio/video codecs
  • Distributed call signaling
  • Network-based call signaling
  • Public Switched Telephone Network (PSTN) interconnection.
Moving beyond H.323

Perhaps the most controversial, and misunderstood, aspect of the entire PacketCable spec has centered around the network-based call signaling model that will be used. Initially, H.323, the International Telecommunication Union set of standards for low-latency videoconferencing, was the choice, but only for lack of a protocol designed specifically for carrier-class, commercial IP telephony.


The H.323 standard, says Bukovinsky, focused on toll bypass and was the protocol enabling inexpensive, IP-based international calls-perhaps the first commercial application of IP telephony.

Yet cable operators, says Bukovinsky, "are the first service providers that are in a position to start looking at local telephone service." H.323 didn't address that type of service because there wasn't a provider in a position to deploy local service. Local service over a cable plant required a new protocol that would facilitate all calling features, such as call waiting and voice mail, so that cable operators could effectively compete with incumbent regional Bell operating companies.

Originally developed for an enterprise network, H.323 over the last four years was being "morphed into a suite of protocols that will support the requirements of a (local) service provider," says Bukovinsky. But he says this was like "trying to take a VW and push it into the Indy 500." Consequently, a new protocol was needed to address the "next generation" of telephony services.

Along came the Simple Gateway Control Protocol (SGCP), developed jointly by Cisco and Bellcore (now known as Telcordia) and IP Device Control (IPDC), which was developed by a team of vendors, led by Level 3. Both efforts, according to Bukovinsky, were "essentially targeting the same space," i.e., how to control PSTN and residential gateway devices.

Consequently, the two standards were merged, and the Media Gateway Control Protocol was born. One functionality gained by adding IPDC to SGCP, according to Bukovinsky, was a scripting capability. Iteration 1.2 of the MGCP protocol is currently CableLabs' choice for network-based call signaling.

Because it's a leaner protocol than H.323, MGCP allows for quicker product development, says Bukovinsky. "You can literally have a prototype up and running in a month, and a commercial product in six months." To prove his point, Bukovinsky points out that in July, at least four, and perhaps as many as six, vendors will introduce call agents developed around the MGCP-based approach. He also notes that four to six vendors are making DOCSIS-based client devices that also support the MGCP approach.

Why is the decision about call signaling so important? Because it's the heart of the telephony service. Network-based call signaling describes a protocol for the call agent, or call management server, to communicate with the client or phone. For example, the call agent must recognize events such as the client going off-hook. The agent must then provide dial tone.

The call agent is a crucial aspect of the IP telephony model because it allows for intelligence to be distributed throughout the network, rather than resident within a specific device. The H.323 spec, in contrast, relies upon client-based intelligence, which makes those devices more expensive, and therefore, less desirable.

Distributed call signaling

Based upon AT&T Labs' Distributed Open Signaling Architecture (DOSA), this application addresses "end point intelligence" and the relationship between the gate controller and the client.

DOSA protocols, says Bukovinsky, are actually being used for call signaling, dynamic QoS and security components of PacketCable. With respect to call signaling, DOSA will be implemented, most likely, by a session-initiated protocol (SIP). The distributed call signaling application would essentially co-exist with network-based call signaling, thereby giving cable MSOs a choice of signaling schemes.

A network-based approach would allow for more control over what services are offered, as well as control over the introduction of new services. Distributed call signaling "pushes the implementation of features out to client devices," says Bukovinsky. Each cable operator's business plan for telephony will dictate which method will likely be deployed.

PSTN interconnection

This application controls the various interfaces to the PSTN, including trunk circuits and the SS7 signaling network. In addition, it determines how signaling and media information is conveyed through the PSTN gateway and back into the packet side of the network. A draft is due at the end of the second quarter.

Audio/video codecs

Codecs are needed for analog-to-digital translations, and voice quality, bandwidth utilization and redundancy are the three factors that influence codec selection. H.263 has been selected as the video codec, while G.711, G.726 and G.729 (A and E) were selected for audio codecs. G.711 uses a 64 kbps bit rate, while G.726 and G.729 use much lower bit rates, between 8 and 16 kbps. Switching to a lower bandwidth codec will allow for the multiplexing of more calls on the network.

No stickers

As various pieces of the PacketCable spec are completed, says Bukovinsky, CableLabs will hold interoperability events over its Virtual Private Network. These events will commence in July and run into next year. However, while CableLabs will announce which manufacturers' equipment it will be testing for PacketCable compliance, it will not be actually certifying or announcing whether or not equipment passes interoperability tests.

The fruits of PacketCable may even be realized before the spec is finally completed in August. Videotron, according to Francois Laflemme, vice president of IP telephony, will begin a field trial of IP-based telephone service either this month or next, with another trial planned this fall. Full-blown service is scheduled to be introduced early next year when Videotron sets its sights on the 2 million-plus homes its network passes in Quebec.

Laflemme noted that Videotron will be using equipment from Cisco and Telcordia, two companies deeply involved with writing the PacketCable spec. Laflemme pointed out that the company is "quite active" with PacketCable, but moved forward in selecting back office functions, such as activation and billing, in advance of the spec.

Fellow Canadian operator Cogeco plans to offer VoIP services in Ontario at an unspecified time this year, according to Denis Chartier, director of multimedia and telecommunications services for Cogeco. While Chartier declined to describe what features will be included in the rollout, he says, "Our end goal is to get on the market (with) a new set of (telephony) features." Cogeco will be offering the service in areas where it's offering Internet access service.

The two Canadian telephony efforts will effectively serve to put the basic principles of PacketCable to a real-world test. If all goes well, it won't be long before consumers might be dialing zero and saying, "Hello, (cable) operator?"