Cisco: Telephony's Touchstone?
Excerpt from article titled "Make the Right Call: Business Phone System Roundup", in the April 2002 issue of Communications Convergence Magazine, a CMP Media publication. Visit Communications Convergence online at www.cconvergence.com.
If you want to talk about pure IP convergence, Cisco's AVVID (Architecture for Voice, Video, and Integrated Data) is what the industry's been talking about all along: smart IP phones plugged into an Ethernet network full of conventional switches and routers. Tracking the voice streams is a server (CallManager) running on a Windows 2000 server.
Cisco's focus is on the large enterprise; and they've had some success in this arena. Cisco's shipped over 500,000 IP phones and 6 million-plus VoIP ports, making Cisco No. 1 in VoIP market share by almost any measure. But the talk on the street from consultants: Plenty of issues need to be ironed out. Some are implementation issues. For instance, fax and modem calls on analog lines within an AVVID system didn't work early on and now can't be faster than roughly 33 Kpbs. And system designers have complained that adding analog lines to an AVVID installation is needlessly expensive. Users in situations where phones get greater than average abuse (eg., hospital emergency rooms) have found the 79XX series too delicate, considering that replacement is a several-hundred-dollar proposition.
Other concerns stem from the core issue of converged telephony: You need effective QoS to keep email and websurfing data out of your way. Companies that fail to build a sufficiently robust network are rewarded with sub-par phone service.
The key to AVVID QoS is Layer 2 Class of Service (CoS) settings (defined as part of the 802.1Q spec), which call for the first three bits in the same tag used for VLAN ID to be set somewhere between 0 and 7. This is the mechanism in the field today - it's transitional, though, and will migrate to the similar prioritizing bits of the Type of Service byte in the IPv4 packet header (the benefit: this will be Layer 3 processing and capable of finer-grained packet processing).
Within the current Layer 2 scheme, Cisco expects voice packets to be set as Class 5 and signaling as Class 3. Conventional data traffic tends to be set to 0 (otherwise ASICs onboard Cisco's phones can reset the TOS bits on packets originating from PCs daisy-chained to the phone's built-in switch). Typically, the switches support two packet queues, one for all priority (non-zero CoS) packets and one for normal data traffic. The voice-bearing priority queue gets serviced ahead of the data queue.
None of this works if there are too many voice packets on a given network segment. So a fundamental aspect of building an AVVID network is building it with big pipes and low-latency switching equipment. This frequently means hauling in new equipment. Genuinely reliable system design mandates redundant backbones, lots of servers, and appropriate gateways distributed throughout the network. Enterprises that want to take advantage of in-cable power for running their handsets without having to plug each one into a wall socket will need appropriate Catalyst switches.