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November 2008 | Volume 11/ Number 11
Packet Voice Over Wireless

Can VoIP Sound Better than POTS?

Start pulling on this thread and you unravel some of the most controversial issues in telecom today. First, there is no question that theoretically VoIP can sound better than POTS because you can use wideband codecs (what Polycom (News - Alert) calls HD Audio). On the other hand, when audio packets are delayed or lost, the call quality deteriorates radically. Circuit switched calls almost never suffer from delay or loss, but IP networks are prone to both.

In wired IP networks, packet loss and delay are mainly caused by congestion. The numerous ways to avoid delay and loss fall into four main categories: over-provisioning, packet re-routing, packet prioritization and bandwidth reservation.

All routers are congested at times. If congestion occurs too frequently, the router’s owner increases its bandwidth. Over-provisioning provides enough headroom that a link or router is congested only very rarely. For IP networks over-provisioning has historically been the cheapest way to reduce packet loss and delay.

VoIP service providers are able to monitor their networks in real-time, and to re-route traffic around congested routers. This technique doesn’t work on the subscriber’s access link because only large organizations have redundant Internet access.

The access link is a bandwidth bottleneck between the LAN and the WAN. Since some types of data traffic can easily saturate the access link (BitTorrent (News - Alert), for example), VoIP quality can plummet. This can be fixed by packet prioritization, also known as traffic shaping.

The main protocol associated with packet prioritization on IP networks is Diffserv. The idea here is to tag (News - Alert) packets with a priority, and for the routers to prioritize them accordingly. The problem with this is that on the Internet packets traverse networks owned by different people, and they are reluctant to trust priorities assigned by third parties, so the tags are normally ignored, and packets prioritized on other criteria.

What about the controversies I mentioned earlier? There are two. First, a torrent-based company called Vuze has complained to the FCC (News - Alert) about over-aggressive traffic shaping practices by Comcast and others, since they violate the principle of network neutrality.

A second related controversy concerns the bandwidth reservation features of IMS (IP Multimedia Subsystem (News - Alert)), the omnibus architecture intended to provide the services of the future to cellular and wireline networks. There isn’t room here to describe the controversy, but you will find a good explanation if you Google (News - Alert) “waclawsky ims critique.”

The bottom line is that VoIP can vastly outperform circuit switched call quality, using only best-efforts techniques. IT

Michael Stanford (News - Alert) has been an entrepreneur and strategist in Voice-over-IP for over a decade.

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