IP Communications

By ROBERT LIU
TMCnet Wireless and Technology Columnist

QoS is a network-centric term used to describe a mechanism (like a set of algorithms) that prioritizes IP packets (web, video, voice etc.) based on the importance of the application. Network administrators tag packet headers with any variety of Type of Service bits in which network elements will then honor to, say, prioritize voice traffic over other application traffic in the event of congestion.

In the video realm, that still holds. But unlike voice, video has a very high and extremely variable packet rate with a much higher average maximum transmission unit. Whereas voice services only deal with 64 kilobits, IP video services require four, six or eight megabits of upstream and downstream bandwidth. And while different video networks may share a common QoS mechanism, performance and end-users’ perceptions of quality (which at the end of the day is the only thing that really matters) may vary drastically purely based on network configurations.

“It’s really how you architecture the network and the platforms you put into the network,” explained David Boland, senior manager of next-generation solutions at Juniper Networks.

Motorola, for example, has just released the results of a limited consumer trial that it conducted in Europe using High Speed Downlink Packet Access (HSDPA) – a next-generation wireless standard that is a successor to GSM-based 3G networks. One of the key findings was that video streaming performance degrades when a relatively modest number of users are active. In fact, as little as four active users are sufficient to cause video streaming to freeze if scheduling priorities are not set properly.

To ensure a mobile broadband user experience Motorola recommends early introduction of Quality of Services features for video. “Motorola's HSDPA experience is industry leading and through the identification of these three guidelines, a result of the real-life trials, we can help operators significantly enhance the consumer experience of HSDPA at launch,” said Raghu Rau, senior vice president, global marketing and strategy, Motorola Networks. “The trials are specifically designed to help build optimal performance HSDPA-enabled networks and to effectively deploy services on those networks.”

Because video packets are packed (if you'll excuse the pun) with more information than voice packets, there's a much greater intolerance to packet loss. For example, while voice packet loss of less than 10 -2 is considered an acceptable rate, video packet loss greater than 10 -6 would result in a poor video experience, according to Cisco Systems David Benham.

“In the case of a video service, the general rule of thumb is they don’t want to have more than one video artifact [pixelization, outage, etc.] every two hours,” Benham explained.

That’s not to say that bandwidth isn’t the major determinant in the QoS equation. It is. However, bandwidth of a pipe only influences performance. It doesn’t determine it just as a user’s experience isn’t determined by the server’s processing power.

“Processor performance is not indicative of network performance,” Boland told IPCommunications.com.

In fact, network configuration requirements differ depending on which video transmission format: point-to-point unicasting such as video-on-demand (VoD); multicasting (which in the IP video world is most analogous to broadcasting); and web video (streaming). Each has its own set of requirements to ensure quality. For the purpose of simplicity, this article will examine the most resource-intensive and stringent of these formats: unicasting. The others along with a fourth format, video conference, will be examined in subsequent articles on the IPcommunications.com site.

“The higher the bandwidth per service, the more congestion points and the more difficult it is to deliver the services with a higher quality of experience,” Boland continued. “Experience is more subjective.”

As opposed a mechanical function such as QoS, the networking professional believes it’s more important to examine network topography and how the hardware is built when dealing with the overall video experience. And in order to ensure the best possible video experience, the optimal approach is to incorporate a service delivery platform (SDP) that includes a Call Admission Control (CAC) and QoS Policy Manager.

This week, Juniper unveiled enhancements to its IPTV platform, which it markets under the brand name, Service Deployment System (SDX). Network Resource Manager is the newest SDX feature enhancement and provides end-to-end, policy-based management of video distribution to ensure efficient, quality delivery of service. The SDX Network Resource Manager performs CAC per video session request to assure the appropriate bandwidth is available to deliver the video stream with the required quality of service as specified by the studio and provider content distribution agreements.

From Cisco's perspective, IP video like unicasting or multicasting is all classified as managed video services. Benham claims Cisco supports well over 10 million subscribers of video services running on Cisco gear. To do that successful, the real trick is akin to insurance -- over-provisioning. Even the best built networks with fail-proof QoS will deliver a faulty experience if demand surges above peak concurrency, the Cisco executive said.

“If you get a handful of users on over peak, you’ll get random loss,” Benham said.

In this instance, the most obvious risk is keeping down total cost of ownership by avoiding overly over-provisioning. As such, Benham believes that service providers will increasingly adopt the RSVP Resource Reservation protocol, a network-control protocol that enables Internet applications to obtain differing qualities of service (QoS) for their data flows.

“VoD companies will start to announce they are implementing the host protocols for RSVP,” Benham said.

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Robert Liu is Executive Editor at TMCnet. Previously, he was Executive Editor at Jupitermedia and has also written for CNN, A&E, Dow Jones and Bloomberg. For more articles, please visit Robert Liu's columnist page.