|
OK, I got your attention. Of course, service level agreements are widely available and that’s one form of quality of service (QoS). But the popular meaning is certain packets get priority over other packets and there is no such prioritization on the Internet backbone and very little anywhere in the Internet.
QoS at the Core of the Internet
Prioritization only matters when links are saturated. Once you get beyond the access network, every link in the Internet - local, regional, national or international - is carrying multiplexed traffic from many, many users. Multiplexing many, bursty flows results in relatively predictable traffic. Traffic volumes vary by time of day, but links don’t saturate, except as a result of poor engineering or other link failures. Either case generates a rapid response from any ISP that expects to remain in business.
In short, except at the edges; i.e., the access network, Internet links may be heavily loaded but are not saturated. Combined with relatively high capacity links, this means typical delay variations are fractions of a millisecond and packet loss is negligible; i.e., best effort is good enough even for low latency applications like voice telephony. Except during major failures, the effect of QoS in the Internet backbone is negligible.
Consumers won’t pay a premium for performance improvements they can’t see! They might be induced to pay for a brand (after all, people pay premium prices for branded water), but as yet no such brand has emerged. And if one does, consumers will be paying for that brand, not for QoS technology per se.
Broadband Access Links
The one place in the public Internet where limited, highly specific QoS measures make sense is at the consumer end of an asymmetric broadband access link. Typical residential DSL connections offer a few megabits per second (or less) to the home, but only a few hundred Kbps from the home to the Internet. Unlike links between core routers, traffic on such access links is very bursty and bursts can saturate the link. If there are no active peer-to-peer applications then, most of the time, little or no traffic flows on most residential broadband connections. Suddenly, someone sends an email with an attached file or photo which saturates the outgoing link for many seconds as several megabytes of data go out at perhaps 250 kilobits (~31 kilobytes) per second.
Typical residential access links are narrow pipes where the cost of purchasing more capacity is prohibitive, if it’s possible at all. On the other hand, we can control the routing policy for what goes out on the link by deploying an appropriate residential router at our end of the broadband access link. As a first approximation, one would like to give priority to VoIP packets (and gamers may want to give priority to specific multi-player games). Simple priority is a good first step, but may not be enough on slow links.
Slow links have an added problem due to large packet serialization delay. VoIP packets are typically less that 150 bytes while a web page or email is typically delivered in ~1500 byte chunks. At 250 Kbps, a 1500 byte packet takes ~50 milliseconds to pass over the link. If a VoIP packet arrives just after a 1500 byte packet has started, it doesn’t matter that the VoIP packet is priority to be sent next, it will have to wait for the current packet to complete. Intermittent 50 ms delays are handled by a jitter buffer at the other end of the VoIP connection, but only at the expense of an additional 50 ms of delay. If the uplink is slower than 250 Kbps, serialization delays are even longer.
Luckily this is one place where priorities work and can be imposed. Indeed most consumer VoIP devices incorporate simple priority and some include the ability to fragment large packets (so as to reduce serialization delays). And, because it’s useful for both VoIP and gaming, this functionality is showing up in popular residential routers from Linksys, Netgear and the like.
Brands Can Command a Premium, But Internet QoS Never Will
Individuals can benefit from simple priority queuing at their end of a broadband access link. But they are not going to pay for benefits they can’t see, so we’re unlikely to see prioritization from ISPs. Operators interested in premium services should focus on branding and perhaps on facilitating simple priority queuing in the access network.
Brough Turner is Senior VP of Technology, CTO and Co-Founder of NMS Communications. (news - alert) For more information, please visit the company online at http://www.nmscommunications.com.
|
