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UK start up develops way of ensuring service quality on congested networks

Packet based networking has become the standard way to deliver communication services. But the very success of packet networking is challenging telecom operators in how they support the varied requirements of the services carried over their networks. Indeed, some industry voices question whether the internet economy is sustainable, given the rate at which IP traffic is growing.

UK start up Predictable Network Solutions has developed technology that it claims ensures the performance of applications in the face of network congestion. The approach promises to save operators money by enabling them to load their networks more efficiently. It also promises operators new revenues by enabling them to provide – and charge for – services delivered with real time assurance.

"This [solution] came about by asking questions," said Neil Davies, cofounder of Predictable Network Solutions (PNS). "How do you ensure applications when networks are saturated? And how do you make sure money is made from networks in the long term?"

According to Davies, operators tackle traffic load by adding excessive network capacity – 'throwing bandwidth at the problem'. However, this has a knock on effect on the delay, and even the dropping of IP packets over the network. And it is packet delay and loss that dictate a service's performance, especially when the packets carry voice or video data.

"Managing bandwidth to manage traffic load establishes a relationship between loss and delay and operators do not manage [these parameters]," said Davies. In fact, he says that, by managing bandwidth, packet loss and delay are made worse, causing quality to degrade. Instead, PNS' approach is to control quality degradation, what it calls quality attenuation. "Quality attenuation is the combined effect of [packet] loss, delay and jitter," said Davies (see fig 1).

Quality attenuation comprises three elements: the distance between the transmit and receive points; packet size – bigger packets take longer; and the effect of other traffic streams on the transmitted one. The distance between end points and packet size are largely fixed and dictated by the technology. What can be controlled is contention, says Davies.

There are certain points in the network where a packet stream encounters congestion along with other streams. This is a consequence of the way packet networks work: when more traffic is present at a node than can be sent, traffic streams are queued and prioritised at the switching or routing node before being sent on.

PNS' technology manages the contention a stream will experience before the traffic even enters the network. "We apply the policies all at one point," said Davies. "We do that based on what loss and delay characteristics you want for your traffic to achieve an outcome."

Clearly, unless the technology is implemented at all the network's entry points, a stream will encounter traffic that has not been so treated. But that is not an issue, so long as the network has predictable, sustainable quality attenuation properties. And operators' networks largely have such properties.

The company characterises a network in terms of overall loss/ delay. Taking that into account and controlling the congestion for each stream it processes, PNS can control quality attenuation.

Using two ADSL modems over BT's network, for example, the end to end delay budget for voice is 150ms. "You need 60% [of the budget] just to get out of the premises, 30% to get into the premises at the other end, and 10% across the core network," said Davies. As long as the ADSL modem does not overdrive the network, the BT network will not try to shape – or rate limit – the stream. And it is such traffic shaping that introduces undue delay; enough to destroy VoIP call quality.

"We avoid any traffic management policies being imposed [on the stream by the network provider] by preshaping the traffic," said Davies.

"We've found the core is predictable; there is a bound on the delay, jitter and loss that it introduces to packet streams flowing across it," he said. "However, this only applies if you stay within the 'predictable region of operation' for that infrastructure. For an ADSL operator, it means avoiding a rate limiter in the broadband remote access server; for other operators, it means avoiding falling foul of their heavy user management system. We make sure the composite traffic stream has that property."

Network topologies tend to be tree like, he says, such that if the properties of the contention points of the paths to the destination are known, all that shaping can be brought back and used before the packets are introduced to the network (see fig 2). "They are spaced in such a way that they never contend with themselves down the line," said Davies.

But that requires contention to be worked out and broken down, based on the individual links. "You don't have to solve this end to end; you can solve it hop by hop," said Davies, "and this is relatively straightforward."

Operators' networks have a well understood, relatively fixed configuration and, while there may be redundant paths, networks don't change very often. "[The nature of] BT's network hasn't changed in six years," said Davies.

For the ADSL example, by using PNS' traffic preshaping, the upstream's traffic's variability is limited to less than 5ms of the 15 to 20ms budget.

PNS performs traffic preshaping using a low cost scheduling algorithm implemented in software. Hardware acceleration, using an fpga, is only needed for data rates in excess of 10Gbit/s. "This is an edge device," said Davies. "We also have a device – a contention manager – that sits in the centre of the network."

Because the network topology being connected to is known, the biggest streams the contention manager must process are some 200Mbit/s. "You can have a rack of contention managers, each taking a portion of the [routing table's] traffic," said Davies. "It forces all the contention to occur at the contention manager, which then manages it."

PNS has developed a suite of software tools to measure a network's quality attenuation performance. It is providing consultancy to operators, large enterprises and government agencies to assess their networks' performance. BT has evaluated the technology and is using it in trials for specialist services.

Davies admits that it has been hard to sell the concept to operators, partly because they believe their networks are working fine and delivering quality service. "People don't believe they have a problem, although they do," he said.

But what Davies says the technology has shown is that operators are continually upgrading their networks prematurely. "We are helping companies by providing measurements that show they are running their networks at very low utilisations and upgrading very early, otherwise they can't cope with the operational problems they incur."

Tackling quality attenuation is also a must as consumers connect more devices – such as femtocells – to their broadband. "I can make small cells work over ADSL," Davies concluded. "I've got one over the top of the house serving the village."

Roy Rubenstein

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