Lady using older style telephone

Keeping the phones ringing

The advent of next-generation access networks is changing the way our phones work. We talk to the people in charge of keeping them ringing.

It seems so Easy. You pick up the phone, hear a dial-tone and dial your number. But the apparent simplicity of a dial-tone belies the complexity of the global telephony network that ensures we can call around the world without giving it a second thought.

That network, however, is set to become more complex as our phone connections evolve from a simple pair of copper wires into being just one of a number of services on a high-bandwidth next-generation access (NGA) network that may be delivered over traditional copper lines, fibre or even RF links.

The shift to NGA marks another chapter in the migration of the phone network away from today's operating metaphor ' a'circuit-switched connection between you and your caller, connected over lines multiplexed in time and controlled by Signalling System 7. In the new world of NGA, existing handsets plug in to ports on new customer premises equipment (CPE) that mimics the old network's electrical characteristics but carries the call as a series of data packets flowing over a non-deterministic IP network. New handsets create their own IP packets, and use the NGA connection to send them on their way to the rest of the network.

As if making all this work wasn't enough, the companies paying to bring NGA networks to our homes naturally want to make a return on their investment as quickly as possible, while the telecoms regulator Ofcom wants to ensure that the market for services remains at least as free as it is at the moment, despite NGA networks bringing a fresh set of technical challenges to open access.

It's up to NICC, the technical forum for interoperability standards, to tackle these issues, filling in gaps where international standardisation doesn't cover UK issues, as well as creating standards that meet the needs of the regulators as the UK makes the transition to NGA.

The challenge

According to Chris Gallon, head of systems at Fujitsu Telecommunications Europe, telephony can be delivered over NGA in two main ways.

If the bearer is still a copper phone-line and the call is carried in the baseband, companies such as BT are using multi-service access nodes and trunking gateways in their networks to convert the traditional voice connection into IP. This allows voice to use the same high-bandwidth links as the broadband traffic ' job done.

New access networks, such as fibre to the home (FTTH), don't have that copper baseband connection, so the CPE has to do a conversion between the customer's analogue telephones and the IP network into the customer's home. It's at this point that some of the openness enjoyed by users of the existing network comes under threat: unless third-party voice providers can access and control this 'analogue telephony adapter' (ATA) from their own call servers, they have to rely upon the NGA providers to manage calls between the home and the network.

According to Paul Rosbotham, manager of technology regulation at Cable&Wireless Worldwide and director of NICC, the issue mimics that of the introduction of broadband services in the UK. At first BT provided the broadband connection between the home and its exchanges, and third-party providers had to resell what BT offered.

'It was only really when the local-loop unbundling players came in that the prices started to tumble and the bit-rates started to rise,' he says.

The situation with NGA voice is similar ' third parties want to rent as little as possible from the NGA provider yet offer their own services and achieve their own economies of scale.

'The architecture that seems to work well to minimise the impact on the user is to use ATAs in the customer premises to present what looks like normal telephony to the customer,' says Rosbotham. 'But if you are to have competitive provision then your chosen provider needs to be able to control that ATA.'

Call-server standard

So NICC has had to come up with a standard way of addressing these ATAs so that it is easy for third parties to control them from their own call-servers.

Gallon says this has been done by defining a profile (or version) of the IETF's Session Initiation Protocol, a SIP user network interface (SIP UNI) tailored to controlling the voice call between the home and the first call-server in the rest of the network.

'It has to be fit for the UK and agreed upon by everyone,' he said. 'It is difficult, because there are many call-server makers, many call-server architectures and many potential operators, and they would all like to use their own protocols and solutions.

'So NICC has taken on the role of identifying the preferred behaviour of this interface, and then mapping it into the SIP UNI to enable, for example, telephony over fibre, in a way that operators can use their existing call servers.

'A lot of work has been about agreeing which parts of all the international options will be applied in the UK.'

Having specified a standard interface, the next step is to standardise the way that third parties order the service from NGA providers, work which is underway.

'What NICC has defined is the SIP UNI spec, which is fully interoperable, and, if it is implemented, the voice provider will know that its call-server can control the ATA to support a UK telephony service,' says Gallon. 'What isn't bottomed out yet is all the bits and bytes of the provisioning and management interface.'

Equipment compatibility

NICC is doing other work to ensure that UK phones will keep ringing, including developing a document outlining potential equipment compatibility issues. It covers delay issues, jitter buffers, echo cancellation, voice codecs, voiceband data and DTMF issues.

According to Trevor Linney, chairman of the NICC QoS group, the idea is to to help equipment makers and operators understand how to move from legacy networks to next-generation networks (NGNs) using multiservice access nodes or NGA networks, where traditional services are being emulated over IP.

'The guidance document shows where the sticking points are going to be and highlights the range of devices that will be affected,' he says.

'We're trying to enable manufacturers to design future devices with an understanding of the environment in which they will have to operate, especially when most of their customers are on NGN, if and when the option to be connected to the old network is no longer available.'

There are various ways in which compatibility issues might emerge, many due to the use of a packet-based network.

'A good example is the way that voiceband modems work,' said Linney. 'On the new network, equipment has dejitter buffers to remove any variations in delay (jitter) from the signal. For voice calls, these buffers are adaptive, which is fine, but if the buffer adapts during a voice (dial-up) modem call, the modem will see a gap in transmission, think it is a loss of data and drop the link.

'This kind of adaptation wasn't needed over the TDM network but becomes a problem over NGN, so we define a fixed dejitter-buffer length to take care of the issue.'

Similar guidance has been developed to tackle echo cancellation issues.

Who's calling?

NICC has also been revisiting its work on caller line identification (CLI), the system that pops up a caller's number in your phone's display, in the light of the shift to NGN.

'CLI is a good example where the international standards and regulatory conditions didn't go far enough,' says Rosbotham. 'The terms of service from Ofcom say 'provide CLI', and give guidance about the existence of services such as 1471, but remains technology-neutral about how this should be achieved.'

A revision of NICC's 'ND1016' document now outlines the detailed rules for providing CLI information on each provider in the call path. This issue is about to become more complex, as IP-based telephony means that there will be more calls in which the 'network number' from which they are made is not the same as the 'presentation number' shown on the recipient's phone.

The NICC document also sets the requirements that the various network protocols need to fulfil to enable providers to meet the rules.

'What we have done is renew the CLI guidelines last published in 2003,' said Rosbotham. 'There was some weird stuff knocking about in the old version of 1016 which meant that CLI didn't work as well as it should.'

Next steps

As the introduction of NGA accelerates, NICC will have more work to do to turn its promise into a reality. According to Rosbotham, there are details to be mopped up in the SIP UNI work, as well as work on connecting IP PBXs to the network in a standardised way, and on a SIP profile to define a basic network to network interface.

'There's an argument there that VoIP operators are connecting to each other already, so why have an NICC spec?' says Rosbotham. 'The answer is that because there isn't a specific UK standard they are having to do a lot of ad hoc work.'

Gallon takes a wider view of NICC's work: 'As next generation networks start to deploy, NICC will be involved where a problem is identified that needs a standardised solution. We will put the landscape together of what it all does. Where we find gaps in standards and capabilities we will work to close those gaps.' *

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