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Preparing for the brave new world of BT’s 21CN
David Groom was at the recent BAPCO roadshow to introduce BT’s 21CN – 21st Century Network – and explain the impact this will have on the emergency response in the future.
There is a huge challenge facing public safety communications officers over the next few years. This much was very clear from the presentation given by David Groom to the BAPCO Roadshow in Reading at the end of October. David is the ex-BT Emergency Planning Policy and Process Manager and his subject was BT’s 21CN.
21CN will replace BT’s existing PSTN network which is now some 25-30 years old, as well as many other services. It was first announced in 2004 and has been trialled in 2006 in South Wales. During 2008 UK-wide migration of customers will start, a process that will cover approximately 15% of customers. By 2009 it is planned that 50% of PSTN lines will have migrated and by the end of the following year the migration of PSTN will be substantially complete.
In planning terms 2010 is not very far away and David Groom is concerned not only that not enough has been done to prepare for the change but, perhaps more importantly, that the people responsible for managing the change within public safety services have not fully understood how their existing communications work, and the impact the new network will have. If they don’t, David Groom fears, they may well find that they are unable to fully understand the impact of the 21CN on their existing facilities and services.
The current PSTN network, how it handles 999 calls and its limitations
999 calls are a priority service and are highly regulated by Ofcom, the purpose of this regulation being to ensure that the network is highly resilient and even in congestion conditions calls get through.
The UK transmission network is built on multiples of 2Mb/s – the basic building blocks of the system. Each system equates to 32 channels and on all systems throughout the UK, two of these channels are reserved for 999 (trunk reservation). When a 999 call is made and one channel is in use, the network automatically designates another channel and so on. 999 calls do not displace other calls in progress and if a call cannot be delivered to the operator because there is congestion in the network, or at the 5ESS switching layer, which delivers calls to the operator, then automatic retry is designed to make sure calls are delivered. When calls route from the 999 operator to the Emergency Services Control Room, then a marker placed in the signalling for this call ensures that the call can force its way past any network controls being operated at the time by BT.
In a major incident, however, this system can easily be overwhelmed. In the early stages of a big incident, even with trunk reservation, all channels in the local exchange may be very quickly taken up with 999 calls. To make matters worse, automatic retry can seriously congest the network – the Dudley earthquake was a prime example of this. And, with new the networks and facilities that are currently available – typically text messaging, VOIP, number portability etc –Calling Line Identification won’t always work to provide location information to the Emergency Services.
As an incident moves to the establishing control phase, the problems multiply with huge volumes of normal voice calls routing into the area of an incident from the rest of the UK, possibly the rest of the world. At the same time, there will also be high levels of originating calls made from within the area of an incident. This can cause serious congestion on fixed and mobile networks and in particular on the local PSTN network including all call boxes and private networks. Mobile networks become completely congested with traffic being taken up on all adjacent cell sites with follow-on impact on the ability of responders to use both fixed and mobile networks. On the plus side, satellite communications are generally not affected.
It is at this point that the control of originating traffic, using protocols such as GTPS and ACCOLC may be invoked. GTPS is the Government Telephone Preference Scheme which is maintained by BT and some other Communications Service Providers. Essential users are defined by Government and if GTPS is switched then access to the PSTN is restricted to those lines registered on the scheme – this does not affect incoming and calls in progress but does restrict the ability to make outgoing calls. There is a hierarchy of GTPS categories – at the top are those lines essential for the management of emergencies, then public access lines such as payphones, and finally lines where there is no entitlement which covers the vast majority. GTPS is to be renamed Fixed Telecommunications Privileged Access Service or FTPAS.
There are issues with GTPS. David Groom described it as a very blunt instrument. Essentially it removes the dial tone to almost an entire exchange and as a result even users with access may retain only 10% of their lines. In practice, he said, organisations would have problems controlling their internal staff using the remaining lines and there is a view that this system may be less help to users than actually taking their chances with the rest.
As a result of this it is rarely used and when it is used, it is used sparingly and in short bursts. However, the existence of this scheme has meant that any PSTN services provided to the emergency services during an incident has to be built for GTPS otherwise, in an emergency, switching would automatically disconnect them.
ACCOLC (Access Overload Control by Class) – which is to be renamed Mobile Telecommunications Privileged Access Service or MTPAS, is the system set up to control mobile networks. If invoked, and this is done either at police or government request, ACCOLC applies to all mobile operators in the UK. The scheme is maintained by the mobile network operators and applications must be supported and agreed by the Cabinet Office. SIM cards have to be programmed accordingly. If ACCOLC is invoked all service is withdrawn except to those with enabled SIM cards which will fall into classes 11-15 and which are allocated to different groups of users.
In practice, explained David Groom, while ACCOLC may help rescue services mobile networks may still be overloaded. Like GTPS it tends to be used sparingly in short bursts.
Aside from these two schemes, there are a number of measures open to operators to establish control of incoming traffic. These include automatic and manual rerouting, appeals to the public to not make calls and also call gapping which can be applied to specific numbers, number ranges, national number codes or international codes.
ï BT’s 21CN – what is it and what needs to be done?
Today, BT’s UK network comprises some 96 discrete but related network platforms, each designed to support different services. David Groom said this structure reflected numerous technology waves – as new technologies and product opportunities emerged it was usually more straightforward to deploy a new platform or overlay. As a result the existing network comprises tens of thousands of elements including switches, routers, concentrators and transmission terminals. Maintaining this type of network is expensive and a significant part of the BT’s operating costs.
Hence the drive to move to a new, single, converged network. 21CN uses IP protocols rather than C7 for signalling although it is important to point out that it is not the internet. Overall, said David Groom, in addition to bringing cost savings to BT and faster services for the customer, it aims is to empower the end-user with control, choice and flexibility including communications to any device.
This is a very ambitious project. There are approximately 30 million lines to transfer and at the height of the programme BT will be migrating 150,000 lines per week. There is a huge amount still do and, as David Groom reflected, plenty that can go wrong.
21CN and emergency response
David Groom confirmed that 999 calls will be provided over 21CN. Initially 5ESS switches will be retained and the principle of trunk reservation will still work as 21CN has a bandwidth manager at the I-node. The signalling marker, which enables a call to route through conditions where call-gapping is invoked will still be required for calls from the 999 Operator to the control room. Trific will no longer be available and automatic retry may not be provided as there were moves to turn off this facility in the existing network. Automatic re-routing has been built into the 21CN specification as have network controls although at this stage it is unclear how they will all operate.
What was not built into the original specification was GTPS or FTPAS as it will be known in future. What is clear is that in future FTPAS will operate to different principles. It will always be ‘on’ and will work by placing a marker onto the specified line which will allow a call from that line to overcome congestion.
So what does all this mean for emergency responders? David Groom explained that 21CN represents a complete change in technology which will bring many benefits, most importantly for this audience greater resilience.
But the changeover will be extremely complex and for some time the two networks will need to work side by side. David Groom said that everyone needed to be aware of what was happening and be prepared because there would be plenty of scope for problems. ‘Everyone needs to be aware that customer resilience may be impacted during this migration period, for instance control rooms may be affected. And while most existing facilities will transfer over, they may operate differently.
‘It is vital that when emergency services come to designing their own back office systems to work within the 21CN environment, they understand fully all the issues relating to managing 999 calls. My concern is that unless great care is taken to ensure that 999 services operate correctly, organisations may find that in the event of a major incident they lose essential communications.’
