BBC Scotland ATM Network (Line Up Feb/Mar 2003) - The cost of fixed lines between broadcast centres has become prohibitive but high-tech IT solutions offer viable and cost-effective alternatives. COLIN MACNAB MIBS reports on a broadcast ATM system North of the Border.
Sometimes communicators seem to take a while to get around to communicating. However, when a broadcast service provider started communicating with a network service provider and a small broadcast equipment manufacturer, the results were very impressive, and highlight a very interesting path for the future of the industry!
The particular case in point is that of a recent installation in BBC Scotland of Audix Broadcast's new Gateway system. This enables the transmission of full rate digital audio over a standard ATM telecoms network, and the current implementation provides eight stereo circuits between Glasgow and Edinburgh. Although a digital replacement for conventional land-lines may not seem that exciting, this is the broadcaster's first step in a very powerful technology which is set to become an everyday tool. Indeed, work is already in progress to apply similar techniques to an ATM network for Broadcasting House in London.
ATM
This particular communications technology – ATM, short for Asynchronous Transfer Mode – is something which has been used by telecoms companies world-wide for some time. It is a fully scalable technology for the transfer of data between locations, with the ability to guarantee the integrity and security of specific pathways between users.
ATM works over a wide variety of different telecom infrastructures using fibre optic, copper, or satellite circuits, and at a various rates. The telecoms provider is not interested in what the data represents, but if you tell them where it has to go – and in the case of ATM when it has to get there – they will make it happen at a cost based on the required bit rate. In the case of the ATM system, it is not necessary to employ data reduction as the available data rate is more than sufficient for any application. There is also no significant time delay imposed on data passing through the network, and ATM protocols even allow the 'round trip time' of your data on your particular network to be measured.
ATM uses a 'cell relay' structure, in the parlance of some telecoms providers. The operating protocols basically divide the data to be transferred over the ATM network among a series of fixed length 'cells' for transmission. The protocols allow instructions along the lines of "transmit immediately", "make the next cell part of this transmission", and "pass other data, you are in the fast lane" to be included in the header of each cell. ("It sounds fine leaving here" does not yet appear to be an option!) The data packets are recombined when it is received, according to the header instructions.
Network Rates As already mentioned, ATM is already in widespread use, notably as the basis of most Internet 'backbone' systems. ATM network data rates are typically 2.5Gb/s on average, with a few 10Gb/s systems in limited use, while rates of 40Gb/s have been achieved in some trials. The operating protocols allow ATM to be used as a Wide Area Network, complete with data collisions slowing the whole system down (in the case of the Internet), or as the equivalent of a switched point to point connection system more recognisable and useful to broadcasters.
The scalable aspect of the ATM network means that any particular connection can be used for anything from a low data rate talkback channel to an uncompressed hi-def video service. Any required data rate can be made available up to the maximum of the particular network, and telecoms providers may offer 'Adaptable Bitrate Contracts' which essentially allow the broadcaster to pay for the data rate used on each specific occasion. However, it is also possible to establish the equivalent of a point to point circuit with a guaranteed bandwidth, if required.
Audix Broadcast In Scotland The telecoms company providing the ATM service to BBC Scotland is called Thus, but was formerly known as Scottish Telcom. In fact, the company started out as Scottish Power's telecom division, and it has a long history of innovative developments in sending 'stuff down wires'. The company also owns and runs the popular Demon Internet brand.
Interfacing between the ATM network and the BBC Scotland premises in Glasgow and Edinburgh are two Audix Broadcast Gateway units, providing four stereo outputs from, and four stereo inputs to, the ATM interface. Plug-in circuit cards determine the format of these audio interfaces, with both analogue (mic/line) and AES-EBU options being available. The latter can be specified with 20 or 24 bit resolution, and includes sample rate conversion facilities and an AES clock reference input.
Although not used in BBC Scotland, the Audix Broadcast ATM system can also be incorporated within Audix Broadcast’s digital studio mixers and newsroom workstations. This enables the creation of a completely integrated broadcast centre with various elements in other buildings or even on the opposite side of the world, connected only via ATM.
In the BBC Scotland installation, the audio signals are routed over the ATM network as a 25Mb/s data stream, through a pair of RJ45 connectors (one for the send and the other, receive). The Gateway unit also incorporates micro-controller and serial ports for external control and test purposes, but was being controlled in this application by a dedicated PC. The audio paths are integrated with the audio routing systems in BBC Glasgow and BBC Edinburgh. Thus the system is completely transparent in operation, and the users in Glasgow or Edinburgh see only sources and destinations on each centre’s internal router. The system functions as if there were fixed permanent lines between the two broadcast centres.
Daily Use
Steve Walker, Communications Developments Manager, BBC Scotland, is very pleased with the way the system is working and summed its operation up with: “Basically, it does what it says on the tin!” Of course, the trick of this particular ‘tin’ is that it joins the broadcast and IT worlds together to serve a new purpose. The level of integration also means that the whole thing is extremely cost-effective and, being based on a system that positively encourages expansion, the potential is enormous.
The cost of line rental can be a seriously large part of any broadcasting budget, but modern technologies - such as ATM - are offering cost effective alternatives without penalising audio quality. Small broadcasters, in particular, can take advantage of this and go directly to telecoms companies for data services where they are charged only for the data bandwidth actually used. Thus, ATM offers all broadcasters a robust, cost effective distribution system which avoids data reduction, coding incompatibilities, and timing delays inherent in many current systems. The Audix Broadcast Gateway system is the first practical installation of such technology in the UK, and it appears to be a roaring success for all concerned.
