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Networking the DAR SoundStation (part 2)

Probably the most common network standard in current use is Ethernet. An Ethernet network can be connected in two ways, either as a ring system or a star system...



Probably the most common network standard in current use is Ethernet. An Ethernet network can be connected in two ways, either as a ring system or a star system. A ring system would suit an office since it uses less cable, and provides an adequate data transfer rate. The ring isn't actually joined in a loop, but is simply a continuous piece of coaxial cable to which all the machines are connected. Every machine transmits data onto the coax and the signal goes off down the wire in both directions where any other machine can pick it up. The signal is absorbed at both ends by terminators similar to the SCSI terminators you may have come across. The bandwidth of a standard Ethernet is 10 megabits per second, and in the ring system this has to be shared among all the users who want to transfer data at the same time. There are obvious disadvantages for digital audio transfer where two or more users could easily clog the system. A better way of connecting the network, which uses more cable, is the star, where each individual machine has its own individual link back to a central 'hub' that distributes the data. Now, each machine has the benefit of a 10 megabit/s link all to itself, so the potential bandwidth is maximised. Figures 1 and 2 show the ring and star systems in comparison. The hub is a standard piece of computer equipment, which in itself is quite complex since each time a packet of data comes in it has to recognise the intended destination of the packet and know on which limb to transmit the data. The ring system uses a twisted pair cable, rather than coax, although the pair may have an overall screen, and these cable types have been dubbed 'thin' for twisted pair and 'thick' for coax.

A large network system, such as a broadcaster may require, can combine both types of wiring. An organisation on the scale of the BBC may require network links between television, radio and production offices, and there is in theory no limit to the number of nodes an Ethernet network could possess. In broadcasting for example, fairly obviously the feed to the transmission suite is of paramount importance, and this could have its own dedicated Ethernet wire which would have a guaranteed bandwidth since no other traffic could gain access. A number of post production suites could sit nicely around a hub with each also having its own feed. At the lower end of the data chain, a number of journalists, or perhaps directors wanting viewing access to rushes or work in progress, could fairly reasonably be expected to work from a box connected to a single ring branching off from the hub. One question you might ask is whether there might be a bottleneck in the ring system shown in Figure 2 where the server connects to the hub. After all, all the other users will want to access data from the server down this one cable. The answer is that the hub can be built into the server an enjoy a sufficient data bandwidth directly from the source. The purpose of Ethernet is to fire a quantity of data down a lengthy cable, and the connection to the server can take a completely different physical form with no 10 megabits per second limit.

Since Ethernet is a very popular standard in the computing world, you might be tempted ask how far you can go with it, and the answer is literally anywhere you like. Ethernet contains the basic protocol that says what the data packets that are sent down the wire will be like, what the header packets will be like, and how individual machines should be addressed. This isn't in itself enough to get a file from one machine to another so on top of that must be another layer. DAR use the TCP/IP protocol which is compatible with virtually all computer systems. So you can connect Suns, Macs, IBM mainframes and even Ataris. You can connect two computers together via a modem link and they can talk TCP/IP to each other. You can get even a satellite TCP/IP link. As you may be aware, the Internet runs on TCP/IP too, so you could connect a Sabre in London to a Sabre in Los Angeles via the Internet and load a project from one Sabre to the other and see it on the screen, and even attempt to play it. It would load the first second or so into it's buffer and you would hear it, but after that the data transfer rate would be far too slow. In ten years time the situation might be different. It would be feasible now however to connect your Sabres together, or even a pair of PCs running Windows 95, via a phone link across the world and transfer an audio file across if you were prepared to wait. The limit is that a phone link can handle around 28kbits/s so to transfer a stereo 16 bit linear file across you would be looking at something like forty or fifty times real time. ISDN isn't all that much better for uncompressed audio since the data rate is only 64kbits/s, a little over twice as fast.

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By David Mellor Thursday January 1, 2004

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