Networking the DAR SoundStation (part 2)

Ethernet

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.