MPEG applications - The MPEG - page 234

FireWire is actually an Apple Computers Inc. trade name for the interface which is formally known as IEEE 1394-
1995. It was originally intended as a digital audio network, but grew out of recognition. FireWire is more than just an
interface as it can be used to form networks and if used with a computer effectively extends the computer's data
bus. Figure 7.27 shows that devices are simply connected together as any combination of daisy-chain or star
network.
Any pair of devices can communicate in either direction, and arbitration ensures that only one device transmits at
once. Intermediate devices simply pass on transmissions. This can continue even if the intermediate device is
powered down as the FireWire carries power to keep repeated functions active.
Communications are divided into cycles which have a period of 125 s. During a cycle, there are 64 time slots.
During each time slot, any one node can communicate with any other, but in the next slot, a different pair of nodes
may communicate. Thus FireWire is best described as a time- division multiplexed (TDM) system. There will be a
new arbitration between the nodes for each cycle.
Figure 7.27: FireWire supports radial (star) or daisy-chain connection. Two-port devices pass on signals destined
for a more distant device - they can do this even when powered down.
FireWire is eminently suitable for video/computer convergent applications because it can simultaneously support
asynchronous transfers of non-real-time computer data and isochronous transfers of real-time audio/video data. It
can do this because the arbitration process allocates a fixed proportion of slots for isochronous data (about 80 per
cent) and these have a higher priority in the arbitration than the asynchronous data. The higher the data rate a
given node needs, the more time slots it will be allocated. Thus a given bit rate can be guaranteed throughout a
transaction; a prerequisite of real-time A/V data transfer.
It is the sophistication of the arbitration system which makes FireWire remarkable. Some of the arbitration is in
hardware at each node, but some is in software which only needs to be at one node. The full functionality requires
a computer somewhere in the system which runs the isochronous bus management arbitration. Without this only
asynchronous transfers are possible. It is possible to add or remove devices whilst the system is working. When a
device is added the system will recognize it through a periodic learning process. Essentially every node on the
system transmits in turn so that the structure becomes clear.
The electrical interface of FireWire is shown in Figure 7.28 . It consists of two twisted pairs for signalling and a pair
of power conductors. The twisted pairs carry differential signals of about 220 mV swinging around a common mode
voltage of about 1.9 V with an impedance of 112 . Figure 7.29 shows how the data are transmitted. The host data
are simply serialized and used to modulate twisted pair A. The other twisted pair (B) carries a signal called strobe ,
which is the exclusive-OR of the data and the clock. Thus whenever a run of identical bits results in no transitions
in the data, the strobe signal will carry transitions. At the receiver another exclusive-OR gate adds data and strobe
to re-create the clock.