MPEG applications - The MPEG - page 229

High forward speeds can be obtained by selecting only the I pictures from the recorded bitstream. These cannot be
transmitted to the decoder directly, because the I pictures contain a lot of data and the decoder buffer might
overflow. Consequently the data rate is diluted by sending null P pictures. These are P pictures in which all of the
vectors are zero and the residual data are also zero. Null P pictures require very few data but have the effect of
converting an I picture into another identical picture. A similar technique can be used to decode in reverse. With a
normal MPEG bidirectionally coded bitstream, reverse decoding is impossible, but by taking I pictures in reverse
order and padding them with null- P pictures a reverse replay is obtained.
7.7 Networks
A network is basically a communication resource which is shared for economic reasons. Like any shared resource,
decisions have to be made somewhere and somehow about how the resource is to be used. In the absence of
such decisions the resultant chaos will be such that the resource might as well not exist.
In communications networks the resource is the ability to convey data from any node or port to any other. On a
particular cable, clearly only one transaction of this kind can take place at any one instant even though in practice
many nodes will simultaneously be wanting to transmit data. Arbitration is needed to determine which node is
allowed to transmit.
Data networks originated to serve the requirements of computers and it is a simple fact that most computer
processes don't need to be performed in real time or indeed at a particular time at all. Networks tend to reflect that
background as many of them, particularly the older ones, are asynchronous. Asynchronous means that the time
taken to deliver a given quantity of data is unknown. A TDM system may chop the data into several different
transfers and each transfer may experience delay according to what other transfers the system is engaged in.
Ethernet and most storage system buses are asynchronous. For broadcasting purposes an asynchronous delivery
system is no use at all, but for copying an MPEG data file between two storage devices an asynchronous system is
perfectly adequate.
The opposite extreme is the synchronous system in which the network can guarantee a constant delivery rate and
a fixed and minor delay. An AES/EBU digital audio router or an SDI digital video router is a synchronous network.
In between asynchronous and synchronous networks reside the isochronous approaches which cause a fixed
moderate delay. These can be thought of as sloppy synchronous networks or more rigidly controlled asynchronous
networks.
These three different approaches are needed for economic reasons. Asynchronous systems are very efficient
because as soon as one transfer completes another one can begin. This can only be achieved by making every
device wait with its data in a buffer so that transfer can start immediately. Asynchronous systems also make it
possible for low bit rate devices to share a network with high bit rate devices. The low bit rate device will only need
a small buffer and will send few cells, whereas the high bit rate device will send more cells.
Isochronous systems try to give the best of both worlds, generally by sacrificing some flexibility in block size.
Modern networks are tending to be part isochronous and part asynchronous so that the advantages of both are
available.
There are a number of different arbitration protocols and these have evolved to support the needs of different types
of network. In small networks, such as LANs, a single point failure which halts the entire network may be
acceptable, whereas in a public transport network owned by a telecommunications company, the network will be
redundant so that if a particular link fails data may be sent via an alternative route.
A link which has reached its maximum capacity may also be supplanted by transmission over alternative routes.
In physically small networks, arbitration may be carried out in a single location. This is fast and efficient, but if the
arbitrator fails it leaves the system completely crippled. The processor buses in computers work in this way.
In centrally arbitrated systems the arbitrator needs to know the structure of the system and the status of all the
nodes. Following a configuration change, due perhaps to the installation of new equipment, the arbitrator needs to
be told what the new configuration is, or have a mechanism which allows it to explore the network and learn the
configuration. Central arbitration is only suitable for small networks which change their configuration infrequently.
In other networks the arbitration is distributed so that some decisionmaking ability exists in every node. This is less
efficient but is does allow at least some of the network to continue operating after a component failure.
Distributed arbitration also means that each node is self-sufficient and so no changes need to be made if the
network is reconfigured by adding or deleting a node. This is the only possible approach in wide area networks
where the structure may be very complex and changes dynamically in the event of failures or overload.