Information Technology Reference
In-Depth Information
Figure 7.23:
Receiving a file which has been divided into packets allows for the retransmission of just the packet in
error.
The error-detection system interacts with this process because if any frame is in error on reception, the receiving
node can ask for a retransmission of the frame. This is more efficient than retransmitting the whole file.
Figure 7.23
shows the mechanism of retransmission where one packet has been lost or is in error. The retransmitted packet is
inserted at the correct place in the bitstream by the receiver.
Breaking files into frames helps to keep down the delay experienced by each process using the network.
Figure
7.24
shows a frame-multiplexed transmission. Each frame may be stored ready for transmission in a silo memory.
It is possible to make the priority a function of the number of frames in the silo, as this is a direct measure of how
long a process has been kept waiting. Isochronous systems must do this in order to meet maximum delay
specifications.
When delivering an MPEG bitstream over an isochronous network, the network transmitter and receiver will require
buffering over and above that provided in the MPEG encoder and decoder. This will enable the receiver to re-
create a bitstream at the decoder which has essentially the same temporal characteristics as that leaving the
encoder, except for a fixed delay. The greater the maximum delay of the network, the greater that fixed delay will
have to be. Providing buffering is expensive and delays cause difficulty in applications such as videoconferencing.
In this case it is better to use a network protocol which can limit delay by guaranteeing bandwidth.
A central arbitrator is relatively simple to implement because when all decisions are taken centrally there can be no
timing difficulty (assuming a well-engineered system). In a distributed system, there is an extra difficulty due to the
finite time taken for signals to travel down the data paths between nodes.
