Extron Electronics VN-MATRIX User Guide - Manual (Page 129)

Appendix B: Understanding Network Performance VN-MATRIX User Guide
I458GB issue 6 Page 129
Network Characteristics
There are a number of characteristics of networks that have to be taken into account when
transmitting image data.
When a conventional analog or digital video signal is sent from a source to a display, the image is
transmitted in real time with negligible delay (or “latency”). The signal itself is continuous with (even in
the case of a digital signal) very little redundancy in the signal (i.e. the great majority of the signal is
image data, so little bandwidth is “wasted”). In general the signal is not subject to any unpredictable
degradation.
Data Packets/Frames
If a digital image stream is sent across a network, it must be “packetized”. The network itself is
indifferent to the nature of data, and simply treats all digital information in a standard way. Before it
can be sent over a network, the data must be re-formatted into packets called “IEEE MAC frames”.
IEEE802.3 Media Access Control frame.
Some points arise from this:
ƒ The data carried within a frame/packet is limited to the MTU (maximum transmission unit) of
the network, typically 1500 bytes.
ƒ The data must include any additional overhead arising from the protocol being used (e.g.
UDP or TCP/IP).
The MTU is the largest physical packet size (measured in bytes) that a network can transmit. Any
messages larger than the MTU are divided into smaller packets before being sent.
The introduction of Gigabit (1 Gb/s) Ethernet has allowed the introduction of “Jumbo” frames with
more than 1500 data bytes. Clearly the overhead involved by limiting the number of bytes to 1500 is
considerable, and using jumbo frames makes more efficient use of the network. However, the
following should be noted:
ƒ Many real world networks only operate at 100Mb/s at local level and these can NOT accept
jumbo frames.
ƒ Even many gigabit networks are not configured to accept jumbo frames. Only networks so
configured can use them.
ƒ The maximum data size of a jumbo frame is 9000 bytes; and typically jumbo frames carry
8000 data bytes.
ƒ While network efficiency goes up with larger frames, the effect of a lost frame or packet is
more serious.