Technical data

130 Theoretical and general applications www.westermo.com
Time switches
Ethernet through design is not deterministic, i.e. you can not guarantee the transfer
time of a data packet from one occasion to another. This previously made it impossible
to use Ethernet for real time applications, such as monitoring transformer stations or
controlling complex machinery, but these limitations no longer exist. In a real time sys-
tem all links must communicate with full duplex while flow control (on the Ethernet
level) must be shutoff, furthermore, it must be possible to prioritise data. All data with
a high priority will be placed at the front of the queue and be communicated with pri-
ority to the recipient. Combined with time synchronisation this creates the possibility
of designing real time applications with Ethernet, also see pages 136 to 137.
What can cause problems for real time applications in a switched network?
A switched network is subject to delays due to the load, speed of the drop link, packet
size, switch architecture and the number of switches between the server and client. A
delay can vary from ten µs to several ms. Most switches are based on the “store and
forward” technology, which receives and checks the entire packet before it is forward-
ed on. Assume that the switch has a drop link speed of 10 Mbit/s (receiving port on
the switch), the packet size is 1522 bytes, this results in a maximum delay of 1.2 ms
due to “store and forward”. However, if you have 100 Mbit/s the maximum delay will
be 1.2 µs. To choose the right technology supplemented with time synchronisation
gives the prerequisites for Ethernet in real time applications.
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