Westermo U200 Operator manal - Manual (Page 7)

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However, the domain of Ethernet has always been controlled by the IT department who
configured office networks normally with an iron fist and dictated to the company how the
network would be designed with complex recovery protocols like spanning tree and SNMP to
help with fault finding and system analysis. If a network failure occurred the IT department
would casually look at repairing the equipment - there was no real rush as it was an office
network. However, with Industrial Ethernet you need very fast repair time, and, with an IT
department not present on the factory floor the maintenance personnel need to be made
aware of the fault, find the error and repair it - quickly.
Industrial rated Switches are intended to be installed in harsh conditions and electrical
environments with the added benefit of fast recovery of a network failure. The switches are
an excellent example of how such Switches should be designed – very high operating
temperatures, fast repair of redundant ring, layer 2 and layer 3 priority switching, time
synchronization capability, etc. Without doubt, Westermo OnTime switches are technically
superior to many similar models available on the market.
3.3 Switches vs. Hubs
A hub consists of a number of ports normally with either RJ-45 (copper) sockets and / or fibre
optic ports that have a number of different styles of fibre optic sockets. Usually a ‘patch cable’
is connected to the hub; the other end is normally connected to a device (PC, Printer etc).
A hub has no intelligence and therefore is unable to identify addresses or any information
contained within
the Header frame of an Ethernet packet. This means that it is not capable of
determining which port to send the frame to. Therefore, every frame is sent to every port .
A network of repeaters and hubs is called a ‘Shared Ethernet’ or ‘Collision Domain’. Various
systems will all compete with each other using ‘Carrier Sense Multiple Access / Collision
Detect’ (CSMA/CD) protocol. This means that only one system is allowed to proceed with
a transmission of a frame within a Collision Domain at any one time.
This is a major
disadvantage when using Hubs and Repeaters within a network.
If a hub sees a collision on a cable segment, it is detected and a ‘jam’ signal is generated.
The ‘jam’ signal is sent to all connected devices. This ensures that every device is aware of
the collision and they do not attempt to transmit during the collision.
All Ports Receive the Same Ethernet Frame
Figure 2, hub
To summarise, hubs operate with the following limitations:
• Only a single speed of operation – no ability to automatically change between 10M or
100M.