Technical data

www.westermo.com Theoretical and general applications 127
Hub or Switch
Why is a switch so much better than a hub and what is the difference between these
products? We have already ascertained that it was the hub that made the installation
of star coupled networks possible, and together with Ethernet, made structured cable
systems popular. The hub does not have an advanced design, everything sent to one
port is transferred to the other ports. This means that everyone hears what everyone
sends and everyone is in the same collision domain.
On the other hand, a switch is more intelligent, either through processors or
through specially designed integrated circuits. This creates the possibility to control and
process data received on a port. The switch learns what equipment is connected to
what port and this is stored in the switch’s MAC-address memory. There are two
types of switch, Cut-through and Store-and-forward. The Cut-through switch exam-
ines the destination address and sends data to the destination port. This results in a
collision if the port is used by other traffic where the most recent packet is lost. These
switches are very fast. The Store-and-forward switch copies the received packet and
places this in the buffer before it localises the destination port and only sends it for-
ward when the port becomes free. Consequently the packet is not lost. Data can also
be prioritised; the network can be divided up into virtual LANs, etc.
The list below shows some of the differences between a hub and switch.
The advantage we usually
emphasize is that a switch
segments the network
(switched Ethernet),
which eliminates collisions.
Hub
Half duplex communication.
Increases the collision
domain.
The whole network shares
the bandwidth.
Low bandwidth utilisation
due to CSMA/CD.
Faster than a Switch
(less latency).
Switch
Half duplex or Full duplex (HDX/FDX).
Segments the network.
Bandwidth as required (self-learning system).
Store and forward
(control of the packet before it is forwarded).
Learns MAC addresses (who is connected where).
Old addresses are forgotten
(time out on the MAC address buffer).
Flow control for FDX and HDX.
Packet buffer on port level.
QoS, prioritisation of data (high priority data is placed
first in the packet buffer).
Virtual network VLAN
(virtually connect together specific ports).
Gbit-switches (powerful switches with a high capacity).
B A C K