Basic Network Cabling information
Ethernet was developed in the late 1970's by the Xerox Corporation at their Palo Alto Research Centre in California. It has been estimated that over 70% of the worlds networks use the Ethernet protocol, so with this in mind it would seem only sensible to discuss how it works.
Collision Detection - A
means of ensuring that when two machines start to transmit data simultaneously,
that the resultant
corrupted data is discarded, and re-transmissions are generated at differing time intervals.
The Basic Ethernet Bus "Coax System RG58"
This is a coax based Ethernet
network where all machines are daisy chained using RG58 coaxial cable
(sometime referred to as Thin Ethernet). Although still in use this is old technology. The image above shows machine 2 wanting
to send a message to machine 4, but first it 'listens' to make sure no one else is using the network.
If it is all clear it starts to
transmit its data on to the network (represented by the yellow flashing
screens). Each packet of data
contains the destination address, the senders address, and of course the data to be transmitted.
The signal moves down the cable
and is received by every machine on the network but because it is only addressed
to number 4,
the other machines ignore it.
Machine 4 then sends a message back to number 2 acknowledging receipt of the data (represented by the purple flashing screens).
But what happens when two
machines try to transmit at the same time? …… a collision occurs, and each
machine has to
'back off' for a random period of time before re-trying.
For the sake of simplicity I have omitted the acknowledgement transmissions from the rest of the animation's on this page.
This animation starts with machine 2 and machine 5 both trying to transmit simultaneously.
The resulting collision destroys
both signals and each machine knows this has happened because they do not 'hear'
their own transmission within a given period of time (this time period is the propagation delay and is equivalent to the time
it takes for a signal to travel to the furthest part of the network and back again).
Both machines then wait for a
random period of time before re-trying. On small networks this all happens so
that it is virtually unnoticeable, however, as more and more machines are added to a network the number of collisions rises
dramatically and eventually results in slow network response. Time to buy a switch!!!
The exact number of machines
that a single Ethernet segment can handle depends upon the applications being
used, but it is
generally considered that between 40 and 70 users are the limit before network speed is compromised.
Using a Hub
An Ethernet hub changes the topology from a 'bus' to a 'star wired bus', here's how it works.
Again, machine 1 is transmitting
data to machine 4, but this time the signal travels in and out of the hub to
each of the other
As you can see, it is still
possible for collisions to occur but hubs have the advantage of centralised
wiring, and they can
automatically bypass any ports that are disconnected or have a cabling fault. This makes the network much more fault tolerant
than a coax based system where disconnecting a single connection will bring the whole network down.
Using a Switch
To overcome the problem of collisions and other effects on network speed, a switch is used.
With a switch, machines can
transmit simultaneously, in this case 1 & 5 first, and then 2 & 4. As you can
switch reads the destination addresses and 'switches' the signals directly to the recipients without broadcasting to all
of the machines on the network.
This 'point to point' switching alleviates the problems associated with collisions and considerably improves network speed.
In the real world however, one or more of these machines will be servers, and as most network traffic is between the clients and a server a serious bottle neck can occur. The answer to this problem is to make server connections faster than the clients. The normal solution is to have the client machines on 100Mbs ports and the servers on 1000Mbs ports (Gigabit Ethernet). This ten to one ratio is usually adequate because not all of the clients will need to access the servers at the same time.