Network Topologies
Introduction
Network topologies can take a bit of time to understand
when you're all new to this kind of cool stuff, but it's very important
to fully understand them as they are key elements to understanding
and troubleshooting networks and will help you decide what actions
to take when you're faced with network problems.
I will try to be as simple as possible and give some
examples you can relate to, so let's get stuck right into this stuff
!
The Stuff :)
There are two types of topologies: Physical
and Logical. The physical
topology of a network refers to the layout of cables, computers and
other peripherals. Try to imagine yourself in a room with a small
network, you can see network cables coming out of every computer that
is part of the network, then those cables plug into a hub or switch.
What you're looking at is the physical
topology of that network !
Logical topology is the
method used to pass the information between the computers. In other
words, looking at that same room, if you were to try to see how the
network works with all the computers talking (think of the computers
generating traffic and packets of data going everywhere on the network)
you would be looking at the logical part of the network. The way the
computers will be talking to each other and the direction of the traffic
is controlled by the various protocols (like Ethernet) or, if you
like, rules.
If we used token ring, then the physical topology would
have to change to meet the requirements of the way the token ring
protocol works (logically).
If it's all still confusing, consider this: The physical
topology describes the layout of the network, just like a map shows
the layout of various roads, and the logical
topology describes how the data is sent accross the network or how
the cars are able to travel (the direction and speed) at every road
on the map.
The most common types of physical topologies, which
we are going to analyse, are: Bus,
Hub/Star
and Ring
The Physical Bus Topology
Bus topology is fairly
old news and you probably won't be seeing much of these around in
any modern office or home.
With the Bus
topology, all workstations are connect directly to the
main backbone that carries the data. Traffic generated by any computer
will travel across the backbone and be received by all workstations.
This works well in a small network of 2-5 computers, but as the number
of computers increases so will the network traffic and this can greatly
decrease the performance and available bandwidth of your network.
As you can see in the above example, all computers are
attached to a continuous cable which connects them in a straight line.
The arrows clearly indicate that the packet generated by Node
1 is transmitted to all computers on the network, regardless
the destination of this packet.
Also, because of the way the electrical signals are
transmitted over this cable, its ends must be terminated by special
terminators that work as "shock absorbers", absorbing the
signal so it won't reflect back to where it came from. The value of
50Ohms has been selected after carefully taking in consideration all
the electrical characteristics of the cable used, the voltage that
the signal which runs through the cables, the maximum and minimum
length of the bus and a few more.
If the bus (the long yellow cable) is damaged anywhere
in its path, then it will most certainly cause the network to stop
working or, at the very least, cause big communication problems between
the workstations.
Thinnet - 10 Base2, also known as coax cable (Black
in colour) and Thicknet - 10 Base 5 (Yellow in colour) is used in
these type of topologies.
The Physical HUB or
STAR Topology
The Star or Hub
topology is one of the most common network topologies found
in most offices and home networks. It has become very popular in contrast
to the bus type (which we just spoke about), because of the cost and
the ease of troubleshooting.
The advantage of the star topology
is that if one computer on the star topology
fails, then only the failed computer is unable to send or receive
data. The remainder of the network functions normally.
The disadvantage of using this topology is that because
each computer is connected to a central hub or switch, if this device
fails, the entire network fails!
A classic example of this type of topology is the UTP
(10 base T), which normaly has a blue
colour. Personally I find it boring, so I decided to go out and get
myself green, red
and yellow colours :)
The Physical Ring Topology
In the ring
topology, computers are connected on a single circle
of cable. Unlike the bus topology, there are no terminated ends. The
signals travel around the loop in one direction and pass through each
computer, which acts as a repeater to boost the signal and send it
to the next computer. On a larger scale, multiple LANs can be connected
to each other in a ring topology by using Thicknet coaxial or fiber-optic
cable.
The method by which the data is transmitted around the
ring is called token passing.
IBM's token ring uses this method. A token
is a special series of bits that contains control information. Possession
of the token allows a network device
to transmit data to the network. Each network has only one token.
The Physical Mesh Topology
In a mesh topology, each
computer is connected to every other computer by a separate cable.
This configuration provides redundant paths through the new work,
so if one computer blows up, you don't lose the network :) On a large
scale, you can connect multiple LANs using mesh topology with leased
telephone lines, Thicknet coaxial cable or fiber optic cable.
Again, the big advantage of this topology is its backup
capabilities by providing multiple paths through the network.
The Physical Hybrid Topology
With the hybrid topology,
two or more topologies are combined to form a complete network. For
example, a hybrid topology could be the
combination of a star and bus topology. These are also the most common
in use.
