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1. Network Topologies
2. Fundamentals A LAN (Local Area Network) is a “Transmission system intended primarily for linking computers and associated devices within a restricted geographical area”.
It covers an area of moderate size, such as an office block, factory or campus.
In practice, its size may range from a few meters to, in rare cases, tens of kilometers.
3. Speed of LANs The raw transmission rate of LANs are high, typically being in the 1-1000 Mbps range.
On some LANs (eg Open System LANs) every device has the potential of connecting to any other device on the LAN.
Smaller LANs typically operate on a Slave/Master basis with slave PCs clustered around a shared master filestore system.
4. Network Sharing A fundamental feature of many LANs is the network itself is shared.
The physical network medium is shared by many machines.
In a traditional network, each machine is usually wired into a switching device.
For example, a telephone is connected to a local switching office.
5. Low Error Rates/Low Cost On LANs, network errors are expected to be relatively few when compared with larger networks.
LANs are relatively inexpensive when compared to the cost of the equipment that connects to it.
However, Each Network Interface Unit (NIU) still costs in the region of Ł10-Ł50.
6. WAN A WAN (Wide Area Network) is a network that is spread over multiple sites (>30Km).
WANs are not limited in size (they can even cross the world).
Public facilities (such as the public switched telephone network) are extensively used.
However, this means that the rate at which data is sent is limited by the bandwidth of these facilities.
7. WAN vs. LAN When comparing WANs with LANs, the main difference is in the data transmission rates.
Delay and error rate parameters are also relevant to some applications.
We can view the technical facilities offered by a WAN as a subset of those offered by a LAN.
What a WAN offers is long distance connectivity.
8. Implementation of a Network How easy a network is to use depends on the sophistication of the software provided.
The most basic level of netware provision is to only have programs designed for specific tasks such as file transfer.
More sophisticated systems incorporate network facilities in the operating system of the computer (thus network operations become a coherent part of the user interface).
9. Network Specifications A network implies that all the computer can communicate with each other.
This requirement can be met in a number of ways but there are certain basic principles common in most networks.
10. Network Structure Each host computer communicates via a Network Interface Unit (NIU).
By host computer we mean computers on which users can run applications.
The term node is often used for the intelligent interface that is part of each host computer (or sometimes for the host computer itself).
The term node can also be used to mean another computer with which the host communicates (such as a server or a router).
11. Network Topology The term topology refers to the way in which the nodes of a network are connected.
The topology of a network will effect its performance (it terms of speed) and its cost (both short and long term).
Cost/resource considerations and the environment in which the network is to be used often determines the choice of topology.
12. A network topology is the way in which a network is connected up.
The simplest network topology you can have is a single link (i.e. cable, optical fibre, radio or any other means of transmitting data) between two computers.
It may not be a very big network, but technically it is still a network. The computers can exchange data and they are autonomous.
13. Switching & Broadcast Networks We can distinguish networks by the way in which they transmit data.
WAN usually use switching networks to send data from source to destination nodes.
LANs, however, often use broadcast networks because they are cheaper to build and maintain.
Broadcast networks send all the data to all nodes (which must out listen for the data meant for them).
14. Common Topologies Some common topologies include:
MESH
STAR
BUS
TREE
RING
BACKBONE
15. Mesh Topology A mesh topology is a network in which the connections between nodes is random.
Mesh topologies include fully connected networks and random networks (e.g. Internet).
Redundant connections in random networks ensure that alternative routes exist for data.
16. Fully Connected Networks Fully connected networks are the fastest types of networks since each device directly connects to every other device.
There is no time delay due to switching.
If there are N hosts in the network, we need N(N-1)/2 bi-directional connections (e.g. 20 hosts needs 190 connections).
This is far too many connections (most of which will be idle most of the time).
17. Sharing Connections Without a fully connected network, connections between nodes must be shared.
One way to do this is to allow nodes to switch data through a random network.
We can view a random network as interconnecting star networks.
Data is passed through this network, via intermediate nodes, until it arrives at its destination.
18. Mesh Topology - Recap A randomly connected network is called a mesh topology. Mesh topologies are most suited to networks that are built up over time.
The Internet is a mesh topology. It has been added to over time with no central control.
Mesh topologies are easily extended.
Often there is more than one possible route for messages to follow. If one path fails then another can be used.
There is no simple algorithm for routing messages. Messages are often sent over sub-optimal paths.
Control and security can be difficult to enforce effectively.
19. Star Networks A star network consists of a special central node (or hub node) to which host computers or terminals are connected.
Any host computer can connect to any other host computer via the hub.
20. Star Topology If all the hosts are connected to a central hub (which can be a switching device or a central server also acting as a router) then we have a star topology.
Hosts communicate by sending their messages to the hub. It then forwards the messages to the destination host.
Star topologies are popular with financial institutions who keep their latest records on a central server. Any changes to the records are updated centrally.
It’s easy to control and monitor access to a central server.
Cabling costs can still be high if network is spread out.
The central hub may be a bottleneck in busy networks.
21. Star Networks The hub switches messages through to the appropriate destination.
The hub may also provide a translation service for devices with different protocols.
Star Networks are vulnerable, however. If the hub fails then the network fails.
Star Networks may require a lot of cabling and can be expensive to install.
22. Bus Networks A bus network consists of a single medium (typically 5 pair twisted-wire cable) to which all the host computers are connected.
Packets are broadcasted on the medium to all nodes on the network.
23. Contention The is an obvious danger that two host computers may attempt to use the network medium simultaneously.
This problem is called contention and is a problem with all Bus topologies.
The nodes must employ Medium Access Protocols, which function in conjunction with other nodes, to permit access only at times when the medium is free.
24. Bus Topology Recap We can make a cheap network by connecting not just two but a number of hosts to a single link. Such a network is said to have a bus topology. It is also called a multi-drop link.
Although physically simple, bus topologies need a complicated protocol in order to ensure that hosts get fair use of the link and do not attempt to use the link at the same time. This can slow things down in a busy network.
25. Fully Connected Network At the other extreme, we do not have to share links at all. Each pair of hosts could have their own exclusive link. This would ensure the fastest possible communication between any two hosts. Such a network is called a fully connected network.
Of course there is one small drawback to this type of network: the number of links.
If N is the number of hosts then we need N(N-1)/2 links.
For 100 hosts, we need 100*99/2 = 4950 links.
The cabling and maintenance costs would be enormous and there would be no room for computers.
26. Tree Networks A tree network (as used in LANs) is a variant of the Bus topology.
Nodes are connected in a tree structure and messages are broadcast across whole tree.
27. Tree Networks Tree topologies have the advantage that they are easy to expand.
Furthermore, if a fault occurs, the effected branch can be easily isolated so that the rest of the network is not effected.
The disadvantage is that signals can be reflected from the ends of branches and cause interference. For this reason, Tree Networks are usually run at lower speeds.
28. Tree Topology - Recap A tree topology has a root and branches that gives it a distinctive tree shape.
Messages pass up the tree until they reach a branching point in common with the destination host. They are then passed down the tree to the destination.
Tree topology is easily extended.
There is a simple algorithm for routing messages.
A large number messages pass through the root, which may become a communications bottleneck.
29. Ring Topology A Ring network consists of nodes connected to each other to form a closed loop.
Nodes accept data from neighbouring nodes in the form of packets.
30. Ring Topology Operation The NIUs (or, in some cases, the hosts themselves) act as repeaters for the packets being forwarded.
This means that the Ring can be expanded to any size (although more hops will be required to get the packets to their destinations).
One big advantage of Ring Topologies is that contention is avoided since each repeater knows if it has to forward an existing packet or is free to accept a new one.
31. Ring Topology - Recap In a Ring Topology hosts are connected to their neighbours to form a loop.
Messages are passed from one host to the next until they reach the destination host.
Typically messages pass the whole way around the ring and are checked and removed by the hosts that sent them.
Less cabling is required because neighbouring hosts are not usually far apart.
A break in one of the links will stop the network from working (but failures can be quickly detected and fixed).
32. Dealing with Contention Contention is dealt with by either using a slotted ring system or a token-passing ring.
With the slotted ring system, blank fixed sized frames are passed around the network.
These frames get filled in with data as they pass a node that wishes to transmit.
The frame goes around the entire network and is copied by the destination node as it passes.
33. Token-Passing Ring A token-passing ring is similar to a slotted ring except that a ‘token’ frame is passed around the ring.
If a ‘token’ arrives at a node, it can be exchanged for a data packet.
The data packet is sent around the entire network and is copied by the destination node as it passes.
When the packet comes back to the sender, the sender puts the token back on the network.
34. Removing Packets/Frames A frame, or packet, will circulate around the network until removed.
In some networks it is the destination node that removes it.
In other networks it is the sender that removes it.
The advantage of getting the sender to remove the frame is that it allows data to be broadcast to any number of nodes.
35. Circulating Frames If, for some reason, a frame is not removed by the sender it will circulate forever and reduced the efficiency of the Ring network.
This is not a problem with Bus networks since terminators (or Head Ends) absorb unwanted packets.
Devices called monitors are responsible for housekeeping by marking frames as they pass. If a marked frame comes back round to the monitor, it is then removed.
36. Backbone Network A backbone network connects many smaller networks via devices called bridges.
This type of network is easy to expand and isolates local traffic.
37. Backbone Network - Recap Rather than connecting hosts directly, a network can be used to connect other networks. Such a network is called a backbone network.
Most messages are sent between nearby hosts which are usually connected to the same local networks.
A message for a host on another network is sent (via bridges) over the backbone to the destination network.
Backbone networks are often used by institutions to connect legacy networks.
Small networks of networks, such as those formed with a backbone network, are often called intranets.
38. Finally ….. The Spanning Tree Algorithm The spanning tree algorithm was developed for backbone mesh networks. By mapping a tree topology onto a mesh topology, routing is greatly simplified.
Each network (lettered A to E above) is connected to other networks by bridges (numbered B1 to B3 above).
The spanning tree algorithm dynamically determines a suitable tree and maps it onto the mesh topology.
39. The Spanning Tree Algorithm When a bridge is manufactured, it is programmed with a unique serial number (there will be only one bridge in the world with that serial number).
Let us say our bridges have serial numbers 1, 2 and 3 (usually serial numbers are much bigger, e.g. 5321293).
The first step is to elect a root bridge:
When the spanning tree is initialised, each bridge broadcasts its serial number.
If a bridge receives a higher serial number from another bridge it will stand down from the election.
Eventually only one bridge will remain - the bridge with the lowest serial number (in our case, bridge number 1).
40. The Spanning Tree Algorithm The second step is to determine which networks are directly connected to the root bridge. The root bridge is then set as the parent bridge to these networks.
Any new bridges connected to these networks are added to the tree.
41. The Spanning Tree Algorithm Each new bridge checks to see what networks are directly connected to them.
Any network that does not already have a parent bridge is connected to the new bridge.
If a network can connected to more than one new bridge then it will be connected to the new bridge that is nearest the root.
If both new bridges are equally near the root then the new bridge with the lowest serial number is selected.
Any new bridges on these new networks are then added and the above process is repeated until there are no more networks to add to the spanning tree.
