Featrues of Compputer Networks

What is a topology? A topology is a description/diagram of how computers can be laid out and connected. There are only three basic topologies that are used but these can be combined in a large network

Topologies For each topologies you must be able to…. Describe the key features of each network and be able to draw a diagram of each network topology Describe the advantages and disadvantages Pick and describe a mix of topologies that would be most suited to a particular situation

Bus Network Computers are connected to a single backbone cable. The computers all share this one cable to transmit to each other and only one computer can transmit at a time. This is fine if most of the time the network is not too busy. It can be very slow with lots of computers trying to transmit at the same time.

Bus Network Advantages Disadvantages Easy to implement and add more computer systems to the network Quick to set up – well suited for temporary networks Cost-effective – less cabling It is difficult to troubleshoot the bus Limited cable length and number of stations – performance degrades as additional computers are added If there is a problem with the main cable or connection, the entire network goes down Low security – all computers on the bus can see all data transmissions Proper termination is required Data collisions are more likely, which causes the network to slow down. A collision is when two computers try to send a packet at the same time

Ring Network Computers are connected to adjacent computers in a ring. Computers take turns in transmitting data. This is controlled by a token around the ring. Computers can only transmit when they have the token.

Ring Network Advantages Disadvantages Data is quickly transferred without a bottleneck – consistent data transfer speeds The transmission of data is relatively simple as packets travel in one direction only Adding additional nodes has very little impact on bandwidth It prevents network collisions. If any of the computer systems fail, the ring is broken and data cannot be transmitted efficiently If there is a problem with the main cable or connection, the entire network goes down It is difficult to troubleshoot the ring Because all nodes are wired together, to add a another you must temporarily shut down the network

Star Network All of the computers have their own cable that connects them to a central computer. The central computer controls the network. The central computer is usually a server, where resources are stored.

Star Network Good performance/fast network speed Easy to set up Advantages Disadvantages Good performance/fast network speed Easy to set up Possible to add more computer systems without taking the network down Any non-centralised failure will have very little effect on the network Minimal network collisions Better security Expensive to install – more cabling required Extra hardware required, such as a hub

Tree / Extended Star A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable (See fig. 3). Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.

Advantages of a Tree Topology Point-to-point wiring for individual segments. Supported by several hardware and software venders. Disadvantages of a Tree Topology Overall length of each segment is limited by the type of cabling used. If the backbone line breaks, the entire segment goes down. More difficult to configure and wire than other topologies.

Mixed topologies Most network use a mix of the three topologies The most common are star and bus.

Mesh A network setup where each computer and network device is interconnected with one another, allowing for most transmissions to be distributed, even if one of the connections go down. This topology is not commonly used for most computer networks as it is difficult and expensive to have redundant connection to every computer. However, this topology is commonly used for wireless networks. Below is a visual example of a simple computer setup on a network using a mesh topology.

Considerations When Choosing a Topology Money. A linear bus network may be the least expensive way to install a network; you do not have to purchase concentrators. Length of cable needed. The linear bus network uses shorter lengths of cable. Future growth. With a star topology, expanding a network is easily done by adding another concentrator. Cable type. The most common cable in schools is unshielded twisted pair, which is most often used with star topologies

IEEE Standards IEEE stands for Institute of Electrical and Electronics Engineers and it is the group of people that define how protocols and devices should function. From now we will be discussing protocols in terms of both the protocol name and the IEEE standard. For example…. Wifi is also know as IEEE 802.11

Common Ethernet speeds and distance

Wifi Wifi - 2.4 - 802.11n Typical speed up to 54 Mbps 45 meters indoor // 90 meter outdoor Wifi – 5Ghz – 802.11ac Typical speed up to 800 Mbps Note – distance from the router increase the amount of noise and therefore reduces the speed significantly Note – Multiple channels can be used on both 802.11n and 802.11ac to increase the speed

Coaxial and Fiber Coaxial cable 10-100 Mbps Range 500M Fibre cable 100Mbps – 100 Gbps 10Km+ in single mode (one signal sent down the wire) – Up to 5000Km 2Km+ in multi mode (multiple signals down the wire)

Devices You need to start learning about the different devices that are used to build a basic computer network. For each device you must be able to define its purpose and use as well as draw the diagram for it in a topology diagram.

PC A typical Network PC. For this side of the course the specification of the PC is not important

Server A device which provides a service to other users such a printer queues or file servers

Hub A hub is a networking device that is used to connect multiple devices together. A hub is considered dumb / static / Inactive Any signal that is passed into the Hub will broadcast down every port of the hub

Switch A hub is a networking device that is almost the same as a hub however… A switch is considered active - dynnamic When a signal is sent into a switch it is only sent down the port to which the receiving device is connected. Because of this a switch must know which device is connected to each port. Switches will also keep an eye out for broadcast storms and will return unknown packets or forward them to a default gateway

Repeater A repeater, as the name implies, is a device that receives a signal on one port and repeats down another port. The reason this is used is to extend the distance that a signal can be transmit Repeaters can be wired or wireless Note that all devices have the capability to repeat a signal

Router A router is a device which connects two networks together. They can “Route” traffic between multiple routers to find an end machine. A router can be thought of as a gateway between your LAN and a WAN

Wireless Access Point A wireless Access Point is the point to which a network can use radio waves to connect to mobile devices. Note that you must draw wireless connections between devices different from wired connections

Building a topology map for Smestow School Computing Department On a sheet of A3, draw a topology map for the 4 computer science rooms + Wifi connection throughout + Correct icons for all devices (consider 10 computers per room for ease) + Colour code connections between devices With a Key