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LAN ARCHITECTURE By Dr. Khalid Hussain 1 Introduction • Physical and Logical Topologies • Topologies – Bus – Ring – Star – Extended Star – Mesh – Hybrid 2 Physical vs. Logical Topology • The actual layout of a network and its media is its Physical Topology • The way in which the data access the medium and transmits packets is the Logical Topology • A glance at a network is not always revealing. Cables emerging from a Hub does not make it necessarily a Star Topology – it may actually be a bus or a ring 3 Physical vs. Logical Topology (2) • Your choice of Logical Topology will affect the Physical Topology – and vice versa • Design carefully – it may be difficult to change part way through the installation • Your choice will determine cable installation, network devices, network connections, protocols (and where you will drill holes in the building !) 4 Factors • • • • • Cost Scalability Bandwidth Capacity Ease of Installation Ease of fault finding and maintenance 5 Bus Topology 6 Bus Topology (2) • Network maintained by a single cable • Cable segment must end with a terminator • Uses thin coaxial cable (backbones will be thick coaxial cable) • Extra stations can be added in a daisy chain manner 7 Bus Topology (3) • Standard is IEEE 802.3 • Thin Ethernet (10Base2) has a maximum segment length of 200m • Max no. of connections is 30 devices • Four repeaters may be used to a total cable length of 1000m • Max no. of nodes is 150 8 Bus Topology (4) • Thick Ethernet (10Base5) used for backbones • Limited to 500m • Max of 100 nodes per segment • Total of four repeaters , 2500m, with a total of 488 nodes 9 Bus Topology (5) Advantages • Inexpensive to install • Easy to add stations • Use less cable than other topologies • Works well for small networks Disadvantages • No longer recommended • Backbone breaks, whole network down • Limited no of devices can be attached • Difficult to isolate problems • Sharing same cable slows response rates 10 Ring Topology 11 Ring Topology (2) • No beginning or end (a ring in fact !!) • All devices of equality of access to media • Single ring – data travels in one direction only, guess what a double ring allows !? • Each device has to wait its turn to transmit • Most common type is Token Ring (IEEE 802.5) • A token contains the data, reaches the destination, data extracted, acknowledgement of receipt sent back to transmitting device, removed, empty token passed on for another device to use 12 Ring Topology (3) Advantages • Data packets travel at great speed • No collisions • Easier to fault find • No terminators required Disadvantages • Requires more cable than a bus • A break in the ring will bring it down • Not as common as the bus – less devices available 13 Star Topology 14 Star Topology (2) • Like the spokes of a wheel (without the symmetry) • Centre point is a Hub • Segments meet at the Hub • Each device needs its own cable to the Hub • Predominant type of topology • Easy to maintain and expand 15 Star Topology (3) • Advantages • Easy to add devices as the network expands • One cable failure does not bring down the entire network (resilience/flexible) • Hub provides centralised management • Easy to find device and cable problems • Can be upgraded to faster speeds • Lots of support as it is the most used • Disadvantages • A star network requires more cable than a ring or bus network • Failure of the central hub can bring down the entire network • Costs are higher (installation and equipment) than for most bus networks 16 Extended Star Topology A Star Network which has been expanded to include an additional hub or hubs. 17 Mesh Topology (Web) 18 Mesh Topology (2) • • • • Not common on LANs Most often used in WANs to interconnect LANS Each node is connected to every other node Allows communication to continue in the event of a break in any one connection • It is “Fault Tolerant” 19 Mesh Topology (3) Advantages • Improves Fault Tolerance Disadvantages • Expensive • Difficult to install • Difficult to manage • Difficult to troubleshoot 20 Hybrid Topology 21 Hybrid Topology (2) • Old networks are updated and replaced, leaving older segments (legacy) • Hybrid Topology – combines two or more different physical topologies • Commonly Star-Bus or Star-Ring • Star-Ring uses a MAU (Multistation Access Unit (see later slide) 22 Types of Logical Topology • • • • Previous slides showed Physical Topologies Only two Logical Topologies (Bus or Ring) Physical Bus or Ring easy to conceptualise Physical Star could be either a Bus or Ring in logical terms • Confused ? See next slides 23 Logical Bus •Modern Ethernet networks are Star Topologies (physically) •The Hub is at the centre, and defines a Star Topology •The Hub itself uses a Logical Bus Topology internally, to transmit data to all segments 24 Logical Bus Advantages • A single node failure does not bring the network down • Most widely implemented topology • Network can be added to or changed without affecting other stations Disadvantages • Collisions can occur easily • Only one device can access the network media at a time 25 Logical Ring • Data in a Star Topology can transmit data in a Ring • The MAU (Multistation Access Unit) looks like an ordinary Hub, but data is passed internally using a logical ring • It is superior to a Logical Bus Hub – see later slide 26 Logical Ring (2) 27 Logical Ring (3) Advantages • The amount of data that can be carried in Disadvantages a single message is • A broken ring will greater than on a stop all logical bus transmissions • There are no • A device must wait collisions for an empty token to be able to transmit 28 Summary • • • • • • Bus Topology Ring Topology Star Topology Other Topologies Logical Topologies Questions and Answers 29