CP2073 Networking Lecture 5

Introduction Physical and Logical Topologies Topologies Bus Ring Star Extended Star Mesh Hybrid CP2073 Networking

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 CP2073 Networking

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 !) CP2073 Networking

Factors Cost Scalability Bandwidth Capacity Ease of Installation Ease of fault finding and maintenance CP2073 Networking

Bus Topology CP2073 Networking

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 CP2073 Networking

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 CP2073 Networking

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 CP2073 Networking

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 CP2073 Networking

Ring Topology CP2073 Networking

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 CP2073 Networking

Ring Topology (3) Advantages Disadvantages 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 CP2073 Networking

Star Topology CP2073 Networking

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 CP2073 Networking

Star Topology (3) Advantages Disadvantages Easy to add devices as the network expands One cable failure does not bring down the entire network (resilience) 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 CP2073 Networking

Extended Star Topology A Star Network which has been expanded to include an additional hub or hubs. CP2073 Networking

Mesh Topology (Web) CP2073 Networking

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” CP2073 Networking

Mesh Topology (3) Disadvantages Advantages Expensive Difficult to install Difficult to manage Difficult to troubleshoot Advantages Improves Fault Tolerance CP2073 Networking

Hybrid Topology CP2073 Networking

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) CP2073 Networking

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 CP2073 Networking

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 CP2073 Networking

Logical Bus A single node failure does not bring the network down 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 CP2073 Networking

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 CP2073 Networking

Logical Ring (2) CP2073 Networking

Logical Ring (3) Advantages Disadvantages The amount of data that can be carried in a single message is greater than on a logical bus There are no collisions Disadvantages A broken ring will stop all transmissions A device must wait for an empty token to be able to transmit CP2073 Networking

Summary Bus Topology Ring Topology Star Topology Other Topologies Logical Topologies Questions and Answers CP2073 Networking