LAN Topologies Part 1

What is topology? Topology is the physical or logical interconnection of communicating devices Physical Topology: LANtopology, workstations (computers) Physical Topology: The physical layout of devices on a network. Every LAN has a topology, or the way that the devices on a network are arranged and how they communicate with each other. It can also be define as the way that the workstations (computers) are connected to the network through the actual cables that transmit data -- the physical structure of the network

Logical TopologySignal Topology. Logical Topology: Also known as Signal Topology. The logical topology is the way that the signals act on the network media, or the way that the data passes through the network from one device to the next without regard to the physical interconnection of the devices. Logical topology Logical topology can also be defined as relationship between nodes (workstations) in the network as seen by the software that handles data delivery from station to station protocols Logical topologies are bound to the network protocols that direct how the data moves across a network. Note: Note: Logical topology of a given network is not necessarily the same as its physical topology.

Categories of LAN Topology

Framing carry data in small units frames To control the transmission of data, all topology carry data in small units called frames Frameheader or a trailer (or both) Each Frame contains a part of data to be sent and a header or a trailer (or both) header The header normally contains the sender and receiver physical address header or trailer error control The header or trailer can also contain control information such as a sequence number or redundant information to be used in error control

Framing

Addressing In all topologies, an addressing mechanism ensure that a frame is received by one or more destinations Each station (node) is assigned a unique address When a station (node ) sends a frame includes Source Address and Destination Address unicast, multicast broadcast When the frame is received by a station, it checks to see if the destination address matches one of its three addresses: unicast, multicast and broadcast

Unicast: Unicast: Unicast addressing means one – to – one communication. A station sends a frame that is received by only one station Multicast Multicast : Multicast addressing means one –to- many communication; a station sends a frame that can be received by a selected number of stations  A group of stations can have a common multicast address, which is different from their unicast addressees  When a station wants to send a frame to a group of stations, it uses multicast address defining that group Broadcasting: Broadcasting: Broadcasting means one – to – all communication; a station sends a frame that can be received by all other stations Broadcast can be considered a special case of multicast communication ; the group includes all of the communication

Mesh Topology

In a mesh topology, each of the node or station (computer) and other devices, are interconnected with one another. That is every node is connected to every other node in the network. This type of topology is very expensive as there are many redundant connections, therefore it is not mostly used in computer networks. It is a preferred topology in wireless networks. Mesh Topology

Number of links in mesh topology Where x is the number of nodes in mesh network

Where p is number of I/O ports a node must have Number of I/O interface per node

Advantages of Mesh Topology Even if one of the components fails there is always an alternative present. So data transfer doesn’t get affected. That is it has multiple links, so if one route is blocked then other routes can be used for data communication. Each connection can have its own data load, so the traffic problem is eliminated. It is easy to troubleshoot mesh topology compared with other topologies. Its performance is not affected with heavy load of data transmission. The arrangement of the network nodes is such that it is possible to transmit data from one node to many other nodes at the same time. Data can be transmitted from different devices simultaneously. This topology can withstand high traffic. Expansion and modification in topology can be done without disrupting other nodes.

Disadvantages of Mesh Topology There are high chances of redundancy in many of the network connections. Overall cost of this network is way too high as compared to other network topologies.other network topologies Set-up and maintenance of this topology is very difficult. Even administration of the network is tough.

Bus Topology

Bus Topology (called bus), interface connectors (T-Connector) In Bus Topology all the nodes (computers or stations) are connected to the single cable (called bus), by the help of interface connectors (T-Connector). Interface connector (T-Connector )medium interface Interface connector (T-Connector ) is also called medium interface This central cable is the backbone of the network Every workstation communicates with the other device through the Bus. The connection is multipoint. All stations share the same medium broadcasted A signal from the source is broadcasted and it travels to all workstations connected to bus cable. Although the message is broadcasted but only the intended recipient, whose destination address matches, accepts it. destination address If the destination address of machine doesn’t match with the intended address, machine discards the signal

terminator A terminator is added at ends of the central cable (bus), to prevent bouncing of signals. barrel connector A barrel connector can be used to extend the bus.

Bus Topology

Addressing in Bus Topology Bus topology allows unicast, multicast and broadcast addressing. Each station checks the destination address of the frame received. If the destination address is unicast, it must match the physical address of the station; otherwise the frame is discarded. If the destination address is multicast, the station checks to see if the station belong to that group. If it does, the station keeps the frame; otherwise the frame is discarded. If the destination address is broadcast, the station keeps the frame because the frame is destined for all stations

Bus Topology Operation

Bus Topology Operation -Explanation unicast frame  Bus topology Operation figures shows the mechanism of sending a unicast frame from one station to another. the unique destination  The frame is received by all stations, but it is discarded by all except the unique destination station A station C station B  In the figure, a frame from station A is destined to station C is received and discarded by station B

Advantages of Bus Topology Easy to connect a computer or peripheral to a linear bus. Requires less cable length than a star topology. It is easy to set-up and extend bus network. Bus topology is not expensive compared with other topologies.

Entire network shuts down if there is a break in the main cable. That is dependency on central cable in this topology has its disadvantages. If the main cable (i.e. bus ) encounters some problem, whole network breaks down Terminators are required at both ends of the backbone cable. Difficult to identify the problem if the entire network shuts down. Efficiency of Bus network decreases as the number of devices connected to the bus increases. That is there is a limit on central cable length and number of nodes that can be connected It is not suitable for networks with heavy traffic. Disadvantages of Bus Topology