Digital Communications-II Lecture – 1I

1. Digital Communications-IILecture – 1I By: Shahbaz Ahmad Department of Computer Science, NTU Faisalabad. shehbazsahi@live.com

2. Network • Advantages of a network • Scope of Network (Type) • LAN • MAN • WAN In Last Lecture

3. The physical topology of a network refers to the configuration of cables, computers, and other peripherals • OR Physical Layout of a network • Physical topology VS.  logical topology • Three basic types • Bus • Star • Ring Network Topology

4. A bus topology connects each computer (node) to a single cable segment • A terminator is required at each end to absorb the signal so it does not reflect back across the bus. • Signals are broadcast to all stations • Each computer checks the address on the signal (data frame) as it passes along the bus. • Only one computer can ‘talk’ on a network at a time Bus Topology

5. Advantages Disadvantages Difficult to administer/troubleshoot Limited cable length and number of stations A cable break can disable the entire network; no redundancy Maintenance costs may be higher in the long run Performance degrades as additional computers are added Bus Topology • Easy to implement and extend • Well suited for temporary networks that must be set up in a hurry • Typically the cheapest topology to implement • Failure of one station does not affect others

6. Star Topology • All of the stations in a star topology are connected to a central unit called a hub • Each station has its own direct cable connection to the hub

7. Advantages Disadvantages Failure of hub cripples attached stations More cable required (more expensive to wire a building for networking) Star Topology • Easy to add new stations • Easy to monitor and troubleshoot • Can accommodate different wiring

8. Ring Topology • Consists of a set of stations connected serially by cable • Signal travels around the circle in a clockwise (or anticlockwise) direction. • Signal is transferred sequentially via a "token" from one station to the next • When a station wants to transmit, it "grabs" the token, attaches data and an address to it, and then sends it around the ring. • Each station on the ring has equal access but only one station can talk at a time

9. Advantages Disadvantages Point to point line configuration makes it easy to identify and isolate faults. Moving, adding and changing the devices can affect the network Communication delay is directly proportional to number of nodes in the network Bandwidth is shared on all links between devices Ring Topology • Performs better than a bus topology under heavy network load • Does not require a central node to manage the connectivity between the computers • Due to the point to point line configuration of devices with a device on either side (each device is connected to its immediate neighbor), it is quite easy to install and reconfigure since adding or removing a device requires moving just two connections.