Aug 1999 CS202 : Computer Architecture and Data Communications (Question 3) Suggested Solutions

(a)	Briefly describe, using diagrams, two different network topologies. In each case, five an example application of networking in which you might expect to find this topology and an advantage of this topology over the others.	[8]
	For any two of these five: Mesh topology : point-to-point connections linking most nodes to most others. Used in public and private data communication networks. Provides alternative routing, robust against failure. Ring topology : single closed loop. Each node connected to two others used in LANs, which is economical; little wiring to set up. Bus topology : common channel for data, all nodes connected to it, used in LANs, easy to add new nodes OR easy to implement broadcast. Tree topology : tree-like structure, remote access networks and corporate networks, better reliability/robustness than star topology OR saving on total branch length and cost. Star topology : all nodes connected to a central controller, LANs with a private branch exchange(PBX), company phone system centralized control/easy implementation of security schemes/ only one critical node.
(b)	For the networks you have described, would the transmission usually be serial, or would it be parallel? Explain your answer.	[3]
	These networks will all typically be implemented with serial transmission. The distances involved are too great for parallel transmission to be effective, (due to effects like clock shew). Parallel transmission would be very expensive over distance, too.
(c)	Of all the network topologies you have studied, most would use (half or full) duplex connections. However, one of them, in its simplest form, needs only simplex transmission. Which one is that, and why?	[3]
	Ring network. Data travels in a single direction around the network. The return-path for data is achieved by passing all the way around the ring.
(d)	A string of twenty characters is to be transmitted over a serial data transmission line. Calculate how many bits will be transmitted in each of the following modes, and use this information to compute the transmission efficiency in each case. (i) Synchronous data transmission, with 4 SYN characters and message begun with STX character and ended with EOT character. (ii) Asynchronous data transmission with 1 start bit and 2 stop bits. List any assumptions that you make.	[6]
	*(i) 8 ( 20+4+2) = 208 bits 160 / 208 =76.9% efficiency (ii) (1 + 8 + 2) * 20 = 220 bits 160/220 = 72.7% efficiency**