of operators for 100% waiting less than 1 min = 612 The number of operators for 90% waiting less than 1 min = 18 Messaging using self-contained packets, be it clay tablets or letters, is perhaps the first type of information tele-communication. In contrast to telephony, packetised telecommunications does not require the establishment of a predefined, static end-to-end link. This flexibility allows for a more robust system design with diversity of channel link capacity. Moreover, data retransmission and queuing techniques permit virtually error-free information transfer. These advantages exist at the expense of system complexity as precise protocols are needed to ensure devices can communicate with each other through a host of diverse intermediate nodes. Nevertheless, advantages are significant enough that packet-switched information transfer is now the dominant mode of telecommunications Review Questions What are the advantages of packet switching compared with circuit switching? 2. What are the relative merits of the OSI and IP stacks? 3. What is the role of the transport layer protocols? 4. What role do the internet layer protocols play? 5. How is the OSI architecture useful in the design of systems operating using the IP approach? What is the role of a link layer protocol such as Ethernet? Can you describe the processes that the Ethernet header supports? Name two factors which determine the size of a buffer at a router. 8. What are the metrics of importance to packet-switched networks? 9. How do switches and routers differ? Broadband Telecommunications Technologies and Management 10. What role does a routing table play? Do you think such a table should be static? Why? A call centre employs 10 operators. On average calls arrive every 15 S and last 120 S. Visit htp://www.math.vu.nl/~koole/ccmath/ErlangC/index.php and answer the following questions: (a) What is the average waiting time? (b) What is the probability that a received call is queued? (c) What is the probability that a received call waits more than 60 