Question 1.
(a) Both circuit and packet switching principles are used in mobile networks. As shown in Figure 1.1, suppose 20 users share a 150 Mbps link. Also suppose each user requires 20 Mbps when transmitting, but each user transmits only 30% of the time.
Figure 1.1 Circuit switched and packet switched users
(i) When circuit switching is used, what is the maximum number of users that can be supported? Explain your answer
[3%]
(ii) Suppose we need support for 13 users, is this possible with circuit-switching, explain?
[3%]
(iii) Suppose packet switching is used for the rest of this question. What is the probability that a given (specific) user is transmitting, and the remaining users are not transmitting?
[3%]
[3%]
(v) When one user is transmitting, what fraction of the link capacity will be used by this user? Write your answer as a decimal.
(iv) What is the probability that one user (any one among the 13 users) is transmitting, and the remaining users are not transmitting?
(vi) What is the probability that any 5 users (of the total 13 users) are transmitting and the remaining users are not transmitting?
(vii) What is the probability that more than 7 users are transmitting?
(b)
(i) Derive the formula for Distribution time for a file in a client-server (CS) and P2P architectures.
(ii) Consider distributing a file of F = 20 Gbits to N peers. The server has an upload rate of us = 30 Mbps, and each peer has a download rate of di = 2 Mbps and an upload rate of u. For N = 10, 100, and 1,000 and u = 300 Kbps, 700 Kbps, and 2 Mbps, prepare a chart giving the minimum distribution time for each of the combinations of N and u for both client- server distribution and P2P distribution. [Hint: 1 Gbits = 1024 Mbits]
(c) As shown in Figure 1.2 below, a mobile phone has a TCP connection with the streaming server using wireless 4G. Assume that both the mobile phone and the server use TCP port 10977 and any intermediary nodes that tunnel the Datagrams use UDP with port 5950.
(i) Describe the functions of the Serving Gateway (S‐GW) and Packet Data Network Gateway (PDN-GW).
[5%, about 200 words]
(ii) Identify the IP source and destination addresses plus the TCP or UDP port numbers for all the packets transmitted between the mobile phone, base station, S-GW and the PDN-GW including all IP tunnels for the encapsulated datagram shown in the Figure
[20%]
Figure 1.2 4G IP tunnelling between a mobile phone and streaming server
(d) A mixture of IPv4 and IPv6 networks are normally used by mobile and fixed network providers, with typically IPv4 at the backbone (core) and IPv6 at the edges. Figure 1.3 below, shows four IPv6 subnets, connected by a mix of IPv6-only routers (shaded blue), IPv4-only routers (shaded red) and dual- capable IPv6/IPv4 routers (shaded blue with red interfaces to IPv4 routers). Suppose that a host of subnet D wants to send an IPv6 datagram to a host on subnet B. Assume that the forwarding between these two hosts goes along the path: D --> E -- > d --> b --> c --> B:
- (i) Which protocol encapsulates the other: IPv4 or IPv6?
[4%]
- (ii) For the IP packets from E to d, what is the IP source and destination addresses. What is the source and destination of the encapsulated IP packet.
[10%]
- (iii) For the IP packets from c to B, what is the IP source and destination addresses. What is the source and destination of the encapsulated IP packet
[6%]
Figure 1.3 Mixture of IPv4 and IPv6 networks
