모바일 컴퓨팅 기말고사
2019. 12. 18.
다음의 질문에 최대한 짧게 답하시오 (핵심없이 길면 감점). 하지만 논리적이고 핵심적인 답변만 점수를 다 받을 수 있습니다.
Question 1 (20 points)
(a) (4 points) Slotted aloha is a variant of the Aloha media-access protocol that divides time into equal sized “slots” and only starts transmissions at the beginning of a slot time. Explain why this approach performs better than pure Aloha (if you’re not sure that your explanation is clear an example may help).
(b) (4 points) In a slotted aloha system, suppose there are 5 nodes that each transmit with probability of p. What is the probability, in a given slot time, that one of the nodes successfully transmits a packet?
(c) (4 points) Why does CSMA perform better than Aloha? Be specific.
(d) (4 points) For IEEE 802.11 MAC, describe one case (set of conditions) where a collision may still occur after collision avoidance
(e) (4 points) Why is SIFS shorter than DIFS ?
Question 2 (15 points)
Consider the following scenario with UDP and TCP over IEEE 802.11 (CBR means Constant Bit Rate)
과목명
모바일 컴퓨팅
점수
이름
학번
where S0 is a station wire-connected to the access point (AP). MS1 and MS2 are wireless nodes associated to the AP, using IEEE 802.11 (DCF basic mode, i.e. no RTS/CTS). The cable capacity is 10 Mb/s, the total wireless channel capacity is 2 Mb/s. Note that all downlink flows share the same queue at the AP.
Scenario 1: Consider the channel to be clear (no noise). S0 sends 1 CBR/UDP flow to MS1 at 5 Mb/s, and 1 CBR/UDP flow to MS2 at 5 Mb/s. (Both flows go through the AP)
(a) (5 points) What are the throughput and packet drop rate for each flow ?
(b) (5 points) What is the probability of a collision on the radio channel ? Explain why.
Scenario 2: S0 sends 1 CBR/UDP flow to MS1 at 5 Mb/s, and 1 CBR/UDP flow to MS2 at 5 Mb/s, (both flows go through the AP). The channel between the AP and MS2 has a high error rate.
(c) (5 points) Comment on the throughputs received at MS1 and MS2. Compare them to the result of Scenario 1.
Question 3 (15 points):
In a cellular system with seven-cell clusters, the average number of calls at a given time is given as follows:
If the system is assigned 49 traffic channels, how would you distribute the channel if (a) (5 points) Static allocation is used based on traffic load
(b) (5 points) An FCA(fixed channel allocation) simple borrowing scheme is used (no traffic load considered)
(c) (5 points) A dynamic channel allocation scheme is used
Question 4 (20 points):
(a) (4 points) Why do IP packets carry a time-to-live (TTL) field? What would happen if they didn’t have it?
(b) (4 points) Name two modifications of IPv6, from the IPv4, that allow a router to process a packet quicker.
(c) (4 points) Explain the use of the “Next Header” field in the context of an IPv6 datagram and how it effectively minimizes the size of a IPv6 header and its processing time, in comparison to the IPv4 header?
Cell Number
Average number of calls/unit time
1
900
2
2000
3
2500
4
1100
5
1200
6
1800
7
1000
(d) (4 points) In Mobile IP, when a packet is sent to a mobile which is not in its home network what mechanism is used to capture the packets destined to mobile? Explain briefly.
(e) (4 points) What is triangular routing problem in Mobile IP? How does Mobile IPv6 address it?
Question 5 (30 points):
Figure 2 shows an IP network with a set of routers (R) and hosts (H). The lines show the IP connectivity between the devices. The lower case letters (a, b and c) are labels for the router interfaces. For example, router R5 has three network interfaces: a, b and c (or R5a, R5b and R5c). Each host only has a single network interface. Hosts H1 and H2 are in the same IP subnet as R1a (called subnet W). Router interfaces R1b and R5a are in the same IP subnet as each other (but different from subnet W). Assume any layer 2 network technology (wired or wireless) may be used to provide the IP connectivity between interfaces. The figure does not indicate physical proximity of devices: that is, although hosts H2 and H3 are near each other in the figure, they may not be near each other in the real network.
Consider the case when any of the hosts may be mobile and Mobile IP is used. Assume hosts may only attach to router R1, R2, R3 and R4 (that is, the other routers do not support hosts visiting their IP subnet). In your answers you may refer to a device or interface by its name (e.g. H1; R5 or R5a). Instead of giving a real IP address, indicate the interface (e.g. H1; R5a).
(a) (2 points) List the Home Agent(s) with respect to H1.
(b) (2 points) List the Foreign Agent(s) with respect to H1.
Host H1 is corresponding with host H8 (that is, H8 is the correspondent node). Assume that during the communication host H1 moves such that it leaves subnet W and enters subnet Y. Mobile IP is used to maintain the connection during movement.
(c) (4 points) Explain two ways in which H1 can detect it has entered a new subnet.
After detecting a new subnet, H1 must perform a Mobile IP registration.
(d) (4 points) Draw a diagram that illustrates the Mobile IP registration procedure. You should show the types of messages sent and the devices involved.
(e) (4 points) Assuming the FA assigns its own IP address as the COA of visiting mobile nodes, show the four entries in the Mobility Binding Table and Visitors List after the registration is complete.
(f) (7 points) Now the registration is complete, indicate the path that packets take for IP packets being sent (indicate the path by listing the complete set of routers, in order, the packets pass via):
1. From H8 to H1
2. From H1 to H8
(g) (7 points) Draw the packet received by router R6 for the following cases (the packet must clearly indicate all source/destination IP addresses carried in the packet header):
1. From H8 to H1
2. From H1 to H8