Advanced Ad Hoc Networks

Homework 1 Solution



1. Textbook, Chapter 6, problem 1
Solution:
    Refer to Figure 3. The region marked A1 is the exposed region with respect to the transmission from node A to node B. Similarly region A3 is the one containing hidden terminals. From the figure,

Area of the shaded portion (shaded by vertical lines)= \frac{2\alphaR^{2}}{2}-0.5R^{2}sin{2\alpha}=0.5R^{2}(2\alpha-sin(2\alpha))
Area A_{2} = 2\tims Area of shaded portion = R^{2}(2\alpha-sin(2\alpha))
Now A_{1}=\pi R^{2}-A_{2}
A_{3}=A_{1}


2. Textbook, Chapter 6, problem 2
Solution:
    A certain level of QoS guarantees can be provided in reservation-based protocols which enables ad hoc wireless networks to be used for real-time applications that require certain minimum QoS guarantees. Packet collisions are reduced which improves the packet delivery ratios of flows.


3. Textbook, Chapter 6, problem 3
Solution:
    Contention-based MAC protocols can be used for civilian applications such as communication in conferences or within offices where the information to be communicated is not time-critical and channel access delay can be tolerated. Though the users may not get guaranteed services, the available bandwidth would be utilized to the maximum. Reservation-based protocols find use primarily in time critical applications such as military communication where unpredictable information transmission and channel access delays would not be tolerated. Though the users would be provided with certain QoS guarantees, bandwidth utilization in such networks would be lower compared to those networks employing contention-based MAC protocols. But since the nature of applications is time critical, the reduction in bandwidth utilization is compensated by the QoS guarantees offered. Scheduling-based MAC protocols can be used in applications where fareness among nodes for channel access and fairness among nodes for channel access and fairness among individual flows is required. Commercial communication networks where fairness among customers trying to access the channel is one such application. Scheduling-based MAC protocols can also be used for enforcing priorities among flows whose packets are queued at nodes.


4. Textbook, Chapter 6, problem 4
Solution:
     The advantages of using scheduling-based MAC protols are:
Increase in network load results in graceful degradation of QoS.
When the mobility is high, the performance is better than that of reservation based protocols.
Bandwidth utilization is better than that of reservation-based schemes.
The disadvantages of cheduling-based protocols when compared to reservation-based protocols are:
Firm QoS guarantees cannot be provided as in reservation-based schemes.
Performance is dependent on the accuracy of neighborhood information available at nodes.
Control overhead is high as regular information exchange between neighbor nodes is required. But in reservation-based protocols, since the decision making is completely local, the control overhead involved is less.


5. Textbook, Chapter 6, problem 6
Solution:
    In MACA, each node maintains just one queue for holding the packets that are ready for transmission. But in MACAW, multiple queues are maintained at every node, one each for each flow. The back-off algorithm is run independently for each queue. The queuing mechanism used in MACAW is better because per flow fairness can be provided in MACAW as opposed to the per node fairness in MACA.