SaraAli-266

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Question# 01

a. What are the disadvantages and advantages of the wireless medium compared tothe high speed wired medium like fibre optic?

ADVANTAGES OF HIGH SPEED WIRED MEDIUM OVER TO

WIRELESS MEDIUM:1: Speed: A fiber-optic connection is faster than wireless by many orders of magnitude. A single

optical fiber can carry about 3 trillion bits per second (bps). The fastest wireless service (fixed

wireless access) approaches 2 million bps.Fiber optics can be more than a million times faster.

2: Data transfer capacity: The capacity to transfer data is superior in optical fibres.

3: Physical limitations: Wireless is useful for delivering broadband to a small number of usersin remote areas. But it is incapable of doing so in densely populated urban areas. Physicallimitations prevent practical wireless speeds from approaching those available over fibre-opticcables. The WLAN is significantly worse than wired in the risk of jamming, potential for

inference, and in the detection of RF signal .

4: Cost: Wireless network connections are prohibitively expensive, typically being 3-4 timesmore expensive, for less data volume and at a much slower speed.

DISADVANTAGES OF HIGH SPEED WIRED MEDIUM OVER

WIRELESS MEDIUM:

1: No mobility of use – Wired medium lacks mobility of use. While using wireless,people inrange can operate anywhere in range. The WLAN equipment can provide customers withconnectivity to real-time information anywhere in their work areas. This flexibility supports

productivity and service opportunities not possible with traditional wired networks.

2: Wire Maintenance - Installation of WLAN equipment can be fast, easy and can eliminatethe need to pull cable through walls and ceilings. They virtually eliminate the cost of wire andcabling media because they use the airways as their transmission media.

3: Difficult installation – While in wireless, no need to run wire and cable which can be

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elaborate and labor intense. The most important benefits of WLAN are flexibility, mobility and portability

b. Describe Multipath Fading.

Multipath is a term used to describe the multiple paths a radio wave may follow between

transmitter and receiver. Such propagation paths include the groundwave, refraction, reflection from the earth’s surface. Radio waves that are received in

phase reinforce each other and produce a strong signal at the receiving side,while those that are received out of phase produce a weak or fading signal.Small alterations in the transmission path may change the phase relationship of the twosignals. In this way, due to multipath fading, the received signal does not remain theactual transmitted signal.

c. What is the main difference between receiver initiative MAC protocols MACA-BIand MARCH?

○ In MACA-BI receiver needs to have traffic characteristic information of the

transmitter. It should know the sender data rate.○ In MARCH receiver does not need to know the sender data rate.

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Question 2.

a. Describe the problems when CSMA/CD is applied to wireless networks. Whatmethods could solve these problems?

REASONS FOR NOT APPLYING CSMA/CD IN WIRELESS

NETWORKS:

CSMA/CD is not applicable to wireless networks because of the following problems:

• Wireless has varying link quality due to noise from devices operating in thesurrounding, multipath fading and signal attenuation.

• In wireless interference is a receiver based phenomena i.e. signals collide at thereceiver. It is practically not possible for the transmitter to know the link status atthe receiver. Thus collision sensing and detection does not work for wireless.

• Packets of longer length are more prone to error than with smaller length.

SOLUTIONS:

The basic technique used in wireless to solve the problems is collision avoidance.

• Fragmentation is used to make packets smaller in length.• RTS/CTS packets are used for reliable data transfer.

• Nodes wait for random amount of time even when channel is free.

a. Consider Figure 1. Transmission is going on between nodes A and B which isseparated by a distance d. R is the transmission range of each node. Mark in thefigure the regions that could contain hidden terminals and exposed terminals.

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Shaded region=Hidden terminals present

Un-shaded region=Exposed terminals present

Question 3.

a. TCP is a connection-oriented end-to-end protocol which is very sensitive to packetlosses. What problem(s) can you identify if TCP is deployed on a wireless node?Moreover, why TCP is not commonly observed in MANET’s?

PROBLEMS OF DEPLOYING TCP ON A WIRELESS NODE:

In wireless congestion is not the primary reason of packet loss. There are certain other reasons

due to which packets are lost or received late. These are given below:

Dynamic reconfiguration and high bit error rate cause loss of packets in wireless. Uplink and downlink are different in wireless. This causes acknowledgements to

arrive late. There are multipath for the packets therefore packets arrive late and disordered. Mobility also causes delayed arrival of packets. If TCP is employed it will move the node to slow start for all these cases. Thus

bandwidth wastage occurs.

REASON FOR NOT DEPLOYING TCP IN MANET’s:

High mobility is a characteristic of MANET. TCP considers congestion as a primary reason for

packet loss. Sometimes wireless link are directional in ad hoc networks leading to delivery of a

packet to a node and failure in the delivery of ACK. Thus it will bring a node to slow start which

is not justifiable for packet loss in MANET.

a. For the problem(s) identified in part (a), can you propose modifications to thestandard TCP that will fix the problem(s)?

Tuning TCP for Wireless:1. Split Connection Approaches: an intermediate node establishes two separate

connections on the wired and wireless links2. End-to-End Approaches: which generally make changes to TCP at both the sender

and the receiver to make them wireless-aware3. Link Layer Approaches: where packets lost due to channel errors are recovered

using TCP-aware link layers

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Question 4.

Assume a wireless ad hoc network where a node S wants to send some data to node D, butdoes not have a route to node D. Naturally node S wants to determine the shortest route to

node D.

a. Node S discovers a route to node D using an unoptimized DSR protocol. However,the DSR-discovered route is not the shortest route. In your opinion, what is the mostlikely reason for not receiving the shortest route? Justify your answer.

The reason for not receiving the shortest path is link failure faced by a packet on its wayto destination. When during propagation of RREQ,any link failure occurs, route error

message is sent to source and an alternative path is used. In this case,there are chancesthat the alternative path provided by source may not be the shortest route.

b. What is the main difference between the Dynamic Source Routing (DSR) and Adhocdistance vector routing protocols (AODV)? Justify your answer.

DSR includes source routes in packet headers. This causes the packet size toincrease as the number of intermediate nodes increase.

In AODV source routes are maintained in tables at the nodes. Thus packet lengthis not an issue in large networks with many intermediate nodes.

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