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Wireless Networks

by: Cassidy Effertz
Cassidy Effertz

GPA 3.64

Raghupathy Sivakumar

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Raghupathy Sivakumar
Class Notes
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This 0 page Class Notes was uploaded by Cassidy Effertz on Monday November 2, 2015. The Class Notes belongs to ECE 6610 at Georgia Institute of Technology - Main Campus taught by Raghupathy Sivakumar in Fall. Since its upload, it has received 10 views. For similar materials see /class/233864/ece-6610-georgia-institute-of-technology-main-campus in ELECTRICAL AND COMPUTER ENGINEERING at Georgia Institute of Technology - Main Campus.



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Date Created: 11/02/15
ECE6610 Wireless Networks 1 What are the relative advantage and disadvantages of the EndtoEnd approach for mobile handoff when compared to mobile IP The advantages of Endtoend approach over Mobile IP are No triangular routing No overheads associated with tunneling packets No constant communication with the Home Agent as the mobile user changes locations No issues with ingress filtering The disadvantage is that 0 Both the sender and the receiver should be modi ed 2 What are the potential problems of ARP in a mobile IP scenario If there is a corresponding host on the home network of the mobile host it will use ARP to determine the layer 2 address of the mobile host If the mobile host moves to another network it will be unable to receive the ARP request and thus communication cannot take place 3 In wireless environment explain how a reliable link layer can affect TCP performance How does the snoop module solve these issues In a lossy wireless environment a reliable link layer will try to retransmit lost packets and thus hide the losses from TCP However retransmissions lead to a variable delay between successive packets This delay affects the rtt calculation of TCP The snoop module which resides in the base station resolves this issue by 0 Caching packets and reforwarding if loss or out of sequence 0 Handling ACKs to prevent DUPACKS 4 In a multihomed mobile clearly explain all the reasons why application layer striping might cause stalling of the traffic due to discrepancies in one pipe How does pTCP avoid this The three reasons why application layer striping is not good are 1 Data rate differential 2 Data rate uctuations and 3 Blackouts See class notes for a complete description of the problems The pTCP approach utilizes a scheme that controls congestion on a per pipe basis TCP v as well controls the aggregate connection behavior pTCP pTCP avoids a aggregate stall from a slow pipe by utilizing the following methodologies 0 Congestion window based striping 7 only allocates data to a particular pipe if the pipe has room Dynamic Reassignment 7 reassign packets that fall out to TCPv congestion window to avoid a stall Redundant Striping 7 after a lost packettimeout a packet is allocated to multiple pipes This reduces the probability that a single pipe will slow stall the aggregate connection 5 How does the rate control mechanism work in WTCP When will underlying CSMN CA MAC affect the rate control mechanism Why Rate control in WTCP uses a finer granularity as opposed to TCP The slow start phase of WTCP has a faster ramp up time WTCP uses a packet pair approach to determine the available rate By measuring the interpacket delay WTCP is able to compute the available network rate In the multiplicative decrease phase rather than using a constant decrease factor WTCP starts out with a small decrease factor and increases the decrease factor each consecutive time congestion is detected Additionally WTCP has a maintain phase based on the interpacket delay When there are losses the CSMNCA MAC 80211 will try try to retransmit lost packets This will cause uneven delays between successive packet receptions These uneven delays affect the rate control mechanism of WTCP as interpacket delay is used to calculate available rate 6 Assume an ftp session running between a wired host and a mobile host Assume TCP reno is used as the transport protocol Assume a single wireless burst loss that occurs when there is no congestion ie the buffers are not currently full and the connection is not in slow start Show how the congestion window behaves when the loss occurs Show in the same graph how WTCP will behave You don t have to give a detailed graph Just show the different trends in TCPreno and WTCP The comparison between WTCP and TCPReno is given in the graph below At t 082 it is assumed that the loss is associated with atimeout WTCP sensing a loss resumes transmitting at the previous rate which is near the capacity of the network By contrast TCPReno senses a timeout and goes to slow start


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