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# Multimedia Database Systems CS 64100

Purdue

GPA 3.68

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This 4 page Class Notes was uploaded by Nick Rowe on Saturday September 19, 2015. The Class Notes belongs to CS 64100 at Purdue University taught by Staff in Fall. Since its upload, it has received 42 views. For similar materials see /class/208101/cs-64100-purdue-university in ComputerScienence at Purdue University.

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Date Created: 09/19/15

Problem B Paper review B Probakaran Multimedia information delivery over wireless channels Multimedia tools and appl 152115126 2001 B1 Major contributions The above paper appeared as one the papers in the special issue on multimedia rystems mobile computing and global information sharing It is reviewed by the editor that appreciates the low available band of a wireless mobile channel thus considers optimized scheduling of multimedia traf c based on constrained linear programming approaches Basically the paper is solving the problem of scheduling multimedia information in wireless environments The bandwidth of wireless networks is limited so only unicast communication is investigated More speci cally the author assumes the multimedia materials are organized page by page with each page including all kinds of text image audio or video Pagez Page 1 D Pa 9 A ii Esme a U 51 of Objects Available Initially Iniormatiun me Figure 6 Delivering multimedia pages over unicast Wireless channels More practical issues make this problem complicated For example if there are N multimedia pages to present in series by a server 1 Every page i might be available only a er a release time r39 2 The importance of page i is identi ed by a parameter w39 3 The transmission and processing time of page i is p so the completion time C 2 r P1 4 The precedence ofpage i may be higher than some other page j denoted by i ltj39 5 The user may interrupt a page being presented and go to the next page this is called preemption 6 More multimedia pages may come before the current n pages being presented so on line dynamic scheduling is needed in this case 7 In broadcast mode the popularity of each page should be taken into consideration So the problem is stated as an optimization problem as follows N Minimize Z ijj 1 1 Subject to the precedence constraints Ck 2 C pk for each pairj k such thatj ltk where C 2 r p 2 CkZCjpkv oerZCkpjforeachpairjk 3 The above problem however is an NPhard problem which can not be solved in polynomial time So approximation is needed in order to get a suboptimal solution To this end the nonlinear constraints are approximated by a linear inequality 2 2 ijCJ 2 p Sps for each S g N 4 jES And a constraint schedule algorithm using standard linear programming techniques is proposed 1 Minimize 1 subject to constraints 2 and 4 2 Find optimal solution to the above linear program and obtain optimal scheduling C1 Cn 3 Schedule the multimedia pages in order of non decreasing Ci In case of ties precedence relations are used to sequence the pages When it comes to the preemption a preempted page i will be processed in a shorter time than pi but there is still a minimum waiting time for a page to become available represented by rmin In this case the above constraint 4 is modi ed by adding a term rmltsgtpltsgt on the right hand side And the same algorithm can be used to solve the new optimization problem The proposed algorithm is also modi ed to coordinate online schedule when a new set of multimedia pages become available for delivering The solution is straightforward just do the above algorithm again when new pages arrive even when there are still some old pages left The broadcast case is dealt with by delivering previous pages repeatedly according the popularity parameters The paper finally implements the presented algorithm in a test involving an Ethernet LAN a wireless LAN access point and a mobile laptop It is shown that the tested multimedia 30 pages can be presented in several seconds for different multimedia types B2 Weakness The idea of the above reviewed paper is quite clear And the proposed scheduling algorithm is easy to implement However in my own view there are still several weaknesses 1 As the title says the paper is working on the multimedia delivery over wireless channels but none of the algorithms presented in the paper addresses the particular characteristic of wireless channels Although the experiment is done in wireless environment there is no evidence that the proposed algorithm is tting well to the low bandwidth and high errorrate of wireless networks According to my own experience in the authors wireless experiment there is no much chance that there will be any transmission error due to wireless link or network congestion because the wireless LAN should work well when the mobile node is within 100m of the access point and the traf c is less than 1 Mbps 2 The algorithms are based on an approximation technique given by l I also read 1 and nd that the whole optimization ideas including the de nition of symbols are borrowed from 1 So I doubt if this algorithm is realistic enough to the wireless environment For example in order to use the approximation technique by l the author replaces the weight Wj by p j in his equation 4 But I don t think the weight of one page is proportional to its processing time 3 For the case of preemption the author adds an item rm to represent the in uence of preemption This makes the algorithm still work But I don t think this is the right way to address the preemption problem because there is no reason that the rmin constraint will include all the effects of preemption and the choice of rmin is not given 4 The given equation is not clear enough in the favor of mathematics For instance the symbols p2N pN2 and pS in equation 3 and 4 of the paper do not make sense and are not explained in the mathematical way B3 Possible future works Base on the above weaknesses I recommend several possible future works 1 Considering the low bandwidth and errorprone nature of wireless channels the proposed algorithm can be further improved with explicit representation of wireless constraint More experiments in wireless environments are needed in this case 2 The proposed algorithm only works in single server and single client case We can actually extend it to the multiserver multiclient case The multimedia pages might be processed in parallel in this case We notice that in l the multiserver problem has already been considered Actually when it comes to the scheduling problems there have already been a lot of results in operation research which can be applied to practical problems such as multimedia delivery in wireless networks 3 We should notice that when the algorithm is executed dynamically it will involve considerable computations Hence a faster optimization algorithm is preferred This again can be borrowed from the rich results in operation research B4 Reference 1 L A Hall et al Scheduling to minimize average completion time Offline and on line approximation algorithms Mathematics of Operations Research 22 513544 1997

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