>>> SOLUTIONS <<< Comprehensive Final Exam for Computer Networks (CNT 6215) Fall 2011

Transcription

1 Comprehensve Fnal Exam for Computer Networks (CNT 65) Fall >>> SOLUTIONS <<< Welcome to the comprehensve Fnal Exam for Computer Networks. Read each problem carefully. There are requred problems each worth ponts. There s also an addtonal extra credt queston worth ponts. You may have wth you a calculator, pencls and/or pens, erasers, blank paper, and one 8.5 x nch formula sheet. On ths formula sheet you may have anythng you want (defntons, formulas, homework answers, old exam answers, etc.) as handwrtten by you n pencl or nk on both sdes of the sheet. Photocopes, scans, or computer generated and/or prnted text are not allowed on ths sheet. Note to tablet (Pad, etc.) users you may not prnt-out your handwrtten text for the formula sheet. You have mnutes for ths exam. Please use a separate sheet of paper for the answer to each queston. Good luck and be sure to show your work! Problem # Each box and flow worth about pont. Sketch the flowcharts of a TCP/IP server and clent showng key sockets functons and nteractons (or flows) between the two flowcharts. You need not lst the functon arguments, you just need to gve the functon names (e.g., bnd(), etc.). Coped from Arthur Dumas, Programmng Wnsock, SAMS Publshng, 995.

2 Problem # Identfyng delays s 4 ponts. U formula s 6 ponts. If clam of neglgble s made must explan why. Derve U (utlzaton) for a Stop-and-Wat protocol. Assume that packets are never lost. For a packet-ack exchange we have the followng delays, t prop, t xmt, t proc, t prop, t ack, and t proc. Of these delays, t xmt s useful (data transfer), the rest are overhead and thus: U t xmt t prop txmt t For a lnk that s wde area (many mles n dstance where t prop > s), for modern processors that can process thousands of nstructons per mllsecond, and for ACK packets x smaller than data packets, t proc and t ack are neglgble and thus U can be approxmated as: U t xmt txmt t prop proc t ack Problem #3 Each secton dentfed s ponts. Note on analytcal modelng s ponts. Brefly (n words or less) summarze Klenrock s 993 IEEE Proceedngs paper On the Modelng and Analyss of Computer Networks. Ths paper s a landscape of analytc models for computer network performance evaluaton. The paper has four major topcs, whch are, ) delay analyss of networks usng the M/M/ model and Klenrock s ndependent assumpton, ) networks as a desgn problem usng classc mnmzaton technques, 3) networks as a control problem (routng and flow control) ntroducng the measure of power for flow control methods, and 4) a dscusson of how ggabt networks are dfferent than prevous lower-speed networks (notably more packets are n flght than buffer sze makng flow control dffcult) ntroducng the concept of latency and bandwdth lmted networks. Problem #4 Identfyng number of trals s 4 ponts. Correct lstng of experments s 6 ponts. Gven factors A, B, and C where factors A and B have levels each and factor C has 3 levels, show a fullfactoral expermental desgn. How many experment trals must be run f for each factor level combnaton a total of 3 replcatons are needed? For a full-factoral desgn we wll have x x 3 = experments, wth 3 trals per experment we wll have 36 trals to run. The expermental desgn (showng here factors and factor levels s): A, B, C A, B, C A, B, C3 A, B, C A, B, C A, B, C3 A, B, C A, B, C A, B, C3 A, B, C A, B, C A, B, C3

3 Problem #5 Markov chan s 4 ponts. Set-up of equatons s 4 ponts. Correct soluton s ponts. For the followng P matrx sketch the correspondng Markov chan. Solve for the steady state probabltes for the three states. The Markov chan s: P We can solve for the steady state probabltes (,, and ) by solve for three equatons dervable from the P matrx, whch are: To solve, we need to break lnear dependence so we replace the thrd equaton wth the conservaton equatons as: The soluton to these three equatons n three unknowns s =., =.8, and =.5. You should recognze ths problem as the Klenrock hppe problem presented n class. Problem #6 Markov chan s ponts. Set-up for,, L, and W are ponts each. Derve W for M/M/ startng wth a Markov chan. We frst draw the Markov chan for the M/M/ queue. Number of customers n the system Customer arrval rate 3 Customer servce rate

4 We solve the Markov chan for L. Once we have L we can easly fnd W usng Lttle s Law (L = W). Solvng for L, from local balance we wrte,,, for We can now solve for wth the knowledge that the sum of must equal one, So, now we have the steady state probabltes as, The mean then follows drectly from the defnton of mean, L Now we can solve, W. Problem #7 Derve W for M/M/ startng wth the P-K formula. The P-K formula s, x x W where x s the frst moment of the servce tme and x s the second moment of the servce tme (whch s not the same as the frst moment squared). For M/M/ wth exponentally dstrbuted servce rate we know that, x and x. Thus, W Whch s W for M/M/ as derved from the Markov chan n the prevous problem. P-K formula s ponts. 4 ponts for x_bar and x_squared_bar. ponts for algebra for result.

5 Problem #8 Faclty s pont. Each functon s 3 ponts. Wrte a smulaton model for an M/M/ queue usng the CSIM functon lbrary. #nclude "csm.h" #defne SIM_TIME. FACILITY Server; double Lambda; double Mu; vod generate(vod); vod queue(vod); vod sm(vod) create("sm"); Server = faclty("server"); Lambda =.; Mu = 3.; generate(); hold(sim_time); report(); vod generate(vod) create("generate"); whle() hold(exponental(. / Lambda)); queue(); vod queue(vod) create("queue"); reserve(server); hold(exponental(. / Mu)); release(server); Problem #9 Descrpton (notng.5 to.) s 4 ponts. Each remanng sub-queston s ponts. Descrbe the Hurst parameter. Who was Hurst? Who was the frst to apply the Hurst parameter to network traffc (and when dd ths occur)? What s the possbly mpact (to capacty plannng of networks) of traffc wth a large Hurst parameter value? The Hurst parameter s a measure of self-smlarty (or long-range dependence) of a tme seres. The Hurst parameter vares from.5 (completely ndependent) to. (completely self-smlar). There are varous ways to estmate the Hurst parameter for a tme seres. Hurst was a hydrologst studyng the hstorcal record of floodng of the Nle rver n order to capacty plan the Aswan Dam. Leland et al. n the early 99s dscovered that network traffc s self smlar and used the Hurst parameter as a measure of ths self smlarty. Bursty self smlar traffc streams when aggregated or multplexed together do not smooth out they reman bursty. Ths means that capacty plannng based on Posson assumptons wll underpredct the requred bandwdth and/or bufferng needed to carry traffc.

6 Problem # Sub-queston (a) s 4 ponts, the remanng two sub-questons are 3 ponts each. Answer the followng questons regardng WSNs: a) What are the necessary and optonal components of a WSN node? Necessary components are: sensng unt, memory (or storage), processor, communcatons, and power source. Optonal components nclude: power generatng unt (solar, vbraton, other), locaton fndng (GPS or other), and a means of moblty. b) Why s source routng used (for example, as n AODV) n WSNs? Internet routng protocols (dstance-vector and lnk-state) are too chatty that s, they generate overhead traffc that would be unacceptable for power-constraned sensor networks. Source routng s reactve routng that generates overhead only when a route s needed AODV s thus ntended to generate less overhead traffc than exstng Internet protocols. c) What are the methods n the second ter (or level) of Anastas et al. taxonomy for energy conservaton n WSNs? Hnt: There are three methods n ths ter. Duty cyclng, data drven, and moblty. Extra Credt What s s 4 ponts. The remanng two sub-questons are 3 ponts each. Descrbe the SIP Catcher. What s ts purpose? How does t work? Who developed t? The SIP Catcher s a proxy for an IP phone. The SIP catcher allows the IP phone that t covers (whch could be a PC runnng a SIP-based VoIP telephony applcaton) to sleep, yet stll rng on an ncomng call. The SIP Catcher catches the ncomng rng message (called tryng ) and wake-ups the sleepng IP phone before forwardng ths message to the IP phone. The SIP Catcher s software runnng n a broadband home router that s nomnally always powered-on n any case. The SIP Catcher was developed by Mguel Jmeno, a past Ph.D. student now at UNINORTE n Colomba. Humor From: I hope that everyone dd well