A Base Station-Coordinated Contention Resolution for IEEE PMP Networks

Transcription

1 A Base Staton-Coordnated Contenton Resoluton for IEEE PMP Networks Wenyan Lu, Weja Ja,2, Wenfeng Du, and Ldong Ln 2 School of Informaton Scence & Engneerng, Central South Unversty, Changsha, 40083, Chna 2 Dertment of Comuter Scence, Cty Unversty of Hong Kong, 83 Tat Chee Ave. Hong Kong, Chna {wenyanlu, tja, wenfeng}@ctyu.edu.hk Abstract. IEEE PMP mode recommends usng truncated Bnary Exonental Backoff (BEB) algorthm to resolve the contenton when multle Subscrber Statons (SSs) comete for the connecton and resource allocaton from the Base Staton (BS). The BEB aroach may not be effectve and transmsson oortuntes (TOs) may be wasted due to ndeendent contenton and backoff of each SS. Ths aer rooses an effcent coordnated backoff algorthm (COB) through BS coordnaton so that a global wndow can be assgned by the BS and the TOs can be effectvely consumed by the SSs. Theoretcal analyss and smulaton results show that COB s adatve to the dynamc change of actve SSs and outerforms the BEB aroach. Introducton The IEEE famly of standards and ts assocated ndustry consortum, WMax (Worldwde Interoerablty for Mcrowave Access), romsng to delver hgh data rates over large areas to a large number of users n the near future, are of great concern n recent years[]. The MAC layer of suorts a rmarly Pont-to- Multont (PMP) archtecture, wth an otonal mesh toology. A PMP network conssts of one Base Staton (BS) and multle Subscrber Statons (SSs). The downlnk (from BS to SS) s generally broadcast. But the ulnk (from SS to BS) s shared by the SSs. IEEE has defned the MAC rotocol stack for BS to assgn the ulnk channel to SSs. But durng ntal mantenance and bandwdth contenton erods, all SSs stll need to contend the ulnk channel. An effectve contenton resoluton s crucal to the whole system. At resent the mandatory method of contenton resoluton that shall be suorted by s based on a truncated Bnary Exonental Backoff (BEB) algorthm. BEB has been wldly nvestgated n IEEE 802. networks[2][4][7][0]. However, the MAC layer of IEEE s much dfferent from that of IEEE 802., whch makes the dsadvantage of BEB more obvous n IEEE It s too cautous that SS enlarges ts backoff wndow whenever there s a collson. Some SSs may be treated unfarly durng the contenton rocess. The radcal reason for these drawbacks s that BEB only use a small amount of contenton nformaton to make a decson for the J. Ma et al. (Eds.): UIC 2006, LNCS 459, , Srnger-Verlag Berln Hedelberg 2006

2 606 W. Lu et al. whole networks. In fact there are many lteratures [2][3][0] tryng to mrove the erformance of BEB wth more system nformaton n IEEE In ths aer, we resent an effcent coordnated backoff algorthm through BS coordnaton. Based on the observaton that n one tme frame the backoff wndow s otmal f t equals to the number of actve SSs, we roose an algorthm to calculate ths number. In each tme frame, all actve SSs wll contend for the transmsson oortunty wth the wndow equal to ths number. Theoretcal analyss and smulatons results show that COB (coordnated backoff) resoluton outerforms BEB resoluton. The remnder of ths aer s organzed as follows. Secton 2 brefly descrbes the BEB based contenton resoluton of IEEE The dsadvantages of BEB are also been onted out. In secton 3, COB resoluton and the algorthm to calculate the number of actve SSs are resented. Secton 4 s the erformance analyss and smulaton results. Fnally, secton 5 concludes the aer. 2 BEB Based Contenton Resoluton n IEEE In IEEE 802.6, both ntal mantenance and Request Contenton are contentonbased. Snce they have the same contenton resoluton, n ths aer we take the Request Contenton as examle. 2. Background of BEB In order to effectvely schedule the data transmsson among SSs, IEEE defnes a comlex data structure. The detal s shown n Fg.. Data are sent and receved n tme frame cycle. Each tme frame s dvded nto ulnk subframe and downlnk subframe. The BS controls the assgnment on ulnk subframe through the UL_MAP whch s broadcasted n downlnk subframe. The orton of request contenton can be defned by a request Informaton Element (IE) whch may consst of multle transmsson oortuntes. A transmsson oortunty s defned as an allocaton rovded n a UL_MAP or art thereof ntended for a grou of SS authorzed to transmt BW requests. An SS whch has nformaton to send s called actve SS. Before enterng ts contenton resoluton rocess, an actve SS frst gets the ntal backoff wndow W and the maxmum backoff wndow W max from BS and then randomly select a backoff value wthn the ntal backoff wndow. Ths random value ndcates the number of transmsson oortuntes that the SS shall defer before transmttng. Backoff value decreases by one on every transmsson oortunty. When ths value reach zero, n the next transmsson oortunty the SS sends out ts BW request. After a transmsson, the SS wats for a Data Grant Burst tye IE n the subsequent tme frame. Once receved, whch means the request s succeed and the contenton resoluton s comlete. Otherwse the BW request transmsson s unsuccessful. The SS now ncreases ts backoff wndow by a factor of two as long as the t s less than W max, the SS shall randomly select a backoff value wthn ts new wndow and reeat the deferrng rocess descrbed above. Ths retry rocess contnues untl the maxmum number of retres f has been reached. Then the BW request shall be dscarded.

3 A Base Staton-Coordnated Contenton Resoluton for IEEE PMP Networks 607 Fg.. Data Structure of BW request 2.2 Dsadvantages of BEB The BEB has been wldly used n DCF, but s not erfect n IEEE Frst, there s blndfold for the SS enlargng ts backoff wndow whenever t encounters a collson. Unlke n IEEE 802., the SS n IEEE doesn t sense whether the channel s busy or not before t transmts a request. Ths leads to a large collson robablty whch s caused by the nature feature of random access method. It doesn t always ndcate that the contenton s severe. When there s a severe collson SS should enlarge ts backoff wndow. But t does not mean that SS should enlarge ts backoff wndow whenever there s a collson. Another dsadvantage s that BEB may be unfar to some SS. In tme frame, when SS j chose ts backoff value b,j n nterval [0, w], f b,j s bgger than the number of transmsson oortuntes O, SS j shall defer the oortunty and contends wth the backoff values b,j -O n tme frame +. But the SSs that contend n tme frame + have no dea of the SS j. So the transmsson oortuntes from to b,j -O n frame + are ncdent to collsons. In another word, the SSs that have b,j >O, have more oortuntes of collsons. 3 Coordnated Backoff Algorthm From the nsuffcences of BEB t s easy to know that lackng global contenton nformaton causes the SSs take an unerfected olcy. If we can use the characterstc of central schedulng of PMP and let the BS control the contenton, we may mrove the erformance. Based on ths deal we resent a coordnated backoff algorthm.

4 608 W. Lu et al. 3. Statement of COB In Our algorthm the contenton s coordnated by BS. In each tme frame BS rovdes a global wndow whch s closed to be an otmal wndow n one tme frame. All SSs wll content wth ths wndow. The detal of the algorthm s as follows. ) In tme frame, BS calculates the global backoff wndows w. Along wth O, w s broadcasted to all SS through UCD message. 2) The actve SS j random select a backoff value b,j n[0 w ]. If b,j > O, SS j do not send n ths frame. If b,j O, SS j sends out ts BW request. 3) BS assgns the bandwdth and records the BW request results. 4) If SS j has been assgned the bandwdth, the contenton rocess s comlete. If SS j s request encounter collson SS j wll content wth the new wndow w + n tme frame Global Backoff Wndow One of the key stes of COB s to rovde a roer global wndow n each tme frame. Ths wndow should make the BS correctly receve the BW request as many as ossble. We defne the success robablty s as the robablty that BS receve a correct BW request n a generc (.e., randomly chosen) transmsson oortunty. It s not hard to know that s s the robablty when anyone of the actve SS sends out a BW request whle the other n- SSs don t send. Note that when w<o, SS can not use those transmsson oortuntes whose ordnal numbers are bgger than w. These transmsson oortuntes are wasted. We wll always kee w O and then we have To maxmze the s we have n n w n s = Cn ( ) = ( ) () w w w w ds n w n n( n ) w n 2 = ( ) + ( ) = 0 (2) 2 3 dw w w w w Form (2) t s very easy to know that the otmal wndow s w= n (3) For the convenence of further dscuss, we also calculate the robablty of collson c for a random chosen transmsson oortuntes. n w w w n n c = ( ) ( ) (4) 3.3 Algorthm of Calculatng the Number of Actve SS The otmal wndow equals to the number of actve SS. whle ths number s changng all the whle. How to estmate the number s crucal to COB method. In -th tme

5 A Base Staton-Coordnated Contenton Resoluton for IEEE PMP Networks 609 frame, denote O - as the number of transmsson oortuntes, C - as the number of transmsson oortuntes that suffer collsons, S - as the number of succeeded transmsson oortuntes, s- =S - /O - as the success rate, ˆ s as the estmated success rate, c- =C - /O - as the collson rate and ˆ c as the estmated collson rate. Usng the technology of exonental smoothness we get the success rate of th tme frame: ˆ = βˆ + ( β) (5) s s s In (5), β s called smooth ndex whch s determned by the varable rate of actve SS. If t changes radly the value wll be large. Suose we know the number of actve SSs n, accordng to (), we can calculate ˆ s as s n w w n ( ) = (6) ˆ Let s = s, wth (5) and (6) we can get an equaton on n. But t s not a snglevalued functon, gven a ˆ s there may be two answers. Whch one s true need further analyze. Smlar to s, we also estmate the collson rate by ˆ = β ˆ + ( β) (7) c c c n n n c = ( ) ( ) (8) w w w From (7) and (8) we get a sngle-valued functon of n. Solvng the equatons we get an estmated number of actve SS denoted as n ˆc. If there are two solutons to (5) and (6) we use the one that s closer to n ˆc as the estmated value n ˆs. Consderng success and collson rate are stochastc we use the average value as the result, nˆ ˆ s + nc nˆ = (9) 2 Takng the number of transmsson oortunty O nto account we set the wndow as w ( ˆ, ) = Max n O (0) Ths wndow s the global ndcator on the contenton. If contenton s server, whch means there are many actve nodes, the wndow s bg so as each actve node sends request n each wth small robablty and eventually acheve better erformance.

6 60 W. Lu et al. 4 Performance Analyses and Smulaton As we have mentoned above, s reflects the use of transmsson oortunty. We use t as the man metrc of erformance. From the vewont of SS, whenever t wants to send data, t hoes ts BW request can be receved by BS as soon as ossble. We use delay d to measure ths character. 4. The Maxmal Success Probablty If we know the number of actve SSs exactly, substtute w=n nto () we have n max ( ) n s = () But unfortunately we have to estmate the number. Suose there s an error between estmated value and actual value. Denotng ˆn= n±δ n, accordng to COB resoluton, all actve SSs shall use / ˆn as ther backoff wndows. So the success robablty can be calculated by: n s = C ( ) = ( ) n ˆ n ˆ n ±Δ n n ±Δ n n n n Snce (2) exress the actual success robablty of COB resoluton, we use s to denote the error between s and smax : n n n Δ s = ( ) ( ) (3) n n±δ n n±δn A numerc examle can show that s s not senstve to n. Suosng n=20, the error of n s 20%, t s easy to fnd out that when n ˆ = 6, s get the maxmal value. The value s whch s only about 2.8% of the smax. So even f there s a bg error between the estmated value and the actual value the s can reman a hgh values. It ndcates that COB s robust. 4.2 Calculaton of Delay When SS sends out a BW request t needs to wat the next tme frame to know f the BW request s succeeded or not. Snce transmsson oortunty s much shorter than a tme frame. We gnore the delay wthn a tme frame. We calculate the delay wth the unt of tme frame. Accordng to COB resoluton, n the frst tme frame the success robablty VO s /w, so the robablty for the delay equals to one tme frame s (- O s /w ) O s /w. The robablty that the delay s k tme frames can be calculated by k Ok O (delay k frame) = sk ( s ) w w k = (2) (4)

7 A Base Staton-Coordnated Contenton Resoluton for IEEE PMP Networks 6 The exectaton values of delay shall be O O ED ( ) = k ( s ) w k k sk k = wk = Suosng O, w, s be constant O, w, s we have k= (5) O O w O ED ( ) = k ( ) k s s s = w w O (6) By (6), we know that delay s determned by O,w, s. The bgger of s s the less of d s. The more of actve SSs s the less of d s. And the bgger of backoff wndow s the bgger d s. 4.3 Smulaton Results To the best our knowledge, there s no lterature to analyze the erformance of BEB n IEEE by now. So we man comare the erformance of BEB and COB by smulaton. We develo a rogram to smulate the rocess of BW request n IEEE The rogram consst of two knds of objects BS and SS. BS send out O transmsson oortuntes n each tme frame and the actve SSs randomly chose a backoff value wthn ts backoff wndow w,j. The BS reads the status and backoff values of each SS and then gve a judgment f the request s succeeded or not. Here we assume the wreless channel s deal, that s, we gnore the ossblty of errors due to nose and also gnore the ossblty of cature technques by whch a SS sometme cature one transmsson n the resence of multle transmssons. The whole smulaton s wrtten wth event drvng model. Frst, we comare the erformances n a steady status. Under ths condton, we kee the number of actve SS as a constant. Each SS generates a new BW request mmedately after a succeed one and SS wll not dscard ts BW request tll t has been successfully transmtted. For smlfy we kee the number of transmsson oortuntes unchanged. We record the number of successful BW request and corresondng delay n each tme frame. Fg. 2 and Fg. 3 are the corresondng value of BEB and COB from tme frame 500 to 550. Here n=32, ntal backoff wndow n BEB s 32 and the smooth ndex β n COB s 0.8. Note that the delays n Fg. 3 are the average delay of all the successful transmsson oortuntes n one tme frame. From Fg 2 we may observe that the success tmes n COB s bgger than that n TEBE. Fg.3 enable us to conclude that: () The delay of COB s less than that of BEB n general; (2) n some fame the delay are very large n BEB whch llustrate the unfar roblem n secton 2.2. Gven an n we smulate the contenton rocess wth 00,000 tme frames. Let n change from 5 to 00, we draw the curve of s and d for BEB and COB resectvely n Fg. 4 and Fg. 5. From Fg. 4, we can observe that When O=32 and n<60 COB s better than BEB both on s and d. But when n>60, BEB erform better than COB. Takng a careful study we can fnd out that when n>60 the delay of BW request s bg (more than 4 tme frames). So the BS should rovde more transmsson oortunty to reduce the delay. When we enlarge transmsson oortunty to 64, COB outerforms BEB. s

8 62 W. Lu et al. Number of Success BWR er tme frame %(%Q Z 2 &2%Q β Tme frame Delay (tme frame) %(%Q Z 2 &2%Q β Tme frame Fg. 2. Varaton of success BWR wth tme frame Fg. 3. Varaton of delay wth tme frame Average success rate BEB O=32,w=32 C O B O =32,β=0.8 BEB O=64,w=32 C O B O =64,β= The number of actve SS Average Delay (Tme Frame) %(%2 Z &2%2 β %(%2 Z &2%2 β The number of actve SSs Fg. 4. Varaton of s wth n Fg. 5. Varaton of d wth n Be used n ractce, the number of actve SS may change dynamcally. It may need a erod of tmes for SS generatng a new BW request. At the same tme, the number of transmsson oortunty that BS rovdes n a tme frame s lmted by some other factor such as bandwdth assgnment method. The number may vary from frame to frame. Takes these factors nto account, we smulate the dynamc feature of IEEE n two asects: on one hand, the arrvng of BW request obeys negatve exonental dstrbuton; on the other hand, the number of transmsson oortunty obeys constant dstrbuton. Furthermore, after tryng sendng a BW request more than f tme fame we dscard t. We don t do t n f retransmsson just n order to comare them easly. And more, the SS who dscards a BW request may generate one after a tme t d. We set N equal to 000. By adjust the value of λ we control the number of actve SS. The exact vales are: w=32, β=0.5, f=25, w max =024, t d =000, λ= 0.005, 0.0, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 (.e., we control the arrve rate aroxmate to 5, 0, 20, 30, 40, 50, 60, 80 er tme frame). We smulate the O n two dfferent ways: ) O s randomly chosen from 0 to 80; 2) O s calculated by COB resoluton. In ths case we lmt the O to 50. Fg. 6, 7 and 8 are the curve of average success rate, average delay and losng rate under three condtons resectvely.

9 A Base Staton-Coordnated Contenton Resoluton for IEEE PMP Networks 63 Average Success rate Arrve rate of actve SS Average Delay Arrve rate of actve SS Fg. 6. Varaton of s wth the arrve rate of actve SS Fg. 7. Varaton of d wth the arrve rate of actve SS From these Fgures, we may observe that: frstly, the average success robablty of COB s larger than BEB, and s much better when O s decded by COB. Secondly, the delay of COB s less than that of BEB when the number of actve SS s less than 2, but the delay of COB tends to be bgger than BEB when the arrval rate of request contnues ncreasng. However, by analyzng the delay and request losng rate carefully, t s not hard to know that the request losng rate of BEB s much more than that of COB. That s to say, we decrease the delay of BEB by throwng away BW request. When the value of f s large enough, the delay of BEB would ncrease remarkably, and wll bgger than COB. If O les on COB, the request losng rate may be much less. Average request losng rate Arrve rate of actve SS Fg. 8. Varaton of d wth the arrve rate of actve SS 5 Conclusons The foundaton of BEB based contenton resoluton s dstrbuted coordnated functon. In BEB, all the SSs judge the degree of contenton ndvdually and take acton ndvdually. Snce each SS only use a small amount of nformaton (the result of ts transmsson), ther contenton olcy may be ncomleton. In ths aer we roose an effectve coordnated backoff algorthm n whch the evaluaton of the contenton condton s conducted by BS. Comarng wth the sngle feedback of SS, BS s able to know of the cometton condton of the whole networks, therefore be able to control the system s cometton n a better way. Smulaton results show ths resoluton outerforms BEB.

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