High Priority Traffic in HCF on Wireless Networks

Transcription

1 High Priority Trffic in HC on Wireless Networks Mo Add, Amnd Pert, Gordon Erly School of Comuting, University of Portsmouth, Lion Terrce, Portsmouth, UK {mo.dd, mnd.ert, gordon.erly Abstrct In this er we nlysis nd comre the erformnce of the high riority trffic in HC using frgment-to-frgment cknowledgement nd block-frme cknowledgment in noisy wireless environment. We will roose mechnism for the block-frme cknowledgement tht will imlement on the IEEE stndrd to suort qulity of service, with no mjor uhevl. The nlysis is vlid for ll tye of trffics low, medium nd high riority clsses. Keywords Wireless Networks, WLAN, Hybrid Coordintion unction (HC), Enhnced Distributed Chnnel Access (EDCA), , Qulity of Service. 1 Introduction Industry hve doted the Internet Protocol (IP) stndrd to route dt over their cororte network, this enbles them to mintin single infrstructure, which in turn reduces their mintennce costs. The imlementtion of Wireless idelity (Wii) networking hs llowed users to ccess the cororte network whilst still mintining mobility. Alictions such s Voice over IP (VoIP) hs rimrily been used on wired networks, imlementing this on Wii network would give greter flexibility to the user. This tye of imlementtion would enble the cororte user to rom nd collect clls from the cororte telecoms network. The drwbck of wireless imlementtion is tht VoIP demnds good qulity of service (QoS) for it to chieve n ccetble level of erformnce for the end user. It is the tye of VoIP liction s discussed bove tht necessittes clssifiction of high riority network trffic roviding good QoS delivery. This er will nlysis nd comre the erformnce of the high riority trffic in noisy wireless environment emloying HC using frgment-to-frgment cknowledgement nd block-frme cknowledgment. A mechnism for the block-frme cknowledgement tht will be comtible with the IEEE stndrd [1] to suort qulity of service [2], with no mjor uhevl will be roosed. The nlysis is vlid for ll tye of trffics low, medium nd high riority clsses. 2 Nottions We refer to the size of the frme generted by the MAC-sub lyer rotocols s L (bits). We lso refer to the size of the frgment by (bits). The number of frgments trnsmitted consecutively is therefore L/. The trnsmission rte of the NIC (Network Interconnection Crd) is (bits/sec). The ositive cknowledgment ACK nd the

2 negtive cknowledgment NAC re of equl size. We lbel the number of bck off slots before trnsmission tkes lce by S. DIS = 34sec, SIS = 16sec, we roose nother inter-frme scing IIS (Immedite Interfrme scing) equls to the length of the time slot = 9sec. The length of the time slots = BS = 9sec. The length of the TC nd CLS re 34Bytes ech. The robbility tht frme will be corruted is clled, nd the robbility tht n cknowledgment or negtive cknowledgement will be corruted is clled q. In this nlysis we will consider single ho trnsmission, s shown in figure 1. DIS SIS T T f TS CLS ACK ACK ACK S igure 1: EDC rt of the HC, the MAC rotocol. The vlue of the DIS deends on the riority clsses. Tble 1, below shows the nottions re used to simlify the equtions of the efficiency for ech scheme. T b = S*BS = 9S (bck off time) T = DIS + TS/ + CLS/ + 2*SIS (Protocol time) T = 2*SIS + ACK/ (Acknowledgment time) T = IIS+ SIS + NAC/ Tble 1. Nottions used to simlify equtions of the efficiency for ech scheme The robbility tht k retrnsmissions re required fter frgment is corruted is define s Prob = k-1 (1-). The exected number of retrnsmissions would be 1/(1-). The robbility tht k negtive cknowledgement retrnsmissions re required fter frgment is corruted is define s Prob =.q k-1 (1-q). The exected number of cknowledgements retrnsmitted would be /(1-q). 3 rgment-by-rgment Acknowledgment As shown in figure 2, the IEEE stndrd, for the relibility ech frgment is cknowledged. If frgment is corruted negtive cknowledge is send bck. In the cse where, the negtive cknowledge is lost, the sender times out nd retrnsmit the frgment. We ssume tht the length of the time out eriod is equl to the cknowledgment time T. Other ossibilities cn be considered. ACK ACK x NAC x NAC x NAC x NAC x ACK ACK v NAC v+1 NAC v+2 NAC v+3 NAC v+4 NAC v+5 NAC v+6 NAC v+7 NAC v ACK ACK igure 2: Trnsmission of sequence of rgments with Errors

3 After k retrnsmissions, when the frgment is corruted, there hve been /(1-) retril eriods of (T + /). After k retrnsmissions, when the negtive cknowledge hs been corruted, there hve been /(1-q)*(T + /). Therefore the efficiency of would be (1): E 1 T T L T b 1 ( T )( 1 q 1 ) (1) 4 Block-rme Acknowledgment An cknowledgement is sent when ll the frgments generted from the sme frme hve been received. rom the IEEE stndrd, ech frgment hs sequence number nd flg indicting the end of the trnsmission. We suggest tht in this cse when frgment is corruted negtive cknowledgment is generted fter frme sce clled IIS, which is less thn the SIS the sce of the cknowledgment, s shown in figure 3. After k retrnsmissions, when the frgment is corruted, there hve been /(1-) retril eriods of (T + /). After k retrnsmissions, when the negtive cknowledge hs been corruted, there hve been /(1-q)*(T + /). The sender receives the corruted negtive cknowledgment s noise, it wits until the chnnel is cler, then fter SIS it trnsmits next frgment, unwre of the revious unsuccessful trnsmission. x NAC x NAC x NAC x NAC x v NAC v+1 NAC v+2 NAC v+3 NAC v+4 NAC v+5 NAC v+6 NAC v+7 NAC v ACK igure 3: Block rme Acknowledgement with Sequence of Corruted rgments. The efficiency of would thn be (2). E 1 T L L * SIS *( ) L * L 1 T T b ( T ' )( 1 q 1 ) (2) 5 esults igures 4,5 nd 6 demonstrte grhiclly the results of the efficiency roosed. The efficiency ercentge for non-frgmented (), rgment-by-rgment ACK (/-A), nd Block rme ACK (B/-A) for noiseless chnnel (=q=0), s function of the frme length is shown in figure 4. The noiseless chnnel rmeter is (=.5, q=0), s

4 E E function of the frme length in figure 5. Here you cn see tht there is miniml effect on rgment-by-rgment ACK (/-A), nd Block rme ACK (B/-A) but there is significnt imct on the efficiency on the non-frgmented frme. The grh in figure 6 increses the robbility tht negtive cknowledgement will be corrut t 0.5 on the noiseless chnnel. Here, there is slight imrovement of the efficiency of the nonfrgmented frme. 512 rme (Bytes) /-A B/-A igure 4: The efficiency (%) for Non-frgmented (), rgment-by-rgment ACK (/-A), nd Block rme ACK (B/-A) for noiseless chnnel (=q=0), s function of the frme length rme (Bytes) /-A B/-A igure 5: The efficiency (%) for Non-frgmented (), rgment-by-rgment ACK (/-A), nd Block rme ACK (B/-A) for noiseless chnnel (=.5, q=0), s function of the frme length.

5 512 E /-A B/-A rme (Bytes) igure 6: The efficiency (%) for Non-frgmented (), rgment-by-rgment ACK (/-A), nd Block rme ACK (B/-A) for noiseless chnnel (=.5, q=0.5), s function of the frme length. 6 uture work We will consider n intelligent sender tht will use the noise from the negtive cknowledgments s indiction to wrong trnsmission. The receiver will hve to send negtive cknowledgment to overcome dulictions. The sender hs time out mechnism to retrnsmit non-cknowledged frgments. We believe tht this will imrove eqution 2 further. We will lso look t the imct of multi-hos on the efficiency of the roosed scheme. eferences [1] IEEE Drft Std e, Medium Access Control (MAC) Enhncements for Quilty of Service (QoS), D4.4, June 2003 [2] Anstsi, G., Lenzini, L., QoS rovided by the IEEE wireless LAN to dvnced dt liction: simultion nlysis. Wireless Networks, (6): ,2000.