A Resource Management Strategy to Support VoIP across Ad hoc IEEE Networks

Transcription

1 A Resource Managemen Sraegy o Suppor VoIP across Ad hoc IEEE 8.11 Neworks Janusz Romanik Radiocommunicaions Deparmen Miliary Communicaions Insiue Zegrze, Poland [email protected] Pior Gajewski, Jacek Jarmakiewicz Faculy of Elecronics Miliary Universiy of Technology Warsaw, Poland {pgajewski, jjarmakiewicz}@wel.wa.edu.pl Absrac This paper describes he concep of he resource managemen sraegy in ad hoc neworks for rescue operaions. The presened sraegy is a resul of he new oulook on he IEEE 8.11 neworks capabiliies and performance enhancemen. The proposed soluion is dedicaed o real ime services suppor and is based on he concep of he Resource Manager ha organizes and conrols he whole raffic in he nework. Novel procedures were developed and applied in order o organize he nework and manage he real ime raffic. A mehod of he available bandwidh measuremen and esimaion was inroduced. Large scale simulaions for differen numbers of Voice over IP (VoIP) sources and various voice codecs have been carried ou. They show he increase of channel uilizaion reaching over 8% and significan growh of he nework capaciy. Keywords - IEEE8.11 WLANs, ad-hoc neworks, VoWiFi, resource managemen I. INTRODUCTION For over en years a permanen developmen of IEEE 8.11 Wireless Local Area Neworks (WLANs) is being observed [1]. Among he many advanages hey offer, users appreciaed he convenience and simpliciy when accessing he nework and esablishing high daa rae wireless connecion. Thanks o a low cos and a small size of devices, nowadays hey seem oo ubiquious. WLAN drivers are embedded in many differen devices like noebooks, mobile phones, Personal Daa Assisans (PDAs), cameras, ec. Despie he fac ha WLANs were originally designed for daa ranspor, oday i is also demanded of hem o be efficien for real ime services suppor. Anoher advanage of WLANs resuls from he ad hoc mode, which is a mehod for wireless devices o direcly communicae wih each oher. Operaing in ad hoc mode allows all wireless devices wihin each oher s range o discover and communicae in a peer-o-peer manner wihou involving he cenral access poin. This mode offers mobiliy and communicaions beween users in areas wihou infrasrucure or in all places wih damaged infrasrucure. From his poin of view, WLANs operaing in ad hoc mode can be a very promising soluion for users, such as he fire brigade, rescue eam, police squad or small miliary uni [,3]. The possible scenario is o use he ad hoc nework for public-safey or search-and-rescue operaions. An imporan issue for such nework is he abiliy o suppor cooperaion beween wo or more emergency services, e.g., he fire brigade, police squad, rescue eam, medical service. On he oher hand, i mus be sressed ha he performance of he nework decreases as he number of wireless users grows. For ha reason, a smar mechanism should be inroduced, which allows opology conrol and nework scalabiliy [4]. The effec of he hidden node is one of he mos difficul problems o solve, because i is inrinsic o he naure of he WLANs. The S/CTS mechanism is no recommended for he ransmission of small packes, e.g. VoIP. A possible soluion is o use an addiional signaling channel, however i requires changes in he physical layer. This issue was widely discussed in [14,15]. When considering he hierarchical srucure of he command sysem of he emergency services, differen ranks of users should be aken ino accoun. This will affec he prioriy of users, as well as he ype of allowed services. Among many wireless soluions, IEEE 8.11 neworks seem o be he mos popular. Alhough he mos common weakness of WLANs is he insufficien suppor of he real ime services [5,6], he auhors formulaed a new oulook on he IEEE 8.11b nework capabiliy and possible performance enhancemen. Despie he fac ha here is a wide range of WLANs specificaions, he issue of nework opimizaion sill remains open. QoS mechanisms were he subjec of he IEEE 8.11e sandard [7]. However, hese mechanisms canno guaranee he qualiy of services, alhough hey slighly improve he nework efficiency [8]. The voice capaciy of IEEE 8.11 neworks is gaining increasing aenion in he lieraure. Mehods of VoWiFi opimizaion, including voice codec negoiaion, audio packes aggregaion as well as he MAC proocol adapaion, can be found in many papers. In [8], he influence of he MAC proocol on he nework performance was shown. This proocol operaes in conenion mode and hus ineviably inroduces he PHY layer overheads, Backoff and proecive periods, frames and reransmissions in some cases. In [9], auhors analyzed he effec of he coding rae and packe

2 size on he voice capaciy of he Disribued Coordinaion Funcion (DCF). In [1], dynamic CW adapaion was suggesed in order o minimize he number of collisions. The idea of he voice coding bi rae adapaion o he available nework bandwidh was described in []. Resuls of experimens confirmed he efficiency of he new scheme. The impac of differen configuraion parameers on he ad-hoc nework performance was presened in [3]. Following parameers were analyzed, he ype of codec, packeizaion inerval and he daa rae. In [4], auhors presened he resuls of he capaciy measuremen of he IEEE 8.11e nework for each access caegory. They also analyzed he effec of he TCP raffic on VoIP sreams. In conclusion, hey saed ha 8.11e sandard can proec he qualiy of VoIP if here is TCP raffic added. However, i can no improve he capaciy of he nework. Alhough proposed mehods can improve nework efficiency, he quesion as o how o guaranee he qualiy of services sill remains open [13]. Furhermore, here is sill a lack of an efficien Call Admission Conrol (AC) mechanism [1,13]. The presen aricle is an aemp o fill his gap. This paper presens he general concep of he resource managemen sraegy and provides informaion on inroduced procedures. All proposed mechanisms are conneced wih each oher and inerac wihin a individual device as well as wihin he whole nework. The res of he paper deals wih he concep and assumpions (Secion ), he descripion of he proposed mechanisms (Secion 3), simulaion resuls and heir discussion (Secion 4), conclusions (Secion 5) and fuure work (Secion 6). II. CONCEPT AND ASSUMPTIONS In he case under consideraion, he aim of he nework opimizaion is o ge as high as possible number of VoIP sreams wih guaraneed voice qualiy. The assumed nework operaes in ad hoc mode and consiss of small group of users, e.g., fire brigade or rescue eam. In emergency siuaions hey ypically use voice communicaion. Therefore he auhors made an assumpion ha here is only one ype of service, namely VoIP. Users have differen ranks, which deermines some differences beween prioriies. Thus, he rade off beween he available bandwidh, he allowed number and he rank of users is inroduced inenionally. The nework model assumes WLAN based soluion. The auhors decided o use he IEEE 8.11b sandard as offering good hroughpu and modulaions more resisan o inerferences, which is a real advanage of he nework operaing in ad hoc mode. MAC QoS mechanisms defined in he IEEE 8.11e sandard were also aken ino accoun. These mechanisms are a good saring poin o enable prioriizaion and bandwidh reservaion in ad hoc nework [1,13]. In paricular, he auhors inroduced he adapaion of a Conenion Window (CW) size o he ype of frame and he rank of he user. Anoher issue concerns he opimal balance beween he raffic load and he services qualiy in ad-hoc neworks. I is expeced ha he proposed range of adapaion and inroducing of new mechanisms will no demand a high cos of implemenaion and will be feasible. Fig. 1 illusraes he concep of efficiency improvemen of WLAN for VoIP suppor. The available bandwidh level is he main facor allowing assessmen of he raffic load in he nework. Cross-layer mechanisms are crucial for nework performance improvemen. They enable he raffic shaping or MAC adapaion if he available bandwidh is oo small or if he level of service is no saisfacory. The CAC mechanism prevens new VoIP calls if he available bandwidh level is oo low. Figure 1. Performance enhancemen of WLAN for VoIP suppor. In he proposed soluion, he nework consiss of differen rank users. For he sake of simpliciy, high rank users shall be denoed as special users while he res shall be referred o as commercial users. I perfecly corresponds o he scenario of he humaniarian aid, when voluneers help people in service. Anoher example can be he siuaion when he fire brigade, police and civilians cooperae wihin small groups while srenghening an embankmen during a flood. However, if he available bandwidh is oo small, special users prevail over he nework. Evenually, he lowes rank users can be compleely blocked. A separae quesion is how o assess he resources of he nework, e.g. channel uilizaion. Since ad-hoc neworks are bandwidh limied, no all measuremen mehods can be applied [15-17]. Closed Nework Conrolled Access raffic queueing VoIP call reques NSOM iniiaion Applicaion Layer Transpor Layer Nework Layer Daa Link Layer Medium Access Crl Physical Layer NSOM Nework Self-Organizing Mechanism AM MAC proocol Acquisiive Mode raffic shaping AM mode on/off FER, daa rae SNR, BER BPCP AM mode iniiaion Closed nework riggering Codec negoiaion Radio parameers masuremen Bandwidh assessmen STD/AM swiching Figure. BPCP alignmen wih proocol sack.

3 Fig. illusraes he exended proocol sack wih crosslayer ineracions. Bandwidh Predicion Conrol Proocol (BPCP) allows monioring of parameers in he physical layer, o measure he channel uilizaion level and also o swich MAC proocol saes, as explained in subsequen secions. raffic shaping relays on codec negoiaion and audio packes aggregaion. Closed Nework Mode is based on he concep of he Resource Manager ha conrols raffic in he nework. A. Bandwidh Esimaion The available bandwidh is crucial for opimizaion of he Wi-Fi ad-hoc nework. Therefore, he auhors proposed o implemen BPCP ha enables o measure he channel uilizaion level and o esimae he available bandwidh. BPCP akes advanage of WLAN card drivers ha enable he measuremen of SNR in he PHY layer and BER calculaion, and passing hese parameers o he Daa Link Layer. If nodes operae in promiscuous mode, hey can receive all he raffic sen across he nework. As a resul, he bandwidh uilizaion is assessed in all nodes of he nework independenly and coninuously for predefined periods called Sampling Inervals. From he PHY layer poin of view, saions can deec he channel sae (idle or busy - which means ransmission) and if hey operae in promiscuous mode, hey can receive and process all frames. The ype of received frames (S, CTS, DATA, ) is recognized in he daa link layer. Knowing he bi rae and he lengh of received frames i is possible o calculae heir ransmission duraion in he radio channel, denoed as AF in (1). AF _ lengh AF = AF _ birae ( bis) ( bis sec) Having knowledge of AF parameers, i is hen possible o deermine he channel uilizaion coefficien for he inerval, e.g. from 1 o 1 (1) d1 + d + + SIFS + DIFS U = () where: d1, d denoes he duraion of he firs and second daa frames; represens he frame duraion, Fig. 3. Figure 3. Transmission scheme in a conenion mode of WLAN. When he curren and he previous channel uilizaion is esimaed, BPCP makes forecass for he nex period. Fig.4 shows he exended WLAN sublayer of he mobile node. This sublayer conains Throughpu Meer In and Throughpu Meer Ou componens o measure all incoming and ougoing raffic. This informaion is used o assess he oal raffic load as well as he available bandwidh. Figure 4. The exended WLAN sublayer of he mobile node. To assess he nework hroughpu in a conenion mode, heoreical analysis was performed and simulaions were made using he OMNET++ v4. simulaion ool. For he purposes of analysis and simulaion, he following parameers were assumed: G.711 voice codec; ypical proocol headers (MAC header = 3B, IPv4 header =B, UDP header = 8B); free space propagaion model and lack of mobiliy. The issue of mobiliy is crucial for deerminaion and is he opic of furher sudy. The values of he MAC parameers are lised in Table I. TABLE I. Parameer MAC PARAMETERS Value DIFS 5 µs SIFS 1 µs Slo Time µs CWmin 3 CWmax 13 Daa Rae Mbi/s PHY header 19 µs MAC header 34 byes 34 µs The main aribues of he G.711 codec are shown in Table II. TABLE II. G.711 CODEC CHARACTERISTICS Codec G.711 Bi rae [kbi/s] 64 Framing inerval [ms] Payload [B] 16 Packes/sec 5 The resuls of he simulaion are presened in Fig. 5. Normal disribuion of a hroughpu esimaor was assumed, as well as a confidence inerval wih α=.1 and β=1.64 (for cumulaive disribuion funcion equal o.9). The period of ime required for he ransmission of one daa frame and he acknowledging frame akes nearly 1,8ms. For ha reason i is possible o send 11 acknowledged frames during one second. Audio packes are generaed by codec periodically every ms. Assuming ha saions work synchronously, i.e., afer he firs one had ransmied a packe, he second one generaes i, hen i is possible o

4 obain he nework hroughpu equal o 1,3Mb/s, Fig. 5. Higher raffic load will cause an increase of he collision rae and a drop in nework efficiency. Throughpu [kbi/s] Analyical Simulaion Number of VoIP sources Figure 5. Wi-Fi nework hroughpu - conenion mode, daa rae Mbi/s. B. MAC Proocol Saes A he beginning of he operaion, special saions can cooperae wih commercial and use sandard access schemes, unil BPCP deecs he insufficien bandwidh and iniiaes he Acquisiive Mode (AM). During AM mode, he Backoff inerval is minimized according o he rank of he user. As a resul, special saions prevail over he nework. Only a small par of he bandwidh can be hard-won by remaining users. To deermine he Backoff inerval, he Conenion Window parameer is used, however differen values have been inroduced, depending on he rank of he user and he ype of frame (Conrol, Daa, Broadcas or Daa). If he available bandwidh is sill oo small, BPCP riggers a mechanism called he Nework Self-Organizing Mechanism, which is responsible for creaing a Closed Nework Mode. From his momen on, Wi-Fi nework operaes in a poin-coordinaed mode. Fig. 6. presens he saes of MAC proocol for he proposed proocol exension collisions is denoed by A. This informaion may be used addiionally by BPCP. Nework hroughpu [kbi/s] Throughpu [kbi/s] A B C Traffic load [kbi/s] Figure 7. Nework hroughpu vs. raffic load (where riggering levels are denoed as follows: A - swich from AM Mode o sd., B - swich from sd. o AM Mode, C - swich o Closed Nework Mode). Collisions [%] Collisions Traffic load [kbi/s] Figure 8. Collisions vs. raffic load (where A denoes he criical level of collisions). Resuls of simulaions performed in order o esimae he accepable number of VoIP connecions, depending on he ype of voice codec and MAC proocol parameers in a conenion mode, were widely discussed in lieraure [9,1,19,]. However, he quesion where and how o implemen he AC mechanism and how o manage he raffic in he nework sill remains open. The AC mechanism is necessary o preven new calls if here is no enough bandwidh. In a conenion mode, saions are no aware of he raffic load and ry o ransmi frames every ime hey have a packe o send. For his reason, a Closed Nework Mode was proposed wih a saion named he Nework Resource Manager () ha manages he nework. A III. CLOSED NETWORK MODE Figure 6. MAC proocol saes. An imporan issue is o deermine he proper level of channel uilizaion for riggering beween MAC AM and Closed Nework Mode. To resolve his problem, he auhors applied he Pareo opimizaion approach. Simulaion resuls obained for Mbi/s daa rae and G.711 voice codec are presened below. Fig. 7 shows he nework hroughpu vs. raffic load. Triggering levels are also presened. If he hroughpu reaches limi denoed by B, he saion swiches from sandard mode o AM. If i reaches anoher limi denoed by C, he saion swiches o Closed Nework Mode. Fig. 8 presens collisions vs. raffic load. The criical level of A. Nework Self-Organizing Mechanism A he beginning, all saions work in a conenion mode wih sandard parameers, Fig. 9. In he background, Neighbor Discovery Procedure is performed, which is based on broadcasing Neighbor Reques and Neighbor Response frames [1]. This procedure allows recogniion of he surroundings by collecing daa from oher nodes, namely: received signal srengh and noise, baery level and rank of he saion. Based on his informaion, each saion deermines is own Readiness coefficien, which describes wheher he saion is ready o play a nework manager role. This mechanism is sill under implemenaion in OMNET++ v4..

5 If BPCP again deecs he insufficien bandwidh coincidence, a saion changes he mode o AM, while Neighbor Discovery Procedure is sill in he background, Fig. 9. When a firs saion deecs he insufficien bandwidh, i iniiaes Nework Self-Organizing Mechanism (NSOM). Only saions wih a cerain Readiness coefficien are allowed o paricipae in his phase. If necessary, informaion on he nework opology is refreshed by sending Neighbour Reques frame, which conains he las Readiness coefficien of he sending saion. When hese frames are exchanged, he saion wih he highes coefficien sends a Reques for frame. From his momen on, he nework operaes in a Closed Nework Mode and all raffic is conrolled by he resource manager ill Timeou elapses and he procedure for Deerminaion sars again, Fig. 1. Ad-hoc Mode BPCP deecs lack of bandwidh AM Mode Iniiaion Sandard Access Scheme MAC Acquisiive Mode Neighbour Discovery Procedure NSOM Iniiaion Deermined Deerminaion Conrolled Access NSOM Closed Nework Mode Figure 9. Nework Self Organizing procedure. B. Real Time Traffic Managemen When he saion is deermined, i sends a Reques broadcas frame informing ha nodes are allowed o call for a bandwidh reservaion. Some saions respond wih Confirm frames if hey have packes o send. The Reques frame is sen periodically o disseminae he lis of queued saions and also he curren queue limi, Fig. 1. A more deailed descripion of he algorihm can be found in [18]. Deermined Queue Disribued Req Timeou Reques Traffic for Queuing NSOM Closed Nework Mode BPCP deecs lack of bandwidh VoWiFi Connecions Figure 1. raffic managemen. Timeou Elapsed Nex Period If he queue limi is reached or Req Timeou has elapsed, he saion sends a Queue frame conaining: queue size: number of STAs in queue, number of cycles: number of queue repeiion, voice codec ype, daa rae, MAC address and order of saions in he queue. Afer receiving he Queue frame, he firs saion on he lis is allowed o ransmi afer DIFS and receives an frame afer SIFS, Fig. 11. The nex saion in queue ransmis daa frame afer DIFS. The number of cycles describes how long nodes will ransmi daa in a given order. Afer each ransmission of DATA and, saions decrease heir TransmissionIndex and are allowed o send afer i reaches zero. Afer a predefined number of cycles, he saion again sends a Reques frame o give a chance o ransmi for saions ha were ou of queue during he preceding period. Queue Disribuio n Queue DIFS DATA 1 SIFS DIFS SIFS SIFS DATA 1s cycle 1s Period... DATA N Figure 11. raffic queue. Timeou Elapsed Queue Organizing VoWiFi New nd Period An unpredicable erminaion may occur, e.g. as a resul of depleion of he baery, which should be aken ino accoun. In such a siuaion nodes will deec a lack of frames from for he assumed imeou. Since his momen on, he saion wih he second highes Readiness coefficien sars playing his role. In order o organize a closed nework and manage raffic, he following managemen frames were inroduced: Neighbor Reques and Neighbor Response - for neighborhood discovering, Reques - for iniiaion of he raffic queuing phase, Confirm - for he bandwidh reservaion, Queue - disribuion of raffic queue. The deailed descripion of he managemen frames srucure can be found in [18]. Because all of hese frames are of a broadcas ype, all receiving saions are forced o process i in he daa link layer, alhough acknowledgemen is no sen. The srucure of new frames is he same as defined in he IEEE 8.11 sandard for managemen frames and consiss of MAC Header and Frame Body conaining informaion fields. The maximum size and capaciy of frames are presened in Table III. TABLE III. Frame Type MANAGEMENT FRAMES SIZE AND CAPACITY Frame max size [B] Number of addresses Reques 4 35 Confirm 4 1 Queue 8 35 IV. VOIP CAPACITY ANALYSIS In order o assess he ime required o organize he raffic, analyical invesigaions were performed. I was assumed ha is deermined, avg. Backoff is equal o 1µs and 1 nodes compee for bandwidh reservaion, Fig. 1. is deermined STA Reques Confirm 1 Confirm Reques Reques disribues he raffic queue Confirm 1 Figure 1. raffic scheduling procedure. Queue VoWiFi The size and he amoun of frames exchanged in his procedure are presened in Table IV....

6 TABLE IV. AVERAGE SIZE AND NUMBER OF EXCHANGED FRAMES Frame ype Frame avg. size [B] Frames number Reques 1 5 Confirm 4 1 Queue 1 1 If he daa rae is se o 1Mbi/s, one cycle required o schedule he raffic akes approximaely 14ms and his period reaches 1ms if he daa rae increases o Mbi/s. Assuming some collisions, his duraion should no exceed ms. Synchronous daa ransmission in a Closed Nework Mode can be verified by using an analyical as well as simulaion model. For he sake of convenience, e.g. in order o apply differen inpu parameers, he auhors used COMNET 3 simulaion ool. The aim of simulaions was o assess he channel uilizaion and he number of possible simulaneous VoIP calls as a funcion of he daa rae. The following assumpions were made: nework saions wih commercial voice codec (G.711) wih aribues defined in Table I, MAC/PHY parameers: SIFS = 1µs, DIFS = 5µs, PLCP Header + Preamble = 19µs, packes wih sandard proocol headers: MAC = 3B, IPv4 =B, UDP = 8B. The channel uilizaion vs. he number of VoIP calls and various daa raes was shown in Fig. 13 and Fig. 14. In he phase of synchronous daa ransmission, here are only wo cases when he channel is idle: DIFS which precedes daa frame ransmission and SIFS beween daa and frames. Channel Uilizaion [%] 1Mb/s Mb/s 5,5Mb/s 11Mb/s Number of VoIP sources Figure 13. Channel uilizaion vs. number of VoIP sreams for G.711 voice codec (64kbi/s). Channel uilizaion [%] Mbi/s Mbi/s 5,5Mbi/s 11Mbi/s Number of VoIP sources Figure 14. Channel uilizaion vs. number of VoIP sreams for G.76 voice codec (3kbi/s). An increasing number of VoIP connecions leads o a linear growh of channel uilizaion, up o 9%. Beer channel uilizaion is unachievable. This is a resul of he fac ha alhough he number of frames sen in a given period increases for higher daa raes, here are sill consan idle periods ha separae frames. The delay of packes resuls from he daa rae and he sequence number of a given saion in he whole queue. Thus, his delay does no exceed wo dozens of milliseconds. When he daa rae grows, he ime needed for ransmission of one frame becomes shorer, while DIFS and SIFS remain on he same level. Therefore i is possible o se up more VoIP connecions, however he channel uilizaion canno exceed 9%.When G.711 codec is used and he daa rae is se up o 11Mb/s, up o 7 VoIP calls are available. V. CONCLUSIONS We have presened he concep of he resource managemen sraegy in ad-hoc neworks for rescue operaions. This sraegy is a resul of he new oulook on he 8.11 WLANs capabiliies and performance enhancemen. A se of novel procedures was developed wih a view of organizing he nework and managing he real ime raffic. These procedures were validaed analyically and by simulaions, and resuls were included. The proposed mehod of he available bandwidh measuremen and esimaion works correcly. The procedure of daa synchronous ransmission in a Closed Nework Mode was verified by simulaion. Resuls of ess allowed esimaing he channel uilizaion achieving over 8% when synchronous ransmission was applied. If he number of saions in a queue is se correcly, he delay of he daa frame ransmission is limied o wo dozens of milliseconds and resuls mainly from he daa rae. The presened resuls were obained under he assumpion ha only UDP raffic is ransferred across he nework. The impac of he TCP flows on he nework performance requires furher analysis. The proposed mechanisms were developed as a resul of a compleely new approach o he suppor of daa ransmission in 8.11 ad-hoc nework. They enrich sandard procedures and enable an efficien uilizaion of he channel. VI. FUTURE WORK In his aricle, we have only presened he resource managemen sraegy o suppor VoIP raffic. We described he procedures enabling he organizing of he nework and real ime raffic managemen. For fuure research i would be ineresing o sudy he effec of he TCP raffic on he nework capaciy for VoIP. Based on his work, we are going o invesigae how o efficienly manage he nework where VoIP sreams are combined wih he TCP flows. The issue of nodes mobiliy is crucial for deerminaion and will be he opic of furher sudy. Furhermore, we would like o devoe aenion o he aspec of he disribued nework managemen. This includes opimizaion of he scheme for deermining he secondary

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