QoS Mechanisms C HAPTER Introduction. 3.2 Classification

Transcription

1 C HAPTER 3 QoS Mechnisms 3.1 Introduction In the previous chpter, we introduced the fundmentl QoS concepts. In this chpter we introduce number of key QoS mechnisms tht enble QoS services. At the end of this chpter, we provide generl frmework for nlyzing the QoS support of ech wireless technology presented in the rest of this book. QoS mechnisms cn be ctegorized into two groups bsed on how the ppliction trffic is treted: 1) trffic hndling mechnisms, nd 2) bndwidth mngement mechnisms (see Figure 3.1). Trffic hndling mechnisms (sometimes clled In-trffic mechnisms) re mechnisms tht clssify, hndle, police, nd monitor the trffic cross the network. The min mechnisms re: 1) clssifiction, 2) chnnel ccess, 3) pcket scheduling, nd 4) trffic policing. Bndwidth mngement mechnisms (sometimes clled Out-of-trffic mechnisms) re mechnisms tht mnge the network resources (e.g., bndwidth) by coordinting nd configuring network devices (i.e., hosts, bse sttions, ccess points) trffic hndling mechnisms. The min mechnisms re: 1) resource reservtion signling nd 2) dmission control. 3.2 Clssifiction The lowest service level tht network cn provide is best effort service, which does not provide QoS support. In best effort service, ll trffic is hndled eqully regrdless of the ppliction or host tht generted the trffic. However, some pplictions need QoS support, requiring better thn best effort service such s differentited or gurnteed service. For network to provide selective services to certin pplictions, first of ll, the network requires clssifiction mechnism tht cn differentite between the different pplictions. The clssifiction mechnism identifies nd seprtes different trffic into flows or groups of flows (ggregted flows or clsses). Therefore, ech flow or ech ggregted flow cn be hndled selectively. 39

2 40 Chpter 3 QoS Mechnisms Wireless Network QoS Mechnisms Trffic Hndling Mechnisms - Clssifiction - Chnnel Access - Scheduling - Trffic Policing Bndwidth Mngement Mechnisms - Reservtion - Admission Control Host Host Intermedite Devices (i.e., ccess point, bse sttion, stellite) Figure 3.1 QoS Mechnisms in Wireless Network The clssifiction mechnism cn be implemented in different network devices (i.e., end hosts, intermedite devices such s switches, routers, ccess points). Figure 3.2 shows simplified digrm of clssifiction module tht resides on n end host nd on n intermedited device. End Host Applictions Clssifiction Dt Trffic Incoming Trffic (from other hosts) Intermedite Device Clssifiction Scheduler Scheduler Dt s Dt s Figure 3.2 Clssifiction

3 Clssifiction 41 Appliction trffic (t the end host) or incoming trffic from other hosts (t the intermedite device) is identified by the clssifiction mechnism nd is forwrded to the pproprite queue witing service from other mechnisms such s the pcket scheduler. The grnulrity level of the clssifiction mechnism cn be per-user, per-flow, or per-clss depending on the type of QoS services provided. For exmple, per-flow QoS service requires per-flow clssifiction while per-clss QoS service requires per-clss clssifiction. To identify nd clssify the trffic, the trffic clssifiction mechnism requires some form of tgging or mrking of pckets. There re number of trffic clssifiction pproches. Some of pproches re suitble for end hosts nd some for intermedite hosts. Figure 3.3 shows n exmple of some trffic clssifiction pproches which re implemented in the different Open System Interconnection (OSI) lyers. OSI Lyer Clssifiction Techniques Appliction User/Appliction Identifiction Trnsport Flow (5-tuplet IP Address) Network IPTOS, DSCP Dt Link 802.1p/Q Clssifiction Physicl Lyer Figure 3.3 Exmples of Existing Clssifiction on Ech OSI Lyer Dt Link Lyer Clssifiction Dt link lyer, or Lyer 2, clssifies the trffic bsed on the tg or field vilble in Lyer 2 heder. An exmple of Lyer 2 clssifiction is IEEE (Institute of Electricl nd Electronics Engineers) 802 user priority. The IEEE 802 heder includes 3-bit priority field tht enbles eight priority clsses. It ims to support service differentition on Lyer 2 network such s LAN. The end host or intermedite host ssocites ppliction trffic with clss (bsed on the Policy, or the service tht the ppliction expects to receive) nd tgs the pckets priority field in the IEEE 802 heder. A clssifiction mechnism identifies pckets by exmining the priority field of the IEEE 802 heder nd forwrds the pckets to the pproprited queues. IEEE recommends mpping the priority vlue nd the corresponding service s shown in Tble 3.1.

4 42 Chpter 3 QoS Mechnisms Tble 3.1 Exmple of Mpping between Priority nd Services Priority Service 0 Defult, ssumed to be best effort service 1 Less thn best effort service 2 Reserved 3 Reserved 4 Dely sensitive, no bound 5 Dely sensitive, 100ms bound 6 Dely sensitive, 10ms bound 7 Network control Network Lyer Clssifiction Network lyer, or Lyer 3 clssifiction, clssifies pckets using Lyer 3 heder. Lyer 3 clssifiction enbles service differentition in Lyer 3 network. An exmple of Lyer 3 clssifiction is IPTOS (Internet protocol type of service), DSCP (Internet protocol differentil service code point). IPv4 nd IPv6 stndrd defined prioritiztion field in the IP heder which cn be used for Lyer 3 clssifiction. RFC 1349 defined TOS field in IPv4 heder. The type of service field consists of 3-bit precedence subfield, 4-bit TOS subfield, nd the finl bit which is unused nd is set to be 0. The 4-bit TOS subfield enbles 16 clsses of service. In IPv6 heder there is n 8-bit clss of service field (see Figure 3.4). Lter the Internet Engineering Tsk Force (IETF) differentited services working group redefined IPv4 IPTOS to be DSCP, which is shown in Figure 3.4. DSCP hs 6-bit field enbling 64 clsses of service.

5 Clssifiction 43 6-bit DSCP Unused DSCP bit Precedence 4-bit TOS 0 IPTOS bit 4-bit heder 8-bit TOS 16-bit totl length (in bytes) version length 16-bit identifiction 3-bit flg 13-bit frngment offset 8-bit TTL (Time To Live) 8-bit Protocol 16-bit heder checksum 32-bit source IP ddress 32-bit destintion IP ddress DATA IPv4 Heder bit 8-bit trffic clss Flow lbel version Pylod length Next Heder Hop limit Pylod length Pylod length IPv6 Heder Figure 3.4 Structure of IPTOS nd DSCP in IPv4 nd IPv Trnsport Lyer Clssifiction (5-tuplet IP Heder) A 5-tuplet IP heder (source IP, destintion IP, source port, destintion port, nd protocol IP) cn be used for trnsport lyer clssifiction. A 5-tuplet IP heder cn uniquely identify the individul ppliction or flow. This clssifiction provides the finest grnulrity nd supports per-flow QoS service. However, the 5-tuplet IP heder clssifiction hs some limittions: It is suitble for edge networks, but it is not suitble for core networks tht crry very lrge mounts of trffic. Mintining queues for ech individul flow cn be n overwhelming tsk.

6 44 Chpter 3 QoS Mechnisms If the trffic psses through firewll tht uses NAT (network ddress trnsltion), the rel IP ddress (i.e., the IP ddress of the trffic source) is hidden from networks outside the firewll. Therefore, the 5-tuplet IP heder exposed to network outside the firewll cnnot uniquely identify the ppliction Appliction or User Clssifiction The ppliction or user cn be uniquely identified by using user/ppliction identifiction (ID). The ID ssignment my be sttic (i.e., the policy or the contrct) or dynmic (i.e., connection signling). For the connection signling, there is centrl sttion or entity in the network tht is responsible for mking the decision whether to llow new session to join the network. First, the ppliction or user sends the connection request to the centrl sttion. Then, if the new connection is dmitted, it will be ssigned unique ID number. s from the ppliction will be ssocited with n ID number. 3.3 Chnnel Access Mechnism In wireless networks, ll hosts communicte through shred wireless medium. When multiple hosts try to trnsmit pckets on the shred communiction chnnel, collisions cn occur. Therefore, wireless networks need chnnel ccess mechnism which controls the ccess to the shred chnnel. There re two types of chnnel ccess mechnisms: 1) collision-bsed chnnel ccess nd 2) collision-free chnnel ccess. Ech type of chnnel ccess mechnism cn provide different QoS services Collision-Bsed Chnnel Access Collision-bsed chnnel ccess is distributed chnnel ccess method tht provides mechnisms to void collisions nd to resolve collisions in cse they occur. A clssic collision-bsed chnnel ccess mechnism developed for wired LANs nd implemented in Ethernet is CSMA/CD (Crrier Sense Multiple Access with Collision Detection). In collision-bsed chnnel ccess schemes, collisions cn occur leding to the need for retrnsmissions. The collision probbility depends on the number of ctive (with pckets for trnsmission) users in the network. High trffic lod increses the number of collisions nd retrnsmissions, incresing the dely. Since we del with stochstic trffic, the number of collisions nd re-trnsmissions is rndom s well, leding to n unbound dely. Therefore, collision-bsed chnnel ccess schemes cn provide best effort service. All hosts in the network receive equl bndwidth nd experience the sme unbounded dely. The service level cn be improved by: Over-provisioning, whereby ll trffic will receive mple of bndwidth nd experience low dely.

7 Chnnel Access Mechnism 45 Adding priority scheme in the collision-bsed chnnel ccess tht is, using different sized bckoff windows for different priority clsses. This will enble the provision of differentited services. An exmple of such solution is described in the proposed IEEE e (Chpter 4). Existing solutions in wireless networks such s IEEE DCF, HomeRF use collision-bsed chnnel ccess protocols similr to Ethernet CSMA/CD, denoted s CSMA/CA where CA stnds for Collision Avoidnce Collision-Free Chnnel Access In collision-free chnnel ccess mechnism the chnnel is rbitrted such tht no collisions cn occur. Only one host is llowed to trnsmit pckets to the chnnel t ny given time. Collision, therefore, will not occur. Exmples of collision-free chnnel ccess techniques re polling nd TDMA (Time Division Multiple Access) Polling A host in the network, or specilized network device such s n Access Point or Bse Sttion, is designted s the poller, which controls ll ccess to the wireless chnnel by the other hosts denoted s pollees. Pollees re not llowed to trnsmit pckets unless they receive polling pcket from the poller. Thus, there is no collision. Some pollees my receive the poll more often thn others. The polling frequency (the number of polls in period of time) reflects the bndwidth lloction. A poller cn dynmiclly llocte bndwidth to pollees by djusting the polling frequency dynmiclly TDMA (Time Division Multiple Access) A TDMA scheme divides the chnnel ccess opportunity into frmes nd ech frme is divided into time slots. A host is llowed to trnsmit pckets in predefined time slot, s shown in Figure 3.5. The number of time slots ssigned to host per frme reflects the bndwidth llocted for the host. This technique requires mster host tht is designted to mnge the time slot ssignment for ll the hosts in the network. This Mster host determines the number of time slots tht ech host will be llowed to trnsmit nd notifies the hosts using some signling mechnism. There re number of time slot ssignment philosophies: Sttic time slot ssignment: Ech host receives fixed time slot ssignment which cn be provided during the connection setup. Dynmic time slot ssignment: The time slot ssignment chnges dynmiclly during the lifetime of the session s function of the trffic lod, ppliction QoS requirements, nd chnnel conditions. This slot ssignment policy is more flexible nd leds to better chnnel utiliztion. However, it leds to signling overhed required to communicte the slot ssignment chnges to the different hosts.

8 46 Chpter 3 QoS Mechnisms Host Trnsmit pckets in ssigned time slots Host Time Slot Frme i Frme i+1 Figure 3.5 Time Division Multiple Access (TDMA) Scheme Collision-free chnnel ccess schemes provide tight chnnel ccess control tht cn provide tight dely bound. Therefore, these schemes re good cndidtes for QoS provision to pplictions with strict QoS requirements. 3.4 Scheduling Mechnisms scheduling is the mechnism tht selects pcket for trnsmission from the pckets witing in the trnsmission queue. It decides which pcket from which queue nd sttion re scheduled for trnsmission in certin period of time. scheduling controls bndwidth lloction to sttions, clsses, nd pplictions. As shown in Figure 3.6, there re two levels of pcket scheduling mechnisms: 1. Intrsttion pcket scheduling: The pcket scheduling mechnism tht retrieves pcket from queue within the sme host. 2. Intersttion pcket scheduling: The pcket scheduling mechnism tht retrieves pcket from queue from different hosts. scheduling cn be implemented using hierrchicl or flt pproches. Hierrchicl pcket scheduling: Bndwidth is llocted to sttions tht is, ech sttion is llowed to trnsmit t certin period of time. The mount of bndwidth ssigned to ech sttion is controlled by intersttion policy nd module. When sttion receives the opportunity to trnsmit, the intrsttion pcket scheduling module will decide which pckets to trnsmit. This pproch is sclble becuse intersttion pcket scheduling mintins the stte by sttion (not by connection or ppliction). Overll bndwidth is llocted bsed on sttions (in fct, they cn be groups, deprtments, or compnies). Then, sttions will hve the uthority to mnge or llocte their own bndwidth portion to pplictions or clsses within the host.

9 Scheduling Mechnisms 47 End Host Intermedite Host Scheduler Intrsttion pcket scheduling (determining pckets from which queue re llowed to be trnsmited) Scheduler Network Intersttion pcket scheduling (determining pckets from which sttion re llowed to be trnsmited) Figure 3.6 Scheduling Flt pcket scheduling: scheduling is bsed on ll queues of ll sttions. Ech queue receives individul service from the network. scheduling mechnism dels with how to retrieve pckets from queues, which is quite similr to queuing mechnism. Since in intrsttion pcket scheduling the sttus of ech queue in sttion is known, the intrsttion pcket scheduling mechnism is virtully identicl to queuing mechnism. Intersttion pcket scheduling mechnism is slightly different from queuing mechnism becuse queues re distributed mong hosts nd there is no centrl knowledge of the sttus of ech queue. Therefore, some intersttion pcket scheduling mechnisms require signling procedure to coordinte the scheduling mong hosts. Becuse of the similrities between pcket scheduling nd queuing mechnisms we introduce number of queuing schemes (First In First Out [FIFO], Strict Priority, nd Weight Fir [WFQ]) nd briefly discuss how they support QoS services First In First Out (FIFO) First In First Out (FIFO) is the simplest queuing mechnism. All pckets re inserted to the til of single queue. s re scheduled in order of their rrivl. Figure 3.7 shows FIFO pcket scheduling.

10 48 Chpter 3 QoS Mechnisms End Host App. App. App. App. First In First Out (FIFO) Scheduling Figure 3.7 FIFO Scheduling FIFO provides best effort service tht is, it does not provide service differentition in terms of bndwidth nd dely. The high bndwidth flows will get lrger bndwidth portion thn the low bndwidth flows. In generl, ll flows will experience the sme verge dely. If flow increses its bndwidth ggressively, other flows will be ffected by getting less bndwidth, cusing incresed verge pcket dely for ll flows. It is possible to improve QoS support by dding 1) trffic policing to limit the rte of ech flow nd 2) dmission control Strict Priority s re ssigned priority order. Strict priority pcket scheduling schedules pckets bsed on the ssigned priority order. s in higher priority queues lwys trnsmit before pckets in lower priority queues. A lower priority queue hs chnce to trnsmit pckets only when there re no pckets witing in higher priority queue. Figure 3.8 illustrtes the strict priority pcket scheduling mechnism. Strict priority provides differentited services (reltive services) in both bndwidth nd dely. The highest priority queue lwys receives bndwidth (up to the totl bndwidth) nd the lower priority queues receive the remining bndwidth. Therefore, higher priority queues lwys experience lower dely thn the lower priority queues. Aggressive bndwidth spending by the high priority queues cn strve the low priority queues. Agin, it is possible to improve the QoS support by dding 1) trffic policing to limit the rte of ech flow nd 2) dmission control.

11 Scheduling Mechnisms 49 End Host High Priority Low Priority Scheduler Strict Priority Scheduling Figure 3.8 Strict Priority Scheduling Weight Fir (WFQ) Weight Fir schedules pckets bsed on the weight rtio of ech queue. Weight, w i, is ssigned to ech queue i ccording to the network policy. For exmple, there re three queues A, B, C with weights w 1, w 2, w 3, respectively. s A, B, nd C receive the following rtios of vilble bndwidth: w 1 /(w 1 +w 2 +w 3 ), w 2 /(w 1 +w 2 +w 3 ), nd w 3 /(w 1 +w 2 +w 3 ), respectively, s shown in Figure 3.9. Host w 3 w 1 w 2 A B C w 2 /s w 1 /s w 3 /s Weight Weight Fir Scheduler s = w 1 +w 2 +w 3 Figure 3.9 Weight Fir Scheduling

12 50 Chpter 3 QoS Mechnisms Bndwidth buse from specific queue will not ffect other queues. WFQ cn provide the required bndwidth nd the dely performnce is directly relted to the llocted bndwidth. A queue with high bndwidth lloction (lrge weight) will experience lower dely. This my led to some mismtch between the bndwidth nd dely requirements. Some pplictions my require low bndwidth nd low dely. In this cse WFQ will llocte high bndwidth to these pplictions in order to gurntee the low dely bound. Some pplictions my require high bndwidth nd high dely. WFQ still hs to llocte high bndwidth in order for the pplictions to operte. Of course, pplictions will stisfy the dely but sometimes fr beyond their needs. This mismtch cn led to low bndwidth utiliztion. However, in rel life, WFQ mostly schedules pckets tht belong to ggregted flows, groups, nd clsses (insted of individul flows) where the gol is to provide link shring mong groups. In this cse dely is of less concern. The elementry queuing mechnisms introduced bove will be the bsis of number of pcket scheduling vritions. Before we move our discussion to the next QoS mechnisms, it is worth mentioning tht in some implementtions the chnnel ccess mechnism nd pcket scheduling mechnism re not mutully exclusive. There is some overlp between these two mechnisms nd sometimes they re blended into one solution. When we discuss QoS support of ech wireless technology in lter chpters, in some cses, we will discuss both mechnisms together. 3.5 Trffic Policing Mechnism Trffic policing is the mechnism tht monitors the dmitted sessions trffic so tht the sessions do not violte their QoS contrct. The trffic policing mechnism mkes sure tht ll trffic tht psses through it will conform to greed trffic prmeters. When violtion is found (e.g., more trffic is sent thn ws initilly greed upon in the QoS contrct), trffic policing mechnism is enforced by shping the trffic. Becuse trffic policing shpes the trffic bsed on some known quntittive trffic prmeters, multimedi (rel-time) pplictions re nturlly comptible to trffic policing. Most multimedi ppliction trffic (voice, video) is generted by stndrd codec which generlly provides certin knowledge of the quntittive trffic prmeters. Trffic policing cn be pplied to individul multimedi flows. Non-rel-time trffic does not provide quntittive trffic prmeters nd usully demnds bndwidth s much s possible. Therefore, trffic policing enforces non-rel-time trffic (i.e., limits the bndwidth) bsed on the network policy. Such policing is usully enforced on ggregted non-rel-time flows. Trffic policing cn be implemented on end hosts or intermedite hosts. Exmples of trffic policing mechnisms include the leky bucket nd the token bucket Leky Bucket The leky bucket mechnism is usully used to smooth the burstiness of the trffic by limiting the trffic pek rte nd the mximum burst size. This mechnism, s its nme describes, uses the nlogy of leky bucket to describe the trffic policing scheme. The bucket s prmeters such s its size nd the hole s size re nlogous to the trffic policing prmeters such s the

13 Trffic Policing Mechnism 51 (A) (B) (C) Rte R < r Rte R > r Rte R > r Bucket is empty Bucket rte r b Bucket is not empty, but not full Bucket is full Non-conformnt trffic - either dropped or sent s best effort trffic Rte R Rte r Rte r Figure 3.10 Leky Bucket Mechnism mximum burst size nd mximum rte, respectively. The leky bucket shpes the trffic with mximum rte of up to the bucket rte. The bucket size determines the mximum burst size before the leky bucket strts to drop pckets. The mechnism works in the following wy. The rriving pckets re inserted t the top of the bucket. At the bottom of the bucket, there is hole through which trffic cn lek out t mximum rte of r bytes per second. The bucket size is b bytes (i.e., the bucket cn hold t most b bytes). Let us follow the leky bucket opertion by observing the exmple shown in Figure We ssume first tht the bucket is empty. Figure 3.10 (A): Incoming trffic with rte R which is less thn the bucket rte r. The outgoing trffic rte is equl to R. In this cse when we strt with n empty bucket, the burstiness of the incoming trffic is the sme s the burstiness of the outgoing trffic s long s R < r. Figure 3.10 (B): Incoming trffic with rte R which is greter thn the bucket rte r. The outgoing trffic rte is equl to r (bucket rte). Figure 3.10 (C): Sme s (B) but the bucket is full. Non-conformnt trffic is either dropped or sent s best effort trffic Token Bucket The token bucket mechnism is lmost the sme s the leky bucket mechnism but it preserves the burstiness of the trffic. The token bucket of size b bytes is filled with tokens t rte r (bytes per second). When pcket rrives, it retrieves token from the token bucket (given such token is vilble) nd the pcket is sent to the outgoing trffic strem. As long s there re

14 52 Chpter 3 QoS Mechnisms (A) Token rte r b Token drined out with rte R Incoming trffic rte R < r Outgoing trffic rte R (B) Token rte r b Token drined out with rte R Incoming trffic rte R > r Outgoing trffic rte R (C) Token rte r b Bucket is empty Token drined out with rte r Incoming trffic rte R > r Outgoing trffic rte r Figure 3.11 Token Bucket Mechnism tokens in the token bucket, the outgoing trffic rte nd pttern will be the sme s the incoming trffic rte nd pttern. If the token bucket is empty, incoming pckets hve to wit until there re tokens vilble in the bucket, nd then they continue to send. Figure 3.11 shows n exmple of the token bucket mechnism. Figure 3.11 (A): The incoming trffic rte is less thn the token rrivl rte. In this cse the outgoing trffic rte is equl to the incoming trffic rte. Figure 3.11 (B): The incoming trffic rte is greter thn the token rrivl rte. In cse there re still tokens in the bucket, the outgoing trffic rte is equl to the incoming trffic rte. Figure 3.11 (C): If the incoming trffic rte is still greter thn the token rrivl rte (e.g., long trffic burst), eventully ll the tokens will be exhusted. In this cse the incoming trffic hs to wit for the new tokens to rrive in order to be ble to send out. Therefore, the outgoing trffic is limited t the token rrivl rte.

15 Reservtion Signling Mechnisms 53 The token bucket preserves the burstiness of the trffic up to the mximum burst size. The outgoing trffic will mintin mximum verge rte equl to the token rte, r. Therefore, the token bucket is used to control the verge rte of the trffic. In prcticl trffic policing, we use combintion of the token bucket nd leky bucket mechnisms connected in series (token bucket, then leky bucket). The token bucket enforces the verge dt rte to be bound to token bucket rte while the leky bucket (p) enforces the pek dt rte to be bound to leky bucket rte. Trffic policing, in coopertion with other QoS mechnisms, enbles QoS support. 3.6 Reservtion Signling Mechnisms The trffic hndling mechnisms (clssifiction, chnnel ccess, pcket scheduling, nd trffic policing) we lredy described enble QoS services in ech device. However, coordintion between devices is essentil to deliver end-to-end QoS services. reservtion signling mechnisms inform the network entities on the QoS requirements of the multimedi pplictions using the network resources. The network devices will use this informtion to mnge the network resources (i.e., bndwidth) in order to ccommodte such requirements. The network devices control the network resources nd provide QoS services by configuring the trffic hndling mechnisms. reservtion cn be pplied to individul flows or ggregted flows. reservtion closely coopertes with the dmission control mechnisms tht will be described in lter section. Figure 3.12 shows schemtic digrm tht describes the coordintion between these mechnisms. The resource reservtion mechnisms include the following functions: Provision of resource reservtion signling tht notifies ll devices long the communiction pth on the multimedi pplictions QoS requirements. Delivery of QoS requirements to the dmission control mechnism tht decides if there re vilble resources to meet the new request QoS requirements. Notifiction of the ppliction regrding the dmission result. Reservtion Protocol (RSVP) is well-known resource reservtion signling mechnism. RSVP opertes on top of IP, in the trnsport lyer, so it is comptible with the current TCP/IP bsed mechnisms (i.e., IPv4, IP routing protocol, nd IP multicst mechnism) nd cn run cross multiple networks. RSVP s min functionlity is to exchnge QoS requirement informtion mong the source host, the destintion host, nd intermedite devices. Using this informtion, ech network device will reserve the proper resources nd configure its trffic hndling mechnisms in order to provide the required QoS service. Once the reservtion process is complete, the sender host is llowed to trnsmit dt with n greed trffic profile. If device or network element on the communiction pth does not hve enough resources to ccommodte the trffic, the network element will notify the ppliction tht the network cnnot support this QoS requirement. In order to chieve end-to-end resource reservtion, ll the network elements long

16 54 Chpter 3 QoS Mechnisms Sender Host Intermedite Host Intermedite Host Receiver Host Appliction Appliction Reservtion Reservtion Reservtion Reservtion Admission Control Admission Control Trffic Hndling Trffic Hndling Trffic Hndling Trffic Hndling Signling Flow Figure 3.12 Reservtion Mechnism the pth (source host, destintion host, nd routers) need to be RSVP-enbled. Originlly, RSVP ws designed for supporting per-flow reservtion. Currently it is extended to support per-ggregte reservtion. 3.7 Admission Control Admission control is the mechnism tht mkes the decision whether to llow new session to join the network. This mechnism will ensure tht existing sessions QoS will not be degrded nd the new session will be provided QoS support. If there re not enough network resources to ccommodte the new sessions, the dmission control mechnism my either reject the new session or dmit the session while notifying the user tht the network cnnot provide the required QoS. Admission control nd resource reservtion signling mechnisms closely cooperte with ech other. Both re implemented in the sme device. There re two dmission control pproches: Explicit dmission control: This pproch is bsed on explicit resource reservtion. Applictions will send the request to join the network through the resource reservtion signling mechnism. The request tht contins QoS prmeters is forwrded to the dmission control mechnism. The dmission control mechnism decides to ccept or reject the ppliction bsed on the ppliction s QoS requirements, vilble resources, performnce criteri, nd network policy. Implicit dmission control: There is no explicit resource reservtion signling. The dmission control mechnism relies on bndwidth over-provisioning nd trffic control (i.e., trffic policing). The loction of the dmission control mechnism depends on the network rchitecture. For exmple, in cse we hve wide re network such s high-speed bckbone tht consists of number of interconnected routers, the dmission control mechnism is implemented on ech router. In shred medi networks, such s wireless networks, there is designted entity in the

17 QoS Architecture 55 network (e.g., sttion, ccess point, gtewy, bse sttion) tht hosts the dmission control gent. This gent is in chrge of mking dmission control decisions for the entire wireless network. This concept is similr to the SBM (subnet bndwidth mnger) which serves s the dmission control gent in 802 networks. In d hoc wireless networks, the dmission control functionlity cn be distributed mong ll hosts. In infrstructure wireless networks where ll communiction psses through the ccess point or bse sttion, the dmission control functionlity cn be implemented in the ccess point or bse sttion. 3.8 QoS Architecture This section shows how ll the QoS mechnisms described in the previous subsections re working together to provide QoS support. Different pplictions tht co-exist in the sme network my require different combintions of QoS mechnisms such s: Applictions with quntittive QoS requirements: These pplictions mostly require QoS gurnteed services. Therefore, explicit resource reservtion nd dmission control re needed. They lso require strict trffic control (trffic policing, pcket scheduling, nd chnnel ccess). Applictions with qulittive QoS requirements: These pplictions require high QoS levels but do not provide quntittive QoS requirements. In this cse we cn use resource reservtion nd dmission control. They lso require trffic hndling which delivers differentited services. Best effort: There is no need for QoS gurntees. The network should reserve bndwidth for such services. The mount of reserved bndwidth for best effort trffic is determined by the network policy. The QoS rchitecture which contins different QoS mechnisms is different for ech network topology. We will focus on the QoS rchitecture for d hoc nd infrstructure wireless networks QoS Architecture for Infrstructure Wireless Networks In infrstructure wireless networks, there re two types of sttions: end sttions (hosts) nd centrl sttion (i.e., ccess point, bse sttion). The centrl sttion regultes ll the communiction in the network tht is, there is no peer-to-peer communiction tht occurs directly between the hosts. The trffic from source host is sent to the centrl sttion nd then the centrl sttion forwrds the trffic to the destintion host. All trffic hndling (clssifiction, trffic policing, pcket scheduling, nd chnnel ccess) nd resource reservtion mechnisms reside in ll sttions (end hosts nd centrl sttion). In ddition, the centrl sttion lso includes n dmission control mechnism. Figure 3.13 shows QoS rchitecture for n infrstructure wireless network. There re some vritions in the signling mechnisms tht configure the trffic hndling mechnisms in ech sttion. We will point out these differences in ech wireless technology chpter.

18 56 Chpter 3 QoS Mechnisms End Host Appliction Access Point or Bse Sttion End Host Appliction fic r T t D Reservtion Reservtion Admission Control Reservtion fic r T t D Clssifiction Clssifiction Clssifiction Trffic Policing Trffic Policing Trffic Policing Scheduling Scheduling Scheduling Chnnel Access Chnnel Access Chnnel Access Wireless Medium Signling Flow Dt Flow Figure 3.13 QoS Architecture of n Infrstructure Wireless Network QoS Architecture For Ad Hoc Wireless Networks All hosts estblish peer-to-peer communiction in the shred wireless medi environment. All trffic hndling nd resource reservtion mechnisms reside in ll hosts. One of the hosts (either dedicted or regulr end host) will be designted to serve s n dmission control gent (i.e., designted SBM [DSBM]). Figure 3.14 shows QoS rchitecture for n d hoc wireless network.

19 QoS Architecture 57 End Host Appliction Designted SBM Appliction End Host Appliction fic tr t D Reservtion Reservtion Admission Control fic tr t D Reservtion fic tr t D Clssifiction Clssifiction Clssifiction Trffic Policing Trffic Policing Trffic Policing Scheduling Scheduling Scheduling Chnnel Access Chnnel Access Chnnel Access Wireless Medium Signling Flow Dt Flow Figure 3.14 QoS Architecture for n Ad Hoc Wireless Network