Delivery in a Broadband Wireless Local Loop MAC Protocol：在宽带无线本地环路的MAC协议交付

1、IP QoS Delivery in a Broadband Wireless Local Loop: MAC Protocol Definition and Performance EvaluationBaiocchi, Cuomo, and BolognesiIEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 18, NO. 9, SEPTEMBER 20001AbstractIn this paper, a complete broadband wireless local loop (WLL) network is presen

2、ted.The proposal is based on the OFDM-CDMA technique, to which an added dynamic reservation/request MAC protocol is proposed. Central to our proposal is the support of different QoS profiles. As a case study, the explicit presentation of the IETF integrated services (IntServ) support over our WLL sy

3、stem is addressed. We prove that our scheme achieves high utilization efficiency, as well as a fair share of the available radio capacity. 2I. INTRODUCTIONFWA (fixed wireless access) ArchitectureA centralized radio node (RN)A group of fixed radio terminals (RT)a customer premises network/equipment (

4、CPN/CPE)3I. INTRODUCTION -2FWA exploits the OFDM-CDMA 1 2 (orthogonal frequency division multiplexing - code division multiple access) technique which provides protection against fading, peak-average power ratio reduction capabilities,and high flexibility in band-width assignment.Duplexing can be ma

5、naged dynamically to provide tight tracking of traffic asymmetry, by sharing the available pool of codes between uplink and downlink (code division duplex).4II. SYSTEM ARCHITECTURENSL = network service layer (e.g. IP)Three layers:(i) Adaptation layer(AL); (ii) MAC layer; (iii) Physical layer.5II. SY

6、STEM ARCHITECTURE -2NSLcorresponds toclassical network functionsaddressing, routingtraffic handling functionspacket flow description and classification,admission control, traffic policing and/or shapingExamples of NSLIP layer enhanced with QoS handling capabilities (e.g. IntServ or DiffServ)ATM traf

7、fic control (e.g. CBR, VBR, ABR, UBR)6II. SYSTEM ARCHITECTURE -3ALmaps NSL traffic classes into MAC service classesTwo service types in the MAC layerGuaranteed bandwidth (GB)Best effort (BE)AL flow mapping tablemapping NSL traffic classes into MAC service classesUpdated by NSL when a new flow is adm

8、ittedSegmenting and reassembling (SAR)7II. SYSTEM ARCHITECTURE -4MAC layerCapacity assignmentSharing radio capacity among flows.Performed at the RN, by a centralized functional entity named MAC scheduler controller (MAC-SC).Two service types in the MAC layerGuaranteed bandwidth (GB)Best effort (BE)P

9、hysical layerCoding and transmitting/receiving signals according to OFDM/CDMA.8II. SYSTEM ARCHITECTURE -59III. PHYSICAL LAYERA. Modulation TechniqueOFDM a multi-carrier techniqueAdvantages of OFDMImmune to channel dispersion compared to a single carrier technique;equalizers require much less computa

10、tional effort than for single carrier systems;intercarrier and intersymbol interference can be eliminated by introducing a guard time interval and a cyclic symbol extension between successive symbols.Disadvantages of OFDMMore sensitive to local oscillator phase noise and to carrier frequency offsets

11、.10III. PHYSICAL LAYER -2The OFDM modulation can support multiple access by means ofOFDM-TDMAEach symbol interval (SI) is used for the transmission of K data symbols of the same user on the K OFDM subcarriersDelay caused by collecting K data symbols from a userOFDM-CDMAOne SI can be used for the tra

12、nsmission of data symbols belonging to K different users; (K = 512 commonly)The contemporary transmission is obtained by multiplying each user data symbol by an orthogonal spreading codeOFDMA Allows an intermediate type of multiplexing by permitting each user to transmit x data symbols on a set of s

13、ubcarriers per SI .(1 x K)Or combination11III. PHYSICAL LAYER -312III. PHYSICAL LAYER -413III. PHYSICAL LAYER -5Advantages of OFDM/CDMA and OFDMALow packetization delayFlexibility in bandwidth assignmentAs the granularity gets finer (i.e., x gets lower), the benefits of multicarrier transmission ten

14、d to disappear for OFDMA, because a smaller and smaller subset of the available subcarriers is actually used by each user. Instead, they are intact in case of OFDM-CDMA.14III. PHYSICAL LAYER -7B. The FWA Physical LayerThis paper assumes an OFDM/CDMA with FDD (frequency division duplex) technique.Thi

15、s paper consider millimeter wave region of the radio spectrum because of the availability of larger bandwidth blocks.The number of OFDM subcarriers is chosen to be 512.Tradeoff: increasing number of subcarriers improves the multipath robustness, reduces the guard interval overhead, and increases the

16、 flexibility in bandwidth assignment; increases the phase noise sensitivity, makes base-band processing (i.e., FFT) more complex. 15III. PHYSICAL LAYER -616IV. MAC PROTOCOLA. MAC Service ClassesTwo service types in the MAC layerGuaranteed bandwidth (GB)Used for services with stringent requirements f

17、or delay and delay jitter, i.e. real time services (e.g. video and audio)Traffic descriptors (TDs) are required (e.g. peak bit rate) for each information flowRelevant admission control and flow parameter compliance checks must be defined in network layerBest effort (BE)To provide for economic use an

18、d efficient useCapacity left from more demanding flows can be filled with traffic with loose requirementsTo accommodate the existing Internet application traffic17IV. MAC PROTOCOL -2B. MAC SignalingThe radio capacity with OFDM-CDMA is structured as K orthogonal codes that can be used simultaneously.

19、Each code is used in a TDMA fashion; a time slot carries a MAC_PDU (TSLOT = TMAC_PDU).RT ID, Other Info., Data LoadTime is structured into frames (TFRAME) lasting N time slots by K * N * MAC_PDUs.The structure is referred to as the TC-matrix (time slots-code matrix).See Fig. 6 for N=318IV. MAC PROTO

20、COL -3The capacity assignment is performed frame by frame.Each RT can transmit (uplink) on several time slot-code pairs (TC-pairs) without restrictions.See the gray slots in Fig. 6 19IV. MAC PROTOCOL -4The basic MAC signaling consists ofthe request channel (ReqCh)an UL ( Uplink Logical) channel to m

21、ake capacity requests the allocation channel (AlCh)a DL (Downlink Logical) channel to answer the requestsThe ReqCh and AlCh is structured in minislots.A ReqCh-AlCh minislot pair is dedicated to each RT.20IV. MAC PROTOCOL -5The ReqCh in UL is structured in minislots:Each minislot contains the bandwid

22、th request.RT ID, Request GB Class, Request BE ClassThe request issued in the kth frame by each RT is just the number of MAC_PDUs of each service class found in the RT at the beginning of the kth frame for which there is no pending request.21IV. MAC PROTOCOL -6The AlCh in DL has the same minislot st

23、ructure as the ReqCh.A ReqCh-AlCh minislot pair is dedicated to each RT.Each minislot contains the allocation reply.Starting Code, Starting Offset, No. of TC-pairsThe RN uses the AlCh to signal to each RTthe number of assigned TC-pairs, the starting code (the row of the TC-matrix)the starting offset

24、 in the code row.Detailed format and dimensioning of ReqCh and AlCh are re-ported in 14.22IV. MAC PROTOCOL 7(1) UL Request Channel(2) DL Allocation ChannelRN to RTRT to RN(N =3)23IV. MAC PROTOCOL -8C. MAC Fair Scheduling Algorithm (FSA)Each RT stores arriving MAC_PDUs into its buffers by separating

25、GB and BE packets:BE traffic is queued up into a single FIFO bufferGB traffic is split among a set of FIFO buffersGB traffic has priority over BE ones.The overall available capacity in each frame (K * N H TC-pairs) is assigned to each RTs according to FSA.FSA shares radio link capacity hierarchicall

26、y among groups of users as to support decreasing QoS targets.24IV. MAC PROTOCOL -9The FSA is a practical realization of the fluid GPS 15 which shares a fixed resource (capacity) among competing users, according to their actual load and to predefined weights.A. K. Parekh and R. G. Gallager, “A genera

27、lized processor sharing approach to flow control in integrated service networksThe single node case, in Proc. IEEE Infocom92, 1992, pp. 915924. Here, the weight is related to the packet flow TDs and is passed to MAC layer by the NSL traffic control.(1) the output of the FSA must be integer;(2) trade

28、off between bandwidth and complexity;25IV. MAC PROTOCOL -10The FSA divided the scheduling operation into two phases.(1) The overall radio link capacity is shared among RTs, according to their overall requests and weights, by the RN MAC-SC;(2) Each RT shares the bandwidth it obtained among the compet

29、ing GB flows and, if possible, the BE traffic.an RT can use a single FIFO buffer for GB traffic yielding the maximum capacity penalty;individual queues can be handled per GB flow, resulting in the most efficient use of capacity although at the price of running a per-flow scheduling algorithm.FSA is

30、applied in each phase.26IV. MAC PROTOCOL -1127IV. MAC PROTOCOL -12Property 1: If packet flows (with different TDs) requiring the same delay bound are FIFO multiplexed, the common delay bound can be met provided the output capacity of the FIFO mux is equal to that required by a GPS scheduler with the

31、 same input.28IV. MAC PROTOCOL -13The parameters used by the FSA are reported in(based on flows QoS requirements and TDs.)29IV. MAC PROTOCOL -14For the GB class (Step 1)The overall capacity to be shared is S* = S - minSBE,j Rqj,BESBE : BW always left for BE trafficj Rqj,BE : sum of all BE requests F

32、SA steps for GB(1) Assign to each RT what is guaranteed : minRqj,GB, Wj,GBRqj,GB : GB request of the jth RTWj,GB : GB weight, given by AC to ensure j Rqj,GB S SBE ( priority)(2) If there are some requests still pending (i.e. requesting more than its guaranteed share), then redistribute residual band

33、width to these RTs, according to their respective weights.For the BE class (Step 2)The overall capacity to be shared is S* = S -j Aj,GBAj,GB : BW assigned to the jth RT for the GB traffic(i.e. fairness for BE)(i.e. left from GB)30IV. MAC PROTOCOL -15ThecompleteFSAalgorithm31IV. MAC PROTOCOL -16The c

34、omplete FSA algorithm Step 1.1assigns up to the floor of the weight to each RT.Step 1.2evaluates whether to assign one TC-pair for the fractional part of the weight . Step 1.3evaluates whether to assign one more TC-pair on account of f_Wj to let small occasional bursts be transmitted even ifStep 2di

35、stributes residual bandwidth to RTs that have still some pending requests, according to a fair sharing (by a random round-robin).Step 3updates the algorithm variables.32V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUDA case study resulting from the application of the FWA system within an IntServ enabled IP

36、 network.We assumeThe existence of an NSLto provide (different profiles of) QoS,to identify packet flows, to possibly attribute them a “weight, expressing the amount of guaranteed capacityexpressing some priority criteriaThe FWA AL and MAC only make use of these general (and minimal) capabilities.33

37、V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -2A. Motivation for the IntServ Case Study in the FWAWork on QoS-enabled IP networks has led to two distinct approaches:the integrated services (IntServ)architecturethe differentiated services (DiffServ) architecture34V. THE FWA SYSTEM AS AN ACCESS RSVP CLOU

38、D -3IntServenables hosts to request per-flow, quantifiable resources, along end-to-end data pathsenables hosts to obtain feedback regarding admissibility of these requests (by using the RSVP resource reservation protocol).lack of scalability (since complexity grows as the number of multiplexed flows

39、)DiffServtargeting per class aggregate flowsno RSVPenables scalability across large networks,35V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -4In this context, an architectural solution is to support the DiffServ paradigm in the core networkwhile a set of edge devices allow the interworking with IntServ

40、 hosts in the access section of the network.QoS is provided by applying the IntServ model end-to-end across a network containing one or more DiffServ domains.36V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -5Creating such an architectural framework requires several parts:(i) an explicit setup mechanism

41、to request resources in accordance to the IntServ paradigm;(ii) a per flow traffic control at the edge of the network;(iii) the configuration of internal nodes (nodes of the DiffServ domains) so that aggregate flows have a well-defined minimum serving rate;(iv) the conditioning of aggregate flows (v

42、ia policing and shaping) so that their arrival rates at any internal node are always less than the allocated capacity at that node.(i) & (ii) : IntServ(iii) : explicit forwarding per hop behavior(iv) : the network boundary traffic conditioners 37V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -6B. IntServ

43、 Support in the FWA System: Admission ControlAssuming that the GB traffic is regulated by means of Dual Leaky Buckets (DLBs)A packet flow can be characterized by only four parameters:the peak rate (P bit/s)the token bucket rate (r bit/s)the bucket depth (b bit)the maximum datagram size (M bit) (wher

44、e Pr and Mb)The amount of information that can be offered by a flow in a time interval of duration, t, is limited by X(t) minPt+M,rt+b38V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -7The maximum delay in the MAC layer to access the TC-MatrixThe bandwidth negociated by the ith flow39V. THE FWA SYSTEM AS

45、 AN ACCESS RSVP CLOUD -8The weight for the jth RTThe admission verifies thatThe required bandwidth in TC-pairs/frame for the ith flow40V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -8The bandwidth Ri to assign for the ith flow41V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -9C. IntServ Support in the FWA Sy

46、stem: Signaling42VI. PERFORMANCE ANALYSISThe simulated FWA comprisesfour RTs and a single RN.Three types of traffic sources:(1) measured MPEG coded traces, used to model real time multimedia GB traffic; (2) measured LAN IP packet traces, used to model the BE traffic;(3) artificial sources with ad ho

47、c synthesized emission pro-files (e.g., CBR and ONOFF).43VI. PERFORMANCE ANALYSIS -244VI. PERFORMANCE ANALYSIS -345VI. PERFORMANCE ANALYSIS -446VI. PERFORMANCE ANALYSIS -547VI. PERFORMANCE ANALYSIS -648VI. PERFORMANCE ANALYSIS -7A. Numerical Results and DiscussionThe actual maximum delay incurred by

48、 GB packets is sensitively less than the target value (32 ms).Delay fairness is achieved, as it is shown by the almost equal values of the delays of different RTs.49VI. PERFORMANCE ANALYSIS -750VI. PERFORMANCE ANALYSIS -851VI. PERFORMANCE ANALYSIS -952VI. PERFORMANCE ANALYSIS -1053VI. PERFORMANCE ANALYSIS -11A. Numerical Results and DiscussionFigs. 1013 represent the probability distribution and the mass functions measured in the case of Simulation 4.GB : Fig. 10 (RT1) & Fig. 11 (RT2)BE : Fig. 12 (RT1) & Fig. 13 (RT2)Note: Pr