1 ip qos delivery in a broadband wireless local loop: mac protocol definition and performance...

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IP QoS Delivery in a Broadband Wireless Local Loop: MAC Protocol Definition and Performance Evaluation

Baiocchi, Cuomo, and BolognesiIEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 18, NO. 9, SEPTEMBER 2000

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Abstract In this paper, a complete broadband wireless local

loop (WLL) network is presented. The proposal is based on the OFDM-CDMA techn

ique, 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 system is addressed.

We prove that our scheme achieves high utilization efficiency, as well as a fair share of the available radio capacity.

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I. INTRODUCTION

FWA (fixed wireless access) Architecture– A centralized radio node (RN)– A group of fixed radio terminals (RT)– a customer premises network/equipment (CPN/CPE)

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I. INTRODUCTION -2 FWA exploits the OFDM-CDMA [1] [2] (orthogo

nal 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 managed dynamically to provide tight tracking of traffic asymmetry, by sharing the available pool of codes between uplink and downlink (code division duplex).

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II. SYSTEM ARCHITECTURE

NSL = network service layer (e.g. IP) Three layers:

(i) Adaptation layer(AL); (ii) MAC layer; (iii) Physical layer.

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II. SYSTEM ARCHITECTURE -2 NSL

– corresponds to• classical network functions

– addressing, – routing

• traffic handling functions– packet flow description and classification,– admission control, – traffic policing and/or shaping

– Examples of NSL• IP layer enhanced with QoS handling capabilities

(e.g. IntServ or DiffServ)• ATM traffic control (e.g. CBR, VBR, ABR, UBR)

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II. SYSTEM ARCHITECTURE -3

AL– maps NSL traffic classes into MAC service

classes• Two service types in the MAC layer

– Guaranteed bandwidth (GB)

– Best effort (BE)

– AL flow mapping table• mapping NSL traffic classes into MAC service

classes

• Updated by NSL when a new flow is admitted

– Segmenting and reassembling (SAR)

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MAC layer– Capacity assignment

• Sharing 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 layer– Guaranteed bandwidth (GB)

– Best effort (BE)

Physical layer– Coding and transmitting/receiving signals

according to OFDM/CDMA.

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III. PHYSICAL LAYER A. Modulation Technique

– OFDM • a multi-carrier technique

– Advantages of OFDM• Immune to channel dispersion compared to a single carrier tec

hnique;

• equalizers require much less computational 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 OFDM• More sensitive to local oscillator phase noise and to carrier fre

quency offsets.

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III. PHYSICAL LAYER -2 The OFDM modulation can support multiple acces

s by means of– OFDM-TDMA

• Each symbol interval (SI) is used for the transmission of K data symbols of the same user on the K OFDM subcarriers

• Delay caused by collecting K data symbols from a user

– OFDM-CDMA• One SI can be used for the transmission of data symbols belon

ging to K different users; (K = 512 commonly)• The contemporary transmission is obtained by multiplying eac

h user data symbol by an orthogonal spreading code

– OFDMA • Allows an intermediate type of multiplexing by permitting eac

h user to transmit x data symbols on a set of subcarriers per SI .(1 ≦ x ≦ K)

– Or combination

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III. PHYSICAL LAYER -3

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Advantages of OFDM/CDMA and OFDMA– Low packetization delay– Flexibility in bandwidth assignment

As the granularity gets finer (i.e., x gets lower), the benefits of multicarrier transmission tend 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.

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III. PHYSICAL LAYER -7 B. The FWA Physical Layer

– This paper assumes an OFDM/CDMA with FDD (frequency division duplex) technique.

– This 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 flexibility in bandwidth assignment;

－ increases the phase noise sensitivity,

－ makes base-band processing (i.e., FFT) more complex.

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IV. MAC PROTOCOL A. MAC Service Classes

– Two service types in the MAC layer• Guaranteed bandwidth (GB)

– Used for services with stringent requirements for 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 flow

– Relevant admission control and flow parameter compliance checks must be defined in network layer

• Best effort (BE)– To provide for economic use and efficient use

• Capacity left from more demanding flows can be filled with traffic with loose requirements

– To accommodate the existing Internet application traffic

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IV. MAC PROTOCOL -2 B. MAC Signaling

– The radio capacity with OFDM-CDMA is structured as

• K orthogonal codes that can be used simultaneously.

• Each code is used in a TDMA fashion; a time slot carries a MAC_PDU (TSLOT = TMAC_PDU).

– [RT ID, Other Info., Data Load]

• Time 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=3

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IV. MAC PROTOCOL -3

– The 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

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IV. MAC PROTOCOL -4

– The basic MAC signaling consists of1. the request channel (ReqCh)

– an UL ( Uplink Logical) channel to make capacity requests

2. the allocation channel (AlCh)– a DL (Downlink Logical) channel

to answer the requests

– The ReqCh and AlCh is structured in minislots.

– A ReqCh-AlCh minislot pair is dedicated to each RT.

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IV. MAC PROTOCOL -5

– The ReqCh in UL is structured in minislots:• Each minislot contains the bandwidth request.

• [RT ID, Request GB Class, Request BE Class]

• The 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.

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IV. MAC PROTOCOL -6– The AlCh in DL has the same minislot structur

e 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-pairs]• The RN uses the AlCh to signal to each RT

– the number of assigned TC-pairs, – the starting code (the row of the TC-matrix)– the starting offset in the code row.

– Detailed format and dimensioning of ReqCh and AlCh are re-ported in [14].

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IV. MAC PROTOCOL –7

(1) UL Request Channel

(2) DL Allocation Channel

RN to RT

RT to RN

(N =3)

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IV. MAC PROTOCOL -8 C. MAC Fair Scheduling Algorithm (FSA)

– Each RT stores arriving MAC_PDUs into its buffers by separating GB and BE packets:

• BE traffic is queued up into a single FIFO buffer• GB traffic is split among a set of FIFO buffers

– GB 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 hierarchically among groups of users as to support decreasing QoS targets.

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IV. MAC PROTOCOL -9 The FSA is a practical realization of the fluid GPS

[15] – which shares a fixed resource (capacity) among compet

ing users, according to their actual load and to predefined weights.

– A. K. Parekh and R. G. Gallager, “A generalized processor sharing approach to flow control in integrated service networks—The single node case,” in Proc. IEEE Infocom’92, 1992, pp. 915–924.

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) tradeoff between bandwidth and complexity;

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IV. MAC PROTOCOL -10

The 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 competing 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 applied in each phase.

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Property 1: – If packet flows (with different TDs) requiring t

he 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 same input.

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The parameters used by the FSA are reported in

(based on flows QoS requirements and TDs.)

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For the GB class (Step 1)– The overall capacity to be shared is

S* = S - min{SBE,Σj Rqj,BE}• SBE : BW always left for BE traffic• Σj Rqj,BE : sum of all BE requests

– FSA steps for GB• (1) Assign to each RT what is guaranteed : min{Rqj,GB, Wj,GB}

– Rqj,GB : GB request of the jth RT– Wj,GB : GB weight, given by AC to ensure Σj Rqj,GB ≦ S – SBE

(~ priority)

• (2) If there are some requests still pending (i.e. requesting more than its guaranteed share), then redistribute residual bandwidth to these RTs, according to their respective weights.

For the BE class (Step 2)– The overall capacity to be shared is S* = S -Σj Aj,GB

• Aj,GB : BW assigned to the jth RT for the GB traffic

(i.e. fairness for BE)

(i.e. left from GB)

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ThecompleteFSAalgorithm

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The complete FSA algorithm – Step 1.1

• assigns up to the floor of the weight to each RT.

– Step 1.2• evaluates whether to assign one TC-pair for the fractional part

of the weight .

– Step 1.3• evaluates whether to assign one more TC-pair on account of f_

Wj to let small occasional bursts be transmitted even if

– Step 2• distributes residual bandwidth to RTs that have still some pend

ing requests, according to a fair sharing (by a random round-robin).

– Step 3• updates the algorithm variables.

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V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD A case study resulting from the application

of the FWA system within an IntServ enabled IP network.

We assume– The existence of an NSL

• to provide (different profiles of) QoS,• to identify packet flows, • to possibly attribute them a “weight,”

– expressing the amount of guaranteed capacity– expressing some priority criteria

– The FWA AL and MAC only make use of these general (and minimal) capabilities.

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V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -2 A. Motivation for the IntServ Case Study in

the FWA– Work on QoS-enabled IP networks has led to t

wo distinct approaches:• the integrated services (IntServ)architecture

• the differentiated services (DiffServ) architecture

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V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -3

– IntServ• enables hosts to request per-flow, quantifiable resou

rces, along end-to-end data paths

• enables 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)

– DiffServ• targeting per class aggregate flows

• no RSVP

• enables scalability across large networks,

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V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -4 In this context, an architectural solution is t

o support – the DiffServ paradigm in the core network– while a set of edge devices allow the interworki

ng with IntServ 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.

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V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -5 Creating such an architectural framework requires

several parts:(i) an explicit setup mechanism to request resources in acc

ordance 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 Diff

Serv domains) so that aggregate flows have a well-defined minimum serving rate;

(iv) the conditioning of aggregate flows (via 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

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V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -6 B. IntServ Support in the FWA System:

Admission Control– Assuming 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)

(where P≧r and M≦b)

– The amount of information that can be offered by a flow in a time interval of duration, t, is limited by X(t) min{≦ Pt+M,rt+b}

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The maximum delay in the MAC layer to access the TC-Matrix

The bandwidth negociated by the ith flow

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The weight for the jth RT

The admission verifies that

The required bandwidth in TC-pairs/frame for the ith flow

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The bandwidth Ri to assign for the ith flow

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V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -9 C. IntServ Support in the FWA System: Sign

aling

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VI. PERFORMANCE ANALYSIS

The 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 hoc synthesized emission pro-files (e.g., CBR and ON–OFF).

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VI. PERFORMANCE ANALYSIS -2

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A. Numerical Results and Discussion– The actual maximum delay incurred by GB pac

kets 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.

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A. Numerical Results and Discussion– Figs. 10–13 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: Probability distribution is magnified 5 times

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Six MPEG sources with different delay requirements are multiplexed in an RT.

(Max. BW penalty, min. target delay)BW penalty factor=overall GB assigned capacity / capacity assigned by GPS

20% less by halved queues

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VI. PERFORMANCE ANALYSIS -18 ◆ Four subsequent frames of requests-assignments

◆ Frame i: Overall GB requests > total capacity

◆ Frame i+1: ◆ Frame i+2: ◆ Frame i+3:

GB Rq Assigned

BE Rq

GB Ex

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VII. CONCLUSION The key contributions of this work are

– the definition of an overall WLL architecture

– a MAC protocol fully exploiting the OFDM-CDMA technique

– the application of these concepts to support the IntServ paradigm for QoS provisioning of in IP networks.

A dynamic bandwidth sharing capability is designed to handle aggregate traffic flows still guaranteeing the single QoS requirements by means of a tradeoff between scheduling complexity and efficiency.

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VII. CONCLUSION –2

Ongoing work is aimed at two goals:(1) integration of information protection mechanisms in

the MAC and adaptation layer (FEC/ARQ) and the impact of ARQ on bandwidth assignment;

(2) modification of the MAC_PDU format, to allow variable length data chunks, so as to significantly reduce padding overhead and find an easier match with the upper IP layer.

The possibility of modifying the multiple access from FDD to code division duplex, in order to accommodate asymmetric traffic load patterns more efficiently, will be also considered.

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