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

1
IP QoS Delivery in a Broadband Wireless Local
Loop MAC Protocol Definition and Performance
Evaluation

• Baiocchi, Cuomo, and Bolognesi
• IEEE 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 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 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 (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 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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II. SYSTEM ARCHITECTURE -4

• 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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II. SYSTEM ARCHITECTURE -5
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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 technique
• 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 frequency offsets.

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III. PHYSICAL LAYER -2

• The OFDM modulation can support multiple access
  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 belonging to K different users (K 512
  commonly)
• The contemporary transmission is obtained by
  multiplying each user data symbol by an
  orthogonal spreading code
• OFDMA
• Allows an intermediate type of multiplexing by
  permitting each 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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III. PHYSICAL LAYER -4
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III. PHYSICAL LAYER -5

• 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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III. PHYSICAL LAYER -6
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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 N3

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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 of
• the request channel (ReqCh)
• an UL ( Uplink Logical) channel to make capacity
  requests
• 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 structure 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
RN to RT
(2) DL Allocation Channel
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
  competing 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 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) 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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IV. MAC PROTOCOL -11
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IV. MAC PROTOCOL -12

• Property 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 same input.

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IV. MAC PROTOCOL -13

• The parameters used by the FSA are reported in

(based on flows QoS requirements and TDs.)
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IV. MAC PROTOCOL -14

• For the GB class (Step 1)
• The overall capacity to be shared is S S -
  minSBE,Sj Rqj,BE
• SBE BW always left for BE traffic
• Sj Rqj,BE sum of all BE requests
• FSA steps for GB
• (1) Assign to each RT what is guaranteed
  minRqj,GB, Wj,GB
• Rqj,GB GB request of the jth RT
• Wj,GB GB weight, given by AC to ensure Sj
  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 -Sj
  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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IV. MAC PROTOCOL -15

• ThecompleteFSAalgorithm

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IV. MAC PROTOCOL -16

• 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 pending 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 two
  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
  resources, 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 to
  support
• the DiffServ paradigm in the core network
• while a set of edge devices allow the
  interworking 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 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 (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) ?minPtM,rtb

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V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -7
The maximum delay in the MAC layer to access the
TC-Matrix
The bandwidth negociated by the ith flow
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V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -8
The required bandwidth in TC-pairs/frame for the
ith flow
The admission verifies that
The weight for the jth RT
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V. THE FWA SYSTEM AS AN ACCESS RSVP CLOUD -8
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 Signaling

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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 ONOFF).

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VI. PERFORMANCE ANALYSIS -2
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VI. PERFORMANCE ANALYSIS -3
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VI. PERFORMANCE ANALYSIS -4
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VI. PERFORMANCE ANALYSIS -5
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VI. PERFORMANCE ANALYSIS -6
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VI. PERFORMANCE ANALYSIS -7

• A. Numerical Results and Discussion
• The actual maximum delay incurred by 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.

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VI. PERFORMANCE ANALYSIS -7
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VI. PERFORMANCE ANALYSIS -8
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VI. PERFORMANCE ANALYSIS -9
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VI. PERFORMANCE ANALYSIS -10
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VI. PERFORMANCE ANALYSIS -11

• A. Numerical Results and Discussion
• Figs. 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 Probability distribution is magnified 5
  times

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VI. PERFORMANCE ANALYSIS -12
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VI. PERFORMANCE ANALYSIS -13
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VI. PERFORMANCE ANALYSIS -14
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VI. PERFORMANCE ANALYSIS -15
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VI. PERFORMANCE ANALYSIS -16
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VI. PERFORMANCE ANALYSIS -17

• 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
BE Rq
? Four subsequent frames of requests-assignments
? Frame i Overall GB requests gt total
capacity ? Frame i1 ? Frame i2 ? Frame i3
GB Ex
GB Rq
Assigned
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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.