quality of service in wireless medium access...

DSP Solutions R & D Center1

Quality of Service in Wireless Medium Access Protocols

Shantanu Kangude

DSP Solutions R & D center

Texas Instruments Inc


Agenda

• Motivation and Definitions

• QoS Aware MAC Protocol Design– Classical MAC mechanisms

– QoS modifications to classical mechanisms

• QoS in Evolving Wireless Technologies– Environment and applicability of mechanisms

– QoS MAC architecture examples

• Summary and Future Directions


Shared Resources in Networks

Network resources are shared among users

– Economicse.g. routers, cable TV

infrastructure

– Inherent naturee.g. wireless medium

Service objectives from “sharing”– Efficiency:

Do not waste while sharing

– Fairness: Do not violate anybody’s right to

the serviceImage Source: http://img.lightreading.com/2003/02/28307_2.gif


Evolution of Fairness Expectations

• Classical– Statistically “equal access” to users with “equal requirements”– Equality over large scales of time

• Current (Comprehensive fairness)– Service as specified/promised

Known pattern of service sharing among different users– Differentiated services to different classes of users– Service guarantees over smaller scales of time

Service expectations currently: Quality of Service (QoS)– Efficiency (as earlier)– Comprehensive fairness


The Importance of QoS

• Different types of traffic have different service requirements– Interactive multimedia (Voice over IP, Video Conferencing)

Strict delay bound requirements

– Streaming multimediaModerate delay bound requirements

– File transfersNo major service constraints

• If all traffic treated equally (classical way)– Non urgent traffic may get service at the expense of urgent traffic

– Strict requirements may not be satisfied


Quality of Service (QoS)

Elements of network wide QoS [1]

• Per hop QoS (QoS capable network “elements”)

Keywords: Classification, Queuing, Scheduling, Priorities, Shaping

• Traffic Engineering/Network wide resource sharing

Keywords: MPLS, DiffServ, IntServ

• Signaling and Provisioning (management)Keywords: RSVP, policies, admission

control

QoS: Capability of a network/network element to guarantee• Differentiated performance to different types of service users• Better performance to certain service classes on bandwidth, delay, and jitter compared to classical performance

Images Source: [1]


Wireless Networks and QoS

• Per hop QoS– Classification and Queuing (identical to

core networks)– Scheduling the access to the shared

wireless medium • Across traffic flows within a node• Across flows all over the shared network

• Network wide mechanisms– Load balancing, routing etc. (applicable to

ad-hoc networks)

• Signaling/Provisioning/Management– Most mechanisms similar to core

networks– Admission control and other

radio resource management (Wireless specific) Image sources: http://www.netustech.com/kor/products/images/6254bs_img.gif

http://www.trl.ibm.com/projects/mmqos/images/framework.jpg


Resource Management for Wireless QoS

Unreliable Wireless Medium: QoS more important• Radio resource management is critical• Keeping reserves of resources is important• Aiming for significantly better performance than limits is

required

QoS issues in wireless networks:

• Network Issues: Independent of the air interface; Solutions may be re-used; E.g. admission control, routing in ad-hoc networks, resource management

• Air interface issues: QoS aware wireless medium access control (MAC) protocols


QoS in Wireless Medium Access

Elements of per-hop QoS are applicable to wireless medium access

Medium Access Control (MAC) ProtocolRules by which nodes and flows within them access a shared medium E.g. Ethernet CSMA/CD

QoS aware MAC protocols: MAC protocols that are• Fair:

• Differentiated service to different traffic classes as per policy/fairness rules• Delay and bandwidth guarantees of admitted flows fulfilled

• Efficient:• Medium is used to the maximum

Image source: Georgia Tech Broadband Wireless Lab


Agenda


• QoS Aware MAC Protocol Design– Classical MAC mechanisms– QoS modifications to classical mechanisms





Ideal MAC: Processor Sharing

Ideally schedule medium access based on• Policies/fairness rules:

Using scheduling algorithms such as weighted fair, strict priority etc.• Requirements of nodes/flows:

Schedule backlogged flows

MAC challenge: Use the medium ‘also’ for communicating/interpreting• User requirements• Scheduling results

MACScheduling

Each node has a queue for its packets

Broadcast Medium

Scheduling Fair (equitable) scheduling across

nodes’ queues


Classification of MAC Approaches

Channel Access Periods• Full reservation• Full contention (rare; e.g. Aloha)• Contention and reservation (most prevalent)

Access Control• Centrally co-ordinated

Access rights are granted by a central controller

• DistributedAir interface “incidents” lead to access right interpretation

• Hybrid: Combination of the above


Dedicated Sub-channel: Centralized Fully Reserved MAC

• Low capacity dedicated sub-channels from each node to the central controller

• Nodes request access to the main channel• Controller grants access to the main channel• Nodes use their grants for transmitting

– Data– More bandwidth requests

Dedicated request/grant sub-channels

Data sub-channels shared through grants


Polling: Centralized Fully Reserved MAC

Central controller requests nodes for data in order

Figure source: Yen-Cheng Kuan, Chih-Chieh Han, {yckuan, simonhan}@ee.ucla.edu

STA2

C F - A c k ( 4 ) + C F - P o l l ( 0 )

F r am e( 0 )

CF-Ack(1) + CF-Poll (2)

Frame(2)

C F - A c k ( 3 ) + C F - P o l l ( 4 )

F r am e( 4 )

CF-Ack(0

) + C

F-Poll

(1)

Frame(1)

CF-A ck(2) + CF-Poll (3)

Frame(3)

STA0

STA1 STA3

STA4AP


Distributed Contention Based MAC

Aloha: Transmit randomly wheneverSlotted Aloha: Transmit randomly, but time aligned (in slots)

Basic contention approach: Randomly choose a slot/slots (transmission opportunities) from multiple slots

Rule for slot selection:Contention resolution (CR) mechanism

CR mechanisms define various contention approaches


Contention Resolution (CR)

• Back-off– Defer transmissions/re-transmissions for later– Choose a number of transmission opportunities to defer

• CR mechanisms Back-off rules

• Most popular: Binary Exponential Back-off– Back-off for a random number from “N” transmission

opportunities (slots)– Double N on every collision– Fall back to default N on successful transmission

TX


Motivation for Hybrid Approaches

Exclusive reservation based MACEfficient: if traffic from nodes deterministicWasteful: in checking non-backlogged nodes

Exclusive contention based MACEfficient: when small number of backlogged nodesWasteful: in collisions

Hybrid approach:– Contention: for requesting change in bandwidth allocation– Reservation: once appropriate bandwidth allocated


Example Hybrid Approach: Contention based Requests

• Reservation requests– Once: for deterministic portion of traffic– Dynamic: for changing bandwidth requirements

• Poll for requests: Contention based• Poll for data: Reservation based


Other Hybrid Approaches

Contention until channel capture

• Finite period to know if collision or not

• No collision implies reservation for a periodE.g. CSMA/CA: Reserved for a specified period if a ‘first’ transmission successful

Multiple approaches as presented may be dynamically combined and used in MAC protocols


Agenda








QoS MAC Objectives

• Classical MAC design: Efficiency centered

• QoS MAC: Comprehensive Fairness along with efficiency

– Various classes of traffic flows with individual requirementsE.g. VOIP, Interactive Multimedia, Streaming video, and data transfers

– Equitable service across nodes within each specific class

– Better than classical service to higher classes

– Better Service More deterministic access Guarantees over smaller scales of time


QoS Influence on Basic Mechanisms

Objectives and Mechanisms

• Differentiated access– Reservation based access:

Reserve access according to scheduling algorithms/policies etc.e.g. weighted fair queuing, strict priority queuing

– Contention based access:Differentiated contention resolution algorithms

• More deterministic access– Reservation based access:

• Available capacity in “reserve”• Short duration reservations (aid in quick dynamic change)

– Contention based access: (difficult to guarantee deterministic access)• Shorter access durations• Strong bias in CR mechanisms for preferential resolution


Differentiated Contention Resolution

• Choose random back-off numbers from different ranges• Higher classes back-off less (shorter ranges)

E.g. Enhanced Distributed Co-ordination Function (EDCF) in 802.11 wireless local networks

Image source: Cisco Website

CWmin = Contention window minimum = Back-off range

Number of slots to defer is randomly chosen from Backoff range


Overall QoS Aware MAC Design

• Hybrid contention and reservation• Reduce contention to flow

admission/changes/closure requests• Biased contention resolution• Polling/Reservation based on requirements and

fairness policies• Polls for small quantum of data at a time• Frequent re-assessment of access allocations


Agenda




• QoS in Evolving Wireless Technologies– Environment and applicability of mechanisms– QoS MAC architecture examples



Network Environment

Network environment influences MAC choices

• Shortest feedback time and data rate– Also interpreted as propagation delay in relation to transmission

delay

– Determines the potential capacity loss in the uncertain period

• Traffic profile– Access requirements of various flows

– Frequency of change of flow requirements


Roundtrip Time and Data Rate

• Data rate * Roundtrip time (RTT): Large E.g. Wireless Metropolitan Area Networks, Cellular networks

– Large amount of data uncertain in contention if the MAC is not synchronized

– Synchronized centrally controlled MAC– Hybrid contention/reservation with

• Synchronized slots for contention• As little contention as efficient

• Data rate * Roundtrip time (RTT): SmallE.g. Wireless Local Area Networks

– Small synchronization mismatch amongst nodes– Carrier sensing and distributed MAC possible– Most flexible MAC options– Hybrid contention/reservation recommended

More contention may be efficient


Traffic Profile

• Deterministic traffic– Deterministic => access instants may be pre-known– Bandwidth change requests are seldom– Reservation based MAC very conducive

• Bursty traffic– Frequent changes in bandwidth requirements– Frequent changes in reservation required– Recommended scenario

• Reservations with a baseline rate• Piggybacked requests for changes in allocations• Contention based

– Requests for changes in allocations– Data


Agenda








QoS MAC: Example Architectures (1)

Data + Multimedia traffic (reasonably bursty)

1-100 MbpsSmall (0-100m)EnvironmentTraffic ProfileData RatesRange/RTT

• Environment provides maximum flexibility in design

• Carrier sensing MAC with

- Polling (HCF)

- Differentiated contention resolution (EDCF)

Solution

Wireless Local Area Network (IEEE 802.11 E)

Data + Multimedia traffic (reasonably bursty)

1-10s of MbpsIntermediate (0-4000m)EnvironmentTraffic ProfileData RatesRange/RTT

• Synchronized centrally controlled MAC with “on demand” access• Polling/Grants for Data as well as bandwidth requests• Contention for small “request” packets in the uplink in specified times

while admitting flows or for non real time data transfers

Solution

Wireless Metropolitan Area Network (IEEE 802.16)


QoS MAC: Example Architectures (2)

Voice and data traffic10 Kbps – 2 MbpsIntermediate (0-4000m)EnvironmentTraffic ProfileData RatesRange/RTT

• Synchronized centrally controlled MAC with “on demand” access• Signaling for change in access allocations

Dedicated control sub-channelsShared paging sub-channels

• Data sub-channelsDedicated: for baseline rate of trafficShared: for the variable portion of the traffic

Solution

Cellular Networks

Aggregated traffic with low variability

10s of Kbps -10s of Mbps

Large (1000s of Kms)EnvironmentTraffic ProfileData RatesRange/RTT

• Centrally controlled synchronized MAC• Dedicated low capacity control channels between each node and controller• Advance flow setup and full reservation• Limited forms of contention based access such as Reservation-Aloha

Solution

Satellite Networks


Agenda








Future Directions• Advanced wireless features changing MAC paradigms

– Adaptive Antenna Systems (AAS)– Multiple Input Multiple Output (MIMO) Systems

• Additional sub-channelizationMulti-dimensional scheduling: codes, space, and time, and carrier

• While scheduling mechanisms are mature, more efficiency is an important goal

– Reduce signaling overhead for changes in allocationsE.g. Use implicit reservations

– Reduce signaling overhead for notification of change in user’s requirements

E.g. Exploit determinism in traffic (predict)


Summary

• QoS in wireless air-interfaces through MAC protocols

• Classical MAC: Various contention based and reservation based MAC mechanisms

• QoS MAC: Differentiated service with tighter guarantees for higher classes

• Network environment and traffic profile determine which mechanisms are efficient

• Some example technologies and future directions were considered


References

[1] Grenville Armitage, Quality of Service in IP Networks, Macmillan Technical Publishers, April 2000.

[2] IEEE 802.11 E draft 8

[3] IEEE 802.16-2004

[4] H. Peyravi, “Medium access control protocols performance in satellite communications,” IEEE Commun. Magazine, vol. 37, no. 3, pp. 62-71, Mar. 1999.


Thank YouQuestions are most welcome

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