quality of service in wireless medium access...
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DSP Solutions R & D Center1
Quality of Service in Wireless Medium Access Protocols
Shantanu Kangude
DSP Solutions R & D center
Texas Instruments Inc
DSP Solutions R & D Center2
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
DSP Solutions R & D Center3
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
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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
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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
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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]
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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)
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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
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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.
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Thank YouQuestions are most welcome