Qos in mobile ad hoc networks
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QoS in Mobile Ad Hoc Networks. Introduction. Mobile ad hoc networks (MANETs) are infrastructureless and intercommunicate using single-hop and multi-hop paths Nodes act both as hosts and routers Topology changes could occur randomly, rapidly, and frequently

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Introduction l.jpg

  • Mobile ad hoc networks (MANETs) are infrastructureless and intercommunicate using single-hop and multi-hop paths

  • Nodes act both as hosts and routers

  • Topology changes could occur randomly, rapidly, and frequently

  • Routing paths are created and deleted due to the nodal mobility

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Applications of MANETs

  • Collaborative computing

  • Communications within buildings, organizations, ad hoc conferences

  • Communications in battlefields and disaster recovery areas

  • Sensor networks

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Quality of Service (QoS)

  • QoS: A set of service requirements that are met by the network while transferring a packet stream from a source to a destination

  • QoS metrics could be defined in terms of one or a set of parameters

  • Examples: delay, bandwidth, packet loss, delay-jitter, etc.

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  • The use of QoS-aware applications are evolving in the wireless environments

  • Resource limitations and variations adds to the need for QoS provisioning

  • Use of MANETs in critical and delay sensitive applications demands service differentiation

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Issues and Difficulties

  • Unpredictable link properties

  • Node mobility

  • Limited battery life

  • Hidden terminal problem

  • Exposed terminal problem

  • Route maintenance

  • Security

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Compromising Principles

  • Soft QoS

    • After the connection set-up, there may exist transient periods of time when QoS specification is not honored

    • The level QoS satisfaction is quantified by the fraction of total disruption

  • QoS Adaptation

    • As available resources change, the network can readjust allocations within the reservation range (dynamic QoS)

    • Applications can also adapt to the re-allocations

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QoS Support in Physical Channels

  • Since wireless channel is time varying, the SNR in channels fluctuates with time

  • Adaptive modulation which can tune many possible parameters according to current channel state is necessary to derive better performance

  • Major challenge: channel estimation – accurate channel estimation at the receiver and then the reliable feedback to the transmitter

  • Wireless channel coding needs to address the problems introduced by channel or multipath fading and mobility

  • Cross-layer issue: Joint source-channel coding takes both source characteristics and channel conditions into account

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QoS Provisioning at the MAC Layer

  • For providing QoS guarantee for real-time traffic support in wireless networks, several MAC protocols based on centralized control have been proposed

  • For multihop networks:

    • The MAC protocol must be distributed in nature

    • It should solve the hidden and exposed terminal problems

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IEEE 802.11 DCF

  • IEEE 802.11 is a CSMA/CA protocol

  • In the distributed control function (DCF) mode:

    • After the node has sensed the medium to be idle for a time period longer than distributed inter-frame space (DIFS), it begins transmitting

    • Otherwise the node differs transmitting and backs off

    • When the medium becomes idle for a period longer than DIFS, the backoff timer is decremented periodically. The node starts transmission as soon as the timer expires

    • To reduce collisions, the sender and the receiver exchange RTS and CTS packets

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QoS Support using IEEE 802.11 DCF

  • IEEE 802.11 DCF is a best-effort type control algorithm

  • The duration of backoff is decided by a random number between 0 and the contention window (CW).

  • Service differentiation can be achieved by using different values of CW

  • When packets collide, the ones with smaller CW is more likely to occupy the medium earlier

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Black Burst Contention Scheme

  • Nodes with best-effort traffic and nodes with real-time traffic use different inter-frame space values

  • When the medium remains idle long enough, right before sending their packets, nodes with real-time packets first contend for transmission right by jamming the media with pulses of energy, called BBs

  • Each contending node uses a BB of different length. The number of slots that forms a BB is an increasing function of the contention delay experienced by the node

  • Following each BB transmission, a node senses the channel for an observation interval

  • Since distinct nodes contend for BBs of different length, each node can determine without ambiguity whether its BB is of longest duration

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  • Multihop Access Collision Avoidance with Piggyback Reservation provides guaranteed bandwidth support for real-time traffic

  • The first packet in a real-time stream uses RTS/CTS dialogs to make reservations in the path

  • The sender schedules the next transmission after the current data transmission and piggybacks the reservation in the current data packet

  • Upon receiving the data packet correctly, the receiver updates its reservation table and sends an ACK

  • ACK serves for the renewal of reservation, not for recovering from packet losses

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QoS-aware Routing at the Network Layer

  • Types of MANET routing protocols:

    • Proactive, table-based routing schemes

    • Reactive, on-demand routing schemes

    • Constraint-based routing schemes

  • These algorithms are based on the discovery of shortest paths

  • QoS-aware routing protocol should find a path that satisfies the QoS requirements in the path from source to the destination

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  • Core Extraction Distributed Ad hoc Routing scheme dynamically establishes the core of the network, and then incrementally propagates the link states of stable high-bandwidth links to the core nodes

  • The route computation is on demand basis

  • Components of CEDAR

    • Core extraction

    • Link-state propagation

    • Route computation

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Integrating QoS in Flooding-Based Route Discovery

  • Ticket-based probing algorithm

    • During the QoS-satisfying path search, each probing message is provided a limited number of tickets to reduce the scope of flooding

    • When one or more probes arrive at the destination, the path and delay/bandwidth information is used to perform reservation for the QoS-satisfying path

    • A simple imprecise model is used for the algorithm

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PANDA Approach

  • Positional Attributes based Next hop Determination Approach (PANDA) discriminates the next hop based on the desired QoS metric

  • Instead of using a random rebroadcast delay, the receiver opts for a delay proportional to its ability in meeting the QoS demands

  • The decisions at the receivers are made based on a predetermined set of thresholds

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QoS Support using Bandwidth Calculations

  • The end-to-end bandwidth can be calculated and allocated during the admission control phase

  • Using TDMA, time is divided into slots, which in turn are grouped into frames

  • Each frame contains two phases: control and data.

  • During the control phase, each node takes turns to broadcast its information to all the neighbors in a predetermined slot.

  • At the end of control phase, each node knows about the free slots between itself and its neighbors

  • Thus bandwidth calculation and allocation can be done in a distributed manner

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Multi-path QoS Routing

  • The algorithms searches for multiple paths between the source and the destination that collectively satisfies the QoS requirements

  • Suitable for ad hoc networks with limited bandwidth

  • A ticket based probing scheme is adopted for the path searching process

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Transport Layer Issues for QoS Provisioning

  • TCP performs poorly in terms of end-to-end throughput in MANETs

    • The assumption used in Internet that packet losses are due to congestion is not valid in MANET environments

  • TCP performance improvement in wireless networks:

    • Local retransmissions

    • Split-TCP connections

    • Forward error corrections (FEC)

  • Explicit feedback mechanisms to distinguish between losses due to errors and congestion is necessary for QoS provisioning in MANETs

  • Efficient techniques for resource management is necessary for QoS provisioning

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Application Layer Issues

  • Application level QoS adaptation belong to adaptive strategies that play a vital role in supporting QoS

    • Flexible user interfaces, dynamic QoS ranges, adaptive compression algorithms, joint source-channel coding, joint source-network coding schemes

  • Adaptive real-time audio/video streaming support can be provided by enhancing:

    • Compression algorithms, layered encoding, rate shaping, adaptive error control, and bandwidth smoothing

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Inter-Layer Design Approaches

  • Efficient intercommunication protocols need to conserve scarce resources – something difficult to achieve following the strict separation of the protocol layer functionalities

  • Inter-layer or cross-layer issues needs to be examined

  • Examples: INSIGNIA and iMAQ

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  • Goal: To support adaptive services which can provide base QoS assurances to real-time voice and video flows and data, allowing for enhanced level of service to be delivered when resources become available

  • Designed to adapt user sessions to the available level of service without explicit signaling between source-destination pairs

  • QoS functionality is decoupled from the routing protocol

  • INSIGNIA uses in-band signaling approach to restore the flow-state in response to topology changes

  • Uses the concept of “soft connection”

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  • Integrated Mobile Ad Hoc QoS (iMAQ) is a cross-layer architecture to support the transmission of multimedia data over a MANET

  • iMAQ framework model

  • Uses a predictive location-based QoS routing protocol

  • The middleware predicts the location and the group partitioning based on the moving pattern

  • Middleware may renegotiate QoS with applications when the resource availability degrades

  • Data is replicated when a network partition is predicted