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

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

Applications of MANETs

• Collaborative computing

• Communications within buildings, organizations, ad hoc conferences

• Communications in battlefields and disaster recovery areas

• Sensor networks

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.

QoS in MANETs

• 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

Issues and Difficulties

• Unpredictable link properties

• Node mobility

• Limited battery life

• Hidden terminal problem

• Exposed terminal problem

• Route maintenance

• Security

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

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

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

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

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

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

MACA/PR

• 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

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

CEDAR

• 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

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

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

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

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

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

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

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

INSIGNIA

• 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”

INSIGNIA Framework

iMAQ

• 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