QoS in Mobile Ad Hoc Networks

1. QoS in Mobile Ad Hoc Networks

2. 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

3. Applications of MANETs • Collaborative computing • Communications within buildings, organizations, ad hoc conferences • Communications in battlefields and disaster recovery areas • Sensor networks

4. 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.

5. 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

6. Issues and Difficulties • Unpredictable link properties • Node mobility • Limited battery life • Hidden terminal problem • Exposed terminal problem • Route maintenance • Security

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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”

24. INSIGNIA Framework

25. 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