Ch 3. IEEE 802.11-based WMNs

Ch 3. IEEE 802.11-based WMNs

Myungchul Kim

[email protected]

– 802.11

• The infrasturcture mode

• The ad hoc mode: a single-hop ad hoc networks

• The wireless distribution system (WDS) mode: each AP as a base station but also a wireless relay node

Introduction

• Limited capacity– Protocol overhead

• 802.11a/g: 54 Mbps

• The max link-layer data rate falls quickly with increasing distance between the transmitter and the receiver.

– Interflow and intraflow interference

• A single-channel multihop ad hoc network

Performance issues and their causes

– Ineffective route selection

• e.g., hop-count metric?

– TCP’s control overhead• One-hop IEEE 802.11a


– Ineffective congestion control

• One-hop IEEE 802.11a

• Channel bit errors


• Flow unfairness– Hidden terminal problem

• 4-node IEEE 802.11a testbed



– Channel sharing problem

• 4-node IEEE 802.11a testbed



– RTT-dependent unfairness

– Bad fish problem

• Interfering links at 1 and 11 Mbps -> the effective throughput of 11Mbps link becomes limited by that of 1 Mbps link


• Link quality-aware routing

• Interference-aware routing– Channel load along the path

– Load based ad hoc routing (LBAR)

High-performance routing

• Interference-aware routing(con’t)– Dynamic load-aware routing (DLAR)

• Use the number of packets queued in a node’s interface queue to estimate its load

– Hyacinth: approximate the channel load observed by a link by explicitly summing the traffic load imposed on that channel by all its interfering links

– Route oscillations


• Multipath routing– Fault tolerence and traffic distribution

– Multiple node-disjoint paths at the receiver for route request

– AOMDV: multipath variant of AODV

– The use of meshed or nondisjoint paths to increase path reliability

– It may not be possible to find multiple paths that do not interfere with each other if the node are using omnidirectional antennas.


• Diversity-aware routing– Multichannel WMNs

– Intraflow interference

– WCETT can select channel-diverse routes

– Disadv of WCETT: no interflow interference and nonisotonic

– MIC: interflow interference and self-interference

– Disadv of MIC: intraflow interference


• Opportunistic routing– When an intermediate receiver fails to receive a packet, the

packet has to be retransmitted by the immediate transmitter even if another neighbor of the receiver successfully receive the packet.

– Delayed forwarding decisions choosing the best receiver


– Channel-switching vs multiple interfaces

– Channel-switching in some predetermined order -> modify the 802.11 MAC

– Assignment of channels to radio interfaces

• Topology-based channel assignment– Constrained graph coloring problem

– Connected low interference channel assignment (CLICA)

• More constrained nodes are visited first

– Das: integer liner program (channels, interfaces, interference)

– Assumption: all network links are equally loaded

Multichannel WMNs

• Traffic-aware channel assignment– As the link load is determined by routing algorithms, in an ideal

solution the channel assignment is also performed in conjunction with the routing.

– Even with complete information about network topology and traffic matrix, the channel assignment problem is NP-hard.

• Dynamic channel assignment– Distributed schems for channel assignment?

Multichannel WMNs

• Dynamic channel assignment(con’t)

Multichannel WMNs

• Inter-channel interference– Two cards placed on the same machine?

– Degradation due to interchannel interference when channel 1 and 6 or channel 1 and 11

Multichannel WMNs

– IEEE 802.11 MAC layer’s unfairness

– A fairness model suitable for WMNs

• Reference model for fairness– Objectives

• The granularity of fairness is an ingree-aggregated flow independent of the number of TCP microflows or mobile devices -> per-customer service granularity

• The spatial resue of channel must be maximized

• Spatial bias must be eliminated

• Channel time rather than throughput should be considered

Flow fairness

• Implicit rate-based congestion control– WTCP measures the ratio of inter-packet spacing on the receiver and

that on the sender, to determine whether to increase or decrease the sending rate

– Not hold in general on 802.11 based WMNs due to bursty traffic

• Explicit rate-based congestion control– In Ad hoc transport protocol (ATP), every intermediate node measures

queuing and transmission delay for each packet passing through it.– ATP sender fails to utilize the network optimally because ATP includes

packet queuing time into the service time measurements that makes it unable to maintain a steady flow rate.

– EXACT routers measure the available bandwidth as the inverse of per-packet MAC contention and transmission time.

– EXACT decouples queuing time from the service time measurements and thus achieves much better network utilization.

Flow fairness

• Ingress flow throttling– Addresses the hidden terminal and channel sharing problem

– Throttle flows at the ingress nodes to the network-wide fair shares

– The offered loads and link capacities are periodically communicated to other WMN routers.

– Run at layer 2 of each router

• Neighborhood RED– Random early detection (RED) to multihop wireless networks

– View the queues on a node and its interfering neighbors as a single distributed queue and apply RED to this distributed queue

– Infer the neighborhood queue size by monitoring the channel utilization

Flow fairness

• Overlay MAC layer approach– On top of 802.11 MAC

– Distributed time division multiple access -> weighted fair queuing

– Disadv: overhead

Flow fairness

• Quality of service– Not sufficient to just differentiate and prioritize traffic at per-

router level?

– Distributed priority scheduling -> 802.11 priority back-off scheme

• Topology planning– The position of regular nodes and the placement of the gateway

nodes

– Clusters, cluster head

Other issues

• Advanced topology discovery– Network management: visualize the location and

interconnectivity of nodes as well as the utilization of network resources at each location

– Research questions

• How to utilize multiple radios?

• How to determine the accurate radio topology of a network considering interferences?

• How to produce a comprehensive RF usage map of the network?

– Localization of network nodes?

– The problem of localization in a multihop setting?

– Finding interference relations between different links?

Other issues

• Long-distance WMNs– Digital Gangetic Plains project: 12 rural villages in India with

directional antennas

– Simultaneous operation of multiple directional antennas -> CSMA MAC considers as interference

– TDMA MAC and a synchronization protocol are proposed

Other issues

• Max-min flow allocation– Max-min fairness on top of unfair 802.11 MAC?

• Interference-aware multipath routing– Multipath routing: Fault tolerance and end-to-end throughput

– Absence of interference?

• Directional antenna-based mesh networks• Secure routing protocols

– Attachs from compromized mesh nodes?

• Fault diagnosis– How to distinguish whether the poor performance is because of

bad channel conditions, interference, software bugs, faulty hardware or compromised nodes.

Open issues

Documents

Ch 3. IEEE 802.11-based WMNs