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Chapter 16Wireless Mesh networks
Overview
IntroductionMesh network definedBenefits of wireless meshCurrent issues and solutionsMesh deployment issuesConclusion
Introduction
Used by Municipalities to enterprises Telecom providers Public safety Military
Popular do to Ease of deployment Increase in network capacity mobility
Mesh Network Defined
A set of fully interconnected network nodes that support traffic flows between any two nodes over one or more paths or routes
Wireless provides connectivity while in motion
Biggest example is the internet
Benefits of wireless mesh
Deployment Each node maintains multiple connectivity Allows for multiple paths
Cost Wired networks are more expensive Many nodes use the same access point
Continuity of services in case of device failure Users expect this because of the fee they pay
Scalability Easy and cost effective to add more capacity
Examples of Deployment
Small in home mesh networks T.v., audio systems
Public safety Police, fire, emergency
responseswiMAX or 3G access
links
Current issues and solutions
Netowork structureIntra-mesh channel re-useMedium access contentionMesh routing and forwardingMesh securityCongestion controlVoice over meshMesh network management
Network structure
Plays a big role in performance of voice services Determines delay encountered by voice packets
Parking lot effect Is a function of the number of hops in a mesh and the
peering ratio As seen the hop depth H is 3 and the peering ratio Rp is 2 The load on the portal link is equal to 7 peer links
Solutions for the parking lot effect include proper dimensioning of the links of mesh network and/or limiting access to the mesh so as to keep the offered load within the limits imposed by the capacity of the available RF links
Intra-mesh Channel Re-use
Links between mesh nodes may use one or more RF channels One RF channel for rapid peer acquisition and mesh
formation May cause interference between links
Re-use worsens parking lot effect The further from a portal, the more a packet has to
compete for the channel In some cases nodes may not see other node
transmissions This leads to interference and packet loss
Intra-mesh Channel Re-use
Due to exponential back off for MAC in IEEE 802.11 protocol, packet loss may lead to significant delays per packet per link
Solutions require increasing the isolation between links Three dimensions are available:
Space(directional antennas) Frequency(multiple RF channels) Coding(using orthogonal codes for mesh links)
Which is best depends on requirements and conditions of a given network deployment
Medium Access Contention
IEEE 802.11 MAC is designed for bursty data traffic in unpredictable RF environments Channel state is re-established for every packet
Done with listen-before-talk and collision avoidance through use of contention window
Medium access is subject to jitter
For voice and video services, QoS extensions have been added to the standard EDCA is an enhancement of the basic distributed
access control mechanism with a priority bias for certain types of traffic like voice, video, and best effort data
Mesh Routing and Forwarding
Routing of packets has an important role in the service level offered by a mesh network. Route set-up is based on finding the most efficient
path to the portal Once links are formed, path does not change except
for node or link failure
Mobile mesh networks do not have static links A lot of research efforts have been invested in finding
a solution to routing in fixed and mobile mesh networks
Mesh routing and Forwarding
Spanning tree routing Networks consisting of wireless bridges naturally
form tree-like structures Rooted in a portal node May be set up using a combination of parameters
Such as hop count and link transit delay Monitor functions which detect link failure and/or
portal loss, may be used to trigger link or tree rebuilding
These structures can be simple and efficient
Mesh Routing and Forwarding
AODV: Ad-hoc, On-demand Distance Vector protocol Fairly simple, but not as efficient as spanning tree When route is needed or needs repair, its originator
floods the network with a request for a destination The latter replies with a unicast that is forwarded
back to the originator Loss of a link triggers a route error message up and
down the route The cost of flooding in AODV depends on the rate of
change of the environment
Mesh Routing and Forwarding
OLSR: Optimized Link State Routing protocol Is a pro-active routing protocol that uses the link state
as a driving factor and includes a multicast capability A subset of nodes called multi-point relays, provides
anchors for neighbor nodes Link state information distribution can remain local
and multicasting is supported naturally Although simple, the implementation of OLSR
requires many different control messages. More efficient than flooding based parents, overall
efficiency is not the best for this protocol
Mesh Routing and Forwarding
HWMP: Hybrid Wireless Mesh Protocol In 2005, the IEEE 802.11 working group started
development of the wireless LAN standard with a layer 2 mesh protocol
Includes simple tree building protocol to handle static mesh, and AODV elements to support mobile mesh
This allows a mesh to use the most efficient routing protocol appropriate for a given deployment or application
Mesh Routing and Forwarding
Regardless of the network structure, there will be alternative routes to a given destination and each node has to decide which next hop to send an a given packet to Requires the use of a consistent set of metrics
All nodes involved must share the same meaning and measurement of the metrics of a given link or path
Path metrics can include Number of hops, airtime needed to reach destination,
complex values that bring together a variety of parameters such as hop count, link load, and SNR
Mesh Routing and Forwarding
The best metric to use depends on the type of mesh network and the operational conditions
Different types of service have different requirements Voice packets are short and must be delivered within
certain time constraints In this case link reliability and hop count are important
routing metrics Background data service packets may be large and
timing is not important In this case a high data rate is more important than link
reliability
Mesh Security
Security in wireless mesh is complicated because of absence of human users that can be used as trusted parties during network initialization Security functionality and the storage of
cryptographic data cannot be fully relied upon Mesh nodes perform a number of functions, each of
which has its own security concerns
Mesh Security
Discovery This function serves to detect other mesh nodes that
belong to the same owner or administrative domain Security is limited to authentication of the information
provided by other nodes Can be provided by means of public key ciphers that
allow the verification of digital signatures This is done depending on operational needs, the
tolerance for overhead and the available budget In commercial mesh, the discovery function is left
unprotected Exposes the network to spoofing and DOS attacks
Mesh Security
Peer link establishment This function creates secure links between mesh
nodes and in the process it validates the non-protected data that nodes obtain during discovery
Secure link establishment requires Nodes are able to identify themselves They can be authenticated They are able to set up a cryptographic session with each
other that protects the flow of data and management information between nodes
Various means are available to secure link set-up A combination of symmetric key ciphers and asymmetric
ciphers are used
Mesh Security
Peer link establishment…continued IEEE 802.11X protocol
Client requests a connection This is done with a security server Validates the ID of a node and generates keys
Mesh Security
Routing Form paths through a network that is used to forward
data and management information Interfering with route set-up causes loss of
connectivity among nodes as well as loss or hiding of data Attacks against routing functions tend to result in DOS
like effects Solutions
Expanding routing protocols Tesla approach Secure the link before routing is initiated
Mesh Security
Forwarding The forwarding function delivers packets to their
destination, either directly or via intermediate nodes that lie on a path known to include the destination
Confidentiality and integrity are most important Symmetric key cryptography is preferred for efficiency Can either be hop-by-hop or end-to-end between the
original node and the end destination If connected to other networks, end-to-end is impractical,
therefore higher layer security solutions like IPSEC must be used
Congestion Control
Congestion occurs when a source produces more than its sink can handle Each link is a sink and all links that feed it are
potential sources Consider each link as potential sink for traffic Each link carries traffic in both directions Must monitor each outgoing link, and using flow
control messages
Congestion Control
Other forms of congestion control Rate limiting the traffic sources Pro-active form of source flow control Downside isinefficiency
Voice over Mesh
Factors that can affect the quality of voice over mesh Choice of codec
Different requirements with respect to jitter Choosing a codec dynamically
Handover Client link and intra mesh links may change Time depends on a number of factors
Other causes of voice quality impairment Radio measurement traffic Location signaling Legacy 802.11 devices that share the channel Non-802.11 devices like microwave ovens and cordless
phones, bluetooth
Mesh Deployment Issues
For fixed mesh networks, deployment can be a complex issue, even though the self organizing capability makes life a bit easier The main factors that drive a fixed mesh deployment
are: The service level to be offered in terms of bits per second
per user and on which basis, best effort or guaranteed The user density– people per area The available sites for mounting the access transceivers The available favorable sites for mounting intra-network
transceivers The available RF frequency band and the number of
channels, shared or otherwise