Transcript Wireless Mesh Network

Wireless Mesh Network
Members
Suman Raj Adhikari
Saroj Raj Regmi
Introduction
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WMNs offer multiple redundant communications paths
throughout the network. Whenever a link fails, the network
automatically routes messages through alternate paths.
Even in adverse conditions devices in WMNs co-operate
with each other in transmitting packets through the
network.
WMNs are believed to be self-configuring and self-healing
networks.
Dramatic increase in link quality just by shortening the
distance between the nodes
Introduction
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Dramatic increase in link quality just by shortening the
distance between the nodes
Reduction in the distance by a factor of two can result at
least four times more powerful signals at the receiver.
Suggests that the links are more reliable without even
having to increase the power of the transmitter in
individual nodes.
The integration of WMNs with other networks such as
Internet, IEEE 802.11, 802.15, 802.16, sensor networks etc
can be accomplished.
Types of nodes in WMNs
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Wireless Mesh Router
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contains additional routing functions to support mesh networking.
usually equipped with multiple wireless interfaces built on either the
same or different wireless access technologies
improves the flexibility of mesh networking
Mesh Clients
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can also work as routers since they also have necessary functions for
mesh networking.
gateway and bridge functions do not exist in these nodes.
usually have only one wireless interface as in Laptop/desktop PC,
pocket PC, PDA, IP phone, RFID reader, etc.
WMN Architecture
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Based on the functionality of the nodes the architecture of
WMNs can be classified into:
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Infrastructure mesh Architecture,
Client mesh Architecture and
Hybrid mesh architecture
Infrastructure Mesh
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In infrastructure mesh architecture, the mesh routers
collectively provide a wireless backbone infrastructure.
Client node is passive in mesh infrastructure.
Infrastructure Mesh
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Via Ethernet links conventional clients with Ethernet
interfaces can be connected to mesh routers.
If the conventional clients have the same radio technologies
as the mesh routers then they can communicate directly with
the mesh routers
If different radio technologies are used then the clients
communicate with their base station that have Ethernet
connections to mesh routers
Client Mesh
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Client meshing provides peer-to-peer networks among client
devices. Here no such mesh router is required. Client will
act like a mesh router by relaying the packets.
Hybrid Mesh
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Mesh router provide the backbone of such network.
With the help of network functionalities such as routing and forwarding
of data packets, clients can actively participate in the creation of the
mesh.
Characteristics
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Dynamic self-configuration and self-organization
Adaptation
Fault tolerance and robustness
Low-Cost
Integration and interoperability
Applications
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Various companies have realized the potential of WMNs and have
started offering mesh networking products for a range of
application scenarios.
Though most of the products are based on commodity IEEE
802.11 hardware, the majority of the commercial systems
implement their own mesh protocols for routing and network
configuration.
This makes integration of mesh routers from different vendors
into a single WMN very difficult.
Use of IP as a common network protocol can resolve the
difficulty.
Applications
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Broadband home networking
Community and neighborhood networking
Enterprise networking
Metropolitan area networks
Transportation systems
Building automation
Standard
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IEEE 802.11s is the most relevant emerging standards for
WMN technology
Efforts are underway in several IEEE working groups, like
802.11, 802.15, 802.16, and 802.20, to define mesh
standards.
Protocol Layers
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Some of the factors that affect the capacity and performance of
WMNs
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network architecture,
network topology,
traffic pattern,
network node density,
number of channels used for each node,
transmission power level, and
node mobility.
In order to develop the protocols we need to clearly understand
the relationship between the above factors and the capacity of
WMNs.
Physical Layer
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The wireless radios of WMNs can support multiple transmission rates
by a combination of different modulation and coding rates.
Adaptive error resilience can be provided through link adaptation.
Orthogonal frequency multiple access (OFDM) and ultra-wide band
(UWB) techniques are the schemes that are being used to support
high-speed transmissions.
If we desire to increase the capacity and mitigate the impairment by
fading, delay-spread, co-channel interference, fading, multi-antenna,
systems such as antenna diversity and smart antenna have been
proposed for wireless communications.
It is more difficult to develop such techniques for WMNs although
these physical-layer techniques are also desired by other wireless
networks.
Physical Layer
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Frequency-agile or cognitive radios are being developed to
dynamically capture the unoccupied spectrum to achieve better
spectrum utilization and viable frequency planning for WMNs.
Since all the components of a radio, such as RF bands, channel
access modes, and channel modulations, are programmable
implementing cognitive radios on a software radio platform is one
of the most powerful solutions.
Although physical test-beds are currently available, the software
radio platform is not a mature technology yet. But as it can enable
the programmability of all advanced physical layer techniques, in
the future it seems to be a key technology for wireless
communications
MAC Layer
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There are differences between the MAC in WMNs and other
wireless networks:
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MAC for WMNs is concerned with more than one-hop communication.
MAC is distributed, needs to be collaborative, and works for multipoint-tomultipoint communication.
Network self-organization is needed for better collaboration between
neighboring nodes and nodes in multi-hop distances.
Mobility is low but still affects the performance of MAC
For WMNs, a MAC protocol can be designed to work on a single channel
or multiple channels simultaneously.
Routing Layer
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The design of the routing protocols for WMNs is still an area of
research although there are many routing protocols that are
available for ad hoc networks.
However, an optimal routing protocol for WMNs must possess
features like:
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multiple performance metrics
scalability
robustness and
efficient routing with mesh infrastructure.
The routing protocols for ad hoc networks is equipped with some
of these features, but non of them possesses all of the above.
Routing Protocols
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Multi Radio Routing
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In a multi-radio link quality source routing (MR-LQSR) a new
performance metric , weighted cumulative expected transmission time
(WCETT) , is incorporated.
Both link quality metric and the minimum hop count are accounted for in
WCETT which provides a good tradeoff between delay and throughput.
Routing Protocols
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Multi-Path Routing:
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Better load balancing and providing high fault tolerance are the two main
objectives of using multi-path routing.
Multiple paths are selected between source and the destination
This provides better fault tolerance as when a link is broken another path
can be chosen.
This enhances efficiency since without waiting to set up a new routing
path, the end-to-end delay, throughput, and fault tolerance can be
improved.
Complexity is the major hurdle of multi-path routing.
Also, the availability of node disjoint routes between source and
destination determines the improvement, given an performance metric.
Routing Protocols
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Hierarchical Routing:
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The hierarchical routing protocols tend to achieve better performance
when the node density is high.
This is because of less overhead, shorter average routing path, and quicker
set-up procedure of routing path etc.
Maintaining hierarchy definitely adds to the complexity and this may
compromise the performance of the routing protocol.
Routing Protocols
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Geographical routing
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This kind of routing scheme forwards packets only by using the position
information of the nodes in the vicinity and in the destination node unlike
the topology based schemes.
This suggests that there is less impact on the geographic routing due to a
topology change than the other routing protocols.
The algorithm used in geographical routing (single-path greedy routings)
suffers from that a delivery is not guaranteed even if a path exists between
a source and the destination as the packet forwarding decision is made
based on the location information of the current forwarding node, its
neighbors, and the destination node.
Planar-graph based geographic routing algorithms have been proposed
which guarantees the delivery. Theses algorithms suffer form a major
drawback: a higher communication overhead.
Transport Layer
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A large number of transport protocols are available for ad
hoc networks and WMNs depend on those transport layer
protocols
Till date, there is no transport protocol that has been
proposed specifically for WMNs.
We know that ad hoc network is also not mature. It also
has various unresolved issues. This suggests further
research in this area.
Application Layer
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There are numerous applications that are supported by
WMNs and they are categorized into various classes.
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Internet access
Distributed Information Storage and Sharing
Information Exchange across multiple wireless networks
The areas of research in application areas include these
classes.
Improving existing application layer protocols, proposing
new application-layer protocols for distributed information
sharing, and developing innovative applications for WMNs
are the areas of more research.
Issues
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Issue in Network performance:
Radio techniques,mesh connectivity, compatibility and
inter-operability etc. are the factors that influence the
performance of WMNs
Security Issues:
Security schemes for WMNs are still not ready. Due to the
distributed nature of WMN, there is no centralized
authority that can be fully trusted and can distribute a
public key.
Other Issue:
Capacity, scalability, and QoS are considered as the major
weakness of current WMN technology
Conclusion
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The nodes in a WMN automatically detect neighbor nodes and
establish and maintain network connectivity in an ad hoc fashion.
The self-configuring nature facilitates the easy and rapid
deployments of WMNs.
They dynamically adapt to the changing environments and can
self heal in case of link or node failure.
Being inherently redundant they have a high level of fault
tolerance and are hence robust.
The low cost of the hardware based on IEEE 802.11 standards is
one of the prominent reasons in the increasing interests in both
research and product development.
Conclusion
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Integration and interoperation with other networks, and bridges to
legacy networks can be easily accommodated due to the fact that
most WMNs are based on Internet Protocol standards.
Based on existing technologies, some companies already have
products for sale, while other companies are still not convinced to
the point of production while others have started to deploy WMNs
in various application scenarios.
Whatever the case be, it we look at the researchs and case studies,
what we can find is that the performance of WMNs is still far
below what is expected. There are various open issues that need to
be resolved.
Reference
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E.M. Royer, a. T. (April 1999). A Review of Current Routing
Protocols for ad Hoc Mobile Wireless Networks'. IEEE Personal
Communications Magazine .
Perkins, C. (2001). Ad-Hoc Networking. MA: Addison Wesley
Professional, Reading.
Wang, Xudong, Kiyon, INC (n.d.). A Survey on Wireless Mesh
Networks. Georgia Institute of Technology
Yan Zhang, J. L. (2007). Wireless Mesh Networking,
Architectures, Protocols, and Standards. FL: Auerbach
Publications
Questions
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Wireless Mesh Network is a radical network form of the
ever evolving wireless network. The Ad-hoc network is
also a form of the wireless network. Compare and contrast
the wireless mesh network and the Ad-hoc network that
are the categories in the multi-hop wireless network.
Explain the characteristics of wireless mesh network and
some of the factors of the mesh networks influencing the
performance. Are these wireless network reliable and
scalable?
Can you explain some of the management functions
needed to maintain the operations of wireless mesh
networks?
Ad-hoc VS WMNs
There are various factors which can differentiate wireless mesh
network from Ad-hoc Network. The major differences between
these two types of network are the network topology and mobility
of node
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Network topology: Ad- hoc network are called as the
infrastructure less network with a highly dynamic topology
where as WMN have a relatively static network topology
Mobility of relay nodes: In Ad-hoc network the relay nodes
have a higher degree of mobility i.e. relay nodes aren't fixed
whereas in WMN's the degree of mobility of relay node is
much lower than in Ad-hoc netowrks.
Ad-hoc VS WMNs
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Deployment: Some planning is required to deploy the WMNs
whereas Ad- hoc network is easy to deploy.
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Infrastructure requirement: Ad-hoc network is infrastructure
less whereas WMN has partial or fully fixed infrastructure.
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Energy constraints: In Ad-hoc network energy constraints is
high whereas it is low in the WMNs.
Characteristics of WMNs
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Dynamic self-configuration and self-organization:
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Adaptation:
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The nodes in a WMN automatically detect neighbor nodes and establish
and maintain network connectivity in an ad hoc fashion.
Typically implemented at the network layer with the use of ad hoc
routing protocols
This self-configuring nature facilitates the easy and rapid deployments
of WMNs.
WMNs dynamically adapt to the changing environments and can self
heal in case of link or node failure.
Fault tolerance and robustness:
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Mesh networks are inherently redundant and hence have a high level of
fault tolerance and robustness.
Characteristics of WMNs
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Low-Cost:
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The low cost of the hardware based on IEEE 802.11 standards is one of
the prominent reasons in the increasing interests in both research and
product development.
Integration and interoperability:
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Integration and interoperation with other networks, and bridges to legacy
networks can be easily accommodated due to the fact that most WMNs
are based on Internet Protocol standards.
Factors Influencing performance
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Radio techniques:
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There are various approaches that have been proposed inorder to
increase the capacity and flexibility of the wireless systems.
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E.G. Directional antennas and smart antennas, MIMO systems, and multi-radio/multichannel systems.
Reconfigurable radios, frequency agile/cognitive radios, and even
software are some of the more advanced technologies that further
improve the performance of a wireless radio.
All these advanced radio technologies are not mature and require better
design in higher layer protocols such as MAC and routing protocols.
Factors Influencing performance
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Scalability:
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Without the support of scalability, which is the critical requirement of
WMNs, the network performance degrades as the network size
increases.
If that is the case, effects such as routing protocols may not be able to
find a reliable routing path, transport protocols may loose connections,
and MAC protocols may experience significant throughput reduction
may be prominent.
All the protocols (MAC layer to application layer) needs to be scalable
in order to ensure scalability in WMNs
Factors Influencing performance
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Security:
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There are many different security solutions proposed for wireless LANs
but the security schemes for WMNs are still not ready.
Due to the distributed nature of WMN, there is no centralized authority
that can be fully trusted and can distribute a public key.
Still, the security schemes that are proposed for ad hoc wireless
networks can be adopted for WMNs.
The architecture for ad hoc network is different, causing differences in
security mechanisms.
If ad hoc networks and WMNs were the same, still since, ad hoc
networks are still in its developmental stage, the security solution it has
is also not mature enough.
This suggests us to keep on looking for more secure and complete
mechanisms
Factors Influencing performance
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Broadband and QoS
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Most applications of WMNs are broadband services with heterogeneous
QoS requirements.
This is the reason why communication protocols must consider various
performance metrics.
Such performance metrics are: delay jitter, aggregate and per-node
throughput, packet loss ratios along with end-to-end transmission delay,
and fairness.
Factors Influencing performance
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Ease of Use:
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The network management tools that efficiently maintain the operation,
monitor the performance, and configure the different parameters of
WMNs needs to be developed.
For the better performance of the network, routing protocol should be
designed in such a way that it is independent of the power management,
self organization behavior, and robustness in the link failure and in
providing the fast user authentication services.
The above mentioned management tools along with the autonomous
mechanisms in networking protocols helps in the rapid deployment of
WMNS.
Factors Influencing performance
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Compatibility and Inter-operability
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There are two types of clients: conventional clients and mesh clients.
WMNs need to be backward compatible with conventional client nodes
i.e. it is a requirement to support both these clients.
This asks for more from the mesh routers. Mesh routers must be capable
of the integration of heterogeneous wireless networks.
Reliability and Scalability?
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Reliability from self configuration and self organization characteristics
of the network. WMN automatically detect neighbor nodes and
establish and maintain network connectivity in an ad hoc fashion,
typically implemented at the network layer through the use of ad hoc
routing protocols.
High level of fault tolerance makes it more reliable, since it has
multiple path for the delivery of data.
This network is scalable for a small network but may be problematic
while setting routing path for larger networks as it may take longer
time to set up the routing path and also the end to end delay become
larger.
Research is still going on to have the better routing protocol that gives
better scalability to the network.
Management Functions
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Mobility Management:
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Location management handles the location and call delivery. Thus
distributed mobility management is needed for WMN's.
Location information can enhance the performance of MAC and
routing protocols , and it can help to develop better location related
applications.
Proposing accurate or efficient algorithms for location service is still an
open research topic.
Mobility management is closely related to multiple layers of network
protocols, so developing multi-layer mobility management schemes is
another areas of research.
Management Functions
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Power Management
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Power management controls connectivity, interference, spectrumspatial reuse, and topology.
Mesh routers do not have a constraint on power consumption.
Mesh clients desire protocols to be power-efficient.
Network Monitoring
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The statistics in the MIB of mesh routers need to be reported to one or
several servers in order to continuously monitor network performance.
Based on the statistical information collected from the MIB, data
processing algorithms can accomplish many other functions such as
network topology monitoring.