Chapter 16 Wireless Mesh networks

1. Chapter 16Wireless Mesh networks

2. Overview • Introduction • Mesh network defined • Benefits of wireless mesh • Current issues and solutions • Mesh deployment issues • Conclusion

3. Introduction • Used by • Municipalities to enterprises • Telecom providers • Public safety • Military • Popular do to • Ease of deployment • Increase in network capacity • mobility

4. 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

5. 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

6. Examples of Deployment • Small in home mesh networks • T.v., audio systems • Public safety • Police, fire, emergency responses • wiMAX or 3G access links

7. Current issues and solutions • Netowork structure • Intra-mesh channel re-use • Medium access contention • Mesh routing and forwarding • Mesh security • Congestion control • Voice over mesh • Mesh network management

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. Congestion Control • Other forms of congestion control • Rate limiting the traffic sources • Pro-active form of source flow control • Downside isinefficiency

27. 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

28. 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