WIRELESS MESH NETWORKS: ISSUES AND SOLUTIONS

1. WIRELESS MESHNETWORKS: ISSUES ANDSOLUTIONS

2. Introduction The primary advantages of a WMN fault tolerance simplicity of setting up a network the broadband capability. A WMN is designed for a static or limited mobility environment In order to improve the capacity of WMNs and for supporting the traffic demands raised by emerging applications for WMNs, multiradio WMNs (MR-WMNs) are under intense research.

3. Comparison between Wireless Ad Hoc and Mesh Networks

4. Comparison between Wireless Ad Hoc and Mesh Networks

5. Challenges in Wireless Mesh Networks Limitation of limited network capacity Theoretical upper limit of the per node throughput capacity: O(1/sqrt(n)) CSMA/CA a string topology : O(1/n) General case: O(w n^(-1/d)) where d is demension

6. Throughput Capacity

7. Greediness of the initial nodes and subsequent flow starvation of the latter hops A rapid degradation of throughput is the exposed node problem

8. Throughput Fairness A single-radio WMN is the high throughput unfairness CSMA/CA Information asymmetry Location-dependent contention Half-duplex character of single channel systems

11. Reliability and Robustness WMN provides high reliability and path diversity against node and link failures Graceful degradation of communication instead of full loss of connectivity MR-WMNs can use appropriate radio switching modules to achieve fault tolerance in communication either by switching the radios, channels, or by using multiple radios simultaneously.

12. Resource Management Energy: low-power and low-data rate interface can be used to carry out-of-band signaling information to control the high-power and high-data rate data interface Bandwidth: the load balancing across multiple interfaces could help preventing any particular channel getting heavily congested and hence becoming a bottleneck

13. Interfaces: bandwidth achieved through each interface can be aggregated to obtain a high effective data rate. Using a multi-radio system in a WMN is the possibility to effect provisioning quality of service through service differentiation

14. Design issues in WMN Network Architectural Design Issues Flat Wireless Mesh Network Simplest WMN Disadvantages include lack of network scalability and high resource constraints Design issues: the addressing scheme, routing, and service discovery schemes Hierarchical Wireless Mesh Network Clients and routers (some routers act as gateway to Internet) The responsibility to self-organize and maintain the backbone network is provided to the WMN routers

15. Hybrid Wireless Mesh Network A practical solution for such a hybrid WMN for emergency response applications is the CalMesh platform[5].

16. Network Protocol Design Issues Physical Layer Design Issues (i) technological considerations and (ii) economic considerations. The main technological considerations include the spectral efficiency, physical layer data rate, and the ability to operate in the presence of interference. CDMA, UWB, MIMO, OFDM

17. Economic considerations: The simplicity of the physical layer technology will lead to inexpensive devices and hence better social affordability of WMNs.

18. Medium Access Control Layer The major issues faced by the popular CSMA/CA-based IEEE 802.11 distributed coordination function (DCF) are: (i) hidden terminal problem, (ii) exposed terminal problem, (iii) location-dependent contention, and (iv) high error probability on the channel. In order to increase the network capacity, multiple radios operating in multiple channels are used. Therefore, new MAC protocols are to be designed for operating in multichannel MR-WMN systems.

19. Another popular research issue for better MAC performance is the use of cross-layer interaction mechanisms that enable the MAC protocol to make use of information from other layers. In general, the MAC layer protocol design should include methods and solutions to provide better network scalability and throughput capacity.

20. Network Layer Since WMN is relatively a static network, the routing can make use of table-driven routing approaches The main issues faced by routing protocol in a WMN are: (i) design of routing metric, (ii) minimal routing overhead, (iii) route robustness, (iv) effective use of support infrastructure, (v) load balancing, and (vi) route adaptability.

21. Transport Layer The biggest challenge is the performance of transport protocols over the WMN. The end-to-end TCP throughput degrades rapidly with hop count. The packet loss, collision, network asymmetry, and link failures can also contribute to the degradation in transport layer protocol performance. Some of the design issues for a transport layer protocol forWMN are: (i) end-to-end reliability, (ii) throughput, (iii) capability to handle network asymmetry, and (iv) capability to handle network dynamism.

22. Application Layer Provide support: Best-effort: Internet data access Time-sensitive: VoIP Provide services discovery mechansims Handle the hetertogeneity of network

23. System-level design issues cross-layer system design, design for security and trust, network management systems network survivability issues.

24. Design issues in multi-radio wireless mesh networks

25. Design issues in multi-radio WMNs Architectural Design Issues topology-based flat-topology-based hierarchical-topology-based. technology-based, Homogeneous Heterogeneous node-based. host-based, infrastructure-based, hybrid

26. Medium Access Control Design Issues Interchannel interference Multiradio channel usage must use nonoverlapping channels. Interradio interference The use of certain low-cost interface cards leads to interference even when they are separated for few feet Techniques such as graph coloring are used for generating channel allocation strategies.

27. Design of MAC protocols. Multichannel carrier sense multiple access (MCSMA) [24], Interleaved carrier sense multiple access (ICSMA) [8], Two-phase time division multiple access (2P-TDMA) [26].

28. Routing Protocol Design Issues (based on) the routing topology, the use of a routing backbone, the routing information maintenance approach.

29. Routing topology Hierarchical routing protocol hierarchical state routing (HSR) [11]. Flat routing protocol Each node has equal responsibility to find a path to the destination and to participate in the pathfinding process of other nodes.

30. The use of a routing backbone tree-based backbone routing, The IP routing mechanism over a spanning tree protocol (STP)-based tree backbone [10]. Simplest approach Poor reliability and lack of network scalability mesh-based backboneless routing Follow a backboneless mesh routing approach.

31. Hybrid topology routing: uses a dynamic backbone topology at certain specific segment of the network Routing information maintenance approach. proactive or table-driven routing protocols: DSDV,WRP, STAR reactive or on-demand routing protocols: AODV, DSR, MRLQSR hybrid routing protocols; ZRP

32. Routing Metric Design Issues The routing metric plays a crucial role in the performance of a routing protocol The design of routing metrics should take several factors such as (i) the network architecture, (ii) the network environment, (iii) the extent of network dynamism, and (iv) the basic characteristics of the routing protocol into account, in order to design an efficient routing protocol for WMNs.

33. The design objectives for a routing protocol and metric are: (i) resource efficiency, (ii) throughput, (iii) freedom from routing loops, (iv) route stability, (v) quick path setup capability, and (vi) efficient route maintenance. The challenges for designing routing protocols for MR-WMNs are: (a) interradio interference, (b) interflow interference, (c) intraflow interference, (d) hidden terminal node problem, (e) exposed terminal node problem, (f) location-dependent contention, and (g) highly dynamic channel characteristics.

34. Topology Control Design Issues Topology control is defined by the network’s capability to manipulate its parameters such as the location of nodes, mobility of nodes, transmission power, the properties of the antenna, and the status of the network interfaces. The topology can be controlled either as a one-time activity during the network initialization phase or as a periodic activity throughout the duration of the network lifetime.

35. LINK LAYER SOLUTIONS FOR MULTIRADIO WIRELESS MESH NETWORKS The primary reasons behind the lack of network scalability in a WMN are: (i) half-duplex character of the WLAN radios, (ii) inefficient interaction between the network congestion and suboptimal congestion avoidance phase at different layers of the protocol stack, (iii) collision due to hidden terminal problem, (iv) resource wasted due to exposed terminal problem and the location dependent contention, and (v) the difficulties in handling a multichannel system. Some of the above-mentioned problems can be solved by an MR-WMN. However, they face several challenges such as (i) adjacent radio interference, (ii) dynamic management of spectrum resources, and (iii) efficient management of multiple radio interfaces.

36. Multiradio Unification Protocol (MUP) The main design goals to minimize hardware modifications, to avoid making changes to the higher layer protocols, to operate with legacy (non-MUP) nodes, and to not depend on the global topology information. One of the primary tasks is monitoring the channel quality between a node and its neighbors such that the node can choose the best possible interface for communicating with a particular neighbor.

38. MUP employs two different schemes for the selection of radio interfaces (or channel as each interface is statically assigned a channel) that are named MUP Random and MUP-Channel-Quality schemes.

39. Multi-radio unification protocol (MUP) Discovering neighbors. Selecting a NIC based on one-hop round trip time (RTT) measurements. MUP selects the NIC with the shortest RTT between a node and its neighbors. Utilizing the selected NIC for a long period. the order of 10–20 s. Switching channels. all NICs are measured again through one-hop probe messages. If an NIC has a certain amount of quality improvement than the existing NIC, then switch.

40. Several issues still need to be investigated further: Hidden node issue is not effectively solved. NIC switching mechanism is not justified. Packet re-ordering is needed after NIC switching. In addition, fixed channel assignment on each NIC also limits the flexibility of MUP.

41. Mesh MAC Protocol (1) Handshake-based channel selection, (2) Channel hopping, (3) Cross-layer channel assignment.

42. Multi-channel MAC (MMAC) Multi-channel MAC (MMAC) [122] Maintaining data structure of all channels in each node. Negotiating channels during ad hoc traffic indication message (ATIM) window. Selecting a channel The criterion is to use a channel with the lowest count of source–destination pairs that have selected the channel.

44. Several problems have not been solved in the MMAC It is assumed that RTS/CTS always work in IEEE 802.11 DCF. Global synchronization in the network is difficult to achieve The channel switching time may be much larger than 224 us [122]. A larger channel switching time will significantly degrade the performance. Channel selection criterion based on the lowest number of source–destination pairs for each channel is not always appropriate.

45. DCA (1) The total bandwidth is divided into one control channel and n equivalent data channels Each node is equipped with two half-duplex receivers, one is used for the control channel and the other can switch between different data channels. All nodes maintain two data structures, a channel usage list (CUL) and a free channel list (FCL) A node builds its CUL by listening to neighboring nodes’ control messages that carry channel usage information.

47. DCA (2) This protocol requires that each node be equipped with one transmitter and multiple receivers. Before sending an RTS, a sender first senses the carrier on all data channels and builds a list of data channels that are available for transmission. If none of the data channels are free, the sender should enter back-off. Otherwise, the sender sends out an RTS with an FCL.

48. On successful reception of the RTS message, a receiver creates its own FCL by sensing all the data channels. If there are free channels in common, the receiver selects the best free channel based on the channel condition. The receiver then sends back a CTS message to inform the sender the channel to be used.

49. Multichannel CSMA MAC

50. RICH-DP RICH-DP requires all nodes in a network to follow a common channel-hopping sequence A node ready to poll any of its neighbors sends out a ready to receive (RTR) message over the current channel hop. Upon successful reception of the RTR message, the polled node starts sending data to the polling node immediately and over the same channel hop, while other nodes hop to the next channel hop. When the data transmission from the polled node is completed, the polling node may start transmitting its own data to the polled node over the same hop. After the data transmissions between the two nodes is over, both nodes resynchronize to the common channel hop. If the polled node has no data to send, the polling node rejoins the rest of the network at the current channel hop.

53. Protocol Outline: SSCH Single interface hopping on multiple channels. time is slotted SSCH (Slotted Seeded Channel Hopping) Each node has its own channel hopping schedule. Each node transmits its schedule to neighboring nodes in the beginning of each slot. To transmit data, a node has to change its hopping schedule to adapt to receiver’s hopping patterns. The seeded hopping ensures through number theory that every pair of nodes have a common channel to exchange their schedulers.

54. Cross-Layer Channel Assignment CA-OLSR

55. During initialization, each node in the network randomly chooses a Node-Number and a channel from a set of all available channels, denoted by C. The random NodeNumber is used for resolving channel conflicts. When two neighboring nodes that choose the same data channel happen to have the same NodeNumber If there is a channel conflict between the current node and an active neighbor, the current node should choose another channel from the available channel set A.

60. New Routing Metrics for Multiradio Wireless Mesh Networks Expected transmission count (ETX) [19] The ETX routing metric is designed to find a path based on (i) the packet delivery ratio of each link, (ii) the asymmetry of the wireless link, and (iii) minimum number of hops.

61. Since ETX is an additive metric, the ETX of an end-to-end path is defined as the sum of the ETX of each of the links in that path. where FDR and RDR are the forward delivery ratio and reverse delivery ratio, respectively. The FDR is the estimated value of the probability of a data packet successfully received at a receiver over a given link. Similarly, the RDR is the estimation of the probability of the ACK packet successfully received at the sender of the data packet over a given link.

62. The ETX value of a given link provides the average number of transmission attempts to be made for sending a packet successfully over a given link.

63. Disadvantages of ETX metric include the following: (1) the ETX may not work efficiently when the traffic load is high. When traffic load is very high, the probe packets may either be lost or queued. (2) Adding a separate queue for the probe packets may prevent this protocol from being used with popular MAC and routing protocols. (3) When nodes are mobile, ETX calculation may not be correct for a specific duration and ETX routing metric depends on the underlying routing protocol to quickly reconfigure path and or communicate the correct ETX value to each neighbor.

64. Multiradio Link Quality Source Routing The main contribution of MRLQSR is the use of a new routing metric called weighted cumulative expected transmission time (WCETT). The major modules in the MRLQSR protocol are: (i) a neighbor discovery module, (ii) link weight assignment module, (iii) link weight information propagation module, and (iv) path finding module.

65. The link weight assigned by the MRLQSR is proportional to the expected amount of time necessary to successfully transmit a packet through that link. It depends on the link data rate and the packet loss rate.

66. The value of ETT is taken as ETT = ETX (S/B) where ETX is the expected number of transmission, S denotes packet length, and B refers to the bandwidth of the link.

67. When an intermediate node forwards a RREQ packet, it attaches the ETT value and channel information for the link through which the packet is received and forwards the packet again to the neighbors.

70. The choice of a is not fixed by the protocol and could either be decided by the network operator as a fixed parameter or be made variable in a dynamic way based on the network load.

71. According to the experiments conducted in an experimental static two-radio-limited WMN test bed [20], it was found that WCETT outperforms ETX routing metric by about 80%.