Wireless Mesh Network and IEEE 802 Standards

     Since the year of 2000, the subscribers have more and higher requirements for network services. They hope that the network can provide convenient and accurate communications services at any time and at any place, so they are paying more and more attention to wireless network technologies. The Wireless Mesh Network (WMN) is a hot topic in the wireless network research. It is a fresh wireless multi-hop Mesh network. The WMN can organize itself, manage itself, and cure itself. It may be compatible and can interoperate with the existing wireless networks. Moreover, the WMN technology can integrate with multiple wireless access technologies, forming one wireless Mesh network with multi-hop wireless links. The WMN may greatly increase the coverage of wireless systems, and improve the bandwidth capacity and communications reliability of the systems. Therefore, it is one ideal solution for wireless broadband access.

1 WMN Architecture
WMN is totally different from the traditional wireless networks. The topology of the traditional wireless access systems is Point-to-Point (P2P) or Point-to-Multipoint (P2MP). In such structures, there is one central node, such as the Base Station (BS) in mobile communications systems, and the Access Point (AP) in 802.11 Wireless Local Area Network (WLAN). The central node connects with wireless terminals through single-hop wireless links, controlling the access of the wireless terminals to the wireless network. On the other hand, the central node connects with the fixed backbone network through wired links, providing the connection to the backbone. However, the topology of the WMN is Multipoint-to-Multipoint (MP2MP). In such a structure, all network nodes connect with each other in a wireless multi-hop mode.

     In the WMN, there are two types of nodes: wireless Mesh router and wireless Mesh client. According to different node functions, the WMN has three architectures: Mesh backbone network, Mesh client network, and hybrid network[1].

     In the wireless Mesh backbone network, routers connect with each other to form a mesh. In addition, the wireless Mesh backbone network connects with external networks through the Mesh routers. Besides the gateway and relay functions of traditional wireless routers, the Mesh router has the routing function for Mesh network interconnection. Therefore, with much lower transmission power, the Mesh network can obtain the same wireless coverage through wireless multi-hop communications.

     The Mesh client network is a small peer communications system formed by the interconnection of Mesh clients. It provides P2P services between user devices. In this network, the user terminal can be any device with wireless network cards and antenna, such as laptop, mobile phone, and Personal Digital Assistant (PDA). Actually, this structure is an Ad hoc network, which can provide a communications support when there is no network or when the existing network cannot be accessed.

     The Mesh client network can access the Mesh backbone network through the Mesh router, which forms the hybrid structure, as shown in Figure 1. This structure provides the connection with other network architectures, enhancing the connection capability and improving the coverage.

2 WMN Standards in IEEE 802 Series
The WMN technology is widely used in the world and its standardization becomes very necessary for large-scale applications. Currently, the WMN standards scatter in such standard drafts as IEEE 802.11s, 802.15.1/2/3/4 and 802.16. The WMN referred in IEEE 802 standards is introduced as follows:

2.1 IEEE 802.11s Mesh Standards
One of the problems that IEEE 802.11 system meets during its large-scale application is the limited coverage capability. Limited by the transmission power, WLAN can only cover within 100 m. APs can be added to increase the coverage, but that also adds the construction cost of the public network. As a new networking technology, the WMN provides a new path to solve these problems.

     The IEEE 802.11s task group studies the protocols that support the Wireless Distributed System (WDS). IEEE 802.11s defines the Medium Access Control (MAC) layer and physical layer protocols for WMN. The purpose is to fulfill the WLAN networking by configuring the multi-hop mode between APs and to increase the coverage of the WLAN. WDS belongs to the IEEE 802.11 network, serving as the trunk. WDS helps the APs to wirelessly bridge with each other while the wireless AP coverage is not affected. The wireless AP or the wireless router that support the WDS technology can work in the hybrid wireless LAN mode, supporting P2P and P2MP data transmission[2].

     IEEE 802 .11s puts forward a WMN reference architecture, as shown in Figure 2. The Mesh Media Access Coordinate Functional Components (MMACFCs) are located above the physical layer and under the Mesh route components. They are responsible for the effective competitive access and scheduling the receiving and the sending of data packets between the multi-hop nodes in WMN. When a safe Mesh link is set up, the Mesh node coordinates with other Mesh nodes so as to crack the problem of wireless media competition and sharing. In this way, the data packets of the node itself and of other nodes are forwarded effectively through the multi-hop WMN. The MMACFC equals to the Distributed Coordinated Function (DCF) in IEEE 802.11 WLAN or the Enhanced Distribution Coordinate Access (EDCA) in IEEE 802.11e. If DCF and EDCA are improved as required, they can also work in the multi-hop Mesh network effectively.

     However, the MMACFC has to solve the following problems[3]:

• Terminal hiding;
• Terminal exposing;
• Traffic control from the source node to the destination node on the multi-hop Mesh path;
• Effective scheduling on the multi-hop forwarding path;
• Distributed access admission control to the multi-hop multi-medium services (video or audio);
• Distributed service management with Quality of Service (QoS);
• Effective processing of local services and forwarding services;
• Upgradeability under different network environments;
• Scheduling of channel access between Mesh nodes;
• Performance improvement by using multiple channels.
  

     The IEEE 802.11s aims to break through the function limits of the traditional AP, and enables it to have the functions of the Mesh router, therefore, the service flow can be forwarded to adjacent APs for the multi-hop transmission. In this way, the WMN is provided with higher reliability, better scalability, and lower investment cost. Therefore, in the new WLAN structure, the APs form the WMN backbone network of the WLAN automatically[4]. IEEE 802.11 Mesh network can be either the backbone or client architecture. In the Mesh client architecture, all the devices in WLAN work in the Ad hoc network mode, and WMN implement node interconnection by automatic configuration, independent of the AP.

2.2 IEEE 802.15 Mesh Standards
The IEEE 802 .15 standard family is developed for the Wireless Personal Area Network (WPAN). The standards define the physical and MAC layers of WPAN. At present, IEEE 802. 15. 1-802. 15. 3 cannot support the Mesh network structure, but it can support the pico-network structure in P2MP mode. However, the scatternet appears the rudiment of the WMN[5].

     The research of the IEEE 802.15.4 standard mainly aims at the application devices that ask for low data rates and long battery life. It provides the WPAN with an integrated network solution. The ZigBee protocol is an upper-layer protocol that runs over the MAC and physical layers of IEEE 802.15.4. The network layer of ZigBee system clearly defines three network topology structures: star; cluster; and Mesh. In the Mesh structure, all wireless nodes are peers in the network and they can communicate with each other directly. The network can select one or several paths for a multi-hop transmission, to send the data information to the central node, as shown in Figure 3. Each node in the Mesh network has multiple paths for transmission to the central node, so the network has strong capability of error tolerance. In the transmission system, the multi-hop replaced the single-hop, which decreases the transmitting power required for the source node.

     The IEEE 802.15.5, developed for the MAC layer of WMN, is still under research. It follows some basic ideas in IEEE 802.15.1-802.15.4, but fully supports the Mesh structure, with no need of support from ZigBee or IP route. In the 802.15.5 standard, the Mesh network is defined as a PAN, and has two networking modes: Full Mesh topology and partial Mesh topology. In the full Mesh architecture, any node can be connected to other nodes directly. In the partial Mesh architecture, certain nodes are allowed to connect to other nodes, and the other nodes only connect to the nodes with much data exchange. The main issues of IEEE 802.15.5 include the following problems:

• Collision-sense beacon scheduling policy;
• Routing algorithm;
• Distributed security issues;
• Operating modes with energy efficiency;
• Support of Mesh nodes and the mobility of Mesh PAN.

2.3 IEEE 802.16 Mesh Standards
IEEE 802.16 standards define the air interface specifications of the Wireless Metropolitan Area Network (WMAN) and provide the “last mile” access for the WMAN. It is a P2MP technology. With the development of the WMN, the IEEE802.16 standard group introduces the Mesh structure into the IEEE802.16d/e standard that has been put forward recently. The WMN is supplementary to the P2MP architecture defined in IEEE 802.16. Any node in the network can form several links with its adjacent nodes and one of the links is selected to transmit the information from the local node or other nodes. Therefore, the link disconnection in WMN is less possible than in P2MP system. Besides, with the increase of number of nodes, the IEEE 802.16 Mesh network can be more robust and have a wider coverage.

     The IEEE 802.16 Mesh network supports two different physical layers and supports adaptive modulation and coding. Therefore, the link rate changes with different channel conditions. For the WMAN based on the wireless Mesh structure, Subscriber Stations (SS) can communicate with each other directly or indirectly, with no need of Base Station (BS) for data relay. Each SS can serve as the transmit point. The signals are transmitted in the hop-by-hop mode, which can increase the system coverage. When new subscribers are added, the network can change its topology dynamically, avoiding the increase of BSs. In addition, the Mesh system can take advantage of special landforms and buildings, and use low transmit power and short links to reduce interference, or use directional antenna to reduce interference and transmit power. The reduction of internal interference improves frequency reuse and frequency efficiency, while the decrease of external interference reduces required protection frequency bands. Moreover, the shorter distance between SSs may reduce the power and size of outdoor antenna, reducing the cost of SS devices. Therefore, the Mesh architecture has advantages in reliability, coverage, subscriber planning and investment.

     In addition, based on the Time Division Multiple Access (TDMA) mode, the centralized scheduling of the IEEE 802.16 Mesh system may provide overall and efficient use of resources[6]. Figure 4 shows the structure of the IEEE 802.16d Mesh system. To further support user mobility, IEEE 802.16e standards are developed based on IEEE 802.16d. The IEEE 802.16e system supports both local and regional mobility, roaming and handoff, and provides a rate up to 150 km/h in moving environments[7]. The Mesh Ad hoc special committee, newly founded in the IEEE 802.16 standard group, is now studying the P2P data transmission support and signal obstacle traversal. The WMN can transmit the data from one point to another bypassing the obstacles, and only a few grids will greatly increase the coverage of a single BS. If the proposals of the Mesh Ad hoc special committee can be adopted, the committee will become the special task group developing IEEE 802.16f standards, and will promote Worldwide Interoperability for Microwave Access (WiMAX) to go ahead for greater
success[8].

2.4 IEEE 802.20 Mesh Standards
The IEEE 802.20 standard working group, also called the Mobile Broadcast Wireless Access (MBWA) group, aims to develop standards for mobile subscribers. IEEE 802.20 has the following goals:

     (1) It will take advantages of both high data rate of the fixed wireless access network and high mobility of the cellular network to solve the contradiction between the low mobility of the fixed wireless access and the increasing demands on high-rate mobile services.

     (2) It will define a global standard configuration for the mobile broadband wireless access network that is built in low cost, allows anytime and anyplace access, and supports diversified IP services.

     IEEE 802.20 standards have high frequency efficiency, ensure higher QoS, and support reliable high-speed wireless data transmission at the 3 GHz band. Therefore, IEEE 802.20 is expected to provide users moving at
250 km/h with a broad-bandwidth data transmission at 1 Mb/s, which will make such applications as videoconferencing available on a high-speed train[9].

     IEEE 802.20 improves the current IEEE 802.16 performance of low mobility with high data rate, and high mobility with low data rate of the cellular network .Basically, the IEEE 802.20 network is a pure IP system. It supports WMN in either indoor or outdoor environments. In the IEEE 802.20 Mesh architecture, the mobile nodes can communicate with each other, and successfully avoid the Triangle Routing. Therefore, it will not only improve the performance of the mobile network, but also fulfill the fast access to the backbone network and provide mobile users with quick and accurate services[10].

3 Conclusion
The WMN has got more and more applications. Therefore, the establishment of related standards is in urgent need. At present, the international standardization organizations, especially IEEE, are cooperating with the academe to promote the standardization of the WMN. IEEE 802.11s, 802.15, 802.16d/e and 802.20 all have specifications for the Mesh networking technology, and have fulfilled such functions as access control, Mesh networking, routing, link congestion control, rapid mobility, handoff support and authentication for security in the second network protocol layer. However, only few research institutions in China have taken part in the development of these standards. Therefore, it is urgent for Chinese research institutes to keep up with the global trend, and to conduct the research on the WMN key technologies immediately.

References
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[3] 孙伟峰. 基于802.11无线—结合网络的QoS研究 [D]. 合肥:中国科技大学, 2007.
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[6] 曾智慧. 基于IEEE802.16标准的无线Mesh结构研究 [D]. 上海: 同济大学, 2006.
[7] IEEE 802.16 Standard Group WebSite [EB/OL].
http://www.ieee802.org/16/.
[8] 方旭明, 戚彩霞, 向征. IEEE 802 系列无线网络网状组网与移动切换技术综述 [J].计算机应用, 2006, 26 (8): 1756-1761.
[9] IEEE 802.20 Standard Group WebSite [EB/OL].
http://www.ieee802.org/20/.
[10] 焦健. 高速移动数据传输新标准IEEE802.20及其切换策略研究 [D]. 北京: 北京交通大学, 2006.

[Abstract] Although the wireless broadband access system is developing rapidly, its development is limited due to some disadvantages, such as small capacity of bandwidth and small coverage. The Wireless Mesh Network (WMN) is one of the important technologies for “last mile” broadband wireless access. The WMN can integrate with multiple wireless systems, such as Wireless Local Area Network (WLAN), Wireless Personal Area Network (WPAN), and Wireless Metropolitan Area Network (WMAN), to improve the performance of the wireless network and enlarge the coverage range. With the wide applications of the WMN, the IEEE 802 standard group is devoted to promoting the development of the WMN technology and establishing the related technical standards. At present, WMN standards have appeared in IEEE 802.11s, 802.15, 802.16, and 802.20.