Wireless Mesh Architecture for IP-Based Base Stations

This work was funded by the National Natural Science Foundation of China under Grant 60632030.

     Due to its unique performance advantages in broadband, wireless convergence, self-organization,
self-management and robustness, Wireless Mesh Network (WMN) is attracting more and more attention, and the WMN architecture is likely to be one potential technology for constructing next generation mobile communication networks.

1 Architecture and Characteristics of WMN
WMN involves two types of nodes: Mesh routers and Mesh terminal users. The backbone of a WMN is composed of routers which are interconnected and distributed in a mesh way.

     WMN can be built with one of the two typical Mesh modes: Mesh mode at the infrastructure end and Mesh mode at the terminal user end. In the former mode, Internet Access Points (IAPs) and terminal users are connected into a wireless closed loop. The IAPs use their routing selection and management functions to select best paths for mobile terminals, while the mobile terminal can, via IAPs, access other networks, such as Wireless Fidelity (Wi-Fi), Worldwide Interoperability for Microwave Access (WiMAX) and sensor networks, hence the compatibility of the network is improved. In the latter mode, terminal users are wirelessly connected into a Point-to-Point (P2P) network. The terminal equipment can run independently without the support of other infrastructure equipment and allow mobile terminals to move at a high rate. Consequently, a broadband network can be quickly constructed. In this mode, the terminal user acts both as a host and a router: on one hand, it runs all related applications; on the other hand, it implements routing protocols and participates in such operations as routing discovery and maintenance.

     According to the different functions of Mesh routers and Mesh clients, the architectures of WMNs can be classified into three groups: backbone WMN (hierarchical), client WMN (flat) and hybrid WMN[1-2].

1.1 WMN Architecture
In the backbone WMN architecture, self-configuration and self-healing links are created with Mesh routers. Taking advantage of the gateway function of Mesh routers, the WMN is connected to the Internet and provides access service for the clients. As shown in Figure 1, the terminal devices are connected to the upper-layer backbone via the Mesh routers at the lower part of the backbone (which act as central access points of access networks), thus the network nodes are interconnected. Meanwhile, each terminal can access other networks via the gateway nodes, so wireless broadband access is achieved. In this way, the cost of network construction is reduced; the coverage and reliability of the network are enhanced. This architecture is compatible with all existing equipment on the market, but any of the two terminal nodes cannot be communicated directly.

     The client WMN is made up of Mesh clients and provides P2P services between user equipment. The clients have routing and self-configuration functions, and support terminal users’ applications. In the client WMN architecture shown in Figure 2, all nodes are equivalent and have the same feature: all of them include the same protocols, such as protocols related to Media Access Control (MAC), routing, management and security.

     These nodes function both as clients and routers that forward services. Obviously, these nodes cannot be compatible with several wireless access technologies, and the singularity of this architecture makes it not suitable for large-scale networking. Therefore, this architecture is only applicable to the situations where a small number of nodes is required and they do not need access to the core network.

     Figure 3 is a hybrid WMN architecture, where Mesh clients are connected to the backbone Mesh network via Mesh routers. In this architecture, the WMN can be interconnected with other networks, including Internet, Wireless Local Area Network (WLAN), WiMAX, cellular and sensor networks. Moreover, the routing function of Mesh clients enhances the network’s connectivity and enlarges its coverage. The terminal nodes enable direct communication between the equipment, for here they are not the devices that support only one wireless access technology but Mesh equipment with forwarding and routing functions. The devices at the terminal nodes are often required to be able to access both Mesh routers at the upper layer and the peer nodes at the same layer.

     Among the standards that support wireless Mesh topology, 802.11s[3] focuses on tree multi-hop Mesh network and multi-hop Ad hoc Mesh network, and defines three types of nodes: Mesh Point (MP), Mesh Access Point (MAP) and traditional WLAN node. In a multi-hop Ad hoc Mesh network, there is no central node to perform centralized control, and all MPs and MAPs in the network are peer. Similar to the mobile nodes in general Ad hoc networks, the MPs and MAPs not only act as data sources, but also forward the packets for other nodes. The wireless P2P network made up of these nodes does not need any network infrastructure.

     The standard 802.16[4] defines two network topologies: Point-to-Multipoint (PMP) and Mesh network. The full coverage of a network can be achieved with the Mesh architecture. As shown in Figure 4, the Mesh network is controlled by a central node called Mesh base station. This central node can be used as an external interface to set up direct links with backhaul equipment outside the Mesh network.

1.2 WMN Characteristics
WMN is an integration of multi-hop and multipoint-to-multipoint architectures, and it has the following main characteristics[5]:

     (1) Multi-hop network architecture: One important objective of WMN is to extend the coverage of current wireless network without reducing channel capacity. Another objective is to provide Non-Line-of-Sight (NLOS) connection for the users within NLOS range. The links in the Mesh network are usually very short, so the interference on them is quite low. As a result, a larger throughput and a higher spectrum multiplexing efficiency can be achieved.

     (2) Support for Ad hoc networking and capability of self-forming,
self-healing and self-organization: The flexible architecture, easy configuration, excellent fault tolerance and good connectivity of WMN enable it to greatly improve the performance of existing networks. Besides, WMN requires a very low upfront investment and can be gradually expanded as required.

     (3) Mobility dependence on the type of Mesh nodes: The Mesh router is usually of low mobility, while the Mesh client can be immobile or mobile.

     (4) Support for multiple types of network access: WMN supports access through backbone network as well as end-to-end communications. Moreover, it can be integrated with other kinds of networks and offer services to the terminal users of these networks.

     (5) Dependence of power consumption constraints on the type of Mesh nodes: Generally, Mesh routers do not have strict constraints on power consumption, but Mesh clients require effective power saving mechanisms.

     (6) Compatibility and interoperability with existing wireless networks, including WiMAX, Wi-Fi and cellular networks.

2 Networking Architecture for IP-based Base Station
Restricted by wired connection between base stations as well as the number of communication nodes between base stations and external networks, the traditional cellular mobile communications system cannot satisfy the requirements for flexible deployment, low cost, high scalability and good reliability[6]. Meanwhile, the next generation mobile communication network architecture tends to be flat and simplified. The number of link nodes will be gradually decreased but the functions of these nodes will be further enhanced. This tendency has already been reflected in the architecture of Long Term Evolution (LTE) access network. For example, the evolved UMTS Terrestrial Radio Access Network (UTRAN) has adopted all IP architecture, cutting down on network entities and interfaces. It also includes many enhanced Node Bs (eNBs) to replace Radio Network Control (RNC) entities in old systems. Besides, the function part of this access network is placed in nodes or core network. All of these changes lead to a significant simplification of the system structure. The eNBs, with enhanced functions, are interconnected with X2 interfaces and interact with each other in terms of data and signaling, thus the signaling exchange delay is shortened and the system efficiency is improved[7]. Furthermore, future base stations are likely to be wirelessly connected. They will function as traditional base stations to provide wireless access and radio resource management for the terminals, and as wireless routers to directly access to IP core network via the gateway.

     Figure 5 illustrates a wireless Mesh architecture for IP-based base station. This architecture adopts a flat networking mode, and the base stations can be directly connected to the IP core network via the gateway. In addition to having functions of a traditional base station, the IP-based base station has a Mesh routing entity. This routing entity makes communication between base stations unlimited to wired connection, and greatly facilitates the self-organization between base stations. As a result, the resources can be coordinated, managed and allocated in a uniform way among multiple base stations, and the system’s overall performance is optimized. This routing entity can also use the load-balancing technology to select proper routing paths for the terminals, which considerably increases the system capacity and reduces the system cost. As shown in Figure 5, IP-based base stations can be directly connected to the gateway to communicate with IP core network, or they can communicate with IP core network by means of grouping into a base station cluster and then connecting the cluster head with the core network. Thus, the coordination between different access networks is realized.

3 Key Technologies Used in Wireless Mesh Networking for IP-Based Base Station
The key technologies used in wireless Mesh networking for IP-based base station[8-11] involve on-demand
self-organization of base station, joint radio resource scheduling, routing, security and network management.

3.1 On-Demand Self-Organization of Base Station
To improve the network’s invulnerability, reliability and resource utility, it is necessary to study self-organization strategies between base stations, including discovery mechanism and negotiation mechanism.
In the self-organization strategies of base station, the concept “base station cluster” is introduced. The cluster head is responsible for coordination and communication between base stations in the cluster.

     Meanwhile, the base stations in the cluster are allowed to directly access and interact with IP core network via the gateway. This not only avoids the  congestion caused by heavy traffic in the cluster head, but also meets the multi-user and multi-service demands in future communications and solves the complex radio resource management problem resulting from user mobility. In this way, the system load balance is achieved.
Depending on the mobility and loads of the mobile terminals, the size and head of the base station cluster can be changed dynamically. In the case of an inter-cluster handoff, the heads of involved base station clusters will interact with each other and complete the handoff control; while in the case of an
intra-cluster handoff, the handoff control is completed by signaling interaction between base stations in the cluster. Moreover, if a base station in the local cell is damaged, the wireless router will relay the services that are originally directed to the damaged base station to other base stations, thus ensuring effectiveness and invulnerability of the communication system.

     In short, the self-organization technology of base station can not only facilitate "intra-network coordination" to realize high speed transmission, but also incarnate the idea of "inter-network coordination". The base stations within the same cluster can communicate with each other via the cluster head, or directly access the IP core network via the gateway, depending on the traffic. The cluster heads of different subnets can exchange information and allocate resources based on service requirements and transmission modes of the users. Supporting heterogeneity and multi-mode transmission, the
self-organization technology adapts to the ubiquitous trend of future mobile communications system.

3.2 Joint Radio Resource Scheduling
In the Mesh architecture for IP-based base station, the new, flat networking approach invalidates the centralized resource scheduling mechanism designed for traditional cellular networks. As a result, it is quite urgent to develop new resource management and scheduling technologies for the flat,
self-organizing and dynamically changing network.

     In developing the joint mechanism, two issues should be addressed: A coordination mechanism and signaling process between radio resource management and base station’s self-organization should be worked out, enabling radio resources to be  optimized autonomously; and the traditional centralized resource scheduling mechanism can manage all radio resources within its jurisdiction in a uniform way, so as to easily achieve the goals of optimally utilizing the global resources and maximizing the system capacity, but this kind of resource scheduling lacks flexibility. In contrast, the distributed resource scheduling strategy can flexibly manage the nodes in a distributed way based on the actual deployment of the network. However, as there is no central management entity, this strategy cannot adjust the entities globally and make unified calculation for some objects, thus an optimal system scheme is unobtainable.

     As for the Mesh architecture for IP-based base station, its radio resource scheduling mechanism should be based on the self-organization strategies between base stations. It should, according to specific self-organization deployment of base stations, adopt a joint resource scheduling mechanism integrated with distributed and centralized approaches to meet different requirements of self-organization and hetero-organization networking modes.

     The joint scheduling strategies should be specific, optimal and multi-object oriented. To utilize radio resources optimally and maximize the system capacity, several factors should be taken into account in designing the joint scheduling strategies: how to improve the radio spectrum efficiency, how to avoid the network congestion and how to reduce the signaling load.

3.3 Routing
As the IP-based base stations are wirelessly connected, the routing issue is raised when the base stations act as wireless routers. Besides, with the expansion of the network, the management and configuration of routing between base stations will become more complicated, and the overheads of building routes will increase accordingly. Although the locations of base stations in Mesh architecture are fixed, and their route selections are almost unchanged, any change in radio environment and load will lead to overheads for reselecting the optimal routes.

     There are several routing protocols available for WMNs, such as single criterion-based routing protocol,
multi-channel routing protocol, multi-path routing protocol, hierarchical routing protocol, cross-layer routing protocol, QoS routing and geographic information based routing. However, further studies have to be made on these aspects: whether these protocols are suitable for IP-based base stations, what problems may arise from using these protocols, and how to design a routing protocol for wireless Mesh architecture for IP-based base station.

3.4 Security
An inevitable issue involved in multi-hop networks is security. With the development of networks, the
network-wide management, including configuration, monitoring and charging, is becoming more and more complex. It is well known that wireless communications are vulnerable to passive attacks (e.g. eavesdropping) and active attacks (e.g. information tampering, denial of service). The cellular communication system has mechanisms to strictly control authentication, authorization and access, but after the network management approach changes from centralized to distributed, more security problems may be raised. As to the Mesh architecture for IP-based base station, the following security challenges have to be faced: On which network entities should the security management be implemented? How to monitor the network and detect invasions? How to avoid the router information being changed? How to ensure the security of air interfaces? How to ensure authorization, identity identification, information encryption, data integrity check and digital signature to be securely implemented?

3.5 Network Management
The next generation wireless mobile networks will be an integration of various wireless technologies. The scale, complexity and heterogeneity of each network will considerably increase, so will the Mesh architecture for IP-based base station. Besides, the complex radio environments impose higher demands and greater challenges on network management.

     With the integration of different kinds of networks, the old, centralized network management mechanism, which is based on single architecture and is relatively steady, has been found extremely defective, and its separate approach will no longer meet the development requirements of communication systems. Therefore, new, effective and comprehensive network management mechanisms should be developed to enable network management to be intelligent, scalable, flexible and robust, as well as to achieve end-to-end network management and Quality of Service (QoS) management.

4 Conclusions
Being a new wireless broadband access network integrated with the advantages of both WLAN and Ad hoc network, WMN has become an effective means for broadband wireless access and can be flexibly applied in various radio environments. With flexible networking approaches and distributed nodes, the WMN architecture can be used to construct high-speed, wide-coverage and easily upgraded wireless networks with few infrastructures and at low costs. However, further studies have to be made on how to make full use of the performance advantages and potentials of this architecture in the next generation mobile communication networks.

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[Abstract] Combining the performance advantages of both Wireless Local Area Network (WLAN) and Ad hoc network, Wireless Mesh Network (WMN) is characterized by large capacity, high rate and wide coverage. It has become an effective means for broadband wireless access and can be applied flexibly in various wireless environments. The wired connections between base stations in traditional cellular mobile communications systems can no longer satisfy the requirements for flexible deployment, low cost, high scalability and good reliability. The wireless Mesh architecture for IP-based base stations is thus introduced as an optional solution to support the flat and simplified next generation mobile communications networks. The key technologies used to support self-organization of IP-based base stations involve several aspects, including on-demand self-organization of base stations, joint radio resource scheduling mechanism, routing, security and network management.