1. Field of the Invention
The present invention relates to a channel allocation method of a wireless network and a system thereof. More particularly, the present invention relates to a distributed channel allocation method of a wireless mesh network and a system thereof.
2. Description of Related Art
In recent years, there is a rapid development in the field of wireless broadband access techniques including Wi-Fi (IEEE 802.11 series), WiMAX (IEEE 802.16 series) and 3G, etc. The wireless mesh network (referred to hereinafter as WMN, IEEE 802.11s) is one of the key techniques integrated with the wireless broadband network. The structure of the WMN illustrated in FIG. 1 is a mesh network based on a wireless transmission interface, and the WMN has a similar operation mode to that of an Ad-hoc network. Since the operation of the WMN is based on the wireless transmission interface, it has the advantage of rapid deployment without restriction of the geographical landforms. The WMN is generally applied to a community network, a temporary network of exhibition halls or shopping stalls, networks established in disaster areas or areas having special geographical environments, and so on.
The operation of the WMN is based on the wireless transmission interface. Taking the IEEE 802.11a/g for an example, its transmission bandwidth of data is 54 Mbps (mega bytes per second), which is the maximum possible transmission bandwidth. However, influenced by a MAC (media access control) contention, 802.11 headers, 802.11 ACK signals and packet errors, an average applicable bandwidth is usually less than half of the maximum bandwidth.
Furthermore, the most serious issue lies in that a data transmission rate of a network link layer may be decreased greatly due to signal interference. Two possible interference problems are shown in FIG. 2: (1) interference in the same transmission path, (2) interference in the adjacent transmission paths. Referring to FIG. 2, the signal coverage of a node 3 includes nodes 2, 4 and 9. Similarly, the node 3 is simultaneously in the signal coverage of the nodes 2, 4 and 9. A first transmission path and a second transmission path are paths for data transmission. The first transmission path is taken for an example. When the node 2 and the node 3 are transmitting data, the node 4 may receive signals from the node 3, resulting in the fact that node 4 cannot transmit data to a node 5 provisionally. Therefore, the bandwidth of the first transmission path is reduced, which refers to the so-called interference in the same transmission path.
On the other hand, referring to the node 9 on the second transmission path, since the node 9 is in the signal coverage of the node 3, the node 9 may receive signals from the node 3 when the node 2 and the node 3 are transmitting data, resulting in the fact that the node 9 cannot transmit data to a node 8 or a node 10 provisionally. The phenomenon indicating an interference of data transmission through the first transmission path with that through another transmission path (a second transmission path) represents the so-called interference in the adjacent transmission paths. Therefore, many studies are performed on the WMN to learn how to improve an applicable bandwidth of the WMN by advancing a structural design thereof.
According to the IEEE 802.11s WiFi Mesh standard, a plurality of wireless transmission interfaces is allowed to use different non-overlapping channels for transmission, so as to increase the transmission bandwidth. Therefore, some studies have been developed to increase a network flow by applying multi-network interface cards (referred to hereinafter as Multi-NIC). A method of increasing the network flow includes allocating a plurality of NICs on each node, and each of the NICs may employ a different non-overlapping channel to communicate with other nodes. The advantage of this method lies in that it is unnecessary to modify any existing hardware structures. Only is an integral channel allocation method required for assisting the existing hardware structure, and the network flow can be substantially improved.
A method and a system for assigning channels in a wireless local area network (WLAN) is disclosed in U.S. Publication No. 2006/0072502 A1, in which the WLAN infrastructure mode (i.e. a client to hub communication mode) is provided. A mobile node (referred to hereinafter as MN) in the network is connected to an access point (referred to hereinafter as AP) by means of one hop, and the other end of the AP is connected to a wired network, wherein each AP has at least two applicable channels, and each AP is at least adjacent to another AP.
Each AP constantly collects the traffic load information and forecasts a possible throughput on each channel. Thereafter, the AP determines an optimal channel for connecting with the MN within the signal coverage of the AP. However, this channel allocation method only takes the optimal channel within one hop between the AP and the MN into account. Therefore, the application of the method is limited.
Most of the early studies focus on modifying an MAC layer protocol of the network to support a multiple channel transmission. The studies aim to find the optimal channel for transmitting every single packet, so as to avoid the interference. On the other hand, a concept of a Multi-NIC disclosed by V. Bahl et al. and P. H. Hsiao et al. in two articles has drawn attention and discussions recently. One of the articles was authored by V. Bahl, A. Adya, J. Padhye, A. Wolman, entitled “Reconsidering the Wireless LAN Platform with Multiple Radios” Workshop on Future Directions in Network Architecture (FDNA-03), while another one was authored by P. H. Hsiao, A. Hwang, H. T. Kung, and D. Vlah, entitled “Load-Balancing Routing for Wireless Access Networks” Proc. of IEEE Infocom 2001. The method disclosed therein is to install a plurality of the NICs on each node of the Ad-hoc network, and each NIC may dynamically determine a channel for communicating with other nodes. The advantage of this method lies in that it is unnecessary to modify any existing hardware structures. Only is the integral channel allocation method required for assisting the existing hardware structure, and the network flow can be substantially improved. Sequentially, a channel allocation method based on a centralize structure was disclosed by A. Raniwala, K. Gopalan, T. Chiueh, entitled “Centralized channel assignment and routing algorithms for multi-channel wireless mesh networks,” ACM Mobile Computing and Communications Review 8 (2) (2003), which is one of the earliest articles having a formal definition of the channel allocation. In the method, a load-aware channel assignment is performed by an evaluation matrix defined by the authors themselves, the entire network is calculated in overall, and a preferable channel allocation is obtained. Thus, a maximum network flow is then achieved.
In recent studies, a channel allocation technique based on a dynamic & distributed structure has been disclosed, wherein channel allocation information is exchanged by using a common channel framework according to the IEEE 802.11s standard. This technique is based on IEEE 802.11 WLAN standard, wherein a plurality of wireless NICs is installed to support a multi-channel transmission. However, the interference still cannot be avoided in the aforementioned techniques.