WiCTP: A Token-based Access Control Mechanism for Wireless Networks

The introduction and growth of large, metropolitan-sized wireless networks has identified a significant problem within the current 802.11 protocol specification; Hidden Nodes. A wireless network may have 2 nodes, A and B, connected to a central Access Point (AP). Node A is within the transmission radius of the AP, but outside the transmission radius of B, and vice versa. Nodes A and B are then said to be hidden from each other. Ordinarily, this does not pose a significant problem, since most nodes on a wireless network are within the transmission radius of each other, but in scenarios where all nodes are hidden it creates a significant decrease in network performance. 802.11 wireless networks use Carrier Sense Multiple Access with Collision Avoidance to determine whether they are able to send collision-free transmissions. They sense the medium to determine if any other nodes are transferring data, and if it is free, they begin their transfer. However, when nodes are hidden from each other, nodes cannot determine if the hidden nodes are transferring. Thus, a hidden node A may start a transfer to a common node C while the hidden node B is also transferring to C. This scenario results in the packets colliding at C and nodes A and B have to retransmit. A and B never know if the other is transferring, so this scenario repeats itself for the life of the network. The standard way of dealing with this problem is to use the Request to Send/Clear to Send (RTS/CTS) protocol. RTS/CTS secures the medium for a node first, then transfer data, reducing the risk of collisions. A node using RTS/CTS will send a RTS packet with a preset duration and destination contained within. Any node hearing this will cease transmission for the duration contained in the packet. The destination will receive the RTS packet and reply to the sender with a CTS packet, which also contains a duration. Any nodes hearing the CTS will cease transmission for the duration in the CTS packet. Thus, the sender node is reasonably sure the medium is free for them to transfer data. Unfortunately RTS/CTS suffers from many problems in scenarios where many nodes are hidden. RTS and CTS packets often collide and require transmission, falling victim to the same problem they are attempting to solve. To combat the hidden node problem, this study proposes a new protocol called Wireless Cyclic Token Protocol (WiCTP). Based on current token-passing access control mechanisms such as Frottle and WiCCP, it uses a token packet to control access to the medium. A master node will pass a token to each node on