Design Approach
4.2 The Operation of the Proposed RMP
Fig. 4: MAC interference among a chain of nodes.
4.2 The Operation of the Proposed RMP
Here, we propose a novel scheduling mechanism without modifying the existing 802.11 MAC protocol. This mechanism is applied to chain-based and multi-radio WMNs. By means of matching radios between mesh routers, we name our pre-planning deployment and scheduling mechanism as a Radio-Matching Protocol (RMP), which can achieve the maximal spatial reuse. In the RMP, mesh routers are equally spaced to form a chain topology, where mesh routers that are not neighbors do not interfere with each other. Every mesh router is equipped with two wireless radio interfaces; one for transmitting and the other for receiving.
In the RMP, there are two types of mesh routers:
T-R mesh router: For T-R mesh routers, the first channel is only for transmitting packets, and the second channel is only for receiving packets.
R-T mesh router: Similarly, for R-T mesh routers, the first channel is only for receiving packets, and the second channel is only for transmitting packets.
Interference range
1 2 3 4 5 6
Transmission range
Fig. 5: A chain topology consists of two kinds of mesh routers that use the RMP scheduling to achieve the maximum spatial reuse.
We form this chain by assigning these two types of mesh routers alternately. That is, the neighbors of a T-R mesh router are R-T mesh routers. Similarly, the neighbors of an R-T mesh router are T-R mesh routers, as shown in Fig. 5 (a). T-R mesh routers and R-T mesh routers are equally spaced such that the same types of mesh routers will not be neighbors. The
(b) The communication process of RMP in different time intervals.
Radio 1
(a) The deployment of mesh routers and associated transmission directions.
1
Fig. 6: Finite state machine of a mesh router using RMP
Fig. 6 shows the finite state machine of a mesh router using RMP. In the RMP, each node should be in one of the three states:
ACTIVE: A node enters this state when its counter counts to 2 or 1, and it can transmit and receive packets without interferences in this state.
LISTEN: A node enters this state when the counter of this node counts to 0. Nodes in this state must keep silence for a period of time.
IDLE: This state is used for initialization and error handling. When an error occurs during ACTIVE or LISTEN state, the node moves to this state to restart.
Each node using RMP may transmit and receive packets for two time slots (in ACTIVE2 and ACTIVE1 states) and is then forced to suspend the transmission for one time slot (in IDLE
state). Each node in this chain contains a simple counter, counting 2, 1, 0, iteratively. That is, this simple counter counts from 2 to 1, 1 to 0, and 0 to 2 again. A node is allowed to transmit or receive a packet only if its counter counts to 2 or 1. Specifically, when a counter counts to 2, the corresponding node can transmit packets to and receive packets from the preceding node in the chain. When it counts to 1, the node can transmit packets to and receive packets from the subsequent node in the chain. After having the medium for a specified amount of time, the counter counts to 0. During this period, the node does nothing but listens.
The RMP initiates with the first node sending an activated packet to the last node in the chain network. The function of the activated packet is to awaken every node in the chain. At the beginning of the chain operation, each node is in IDLE state. A node (except the first node) is activated by an RTS frame generated by its preceding node in order to deliver the activated packet. After that it triggers its counter starting with the value of 2. This counter counts in sequence of 2-1-0 iteratively. A node has right to transmit or receive packets when its counter is not equal to 0. If a node is in LISTEN state, it just keeps silence. With the initiation of the first node, the RMP does not need a centralized control and can achieve distributed operation and synchronization. After the awakening phase of RMP, two nodes with a spatial-reuse distance [14] of three hops can transmit simultaneously without interfering with each other.
Note that 2/3 nodes in this topology will be ACTIVE at the same time to accomplish the maximum network throughput. The communication process of RMP in different time interval is shown in Fig. 5 (b).
The 802.11 DCF uses a 4-way distributed handshake mechanism to resolve contention
receiving packets, node N + 3 could be ACTIVE without interference from node N + 1 because node N + 1 and node N + 3 are using the same type of mesh routers. The interference caused by node N + 1 will not be sensed by node N + 3 because transmission and reception of these two nodes are in two non-overlapping channels. Therefore, we use an alternative radio pattern and an efficient distributed scheduling scheme to achieve the maximum spatial reuse.
Problems caused by CSMA/CA, including the hidden terminal problem, exposed terminal problem and binary exponential backoff problem, which result in severe transmission problems in wireless multi-hop networks could also be resolved by using RMP. In summary, a node using RMP can achieve the optimal utilization of 2/3 under spatial reuse by resolving the interference problem. The detail operation of the RMP is summarized in Fig. 7.
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