Study on multi-channel mesh deterministic access for wireless mesh LAN
© Lee et al.; licensee Springer. 2012
Received: 12 June 2012
Accepted: 25 September 2012
Published: 16 October 2012
IEEE802.11s draft proposes a new medium access control (MAC) function-mesh deterministic access (MDA), which is mainly used for single-channel wireless mesh local area network (LAN). In single-channel environment, collisions between control packets and data packets may occur very often. In order to provide higher performance and network capacity for wireless mesh LAN, this article develops an algorithm for MDA to work well on multi-channel wireless mesh LAN. To reduce the hardware requirements in design, a mesh point (MP) only equips single transceiver to support multi-channel environment. To completely avoid the collision between control packets and data packets, the interval of the meshed delivery traffic indication message is first divided into contention period and data transmission period. We newly define a neighbor MP status table for MPs to support multi-channel environment. The mechanism of reserving MDA opportunity (MDAOP) adopts the four-way-handshaking mode to reduce hidden node problem; we propose channel load first random fit and multi-channel best fit mechanisms to select MDAOP. We also propose a multi-channel MDA (MMDA) algorithm to improve the overall performance of wireless mesh LAN in multi-channel environment. The theoretical analysis gives the upper limit of the throughput for MMDA. The simulation experiments clearly show the results in multi-channel wireless mesh LAN environment that MMDA performs better than the enhanced distributed channel access in throughput, average waiting time, and packet drop ratio both in the saturated and non-saturated mode.
With the popularity of wireless local area network (LAN) , it is inevitable to increase the network coverage. As outdoor wiring is not easy, the multi-hop technology of wireless mesh network becomes more important. IEEE802.11s working group aims to build the wireless mesh network standard. IEEE802.11s draft provides a distributed network environment, so that each node can do self-configuration with the surrounding environment and does not need a service provider. In IEEE802.11s draft, the medium access control (MAC) protocol has great influence for wireless LAN performance and it follows the enhanced distributed channel access (EDCA) of the original IEEE802.11e, containing congestion control, power saving, synchronization[6, 7], and beacon collision avoidance. In addition, IEEE802.11s draft itself also defines a new MAC function-mesh deterministic access (MDA) to improve the performance of wireless mesh LAN.
In the recent IEEE802.11s researches, most literatures focus on the routing and gateway selection. In the routing area, IEEE802.11s draft proposes a hybrid wireless mesh protocol to provide on-demand and proactive routing modes[9–11]. The VoIP performances of several different routing mechanisms in wireless mesh LAN’s are compared. Some routing methods have also been proposed, like the multi-metric ad-hoc on-demand distance vector routing methods, the mechanism of supporting mobility path management, the combined routing and traffic shaping decisions to enhance the efficiency of the distributed coordination function (DCF). Neishaboori and Kesidis use the link/load-sensitive metric for routing, while Lee et al. propose a new routing protocol to select high throughput paths based on channel diversity information and reduce the broadcast overhead of control messages. In the gateway selection, a gateway selection mechanism is proposed to reduce the occurrence of broadcast loop by connecting the external network, while Ashraf et al. combine the gateway load, route interference, and route quality as the basis for selecting the gateway.
According to the studies[20, 21], using multi-channel architecture is more effective than single-channel architecture to enhance performance and reduce collision probability in IEEE802.11 network, but the researches for the IEEE802.11s MAC enhancement are relatively fewer and mainly discuss on the single-channel MAC protocol design. For example, in order to improve the utilization of transmission media, Nandiraju et. al. propose a queue management scheme for multi-hop networks to increase the fairness of multi-hop flows using available buffer. Ranjitkar and Ko propose an aggressive block acknowledge (ACK) scheme to improve the performance of 802.11s mesh network. Vishnevsky et al. make beacons responsible to support MDA for delay-sensitive multimedia applications. Based on MDA, Chen and Emeott develop a scheduled mesh access (SMA) mechanism, which has better anti-interference ability than EDCA. Finally, this article proposes a multi-channel single-transceiver MAC protocol called multi-channel MDA (MMDA) algorithm to further enhance the overall performance of wireless mesh LAN.
This article is organized as follows. The next section is to describe the proposed MMDA algorithm. “MMDA analysis in throughput” section describes the MMDA throughput analysis and “Simulation experiments” section shows the simulation results. Finally, the article ends with some conclusions.
Description of the proposed MMDA algorithm
Mesh DTIM interval architecture
In order to make MPs achieve timing synchronization in wireless mesh LAN, we use the IEEE802.11 ad-hoc timing synchronization mechanism, because MMDA algorithm uses the new defined mesh DTIM interval architecture. An MP sends beacon in accordance with the principles of DCF at the beginning of each beacon interval; the other MPs will cease sending beacon if any MP sends beacon successfully; then the MPs will take the timestamp value in beacon. This article also requires that each MP must build a neighbor MP status table (NMST) to make sure that MPs know their neighbor MPs’ status and the channel state information, including MP ID (identification), the channel ID currently used by MP, offset from MDAOP’s starting position to the beginning of subinterval, duration of MDAOP, periodicity (i.e., the number of MDAOPs in DTP). For example, if periodicity is equal to 4, it means that DTP are divided into four subintervals, while the four MDAOPs are located in the same distance from the beginning of these four subintervals. We also need to modify MDAOP reservation element, because the MDAOP reservation element of the original MDA control packet is designed for single-channel environment. Therefore, it is necessary to add a new Channel ID field into MDAOP reservation element to support the operation of multi-channel environment.
Four-way-handshaking mechanism for MMDA algorithm
NMST records the corresponding information of MDAOPs in Figure 4
If an MP wants to send data to a destination MP, the steps of MMDA algorithm can be summarized as follows:
Step1: To check NMST to find available durations to satisfy the required size of MDAOP in every channel.
Step2: To select the position of these durations to establish MDAOP.
Step3: To start four-way-handshaking mechanism in CP.
Step4: To start transmission in the MDAOP on the selected channel in DTP after the completion of four-way-handshaking mechanism.
Step5: To use the established MDAOP to periodically send data to the destination MP.
MDAOP selection mechanism for MMDA algorithm
An MP is called non-MDA-MP (NMP) if it does not support MDA mechanism; conversely an MP is called MMP if it supports MDA mechanism. An NMP may occupy other MP’s MDAOP, because it arbitrarily selects duration on the channel and does not know the distribution of MDAOPs on the channel. An MMP must wait until NMP finishing transmission; therefore, an MP can only transmit data in the remaining MDAOP duration, which degrades the throughput of MP. Furthermore, the MP’s MDAOP may fully be occupied by NMP in the worst case and make the MP be unable to transmit any data in its own MDAOP. In order to avoid the aforementioned situation, NMP also needs NMST and listens the control packets on CH1 in CP, so NMP knows the distribution of MDAOPs and obtains the channel state information. However, NMP still has to use the proposed four-way-handshaking mechanism to obtain the time slots on CH1 in CP, and then NMP can use the time slots to transmit data in DTP.
By using NMST, NMP can avoid selecting other MMP’s MDAOP, but MMP still records the information of the received NMP’s control packets into its NMST, including NMP’s channel ID, offset, and duration, but the MDA-support field is marked as 0, where duration here means the slot number used for NMP to transmit data. The information will be removed from NMST after the end of mesh DTIM interval, which means NMP has released the time slots.
When a source MMP wants to create MDAOP to send data to its target MMP, it first gets the channel state information according to its own NMST and avoids selecting the MDAOPs used by other MPs. Different MDAOP selection mechanism may affect the performance of wireless mesh LAN. Cicconetti et al. propose two MDAOP selection mechanism: random fit and best fit, while these two mechanisms only apply to single-channel network structure but not multi-channel network structure. We want to improve these two mechanisms to work on multi-channel wireless mesh LAN; therefore, MMDA algorithm also have two MDAOP selection mechanisms: channel load first random fit (CLFRF) and multi-channel best fit (MCBF).
MDAOP teardown and relocation mechanisms for MMDA algorithm
The source MMP must release the occupied MDAOP when it completes transmitting data. Before sending teardown packet, the source MMP will enter the backoff mechanism at the beginning of CP. After backoff mechanism finished, the source MMP sends a teardown packet to the destination MMP for releasing the occupied MDAOP. The neighbor MMPs of the source MMP will update NMST and release the occupied time slots of the MDAOP. After receiving the teardown packet, the destination MP needs to forward it to its neighbor MMPs to release the occupied time slots of the MDAOP as well. Because the original teardown packet is designed for single-channel environment, it is not applied to our MMDA algorithm. Therefore, we add the new Channel ID field into the format of the teardown packet to support the operation of multi-channel environment.
When an MMP wants to establish MDAOP, it may find according to NMST that all of the remaining durations on each channel cannot meet the MDAOP size. However, after checking NMST, we will find that some channels’ total remaining time slots can meet the MDAOP, which is caused by serious fragmentation on every channel. To avoid the waste of channel space, we use MDAOP relocation mechanism to deal with. However, to avoid too complex relocation action, resulting in too many MDAOPs to be moved, we hope to move only one MDAOP to solve this problem instead.
First MP will choose the lightest load channel according to NMST; MP knows the distribution of MDAOPs on the channel, and further finds out the MDAOP having enough front and back time slots to meet the MP’s own MDAOP size. MP will send an MDAOP relocation request packet to the owner of the MDAOP in the next CP of mesh DTIM interval, if the MDAOP meet the requirements. The requested MP will reply an MDAOP relocation reply packet if it can perform the MDAOP relocation; otherwise it will reply an MDAOP relocation reject packet if it cannot perform the MDAOP relocation. After finishing the MDAOP relocation, MP may have the sufficient number of time slots to establish MDAOP.
MMDA analysis in throughput
Number of MPs
MP transmission range
In Figures11 and12, the average waiting times of MMDA (CLFRF), MMDA (MCBF), and EDCA increase when traffic load increases, because MP suffers more contentions to transmit data when packet arrival rate increases. For MMDA (CLFRF) and MMDA (MCBF), MP needs to wait until the next mesh DTIM interval, if it cannot complete MMDA four-way-handshaking before the end of CP. Therefore, MMDA (CLFRF) and MMDA (MCBF) need to wait for several mesh DTIM intervals to transmit data packets when traffic load increases, which significantly make the average waiting time increase. Figures13 and14 show the packet drop ratio also increases, because packets will be dropped for time out or reached the retransmission limit when traffic load increases.
This research makes the MDA of IEEE802.11s draft to work well on multi-channel wireless mesh LAN to provide higher performance and network capacity. MMDA algorithm uses four-way-handshaking mechanism to reduce hidden node problems, and mesh DTIM interval is divided into CP and DTP to completely avoid the collision between control packets and data packets. MMDA algorithm is designed with only single transceiver, so it can reduce the hardware resource requirements and design complexity. NMST helps MP to know the distribution of MDAOPs on each channel and makes MP easily to select an available MDAOP. These two MDAOP selection mechanisms, CLFRF and MCBF, make MP to select the location of MDAOP more effectively. Because of these designs, MMDA algorithm works well on multi-channel wireless mesh LAN. The theoretical analysis gives the upper limit of the throughput for MMDA. The simulation experiments clearly show the results in multi-channel wireless mesh LAN environment that the throughput of MMDA is better than EDCA both in saturated and non-saturated modes. In addition, MMDA also has the lower average waiting time and packet drop ratio than EDCA. Obviously, the proposed MMDA algorithm can effectively improve overall performance of multi-channel wireless mesh LAN.
This study was supported in part by the National Science Council (NSC) of Taiwan under Grant no. NSC 99-2221-E-011-119.
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