Bih-Hwang Lee and Hung-Chi Chien Department of Electrical Engineering,
National Taiwan University of Science and Technology, Taipei, Taiwan
Huai-Kuei Wu
Department of Information Networking and System Administration, Ling Tung University,
Taichung, Taiwan
Abstract—IEEE802.11s draft proposes a new medium access control function-mesh deterministic access (MDA), which is mainly used for single-channel wireless mesh local area network (LAN). To provide high performance and network capacity for wireless mesh LAN, this paper develops an algorithm for MDA to work well on multi-channel wireless mesh LAN. To completely avoid the collisions 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 define a neighbor MP status table for MPs to support multi-channel environment. The mechanism for reserving MDA opportunity (MDAOP) uses the four-way-handshaking mode to reduce hidden node problem. We propose a multi-channel MDA (MMDA) algorithm to improve the overall performance of wireless mesh LAN in multi-channel environment.
Keywords: IEEE802.11s, mesh deterministic access (MDA), wireless mesh LAN
I. INTRODUCTION
With the popularity of wireless local area network (LAN), it is inevitable to increase the network bandwidth. As outdoor wiring is not easy, so 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 [1] provides a distributed network environment so that each node can do self- configuration with the surrounding environment and doesn’t need a service provider. In IEEE802.11s draft, the medium access control (MAC) protocol has great influence for wireless LAN performance [2] and it follows the enhanced distributed channel access (EDCA) of the original IEEE802.11e [3], containing congestion control [4], power saving [5], synchronization [6,7] and beacon collision avoidance [8]. 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 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 (HWMP) to provide on-demand and proactive routing modes [9-11]. Some routing methods for wireless mesh LAN’s
performance have been proposed, like the multi-metric (MM) AODV routing methods [12], the mechanism of supporting mobility path management [13], the combined routing and traffic shaping decisions to enhance the efficiency of the distributed coordination function (DCF) [14]. Vishnevsky et.
al. make beacons responsible to support MDA for delay-sensitive multimedia applications [15]. Based on MDA, Chen and Emeott develop a scheduled mesh access (SMA) mechanism, which has better anti-interference ability than EDCA [16]. This paper proposes a multi-channel single-transceiver MAC protocol called multi-channel MDA (MMDA) algorithm to further enhance the overall performance of wireless mesh LAN.
II. THE PROPOSED MMDA ALGORITHM
In the original MDA, a mesh delivery traffic indication message (DTIM) interval is defined as the time between two consecutive beacon frames. According to IEEE802.11s draft, the slot size of the mesh DTIM interval is set to 32µs, in which a mesh point (MP) can reserve MDAOP (MDA opportunity) to transmit data. An MDAOP can be reused in every mesh DTIM interval until an MP finishes transmission and releases it. MDA is established by using two control packets: MDA Setup Request and MDA Setup Reply. Before a source MP transmits data, it sends MDA Setup Request packet to a destination MP for requesting MDAOP setup. If the destination MP can satisfy the request of MDAOP setup, it will reply MDA Setup Reply packet to the source MP, i.e., MDA mechanism is completed and the source MP can periodically use MDAOP to transmit data. However, in IEEE802.11s draft, not every MP in wireless mesh LAN can support MDA mechanism.
In MMDA algorithm, we further split the mesh DTIM interval into contention period (CP) and data transmission period shown as Fig. 1. CP provides MP to use contention mechanism for reserving MDAOP, and DTP lets the successful MP to establish MDAOP at the selected channel to transmit data, which is to make sure that control packets and data packets can be transmitted in different intervals and avoid the collisions between them. Because MMDA algorithm allows an MP to only equip a single transceiver in wireless
Step3: To select the position of these durations to establish MDAOP.
mesh LAN, only a channel (e.g., CH1) is listened by all MPs in CP; then MPs know the channel information in DTP, but CH1 can be used to transmit data in DTP. The maximum size of an MDAOP to be reserved is 4096µs (or 128 slots) in DTP, which can be used by MP periodically at the same location of each DTP.
Step4: To start four-way-handshaking mechanism in CP.
Step5: After the completion of four-way-handshaking mechanism, the transmission is started in the MDAOP on the selected channel.
Step6: To use the established MDAOP to periodically send data to destination MP.
MMDA allows MP to reserve different MDAOPs to transmit data on the same channel or different channel in DTP, which means that MP can use more than two MDAOPs in DTP, but it cannot overlap in time and MP must complete corresponding times of MMDA’s four-way-handshaking mechanisms before the end of CP. Fig. 2 shows an example of multiple MDAOPs used by MPA on different channels in DTP.
Fig. 1 The mesh DTIM interval of MMDA algorithm
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 principle of DCF at the beginning of each beacon interval; the other MPs will stop sending beacon if any MP sends beacon successfully; the MPs
will take the timestamp value in beacon. Fig. 2 MDAOPs used by MPA on different channels in DTP Before introducing MMDA algorithm, in order to make
MPs know their neighbor MPs status and the channel state information. This paper requires that each MP must build a neighbor MP status table (NMST), including the MP identification (ID), 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), MP supported for MDA mechanism or not.
III. MMDA Algorithm Analysis
In this section, we analyze the proposed MMDA algorithm based on the saturated contention in CP. To simplify the complexity of analysis, we only consider the two-hop wireless mesh LAN environment. We assume that there are totally n MPs operated in CP with the saturation mode. W0 is the initial contention window, and pc is the collision probability caused by at least one of n-1 MP transmissions. The transmission probability pt of MP is obtained by Eqs. (1) and (2) [17].
Similarly as the original IEEE802.11 Ad-hoc networks, wireless mesh LAN also has hidden node problem [18]. In order to effectively solve this problem, MMDA algorithm uses four-way-handshaking mechanism instead of two-way- handshaking in the original MDA. In other words, MMDA uses four control packets, including MDA Setup Request, MDA Setup Reply, MDA ACK (Acknowledge) and MDA ADV (Advertisement), to reserve MDAOP, where MDA ACK and MDA ADV are newly defined in this paper. When an MP has data to transmit, it needs to decide the MDAOP size used in DTP to obtain the required bandwidth.
0 respectively, by assuming W0 is a fixed system parameter.
Therefore, the probabilities of successful transmission and idle channel can also be considered as a function of n and expressed as Eqs. (3) and (4), respectively. Similarly, we consider the other parameters as the functions of n later in this paper. The pcoll represents the collision probability caused by at least two MPs transmission.
MMDA algorithm is described as follows:
Step1: MP wants to send data to destination MP.
Step2: To check NMST to find available durations to satisfy the required size of MDAOP in every channel.
Table I. Simulation parameters
MP transmission range 60 meters
Channel capacity 1 Mbps
dot11MeshRetryTimeout 500 ms
dot11MeshMaxRetries 6
Simulation time 150 sec
( ) (1 ( ))n
i t
p n = −p n (4)
MP can know which duration is used in DTP according to the content of NMST, and then MP can reserve MDAOP by completing MMDA four-way-handshaking mechanism. Let us denote Tcp to be the duration of CP, which obviously affects the number of successful contentions in CP. Let us define Nmin
as the minimum value of n and Na; then, the critical duration of CP Tcp_c(n) can be obtained by Eq. (5). Hence, the available MDAOPs will be completely reserved by MPs within CP, if the Tcp is longer than Tcp_c. Let us define Tm as the mth sub-critical duration of CP that m MPs can successfully reserve MDAOPs, which can be obtained by Eq. (6). Let us define G(n) as the number of MPs that can successfully reserve MDAOPs in a CP, which can be obtained by Eq. (7).
Finally, the throughput of MPs, S(n), and the average throughput of each MP, s(n), can be obtained by Eqs. (8) and (9), respectively.
Figs. 3 to 5 show the results for CBR traffic in non-saturated mode, in terms of average throughput, average waiting time and packet drop ratio, respectively. Fig. 3 shows that both MMDA (CLFRF) and MMDA (MCBF) perform better than EDCA in average throughput. EDCA has poor performance because EDCA needs to contend in both CP and DTP, and it can just reserve TXOP and causes more collisions and increases network overhead. In Fig. 4, when traffic load increases, the average waiting times of MMDA (CLFRF), MMDA (MCBF) and EDCA also increase, because MPs need to do more contentions as packet arrival rate increase. For MMDA (CLFRF) and MMDA (MCBF), if MP cannot complete MMDA four-way-handshaking before the end of CP, it needs to wait until the next mesh DTIM interval. Therefore, when traffic load increases, the average waiting time increases significantly, because MMDA (CLFRF) and MMDA (MCBF) need to wait several mesh DTIM intervals to transmit data packets. Fig. 5 shows the packet drop ratio increases as traffic load increases, because the packets are dropped for time out or reached the retransmission limit.
min
This paper 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 adopts four-way-handshaking mechanism to reduce hidden node problems. The mesh DTIM interval is divided into contention period and data transmission period 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. Because of those designs, MMDA algorithm works well on multi-channel wireless mesh LAN.
The simulation results clearly show that in multi-channel wireless mesh LAN environment, the throughput of MMDA is better than EDCA both in saturated mode. In addition, MMDA also has the lower average waiting time and packet drop ratio than EDCA. Therefore the proposed MMDA algorithm can effectively improve overall performance of multi-channel wireless mesh LAN.
IV. SIMULATION RESULTS
We consider a wireless mesh LAN has random topology with total 32 MPs. We assume that all MPs are fixed, regardless of background noise and do not consider propagation delay. The other simulation parameters are listed in Table 1.
We consider multi-hop environment at non-saturated mode.
The packet arrival distribution of non-saturated mode is assumed to be Poisson distribution. In the constant bit rate (CBR) traffic class, the packet size is 512 bytes. The variable bit rate (VBR) traffic class is exponential distribution, so we set the average, maximum and minimum packet sizes are 256, 512, and 64 bytes, respectively. We compare the performance that MMDA adopts CLFRF mechanism, known as MMDA (CLFRF), MMDA adopts MCBF mechanism, known as the MMDA (MCBF) and EDCA performance. The mesh DTIM interval is set to 30ms, and the length of CP is 6ms. Traffic type is set to CBR and MDAOP duration is fixed to 128 slots.
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