Scheduled Mesh Access Mechanism for an IEEE 802.11 Mesh Network

Ye Chen Motorola Labs,

1301 E. Algonquin Rd., Schaumburg, IL 60196 Ye.Chen@motorola.com

Steve Emeott Motorola Labs,

1301 E. Algonquin Rd., Schaumburg, IL 60196 Steve.Emeott@motorola.com

Abstract— One of the key issues for an IEEE 802.11-based mesh network is the effectiveness of its distributed listen-before-transmit media access control approach. In this paper we proposed one technique, called Scheduled Mesh Access (SMA), for reducing the impact of interference on a mesh network. The SMA mechanism reserves time on the media for future transmissions, and then accesses the media at reserved times using the 802.11 channel access procedures. We study the ability of mesh network employing SMA to avoid non-mesh interference and schedule around self interference. Results indicate that SMA is more robust to interference than the Enhanced Distributed Channel Access (EDCA) function (defined by in the IEEE 802.11e amendment to the IEEE 802.11 standard), which does not includes a mesh-aware scheduling mechanism.

Keywords-, 802.11, mesh, hidden terminals, self interference, Scheduled Mesh Access

I. INTRODUCTION The success of IEEE 802.11-based mesh networks, such as

metropolitan Wi-Fi (metro Wi-Fi) systems being deployed world wide, heavily depends upon the coverage and capacity that IEEE 802.11 technology can deliver. The key challenge for such system is to develop an efficient Media Access Control (MAC) solution, which specifies how every node in the mesh network shares the channel. The MAC mechanisms specified in IEEE 802.11 standard, which was updated to support Quality of Service (QoS) by the IEEE 802.11e amendment [1], include an enhanced distributed channel access (EDCA) mechanism and Hybrid Coordination Function Controlled Channel Access (HCCA). The former is a natural fit for a mesh system due to its inherent distributed access nature while the latter requires a centralized control mechanism and is difficult to realize in a distributed mesh system. Although EDCA is considered a natural fit for a mesh, the major concern is the effectiveness of EDCA to withstand and overcome self-interference such as hidden terminals, which are fairly common phenomena in an IEEE 802.11-baesd mesh system operating on a single channel.

Already aware of such concern, a solution called mesh deterministic access (MDA) was proposed to the IEEE 802.11s task group in [2] and was included in the first 802.11s draft standard. The MDA mechanism was designed to combat the self-interference problem between mesh nodes via a reservation-notification mechanism combined with deterministic channel access. MDA is expected to be effective for well-behaved traffic such as constant bit rate (CBR) voice. However, the deterministic access scheme employed by MDA can be complex to implement in an environment subject to channel errors, transmit rate variations and interference from non-MDA stations. To resolve some these implementation

concerns, we propose an enhancement called Scheduled Mesh Access (SMA), which replaces the deterministic access scheme with a more flexible contention based mechanism. In this paper, we describe the proposed SMA scheme and evaluate the effectiveness of SMA in combating self and non-mesh interference utilizing a simulation model. The results of this study not only benchmark the performance of SMA, but also provide a reference against which future scheduled or deterministic access schemes may be compared.

The rest of the paper is organized as follows: first we review the existing literature investigating the performance of EDCA in a mesh network. Then we describe the MDA mechanism specified in [2] and present a few issues. Next, we describe our SMA channel access enhancements followed by the implementation prototype for the SMA mechanism. Afterwards, we present a simulation model consisting of both a mesh network and an infrastructure access point with overlapping coverage. Simulation results for the use case scenario are presented. Finally, we conclude the paper and summarize the investigation.

II. EXISTING LITERATURE A great deal of research has been conducted to evaluate the

performance of exiting MAC solutions for IEEE 802.11 based systems, such as EDCA [3] [4] and HCCA [5] [6]. Most of the work focuses on an 802.11 system organized around an infrastructure Basic Service Set (BSS), which consists of an access point (AP) and a number of stations (STAs). For example, in [4], the authors analyze the performance of EDCA and study the impact of different countdown procedure used by EDCA and the legacy DCF, as well as the retransmission limit. Most of the literature evaluating HCCA focuses on the scheduling technique [5] and target at multimedia traffic such voice and video [6].

The performance of EDCA in an ad hoc system is studied in [7-8]. Both papers investigate schemes for adapting the dynamic contention window, but neither studies the impact of non-mesh interferers on performance. HCCA requires a centralized control mechanism, so it is difficult to extend such solution to a distributed mesh system. Very little published information is available about the performance of deterministic access mechanisms that could be deployed in a mesh, nor the impact of interference on these or other mechanisms.

III. MESH DETERMINISTICS ACCESS (MDA)

A. Background The basic idea behind MDA as defined in [2] is to allow

mesh points (MPs) to make reservations for future

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transmissions. The reservation is in a form of duration of time called MDA Transmit Opportunity (MDAOP) occurring periodically. When a reservation is established, both the sending and receiving MP should broadcast the schedule to their neighbors. Each node implementing MDA should scan the channel for advertisements, either in beacons or in special broadcast action frames. Each advertisement identifies times when the mesh point transmitting the advertisement will be busy either as a transmitter or receiver. During the advertised busy times, nodes implementing MDA should not initiate any new transmissions. The advertised busy times remain in effect until a new advertisement is received from the same mesh point providing a new set of busy times. An abstract picture of the signaling frame exchange used to establish an MDA reservation is shown in Figure 1.

Neighbor MP MP A (transmitter) MP B (receiver)

setup request

setup reply

Beacon @ offset 1: MDA advertisement

Beacon @ offset 2: MDA advertisement

Neighbor MP

Beacon @ offset 1: MDA advertisement

Beacon @ offset 2: MDA advertisement

Figure 1. Simple frame exchange of the MDA Signaling

The MDA mechanism only applies to mesh nodes, which may be a mesh access point serving as an AP for subscriber devices or a mesh point that is only used to relay mesh traffic.

B. Issues with Deterministics Access As defined in [2], the deterministic access scheme

employed by MDA has two issues. First, a MP may use a special set of contention parameters, the MDA contention parameters, during an MDAOP. To avoid conflicts with nodes not deploying MDA and to avoid misuse of the MDA contention parameters, these parameters may not be used outside an MDAOP. Second, because MDA contention parameters are not used outside an MDAOP and because nodes must avoid transmitting during a neighbor’s MDAOP, all MDA transmissions must be completed within an allocated MDAOP or deferred until node’s next MDAOP.

The issue with limiting MDA transmissions to occur during an MDAOP can be illustrated with a figure (see Figure 2). In Figure 2 three MDA transmission attempts are made by an exemplary node during three consecutive MDAOPs.

During the first MDAOP, MDAOP1, we show the channel being busy, perhaps due to traffic from non-MDA devices (e.g. non-mesh STAs). Because the channel is busy, the MDAOP owner cannot obtain the channel until Dstart1 after the scheduled start time due. Once the channel becomes free, the remaining time (between Tstart1 and the scheduled end time of MDAOP1) is insufficient to complete a frame exchange. Therefore the MDAOP owner cannot start any transmission for MDAOP1, and the frame scheduled to be transmitted must wait for the next MDAOP. If the frame is delay sensitive (e.g. voice), it might have to be discarded before the next MDAOP occurs.

The second MDAOP, MDAOP2, shows a normal case. The transmission TX2 starts with Dstart2 delay but the remaining

time is enough to complete a full frame exchange. During MDAOP3, the channel error results in an aborted frame exchange. If the remaining time of MDAOP3 is not enough to accommodate the retransmission of TX3, the node has to wait until its next MDAOP.

Tstart1

MDAOP1

Tx2 Tx 3

Channel error

Dstart1

MDAOP Schedule

Channel busy Time

Tstart2

MDAOP2 MDAOP3

Dstart2

Figure 2. An example of MDA transmission within MDAOP.

The issue with utilizing different contention parameters within/outside the MDAOP can be explained as follows. Since the special MDA contention parameters are used within an MDAOP, allowing a node to contend for the channel after an MDAOP might create a sudden change in contention parameters. Since the carrier sense channel access and backoff scheme employed by 802.11 is a continuous process, it would be complicated to implement such sudden change in some low cost IEEE 802.11 chipsets, which may require a hardware reset to change the contention parameters for one of its QoS enabled channel access buffers. For example, if an MP is performing backoff using the special contention parameters before the end of the MDAOP, the MP would have to change the backoff contention parameters as soon as it is outside MDAOP

IV. SCHEDULED MESH ACCESS (SMA) To address the issues identified for the MDA mechanism,

we define an enhanced mesh channel access method called scheduled mesh access (SMA) for mesh devices to access the wireless channel.

A. Overview The SMA mechanism combines a channel access scheme

with the MDA reservation notification mechanism. In fact, the SMA mechanism studied in this paper utilizes the same signaling procedure as the MDA mechanism, as shown in Figure 1. We will call the time periods reserved by a node implementing the proposed mechanism SMA reservations.

The channel access scheme, however, is different from MDA in three ways. First, the principle use of a SMA reservation is to distribute the transmissions by mesh points implementing SMA across time to reduce self interference. Second, in the event that a node is unable to transmit during its allotted SMA reservation, it should continue accessing the channel until it can do so with one exception. Specifically, nodes implementing SMA must not transmit during any part of the SMA reservations advertised by their peers, either by disabling queues or some other equally effective means. Third, in order to simplify implementation and ensure fairness, the contention parameters used by nodes to transmit SMA data are based upon the access category of the frame and not the access mechanism used.

To better illustrate the channel access rules of the SMA mechanism, an example similar to the MDA channel access scenario depicted in Figure 2 is shown in Figure 3. Again, we show three consecutive SMA reservations. During the first

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SMA reservation, the MP starts transmission Dstart1 after the scheduled start time. Although the remaining time is not enough to finish the transmission within SMA reservation, the transmissions is allowed to continue the channel access outside the SMA reservation if the channel is available. For the third SMA reservation, if the retransmission caused by the channel error cannot be finished within SMA reservation 3, the MP is allowed to continue to access the channel and finish the retransmission.

As mentioned above, one limitation placed on SMA stations is that it is mandatory for each MP to honor every other MP’s SMA reservation. Since the MP uses the same contention parameters to access the channel within/outside SMA reservation, there is no issue when the channel access transitions inside and outside the SMA reservation.

SMA reservation Schedule

Tx 1Channel busy Tx2 Tx 3

SMA RSV 1

SMA RSV 2

SMA RSV 3

Channel error Time

Dstart1 Dstart2

Tstart1 Tstart2 Figure 3. An example of the SMA transmission.

The SMA channel access enhancements [9] introduced in this paper have been presented to the IEEE 802.11s task group for inclusion in the draft standard. A subset of the results presented in Section VI has been presented [10] [11].

B. Implementation of the SMA Mechanism In this section, we discuss an approach for implementing

the SMA mechanism. A block diagram of a SMA-enabled transmitter is shown in Figure 4. The block diagram consists of 4 blocks. Two of the blocks, namely the scheduled access buffer and the QoS-enabled channel access function, take data received from upper layers and buffer the data until its ready to be transmitted. The other two blocks, namely the scheduler and the contention manager, control when data in the buffer is ready to be released for transmission and when the channel access function is enabled. A detailed description of each block is provided next.

Scheduled Access Buffer

Release Buffered Frames

Disable Queues with non-SMA

traffic

QoS-Enabled Channel Access

Function

Resume/suspend channel access

Scheduler Contention Manager

To Transmitter

From upper layer or relay

frame Figure 4. Block diagram of the SMA mechanism (data plane).

The “scheduled access buffer” receives data frames from upper layers or relays frames and releases these frames to the QoS enabled channel access function at scheduled times. The buffer holds data frames associated with a reservation until the scheduler sends a control signal indicating that a start of a reserved period has arrived. Upon receiving the control signal, the buffer releases frames associated with the reservation to the QoS enabled channel access function for transmission. Frames not associated with a reservation may be forwarded directly to the channel access function without waiting for a schedule, as

these frames must contend for the media during periods when no reservations have been made

The “scheduler” is responsible for issuing control signals indicating when the scheduled access buffer may release frames to the channel access function and indicating when the channel access function should suspend transmitting frames not associated with a reservation. The scheduler keeps track of reservations made by the mesh point with other devices, and sends a control signal to the scheduled access buffer indicating when these reservations begin. When the channel access function used EDCA to arbitrate internal contention, the scheduler should also send a control signal to the channel access function to disable queues that are not affiliated with access categories for which the reservation has been made. At the beginning of an SMA reservation, the “scheduler” block will instruct the “QoS-enabled channel access function” to disable the channel access for the non-SMA traffic to eliminate the internal contention for the SMA traffic. As a result, the SMA traffic is granted the higher priority over non-SMA traffic during its own reservation. At the end of a reservation, the “scheduler” block will instruct the “the “QoS-enabled channel access function” to enable the channel access for the non-SMA traffic. The other main function of the “scheduler” block is to map the SMA reservation with traffic flow as well as scheduling the SMA reservation based on the neighbor MPs’ SMA reservation information.

The “contention manager” block is used to control the contention of the MP based on the neighbor MPs’ SMA reservations. At the beginning of a neighbor MP’s SMA reservation, the contention manager will instruct the “QoS-enabled channel access function” to suspend the external contention (channel access) for all traffic and consider the channel unavailable. In the end of a neighbor MP’s SMA reservation, the contention manager will allow the “QoS-enabled channel access function” to resume the external contention and back to its normal operation.

The “QoS-enabled channel access function” block implements the channel sensing and backoff functions that govern when the mesh point may transmit data. It may also implement internal contention to arbitrate in cases where multiple frames from different access categories are buffered to be transmitted. For example, the “QoS-enabled channel access function” could be based upon the EDCA defined in the 802.11-2007 standard [1]. The extensions needed to support SMA include control lines that take inputs from both the scheduler and the contention manager to determine when its appropriate to disable contention because another neighboring MP has a reservation or when to disable one or more access categories because one access category hold traffic associated with a reserved times while others do not.

V. SMA USE CASE AND SIMULATION MODEL In this section, we present the simulation model used in our

research.

A. Topology of Mesh Use Case Scenario The topology of the use case under study is illustrated in

Figure 5. The exemplary mesh network consists of 1 mesh

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point portal (AP2) in the center with 2 mesh APs on both the left and right side of the portal. The maximum hop count for a STA is 3 counting the access hop. This is by no means meant to represent a recommended or preferred topology, but is rather just an exemplary configuration. All STAs are configured to communicate over the same channel in the 5.8 GHz band with either a mesh AP or mesh portal. Each voice station sets up a two-way voice call with a voice gateway connected to the mesh portal via an Ethernet link. The mesh is large enough to include the following hidden terminals pairs among mesh APs: AP6 is hidden from AP2, AP1 is hidden from AP7, and AP2 is hidden from AP3. All voice STAs circle around APs to vary location with trajectories depicted by the blue, green, and red curves in Figure 5.

In addition to the baseline mesh system, a non-mesh infrastructure BSS serving three stations is included in the scenario as an extra interference source. The BSS is highlighted by the dotted circle and added to the neighborhood of AP6, AP1, and portal AP2. It is directly connected to an Ethernet switch via wired link. Since the infrastructure BSS is not part of the mesh, AP6 can not hop through the new AP to the distribution system. The purpose of including the BSS in the simulation model is to investigate the impact of interference generated by the infrastructure BSS on the mesh.

Mesh AP2 /Mesh Portal

Mesh AP7

Mesh AP3

Mesh AP1

Mesh AP6

Wired DS

Infrastructure BSS on Same Channel

Figure 5. The topology of the indoor 802.11-based mesh system.

B. Model Implementation Details We implemented the SMA mechanism described in Section

IV in the OPENT simulator. We also implemented the 802.11e EDCA mechanism for purposes of comparison. We already pointed out the issues related to the deterministic-based mesh access solution such as MDA in Section III. Due to the predictably long access and retransmission delays that occur when forcing all transmissions to take place within a fixed time window (MDAOP) in an environment subject to channel errors, transmit rate variations and interference, we did not implement MDA. Instead, we provide results illustrating the delays involved in contending for a channel during an access window as required by the MDA mechanism.

C. Simulation Settings To evaluate the performance of a mesh system employing

both EDCA and SMA, we developed a comprehensive simulation model of the 802.11 protocol, built upon OPNETTM simulation platform and including QoS and mesh functionality. Some of the key simulation parameters are listed as follows: the simulated 802.11 mesh is an 802.11a system operating at

5180 MHz assuming an indoor propagation model. Both mesh and access link are set to a fixed 6 Mbps. The vocoder located at each station is CBR 8 Kbps with 20 ms framing period. Fixed routes are established between mesh APs. The contention parameters (Arbitration Inter-Frame Space (AIFS)/CWmin/CWmax) are (DIFS,3,7) for the AP and (DIFS+aSlot,7,15) for the STA.

D. Interference Analysis The mesh network experiences two types of interference.

The first type is self interference from devices that are either part of the mesh, namely mesh AP, or non-mesh devices associated with these devices. The second type is caused by overlap between the mesh and other networks, such as the infrastructure BSS depicted in Figure 5. The hidden terminals pairs listed in the previous section are the major sources of self interference in the mesh. The infrastructure BSS and the STAs associated with the BSS are, at least from the perspective of the mesh, the major source of uncoordinated.

When a mesh employs SMA, self interference from other SMA devices can be minimized. However, the SMA mesh must still cope with non-SMA and non-mesh interference, such as the stations associated with the mesh AP and the uncoordinated infrastructure BSS. Since SMA is only implemented at the mesh APs, all the access traffic including both uplink and downlink access traffic from stations should be considered as interference for the mesh traffic using SMA. The infrastructure BSS also provides extra non-SMA traffic, which is added to the channel using the EDCA MAC solution.

VI. RESULTS The simulation results include two sets of results: the first

set shows the impact of traffic from non-MDA devices on mesh points attempting to access the channel according to a schedule. The second set is a comparison between an exemplary mesh network using SMA combined with EDCA versus EDCA alone. This exemplary scenario is presented with and without infrastructure BSS interference present. In the following context, a mesh system using EDCA with SMA disabled is referred as an EDCA-mesh and a mesh system with SMA enabled is referred as an SMA-mesh.

A. Channel Access Delays In this section, we present results that illustrate the channel

access delay that can result from traffic on the channel due to devices that do not support a reservation mechanism (e.g. non-SMA or non-MDA devices).

Figure 6 shows the percentage of transmissions in the exemplary mesh network with delayed start time (Dstart in Figure 2 and Figure 3) more than 90 µs and 340 µs at AP2 for different loading. In the study, the duration of the SMA reservation (similar to MDAOP) is calculated by the time required to complete a voice frame exchange (voice frame+ACK) using 6 Mbps rate plus extra 10 slots (90 µs). If the delayed start time is greater than 90 µs, an MDA-enabled MP cannot finish the transmission within the MDAOP, assuming a 340 µs SMA (or MDA) reservation and a 250 µs frame exchange. When the delayed start time exceeds 340 µs,

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the MP cannot even start within the reservation. If MDA were utilized, a missed MDAOP will cause the MP to give up the current MDAOP and wait for the next MDAOP. Fortunately, the SMA mechanism allows the MP continues to access the channel outside the SMA reservation and avoid deferring to the next reservation (for delay sensitive traffic like voice it might result in a discarded packet). For both cases, the percentage increases as the loading goes up. For 90 µs delay at most loading, the percentage is greater than 30%.

The Percentage of SMA Trasmission with Delayed Start Greater than 90 µs and 340 µs

00.10.20.30.40.50.60.70.80.9

5 6 8 10 12 13 15 16Number of Voice Stations

The

perc

enta

ge

Start Time Delay > 90 µsStart TimeDelay > 340 µs

Figure 6. The delayed start time results at portal (AP2).

Note that if we consider the potential retransmission during the MDAOP due to the channel error, the percentage of the transmission cannot accommodate within an MDAOP will be even greater.

B. Comparison Between SMA-mesh and EDCA-mesh Packet loss rate is one of the key metrics to evaluate the

performance of a MAC solution. In Figure 7, the packet loss rate during the simulation is shown for a system using EDCA and SMA with/without the interference BSS. Note that the voice calls are set up randomly between 5 to 10 seconds. Therefore the meaningful results start after 10 seconds for all the figures in this section.

0 5 10 15 20 25 300

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10

Time(sec

Pa

cket

Los

s R

ate

(%)

Packet Loss Rate

EDCA-meshSMA-meshEDCA-mesh (with interference BSS)SMA-mesh(with interference BSS)

Figure 7. Global packet loss rate comparison.

It is obvious that the packet loss is much lower when SMA is used compared to EDCA. The average packet loss rate after retransmission is over 3% when EDCA is used while it is less than 0.01% when SMA is used for the same system when the

interference BSS is not presented. When the interference BSS is presented, due to the extra non-mesh interference, the packet loss rate for EDCA-mesh increases to almost 7%. We notice that the extra interference has very little impact on SMA-mesh as the packet loss rate almost the same for both cases.

To understand the reason behind the much lower packet loss rate using SMA, the total retransmission attempts occurred at mesh AP 1 is depicted in Figure 8.

0 5 10 15 20 25 300

1000

2000

3000

4000

5000

6000

7000The Number of Retransmission Attempts(sum)at Mesh AP1

Th

e N

umb

er o

f Re

trans

mis

sion

Atte

mp

ts(s

um)

Time(sec

EDCA-meshSMA-meshEDCA-mesh (with interference BSS)SMA-mesh(with interference BSS)

Figure 8. Total retransmission attempts (sum) at mesh AP1.

Note that the retransmission results are shown in the sum mode, which accumulates the number of retransmission attempts over time. When the interference BSS is not presented, for 20 seconds effective transmission time, mesh AP1 suffers about 3500 attempts in the system using EDCA compared to less than 50 attempts in the system using SMA. EDCA is about 70 times more likely to suffer retransmission. The almost straight-line curves suggests constant attempt rate. For example, the retransmission attempt rate for EDCA-mesh is about 175 attempts per second. The less retransmission attempts, the better system capacity and less packet loss. When the interference BSS is presented, the total number of retransmission attempts at AP1 nearly double to almost 7000 attempts. The retransmission attempts of the SMA-enabled mesh slightly increases to over 50.

The retransmission attempt results provide one type of performance comparison, but do not directly explain why SMA-mesh has a lower packet loss rate. Figure 9 shows the total number of collisions observes at Mesh AP1 again using the sum mode. When the interference BSS is not presented, the total number of collisions for 20 seconds transmission is more than 12000 for EDCA compared to less than 2000 collisions when SMA is employed. The presence of interference BSS caused more collision but impacts the SMA-mesh less than the EDCA-mesh. It is straightforward that fewer collisions mean less self-interference and therefore less packet loss.

As described in Section IV, SMA reduces the self-interference and collision via coordinating transmission. The collision and retransmission attempt results shown in Figure 8 and Figure 9 prove that the SMA mechanism indeed reduces the self-interference very effectively.

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In addition to studying the impact of non-mesh interference on a mesh, we can also investigate the impact of the mesh on the performance of the infrastructure BSS in Figure 10.

0 5 10 15 20 25 300

0.5

1

1.5

2

2.5

3x 10

4 Total Number of Collisions Observed at Mesh AP1

Time(sec)

Tot

al N

umbe

r of C

ollis

ions

EDCA-meshSMA-meshEDCA-mesh(with interference BSS)SMA-mesh(with interference BSS)

Figure 9. Total number of collisions observed at mesh AP1.

0 5 10 15 20 25 300

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umbe

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Atte

mpt

s (s

um)

Total Number of Retry Attempts (sum) at Interference AP(AP4)

EDCA-meshSMA-mesh

Figure 10. Total number of retransmission attempts at interference AP (AP4).

Figure 10 illustrates the impact from the mesh on the total number of retransmissions by infrastructure AP (AP4). An infrastructure BSS operating near an SMA-mesh performs better compared to an infrastructure BSS operating near an EDCA-mesh because the SMA mechanism improves the bandwidth efficiency of the mesh, which has a positive impact on the availability of the channel, since both mesh and the infrastructure BSS are operating at the same channel. The reduction of the retransmission attempts at the interference BSS AP demonstrates such an effect.

VII. SUMMARY In this paper we proposed a mesh channel access technique

called Scheduled Mesh Access (SMA). The SMA mechanism is designed to provide a schedule to allow mesh points to avoid self interferences and is designed to work around interference from non-mesh stations. We conducted and presented results from an interference study for an exemplary 802.11-based

mesh system operating at a single channel and compared two cases with and without SMA enabled. The results demonstrate that SMA combined with EDCA delivers better performance than EDCA alone at a reasonable implementation complexity. Whether an infrastructure BSS network is included in the simulation model, the SMA solution yields much lower packet loss rate, fewer retransmission attempts, and total number of collisions than an EDCA only solution. The improvement of using SMA over EDCA is due to the coordinated transmission under SMA, which reduces the collision probability and contention among the devices in the system. The simulation results also suggest that the mesh system employing SMA helps improve the performance of the non-mesh BSS since the SMA mechanism improves the bandwidth efficiency within mesh, which has the positive impact on the non-mesh interference BSS operating at the same channel. The conclusion we draw from the interference study is that SMA is an effective solution to combat interference in a single channel mesh, and can deliver better performance, depending upon the topology of the mesh, compared to the conventional 802.11e EDCA solution. The results of this study not only benchmark the performance of SMA, but also provide a reference against which future scheduled or deterministic access schemes may be compared.

REFERENCES [1] ANSI/IEEE Std 802.11: “Wireless LAN Medium Access Control

(MAC) and Physical Layer (PHY) Specifications,” 2007 [2] IEEE 802 11-06/0328r0, “Joint SEE-Mesh/Wi-Mesh Proposal to 802.11

TGs”, Feburary 27th, 2006. [3] Zhen-ning Kong, et.al. “Performance analysis of IEEE 802.11e

contention-based channel access”,EEE Journal on Selected Areas in Communications, Volume 22, Issue 10, Dec. 2004 Page(s):2095 – 2106

[4] Zhifeng Tao, et.al, “Throughput and delay analysis for the IEEE 802.11e enhanced distributed channel access,” IEEE Transactions on communications, Volume 54, Issue 4, April 2006

[5] Bin Muhamad Noh, et.al,”A Packet Scheduling Scheme for Audio-Video Transmission with IEEE 802.11e HCCA and its Application-Level QoS Assessment”, 2006 Asia-Pacific Conference on Communications, Aug. 2006 Page(s):1 – 5

[6] Van der Schaar, M, et.al, “Optimized scalable video streaming over IEEE 802.11 a/e HCCA wireless networks under delay constraints.,” Mobile Computing, IEEE Transactions on, June 2006

[7] Gannoune, L, et.al. “Dynamic tuning of the contention window minimum (CW/sub min/) for enhanced service differentiation in IEEE 802.11 wireless ad-hoc networks,” Personal, Indoor and Mobile Radio Communications, 15th IEEE International Symposium on, Sept. 2004

[8] Hao Zhu, et.al. “ EDCF-DM: a novel enhanced distributed coordination function for wireless ad hoc networks,” Communications, 2004 IEEE International Conference on , Volume 7, 20-24 June 2004.

[9] Ye Chen, Steve Emeott, Dee Denteneer, and Guido R. Hiertz, “MDA Channel Access and Retransmission Behavior Comment Resolution”, IEEE submission 11-07/2536r0, September 18th, 2007.

[10] Ye Chen, Steve Emeott, “MDA Simulation Study: Robustness to non-MDA Interferers’, IEEE submission 11-07/356r0, March 5th, 2007. Available at https://mentor.ieee.org/802.11/documents

[11] Ye Chen, Steve Emeott, Dee Denteneer, and Guido R. Hiertz, “MDA Simulation Study: MDAOP Stretching and Other Concerns”, IEEE submission 11-07/2537r0, September 18th, 2007.

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2008 proceedings.

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