Achieving Transmission Fairness in Distributed Medium Access Wireless Mesh Networks: Design Challenges, Guidelines and Future Directions

International Journal of Wireless & Mobile Networks (IJWMN) Vol. 5, No. 3, June 2013
DOI : 10.5121/ijwmn.2013.5301 1
Achieving Transmission Fairness in Distributed
Medium Access Wireless Mesh Networks: Design
Challenges, Guidelines and Future Directions
Salitha Priyanka Undugodage and Nurul I Sarkar
School of Computing and Mathematical Sciences
Auckland University of Technology, Auckland, New Zealand
Nurul.sarkar@aut.ac.nz
ABSTRACT
Wireless mesh networking gained an international interest over the years as a result to high recognition in
the wireless industry as a cost effective, scalable, wider coverage and capacity capable wireless technology.
The contention based distributed medium access in wireless networks has advanced not only in supporting
the quality of multimedia but also achieving high throughput and to minimize packet delay overheads in
legacy systems. Unfortunately, the impact of such enhancement has not been fully justified with mesh
network environments yet. The medium access frames are required to be contended over multi-hops to
overcome the challenges of improving overall system performance through concurrent transmissions. The
goal of this paper is to discuss the issues and challenges of transmission fairness and the effect of
concurrent transmission on system performance. To mitigate transmission fairness issues, we review
existing open literature on mesh networking and provide guidelines for better system design and
deployment. Finally, we conclude the paper with future research directions. This study may help network
designer and planner to overcome the remaining challenging issues in the design and deployment of WMNs
worldwide.
Keywords
Wireless Mesh Networks (WMN), Frame Aggregation, Block Acknowledgement (BA), Reverse Direction
Grant (RDG), Carrier Sensed Threshold (CST)
1. INTRODUCTION
The Wireless Mesh Network (WMN) is a set of wireless nodes where each node can
communicate directly with one or more peer nodes. WMN has been standardized by IEEE 802.11
Task Group “s” to develop a set of standards for WMNs under the IEEE 802.11s. Further the
IETF (Internet Engineering Task Force) had also setup wireless mesh networking called Mobile
Ad-Hoc Network (MANET) with a separate set of standards. Both MANETs and WMN nodes
exploit the redundancy of connected nodes and have the ability of self-organize, self-discover,
self-heal, and self-configure. However, in real-world applications, MANETs are implemented
with mobile and more power constrained nodes, and the infrastructure is less self-organized. In
contrast, WMNs are typically a collection of more organized stationary nodes and may use
multiple radios for the purpose of wireless mesh backhauling for Wireless Local Area Network
(WLAN) with one radio and the other radio for Access Point (AP) functionality [1]. Although
WMNs could extend the wireless coverage as a cost-effective backhaul solutions it has many

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challenges, especially when increasing the per user data rate of multiple concurrent sessions
between multi-hop mesh nodes in serving as backhaul WLAN technologies. These challenges are
as a result of 802.11’s shared medium access constrains in achieving transmission fairness,
especially in multi-hop networks.
In this paper we address some of the key issues of such constrains and provide guidelines for
network researchers and designer for efficient system design and deployment of such system.
The remainder of this paper is organized as follows. In Section 2 we highlight 802.11 Physical
layer (PHY) and Medium Access Control (MAC) layer standards in WMNs focusing on
distributed medium access protocols. The issues and challenges in designing WMNs are also
discussed. Section 3 presents WMN architecture highlighting the transmission fairness issues in
a multi-hop contention based shared medium access. In Section 4, we discuss MAC
enhancements for multi-hop WMNs medium access efficiency. Section 5 discusses transmission
fairness focusing on optimum concurrent transmission in a mesh network. An amendment to the
shared MAC with a reverse direction MAC frame pull mechanism to optimize concurrent
transmission is also discussed. In Section 6, we present guidelines for WMN design and
deployment and future research directions. Finally, a brief discussion in Section 7 concludes the
paper.
2. WMN DESIGN CHALLENGES IN DISTRIBUTED MEDIUM ACCESS
One of the primary objectives of 802.11s WMN standardization was to define the 802.11 PHY
and MAC layers to create a Wireless Distribution System (DS) which is capable of automating
topology learning and wireless path configuration for self-learning, self-forming and self-healing
wireless paths. The standard defines dynamic and radio-aware path selection mechanism to
delivery of data on both single-hop and multi-hop networks. Any wireless node complying with
these functionalities are said to be wireless mesh capable nodes which forms a WMN or a mesh
cloud. One of the key issues in WMN standardization is the adaptation of legacy distributed
medium access schemes to share the medium which has inherent unfairness in achieving
concurrent transmissions between mesh nodes in a multi-hop mesh network. However, it is
important that WMN standards should address these challenging issues without compromising
the compatibilities of WMNs to continue to evolve as a cost-effective backhauling technology for
WLANs [2] [3] [4].
2.1 Mesh Network PHY and MAC layer Standards
The IEEE 802.11 PHY and MAC Layer standards were first introduced in 1997. Since then
multiple standards had evolved under different IEEE Task Groups as “a” (TGa) and “b” (TGb) in
1999, “g” (TGg) in 2003 and “n” (TGn) 2007. These WLAN standards had evolved with the
number of enhancements into the PHY and MAC layers mainly to improve raw data speed and
propagation range while maintaining backward compatibility with the previous standards.
Consequently, the 802.11g APs are backward compatible in connecting 802.11b Stations (STAs).
Similarly IEEE 802.11n APs are backward compatible in communicating to 802.11a/b/g STAs. A
WLAN operating in multi-mode supporting more than one mode is said to be in “mixed mode”
whereas a WLAN is said to be operating in “Green field” if all STAs only support native highest
performing mode. The most capabilities of Green field operation are compromised when
operating in mixed mode. In theory 802.11s could operate on any 802.11 PHY layer standard

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supporting either mixed mode or green field but it is sensible for all mesh nodes to be deployed
in a same mode (e.g. green field network) for greater performance [5].
Figure 1 shows 802.11 MAC protocol structure. The MAC layer defines the data link between
two mesh nodes and exchanges MAC Service Data Units (MSDUs) packed into MAC Protocol
Data Units (MPDU) and carried over the PHY Protocol Data Unit (PPDU) as per the original
802.11 MAC standards. The main concern observed in a wireless mesh is that the standard
requires every successfully non multicast and broadcast frames received at each mesh node to be
acknowledged causing considerable packet delays in multi-hop communications (a frame needs
to cross multiple hops in reaching the destination) [6] [7].
Figure1: Media access control (MAC) protocol structure.
2.2 WMN MAC Layer Protocol Design Challenges
IEEE 802.11 medium access protocol is based on Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA) to avoid frame collisions in a shared wireless channel. This medium
access layer is similar to IEEE 802.3 wire-line medium access which is based on Carrier Sense
Multiple Access with Collision Detection (CSMA/CD) where medium access coordination
mechanism is to detect rather than avoid collisions only. CSMA/CD is not suitable for wireless
network because network interface cards cannot transmit and listen on the same wireless channel
simultaneously. The Receiver (Rx) must receive the incoming frame fully before the wireless
interface could switch from “receive” mode to “transmit” mode to transmit a frames which
makes CSMA/CA ideal in wireless contention based shared access. The idea of carrier sense is
listen before transmit in assessing nearby node engage in transmission. The Transmitter (Tx) will
refrain from transmission if received energy level at a Tx at any time slots duration higher than a
fixed Carrier Sensed Threshold (CST) to avoid collision [8] [9]. Although this is acceptable in an
AP centric WLAN implementation, it is a major concern in WMNs where exposed mesh nodes
suspend any concurrent transmission to avoid collisions. To overcome this problem it is possible
to separate mesh nodes so that they do not necessarily exposed to each other. However this may
lead to increased collisions at the Rx end if a Transmit mesh node estimates a lower energy level
from another hidden mesh node that could transmit at the same time slot. This is known as
Hidden Station Problem which is a well-known issue in CSMA/CA medium access. The IEEE
has standardized the 802.11 Distributed Coordination Function (DCF) for contention based
medium access minimizing the hidden station problem by either 2-Way handshaking where each

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MAC frame is acknowledged by an acknowledge frame (ACK) or 4-Way handshaking with an
additional contention free medium protection called RTS/CTS handshaking or both by providing
any hidden nodes access the shared channel. In fact, as shown in Figure 2 RTS/CTS is a virtual
carrier sense mechanism to block any attempt to transmit by any exposed nodes for a specified
duration called Network Allocation Vector (NAV) exclusively allocating the channel to the
nodes that raise the RTS and CTS as shown in Figure 2. However, a complete elimination of
hidden mesh nodes as well as exposing all mesh nodes in a WMN refraining concurrent
transmission between mesh nodes could be challenging, as it leads to unfairness in sharing
channel for multi-hop transmissions [10]. The multi-hop network throughput scenario/analysis is
discussed next.
Figure 2: RTS/CTS and NAV timing diagram.
2.3 Multi-Hop Network Throughput Analysis
To eliminate the hidden station problem and to avoid collisions, mesh nodes must maintain
received power levels within carrier sensed threshold (CST). However this could prevent the
nodes that are exposed utilizing the medium for concurrent transmission due to the DCF
contention access scheme which is called Exposed Station Problem which is a major barrier to
exploit concurrent transmissions in multi-hop mesh networks. Inability to perform concurrent
transmission between mesh nodes in a multi-hop network would increase the MAC frame
transmission waiting times with the increase of mesh node density resulting in rapid throughput
degradation. In other words, the balance between hidden and exposed nodes is crucial in
optimizing the concurrent communications in a WMN [11] [12] [13].
When all mesh nodes are in the same collision domain and if N numbers of nodes are exposed to
each other, the probability of successful frame transmission would be 1/N. Assuming N number
of hops or N +1 number of nodes in an exposed collision domain and no packets are losses at
relay nodes between source to destination as well as negligible propagation time between nodes
then the single-hop normalized end to end throughput would be:
Where Tp = Time to Transmit / Receive payload at a node
Ti = Intermediate relay node transmission latency

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Figure 3: Multi hop concurrent transmission delay
The above expression shows that the packet delay overheads could be minimized to improve the
raw data transmission efficiency by minimizing Ti which will in return increase the throughput
between mesh nodes. Applying this model to the concurrent transmission scenario as shown in
Figure 3 for a contention based shared medium access scheme where Tp is unbounded, indicating
that constrains in concurrent transmission between multi-hops could limit the per user throughput
when multiple user consume network bandwidth [14] [15] [16] [17].
2.4 IEEE 802.11 DCF Access Mechanism
In 2-way handshaking (Figure 4) when the medium is idle and the nodes contending for the
medium will access the channel immediately after the period of Short Inter-Frame Space (SIFS).
If expecting any acknowledgement frames for prior transmissions and wait further duration up to
DCF Inter-Frame Space (DIFS).
Figure 4: Inter-frame spacing and back-off.
If the medium is not idle, nodes will continue to wait a random back-off period set up in the
Back-off Counter (BC). The node transmits when BC expires to minimize any possible collisions
in transmissions. BC is a uniformly distributed random number between 0 and a Contention
Window (CW) defined. The CW size is initially assigned CWmin, and increases by doubling CW
with an upper bound of CWmax when collision is experienced but every successful transmission
will reset CW back to CWmin. CW size is measured in terms of slot time which is defined for
different 802.11 PHY standards [12] [2]. Figure 4 illustrates the basic concept of inter-frame
spacing and back-off mechanisms.
These inter-frame spacing and random back-off introduce delay overheads where medium left
unutilized before each transmission but built into the DCF scheme to minimize possible
collisions [9]. Figure 5 illustrates the overheads associated with 2-way handshaking. This delay
overhead is even worst with 4-Way handshaking where additional RTS/CTS NAV delays further

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contribute to longer underutilised medium assuring the medium protection for transmission
without collision at a cost of overall raw data rate degradation. In fact 4-Way handshaking or
RTS/CTS is usually recommended to be used only when long frames are to be transmitted where
a retransmission degrades system performance in case of a frame losses due to collisions [12].
Figure 5: Overheads in 2-way handshaking.
3. WIRELESS MESH NETWORK ELEMENTS
Figure 6 shows the architecture of a typical wireless mesh network. The 802.11 standard defines
the WLAN Basic Service Set (BSS) where a set of WLAN STAs that are associated to an AP or
each other in an ad hoc manner. Similarly in a WMN, the mesh nodes called as Mesh Point (MP)
are associated to each other based on the 802.11s standardized Mesh Basic Service Set (MBSS).
In other words the MBSS is a set of MPs that are associated to each other forming a transparent
single broadcast domain mesh cloud. However, unlike WLAN BSS STAs the MPs in a MBSS
has the relaying capability and MPs could exchange MAC frames over multiple wireless hops by
maintaining established mesh links with peering MPs in its neighborhood. The MBSS mesh
topology formed by MPs searches for potential MPs present in the neighbourhood by either
active scanning or passive listening over air waves and exchanging the Mesh Profile which
consisting of a Mesh ID, Path selection protocol identifier, and Link metric identifier. In fact, the
Mesh Profile that matches each other got associated them-self forming partial or a full mesh
topology. Once associated the MPs establish mesh links and continue to exchange beacons
frames for topology maintained and concatenated set of mesh links established via reachable
MPs maintained mesh paths in a mesh topology [7] [6].
3.1 WMN Mesh Functionality and Routing
MPs in a WMN could have one or multiple optional functions other than the mandatory mesh
function, such as the AP function which allows an MP to function as an AP to connect 802.11
WLAN STAs and such a mesh node is called a Mesh Access Point (MAP). A MP that could
translate 802.11s MAC frames to 802.11 WLAN MAC frames is called a Mesh Gateway (MG).
Having gateway functionality and an MG may have external gateway functionality as well to
connect an MP to an external 802.3 LAN or wired backhaul such an MP is called a Mesh Portal
Point (MPP) as illustrated in Figure 6.

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Figure 6: Wireless mesh network topology.
The MPs learn the mesh topology through routing protocols and an interesting feature in 802.11s
WMN standard is that the definition of its own routing protocols for frame forwarding and path
selection in the MAC layer itself without depending on network layer or usual TCP/IP routing
Protocols.
The IETF routing and forwarding standard for MANET called Hybrid Wireless Mesh Protocol
(HWMP) which provides both on demand routing with Ad-hoc On-demand Distance Vector
(AODV) and proactive tree-based routing with Optimized Link State Routing (OLSR) is used in
802.11s WMNs as well. Although WMN framework allows multiple routing protocols to be
implemented in a MANET and only one of them could be active in a Mesh cloud [8] [4].
3.2 Spatial Bias Multi-Hops WMN
In a multi-hop mesh topology, a user performance depends on the number of hops the frame had
to travel in reaching the destination. Higher the number of hop counts, the lower the overall
throughput achieved due to contention overheads at each hope resulting unfairness in spatial
resources use for an MP which has higher number of hops to the destination. The scenario called
spatial bias where more the mesh hops in a mesh path, higher the frames affected which is
scalability concern in designing WMN. Research has shown that the bandwidth starvation due to
spatial bias in multi-hop could be optimized by dynamically adjusting the packet size and the
minimum contention period based on congestion experienced due to spatial bias [6].
4. MAC ENHANCEMENT SCHEMES
Figure 7 shows the access control mechanism based on arbitration inter-frame space (AIFS) in
EDCA. The 802.11standards had not considered priority base Quality of Service (QoS) and
capable of serving only best effort delivery data over WLANs. But with the increasing demand
for carrying multimedia traffic over WLANs, the IEEE to form separate Task Group “e” (TGe)
introducing standards for Wireless Multimedia (WMM) under the 802.11e standard which
extend DCF with QoS capabilities. In the 802.11e standard, different traffic types are classified
based on 8 different priority values mapped onto the 4 FIFO queues, called Access Classes (ACs)
where each AC behaves like a virtual node. This WMM medium control coordination scheme is
called Enhanced Distributed Channel Access (EDCA) and the contention time DIFS is defined
for each corresponding traffic AC as the AIFS as illustrated in Figure 7 [18] [13].

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Figure 7: Access control based AIFS in EDCA.
Higher priority traffic category will have a shorter AIFS than a lower priority traffic category
which means lower priority traffic must wait longer time than high priority traffic before
accessing the medium. Although the probabilistic priority mechanism for allocating bandwidth
based on traffic categories has no guarantees of delivery between MPs in a mesh cloud, the
EDCA is the mandatory medium access scheme in 802.11s WMNs. As a result 802.11s standard
specifies another medium access scheme called Mesh Deterministic Access (MDA) as an option
in the Mesh Coordination Function (MCF). MDA is also a distributed and reservation based
deterministic medium access scheme and capable of providing prioritized QoS with delivery
guarantees. The advantage of MDA compared to EDCA is that the mesh nodes could negotiate a
periodic transmission opportunity for collision free transmissions. However, MDA capable mesh
nodes need to be synchronized each other and therefore it becomes more complex due to the ad
hoc nature of the mesh topology. Further in 802.11s based WMNs, synchronization is optional
due to its distributed nature and not all mesh nodes are required to participate in the MDA
scheme which could impact the presence of contention from non-MDA mesh nodes in the
neighbourhood [19] [17] [20]. The various MAC enhancement mechanisms for WMNs are
discussed next.
4.1 Transmission Opportunity and Frame Aggregation
IEEE 802.11e WMM standard also adds additional MAC enhancements such as aggregating
frames to be transmitted during the opportunity gained by contention scheme which is named
Transmission Opportunity (TXOP) and Block Acknowledgement (BA). It enables the receiver to
acknowledge the successful reception of multiple frames using a single BA frame. TXOP is a
bounded time interval defined by a maximum duration in which a series of frames are
transmitted. TXOP Limit, which depends on the AC, is the maximum time a node could hold a
channel after a successful contention. Frame aggregation concept allows Aggregated MAC
Service Data Unit (A-MSDU) to be sent to the same receiver concatenated into a single MPDU
and transmitted either when transmit queue reaches the maximal A-MSDU threshold or any
frame timeout condition. The BA contains a bitmap to selectively ACK individual frames in an
aggregated frame burst allowing a block of frames separated by an inter frame spacing of SIFS
with same AC to be transmitted without waiting for acknowledgment. A-MSDU transmitted is
followed by a BA Request (BAR) frame to enquire which frames have been received

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successfully which is answered with a BA frame for every successful frame delivery [5] [21] [22]
[23].
4.2 IEEE 802.11n MAC Enhancements
IEEE 802.11n WLAN standard added further enhancement for the frame aggregation with
another level of aggregation called Aggregated MAC Protocol Data Unit (A-MPDU),
aggregating MPDU sub frames to a single PHY frame. Unlike A-MSDU there is no waiting time
for an A-MPDU and the number of MPDUs aggregated depends on the number of frames in the
transmit queue at the time of gaining the TXOP. MSDUs within an A-MSDU are addressed to
the same receiver whereas MPDUs within an A-MPDU need not be to the same receiver. A blend
of both A-MSDU and A-MPDU over two stages will maximize throughput efficiency. Further
BAR is made optional and Rx could respond with BA after each aggregated frame without
waiting for a BAR which removes the BAR overhead and eliminate the possibility of
retransmission. This could be any failure to receive BAR and allows multiple aggregated frames
to be acknowledged by a single BA. BA could be either expected immediately as a response to
the BAR or could be a delayed BA [24] [25] [26].
Another medium access enhancement introduced in 802.11n is the reduced and zero inter-frame
spacing (RIFS and ZIFS) to minimize the overhead between frames. Inter frame spacing is
required within TXOP between frames and between the last frame and BAR. This is reduced
from SIFS to RIFS where (RIFS << SIFS) between multiple aggregated frames or completely
removed which means RIFS = ZIFS eliminating the overhead due to inter-frame spacing resulting
more bit transmission using TXOP [5] [27] [28].
4.3 Reverse Direction (RD) Flow
IEEE 802.11n standard has also been enhanced frame aggregation called Reverse Direction (RD)
flow, which improves the TXOP effectiveness by allowing frame transfer from responder to the
originator during originator’s TXOP. RD flow initiates with RTS/CTS handshake and the peers
make a request inside the RTS/CTS NAV duration. RD flow requires the TXOP originator to
grant permission to the responder to send data frames aggregated in the reverse direction while
being responsible for channel ownership. Gain in throughput performance would be achieved in
RD flow by granting responder node to transfer frames without contention related overheads [29]
[30].
4.4 Green Field High Throughput Mode
IEEE 802.11n standard is backward compatible with previous generations 802.11a/b/g and
operates in three modes, namely Legacy Mode, Mixed Mode and Green Field Mode. In Legacy
mode, frames are transmitted in the legacy 802.11a/g MAC format frames with no 802.11n MAC
features. In the Mixed Mode, 802.11a/g frames are transmitted with a preamble compatible with
the legacy 802.11a/g such that it can be decoded by legacy 802.11a/g devices while transmitting
802.11n frames with an initial training sequence format which occupies less air time to reduce
per-transmission overheads. Therefore, medium protection RTS/CTS handshaking is required to
permit communication with legacy stations to ensure legacy devices sense the channel busy state.
Thus, 802.11n devices have to pay significant throughput penalties when legacy devices are
served in mixed mode. The Green Field mode is exclusively for 802.11n devices only with high
throughput (HT) format preamble is used in MAC frames for HT transmission. If no legacy

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devices served the 802.11n WLANs operate in maximum HT performance then it is said to be a
green field network [28] [31] [32] [33].
5. TRANSMISSION FAIRNESS STRATEGIES
The DCF and the enhanced EDCA contention based distributed access schemes have been
successful in all 802.11 standards irrespective of other contention free schemes used in the
standards to avoid contention in 802.11n [34].
In contention based distributed medium access schemes for multi-hop networks, nodes are
required to accommodate multiple concurrent transmissions. It is important to distribute/separate
the nodes to multiple collision domains so that the nodes can be exposed in that collision domain.
To carry frames across the collision domains, an overlap between collision domains is required
where a node within overlapping area would be the transit node for interconnecting two or more
collision domains [35] [36] [37].
Table 1 compares the four main distributed medium access mechanisms for WMNs. The
comparison is based on various factors, including IEEE standards, MAC type, operating mode,
frame aggregation, support for block Ack (BA), reverse direction (RD), and NAV, and priority.
Table 1: Comparison of distributed medium access schemes
Scheme DCF EDCA MDA HT EDCA
IEEE Standard 802.11a/b/g 802.11e 802.11s 802.11n
MAC type Contention Contention No
contention
Contention
Mixed mode
supported
Yes Yes Yes Yes
Frame
aggregation
No Yes Yes Yes
Support BA No Yes Yes Yes
Support for
Reverse
Direction flow
No No No Yes
RIFS/ZIFS No No No Yes
Support NAV
Protection
Yes Only Long
frames
Yes Mixed
mode
Synchronization Optional Optional Manda-
tory
Optional
AC Priority QoS No Yes Yes Yes
Exposed STA
avoid scheme
No No Yes No
Concurrent Tx
scheme for MPs
No No No No

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Figure 8: Transit mesh point exposed areas.
A transit node will be exposed to multiple collision domains to contend in both collision domains
to have longer span of time for a transmission as shown in Figure 8. This scenario is called
“Neighborhood capture problem” where the transit node will hardly find the free medium to
access both collision domains [38] [39] [40] [41].
5.1 Channel Estimated Power Management
In contention based medium access, the frames are lost as a result of collisions or transmission
errors. The transmission errors occurred due to poor channel conditions. The dynamic link
adaptation using modulation and coding scheme (MCS) and forward error correction (FEC)
compromises the raw data rate by mitigating transmission errors. However, frame losses are
normally occurred due to collision at the receiving end when a hidden node attempts to transmit
data as illustrated in Figure 9. In a WMN setting, the Tx power and CST levels at each node is a
decisive factor in fixing the propagation range of exposed MPs. To select the optimum CST level
one could avoid hidden MPs as well as limit the exposed MPs [42] [43] [44].
Figure 9: Transit node receiver end collision.
To optimize concurrent transmission between MPs, a strict power management and channel
condition estimation at each MP in WMNs is required to ensure MPs are exposed to well manage
collision domains.
To analyse MP power management, let us look at the well-known formula for Free Space Power
Loss PL is given by
Where d is the distance (in km) between the nodes, f is the signal frequency in MHz and POut is
the MP Transmit power and the RSSI is the receiving MP Received Signal Strength Indicator in
dBm.

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Even at low RSSI levels due to poor channel conditions or high PL, the MPs could establish a
transmission by adopting low bit rate MSC schemes. However, to avoid two MPs exposed to
each other it is required to lower the transmit power PL to a level such that RSSI level at the
receiver is insufficient to establish any transmission even with lowest possible MSC scheme [45]
[38].
Figure 10: Transmit power management.
Figure 10 demonstrates the transmit power management strategy in WMNs. In 802.11, every
frame transmitted is expected to be acknowledged when delivered. If the frame is not
acknowledged within a predefine timeout duration, the frame is considered to be lost. The reason
for acknowledgement failure which could be either a transmit frame lost due to poor channel
conditions or a frame collision that is not known by the sender. However with the introduction of
802.11e MAC enhancements, the BA could be used by the sender to assess the exact reason for
an unsuccessful frames delivery. If return indicate many error frames transmitted in the Bitmap,
the channel suffers from poor channel conditions. Further if BA is never returned during the BA
timeout period then frames could have been collided. This clarity on transmit frames is useful in
a WMN to adjust the transmit power levels and CST of an MP to avoid collisions as a result of
an optimum collision domain separation [30] [46] [11] [33].
5.2 Design of RD Pull Collision Avoidance Scheme
WMN in Greenfield mode will ensure all MPs support 802.11e MAC layer enhancements as well
as 802.11n HT features consistently. In a Greenfield mode, a WMN can be implemented in
multiple collision domains where each collision domain is interconnected to the neighboring
collision domains through one or more transit MPs. This strategy would allow concurrent
transmissions without interfering nodes using contention based medium access mechanisms in
neighboring collision domains. However, this may lead to transit MP starvation due to
neighborhood capture problems. We suggest that the reverse direction (RD) pull mechanism can
be used to avoid collisions due to neighborhood capture problems (discuss below). This strategy
can be used without modifying the EDCA contention based medium access scheme in WMNs,
especially when modeling using a credible simulation package, such as OPNET or ns-2 for
performance evaluation [47] [48]. The RD algorithm is described below.
• Neighborhood capture most transit MPs in receiving state and request for RD flow from the
sender during sender’s TXOP using either CTS frame or BA frame.
• The reverse direction grant (RDG) request could be initiated by the neighborhood captured
MPs when its transmit buffer reaches threshold limit. The system may experience collisions
when contending for the shared channel or any higher layer strict delivery conditions.

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• Receiver’s RDG request is responded by the TXOP’s own MP with a RDG allocating any
excess TXOP to pull the MAC frames in RD. If the excess TXOP is insufficient to pull all
MAC frames waiting to be transmitted, another RDG request can be processed before the
end of TXOP.
• If an MP had received a RDG request it will contend to the shared channel to respond with a
RDG in order to pull frame in RD.
Although the RD pull mechanism may halt concurrent transmissions, addressing the
neighbourhood captured problems will in fact optimize the concurrent transmission in WMNs
[14] [47]. This is an important strategy in achieving high throughput in WMNs. The network
design guidelines to improve system performance are discussed next.
6. DESIGN GUIDELINES AND DISCUSSION
The three design guidelines for optimizing multi-hop WMNs using concurrent transmissions are
discussed below.
(1) Network design using Green field mode: It is important to operate all mesh network MPs
in green field mode only. Use all 802.11e wireless multimedia (WMM) and 802.11n HT
features to maximize system performance as well as capitalise on novel features such a
reverse direction (RD) flow.
(2) Network design by splitting: Split the WMN into multiple collision domains in such a way
that MPs in each collision domain can transmit frames within the domain independently to
exploit concurrent transmissions. This could be done by varying the transmitter power and
CST so that MPs in different collision domains do not associate each other even at the lowest
possible modulation and coding scheme (MCS). This strategy of WMN design will optimize
network throughput performance.
(3) Network design by exposed collision domain: Ensure that at least two MPs can be exposed
to any two collision domains to transit traffic between the two collision domains. This
strategy will avoid a single point of failure of a single MP. However, the impact on such a
transit MP due to neighbourhood capture problem for being exposed to more than one
collision domains needs to be addressed. An effective solution would be to use RD pull
mechanism that could pull traffic from transit MPs by granting the RD flow to achieve
transmission fairness and optimising concurrent transmissions.
There are many challenging issues in the design, implementation, and deployment of WMNs.
Some of the issues are discussed in Section 2 of this paper. Now the question may arise about the
better ways of designing WMNs for optimum performance. However, high performance WMN
can be (re)design if some obvious guidelines were adopted. In this section we formulated three
guidelines for network designers and researchers for optimizing multi-hop WMNs. First, network
should be designed using Green field mode only. This means that WMN design using high
performance 802.11n devices only. Second, network should be designed by splitting a WMN into
multiple collision domains to exploit concurrent transmissions. Third, network should be
designed by exposed collision domains. This means that at least two MPs can be exposed to any
two collision domains for better system performance.

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7. CONCLUSION AND FUTURE WORK
The existing distributed medium access schemes and MAC-layer enhancements for improving
concurrent transmission among mesh points (MPs) in WMNs are discussed. The evolution of the
802.11 standards PHY and MAC layers with the assumption that contention based distributed
medium access protocols will continue to be the most accepted mechanisms in sharing the
medium not only in WLANs but also in multi-hop WMNs. We reviewed existing mechanisms for
improving the performance of a typical WMN by concurrent transmission among MPs. These
mechanisms can be added to the 802.11 standards especially transmit opportunity (TXOP), frame
aggregation, block acknowledgement, reduce inter frame spacing and reverse direction flow.
Based on the findings from open literature we identify a high throughput green field WMN
separated into multiple collision domains would be the best design strategy to optimize
concurrent transmissions in WMNs. We also suggest that reverse direction (RD) pull mechanism
can be used to avoid neighbourhood capture issues at a MP specially when handling transit
traffic between collision domains. Development of an extensive simulation model of a large
WMN with multiple transit mesh nodes handling contention and concurrent transmission is
suggested as future work.
Acknowledgment
This research was supported in part by the Faculty of Design & Creative Technologies under
Summer Studentship grant 2012-2013, Auckland University of Technology.
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