89.a high-throughput mac protocol for wireless ad hoc networks

7/27/2019 89.a High-Throughput MAC Protocol for Wireless Ad Hoc Networks

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IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 7, NO. 1, JANUARY 2008 135

A High-Throughput MAC Protocol forWireless Ad Hoc Networks

Wanrong Yu, Jiannong Cao, Senior Member, IEEE, Xingming Zhou, Xiaodong Wang, Member, IEEE,

Keith C. C. Chan, Member, IEEE, Alvin T. S. Chan, Member, IEEE, and H. V. Leong, Member, IEEE

Abstract One way to improve the throughput of a wireless adhoc network at the media access (MAC) layer is to allow as muchas possible concurrent transmissions among neighboring nodes.In this paper, we present a novel high-throughput MAC protocol,called Concurrent Transmission MAC(CTMAC), which supportsconcurrent transmission while allowing the network to have asimple design with a single channel, single transceiver, and singletransmission power architecture. CTMAC inserts additional con-trol gap between the transmission of control packets (RTS/CTS)

and data packets (DATA/ACK), which allows a series of RTS/CTSexchanges to take place between the nodes in the vicinity ofthe transmitting or receiving node to schedule possible multiple,concurrent data transmissions. To safeguard the concurrent datatransmission, collision avoidance information is included in thecontrol packets and used by the neighboring nodes to determinewhether they should begin their transmissions. Also, to isolate thepossible interference between DATA packets and ACK packets,a new ACK sequence mechanism is proposed. Simulation resultsshow that a significant gain in throughput can be obtained bythe CTMAC protocol compared with the existing work includingthe IEEE 802.11 MAC protocol.

Index Terms Wireless ad hoc networks, media access control,concurrent transmission, throughput.

I. INTRODUCTION

DUE to the characteristics of being infrastructureless,mobility, and robustness, mobile ad hoc networks(MANETs) have gained significant attentions in the past

several years. The deployment and rerouting of traffics are

flexible in MANETs, but how to efficiently utilize the scarce

shared wireless radio channel remains a great challenge in

practice. Extensive research efforts have been dedicated to

improving the throughput of a MANET.

IEEE 802.11 DCF [1] has been regarded as the basic MediaAccess Control (MAC) protocol for MANETs. It is based

on CSMA/CA (Carrier Sense Multiple Access with Collision

Avoidance), with extensions to allow for the exchange of

RTS/CTS (Request-To-Send/Clear-To-Send) packets between

Manuscript received February 17, 2006; revised October 20, 2006 and July22, 2007; accepted September 9, 2007. The associate editor coordinating thereview of this paper and approving it for publication was X. Shen. This work issupported in part by the Hong Kong Polytechnic Universities under the ICRGgrant A-PF77 and 4-6941,Hong Kong RGC CERG grant PolyU 5105/05E andthe National Natural Science Foundation of China under Grant No. 60273068.

W. Yu, X. Zhou, and X. Wang are with the School of Computer, NationalUniversity of Defence Technology, Changsha, China (e-mail: {wlyu, xmzhou,

xdwang}@nudt.edu.cn.).J. cao, K. C. C Chan, Alvin T. S. Chan, and H. V. Leong are withthe Department of Computing, HongKong Polytechnic University (e-mail:{csjcao, cskcchan, cstschan, cshleong}@comp.polyu.edu.hk.).

Digital Object Identifier 10.1109/TWC.2008.06094.

A

iu j nmv

Fig. 1. Inefficiency of the standard IEEE 802.11 approach.

the transmitter and the receiver before the actual transmission

of DATA packets.

Although it has the advantage of simplicity, the IEEE 802.11

DCF can be overly restrictive. It prohibits any concurrent

transmission among neighboring nodes even when the trans-

mission is possible. In MANETs, a packet can be received

successfully even if there exist other overlapping or interfering

packets, only if its instantaneous power is larger than the

instantaneous joint interference power by a minimum certain

threshold factor [2]. This effect is the capture effect, and

the threshold factor can be called Signal-to-Interference-and-Noise Ratio (SINR).

Fig. 1 illustrates such a scenario (the circles indicate the

transmission ranges of central nodes). When the node isends packets to node j, after exchanging control packetsRTS/CTS between node i and j, the transmissions u v andmn are both prohibited until the end of transmission ij.Nodes i and js RTS/CTS packets will silence all of theirneighboring nodes. In fact, however, one node can receive

packet successfully as long as its required SINR is fulfilled.

Therefore, in Fig. 1, without considering the ACK packets,

as long as the required SINR of both node j and v is kept,then the DATA packets of transmissions ij and uv canbe transmitted simultaneously. Furthermore, if node js SINRis not violated by node ms transmission, then node m cantransmit the DATA packet simultaneously with node i. Duringthe above discussion, we talked only about the DATA packets,

but the ACK packets also could collide with each other or

with the DATA packets. Fortunately, this problem is easy to

solve. As proposed later in this paper, we can hold all the

three ACK packets until the transmission of the three DATA

packets completes.

The above observation motivates people to exploit potential

concurrent transmissions between neighboring nodes in aMANET, which is the main topic of this paper. We propose

the CTMAC protocol, which is a distributed, asynchronous

and adaptive MAC protocol. It requires a very simple standard

1536-1276/08$25.00 c 2008 IEEE


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136 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 7, NO. 1, JANUARY 2008

TABLE I

SYMBOLS AND ABBREVIATIONS USED IN THE PAPER

AS the abbreviation of access slot

ACG the abbreviation of additional control gap

ANLi the Active Neighbor List of node i

NACG the number of AS in the ACG

Dthe tag in the ANL, indicating whether the sender ofthe recorded packet is transmitter or receiver

Mthe tag in the RTS packet, indicating whether the ANLof the sender is empty or not

Ptxthe transmission power, definite and common to allnodes

Prx the average power of the received control signal

Gij the channel gain between two nodes i and j

P(v)MT I

the maximum tolerable interference from one neigh-boring node of node v

P(v)current

the accumulated total interference power at node v dueto current scheduled transmissions

P(v)maximum

the maximum accumulated total interference at nodev

P(v)addable

the total future interference that node v can toleratewithout its SINR being violated

PrxThresholdthe minimum reaching power for a node to decode thepacket correctly

Pbackgroundthe background noise of one node, ,which is thecommon to all nodes

Tdata the start time of transmitting DATA packet

Tack the start time of transmitting ACK packet

Tramain the remaining time of current ACG

TAS the length of access slot duration

IEEE 802.11 circuitry, and works on the single channel andsingle transmission power architecture. CTMAC works on

contention-based CSMA/CA mechanism and performs expo-

nential backoff with the same minimum and maximum value

for contention window (CW) size, as in the standard IEEE

802.11. CTMAC has the following key features.

First, after successfully exchanging the control packets (i.e.,

RTS/CTS), the transmitter does not immediately send the

DATA packet as in the IEEE 802.11 specification. Rather, an

additional control gap (ACG) is inserted between RTS/CTS

and the corresponding DATA packet. The ACG offers the

nodes in the vicinity of a transmitting or receiving node the

chance to exchange their own control packets and scheduleconcurrent data transmission without violating existing sched-

uled transmissions.

Second, unlike the IEEE 802.11 approach and the existing

schemes [3, 4, 5], CTMAC does not use the control packets to

silence neighboring nodes. Instead, collision avoidance infor-

mation is inserted in the control packets. This information is

used, in conjunction with the received signal strength of these

packets, by the potentially interfering nodes in the vicinity of a

transmitting/receiving node to dynamically determine whether

it is possible to schedule their transmission.

Third, to avoid the collision between ACK and DATA

packets of different transmissions, CTMAC introduces a newmechanism to sequence the ACK packets of concurrent trans-

missions. Thus, only the interference between various DATA

packets needs to be considered.

Finally, in CTMAC, the concurrent transmission is con-

trolled locally by the nodes in the vicinity of a transmit-

ting/receiving node, depending on the information they over-

heard about the already scheduled transmissions. So CTMAC

is asynchronous, and does not need any central control. As

such, it is suitable for large scale wireless ad hoc networks.To the best of our knowledge, CTMAC is the first protocol

that works with single-channel, single-transceiver and single-

transmission power to increase the network throughput whilepreserving the collision avoidance property of the 802.11

scheme. These singles mean the CTMAC is simple and has

fewer requirements for the hardware of the station.The rest of the paper is organized as follows. In Section

2 we present and analyze the related works on throughput

enhancement at MAC layer and discuss their limitations. The

assumptions required for designing the CTMAC protocol is

described in Section 3. The proposed CTMAC protocol is

detailed in Section 4, followed by the discussions on the

simulation results in Section 5. Finally, Section 6 concludes

the paper with a discussion on the future work.

I I . BACKGROUND AND RELATED WOR K

Effort has been made on increasing the throughput of

MANETs through scheduling of concurrent transmissions at

the MAC layer. The existing works can be divided into two

main classes. In the first class, to improve the network through-

put, additional control gaps are inserted between RTS/CTS and

DATA packets for successfully scheduled transmission. Thus,

neighboring nodes have more chances to exchange their con-

trol packets during the ACG to schedule parallel transmissions.

The MACA-P protocol proposed in [6] and its enhancement in

[7] allow parallel transmissions when two neighboring nodesare either both receivers or both transmitters, but not a receiver

and a transmitter. However, these works do not consider

the tolerable interference during a nodes receiving period,

so neighboring sending and receiving sessions of different

transmissions are prohibited. In the proposed CTMAC, this

problem is solved by inserting collision avoidance information

into the control packets. Accordingly, concurrent transmissions

are scheduled based on this information. In fact, MACA-P can

be regarded as one special case of CTMAC.In the second class, transmission power control (TPC) is

used per-packet to increase the spatial channel reuse. TPC is

first motivated by the observation that maximum transmissionpower is not always necessary for the packets to be received

correctly by the destination node. TPC-based schemes can

further be divided into two sub-categories: single-channel

based and multi-channel based.Multi-channel based TPC schemes [8, 9, 10] can obtain

great throughput enhancement compared with IEEE 802.11.

These schemes allow for concurrent transmissions in the same

vicinity of a receiver by locally broadcasting collision avoid-

ance information over one or more separate control channels.

Although the simulation results for the protocols in [8, 9, 10]

indicate their impressive improvements in throughput over the

IEEE 802.11 scheme, there are several problems with theseschemes, which make their practicality questionable:

The latency in change of transmission power is huge [11],

which makes the TPC-based solutions very difficult to use

in practice, if not impossible.


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YU et al.: A HIGH-THROUGHPUT MAC PROTOCOL FOR WIRELESS AD HOC NETWORKS 137

The channel gain is assumed to be the same for both

the control channel and data channel. This requires the

control channel to be within the coherence bandwidth

of data channel. However, holding this assumption will

violate another assumption that one node can transmit or

receive simultaneously on two channels.

The node must be equipped with two or more transceivers

for multi-channel based schemes. The complexity and

cost of additional hardware may devalue the improvement

in throughput.

The multi-channel protocols are not backward compatible

with the IEEE 802.11 standard, which makes it difficult

to deploy such protocols in real networks.

Besides the first one, single-channel based TPC schemes avoid

other problems of multi-channel based TPC schemes. In the

single-channel based TPC schemes, all control and data pack-

ets contend for the same channel. Basically, nodes exchange

their RTS/CTS packets at a maximum power (denoted by

Pmax), but send their DATA/ACK packets at the minimumpower needed for reliable communication (denoted by Pmin).The value of Pmin is determined by the receiver based on therequired QoS (i.e., SINR), the interference level at the receiver,

and the channel gain between the transmitter and the receiver.

However, these single-channel based TPC schemes [3, 4, 5]

at best can give a throughput comparable to that of the IEEE

802.11 scheme [5]. POWMAC [12] is an enhanced single-

channel based TPC scheme, which combines the approaches of

additional control gap and TPC. However, besides the problem

of huge latency caused by changing of transmission power,

POWMAC adds additional control packet for all nodes which

is not always necessary.

III. PRELIMINARIES

We assume that each node is equipped with the basic IEEE

802.11-compliant hardware. Since most of existing products

follow the specification of IEEE 802.11, this assumption is

widely supported.

One essential requirement of CTMAC is that the scheduled

concurrent transmissions should not interfere with each other.

To achieve this, CTMAC needs to maintain some necessary

information. The key information to guide the concurrent

transmission scheduling is the knowledge about existing trans-

missions in the vicinity of a node. In CTMAC, each node

maintains a new data structure, called Active Neighbor List

(ANL), to record this information. ANLi records node isknowledge about other active nodes (i.e., nodes that are

currently receiving, transmitting, or scheduled to do so) in isvicinity. For every active node u in is vicinity, ANLi containsthe following information:

{Uaddress, Giu, T(uv)data , T

(uv)ack , D, P

(u)MT I}

where

Uaddress is the address of the active node u.

Giu is the estimated channel gain between nodes i andu, computed as follows: Giu = P

(u)rx /Ptx, where P

(u)rx is

the signal power of the received control packet of node

u, and Ptx is the single transmission power, which iscommon to all nodes.

T(uv)data and T

(uv)ack are the starting times of the DATA and

ACK packets of the transmission between nodes u and v(assuming the corresponding communication node of u is

node v). T(uv)data and T

(uv)ack included in the control packets

are relative time and indicate the remaining duration from

current time. However, the recorded time in ANL is the

global time, computed by adding T(uv)

data

and T(uv)

ack

to

the current absolute time of the nodes that receives the

packet.

D is the tag used to distinguish the transmitter andreceiver. If the control packet received is from a trans-

mitter, then the D tag is set to 1. If the control packetreceived is from a receiver, then the D tag is set to 0.According to the type of control packets, one node can

easily distinguish the transmitter and receiver.

MTI is the maximum tolerable interference of a receiver

node u, denoted by P(u)MT I. This is the maximum addi-

tional interference that node u can tolerate from a poten-tial interfering node during

us DATA packet reception.

As will be explained later, this value is computed and

advertised by node u.

To distinguish different roles in transmissions, we call a

successfully scheduled transmission either a master transmis-

sion or a slave transmission. During the exchange of control

packets (RTS/CTS) for scheduling a transmission, if both

the transmitter and receiver have no active nodes of other

scheduled transmissions in their vicinity (their ANLs are both

empty), then this is a master transmission, and the transmitter

and receiver are called master transmitter and master receiver

respectively. After the exchange of control packets, the ANL

of a master node can be not empty. In fact, a master nodeneeds to maintain its ANL according to the overheard control

packets during its ACG. Other scheduled transmissions must

adjust their transmitting times of DATA or ACK packet

according to the overheard information of the neighboring

master transmission, so they are called slave transmissions and

the participating nodes are called slave transmitters and slave

receivers respectively. For example, considering transmissions

ij and uv in Fig. 1, if node i and j first occupy the channelto exchange the control packets, then node i and j are masternodes because their ANLs are empty during the exchange of

control packets, as illustrated in Fig. 2 later. Node u and v can

exchange their control packets during the ACG of transmissionij, so they are slave nodes of transmission ij and shouldsynchronize to the master transmission. Note that the use of

the terms master and slave does not imply any form of

centralized control. Each node has equal chance to become a

master node if it happens to schedule a transmission to which

others nodes synchronize.

IV. THE PROPOSED CTMAC PROTOCOL

We now describe the details of CTMAC protocol, which are

divided into three main parts. We first introduce the packets

exchange process in subsection A, which is an extension ofthe 802.11 RTS/CTS mechanism. Then, in subsection B, we

describe how a node maintains its ANL and the detailed

concurrent transmission control rules and mechanism. In sub-

section C, we introduce how one node adjusts the size of its


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i

j

u

RTS

vCTS

ATS

ACK

DATA

DATARTS

CTS

ACG

ACK

Backoff

ij

dataT ijackT

Fig. 2. Basic operation of CTMAC.

additional control gap (ACG) according to the current status

of the network.

A. Basic operation of CTMAC

First, let us consider the example shown in Fig. 1 to see the

basic operation of CTMAC, which is illustrated in Fig. 2. To

simplify the description, we only consider the transmissions

ij and uv. Node i first transmits an RTS packet to nodej, including information such as the scheduled start times ofis DATA packet (Tijdata) and js ACK packets (T

ijack). To

avoid the requirement of synchronized clocks, both values are

specified relative to the receiving time of the associated control

packet. Node j replies with a CTS packet to node i, includingsimilar information. As explained later, this information is

needed so that a node in the vicinity of i or j can determinewhether or not it can receive a data packet from some other

node simultaneously while i is transmitting data to j. After theRTS/CTS packets are exchanged, node i refrains from sendingits data packet for the duration specified by ACG. During this

period, u and v can exchange control packets and scheduletheir transmission if possible. If the transmission between node

u and v is successfully scheduled, then the two transmissionscan be done concurrently and the starting time of node usDATA packet will be the same as node is DATA packet.If more than one neighboring nodes contend for the channel

during the ACG, then the access method used by these nodes

is contention-based CSMA/CA.

As illustrated in Fig. 2, the DATA packet of node u is shorter

than that of node i. This is another requirement of CTMAC.In CTMAC, the DATA packet of a slave transmission must

be less than or equal to the DATA packet of corresponding

master transmission. Due to the existence of frame fragmen-

tation at the MAC layer, large network layer payload will be

fragmented to small frames, so we can use CTMAC only for

those DATA packet whose length equal frame threshold. While

for other short DATA packets, we still use the traditional IEEE

802.11 scheme.

In CTMAC, the CTS packet of the original IEEE 802.11

DCF is extended and classified into two types: normal CTS

and negative CTS packet. The use of the normal CTS packet

is similar to IEEE 802.11, i.e., to let the receiver tell thetransmitter that it is ready for the coming DATA transmission.

However, in CTMAC, for a slave transmission, the receiver

may modify the values of Tdata and Tack declared by thetransmitter in the RTS packet and includes new values in

i j mn

p1 p2

Fig. 3. Possible collision between ACK and DATA packets.

its CTS packet. For the master receiver, since its ANL is

empty, it always accepts the values declared by the master

transmitter without modification. The use of the negative CTS

packet in CTMAC is as follows. If the slave receiver finds it

impossible for the slave transmission to continue according

to the concurrency control rules (see later), it will send a

negative CTS packet to notify the slave transmitter to cancel

the proposed transmission.

However, just extending the CTS packet is not enough for

concurrent transmission control. First, if the slave receiver

modifies the value of Tdata and Tack, the slave transmitterhas no way to notify its neighboring nodes of the adjustment.

Thus, the neighboring nodes may make wrong decisions

leading to collisions with scheduled transmissions. Second,

the so called cascading lock problem remains unsolved, which

may lead to unnecessary channel reservation. For example,

consider the following chain of nodes,

r1 t1 r2 t2 r3 t3.

Assume that nodes t1 and r1 have successfully exchangedRTS/CTS packets and scheduled a transmission, and node t2sends a RTS packet to node r2 during the ACG of t1r1transmission. If the SINR of node r2 is violated by thetransmission of node t1, r2 will send a negative CTS packetto node t2 so that t2 will not begin its transmission. Then,when node t3 sends the RTS packet to node r3, if its SINRis also violated by the transmission of node t2 (although ithas been canceled, r3 knows nothing about it), it will send anegative CTS packet which prohibits t3 from transmitting. Butin fact, sine the transmission t2r2 has been canceled, and thetransmission t3r3 actually can be scheduled successfully.

To solve the above problem, a packet called ATS (abort-

to-send or adjust-to-send) is used by a slave transmission to

inform the neighbors of necessary information. ATS has two

meanings according the two situations when it is used. Thefirst is when the slave receiver disagrees with the values of

Tdata or Tack scheduled by the slave transmitter. It includesthe new values in the CTS packet, so the slave transmitter

uses ATS (adjust-to-send) packet to inform its neighbors of

the adjustment. The second situation is when it is impossible

for the slave receiver to get the data packet from the slave

transmitter. The slave receiver responds with a negative CTS

packet, so the slave transmitter uses ATS (abort-to-send)

packet to notify its neighbors to cancel the proposed schedule.

Another problem is that, at the MAC layer, besides the inter-

ference between the DATA packets of different transmissions,

the ACK packets and DATA packets of different transmissionsmay also collide with each other. As illustrated in Fig. 3,

even when the DATA packets of transmissions ij and mncan be transmitted simultaneously without collision with each

other, the two transmissions may still fail due to the collision


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between the DATA and ACK packets. In fact, if the length of

packet p2 is less than the length of packet p1, node n will sendACK packet before node j finishes receiving its DATA packet.It is obvious that the interference at node j due to node nstransmitting is greater than that due to node ms transmitting,thus the receiving of DATA packet at node j may not workcorrectly during the transmission of node ns ACK packet.

To ensure the correct completion of the concurrent transmis-sions, we propose a new ACK packet sequence mechanism.

The ACK packets of one master transmission and all the slave

transmissions synchronized to this master transmission are

transmitted one by one in sequence. This helps eliminate the

collisions between the DATA and ACK packets or between

the ACK packets themselves of the concurrent transmissions.

The detailed process is as follows. When a slave transmis-

sion computes its Tack, it postpones the starting time of itsACK packet to the completion of the ACK packets of all

the scheduled transmissions within the vicinity of the slave

transmitter or receiver. Since the size of the ACK packet is

definite and identical to all nodes and the Tack of the scheduledtransmissions is recorded in the ANL, it is easy for the slave

transmission to determine its own Tack.The sequencing of the ACK packets isolates the DATA/ACK

packets and different ACK packets in time, thus enables more

concurrent transmissions. Meanwhile, it greatly simplifies the

CTMAC protocol because we only need to consider the

possible collisions between the DATA packets. As the size

of ACK packet is very small and identical to all the nodes,

the performance is affected only slightly.To avoid possible collisions between the RTS packet and

the sequenced ACK packets, participate nodes (include masternodes and slave nodes) of concurrent transmissions will start to

transmit new RTS packet after the finish of last potential ACK

packet of current concurrent transmissions. When participate

nodes exchange their control packets, they can know the

remaining time of current master transmissions ACG, which

determines how many slave transmissions are possible. Each

slave transmission needs one ACK packet and the time to

transmit the ACK packet is common to all nodes. Then, based

on the Tack of its own transmission, one participate node candetermine when the last potential ACK packet will finish.

B. Concurrent Transmission ControlThe information recorded in the ANL is the basis of concur-

rent transmission control, so we first describe the mechanism

for a node to maintain its ANL in this section. Then we

introduce how one node compute the collision avoidance

information inserted in the control packets, followed by the

four rules for concurrent control.In RTS/CTS based MAC protocols, nodes need to ex-

change control packets before transmitting the payload data.

Therefore, the key to schedule concurrent transmissions is to

offer nodes more chances to exchange control packets. In

IEEE 802.11, after receiving the RTS/CTS packet destined

for other nodes, the node should set the network allocatevector (NAV) and wait for the current transmission to finish.

This conservative reservation can assure the success of the

scheduled transmission, but the channel is occupied by a single

transmission, limiting the throughput of network.

To exploit potential concurrency in the network, in CTMAC,

an additional control gap (ACG) is inserted between the

RTS/CTS/ATS packets and the DATA packet. Thus, after

a node receives control packets destined for other nodes,

it does not need to immediately postpone its transmission.

Rather, it records necessary information in its ANL. Then,

when a node with a non-empty ANL gains the chance to

send control packets, it will first check its ANL to see if

the recorded remaining time before starting the scheduled

transmission is long enough for it to complete the exchange

of a control packet. If so, it can start transmitting the control

packet. Otherwise, it will postpone the transmission until the

scheduled transmission finishes.

ACG makes concurrent transmissions possible, but does

not ensure their success. If the concurrent transmissions

collide with each other, then all transmissions will fail and

retransmissions are needed. For the concurrent transmissions

to proceed successfully, the interferences between them should

be tolerable. In CTMAC, we add some information about

signal power into the control packets. This information is

recorded in the ANL of the node that receives the control

packets. Before we describe the details of the concurrent

transmission control mechanism, we first introduce how the

node maintains its ANL.

A node updates its ANL only when it receives control

packets that are destined for other nodes. In the IEEE 802.11

standard, nodes also need to decode the MAC layer header

of each coming packet. So, except for a few memory spaces,

maintaining the ANL based on overheard control packets will

not introduce new cost. Based on the packet received, a node

takes one of the following four actions:ADD: When the node receives a RTS or a normal CTS

packet, it adds an entry in its ANL to record the information

obtained from the packet. However, if the received packet is

a negative CTS packet, the node will do nothing.

MODIFY: If the received packet is an ATS packet carrying

a Tdata or Tack value different from the previous RTS fromthe same node, the node will update the corresponding entry

with the new value.

DELETE: When an aborting ATS is received, the node

deletes the corresponding entry from its ANL.

EMPTY: the empty action can be divided into two types,

depending on whether the node participates in transmissionor not. If the node is a transmitter/receiver of a success-

fully scheduled transmission, after receiving/sending the ACK

packet, it will empty its ANL because the current round of

transmission has finished. If a node only overhears control

packets destined for other nodes, it will empty its ANL

on receiving a DATA/ACK packet that belongs to another

transmission. For possible collision or discarding of packets,

each node should first checks its ANL before sending the

RTS/CTS packet. If the information stored in the ANL has

timeout, then the node empties its ANL.

For a receiver node to schedule its transmission, it needs to

tell the neighboring nodes how much additional interference itcan tolerate. Next, we explain how the receiver computes its

maximum tolerable interference PMTI. Let the backgroundnoise experienced by one node is Pbackground, which iscommon to all nodes. Then, if there are no other noise besides


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the background noise, for a node to decode the received

packet correctly, the minimum reaching power (denoted by

PrxThreshold) of the packet is:

P(v)rxThreshold = PbackgroundSINR (1)

Let P(v)current denote the accumulated interference power due

to current scheduled transmissions in the vicinity of node v.It is defined as,

P(v)current =

j

Gvj Ptx (2)

where j is a transmitter recorded in node vs ANL. If node vis a master receiver, its P

(v)current will be zero because its ANL

is empty at the moment of computing P(v)current.

Then, the total future interference that node v can toleratewithout violating its SINR given by:

P(v)addable =

P(uv)rx

SINR

PrxThreshold P(v)current (3)

where P(uv)rx is the reaching power of the control packet from

the corresponding transmitter, node u.The maximum tolerable interference (MTI) that each future

neighboring node can add to node v is calculated as:

P(v)MTI =

P(v)addable

N(v)ACG(1 + )

(4)

where N(v)ACG is the number of access slots (AS) in the ACG

of node v (see subsection C for more details); (


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receiver should send back a CTS packet with new values of

Tdata and Tack. To achieve this, we add an M tag in the RTSpacket to indicate whether the ANL of its sender is empty or

not. The detailed format of RTS/CTS/ATS packet is shown

below:

RT S(ij) = {i, j, T(ij)data, T

(ij)ack , M }

CT S(ji) = {i, j, P(j)MT I, T(ij)data, T

(ij)ack }

AT S(ij) = {i, j, T(ij)data, T

(ij)ack }

C. Adaptation of NACG

The additional control gap (ACG) offers the nodes in the

vicinity of a scheduled transmission the chance to exchange

their own control packets (RTS/CTS/ATS), and thus improves

the throughput of the network. However, on the other hand, the

additional waiting time introduced by ACG may also decrease

the throughput if the duration of ACG is not utilized well. As

the simulation results in [7] showed, if the size of ACG is

too small, the neighboring nodes do not have enough time to

exchange their own control packets, and the enhancement of

performance is insignificant. On the other hand, if the size of

ACG is too large, the negative effect of ACG will devalue the

benefits gained through concurrent transmissions. Therefore,

the size of ACG has a significant impact on the performance

of the CTMAC protocol.

It is desirable that the size of ACG is adjusted according

to the current network status to achieve better performance.

In [7], the authors discussed the relationship between the

performance of MACA-P and the size of ACG. In MACA-P, the size of ACG is fixed and there is no adaptation

mechanism. In [12], the author proposed to adjust the size

of ACG according to the cumulate power of interference

signal during one nodes transmission. We notice that two

types of nodes contribute to the cumulate interference of a

node: nodes in and out of its transmission range. However,

only the nodes within the transmission range of the node can

receive its control packet and thus utilize its ACG to schedule

concurrent transmissions. Therefore, we only need to consider

the nodes in the transmission range when adjusting the ACG.

Fortunately, these nodes are recorded in the ANL, and we can

tune the size of ACG adaptively according to the number ofentries in the ANL.

In CTMAC, the ACG consists of an adjustable number

(NACG) of access slot (AS). The duration of an AS is fixedand equals the sum of the time required to transfer the RTS,

CTS, and ATS packets, plus the maximum backoff time when

the CW equals 31. It is long enough for one potential slave

transmission to exchange its control packets and make itself

scheduled if possible.

The initial and minimum value of NACG for the mastertransmitter is 1, which allows one potential slave transmission

to be scheduled. After initialization, the NACG of the master

transmission is updated adaptively according to the currentinformation in the ANL. If the number of concurrent slave

transmissions is larger than or equal to the current value of

NACG , then NACG is increased. Otherwise, the NACG isdecreased. To prevent the fluctuation, the step of increment

TABLE II

PARAMETERS USED IN THE SIMULATION

Propagation model TwoRayGround

RadioModel Accnoise

Data packet size 2KB

Data rate 2 Mbps

SINR 6 dB

Receive sensitivity -94dBm

Receive threshold -82dBm

Transmit power 15dBm

Transmission range 400m

Carrier-sense range 800m

or decrement is 1. However, it is not the case that the larger

the value ofNACG the better, because waiting excessively willovercome the performance gain through concurrent transmis-

sions. In CTMAC, the maximum value of NACG is set to 3,which allows at most three slave transmissions to be scheduled

concurrently with the master one. For the slave node, it can

calculate its own NACG according to the remaining time ofits master transmission.

V. PERFORMANCE EVALUATION

We have evaluated the performance of the proposed CT-

MAC protocol via simulation, in comparison with the IEEE

802.11 scheme and MACA-P [7]. We implemented CTMAC

and MACA-P by extending the 802.11 DCF MAC protocol

available in the GloMoSim [14] simulator. For simplicity, data

packets are assumed to be of a fixed size of 2KB. We focus on

one-hop throughput, thus the packet destination is restrictedto one hop from the source. Table II shows the settings of

the simulation parameters, which is compatible with standard

802.11 and corresponds to realistic hardware settings [15].Before analyzing the simulation results, we first consider

the cost of CTMAC for any possible throughput enhancement.

Compared with IEEE 802.11, the RTS packet is implemented

with an extra 2-byte field for Tack and 1-bit for M tag in theheader (remembering that the original RTS has one duration

field can be used for Tdata). CTS packet is implemented withan extra 12-byte field for Tack, PMT I and the sender address.Since the original 802.11 RTS/CTS packet is about 40 and

36 bytes respectively (including the physical header whichis 24 bytes), the increase in the size of control packets is

around 18%. Moreover, the ATS packet is 36 bytes including

the physical header, so in the worst condition, i.e., the ATS

packet is not used for all transmissions, it imposes additional

67% signaling overhead. However, all these costs are well

deserved. For the typical 2K size data packet, the overall cost

of control packets including the ATS packet is about 6%, while

the successful scheduling of two concurrent transmissions

will improve the throughput about 75%, as illustrated later

in the line topology. If there are more possible concurrent

transmissions, the enhancement of throughput can be even

more.

A. Line Topology

We first use the topology shown in Fig. 5 to highlight

the operational details of CTMAC, though this simplified


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i j nm

50m 200m

m moving towards n

150m

Fig. 5. Line topology of two transmissions.

scenario does not exhibit the best performance of CTMAC.The distances between nodes are also shown in the figure.

Node i is transmitting packets to node j, node n is transmittingpackets to node m. Persistent load is used in this experiment,i.e., i and n always have packets to send. Since nodes i, j,m, n are all in each others transmission ranges, there can beonly one transmission at anytime under the scheme of IEEE

802.11.

In CTMAC, both transmissions may proceed simultane-

ously. At the beginning, the distances between node m to nodei and n are approximately the same, and node is transmissionprohibits node m from correctly receiving node ns DATA

packet, because its SINR is violated. Due to the existenceof ACG and the delay introduced by it, the performance of

CTMAC is less than IEEE 802.11, but the degradation is

small. Then node m begins to move towards n at the speedof 10m/s and node is influence on node m is decreasing.After 5 seconds, node ms SINR is still kept while node iis transmitting, so these two transmissions can take place at

the same time. From Fig. 6, we can see CTMAC outperforms

802.11 greatly after the 5th second.

Note that during 5-6 seconds, the gain of performance is

half of the expected result, and this is due to the consideration

of when node m computes the PMT I. In fact, from 5th

second, node i doesnt violate node ms SINR any more, butthe existence of decrease the PMT I of node m. When thetransmission nm is the master transmission, the transmissionij is blocked. If the transmission ij is the master one, thetransmission nm can still be scheduled. Roughly, for half ofthe time, node i starts before node n while for another half ofthe time node n starts before node i. Therefore, the throughputenhancement is about 50% during this period. From the 7 th

second, even when taking into account , the PMT I of nodem is large enough for node i to start transmitting, so we canget 100% of expected enhancement.

B. Random Grid Topologies

We next study the performance of CTMAC under more

generic network topologies. First we consider a random grid

topology where nodes are placed within a square area of

length 800 meters. The square is split into n n smallsquares, with one node placed in the small square randomly.

Assume there are m transmission pairs where the transmitteris saturated. The destination nodes of all transmissions are

chosen randomly from nodes in the neighboring grids of the

corresponding transmitters. Since all the nodes are within the

carrier sense range of each other, only one transmission can

proceed at a time under the IEEE 802.11 scheme. To furtherstudy CTMAC, we also implemented the MACA-P protocol

and compared its performance with our CTMAC. MACA-P

also works on IEEE 802.11 compatible hardware and without

the help of TPC, so it is fair to do this kind of comparison.

2 4 6 8 10 12 14 16 18 200

0.5

1

1.5

2

2.5

3

3.5

Time (sec)

Throughput(Mbps)

CTMAC

802.11

Fig. 6. Performance of the CTMAC and the 802.11 protocols (line topology).

Because MACA-P fixes the size of additional control gap, we

choose the optimal values for different scenarios according to

[7]. In our simulation of MACA-P, the optimal size of control

gap for different m is 256B (m=2), 448B (m=3) and 640B(m=4).

The results of the performance evaluation are illustrated in

Fig. 7, where the results are shown for different values of

m. We can see that the density of nodes greatly affects thenetwork throughput under CTMAC scheme. With the increase

of node density, the average distance between the transmitter

and the receiver is decreasing, because the receiver is in

the neighboring grids of its transmitter. At the same time,

the average distance between nodes of different transmissions

is increasing. Thus, the number of schedulable simultaneous

transmissions increases, resulting in the enhancement of the

network throughput. Since MACA-P only schedules concur-

rent transmissions when neighboring nodes are both trans-

mitters or receivers, its performance enhancement is alwaysless than CTMAC. Furthermore, even when the requirement is

fulfilled for MACA-P, two concurrent transmissions may still

interference each other if they are too close, so MACA-P may

schedule concurrent transmissions improperly. In fact, all the

successfully processed concurrent transmissions in MACA-P

are sure to be scheduled in CTMAC, but CTMAC can schedule

much more possible concurrent transmissions than MACA-P.

This is the main reason that why CTMAC performs better.

At any moment of time, the number of contending transmis-

sions in the system also has an impact on the throughput of

the network, because there will be more potential concurrent

transmissions if there are more contending transmissions.From Fig. 7, we can see that under the same node density,

more contending transmissions mean greater throughput en-

hancement for CTMAC and MACA-P, but CTMAC performs

better.

C. Cluster Topologies

Traffic locality is the key to determine the feasibility of large

ad hoc networks. Therefore, we also studied the performance

of CTMAC under clustered topologies. In such a topology, a

node communicates mostly with nodes within the same cluster,

and rarely with nodes in the other clusters. These scenariosare common in real environments.

To generate a cluster topology, we consider an area of

dimensions 400*400 (in meters). Sixteen nodes are split into

four equal groups, each occupying a 100*100 square in one


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2 3 4 5 6 7 80

0.5

1

1.5

2

2.5

Number of nodes(n*n) (m=2)

Throughput(Mbps)

CTMAC

MACAP

802.11

3 4 5 6 7 80

0.5

1

1.5

2

2.5

3


Throughput(Mbps)

CTMAC

MACAP

802.11

3 4 5 6 7 80

0.5

1

1.5

2

2.5

3


Throughput(Mbps)

CTMAC

MACAP

802.11

Fig. 7. Performance of the CTMAC and other protocols (random grid topology).

0 20 40 60 80 1000

0.5

1

1.5

2

2.5

3

Packet generation rate (packets/sec)

Thro

ughput(Mbps)

CTMAC802.11

0 20 40 60 80 1000

1

2

3

4

5

Packet generetion rate (packets/sec)

Thro

ughput(Mbps) CTMAC

802.11

Fig. 8. Performance of the CTMAC and the 802.11 protocols as function of k.

of the corners of the whole area. For a given transmitter, the

receiver is selected from another cluster with a probability of

p or from the same cluster with a probability of 1 p. Wesimulated the scenario of four transmissions in the network,

with the packet generation rate of k packets per second foreach transmitter.

Under this scenario, all nodes are in the transmission range

of each other, so only one transmission can be scheduled at any

time under the IEEE 802.11 scheme. For the MACA-P, also

only one transmission can process at one moment. However,

its throughput can be even less than IEEE 802.11 due to the

performance loss introduced by the additional control gap. If

the size of control gap is set to zero, the MACA-P degenerates

to the IEEE 802.11. So we only plot the results of CTMAC

and 802.11 in Fig. 8.

Part (a) of Fig. 8 compares the performance of CTMACand IEEE 802.11 when p = 0.25. With the increasing of thenetwork traffic, the CTMAC can achieve about 70% increment

in throughput over the IEEE 802.11 scheme. The result when

p = 0 is illustrated in part (b) of Fig. 8. In this case,CTMAC approaches its best performance, achieving about

150% increment over the IEEE 802.11 scheme.

D. Random Topologies

Finally, we evaluate the performance of CTMAC under the

random topology. We consider a 1000m*1000m area, with

100 nodes placed randomly. Assuming there are m end-to-endflows in the network and the source and destination nodes of

each flow are chosen randomly too. For each flow, the source

node is saturated and always has packet to send. To better

understand the performance the upper layer of MAC layer

4 5 6 7 8 9 100.5

0.6

0.7

0.80.9

1

1.1

1.2

Number of traffics (m)Endtoendthro

ughput(Mbps)

CTMAC

MACAP

802.11

Fig. 9. Performance of the CTMAC and other protocols (random topology).

can achieve through CTMAC, we use the metric of end-to-

end throughout in this simulation. The size of control gap

used for MACA-P is 640B, because we start from m = 4.The simulation results are plotted in Fig. 9. When m

is small, the throughput of MACA-P is less greatly than

802.11 and CTMAC. This is because the lack of dynamic

adjusting mechanism for the ACG. CTMAC overcomes this

problem and achieves similar performance as 802.11 when

m is small. With the increase of m, there are more chancesfor concurrent transmissions, so MACA-P and CTMAC both

perform better than 802.11. The larger the m to be, the moreimprovement MACA-P and CTMAC can achieve. However,

CTMAC always gains better performance than MACA-P.

VI. CONCLUSIONSIn this paper, we proposed CTMAC, a high-throughput

media access control protocol for MANETs. Similar to the

IEEE 802.11 scheme, CTMAC is based on a single-transceiver

circuitry, and operates over a single channel for data and


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control packets. Unlike existing transmission power control

approaches, CTMAC works on single transmission power and

can be applied to most existing hardware products.

Through simulation, we have evaluated and compared the

performance of the proposed CTMAC with the IEEE 802.11

scheme and the MACA-P protocol. The simulation results

showed that CTMAC can outperform the exiting protocols

and improve the network throughput by up to 150%. To the

best of our knowledge, CTMAC is the first single-channel,

single-transceiver and single-transmission power protocol that

increase network throughput while preserving the collision

avoidance property of the IEEE 802.11 scheme.

Besides tuning the parameters of CTMAC and investigating

its performance under various scenarios, our future work will

also address other techniques for capacity enhancement.

ACKNOWLEDGMENT

The authors would like to thank the editor and the anony-

mous reviewers for their helpful comments and suggestions inimproving the quality of this paper.

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[1] IEEE Computer Society LAN MAN Standards Committee, Ed., Inter-national Standard ISO/IEC 8802-11; Part 11: wireless LAN MediumAccess Control (MAC) and Physical Layer (PHY) specifications.ANSI/IEEE Std 802.11, 1999 Edn. The IEEE,1999.

[2] J. H. Kim and J. K. Lee, Capture effects of wireless CSMA/CAprotocols in Rayleigh and shadow fading channels, IEEE Trans. Veh.Technol., vol. 48, no. 4, pp. 12771286, July 1999.

[3] P. Karn, MACAa new channel access method for packet radio, inProc. 9th ARRL Computer Networking Conference, 1990, pp. 134140.

[4] M. B. Pursley, H. B. Russell, and J. S. Wysocarski, Energy-efficient

transmission and routing protocols for wireless multiple-hop networksand spread spectrum radios, in Proc. EUROCOMM 2000, May 2000,pp. 15.

[5] E.-S. Jung and N. H. Vaidya, A power control MAC protocol for adhoc networks, ACM/Kluwer Wireless Networks (WINET), vol. 11, no.1-2, pp. 5566, Jan. 2005.

[6] A. Acharya, A. Misra, and S. Bansal, MACA-P: a MAC for concurrenttransmissions in multi-hop wireless networks, in Proc. IEEE PerCom2003, Mar. 2003, pp. 505508.

[7] A Acharya, A Misra, and S Bansal, Design and analysis of a coop-erative medium access scheme for wireless mesh networks, in Proc.BroadNets 2004, Oct. 2004, pp. 2529.

[8] J. Monks, V. Bharghavan, and W.-M. Hwu, A power controlled multipleaccess protocol for wireless packet networks, in Proc. IEEE INFOCOM2001, Apr. 2001, pp. 219228.

[9] S.-L. Wu, Y.-C. Tseng, and J.-P. Sheu, Intelligent medium access for

mobile ad hoc networks with busy tones and power control, IEEE J.Select. Areas Commun., vol. 18, no. 9, pp. 16471657, Sept. 2000.

[10] A. Muqattash and M. Krunz, Power controlled dual channel (PCDC)medium access protocol for wireless ad hoc networks, in Proc. IEEEINFOCOM 2003, Apr. 2003, pp. 470480.

[11] V. Kawadia and P. R. Kumar, Principles and protocols for power controlin ad hoc networks, IEEE J. Select. Areas Commun. (special issue onad hoc networks), vol. 23, no. 1, pp. 7688, Jan. 2005.

[12] A. Muqattash and M. Krunz, POWMAC: a single-channel power-control protocol for throughput enhancement in wireless ad hoc net-works, IEEE J. Select. Areas Commun., vol. 23, no. 5, pp. 10671084,May 2005.

[13] T. S. Rappaport and L. B. Milstein, Effects of radio propagation pathloss on DS-CDMA cellular frequency reuse efficiency for the reversechannel, IEEE Trans. Veh. Technol., vol. 41, pp. 231242, Aug. 1992.

[14] GloMoSim. [Online]. Available: http://pcl.cs.ucla.edu/projects/glomosim.

[15] Data sheet: Cisco Aironet 350 Series client adapters. [On-line]. Available: http://www.cisco.com/en/US/products/hw/wireless/index.html.

Wanrong Yu received the B.Sc. degree, the B.A.degree, and the Ph.D. degree in computer sciencein 1999, 2002, and 2006 respectively, all from theSchool of Computer, National University of DefenceTechnology, Changsha, China. From 2005.3-2006.3,he visited the Department of Computing, HongKongPolytechnic University, working as a visited Ph.dCandidate. Now he is a lecture in the School ofComputer, National University of Defence Technol-

ogy. He is active in research and has published over20 research papers. His research interests lie in MAC

protocols,cross design for wireless ad hoc networks, wireless sensor networkand wireless mesh networks.

Jiannong Cao is a professor in the Department ofComputing at Hong Kong Polytechnic University.He received the B.Sc. degree in computer sciencefrom Nanjing University, China, in 1982, and theM.Sc. and Ph.D. degrees from Washington StateUniversity, USA, in 1986 and 1990, all in com-puter science. From 1982 to 1983, he studied as aMSc. student in the Graduate School of the Chi-nese Acadmy of Science in Beijing. Before joinedthe Hong Kong Polytechnic University in 1997, hehas been on faculty of computer science in James

Cook University and The University of Adelaide in Australia, and the CityUniversity of Hong Kong. He is an adjunct professor of BJTU, NUDT andNWPU, and a guest professor of CSU in China. He also held several visitingpositions, including a visiting research professor in the National Key Lab forNovel Software Technology, Nanjing University of China, a visiting fellow inthe School of Computer Engineering, Nanyang Technological University ofSingapore, a visiting scholar at the Institute of Software, Chinese Academy ofScience, and Peking University Overseas Scholar Lecture Program. He is theCoordinator in Asia of the Technical Committee on Distributed Computing(TPDC) of IEEE Computer Society . He is a vice chairman and memberof the Technical Committee on Computer Architecture, a member of theTechnical Committee on System Software and the Technical Committee ofPervasive Computing, and a senior member of China Computer Federation.He is a senior member of the IEEE (including the IEEE Computer Societyand the IEEE Communication Society), and a member of the ACM. He is

also a member of the IEEE Technical Committee on Parallel Processing, IEEETechnical Committee on Fault Tolerant Computing, and ACM SIG-Mobile.He has served as associate editor and member of editorial boards of severalinternational journals, a reviewer for international journals conference pro-ceedings, and also as chair and member of organizing programme committeesfor many international conferences. His research interests include mobile andpervasive computing, wireless networking, parallel and distributed computing,fault tolerance, and distributed software architecture and programming.

Xingming Zhou has been with National Universityof Defence Technology since 1978, where he is cur-rently the head of scholar committee of National KeyLaboratory on Parallel and Distributed Processing.He has been the Academician of Chinese Academyof Science (CAS) since 1992. He has been the co-chair of many international conferences and work-

shops on parallel computing, network computing,and mobile networks. His current research interestsinclude high performance parallel and distributedcomputing, network computing.

Xiaodong Wang received the B.Sc. degree, the B.A.degree, and the Ph.D. degree in computer science in1994, 1998, and 2001 respectively, all from the Na-tional University of Defence Technology, P.R.China.He is a member of IEEE Communication Societyand China Computer Federation. His main researchinterests are wireless networks and mobile comput-ing, including mobile ad hoc networks and wirelesssensor networks.


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Keith C. C Chan has a B.Math. (Hons.) degreein Computer Science and Statistics and a M.A.Sc.and Ph.D. degree in Systems Design Engineeringfrom the University of Waterloo, Ontario, Canada.He has a number of years of academic and industrialexperience in software development and manage-ment. Before joining The Hong Kong PolytechnicUniversity, he was with the IBM Canada Laboratory,Toronto, Canada, where he was involved in the

development of Image and Multimedia software aswell as software development tools. He joined the

Department of Electrical and Computer Engineering at Ryerson PolytechnicUniversity, Ontario, Canada as an Associate Professor in 1993, beforereturning to HK to join the Hong Kong Polytechnic University (PolyU) in1994. He is currently a Professor and Head of the Department of Computingand is in charge of the Intelligent Home Group. He is also an AdjunctProfessor of the Institute of Software, The Chinese Academy of Sciences,Beijing, China. He is active in consultancy and has served as a consultantto government agencies as well as large and small to medium enterprises inHong Kong, China, Singapore, Malaysia and Canada. His research interestsare in software engineering, data mining and computational intelligence.

Alvin T. S. Chan obtained extensive industrial ex-perience before joining the Hong Kong PolytechnicUniversity in 1998. After graduating from the Uni-versity of Leeds in 1990, he joined Chartered Indus-trial of Singapore as a software engineer responsiblefor the development of software for military radiosystems used in combat and hostile environments.Having completed his PhD study at the Universityof New South Wales in 1995, he was employed as

a research scientist by the Commonwealth Scien-tific and Industrial Research Organization (CSIRO),

where he was involved in the design and deployment of a wide area video-on-demand system based on ATM. He returned to Singapore in 1997 and wasemployed as a Program Manager by the Center for Wireless Communications,at the National University of Singapore. His current research interests focuson several key areas including mobile computing, context-aware computingand smart card applications.

H. V. Leong graduated from the University ofCalifornia at Santa Barbara in 1994 and joined theHong Kong Polytechnic University. He received hisundergraduate and first graduate degree from theChinese University of Hong Kong. Since then, hehad been working as teaching and research assistantsthroughout his graduate career and held severalscholarships and fellowships. He is active in researchand has published over 100 refereed research papers.His research interests lie in distributed systems,distributed databases, and mobile computing. He

is also interested in applications originating from the domains of parallelprogramming, internet, and geographical information systems. He is a memberof the ACM, IEEE Computer Society and IEEE Communications Society.

89.a high-throughput mac protocol for wireless ad hoc networks

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