Many-to-One Throughput Capacity of IEEE 802.11 Multi-hop Wireless
  Networks
Microsoft Word - Transaction _Mar 2, 2007__1.doc
IEEE TRANSACTIONS ON MOBILE COMPUTING,  MANUSCRIPT ID 1 
Many-to-One Throughput Capacity of IEEE 
802.11 Multi-hop Wireless Networks 
Chi Pan Chan, Student Member, IEEE, Soung Chang Liew, Senior Member IEEE, and An Chan, 
Student Member, IEEE 
Abstract—This paper investigates the many-to-one throughput capacity (and by symmetry, one-to-many throughput capacity) 
of IEEE 802.11 multi-hop networks.  It has generally been assumed in prior studies that the many-to-one throughput capacity is 
upper-bounded by the link capacity L. Throughput capacity L is not achievable under 802.11. This paper introduces the notion of 
“canonical networks”, which is a class of regularly-structured networks whose capacities can be analyzed more easily than 
unstructured networks. We show that the throughput capacity of canonical networks under 802.11 has an analytical upper 
bound of 3L/4 when the source nodes are two or more hops away from the sink; and simulated throughputs of 0.690L (0.740L) 
when the source nodes are many hops away. We conjecture that 3L/4 is also the upper bound for general networks. When all 
links have equal length, 2L/3 can be shown to be the upper bound for general networks. Our simulations show that 802.11 
networks with random topologies operated with AODV routing can only achieve throughputs far below the upper bounds. 
Fortunately, by properly selecting routes near the gateway (or by properly positioning the relay nodes leading to the gateway) to 
fashion after the structure of canonical networks, the throughput can be improved significantly by more than 150%. Indeed, in a 
dense network, it is worthwhile to deactivate some of the relay nodes near the sink judiciously. 
Index Terms—wireless mesh networks, many-to-one, one-to-many, data-gathering networks, 802.11, sensor networks, 
throughput capacity, wireless multi-hop networks. 
——————————      —————————— 
1 INTRODUCTION
any-to-one communication is a common communi-
cation mode in many multi-hop wireless networks. 
Two relevant applications are sensor networks and 
multi-hop wireless mesh networks. In sensor networks, 
there is often a “data processing center” to which data 
collected at distributed sensors are to be forwarded. In 
multi-hop wireless mesh networks, there is an Internet 
gateway connecting the mesh network to the core wired 
Internet – the client stations and the Internet gateway 
form a many-to-one relationship.  
This paper investigates the many-to-one throughput 
capacity of IEEE 802.11 multi-hop networks. In this set-
ting, there are multiple source nodes generating traffic 
streams to be forwarded to a common sink node via relay 
nodes. The relay nodes could be sources themselves. By 
symmetry, the throughput capacity thus found is also the 
same as that in a one-to-many scenario in which a source 
node generates multiple distinct data streams to be for-
warded to their respective sinks (note: this is not to be 
confused with the multicast scenario in which the same 
data is to be forwarded to multiple sinks). For conven-
ience, we shall refer to the sink in the many-to-one sce-
nario as the “center” of the network.  
There have been many related studies on the capacity 
of general wireless networks. Gupta and Kumar [1] ana-
lyzed the capacity in many-to-many situation. It provides 
the basic model that can be adapted for use in the analysis 
of the many-to-one communication. As a loose bound, it 
is obvious that the many-to-one throughput capacity is 
upper-bounded by L [1]-[3], where L is the single-link 
throughput capacity, since this is the rate at which the 
sink can receive data. There is a high probability, how-
ever, that the throughput capacity is lower than L for a 
random network [3]. This paper follows the approach 
used in [1]-[3] in characterizing which nodes can transmit 
together without packet collisions. The main difference is 
that here we are interested in the capacity throughput 
obtained under the IEEE 802.11 distributed MAC protocol 
[4]. Specifically, we integrate into our analysis the effects 
of carrier sensing,  the existence of an ACK frame for each 
DATA frame transmission, and the distributed nature of 
the CSMA protocol,  while [1]-[3] do not and their bounds 
are obtained with the implicit assumption of perfectly 
scheduled transmissions.     
There are three main contributions to this paper: 
1. We introduce the notion of “canonical networks”, 
which is a class of regularly-structured networks 
whose capacities can be analyzed more easily than 
general unstructured networks. We find that the 
throughput capacity of canonical networks under 
802.11 is upper bounded by 3L/4 when the source 
nodes are at least two hops away from the sink. We 
conjecture that this is also the upper bound for general 
networks. Indeed, when all the links in the network 
are of equal length, canonical networks and general 
networks have the same upper bound of 2L/3.   
xxxx-xxxx/0x/$xx.00 © 200x IEEE 
———————————————— 
• All authors are with the Department of Information Engineering, The 
Chinese University of Hong Kong, New Territories, Hong Kong. E-mail: 
C.P. Chan : cpchan4@ie.cuhk.edu.hk , S. C. Liew : soung@ie.cuhk.edu.hk, 
A. Chan : achan5@ie.cuhk.edu.hk.  
Manuscript received (insert date of submission if desired). Please note that all 
acknowledgments should be placed at the end of the paper, before the bibliography. 
2 IEEE TRANSACTIONS ON MOBILE COMPUTING,  MANUSCRIPT ID 
2. We find that canonical networks give much insight on 
how a many-to-one network should be designed in 
general. Our simulations show that 802.11 networks 
with random topologies operated with AODV routing 
can only achieve throughputs far below the upper 
bound of canonical networks. However, if we route 
the traffic in accordance to the optimized routes ob-
tained from an optimization algorithm, the routes near 
the center have a structure similar to that of the opti-
mal canonical network structure. In other words, as a 
principle, routing or network design near the center 
should be fashioned after the canonical network. Our 
further investigation shows that a “manifold” canonical 
network structure near the center may yield through-
put improvement of more than 150% relative to that 
obtained by using AODV routing in a general network 
structure. Indeed, in a dense network, it is worthwhile 
to deactivate some of the relay nodes near the sink ju-
diciously.  
3. We find that ensuring the many-to-one network is 
hidden-node free (HNF) in our design leads to higher 
throughputs as compared to not doing so. This is in 
contrast to the many-to-many case, in which the large 
carrier-sensing range required to ensure the HNF 
property may lower the network throughput due to 
the increased exposed-node problem [5]. This observa-
tion is used as a design principle in much of the study 
in 1 and 2 above.  
 The rest of this paper is organized as follows. Section 
II provides the definitions and assumptions used in our 
analysis. Section III derives the throughput capacities of 
canonical networks, and presents simulation results to 
support our findings. In addition, we demonstrate the 
desirability of ensuring the HNF property in many-to-one 
networks. Section IV investigates general networks not 
restricted to the canonical network structure. We show 
that the optimal routing in general networks results in a 
subset of selected routes that form a structure near the 
center that resembles the optimal canonical network. We 
then apply this insight to demonstrate the desirability of 
designing the network according to a “manifold” canoni-
cal-network structure near the center. Section V concludes 
this paper. 
2 DEFINITIONS AND ASSUMPTIONS 
Let us first provide some definitions used in our analy-
sis.  
Definition 1: The source nodes are nodes that generate data 
traffic. 
Definition 2: The sink node is the center to which the data 
collected at the source nodes are to be forwarded. 
Definition 3: The relay nodes relay data traffic from the 
source nodes to the sink node.  
Note that a node can be classified as one of the follow-
ings: 1) a source node; 2) a sink node; 3) a relay node; or 4) 
both a source node and a relay node.  
Definition 4: Given a network topology, the uniform throu-
ghput capacity uC with respect to a set of source nodes and 
a sink node is the maximum total rate at which the data 
can be forwarded to the sink node, with equal amount of 
traffic from each source node to the sink node. The throu-
ghput capacity, mC , on the other hand, does not require 
equal amounts of traffic from sources to sink. Thus, in 
general, m uC C≥ . 
Fig. 1 shows a simple example of a network consisting 
of three nodes. Suppose that node 2 is the sole source 
node and node 1 is the relay node that forwards packets 
from node 2 to node 0. Node 1 does not generate traffic 
by itself.  Then, / 2mC L= , where L is the capacity of one 
link. This is because node 1 cannot receive and transmit at 
the same time (typical assumption of half-duplexity of 
wireless links). Also, since there is only one source node, 
m uC C= .  
If node 1 is also a source node in addition to being a re-
lay node, then mC L=   (obtained when only node 1 is 
allowed to transmit), and 2 / 3uC L= , with nodes 1 and 2 
having a throughput of L/3 each. Since node 1 needs to 
serve as the relay node for node 2, node 1 will need to 
transmit twice as often as node 2. So, proper scheduling is 
required.  
Now, if we generalize the above linear network [7] to 
the one consisting (n+1) nodes, in which there are n 
sources nodes with (n-1) of them also being relay nodes. 
Then, uC  can be obtained as follows. Node 1 will trans-
mit to node 0, the sink node, at rate uC . Node 2 will 
transmit to node 1 at rate ( 1) /uC n n− , and so on. In gen-
eral, node (i+1) transmits to node i at rate ( ) /uC n i n− . We 
note that when node i transmits, nodes (i+1) and (i+2) 
cannot:  node (i+2) cannot transmit because the reception 
at node (i+1) will be corrupted by the transmission by 
node i. So, considering transmissions of nodes 3, 2, and 1 
(which       form      the       bottleneck), we             have 
( 1) / .u
C n i n L
− + =∑  That is, /(3 3) / 3uC Ln n L= − ≈  for 
large n.  
We note that L/3 is also the mC  if node n were the only 
source node. As a matter of fact, / 3u mC C L= =  if the 
source nodes in the linear network were nodes i for 3i ≥  
only. Thus, for reasonably large n, if the traffic from 
nodes 1 and 2 is only a small fraction of the total traffic to 
the sink,   we could treat nodes 1 and 2 as pure relay, non-
source, nodes. Once we do that, we then do not have to 
distinguish between uC and mC .  
We next consider a general many-to-one network, such 
as that in Fig. 2. For the study of many-to-one networks in this 
paper, we focus on the case where the source nodes are two or 
more hops away from the sink. This is a good approximation 
when the nodes within one hop to the sink only generate 
a small fraction of the total traffic.  
Definition 5: The throughput capacity with respect a 
multi-access protocol p (e.g., IEEE 802.11), pC , is the total 
rate at which the data can be forwarded to the sink 
nodeusing that protocol, assuming the source nodes are 
two or more hops away from the sink.   The  transmission  
CHAN ET AL.:  MANY-TO-ONE THROUGHPUT CAPACITY OF IEEE 802.11 MULTI-HOP WIRELESS NETWORKS 3 
schedule by the links is dictated by the protocol.  
This paper focuses on the throughput capacity under 
the 802.11 CSMA protocol, 802.11C . Henceforth, by 
throughput capacity, we mean 802.11C . For illustration, let 
us consider the two-chain linear topology shown in Fig. 3. 
Suppose that only nodes 2 and 2’ are the source nodes. 
Under “perfect scheduling”, nodes 1 and 2’ will transmit 
together; and nodes 1’ and 2 will transmit together. This 
results in a throughput capacity of L. Under 802.11, how-
ever, the transmissions are usually not perfectly aligned 
in time. In addition, a DATA frame is followed by an 
ACK frame in the reverse direction. Suppose nodes 1 and 
2’ transmit together. Say, the transmission of the DATA 
frame of node 1 completes first, while the transmission 
node 2’ is ongoing. When node 0 returns an ACK to node 
1, this ACK also reaches node 1’, the receiver of the 
transmission from node 2’, causing a collision there. Thus, 
under 802.11, simultaneous transmissions by nodes 1 and 
2’ will usually result in a collision unless the completion 
times of their DATA transmissions are perfectly aligned, 
which is rare. In this case, 802.11C is at best 2L/3, since at 
best node 2 and 2’ can transmit together, and nodes 1 and 
1’ will need to transmit at separate times.  
For many-to-one networks, the capacity bottleneck is 
likely to be near the sink node because all traffic travels 
toward the sink node. Specifically, nodes near the sink 
node are responsible for forwarding more traffic, and 
these nodes contend for access of the wireless medium 
because they are close to each other. To obtain an idea on 
the upper limit of the throughput capacity under 802.11, 
we consider a class of networks referred to as the canoni-
cal networks. An example of a canonical network is 
shown in Fig. 4. We show that 3L/4  is the upper bound 
of the throughput capacity of canonical networks, and 
conjecture that this is also the upper bound for networks 
with general structures. We will motivate the study of the 
canonical networks shortly. In the special case in which 
all links have equal length, then the throughput capacities 
of the canonical network as well as general networks are 
upper-bounded by 2L/3. We now define the canonical 
networks.  
Definition 6: A chain is formed by a sequence of at least 
three nodes leading to the center sink node. Traffic is for-
warded from one node to the next node in the sequence 
on its way to the sink node. A linear chain is a chain 
which is a straight line.  
In Fig. 4, for example, there are eight linear chains. 
Definition 7: An i-hop node is a node that is i hops away 
from the sink node in a chain (see Fig. 4). 
Definition 8: A canonical network is formed by a number of 
linear chains leading to a common center sink node; the 
nodes in different chains are distinct except the sink node. 
In addition, the distance between an i-hop node and an (i-
1)-hop node, di, is the same for all the linear chains (see 
Fig. 4).  
Definition 9: A ring is a circle centered on the sink node. 
An i-hop ring consists of all the i-hop nodes of the differ-
ent linear chains in a canonical network (see Fig. 4). 
Motivation for the Study of Canonical Networks  
Canonical networks have regular structures and can 
be analyzed more easily than general networks. We con-
jecture that the upper bound of throughput capacity ob-
tained for canonical networks is also the upper bound for 
general networks, because intuitively canonical networks 
model a rich class of networks the optimal of which may 
yield very good throughput performance. Consider the 
following intuitive argument. (i) In a densely populated 
network (say, infinitely dense), we may choose to form 
linear chains from the source nodes to the center sink 
node for routing purposes. Since the direction of traffic 
flow is pointed exactly to the center, there is no “wastage” 
Fig. 1. Simple network example. 
Fig. 2. A random many-to-one network. 
Fig. 4. A Canonical Network. 
Fig. 3. A two-chain many-to-one network with equal link length.
Sink Source 
Node 0 Node 1 Node 2 
Relay Sink Source 
Node 0 Node 1 Node 2 
Relay Relay Source 
Node 1’ Node 2’ 
ring 
3-hop node 
d0 d1 d2 
2-hop node 
1-hop node 
chain 
4 IEEE TRANSACTIONS ON MOBILE COMPUTING,  MANUSCRIPT ID 
with respect to the case in which the routing direction is 
at an angle to the center. (ii) We have defined the class of 
canonical networks to be quite general in that we do not 
restrict the number of linear chains in it. Neither do we 
limit the distance di. In deriving the capacity of the ca-
nonical network later, we allow for the possibility of an 
infinite number of linear chains and arbitrarily small di. 
This provides us with a high degree of freedom in identi-
fying the best-structured canonical networks. The above 
intuitive reasoning will be validated by simulation results 
later. In addition, we will show later that in a random 
network with many nodes (so that there is a high degree 
of freedom in forming routes), establishing a canonical-
network-like structure near the center for routing pur-
poses will generally lead to superior throughput per-
formance.  
In this paper, unless otherwise stated, we further as-
sume the following: 
Assumptions: 
(1) The nodes and links are homogenous. They are con-
figured similarly, i.e., same transmission power, carrier-
sensing range (CSRange), transmission rate, etc. 
(2) ACK is sent by the receiver when a packet is received 
successfully, as per the 802.11 DCF operation.  
(3) The following constraints apply to simultaneous 
transmissions [1][6]. Consider two links (T1 ,R1) and 
(T2 ,R2). For simultaneous transmissions without collisions, 
they must satisfy all the eight inequalities below: 
2 1 1 1
2 1 1 1
2 1 1 1
2 1 1 1
1 2 2 2
1 2 2 2
1 2 2 2
1 2 2 2
T R T R
R R T R
T T T R
R T T R
T R T R
R R T R
T T T R
R T T R
X X X X
X X X X
X X X X
X X X X
X X X X
X X X X
X X X X
X X X X
− > + ∆ −
− > + ∆ −
− > + ∆ −
− > + ∆ −
− > + ∆ −
− > + ∆ −
− > + ∆ −
− > + ∆ −
 (1) 
where Xi is the location of node i, |Xi – Xj| is the distance 
between Xi and Xj, ∆ > 0 is the distance margin (see next 
paragraph). These are the physical constraints that pre-
vent DATA-DATA, DATA-ACK and ACK-ACK colli-
sions. 
The received power function can be expressed in the 
form of 
 ( ) /tP d P d
α∝ , (2) 
where Pt is the transmission power, d is the distance and 
α is the path-loss exponent, which typically ranges from 2 
to 6 according to different environments [8]. By the as-
sumptions that all the nodes have the same transmission 
power and α = 4, and Signal-to-Interference Ratio (SIR) 
requirement of 10dB. Then at R1 , we require 
(|  -  |)
(|  -  |)
P X X
P X X
>    (3) 
giving 
|  -  |
 10   1.78
|  -  |
> =    
In other words, ∆ = 0.78. Unless otherwise stated, we as-
sume ∆ = 0.78 throughout this work.  
(4) In 802.11 networks, there are two types of packet colli-
sions: collisions due to hidden nodes (HN) (see explana-
tion of assumption (5) below or [6]), and collisions due to 
simultaneous countdown to zero in the backoff period of 
the MAC of different transmitters. In much of our throu-
ghput-capacity analysis, we will neglect the latter colli-
sions and assume that they have only small effects toward 
throughput capacity, a fact which has been borne out by 
simulations and which can be understood through intui-
tive reasoning, particularly for a network in which a node 
is surrounded by only a few other active nodes who may 
collide with it. As will be shown later in this paper, this is 
generally a characteristic of a network with good throu-
ghput performance (see results of Fig. 14 and Fig. 18, for 
example). Also, an upper bound on throughput capacity 
obtained by neglecting the countdown collisions is still a 
valid upper bound. It is a good upper bound so long as it 
is tight. We will see later that the upper bounds we obtain 
are reasonably tight when verified against simulations 
results in which countdown collisions are taken into ac-
count. In the remainder of this paper, unless otherwise 
stated, the term “collisions” refers to collisions due to HN 
(i.e., caused by the failure of carrier-sensing) rather than 
simultaneous countdown to zero.  
(5) In this paper, unless otherwise specified, we assume 
the so-called Hidden-Node Free Design (HFD) [6] in the 
network. That is, we design the network such that simul-
taneous transmissions that will cause collisions can be 
carrier-sensed by transmitters and be avoided. A reason 
for this assumption is that for many-to-one communica-
tion, eliminating hidden nodes is worthwhile (see simula-
tion results in Section III-C). According to [6], HFD re-
quires 
(i) Use of Receiver Restart (RS) Mode, and 
(ii) Sufficiently large CSRange. 
This paper assumes the 802.11 basic mode and 
RTS/CTS are not used. We briefly describe the HFD re-
quirements for understanding of the analysis later. More 
details can be found in [6]. Fig. 5 is an example showing 
that no matter how large CSRange is, the hidden node 
(HN) phenomenon can still occur in the absence of RS. In 
the figure, T1 and T2 are more than CSRange apart, and so 
simultaneous transmissions can occur. Furthermore, the 
SIR is sufficient at R1 and R2 so that no “physical colli-
sions” occur. But HN can still happen, as described below. 
Assume T1 starts first to transmit a DATA packet to R1. 
After the physical-layer preamble of the packet is re-
ceived by R2, R2 will “capture” the packet and will not 
attempt to receive another new packet while T1’s DATA is 
ongoing. If at this time T2 starts to transmit a DATA to R2, 
CHAN ET AL.:  MANY-TO-ONE THROUGHPUT CAPACITY OF IEEE 802.11 MULTI-HOP WIRELESS NETWORKS 5 
R2 will not receive it and will not reply with an ACK to T2, 
causing a transmission failure on link (T2, R2). This is the 
default receiver mode assumed in the NS-2 simulator [10] 
and most 802.11 commercial products.  Note that the ex-
ample in Fig. 5 is independent of the size of CSRange. 
This HN problem can be solved with the Receiver Re-
start Mode (RS) which can be enabled in some 802.11 
products (e.g., Atheros Wi-Fi chips; however, the default 
is that this mode is not enabled). With RS, a receiver will 
switch to receive the stronger packet if its power is Ct 
times greater than the current packet (say, 10 dB higher). 
The example in Fig. 5 will not give rise to HN with RS if 
CSRange is sufficiently large.  
RS Mode alone, however, cannot prevent HN without 
sufficiently large CSRange. To see this, consider the ex-
ample in Fig. 6. Assume T1 transmits a DATA to R1 first. 
During the DATA’s period, T2 starts to send a shorter 
DATA packet to R2. With RS Mode, R2 switches to receive 
T2’s DATA and sends an ACK after the reception. If T1’s 
DATA is still in progress, R2’s ACK will corrupt the 
DATA at R1, since the distance between R1 and R2 is 
within interference range (
max(1 )d+ ∆ ). To prevent T2 from 
transmission (hence the collision), the following must be 
satisfied:  
1 2|  -  |   T TX X CSRange≤ . (4) 
Reference [6] proved that in general if CSRange > (3+∆) 
dmax, where dmax is the maximum link length, then HN can 
be prevented in any network. However, for a specific 
network topology, e.g., the canonical network, the re-
quired CSRange can be smaller. 
Throughout this work, we primarily focus on the pair-
wise-interference model [1][6]. The concept of CSRange 
and the constraints in (1) rely on this assumption. An 
analysis which at the outset takes into account the simul-
taneous interferences from more than one source will 
complicate things significantly. So, given a network to-
pology, our approach is to first identify the capacity 
based on pair-wise interference analysis only, and then 
verify the capacity is still largely valid under multiple 
interferences (this verification is done in Section III-D). 
3 CANONICAL NETWORKS 
In this section, we derive the throughput capacities of 
canonical networks. Section A analyzes two kinds of ca-
nonical networks: equal link-length and variable link-
length networks. Simulation results are presented and 
discussed in Section B. Section C compares the perform-
ance of HFD and non-HFD networks, and Section D veri-
fies the results under multiple interferences. 
3.1 Theoretical Analysis 
(1)      Equal Link-Length Networks 
We first consider the case where all links have the 
same length d, i.e., d0 = d1 =… =d. Theorem 1, which fol-
lows from  Lemma 1 and Corollary 1 below, proves that 
the throughput capacity in this network is upper-
bounded by 2L/3, where L is the single-link throughput. 
Lemma 1: Given three nodes on the periphery of a circle of 
radius d, we can identify two nodes with distance smaller 
than (1+∆)d between them. 
Proof: The three nodes form the vertices of a triangle. 
Consider the equilateral triangle inscribed on the circle of 
radius d , and let t be the length of one side (see Fig. 7). 
Then 
2 cos = 1.731 (1+ )
t d d d
= < ∆  
That is, it is not possible to inscribe a triangle with all 
sides no less than (1+∆)d  on the circle.  
Corollary 1: At any time, at most two 2-hop nodes can 
transmit at the same time. 
Proof: With reference to Fig. 8, suppose that three 2-hop 
nodes can transmit together. In order that the ACK of any 
1-hop node to not interfere with the reception of DATA 
packet of another transmission, the distances between the 
three 1-hop nodes must all be larger than (1+∆)d.  By 
Lemma 1, this is not possible.  
Theorem 1: For equal-link-length canonical networks, 
802.11 2 / 3C L≤ , where L is the link capacity. 
Proof: Define “airtime” usage of a node to include the 
transmission time of DATA packets as well as the ACK 
from the receiver [7]. Let Sij be the airtime occupied by the 
transmission   of  the  i-hop node on the  j-th  chain over  a 
Fig. 5. Lack of RS Mode leads to HN no matter how large
CSRange and SIR are.  
Fig. 6. With RS Mode, CSRange not sufficiently large still leads to 
HN due to insufficient SIR . 
Fig. 7. Equilateral triangle inscribed in a circle. 
R1 T1 T2 R2 
dmax dmax 
DATA 
DATA 
CSRange 
>(1+ ∆ )dmax 
T1 R2 R1 DATA ACK 
dmax dmax <(1+ ∆ )dmax 
CSRange 
6 IEEE TRANSACTIONS ON MOBILE COMPUTING,  MANUSCRIPT ID 
long time interval [0, Time]. 
Let S1 = the union of airtimes occupied by all 1-hop nodes 
S1j. Similarly, let S2 = the union of airtimes occupied by all 
2-hop nodes S2j . That is, 1 11 12 1... NS S S S= ∪ ∪ ∪  and  
2 21 22 2... NS S S S= ∪ ∪ ∪ . We further define xij = |Sij|/Time. 
By definition, 
1 2| |S S Time∪ ≤  (5) 
According to assumption (3), when any 1-hop node trans-
mits, none of the other 1-hop nodes or 2-hop nodes can 
transmit at the same time if collisions are not to happen. 
Thus, if carrier-sensing works perfectly and collisions due 
to simultaneous countdown to zero in the 802.11 backoff 
algorithm are negligible (see assumptions (4) and (5) in 
Section II), then  
1 2S S∩ = ∅  (6) 
and   
                          
1 1i jS S∩ = ∅           for  i ≠  j . (7) 
This implies 
1 2 1 2| | | | | |S S S S Time+ = ∪ ≤  (8) 
1 11 12 1| | | | | | ... | |NS S S S= + + +  . (9) 
By Corollary 1,  
 21 22 2
| | | | ... | |
NS S SS
+ + +
≥  . (10) 
Recall that we assume that the 1-hop nodes are relay nodes that 
do not generate data (see Definition 5 and the justification be-
fore that in Section II). All traffic transmitted by 1-hop nodes 
must therefore come from 2-hop nodes.  By the “no collision” 
assumption, the sum of the airtimes of 1-hop nodes must not be 
greater than the sum of airtimes of 2-hop nodes. We have 
11 12 1 21 22 2| | | | ... | | | | | | ... | |N NS S S S S S+ + + ≤ + + +  (11) 
From (8)-(10), we have 
11 12 1 21 22 2| | | | ... | | (| | | | ... | |) / 2N NS S S S S S Time+ + + + + + + ≤ . 
Applying (11), we get  
11 12 1
11 12 1
( ... )
( ... ) 1
x x x
x x x
+ + +
+ + + + ≤  
giving 
      
11 12 1
x x x+ + + ≤  
where 
11 12 1( ... )Nx x x L+ + +  is the throughput. 
We now show a specific schedule on a 2-chain network 
which achieves the capacity of 2L/3. Consider the topol-
ogy shown in Fig. 9. There are two chains, having link 
distance d and CSRange = 2.9d, which removes HN. Recall 
that the general HFD has two requirements, (i) RS mode 
and (ii) CSRange > (3+∆) dmax [6]. For the topology in Fig. 9, 
it turns out that CSRange = 2.9d is enough. 
The numbers shown on the links in Fig. 9 represent a 
possible transmission schedule. Links with same number 
transmits at the same time. Following this pattern, the 
throughput capacity of 2L/3 is “potentially” achievable. 
Our simulation results in Subsection B below show that 
the 802.11 protocol throughput capacity is below but close 
to this upper bound.  
The reader may be curious as to why we did not use a 
“symmetric” 2-chain network (where the angle between 
the chains isπ ) as the illustrating example above. It turns 
out that the symmetric structure cannot achieve the 
throughput of 2L/3 if there are source nodes four or more 
hops away. To see this, first we note that for a symmetric 
2-chain network, CSRange must be at least 3d to ensure 
HFD in the areas around the sink node (see discussion of 
the example in Fig. 3 in Section II). Given CSRange=3d, 
each of the chains (assuming a long chain with more than 
four hops (or five nodes)) cannot have throughput of L/3, 
as can be easily verified by analysis of one linear chain [7], 
[9]. 
Before going to the next subsection, we note that Theo-
rem 1 actually applies not just to canonical networks (the 
proof does not require it), but general networks in which 
(i) all links are of the same length; and (ii) source nodes 
are two hops are more away from the center. In other 
words, the chains leading to the data center need not be 
straight-line linear chains. Thus, Theorem 1 can be stated 
more generally as Theorem 1’ below: 
Theorem 1’: For equal-link-length general networks, 
802.11 2 / 3C L≤ , where L is the link capacity. 
Proof: Same as Theorem 1 since Lemma 1 and Corollary 1 
apply to general networks with equal link length also.  
(2)     Variable Link-Length Networks 
In this subsection, we consider canonical networks in 
which the distance between adjacent rings can be varied 
(i.e., d0 , d1 ,…  may be distinct). With this assumption, the 
capacity is upper-bounded by 3L/4. This is proved in 
Fig. 8. At most two simultaneous transmissions from 2-hop nodes.
Fig. 9. Example of equal-link-length topology, CSRange=2.9d.
1 2 3 
O N11 N12 
>(1+∆)d >(1+∆)d 
<(1+∆)d 
CHAN ET AL.:  MANY-TO-ONE THROUGHPUT CAPACITY OF IEEE 802.11 MULTI-HOP WIRELESS NETWORKS 7 
Theorem 2 after Lemma 2 in the following. 
Lemma 2: At any time, at most three 2-hop nodes can 
transmit at the same time. 
Proof: Assume the contrary that we can have four 2-hop 
nodes belonging to four different chains transmitting at 
the same time. With respect to Fig. 10, consider the four 
straight lines formed by the four nodes to the center (note: 
the network could have more chains, just that we are fo-
cusing on the four chains of the four 2-hop nodes in focus 
here). Four angles are formed between adjacent lines. Let 
θ < / 2π  be the minimum of the four angles.  Four angles 
are also formed between non-adjacent lines. Let β π≤ be 
the angle encompassing θ (see Fig. 10).  
For simultaneous transmissions of 2-hop nodes, the 
transmitters should not be able to carrier-sense each other. 
This implies an upper bound for CSRange as follows: 
0 12( )sin 2
CSRange d d
< +  . (12) 
In addition, by assumption (5), to prevent collisions of 1-
hop nodes and 2-hop nodes, they should be able to car-
rier-sense each other. This implies a lower bound for 
CSRange. By (4), 
0 1 0 0 0 1( ) 2 ( )cosCSRange d d d d d d≥ + + − +  β . (13) 
By assumption (3), the receivers of simultaneous transmis-
sions should not violate the physical constraints.    By (1), 
1 0(1 ) 2 sin 2
+ ∆ <  . (14) 
Since there are four chains, / 2θ π≤  and β π≤ .  From 
the definitions of θ and β, we have 
 2θ β π≤ ≤ . (15) 
From (13) and (15), 
0 1 0 0 0 1( ) 2 ( )cos(2 )CSRange d d d d d d≥ + + − + θ . (16) 
Let d1= α d0. We can form two inequalities from (12), (14) 
and (16): 
 2(1 cos )
 , (17) 
(2cos 1) 1 2cos1 2cos
1 2cos 1 2cos
(2cos 1) 1 2cos1 2cos
1 2cos 1 2cos
⎧ θ − + − θ− θ
α > + −⎪
− θ − θ⎪⎪
θ − + − θ− θ⎪α < − −⎪ − θ − θ⎩
   . (18) 
Fig. 11 shows the plot of (17) and (18) when ∆ = 0.78. The 
shadowed region is the area of solution. From the plot, 
 1.73 / 2θ π> > . 
This leads to a contradiction. Thus, there can be at most 
three simultaneous 2-hop transmissions. 
Theorem 2: For variable-link-length canonical networks,  
802.11 3 / 4C L≤ , where L is the link capacity. 
Proof: Similar to the proof of Theorem 1, from Lemma 2,  
 21 22 2
| | | | ... | |
NS S SS
+ + +
≥   . (19) 
Hence, 
11 12 1
11 12 1
( ... )
( ... ) 1
x x x
x x x
+ + +
+ + + + ≤  
or       
11 12 1
x x x+ + + ≤  , 
where 
11 12 1( ... )Nx x x L+ + +   is the throughput 
Fig. 12 shows an example of a canonical network. The 
CSRange has to be set larger than 2.62d0 and smaller than 
3.417d0. The numbers on the links show a possible trans-
mission schedule that achieves capacity of 3L/4. 1  Our 
simulation results in Subsection B below show that 802.11 
throughput capacity is below but close to this upper 
bound. 
In the analysis of canonical networks, we have as-
sumed that the loss exponent is 4,  corresponding  to  ∆ = 
1 For the one-to-many network (i.e., the sink becomes the source, and 
the sources become the sinks with respect to the many-to-one case here), 
some parameters should be changed to attain the capacity of 3L/4. Spe-
cifically, CSRange = 1.7d0, and di = 0.7d0 for i=1, 2, … The derivation 
method for the capacity of the one-to-many case is similar to that in the 
many-to-one case here. 
Fig. 11. Plot of Inequalities (17) and (18). 
Fig. 10. Example of 4-chain canonical network. 
d0 d1 
d1 (12) 
(13) (14) 
8 IEEE TRANSACTIONS ON MOBILE COMPUTING,  MANUSCRIPT ID 
0.78. In outdoor environment, the typical value of loss 
exponent is in the range 2 to 4. Similar analytical tech-
nique can be used to find their throughput capacities. 
Since smaller loss exponent implies larger ∆ (larger inter-
ference), the throughput capacity under the assumption 
of loss exponent 4 serves as an upper-bound for the 
throughput capacity in outdoor environment.  
3.2 Simulation 
We use the network simulator NS2 [10] to simulate the 
canonical network shown in Fig. 12. As shown in Subsec-
tion A, for the 3-chain canonical network, 802.11 3 / 4C L≤ . 
In the simulation, the RS Mode is enabled. Table I shows 
the details of the simulated configuration. Only the n-hop 
nodes at the boundary are source nodes that generate 
data. Offered load control is applied to prevent them 
from injecting too much traffic into the network. For the 
interested reader, it has been shown in [7] that offered-
load control can yield higher throughput in multi-hop 
networks. 
Fig. 13 shows the simulation result assuming the set-
up of Table I. The x-axis is the number of nodes per chain, 
including the sink. Given a number of nodes per chain, 
we vary the offered load in the simulation to identify an 
offered load that achieves the highest average throughput. 
When the number of nodes per chain is 3, i.e., the 2-hop 
nodes are the source nodes, the throughput is 4.62Mbps 
(0.740L), which is very close to the theoretical capacity 
3L/4, where the link capacity L is around 6.24Mbps as 
obtained by simulating one single link. But when the 
number of nodes per chain increases, the throughput 
drops to 4.30Mbps (0.690L). 
 An explanation for this phenomenon is that the sched-
uling scheme of IEEE 802.11 does not result in the optimal 
transmission schedule presented in Fig. 12 needed to 
achieve the 3L/4 upper bound. That is, the incorporation 
of random backoff countdown time in 802.11 causes im-
perfect scheduling. Consider Fig. 12, it is possible for 2-
hop and 3-hop nodes of different chains to transmit at the 
same time in 802.11, since they are out of the carrier-
sensing range of each other. To achieve capacity 3L/4, 
however, all the 2-hop nodes must transmit together. 
However, a 3-hop transmission may prevent this, result-
ing in only some of the 2-hop nodes transmitting together. 
In other words, there are times when not all 2-hop nodes 
transmit together, meaning |S2| cannot reach the lower 
bound in (19). Meeting the lower bound, however, is es-
sential to achieving the optimal throughput 3L/4.  
Fig. 14 shows the simulation results of canonical net-
works with different numbers of chains but with equal 
link length. The simulated configuration is shown in Ta-
bles II and III. For the 2-chain canonical network, we use 
the network structure in Fig. 9. The angle between two 
chains are slightly less than π . The reason of not using a 
symmetric structure has been given in Subsection A 
above. For other cases, the chains are evenly placed on the 
network. The CSRange for each topology is determined 
by minimizing its value while preventing HN. The 
throughput is obtained by varying the offered load and 
choosing the highest one. From the graph, the highest 
throughput is 3.86Mbps (0.619L), which is slightly smaller 
than the theoretical capacity of 2L/3. This is due to the 
imperfect scheduling by 802.11, which has been discussed 
in the previous paragraph. 
In Fig. 14, the throughput converges to around 
2.0Mbps (0.321L) when the number of chains increases. 
The convergence can be explained as follows. From the 
analysis in Subsection A, we see that the bottleneck is 
around the center. When the number of chains is large, 
the area near the center will become dense. The possible 
transmission patterns are similar in this area, and thus the 
throughput converges. In addition, note that the con-
verged value, 0.321L, is considerably smaller than the 
value achieved when the number of chains is three, 
0.619L. This is again due to imperfect scheduling of 802.11 
MAC protocol. An interesting insight is that when the 
number of chains is small, the possible transmission pat-
terns arise from “random” 802.11 MAC scheduling is 
more limited. And by limiting this degree of freedom, 
higher throughput can actually be achieved because ran-
dom transmission patterns that degrade throughputs are 
eliminated.  
The above observation has two implications: (i) For 
network design, we may want to design the network in such a 
way that the number of routes leading to the center is limited. 
(ii) Even for a general, non-canonical, network densely popu-
lated with nodes and with many routes leading to the center, it 
is better to selectively turn on only a subset of the nodes to limit 
the routes to the center. This principle will be further dis-
cussed in Section IV.  
TABLE I  
SIMULATION CONFIGURATION FOR VARIABLE-LINK-LENGTH CA-
NONICAL NETWORKS 
Number of chains 3 
d0 250m 
d1 242m 
di      for i>1 250m 
Transmission Range 250m 
Carrier Sensing Range 675m 
Routing Protocol AODV 
Propagation Model Two Ray Ground 
Packet Data Size 1460 bytes 
Fig. 12. Example of 3-chain canonical network, CSRange=2.7d.
1.732d0 
0.973d0 
2.62d0 
3.417d0 
CHAN ET AL.:  MANY-TO-ONE THROUGHPUT CAPACITY OF IEEE 802.11 MULTI-HOP WIRELESS NETWORKS 9 
3 4 5 6 7 8 9 10
Number of nodes per chain
Fig. 13. Simulated throughput of a 3-chain canonical network with 
offered load control. 
TABLE II  
SIMULATION CONFIGURATION FOR EQUAL-LINK-LENGTH CA-
NONICAL NETWORKS 
Number of nodes per chain 8 
di      for all i 250m 
Transmission Range 250m 
Carrier Sensing Range Refer  to Table III 
Routing Protocol AODV 
Propagation Model Two Ray Ground 
Packet Data Size 1460 bytes 
TABLE III  
CARRIER SENSING RANGE FOR  EQUAL-LINK-LENGTH CANONICAL 
NETOWKRS 
Number of chains Carrier Sensing Range 
2 725m 
3 875m 
4 750m 
5 725m 
6 875m 
7 800m 
8 750m 
9 875m 
10 825m 
>10 900m 
2 6 10 14 18
Number of chains
Fig. 14. Simulated throughput of equal-link-length canonical net-
works with offered load control. 
3.3 HFD versus Non-HFD Performance 
In the preceding sections, we have assumed HFD net-
works to simplify the analysis by eliminating the effect of 
collision. We now investigate the performance of HFD 
versus that of non-HFD networks. As a reminder, HFD 
requires 
(i) Use of Receiver Restart (RS) Mode, and 
(ii) Sufficiently large CSRange. 
From [11], we know that increasing CSRange increases 
the number of exposed nodes (EN) and decrease the 
number of hidden nodes (HN), and vice versa. When HN 
is removed, say with HFD, the EN phenomenon will be 
more severe, which lowers the throughput.  However, 
that is the case for many-to-many data delivery only. For 
this paper, we are interested in many-to-one data delivery. 
Table IV shows the simulation results with same configu-
ration as in Table II with varying CSRange. The shaded 
entries correspond to HFD. From the table, when the 
number of chains is between 2 to 10, the highest through-
put is achieved if we choose the smallest CSRange within 
HFD. This shows that the best HFD configuration gener-
ally works better than non-HFD. 
TABLE IV 
SIMULATION RESULT FOR EQUAL-LINK-LENGTH   CANONICAL 
NETWOKRS 
No. of Chains Through- 
(Mbps) 2 3 4 5 6 7 8 9 10 
975 2.388  2.981  3.355  2.833  2.863  3.022  2.891  3.054  3.114  
925 2.793  2.993  3.329  3.518  2.837  2.805  2.943  3.270  3.108  
875 2.797  2.999  3.508  3.535  3.393  3.272  3.163  3.384  2.883  
825 2.795  2.490  3.513  3.483  2.615  3.681  3.575  3.053  3.366  
775 2.808  2.473  3.724  3.540  2.760  2.754  3.709  3.367  3.269  
725 3.589  2.226  3.210  3.854  2.095  2.264  3.147  3.199  2.686  
675 3.170  2.288  2.398  2.799  2.142  2.261  2.176  2.367  2.633  
625 3.166  1.806  2.219  2.657  1.735  2.020  2.670  1.906  2.156  
575 3.183  1.788  2.168  2.202  1.657  1.609  2.280  1.929  2.041  
bold: highest throughput;  shaded: HFD 
The better performance of HFD could be explained as 
follows. When CSRange is decreased, the number of HN 
increases and the number of EN decreases. More links 
could be active when there are fewer EN, thus the 
throughput in multiple-source-multiple-destination net-
work could be higher in the non-HN free situation. In a 
many-to-one network, however, all the traffic is directed 
toward the same destination. With a non-HN free design, 
although the total throughput on a link basis (point-to-
point throughput) may be increased, the many-to-one 
throughput (or the end-to-end throughput) could not 
benefit from the increase, because all the traffic in the end 
will flow toward the bottleneck and be dropped there due 
to HNs. We will see later that this observation suggests a 
design in which the area near the center should be made 
HN-free, while areas far away from the data center need 
not be HN-free.  
3.4 Multiple Interference 
Thus far, we have considered pair-wise interferences 
only. The analysis of pair-wise interferences is appealing 
from the simplicity viewpoint. However, it may not have 
taken into account the fact that the interferences from 
several other simultaneously transmitting sources may 
add up to yield unacceptable SIR even though each of the 
interferences may not be detrimental. In this section, we 
extend our analysis to take into account the effect of mul-
tiple interferences. For brevity, we refer to the throughput 
10 IEEE TRANSACTIONS ON MOBILE COMPUTING,  MANUSCRIPT ID 
capacity obtained by assuming pair-wise interferences as 
pair-wise-interference throughput capacity, and the 
throughput capacity with mul-tiple interferences taken 
into account as multiple-interference throughput capacity.  
The multiple-interference throughput capacity is in 
general less than or equal to that of the pair-wise 
throughput capacity. The question then is whether the 
pair-wise-inter-ference capacity is a tight bound for mul-
tiple-interference capacity. We show in the following that 
this is indeed the case. In the following, we focus on the 3-
chain network. The analytical argument and the qualita-
tive results for the 2-chain network are similar.  
Consider the canonical network in the Fig. 15, 
where d0=d2=d3=d4, and d1=0.9d0. In some cases, the SIR 
may not satisfy the constraint 10dB. For example, 
when N11 is receiving DATA from N12, and at the same 
time N21 and N31 are replying ACK to N22 and N32, the 
SIR is at most 
11 11
21 31
4 4 4
( ) (0.9 )
6.859
1 1( ) ( ) ( )
1.7321 1.7321
P N d
PP N P N
where PX(Y) is the received power from node Y to node X, 
Pt is the transmission power. 
This situation, however, occurs only if multiple ACKs 
are transmitted simultaneous in nearby links near the 
center. The probability of this occurring is low, since the 
transmission time of ACK is much lower than that of 
DATA. If we ignore the simultaneous transmissions of 
ACKs in these nearby links, we can show that the SIR due 
to multiple interferences is still more than 10dB, given 
that the SIR due to pair-wise interferences is more than 
10dB, as follows.  
1. 1-hop node to sink node 
When the sink node is receiving DATA from N11, the 
nearest three active links that cause largest interference 
are: N23 to N22, N33 to N32 and N14 to N13. If no two ACKs 
are transmitted simultaneously by these three links, the 
“worst-case” interference power at N0 (which includes 
ACK from N22 DATAs from N33 and N14, and transmis-
sions by other nodes) is at most 
Hence, the SIR is at least 1/0.09949=10.513 
2. 2-hop node to 1-hop node 
Consider the link N12 to N11. The nearest three active 
links are: N22 to N21, N32 to N31, N15 to N14. Similar to 
above, the SIR is at least 
3. 3-hop node to 2-hop node and others 
The interference is less than the above cases. This part 
is skipped because the analytical approach is similar. 
In the above, we have argued analytically the consid-
eration of multiple interferences will not have substan-
tially different performance than that of pair-wise inter-
ference. We have focused on the 3-chain network with 
variable link distance because this structure provides the 
highest capacity bound among the canonical networks. 
We now present simulation results for general canoni-
cal networks with arbitrary number of chains. We have 
modified the NS2 simulator to take into account the ef-
fects of multiple interferences (the modified NS2 code can 
be downloaded from the website in [12]). The throughput 
results are shown in Fig. 16. The multiple-interference 
throughput is only lower than the pair-wise-interference 
throughput by a small margin, and therefore the pair-
wise-interference throughput serves a good bound for 
multiple-interference throughput. 
4 GENERAL NETWORKS 
In this section, we consider the throughput of general 
networks. Since general networks may not have the regu-
lar structure of canonical networks, the throughput capac-
Fig. 15. Example of 3-chain canonical network, CSRange=2.7d.
Fig. 16. Simulated throughput of 3-chain canonical network 
with offered load control. 
0 0 0 0 0 022 33 14 25 35 16
4 4 4 4 4 4 4 4
( ) ( ) ( ) ( ) ( ) ( ) ...
1 1 1 1 1 1
( ...) 0.0995
1.9 2.9 3.9 4.9 4.9 5.9
N N N N N N
P N P N P N P N P N P N
+ + + + + +
= + + + + + + ≈
11 11 11 11 11 11
N 21 N 32 N 15 N 24 N 34 N 17
4 4 4 4 4 4 4
(N ) (N ) (N ) (N ) (N ) (N ) ...
(0.9 )
1 1 1 1 1 1
( ...)
1.7321 2.5515 3.9 4.4844 4.4844 5.9
10.5259
P P P P P P
+ + + + + +
+ + + + + +
1.732d0 
0.9d0 
2.55d0 
3.29d0 N11 
3 4 5 6 7 8 9 10
Number of nodes per chain
multiple
interference
pairw ise
interference
CHAN ET AL.:  MANY-TO-ONE THROUGHPUT CAPACITY OF IEEE 802.11 MULTI-HOP WIRELESS NETWORKS 11 
ity could be lower than 3L/4. We propose a method to 
find the capacity by selecting Hidden-node Free Paths 
(HFP).  
4.1 Discussion of HFP 
In Section III-C, we found that the network with HN-
free outperforms that with HN in terms of throughput 
capacity. We could have three schemes which satisfy the 
HN-free condition for general network analysis. As one of 
the requirements of HFD, we assume RS Mode is used in 
all the analyses and experiments in the remaining of the 
paper. We assume that all nodes use a common fixed 
CSRange in each of the following schemes (assumption (1) 
in Section II); however, the schemes set the fixed CSRange 
differently. 
Scheme 1: CSRange is set to 3.78‧TxRange, where 
TxRange is the transmission range. This is a sufficient 
condition of HN free for any networks [6]. 
Scheme 2: CSRange is minimized according to the 
network topology so that no hidden node exists with 
respect to any two links in the network. This scheme, 
for example, was used in the analysis of canonical 
networks. 
Scheme 3: HFP - We select a subset of links to form 
paths to the center which are hidden-node free and 
achieve the highest possible throughput. Since some 
links are not used, the CSRange can be smaller than 
scheme 1 and 2 (i.e., only the links in the path are 
considered when fixing CSRange.) 
 Based on Table IV, the highest throughput is achieved 
when we choose the smallest CSRange within HFD. So 
we have the following predictions for the throughputs of 
the different schemes above. The throughput of scheme 1 
cannot be higher than that of scheme 2 (because the 
CSRange of some links are forced to adopt a higher value 
than necessary in scheme 1). Also, the throughput of 
scheme 2 cannot be higher than that of scheme 3 (because 
scheme 3 requires the HN property to be maintained only 
for links along the paths, and the paths that will be used 
are optimally chosen with regard to the throughput; 
whereas scheme 2 requires all links to be HN-free, even 
for links that are not used). For an example where HFP 
can achieve a higher throughput than scheme 2, we add 
two nodes to the 3-chain canonical network in Fig. 12 to 
yield the network in Fig. 17. In the network, link BB’ in-
terfere with link AA’. If we set CSRange to be less than 
3.417d0, node B will become a hidden node of link AA’. If 
we set CSRange larger than 3.417d0, the capacity upper-
bound 3L/4 cannot be achieved. On the other hand, if we 
use HFP, we could select the links in the canonical net-
work only. So node A could be “switched off” and there 
will not be hidden-node problem if we set CSRange to 
2.7d0.  
4.2 Experiments and Discussions 
To conserve space, this paper will not go into the de-
tails of the formulation of the HFP problem, and the HFP 
experimental methodology. For the interested readers, 
such details can be found in the Appendix of our techni-
cal report [12]. In a nutshell, our approach extends that of 
[13] by additionally taking into consideration the effects 
of carrier sensing and HFD requirements. We also pro-
vide a branch-and-bound heuristic algorithm for the re-
sulting integer linear program (ILP). Here we only pre-
sent the performance results of experiments on schemes 1, 
2, and 3 and their implications. Solving the ILP of scheme 
3 is computationally intensive. The experimental results 
of scheme 3 in this subsection are therefore obtained us-
ing our branch-and-bound heuristic. Schemes 1 and 2 are 
still solved in an optimal manner. As will be seen, even 
with a suboptimal heuristic, scheme 3 still yields better 
results. 
In our experiments, we put the nodes inside a disk of 
radius one. A sink node is placed at the center of the disk, 
and six source nodes are placed evenly at the boundary of 
the disk spaced evenly apart. For each source node, a 
node is randomly generated within the transmission 
range 0.4. More nodes are generated similarly with refer-
ence to the newly created node until a node is within the 
transmission range from the sink node. In this way, we 
could ensure that there is a path from any source node to 
the sink node. By setting the transmission range to 0.4, the 
data from the source nodes will need at least three hops 
to reach the sink node. 
Table V shows the experiment results for five ran-
domly generated networks, Net1, Net2, …, Net5 . T1, T2 
and T3 are the throughputs of the three schemes. In ob-
taining Ti, we vary the offered load at the source nodes 
until the highest throughput is obtained [7]. From Table V, 
scheme 3 has improvements of 4.8% to 43.8% over scheme 
1, and 4.8% to 23.2% over scheme 2. As related earlier, we 
did not solve scheme 3 optimally, but rather used a heu-
ristic. Therefore, the CSRange (CS) found for HFP in the 
experiments may not be the shortest possible CSRange.  
Nevertheless, the result shows that the solutions of 
scheme 3 exhibit some properties similar to the canonical 
network, as shown in Fig. 12. We discuss the similarities 
in the following paragraph. 
First, for scheme 3, CSRange/TxRange (CS3/TX) for 
Fig. 17. Example of HFP. 
1.732d0 
0.973d0 
2.62d0 
3.417d0 
12 IEEE TRANSACTIONS ON MOBILE COMPUTING,  MANUSCRIPT ID 
Net1 to Net5 is in the range of 2.62 to 3.417, which is the 
CSRange region we mentioned near the end of Section III-
A for achieving the capacity of 3L/4 in a canonical net-
work.  Second, exactly three paths leading to the sink 
node are used, which is the same as the 3-chain canonical 
network (Fig. 18). This gives us an intuition that the ca-
nonical network is in a sense optimal – that is, we may 
want to form a structure similar to the canonical network 
by turning on only some of the relay nodes. 
TABLE V  
RESULT FOR THROUGHPUT OF RANDOM NETWORKS 
 T1 T2 T3 T3/T1 T3/T2 CS3 CS3/TX 
Net1 0.4 0.5 0.575 1.438 1.15 1.253 3.133 
Net2 0.412 0.439 0.541 1.313 1.232 1.265 3.162 
Net3 0.429 0.451 0.536 1.25 1.189 1.265 3.163 
Net4 0.429 0.5 0.6 1.4 1.2 1.205 3.012 
Net5 0.5 0.5 0.524 1.048 1.048 1.287 3.216 
Neti: Network i 
TX: Transmission range, set to 0.4 in experiments 
T1: Throughput when CSRange=3.78 TX (Scheme 1)  
T2: Throughput when CSRange is minimized with respect to links in 
the network (Scheme 2). 
T3: Throughput when only some links in the network are activated 
(HFP) (Scheme 3) 
CS3: CSRange for Scheme 3 
4.3 Applying Canonical Network to General 
Networks 
The preceding subsection shows that HFP outperforms 
other HN-free schemes in terms of throughput. We also 
observe from the results that (i) HFP solutions for a ran-
dom network exhibit structures similar to that of the 3-
chain canonical network near the center. Furthermore, 
from simulation results in Section III-B (see Fig. 13), we 
observe that (ii) IEEE 802.11 scheduling in the canonical 
network achieves throughput close to that of perfect 
scheduling. Observations (i) and (ii) lead to the following 
general engineering principle: 
Centric Canonical-Network Design Principle 
 In a general multi-hop network densely populated 
with relay nodes, instead of solving the complex 
HFP optimization problem, as a heuristic, we may 
select routes near the center so that the structure 
looks like that of a 3-chain canonical network.  
 If we have the freedom for node placement near the 
center during the network design process, then the 
nodes around the center should be structured like a 
3-chain canonical network.  
Note that there is no restriction on nodes far away from 
the center, and that they can be randomly distributed (see 
Fig. 19 for illustration).  
This subsection investigates the application of the Cen-
tric Canonical-Network Design Principle. For our simula-
tions, we assume there is a disk with radius 2000m. 
Within the disk, there is an inner circle with radius 980m. 
As illustrated in Fig. 19, the inner circle is structured as a 
canonical network. The nodes outside the inner circle are 
placed randomly with the constraint that the smallest 
distance between any two of them is not shorter than 
125m. The nodes outside the inner circle act as source 
nodes and relay nodes at the same time, while the nodes 
inside the inner circle act merely as relay nodes. We refer 
to the network structure in Fig. 19 as centric canonical net-
work, alluding to the fact that only the vicinity of the cen-
ter looks like a canonical network. Henceforth, we shall 
refer to vicinity of the center as the canonical network and 
the randomly-structured part beyond that as the random 
network. The number of nodes beyond the inner circle is 
284. We use the default setting in NS2, CSRange of 550m 
and TXRange of 250m, for performing the simulations. 
AODV routing is assumed. For the canonical network, 
with respect to Fig.12, we set d0=200m. Since 
550m/200m=2.75, which is within the range 2.62 to 3.417 
(see Fig. 12), the canonical network is HN free.  The ran-
dom network, however, is not necessarily HN-free in our 
experiments. The assumption is reasonable, and corre-
sponds to the real situation in which we only try to de-
sign the network architecture near the center judiciously 
by careful node placement.  
As a benchmark, we have also conducted simulation 
experiments for a random network in which the inner 
circle is populated by 146 randomly placed nodes with no 
constraint on the node-to-node distance. In all our simula-
tions below, the offered load to the source nodes are var-
ied until we find the largest throughput for each network 
structure [7]. Simulation of 802.11 with AODV yields a 
Fig. 18. Random Networks and HFP.. 
CHAN ET AL.:  MANY-TO-ONE THROUGHPUT CAPACITY OF IEEE 802.11 MULTI-HOP WIRELESS NETWORKS 13 
throughput of 1.16 Mbps for the benchmark random net-
work, and a throughput of 2.79Mbps for the centric ca-
nonical network. That is, the throughput of the centric 
canonical network is more than 100% higher. This dem-
onstrates that a carefully designed structured network around 
the data center yields superior performance. 
Although the improvement is significant, 2.79 Mbps is 
still a bit lower than the 4.30Mbps simulated throughput 
of the 3-chain canonical network in Section III. It turns out 
that the centric-canonical network actually fails to take 
another bottleneck into account. That is, in addition to the 
bottleneck around the center, there is also a bottleneck at 
the “confluence” of the random network and the canoni-
cal network, where the canonical network may branch off 
to many paths in the random network, and the nodes on 
these branches may interfere with each other in a negative 
way to bring down the throughput.  
To mitigate the bottleneck at the confluence, we mod-
ify the canonical network as in Fig. 20. As shown, each 
chain in the canonical network only branches out further 
into two chains before meeting the random network. We 
refer to this design as the manifold canonical network, in 
reference to the fact that there are actually two “layers” of 
canonical networks. The first one is at the center, with 
three more before meeting the random network. We refer 
to this design principle as the Manifold Canonical-Network 
Design Principle.  
In our simulations, the manifold canonical network is 
placed inside an inner circle of radius 1026s. The nodes 
beyond the manifold canonical network are randomly 
generated with the same constraints as the nodes gener-
ated beyond the inner circle of the centric canonical net-
work. As the inner circle is larger than previous networks 
and the number of nodes (which are relay nodes) in the 
manifold canonical network is 31, to keep the total num-
ber of nodes in the network constant, the number of ran-
domly generated nodes (which are also the source nodes) 
outside the inner circle is decreased from 284 to 269. We 
set CSRange 550m and d0=200m in the manifold canonical 
network in our simulation (see Fig. 12). Simulation of 
802.11 with AODV routing yields a throughput of 
3.34Mbps, which is 20% higher than that of the centric 
canonical network. For fair bench-marking, we again per-
form the simulation with the inner circle replaced by ran-
dom node placements, but this time with the inner circle 
having a radius of 1026m, as in the manifold canonical 
network. The simulation of the benchmark network 
yields a throughput of 1.31Mbps. We find that the 
throughput of the manifold canonical network is more 
than 150% over that by the pure random benchmark net-
work. 
We have also investigated the robustness of the mani-
fold canonical network with respect node positioning. 
Simulations show that 5% position error of the nodes in 
the two “layers” of the canonical network only decreases 
the throughput by 10% on average, as summarized in 
TABLE VI.  
TABLE VI  
COMPARISON OF THROUGHPUTS OF MANIFOLD CANONICAL NETWORKS WITH 
AND WITHOUT NODE POSITION ERROR 
Throughput without 
position error (Mbps) 
Throughput with 
position error (Mbps) 
Ratio 
3.44 3.45 1.003 
3.35 3.11 0.928 
3.32 3.18 0.958 
3.29 2.94 0.894 
3.37 2.96 0.878 
3.36 2.82 0.839 
5 CONCLUSION 
In this paper, we have studied the throughput capacity 
of many-to-one multi-hop wireless networks based on the 
IEEE 802.11 MAC protocol. We have defined a class of 
canonical networks whose throughput capacity serves as a 
benchmark for general networks. Specifically, the throu-
ghput capacity of canonical networks under 802.11 is up-
per bounded by 3L/4, where L is the single-link capacity, 
when the source nodes are at least two hops away from 
the sink.  
If we restrict our attention to networks in which all 
links have the same length, the upper bound is further 
reduced to 2L/3. While the 3L/4 result in the previous 
paragraph has been established for canonical networks 
only, the 2L/3 result applies to general networks so long 
as (i) source nodes are at least two hops away from the 
data center; (ii) all links have the same length.  
Our 802.11 simulation results yield throughputs are 
Fig. 19. Example of a centric-canonical. 
Fig. 20. Example of a manifold canonical network. 
-2000
-1000
-2000 -1000 0 1000 2000
-2000
-1000
-2000 -1000 0 1000 2000
14 IEEE TRANSACTIONS ON MOBILE COMPUTING,  MANUSCRIPT ID 
around 0.690L (for variable-link-length canonical net-
works) and 0.619L (for equal-link-length canonical net-
works) under the worse-case scenario when the source 
nodes are very far away and their traffic needs to go 
through many hops before reaching the sink node. That is, 
the simulated throughputs are reasonably close to the 
theoretical upper bounds of 3L/4 and 2L/3, respectively. 
This is a quite positive result considering the fact that 
802.11 schedules transmissions in a rather random man-
ner, while the examples we gave in Section III-A to 
achieve throughputs of 3L/4 and 2L/3 require very spe-
cific transmission orders.  
The above results also imply that using variable link 
length is more desirable than using fixed link length. 
When the network is very dense (say, infinitely dense), if 
each node chooses a routing path with maximum hop 
distance in each hop, an equivalent network with fixed 
link length dmax, may result, where dmax is the maximum 
hop-distance governed by the transmit power and re-
ceiver sensitivity. This max-hop-distance routing is not 
optimal for the many-to-one traffic pattern.  
This paper has considered both canonical networks 
with and without hidden nodes. Our results indicate that 
hidden-node free designs (HFD) yield higher throughput 
capacity. This is in contrast to the many-to-many case 
where HFD may not yield better throughputs [5] [6] and 
may actually decrease the overall system throughput.   
For general networks, we have used the concept of 
HFP (Hidden-node Free Path) to set up routes that yield 
optimal throughput. HFP routing, however, requires 
solving a complicated integer linear program, which may 
not be practical. Fortunately, our experimental results 
indicate that the routes selected by the HFP algorithm 
resemble the structure of the canonical network near the 
center. This gives rise to simple network design principles 
that attempt to approximate the canonical network struc-
ture in the center. Specifically, we have shown that a 
manifold canonical network structure near the sink can yield 
superior throughput that is as much as 150% higher than 
that of a dense random network.  A key insight is that in a 
network densely populated with nodes, deliberating turn-
ing off some nodes in the area near the sink node so as to 
approximate the canonical network structure can actually 
give rise to better throughput performance. 
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Chi Pan Chan received his B.Eng and M.Phil. degrees in Informa-
tion Engineering from The Chinese University of Hong Kong in 2004 
and 2006. His research was mainly related to capacity analysis in 
multi-hop wireless networks. He is now involved in the software in-
dustry in the field of multimedia and networking. 
Soung Chang Liew received his S.B., S.M., E.E., and Ph.D. de-
grees from the Massachusetts Institute of Technology. From March 
1988 to July 1993, Soung was at Bellcore (now Telcordia), New Jer-
sey, where he engaged in Broadband Network Research. Soung is 
currently Professor and Chairman of the Department of Information 
Engineering, the Chinese University of Hong Kong. Soung’s current 
research interests focus on wireless networking. Recently, Soung 
and his student won the best paper awards in the 1st IEEE Interna-
tional Conference on Mobile Ad-hoc and Sensor Systems (IEEE 
MASS 2004) the 4th IEEE International Workshop on Wireless Local 
Network (IEEE WLN 2004). Separately, TCP Veno, a version of TCP 
to improve its performance over wireless networks proposed by 
Soung and his student, has been incorporated into a recent release 
of Linux OS. Publications of Soung can be found in 
www.ie.cuhk.edu.hk/soung. Besides academic activities, Soung is 
also active in the industry. He co-founded two technology start-ups in 
Internet Software and has been serving as consultant to many com-
panies and industrial organizations. He is currently consultant for the 
Hong Kong Applied Science and Technology Research Institute 
(ASTRI), providing technical advice as well as helping to formulate 
R&D directions and strategies in the areas of Wireless Internetwork-
ing, Applications, and Services. 
An Chan received the B.Eng degree in Information Engineering from 
The Chinese University of Hong Kong, Hong Kong in 2005. He is 
currently working toward a M.Phil degree in the same field at The 
Chinese University of Hong Kong. His research interests are in QoS 
over wireless network and advanced IEEE 802.11-like multi-access 
protocols. He is a graduate student member of IEEE.
