Blanket WLAN: A Distributed Network Evolution

Blanket WLAN: A Distributed Network Evolution
Paper #: 53
ABSTRACT
This paper proposes a novel WLAN configuration
designed to address the needs of networks with wireless
transmitters that can provide coverage to a very limited
area. To expand such coverage over a much larger area we
propose a network with a dense deployment of Network
Access Points (NAPs) and where a station simultaneously
connects to multiple NAPs. Unlike the existing virtual
WLAN solutions, our approach heavily emphasizes
distributed decision making. Further, our approach
preserves much of the current CSMA protocol in WLANs
while effectively utilizing the legacy backhaul over which
our network may be deployed.
In this work we discuss some basic issues associated with
such a network. Specifically we define an architecture
which can leverage both the backhaul communication and
the backhaul topology. Additionally, we propose two
simple, but effective solutions to manage End-Node (EN)
mobility in a fully distributed fashion. Finally, we
demonstrate effective data communication protocols that
allow the network to take advantage of the spatial diversity
offered by the multi-NAP communication system while
preserving the essential distributed scheduling and
coordination benefits of CSMA.
Keywords
AP, CSMA, BWLAN, NAP, EN
1. INTRODUCTION
Consider a wireless network comprising Network Access
Points (NAPs), most of which provide very limited spatial
coverage. The network as a whole, is, however, required to
provide full coverage of a relatively large area, compared
with the coverage of a single NAP. We think of this
network as a LAN network, as its coverage is typically
extended over a single building or certain substantial area
of a building. Moreover, the entire network is under the
control of its user, rather than a 3rd party operator like
cellular networks.
The proposition here is, essentially, to take a common
WLAN approach and replace a relatively high-power
Access Point (AP) with a large number of low-power ones,
which we call NAPs to distinguish them from APs.
Several underlying trends in today’s technology motivate
the need of creating such networks. Some trends are
outlined below:

Enable new spectra and access technologies
As communication engineers begin to explore everhigher electromagnetic spectra in search of new bandwidth,
the need for such a network might arise naturally. A simple
example of high spectra is the Visible Light
Communications (VLC). The recent status of VLC
development can be found in [1], as well as other
references. Consider each light-bulb in a room as a NAP,
as it has a very limited cone of coverage. A full coverage of
an area significantly larger than a single cone will require
utilization of multiple light-bulb NAPs. Such challenges of
covering a large area with limited-coverage devices may
also arise from other spectra like the 60 GHz RF (see, e.g.,
[2]) and ultraviolet.

Solve coverage issues
Modern WLAN systems suffer from the inconsistent
coverage problem. A usual way of closing a coverage hole
is increasing the transmission power. This results in the
increase of the overall coverage area. On the other hand, it
follows from security requirements that wireless signals
should not be propagated beyond a desired coverage area.
To some extent these challenges are being addressed with
careful wireless network planning. However, a considerate
site planning is in general beyond the capabilities of many
homeowners and even enterprises. Moreover, the extent to
which this approach addresses the coverage and leakage
issues is often less than perfect.
Low-coverage NAPs provide an alternative solution to
this problem. Rather than resorting to a distant high-power
AP, the network operator simply places a low-coverage
NAP where it is needed. Because the new NAP’s coverage
is by definition highly-limited, signal leakage outside the
desired area is minimized.

Enable low-power, reliable applications
A feature of the network with low-coverage NAPs is that
any End-Node (EN) in the network is likely close to
several NAPs. This means that the EN communicating with
the network requires lower power than in a traditional
network. Additionally, reliability can be improved if the
media access protocol can take advantage of the multiple
available paths.
We note that the concept described here is not entirely
novel to the wireless LAN industry. Several WLAN
solution providers offer an enterprise solution designed to
provide consistent coverage using a large number of simple
APs. Such a solution is called virtual WLAN, and the
solution providers include Meru [3], [4], Extricom [5],
Cisco, Aruba, and others. More proposals about virtual
WLAN can be found in [6]—[8]. A major feature of virtual
WLAN appears to be1 the presence of a centralized
decision making entity which coordinates the operations of
the WLAN APs. While an effective solution, this limits the
ability of such a network to scale easily, as it places severe
bandwidth and latency restrictions on the backhaul.
Though an enterprise deploying a state of the art WLAN
may be able to supply an appropriate backhaul, its
deployment at a typical home or small enterprise
environment may be difficult.
In this paper we propose an alternative approach – one
where the control of the network is mostly in a distributed
fashion. Specifically, we decentralize most of the
operations in the virtual WLAN into NAPs. Our proposed
network is aimed at restricting the backhaul load, and
hence obtaining the scalability of such a network. On the
other hand, our proposed network is able to provide
consistent network coverage over certain areas, like the
virtual WLAN solutions. In this sense, our proposed
network is called the Blanket WLAN (BWLAN).
modifications of the current CSMA protocol such that the
new protocol fits in the BWLAN, which is illustrated
through simulations. Section 6 concludes this paper.
2. BLANKET WLAN DEFINITION
Figure 1 illustrates the structure of a BWLAN, which
serves a number of ENs. These ENs communicate with a
set of NAPs through the wireless medium. The NAPs are
interconnected through some backhaul network which
could be either wired or wireless. As mentioned above, the
backhaul network is critical as it must be able to support all
the protocols which enable the BWLAN operations.
There are several issues resulting from the
decentralization of the network. The first one is on the
network control, especially when the EN moves. As noted,
each NAP in the BWLAN has a small service area. This
leads to frequent changes of serving NAPs for a particular
EN, in the case of EN movement. Unlike the virtual
WLAN approach where a centralized entity takes the
responsibility of handling all these changes, the NAPs in
the BWLAN need to have the functionality of efficiently
dealing with such changes.
The second issue is data transmission in the BWLAN.
Because of the dense deployment of NAPs, each EN is
connected with multiple NAPs in a BWLAN. Although
these NAPs serve a common EN, the Distributed
Coordination Function (DCF) in the CSMA protocol of
traditional WLANs causes unnecessary contentions among
these NAPs. These contentions may be severe when the
number of participant NAPs becomes large. In this paper,
we propose some modifications on the current CSMA
protocol so as to minimize the contentions among NAPs.
Furthermore, our approach could take advantage of the
spatial diversity to increase the transmission reliability over
the traditional WLANs.
The rest of the paper is organized as follows. Section 2
provides the definition of BWLAN. The architecture of
BWLAN is introduced in Section 3. Section 4 discusses the
BWLAN control protocols for EN mobility. The
performance of these protocols is also simulated and
analyzed in Section 4. In Section 5, we propose two
1
We say “appear” as these are publically available industry white
papers and the information about the details of their internal
solutions is limited.
Figure 1: BWLAN Structure
The BWLAN structure also contains centralized services
and databases, as certain network operations, e.g.,
admission control, are best handled via a centralized
gateway. Still, the major functionality associated with
maintaining communication with an EN in this network is
implemented in a distributed fashion.
The configurations of the BWLAN are described below.

Single channel operation: All NAPs in a BWLAN
shall operate on the same frequency channel. Note
that this does not preclude operation on multiple
channels – such operation would be treated as multiple
BWLANs separated in frequency. However, unlike
most existing WLANs, this approach has an effective
frequency re-use factor of 1. The rough equivalent for
most WLANs is 3, which means that this approach has
the potential of improving the spectral efficiency of the
network by a factor of 3.



Transparency to an EN: Given the low-coverage area
of NAPs, it is inconvenient to have an EN associate
with a single NAP as such associations require
frequent network management operations in the case
of moving EN. Instead, an EN is associated with the
whole network, which acts as a single virtual network
entity. This concept is in common with existing
Virtual WLAN solutions [3]—[5].
Virtual private network for each EN: Each EN
communicates with a set of NAPs. These NAPs shall
create a virtual network dedicated to serving the given
EN. This group of NAPs will be referred to as the
EN’s “blanket.” Figure 2 illustrates 3 different layers
of NAPs with respect to an EN: i). a set of NAPs in
active communication with the EN (which is the
precise definition of a “blanket”); ii). a set of NAPs
which are able to receive from an EN but do not
actively transmit to it; and iii). a set of NAPs which are
aware of the EN’s presence, but are unable to receive
from it.
Dynamic network separation: The notion of a
blanket allows the network to locate each EN and
transmit to the EN only from the NAPs that can reach
it. This, combined with the likely effect of low-power
transmission by the EN, reduces the overall contention
for the wireless medium and permits spatial re-use
within the network. That is, multiple devices can
transmit on the same channel at the same time without
a collision – if they are far enough away from each
other.
Design of a network with the above features presents
certain challenges. In the rest of this paper we define the
general architecture of a BWLAN. In addition, we propose
two simple protocols for establishing and maintaining a
blanket, mainly addressing EN mobility. Finally, we
illustrate the way of efficiently utilizing the NAPs to serve
a common EN.
3. BWLAN ARCHITECTURE
We begin by defining the architecture of the BWLAN,
which addresses both the EN-NAP communication over the
wireless medium and the communication over the backhaul
network (cf. Figure 1). The wireless medium is assumed to
utilize the same structure as existing WLAN systems (e.g.,
[9], [10]). As mentioned, our approach is to modify the
operations of the components in the wireless medium as
little as possible.
The backhaul, on the other hand, shall require much
more attention. In particular, we define the notion of a
backhaul neighborhood, whereby the latency of general
backhaul communication may be significantly higher than
the latency of localized backhaul communication within a
neighborhood of NAPs. This assumption reflects the
topologies of certain backhaul architectures such as powerline networks and hierarchical LANs (with individual
routers supporting a neighborhood and interconnected to
create the complete backhaul). Furthermore, we wish to
produce a design which is maximally agnostic to the details
of a particular wireless communication protocol and
therefore can work with a multitude of underlying wireless
communication technologies. The proposed architecture is
then shown in Figure 3, with its major components
described below.
MAC (at both EN and NAP)
We shall assume the IEEE 802.11 MAC (cf. [9], [10])
for the BWLAN.
NAC (Network Access Function)
The NAC is effectively the MAC protocol by which
NAPs communicate over the backhaul. In BWLAN, this
consists of two protocols: one for gateway and the other for
NAP. More details on this are provided below.
Inter-Working Function (IWF)
Figure 2: Relationship between EN and NAP sets
The IWF allows the NAP’s MAC to hide BWLANspecific details from the ENs, e.g., use of virtual cell
ID/MAC address as provided from neighborhood NAC. It
also provides the capability to extract relevant information
from received MAC frames for potential use by the
decision making functions in the NAC entities. MAC data
frames will typically pass through the IWF transparently to
the higher layer entities, including the gateway. The
interface between the NAP and the gateway is referred to
as the X2 interface.
Hence, the IWF maintains a list of ENs served by this
NAP, and their corresponding blanket (virtual) MAC
addresses.
Neighborhood NAC
Each NAP contains a neighborhood NAC function. The
neighborhood NAC maintains the contact information of a
list of its neighbor NAPs. It is a local NAP function
interfacing with the MAC IWF and supporting a distributed
control plane protocol with other NAPs via the backhaul
interface. The backhaul interface between all neighborhood
NACs is referred to as X1 interface.
The neighborhood NAC function can be topologyspecific and adapt to the restrictions and take advantage of
opportunities offered by each specific backhaul network.
Gateway NAC
Each EN is associated with a gateway node that
maintains a forwarding table to track the set of NAPs that
are aware of the EN. Hence, the gateway NAC maintains
global information about the BWLAN. The gateway relays
information between the set of NAPs communicating with
the EN and the core network, through the X3 interface.
The gateway node can be a centralized external entity as
shown in Figure 3. Alternately, one of the NAPs could take
the role of the gateway, acting as an anchor for forwarding
data frames to/from the appropriate NAPs serving specific
ENs.
Figure 3: BWLAN Architecture
to it. The separation of the IR state and the AT state
follows from the data plane protocols, and they may be
merged into a single state under certain conditions.
PHYs (not shown in Figure 3)
The PHY layers of both the wireless access technology
and the backhaul communication technology are assumed
to be unaffected by our design.
4. BWLAN CONTROL
The blanket mobility function is one of the main control
functionalities of the BWLAN. This function is responsible
for the maintenance of a blanket, as well as the support of
EN mobility and data communication between the EN and
the blanket. The function is primarily located within the
IWF and the neighborhood NAC and relies heavily on the
X1 interface.
4.1 The State Machine
For any EN in the BWLAN, the IWF within each NAP
stays in one of the following states.

Not Aware (NA): The NAP is unaware of the EN, i.e.,
it has no information about it.

Active Transmit (AT): The NAP is actively
transmitting to the EN.

In Range (IR): The NAP is aware of the EN and is
receiving from the EN, but is not actively transmitting

Neighbor Range (NR): (optional) The NAP has a
neighbor NAP which is in the AT state or the IR state
for the EN.
A blanket associated with an EN is composed of the set
of NAPs for which the given EN is in the AT state. There
is a unique virtual MAC address for each blanket in the
BWLAN.
Note that if a NAP is in the IR state with respect to an
EN, it belongs to the middle set of the NAPs in Figure 2.
Similarly, if a NAP is in the NR state with respect to an
EN, it belongs to the outer set of the NAPs in Figure 2.
Hence, the states defined above directly reflect the
BWLAN nature as illustrated in Figure 2.
The state transition diagram depends on the mobility
protocol utilized. We propose two types of EN mobility
protocols: a poll mobility protocol and a push mobility
protocol. Both protocols require each NAP to maintain a
list of neighbor NAPs, which are in the vicinity of the
given NAP. The notion of vicinity should be tied to the
physical distance, but can, in fact be left up to the system
designer.
Next, we propose the protocols for supporting mobility
of the EN through the BWLAN. The protocols assume that
the EN has some mechanism for discovery, initial access
and admission into BWLAN and that these have been
accomplished. Thus, these protocols deal with the steady
state behavior of an EN within the BWLAN.
4.2 Poll Mobility Protocol
The first protocol does not have the NR state. It operates as
follows:

Upon receiving a burst from an unaware EN, the IWF
of a NAP uses the neighborhood NAC to poll the
NAPs on its neighbor list.
o
o


If at least one of the neighbors is aware of the
EN, it responds to the NAP of interest with all
the relevant information about the EN, e.g.,
access control parameters, QoS parameters,
active connections, security parameters, etc.
Then, the NAP adds the EN to the IR set or
the AT set, depending on the data plane
protocol.
If none of the neighbor NAPs respond with
the EN information, the EN is flagged as
“new” and a network admission protocol
needs to be utilized.
Transition between the AT state and the IR state
depends on data plane operation and may also be
forced by the network.
Figure 4: State transition diagram for EN within a NAP
under the poll mobility protocol.
However, our analysis shows that this protocol suffers
from a drawback. The latency of the polling process is
required to be low as the NAP may need to transit to the
AT state rather quickly in order to provide instantaneous
services. To address this we propose an alternative protocol
which proactively makes the EN information available at
the NAPs that might serve the EN in the near future.
4.3 Push Mobility Protocol
The second protocol operates as follows:

A NAP may drop an EN, i.e., transiting to the NA
state, if instructed by the network. Otherwise, each
transmission from the EN or to the EN successfully
received by the NAP resets a timer for that EN. The
timer value may depend on whether the EN is in
power-saving mode. If the timer expires, the NAP
sends a Range Probe to the EN to check if the EN is
still within range of the NAP. If no response is
received, the EN is dropped.
Figure 4 shows the state transition diagram for an EN
within a NAP with the poll mobility protocol. It follows
from the figure that the network can exercise control over
whether a NAP supports and EN or not.
This protocol has several advantages. It provides for a
natural way to change the set of serving NAPs. NAPs begin
to serve an EN when necessary, i.e. when the EN can
communicate with them, and stop service when no longer
necessary. Furthermore, a significant portion of the
information exchange can be limited to neighboring NAPs
(using the Neighborhood NAC), which can reduce the
overall backhaul load in some types of backhaul networks.

Each NAP informs its neighbor NAPs of the ENs for
which it is in the AT state or the IR state. The IWF
provides a list of such ENs to the neighborhood NAC
which manages such updates. We shall refer to this
message as an Update Report from Neighbor (URN).
The URN either carries all access information for each
EN, including access rights and cryptography keys, or
the NAP has a means of obtaining these from the
network.
o
The URN may be issued periodically and/or it
may be triggered by a status change from one
of the ENs.
o
The contents of the URN may include the
complete EN information, or only relevant
changes, or mixture.
Upon receiving an URN, a NAP checks the list of ENs
in the message against the list of its aware ENs (i.e.
those in the AT, IR or NR state). If the NAP is already
aware of all the ENs in the URN, a NAP does nothing.
If the URN contains an EN that it is unaware of, the
NAP adds the EN to its NR set and places it in the NR
state.

Upon first receiving a transmission from an EN that a
NAP has not previously communicated with, the
NAP’s IWF checks its NR set for this EN.
o
If the EN is in the NR set, the IWF informs
the gateway that it is ready to communicate
with the EN. Based on the response from the
gateway, the EN may be moved into the IR
state or the AT state.
o
If the new EN is not in the NAP’s NR list, a
fault occurs. The NAP can revert to the poll
mobility protocol – it polls its neighbors
before resorting to the gateway.

Transition between the AT state and the IR state
depends on data plane operation and may also be
forced by the network.

A NAP maintains a timer for each EN in the AT state
or the IR state. If the timer expires, the EN is transited
to the NR state from either the AT state or the IR state.

A NAP may drop an EN if instructed by the network

For each EN in the NR state, the NAP maintains a
timer, different from the timer for the AT and IR
states. If this timer expires without a transmission
from the EN or to the EN, the NAP sends a Range
Probe to the EN to check if the EN is still within range
of the NAP. If no response is received, the EN is
dropped.
Figure 5: State transition diagram for EN within a NAP under
the push mobility protocol
The state transition diagram for the push protocol is shown
in Figure 5 which also shows how the network can exercise
control over whether a NAP supports and EN or not.
4.4 Performance Evaluation
4.4.1 Test Bench Description
We set up a test bench to examine the performance of our
proposed mobility protocols. One of the purposes is to
illustrate that the BWLAN is able to handle EN movement
by providing continuous connections, especially when an
EN is entering or exiting the coverage area of a NAP. This
feature indicates the advantage of BWLAN over traditional
WLANs.
As mentioned above, one advantage of our proposed
distributed approach over the centralized approach (cf. [3]-[5]) is the relief of the backhaul traffic. Hence, our test
bench shall focus on the backhaul load of BWLAN.
Our test bench is built on OPNET. In the test bench, we
set up a BWLAN composed of 63 NAPs uniformly
distributed in a square space of 70 meters by 70 meters.
The distribution of the NAPs is shown in Figure 6. The
distance between two neighbor NAPs is 10 meters. A
Figure 6: Floor plan of the test bench blanket
gateway is resided at one of the NAPs. All these NAPs are
connected with the gateway, which in turn is connected
with an outside network. The connection between NAPs
and gateway is wired, and all the inter-NAP
communication is through the gateway. In our simulations,
each inter-NAP packet is assumed of size 32 bytes, which
contains the information of blanket ID, EN MAC address,
etc.
In the poll mobility protocol, the time-out for the
transition from the AT state (or the IR state) to the NA state
is set as 25 seconds. In the push mobility protocol, the
time-out for the transition from the AT state (or the IR
state) to the NR state is 5 seconds, while the time-out for
the transition from the NR state to the NA state is 20
seconds. Each NAP in the BWLAN regards 8 closest NAPs
as its neighbors.
The connection between NAPs and ENs follows the
IEEE 802.11g protocol, and all the NAPs operate on a
common channel. The maximum transmission rate between
NAPs and ENs is chosen as 24 Mbps.
Since in our simulations, the receiver sensitivity is -86
dBm for data packet and -95 dBm for beacon packet, the
transmission power of a NAP increases 9 dBm when it
joins a blanket or begins to serve an EN. This scheme
guarantees the successful data transmission when a NAP is
serving an EN. On the other hand, it does not affect the
blanket maintenance, as an EN could still detect beacons
from NAPs beyond its serving blanket. In our simulations,
the transmission power of NAPs ranges from -36.5 dBm to
-29.4 dBm when it is not in a blanket, while the
transmission power ranges from -27.5 dBm to -20.4 dBm
otherwise.
depends not only on EN’s moving speed but also on the
message rate from EN to NAPs. If an EN sends messages
to NAPs at a higher rate, then the receiving NAPs which
are not in the blanket will send polling messages more
frequently, resulting in an increased backhaul load. In our
simulations, the message rate from the EN to NAPs is 26
messages/sec. The backhaul load for the poll protocol will
decrease with the message rate. On the other hand, the
backhaul load of the push protocol does not rely on the
message rate from the EN to NAPs.
Overall, the backhaul load of either protocol is small (on
the magnitude of kbps), compared with the capabilities of
most current networks. This implies that a BWLAN with
the mobility protocols could be set up on top of most
existing backhaul networks.
Note that transmission power of NAPs is closely
correlated with blanket size, as the larger transmission
power, the larger blanket size.
4.4.2 Performance Results
We first examine the case of a single EN. Suppose an EN
moves within the coverage of the BWLAN, at a pedestrian
speed. Its moving trajectory is shown as the line in Figure
6.
Figure 7 shows the backhaul loads of our proposed
mobility protocols, in terms of the moving speed of EN. In
plotting this figure, the transmission power of serving
NAPs is -24 dBm. This results in an average of about 12
NAPs in a blanket during the whole simulation period. To
reduce the backhaul load in the poll protocol while still
guaranteeing the connectivity of EN, the interval between
two polling messages from a NAP is restricted to be no less
than 50 ms.
It is seen from the figure that the backhaul loads almost
linearly increase with the moving speed of EN. This is
because with an acceleration of EN, the frequency that
NAPs join and exit the blanket is linearly increased, and
hence the communication amount between NAPs and the
gateway is linearly increased.
Also, it is observed that the poll protocol results in a little
bit higher backhaul load than the push protocol at high EN
speeds. According to the poll protocol, the backhaul load
Figure 7: The backhaul load vs. EN moving speed
Figure 8 shows the effects of blanket size on the
backhaul load. Here, the blanket size is configured by
adjusting the transmission power of NAPs. In the
simulations, the moving speed of EN is set as 0.7 m/s. As
expected, the backhaul load is shown to increase with the
blanket size.
Next, we illustrate the backhaul load when the BWLAN
is serving more than one EN simultaneously. For
comparison, the speed of all ENs is fixed at 0.7 m/s and the
trajectory of these ENs is predetermined. The transmission
power of serving NAPs is chosen as -25.5 dBm.
Figure 9 shows the backhaul loads under different
number of ENs. It is seen from the figure that the backhaul
load increases with the number of ENs almost linearly.
This establishes the scalability of our mobility protocols.
Figure 10: The connection status when the regular APs are
used
5. MODIFICATIONS OF CSMA
PROTOCOL FOR BWLAN
Figure 8: The backhaul load vs. blanket size.
Having illustrated simple protocols can solve the mobility
problem in the BWLAN with minimal impact to the
underlying MAC/PHY and with small load on the
backhaul, we turn our attention to the actual data
transmission in this network.
As discussed, each EN in a BWLAN is connected with
multiple NAPs. These NAPs compose a blanket for the EN.
One assumption is that all these NAPs are provided with
identical data that needs to be sent to the EN.
Figure 9: The backhaul load vs. number of blankets
We examine the simulations above and find there is not a
single connection loss when ENs move inside the
BWLAN. In other words, our BWLAN approach
overcomes the handoff problem in traditional WLANs.
Figure 10 shows the real-time transmission rate if the
NAPs in Figure 6 are replaced by the regular APs defined
in WLANs. It is calculated that the EN experiences
connection loss in almost 53% of time, if the transmission
power of NAPs is -25.5 dBm and the moving speed of EN
is 0.7 m/s. Some recent work [11]—[13] proposed schemes
to reduce the handoff period, which could relieve the
amount of disconnection time.
It follows from the BWLAN architecture that the
wireless medium between the NAPs and the EN is WLAN.
According to the DCF in the CSMA protocol of current
WLANs, all the NAPs compete for the channel resource
for their transmissions, even though they aim to deliver the
same data to a common EN. This unnecessary contention
among the NAPs results in a huge waste of channel
resources and a large amount of repeated packets. Our
simulations show that such a contention could significantly
reduce the network throughput, when the number of NAPs
in a blanket is large. Therefore, it is desirable to have the
NAPs in a blanket coordinate on their transmissions.
The coordination among the NAPs is easy to implement
in a centralized network. A simple example is the TDMA
system, where the NAPs are assigned different time slots
for their transmissions. In a more sophisticated network,
the NAPs could transmit at the same time in a cooperative
way. Specifically, these NAPs perform as a distributed
MIMO system (cf. e.g., [14]). The accurate synchronization
among the NAPs is required to support the distributed
MIMO system. Further, much instantaneous channel state
information needs to be exchanged to optimize the MIMO
operations. These necessitate a low latency and high date
rate backhaul. However, such a requirement is not satisfied
in the BWLAN, and it is even against the design purpose of
BWLAN.
Our approach to addressing the contention issue is by
means of some modifications on the current CSMA
protocol. With the modified CSMA protocol, the NAPs in a
blanket could have coordinated data transmissions, rather
than contentious data transmissions. Comparing with the
traditional WLANs, the BWLAN with the modified CSMA
protocol can take the advantage of the spatial diversity to
provide more reliable transmissions.
transmission and is halved after each successful
transmission. It is clear that if the BWLAN applies the
EIED back-off algorithm, more NAPs in a blanket will
contribute to the real transmissions to the EN, in the benefit
of the gentle decrease of the contention window size. This
will increase the spatial diversity, and hence increment the
throughput.
Our modifications include a changed back-off protocol
and a coordinated acknowledgement mechanism, which are
discussed in the following subsections. Note that these
modifications are on the NAPs only. The EN and the
gateway are (almost) remained intact.
With the assumption that all the NAPs in a blanket are
provided with identical data to be sent to an EN, each NAP
in a blanket tries to send all the data to the EN in the
traditional CSMA protocol. This causes lots of redundant
transmissions, and hence reduces the throughput.
It should be mentioned that the contention among NAPs
does not occur in the virtual WLANs, as the centralized
entity of a virtual WLAN controls the actual transmissions
of AP. This guarantees an EN is served by a single AP at
any time.
5.1 Back-off Protocol
We begin with the back-off protocol. The present IEEE
802.11 DCF utilizes a Binary Exponential Back-off (BEB)
algorithm to adjust contention window size. Specifically, at
the first transmission attempt, the contention window CW
is set to the minimum contention window CWmin. After
each unsuccessful transmission, (CW+1) is doubled until it
reaches the maximum contention window CWmax. After
each successful transmission, CW is reset to CWmin.
However, such BEB back-off algorithm is not suitable in
the BWLAN. Suppose one NAP seizes the channel and
makes a successful transmission at the beginning. Other
NAPs overhear the channel is busy and hence, they double
their contention window size. On the other hand, the NAP
with a successful transmission resets its contention window
to CWmin. Subsequently, this NAP will win the next
contention with a high probability due to its small
contention window size. Finally, only a small portion of the
NAPs in a blanket contribute to the actual transmissions to
the EN. This deviates from the purpose of utilizing multiple
NAPs to serve a single EN, and it reduces the diversity of
BWLAN.
The main reason why the BEB back-off algorithm will
result in only a small portion of the NAPs in a blanket
serving the EN is that the contention window is reset to
CWmin on each successful transmission. This sharp
decrease of contention window leads to the current
transmitting NAP more competitive for the next
transmission.
Many alternative back-off algorithms have been
proposed to achieve a better performance over BEB
algorithm. One of the proposed back-off algorithms is
called the Exponential Increase Exponential Decrease
(EIED). According to EIED (cf. e.g., [15], [16]), the
contention window size is doubled after every unsuccessful
5.2 Coordinated Acknowledgement Mechanism
We propose a mechanism with very little modification
on the underlying CSMA protocol, such that the
throughput of EN is largely increased. This mechanism is
to make use of the ACK/NACK feedback from the EN.
Specifically, each NAP in a blanket pays attention to the
ACK feedback from the EN, though the feedback does not
correspond to a transmission from this NAP. This
information helps the NAP know which packet has been
received by the EN. With the availability of such
information, the NAP could delete that packet from its data
buffer. The redundant transmissions within a blanket are
largely reduced via this way.
To implement such mechanism, we should require the
packet ID (e.g. sequence number) be contained in the ACK
message, since otherwise the NAP cannot identify the
received packet at EN. This poses a small modification on
the existing CSMA protocol.
5.3 Performance Evaluation
In this subsection, we shall examine the performance gain
from the two modifications stated above. We apply the
same simulation settings as described in Section 4.4.1, and
focus on the downlink throughput. Further, the push
mobility protocol is applied in the following simulations.
Figure 11 shows the downlink throughput in terms of the
transmission rate at NAPs. In plotting this figure, the
moving speed of EN is 0.7 m/s and the transmission power
of serving NAPs is -24 dBm. It is shown from the figure
that the downlink throughput decreases with the
transmission rate in the original CSMA protocol. This is
because the increased number of redundant packets and
transmission collisions crushes the whole system. On the
other hand, the downlink throughput linearly increases with
the transmission rate, provided that the two modifications
have been applied. The actual throughput is only slightly
less than the transmission rate.
The downlink throughput with the modification of ACK
mechanism only is also plotted in the figure. It is seen that
the performance is good at low transmission rates. In the
high transmission rate region, it does not perform as good
Figure 11: Downlink throughput vs. transmission rate
Figure 12: Downlink throughput vs. blanket size
as that when the EIED back-off algorithm is applied.
Figure 12 shows the downlink throughput in terms of the
blanket size. The transmission rate of NAPs is set as 5.12
Mbps in the simulations. As expected, the downlink
throughput decreases with the blanket size in the original
CSMA protocol, while the modified CSMA protocol
provides steady downlink throughput, no matter of the
blanket size.
Finally, we examine the downlink throughput of the
whole network when there is more than one EN. In the
simulations, the transmission power of serving NAPs is 25.5 dBm and the moving speed of ENs is 0.7 m/s. The
transmission rate from all NAPs is fixed as 1.024 Mbps.
Figure 13 shows the overall network downlink
throughput, in terms of the EN number in the BWLAN. For
comparison, we also plot the upper bound of the network
downlink throughput in the same figure. It is seen from the
figure that our BWLAN is able to achieve the throughput
upper bound when it simultaneously serves a small number
of ENs.
Though there is a gap between our achieved network
throughput and the upper bound in the condition of a large
number of ENs, our achieved throughput still increases
with the EN numbers. Furthermore, this gap can be reduced
with an incremented density of NAPs in the BWLAN.
6. CONCLUSIONS
In this paper we presented a novel approach to
constructing a wireless network out of low power/low
coverage access points. We noted that such a network can
be effectively managed using simple distributed algorithms
Figure 13: Network downlink throughput vs. number of
blankets
while supporting seamless mobility, taking advantage of
the spatial diversity created by such a performance and
retaining the key features of the CSMA protocol which
forms the backbone of most modern WLANs and WPANs.
Furthermore, we argues that all of this can be done while
placing reasonable demand on the backhaul supporting the
multiple access points.
Admittedly, the results presented in this paper do not
fully address the problems associated with deploying such
a network. While many such problems can be addressed by
the same means as those which exist today (e.g. admission
control), others require further work, which we hope to
present in follow-up papers. Among the latter is the
challenge of addressing the overlapping blankets, and, with
mobility, addressing colliding blankets. The advantages
that can be derived from the potential for frequency re-use
inherent in BWLAN should be quantified. And finally, and
critically, more spectrally efficient cooperation techniques
should be investigated.
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