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8/3/2019 Towards HD Mechanism
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Towards a Hierarchical-Dynamic Mechanism for
Bandwidth Management in Wireless Mobile
Networks
Rafael Baquero S., Jose G. Rodrguez G. and Sonia Mendoza C.
Departamento de Computacion, CINVESTAV
Av. I.P.N. No. 2508, Col. San Pedro Zacatenco
Mexico, D. F., Mexico, 07360
Phone: (+5255) 5747-3800 Ext. 3758
Fax: (+5255) 5747-3800 Ext. 3757rbaquero@computacion.cs.cinvestav.mx, rodriguez@cs.cinvestav.mx, smendoza@cs.cinvestav.mx
AbstractIEEE 802.11 has become a de-facto standard forproviding wireless access in many scenarios. One of the char-acteristics of 802.11 networks is bandwidth variations. These
variations have an adverse impact on applications such as voice,video, and biomedical measurements among others. Due to thesebandwidth variations, traditional schemes cannot provide QoSguarantees adequately in this type of scenarios. Additionally,current QoS schemes do not provide a means for applications tobe aware of each other networking needs. This awareness wouldallow an application to limit its network use depending on band-width availability and the requirements of other applications. Inthis paper we present a novel Hierarchical-Dynamic Mechanismfor Bandwidth Management in Wireless Mobile Networks. Thismechanism organizes applications into groups, where each grouphas a different hierarchy in regard to the rest of the groups.
I. INTRODUCTION
A byproduct of the constant increase in the transistor densityof digital integrated circuits has been a constant drop in the
price of low powered computing devices [1]. Together with
advances in other fields such as battery and communications
technologies has led to a boom in the availability of mobile
devices. Currently there are no signs that point towards an
end to this boom. On the contrary, advances in areas such
as ubiquitous computing suggest that the number of mobile
wireless capable devices will likely increase.
Next Generation Wireless Networks (NGWN) will utilize
several different radio access technologies, seamlessly in-
tegrated to form one access network [2]. Due to its low
cost, high availability, and ease of installation, IEEE 802.11
based networks will, in all likelihood, be a part of any suchheterogeneous wireless networking solution.
Frequent signal strength variations are part of the normal
operation of wireless networks. These variations can be due
to different factors such as RF interference, multi-path self
interference, signal absorption by foreign objects, etc [3][4].
Certain types of data, such as video, voice, and other, require
network parameters to be kept within certain bounds in order
to remain useful. One of these constraints is that a minimum
bandwidth be available [5]. The increasing wide spread use of
mobile devices, availability of wireless networks and process-
ing capabilities of these devices means that the load placed on
these wireless links will also, in all likelihood, increase.However not all types of data have the same degree of
importance for a given user. To illustrate this let us make
a simple imaginary experiment. Assume that a given user is
simultaneously engaged in 2 live video conferences on his
802.11 connected mobile device while concurrently sending
several biomedical measurements to a hospital. Furthermore,
assume that due to changes in the environment or because
of physical displacement of the mobile unit bandwidth drops
from 12 Mbps to 1 Mbps. Depending upon the minimum
bandwidth requirements for each of these data feeds, there may
not be enough available bandwidth to satisfy the requirements
of all of them.
Current 802.11 implementations will either allow data flowsfrom all sources to compete for the wireless link with equal
or different opportunities (802.11e), but all applications will
retain some data transmission capabilities. This means that in
order to determine its available bandwidth an application must
use part of the link bandwidth, even if the part used is below
the minimum required bandwidth for this application.
If available bandwidth is insufficient to satisfy the minimum
requirements of all currently running applications then a
compromise must be reached. This compromise consists of
preventing some applications from making use of the network
in order to allow other applications to satisfy their minimum
requirements. Which applications should be prevented from
making use of the network and which should be alloweddepends upon the user and the nature of each application. For
instance, in our previous example with two video conferences
and biomedical measurements, it is likely that the user will
prefer to sacrifice the video conferences in order to keep the
biomedical measurements at a minimum quality. On the other
hand, if available bandwidth is sufficient, shutting down video
while allowing audio and biomedical measurements to proceed
could also be a viable alternative, but this is something that
must be configured depending on each individual users needs.
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The need to give preference to some applications over others
means that a user defined hierarchy must be established among
applications. We propose a mechanism based on establishing
such a hierarchy among network enabled applications.
The rest of this article is organized as follows. In section
II we describe some works related to our proposed frame-
work. Section III provides a description of our Hierarchical-
Dynamic Mechanism for Bandwidth Management in Wireless
Mobile Networks. To test the viability and performance of
our mechanism we implemented this mechanism on a Linux-
based netbook-type device. The results of this implementation
are described in section IV. Section V describes some future
work that will be performed in order to improve this mech-
anism. Finally section VI provides the conclusions regarding
the development of our Hierarchical-Dynamic Mechanism for
Bandwidth Management in Wireless Mobile Networks.
I I . RELATED WORK
Quality of Service in wireless systems and modifying the
behavior of computer programs depending on resource avail-
ability have been the subject of ample study. In the followingparagraphs we will describe some of the work carried out in
this field.
A. IEEE 802.11
Due to the extreme ratio between transmitted and received
signal power on a wireless medium it is not practical to attempt
to detect collisions. Therefore 802.11 Medium Access Control
seeks to avoid collisions instead of detecting them as is done
in standard Ethernet. This is achieved through a carefully
orchestrated choreography based on the transmission of data
followed by an idle time period known as an Inter Frame
Spacing (IFS) time interval. The original 802.11 defined three
IFS intervals (shortest first): Short Inter Frame Spacing (SIFS),Point Coordination Function Inter Frame Spacing (PIFS),
and Distributed Coordination Function Inter Frame Spacing
(DIFS). The specific duration of each o these intervals is
dependent upon the particular physical (PHY) layer employed.
Prior to the ratification of the 802.11e amendment, 802.11
wireless networks employed three Medium Access Con-
trol (MAC) mechanisms: Distributed Coordination Function
(DCF) for contention based access, Point Coordination Func-
tion (PCF) for contention free access, and Request to Send/
Clear to Send (RTS/CTS). These MAC protocols do not
provide a means of differentiating traffic streams and, as a
result, can not provide special considerations to traffic with
requirements in bandwidth, delay, jitter, and packet loss. Asa first step towards solving these issues IEEE 802.11-2007
introduced two new MAC mechanisms: Enhanced Distributed
Channel Access and Hybrid Coordination Function Controlled
Channel Access (HCCA) [3][4][6].
EDCA. 802.11e introduces up to eight priority traffic classes
which map directly to the differentiated services code point
(DSCP). These traffic classes are introduced through four Ac-
cess Categories (AC) which can support eight User Priorities
(UP). Each AC is an enhanced variant of the DCF which
contends for transmission opportunities (TXOPs). A TXOP
is a time interval when a particular station has the right to
use the wireless medium WM. An AC with a higher priority
selects a value for its backoff timer from a smaller range than
an AC with lower priority. Additionally, a new IFS period,
called an Arbitration Inter-Frame Space (AIFS), is introduced
for each AC instead of the DIFS. The AIFS is at least DIFS
and can be enlarged individually for each AC.
In this manner each AC behaves like a virtual station within
the STA contending for the medium in a form analogous to
DCF.
HCCA. HCF-controlled channel access uses a QoS-aware
centralized coordinator called a hybrid coordinator (HC). HCF
is similar to PCF and provides contention free access to the
WM. The HC has knowledge of the amounts of pending traffic
belonging to different Traffic Streams (TS) and/or Traffic
Categories (TC) in the QoS STAs (QSTAs) which allows it
to allocate TXOPs accordingly.
B. Control Theory for Resource Management
Li and Nadherdst used adaptable and fuzzy control algo-rithms for resource management [7][8]. In their proposal an
application is divided into a set of functional components
called Target Tasks. A state space representation of the Target
Task is obtained and current values for the states are estimated
with the use of an observer component called an Observation
Task. The rate at which resources are requested by the Target
Task is controlled by means of an algorithm implemented in an
Adaptation Task. One of the requirements set for their system
is a fair resource allocation, however fairness is not always a
desirable feature mentioned before.
In [9] Hellerstein et al describe control theoretical concepts
applied to computing systems and illustrate the concepts intro-
duced by applying different control objectives to the ApacheHTTP Server and IBM Lotus Domino Server among others.
C. DiffServ
Vergados et al used DiffServ for the transmission of biomed-
ical measurements over wireless links in mobile emergency
telemedicine systems [10]. Their paper provides, among other
things, a description of the bandwidth requirements of different
types of biosignals.
III. HIERARCHICAL-DYNAMIC MECHANISM DESCRIPTION
As mentioned before our Hierarchical-Dynamic Mechanism
for Bandwidth Management in Wireless Mobile Networks
allows users to establish a network usage hierarchy amongapplications. However several applications may be of equal
importance to the user and therefore may be considered to
form a group, in such a way that if there is not enough
bandwidth available for the entire group of applications then
the entire group should be prevented from using the network.
To provide this functionality our mechanism includes the use
of an applications groups stack, which allows users to group
applications while simultaneously establishing a hierarchy
among the groups (see Figure 1). When bandwidth becomes
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App 1.1 App 1.2 App 1.n...
Link
Supervisor
Application Stack Layer 1
App k.1 App k.2 App k.m...
Application Stack Layer k
Wireless Link
System
Manager
Feedback
Bandwidth
Monitor
System
Administration
.
.
.
Fig. 1. Mechanism Diagram
scarce, lower hierarchy groups are prevented from using the
network, thus freeing resources for higher hierarchy groups.
The number of groups prevented from using the network
depends upon the currently available bandwidth. On the other
hand, as available bandwidth increases lower hierarchy groups
are once again allowed to make use of the network.
A Two-Point Control or On-Off Control scheme is a simple
type of control system where the actuator is either on full force
or off [11]. Our mechanism employs such a control scheme
and is based on switching on or off applications use of the
network depending upon bandwidth availability in the wireless
link. This type of control scheme is one of the easiest toimplement and requires fewer computational resources when
compared to other more sophisticated control schemes.
The description of our mechanism is carried out in three
stages. First we define some terms which are necessary to
understand the components that form our mechanism and to
understand its operation. Afterwards we show the components
that constitute our mechanism and finally we provide a de-
scription of how these components interact with each other.
A. Definitions
Networking Application (NA). An application that re-
quires use of the network to communicate with a remotecounterpart.
Network Permission Status (NPS). Indicates whether an
application is denied or allowed network use.
Networking Applications List (NAL). This is a list of the
NAs running at any given time. Two further lists are
derived from this list:
1) Current Application Permission List (CAPL). This
is a list of NAs currently being supervised and the
NPS of each application.
2) Application Permission Update List (APUL). Con-
tains an updated version of the CAPL.
Total Available Bandwidth (TAB). The currently available
bandwidth in the wireless link.
Application Minimum Required Bandwidth (AMRB). This
is the minimum bandwidth required by a single NA for
correct operation.
Total Minimum Required Bandwidth (TMRB). The TMRB
is the sum of the AMRBs of a subset of the Networking
Applications List. The subset of applications may be
improper.
Layer i Minimum Required Bandwidth (LiMRB). Thisis the TMRB of the subset formed by the NAs of
Application Stack Layer i.
Station (STA). Following IEEE Std 802.11 definition
a STA is any device that contains an IEEE 802.11-
conformant medium access control (MAC) and physical
layer (PHY) interface to the wireless medium (WM) [6].
Access Point (AP). Any entity that has STA functionality
and provides access to the distribution services, via the
wireless medium (WM) for associated STAs [6].
B. Mechanism Components
Our Hierarchical-Dynamic Mechanism for Bandwidth Man-
agement in Wireless Mobile Networks is composed of thefollowing modules:
Networking Application Groups Stack (NAGS). The
NAGS allows to hierarchically group NAs into layers of
equal importance. Each group of NAs becomes a layer of
the stack. This scheme allows the user both to prioritize
applications and simultaneously to group applications in
such a manner that, if there is enough bandwidth available
for the entire layer, all NAs will be allowed to make use of
the network and if there is not enough bandwidth for the
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entire group, all of the NAs in the layer will be prevented
from using the wireless link. A layer can of course consist
of a single NA.
Bandwidth Monitor (BM). As its name implies, the BM
constantly monitors TAB.
System Manager (SM). The SM maintains an updated
CAPL both by supervising the currently running NAs and
by establishing the NPS of each NAGS layer based upon
the TAB. Updates are delivered to the Feedback and Link
Supervisor modules by means of an APUL.
Link Supervisor (LS) NAs that have been designed to
operate with the Hierarchical-Dynamic Mechanism are
notified when to stop or resume network link usage by
means of feedback. To allow network use management
of NAs that have not been designed to operate with the
system, it is necessary to have a component external to the
application that allows or prevents network use by these
Networking Applications. This function is performed by
the Link Supervisor.
Feedback This module notifies applications when they
must stop transmission and when to restart their networkuse. The use of feedback allows NAs to perform tasks
such as notifying remote counter parts before ceasing
their transmissions.
User Preferences (UP) The UP module allows users to
assign NAs to layers.
Now that we have described the components of our
Hierarchical-Dynamic Mechanism for Bandwidth Manage-
ment in Wireless Mobile Networks we need to describe how
these components interact with each other.
C. Component Interaction
The central part of our mechanism is the System Manager
module. The System Manager module performs the followingtwo basic tasks:
1) Constantly update the NAL. The means by which the
SM obtains the NAL is dependent upon the operating
system and how the mechanism is implemented on a
particular architecture and will therefore not be detailed
here. However, since multitasking operating systems
need to keep track of the applications currently being
executed it is a reasonable assumption that such a list
can be assembled.
2) Adjust the NPS of each layer of NAs depending upon
the TAB and on the current TMRB.
The System Manager periodically obtains a list of all
currently running applications on the STA and updates theNAL accordingly. The AMRB for each NA is obtained by
one of two means:
1) For applications that have been designed to work with
the mechanism. The NA dynamically notifies the SM
module of its current AMRB. The AMRB need not re-
main fixed, but can be updated throughout the execution
of the NA.
2) For applications that have not been designed to work
with the mechanism. The AMRB is set by the user
through the UP module. This value can be updated by
the user at any moment.
TAB is obtained by the SM through the Bandwidth Monitor.
Based upon the TAB, the SM determines which layers of the
NAGS can be allowed to use the wireless link in such a way
that the sum of the LiMRBs of the layers is less than or equal
to the TAB. Once the layers of the NSGS that will be allowed
network use is determined the APUL is defined and passedto the Feedback and LS modules. Assuming the NAGS has a
total of k layers, the NAs that will have their NPS set to On
is determined in the following way:
1) RequiredBandwidth := 0.
2) NPS := Off for all NAs.
3) i := 1.
4) RequiredBandwidth := RequiredBandwidth + LiMRB.
5) if RequiredBandwidth TAB then NPS := On for every
NA in NAGS Layer i, else end.
6) i++
7) if i k goto 4, else end.
Once the APUL has been defined through the previousprocedure, it is passed to the Feedback and LS modules.
The Feedback module compares the APUL to its CAPL and
notifies each NA whose NPS has been toggled that it must
shutdown its network transmission or that it can recommence
network use. If a NA must shutdown its network use, the LS
module waits for a fixed time delay and afterwards proceeds
to stop the NA from making any further network use. The
purpose of this delay is allowing a NA to cease network use
gracefully when notified to do so by the Feedback module. On
the other hand, if network use by an NA can be recommenced,
then the LS immediately allows network use by the NA.
After the Feedbak and LS modules have made their respective
notifications and wireless link supervision adjustments theyupdate their local CAPLs. Finally, the UP module allows users
to assign applications to a NAGS layer and define the AMRB
for applications that have not been designed to be used with
the mechanism.
To illustrate the interaction of the modules let us consider
a hypothetical usage scenario. Assume that a mobile station
(STA) is connected to an access point (AP). Initially STA and
AP are near each other with no obstacles between them and
the total available bandwidth is 54 Mbps. Several Networking
Applications are running simultaneously on the STA on a
three layer Networking Applications Group Stack. Layer 1
Minimum Required Bandwidth is 0.5 Mbps, Layer 2 Minimum
Required Bandwidth is 1.5 Mbps, and Layer 3 MinimumRequired Bandwidth is 2 Mbps. The STA is required to move
near the edge of the coverage area where Total Available
Bandwidth will drop to 1 Mbps. Afterwards the STA moves
once again to the vicinity of the AP and bandwidth climbs to
54 Mbps.
In the previous scenario, as long as TAB remains above
4 Mbps all NAs are allowed to make network use without
any type of restrictions, in other words the mechanism is
transparent to the NAs. When bandwidth drops below 4 Mbps,
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the System Manager module determines that there is not
enough TAB to satisfy the demands of all the NAs, but there
is enough to satisfy the demands of the NAs in Layers 1 and
2. Therefore the NPS of Layer 3 applications is set to Off
in the Application Permission Update List, while the NPS
of Layer 1 and 2 NAs is set to On. The APUL is passed
to the Feedback and Link Supervisor modules so that Layer
3 NAs are prevented from making any further network use.
Notice that, at this point, the available bandwidth for Layer
1 and 2 NAs is 4 Mbps. This is above the TMRB for all
the applications from Layers 1 and 2, which allows them to
improve the quality of their transmission due to the bandwidth
surplus.
As the STA moves further away from the AP NAs from
Layers 1 and 2 are allowed to make use of the network until
TAB drops bellow 2 Mbps. At this stage the SM module once
again determines that TAB is not enough to satisfy the TMRB
of Layer 1 and 2 NAs. Therefore the NPS of Layer 2 NAs
is toggled to Off in the APUL and the list is passed to the
Feedback and LS modules to shutdown network use by Layer
2 NAs. Notice again how the bandwidth available to Layer 1NAs is increased which allows them to improve the quality of
their transmissions.
During the return trip the reverse process occurs. When TAB
raises to 2 Mbps the SM detects that the available bandwidth
is enough to satisfy the requirements of Layer 1 and Layer 2
NAs which results in the NPS of Layer 2 NAs being toggled
to On. This results in the bandwidth available to Layer 1 NAs
dropping which may force them to reduce the quality of their
transmission but it allows Layer 2 NAs to make use of the
network. As the STA further approaches the AP and TAB
raises to 4 Mbps the NPS of Layer 3 NAs is toggled to On
allowing them to make use of the network. Once again the
bandwidth available to Layer 1 and 2 NAs is reduced but inexchange Layer 3 NAs are allowed to make use of the network.
IV. TES T PLATFORM
To test the mechanism functionality an experimental partial
implementation was carried out. The purpose of this imple-
mentation was not only to test the mechanism but also to
provide a test bed for future development of our Hierarchical-
Dynamic Mechanism for Bandwidth Management in Wireless
Mobile Networks.
The first step in the implementation was to select the
hardware to be used. The hardware had to satisfy the following
the following requirements: WiFi support.
Easily transportable device. In order to carry out tests
which involved physical location changes, the device
selected would have to be easily transported.
Battery powered with reasonable battery duration.
Availability of a modifiable OS that could be modified.
Ease of OS installation.
These criteria quickly narrowed the choice to a netbook type
personal computer.
The next stage was the selection of an operating system.
Selecting an operating system appropriate for the mechanism
implementation was a far from trivial task. There is a wide
variety of operating systems, both experimental and stable,
each with its own benefits and drawbacks. The basic criteria
for the test bed operating system selection was the following:
Wireless networking support.
Source code available. Available version for installation in a netbook.
Stable and well tested.
Well documented.
Customizable to a bare bones minimal system to avoid
applications, other than our own, from using sockets for
IPC.
After considering the advantages and disadvantages of vari-
ous operating systems Linux was selected as testbed. In order
to have full control of the programs installed on our test
platform and to avoid any customization made by distribution
developers our test platform was built from source following
the procedure outlined in the Linux From Scratch project [12].
Once the test platform had been developed, we proceeded tomake a simple implementation of our Hierarchical-Dynamic
Mechanism. The following section describes this implementa-
tion.
V. TES T IMPLEMENTATION
To test the behavior of our mechanism we required a
means to place load on the wireless link and to measure
the throughput of several simultaneously running NAs. In
order to do this we developed a simple client and server pair
of programs which measure available bandwidth by sending
test data. The client NA can communicate with the SM
and Feedback modules using sockets to notify its bandwidth
requirements and to obtain changes to its NPS. The client NAconstantly updates a text file reporting measured bandwidth.
With a set of NAs developed, the components of our
Hierarchical-Dynamic Mechanism were implemented as fol-
lows:
Networking Applications Group Stack. The NAGS is
defined by means of a plain text CSV configuration file,
the Application Stack Definition File (ASDF). Each entry
in the ASDF is a record used to describe an NA and its
requirements. The first field indicates the name of the NA,
the next field is used to indicate the layer in the NAGS to
which the NA is assigned, and finally the last field sets
the AMRB of the NA.
Bandwidth Monitor. Wireless Extension is an API thatallows applications to obtain statistics about the wireless
link in Linux systems [13]. The BM employs this API
to obtain the speed of the wireless link and passes this
information to the SM. A good starting point to use the
Wireless Extension API is the iwconfig program, part of
the Wireless Tools package.
System Manager. Upon start, the SM first reads the ASDF
and then enters a loop in which it performs the following
tasks:
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1) Obtains the currently running process list and cre-
ates the NAL by comparing the process list to the
entries of the ASDF.
2) Forms the layers of the NAGS.
3) Determines the NPS of each application based upon
the TAB reported by the BM module and creates
the APUL. The APUL also indicates if the NA is
feedback-enabled. This information is required by
the Feedback and LS modules.
4) Compares the CAPL to the APUL and, if changes
have occurred, sends the APUL to the Feedback and
LS modules.
Feedback Monitor. Upon reception of the APUL the
Feedback module compares this list to its CAPL. If the
NPS of any feedback-enabled NA has changed, the NA is
notified of its current NPS. To allow coordination with the
LS, a configurable time delay is employed when notifying
NAs that their NPS has been toggled to On.
Link Supervisor. The LS module consists mainly of a loop
which reads the bandwidth usage output files of the NAs
to determine if an NA is making use of the network.Upon reception of an APUL the LS compares this list
to its CAPL. If the NPS of a feedback-enabled NA has
been toggled to Off, the LS waits a configurable delay
and if the NA has not ceased its network use it will be
paused by means of an IPC STOP signal. Non feedback-
enabled NAs are paused immediately. On the other hand,
if the NPS of an NA has been toggled to On it will
immediately receive a CONT signal to resume its network
transmission.
V I . CONCLUSIONS AND FUTURE WOR K
In this work we proposed a novel mechanism for wireless
link management in varying bandwidth type networks. A testplatform was set up to allow further development and a basic
implementation of the proposed mechanism was performed.
There are many feedback schemes that can be employed to
provide applications with a collective awareness of networking
resource requirements and networking resource availability.
For instance, instead of a Full-Cut feedback scheme the
amount of bandwidth employed by lower layer applications
could be limited instead of completely cutoff while simulta-
neously notifying applications of their allocated bandwidth by
means of feedback. This would allow an application to choose
between lowering the quality of their transmissions or shutting
down transmission completely depending upon their assigned
bandwidth.On the other hand, in order to make a more efficient use
of the wireless link, inbound traffic must also be limited. This
means that a component must be added to the AP in order to
limit or prevent the inbound traffic of applications that have
been cutoff from entering the wireless link. Finally, a remote
component which allows applications to coordinate their net-
work transmissions with local applications depending on their
current network resource availability could also represent an
improvement of the mechanism.
ACKNOWLEDGMENT
The work of Rafael Baquero S. is supported by CONACyT,
Mexico.
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