Towards HD Mechanism

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. [email protected], [email protected], [email protected]

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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