LINKING SIMULATION AND PRACTICE

Chris McDonald

Computer Science & Software Engineering, The University of Western Australia, Crawley, Australia

Keywords: Simulation, Wireless Networks, Computer Science Education.

Abstract: The current generation of Computer Science students are far more likely to engage with computer

networking through their own mobile, wireless devices than they are using fixed, wired desktop computers.

Traditional approaches to teaching computer networking evolved when the Internet was composed of fixed

wired infrastructure, and this historical background still forms most of the material in contemporary

textbooks on computer networking. Today's students have strong expectations that their computer

networking courses will have a significant focus on the networking devices and applications that they use

daily - increasingly mobile and wireless. This paper describes a computer-networking project, examining

delay tolerant networks, that has successfully assessed students' understanding of networking protocols

using both simulation and actual implementations on Apple iPods. Importantly, students developed identical

source code to be compiled and then executed by both the simulator and the iPods. The paper then reflects

on the use of simulation in computer networking projects, identifying some deficiencies in students'

understanding that may be masked by the use of simulation alone.

1 INTRODUCTION

It can be argued that computer networking continues

to be the field of computing making the greatest

impact on our daily lives, with the Internet becoming

the ubiquitous carrier of global applications such as

user-contributed videos and music, instant

messaging, location-aware mapping and

applications, and the myriad of competing and often

contradictory information sources. Our students'

almost insatiable desire for access to the Internet

often pushes aside the desire to actually understand

the technical aspects of how it all works, and even

why it all works.

Contemporary students, collectively described

as Generation-Y, the M-generation, and Millennials,

are more likely, or wish, to access many Internet

services using handheld and mobile devices,

including laptop computers, network-enabled

personal digital assistants (PDAs) and palmtop

computers or tablets (Junco, 2007; Oblinger, 2005).

For current students in Computer Science courses

the Internet's “last-mile” problem is being solved,

not through traditional fibre or copper connections to

the home, but by near-ubiquitous wireless access.

Consequently, students undertaking courses in

computer networking today are expecting their

courses to contain, if not focus on, details of wireless

and mobile networking, developed this century, and

not to remain focused on the traditional, often

inaccessible, wired backbone infrastructure and their

protocols, developed in the 1970s.

To maintain students' engagement and, more

importantly, to increase their understanding of the

design and implementation of computer networks, it

is now necessary to convey many of the fundamental

concepts by drawing upon mobile and wireless

networking examples. This paper presents our

recent successes with student projects employing a

combination of simulations and real exercises using

the wireless communication facilities of Apple

iPods.

2 THE CNET SIMULATOR

The cnet network simulator has undergone a

sequence of developments over eighteen years,

being used as a primary teaching tool in both

undergraduate and graduate computer networking

courses (McDonald, 1991). Today cnet supports

student-developed protocols and experimentation

with a variety of data-link layer, network layer, and

transport layer networking protocols in networks

258

McDonald C. (2009).

LINKING SIMULATION AND PRACTICE.

In Proceedings of the First International Conference on Computer Supported Education, pages 258-264

DOI: 10.5220/0001979702580264

 SciTePress

comprising any combination of wide-area-

networking (WAN), local-area-networking (LAN),

or wireless-local-area-networking (WLAN) links.

Students working in open or closed laboratory

sessions, or professors, wishing to demonstrate

networking concepts in the classroom may develop

or extend a wide variety of network protocols and

employ cnet controls and statistical routines to

gather and evaluate their experiments. cnet has been

selected by William Stallings to complement the

materials in his award winning networking textbook

(Stallings, 2007).

cnet has most recently been extended to support

wireless networking and, in particular, now supports

ad-hoc and mobile networking as characterized by

delay tolerant networking (see section 4). Using

cnet's event-driven framework students may control

the mobility (speed and direction) of hundreds of

mobile nodes, having them move across a physical

landscape and occasionally pausing at defined

landmarks or when other mobile nodes are detected.

cnet permits full control of one or more wireless

antennae on each mobile node and the physical

characteristics, such as transmission frequency,

output power and gain, input sensitivity, and

whether the node's radio card is in a low power

“sleep” mode, may all be controlled, either as fixed

parameters to a single simulation or dynamically for

a location- and power-aware protocol..

Students also have programmatic control over

the propagation of their nodes' wireless signals. For

example, once cnet has considered the physical

properties of each transmission (to determine

strength of arrival over the distance between nodes)

and managed physical layer collisions, student-

written code may further determine the successful

arrival of a signal through obstacles (such as walls),

across different terrain types (such as concrete or

grass), or at certain angles (using omni-directional

antennae).

cnet was first developed as a simulator running

on a variety of Linux and UNIX platforms and

Apple's Mac OS-X. Support has recently been added

for student-written protocols to be cross-compiled

for the Apple iPhone and iPod Touch platforms. The

motivation of this has been so that students may first

design, implement, test, and evaluate wireless

network protocols within the robust simulator, and

then cross-compile their unmodified source code for

execution on the physical, mobile devices. It is

believed that, in combination, the use of both

simulation and actual exercises can dramatically

increase a student's understanding of wireless

networking, particularly the nature of physical errors

and transmissions. Recent developments have been

generously supported by a ACM-SIGCSE Special

Project Grant and through support from Apple Inc.

cnet's full source-code and examples are released

under the GNU Public Licence (GPL).

3 PROTOCOLS ON IPODS

The Apple iPod Touch is a handheld mobile device

with a 670MHz ARM11 processor, 128MB of RAM

in which the operating system and applications

execute, and of flash memory providing a solid-state

disk. The iPod runs a modified version of Apple's

OS-X operating system and provides a multi-touch

GUI for all input. Apple promotes the devices as

music and video players, also capable of executing

approved applications. However, it is possible to

bypass the supported, constrained, environment of

the device and to develop general programs for it,

taking advantage of the IEEE802.11 (Wi-Fi)

networking interface and general TCP/IP protocol

stack (an activity requiring the unsanctioned

“jailbreaking” of the device).

Our school purchased twenty 8GB iPod Touch

devices for experimentation in this networking

project. Our students have long developed their cnet

simulations on their own home or laptop computers,

as well as on our laboratory equipment. For this

project, students were not expected to have

undertaken the difficult task of installing all files

necessary for cross-compilation for the iPods.

Instead, the cnet simulator was modified to accept an

additional command-line argument to request that

students' protocol files be sent to a purpose-written

remote cnet/iPod cross-compiler. The round-trip

between the students' personal machines and the

server was typically only three seconds. The

products of this cross-compilation phase are a

directory of files, forming an application bundle,

including the actual executable application, an XML

configuration, and basic icons. The salient point is

that identical source code is developed and compiled

for protocols exercised by both the simulator and the

iPods.

The Students then loaded this new iPod

application, their developed protocol, on (typically)

six iPods and walked around one floor of our

Computer Science building. Under execution,

students' protocols perform as they did on the

simulator, generating fictitious messages and control

packets forming the implemented protocol, and

responding to screen-based input via cnet's event

buttons or the virtual keyboard. Wireless frames

LINKING SIMULATION AND PRACTICE

259

from the students' protocols are actually transmitted

via the iPod's Wi-Fi interface, as encapsulated UDP

frames broadcast to all devices within range. A non-

standard artefact of the iPod's networking protocol

stack is that frames broadcast from the Wi-Fi

interface are also received by the transmitting

device, and cnet's on-device implementation was

modified to filter these frames before they reached

students' protocols.

4 DTN PROTOCOLS

For the assessed project described in this paper, we

required students to implement a delay tolerant

network (DTN) protocol. A DTN is one in which

mobile wireless nodes experience frequent, lengthy

delays in communicating with other nodes because

there is no guaranteed connectivity between

communicating end points (Jain, 2004). The nodes

themselves, typically personal digital assistants

(PDAs), “smartphones”, or tablets, generate network

traffic for the other nodes in the network, and

transmit their messages using their own built-in

wireless antennae. The lack of fixed infrastructure

means that the nodes are collectively and

cooperatively responsible for the delivery of their

own messages, and for the messages of other nodes.

DTNs are often described as potential

mechanisms for communication between low-earth

orbiting (LEO) satellites, for communication

between devices in environments lacking fixed

infrastructure, and for emergency workers

communicating after natural disasters. The most

frequently cited example of DTNs is the Wizzy

Digital Courier service which exchanges email

between remote villages and schools in South Africa

using motorcycles and a milk truck (Wizzy, 2007).

In an environment where wireless network

coverage is sparse, such as (unfortunately) many

university campuses, the mobile devices will,

themselves, carry and exchange messages for each

other. The devices (actually, the people carrying the

devices) form a social network with other devices

that they meet while walking around campus, and

periodically try to keep in contact with them by

generating and sending self-contained messages.

The nature of the network is such that the reliable

and timely delivery of the messages is not critical;

thus protocols and applications in this domain must

be delay tolerant. Messages eventually arriving at

the intended destination may arrive out of order and

do not need to be acknowledged.

There are many metrics used to evaluate the

effectiveness of a DTN. For example, information in

messages can lose its value over time, and if our

DTN cannot deliver messages in a timely fashion,

then it will be considered ineffective (and hence will

not be used). The latency with which messages can

be delivered will depend on a number of factors,

including the number of nodes in the communication

region (and thus the node density), and the speed at

which nodes move (and thus come into contact with

other nodes). Similarly, later messages from a source

node could be considered more important than

earlier ones, and devices have limited storage

available for carrying the messages of other nodes.

In combination, these two factors suggest that nodes

may have to occasionally drop messages due to a

lack of storage. To evaluate the effectiveness of a

DTN implementation we thus need to evaluate it

under a range of operating conditions.

5 THE DTN PROJECT

The Computer Networks course at The University of

Western Australia is offered to students in their third

undergraduate year, and is taken by a wide variety of

students undertaking bachelor degrees in Computer

Science, Software Engineering, and Electronic

Engineering. In recent years the course has had

enrolments of between 120 and 65 students. The

course comprises one eighth of a year's work, and is

assessed with a practical project contributing 40%

and two examinations contributing 60%. The

practical project, described in this section, ran for 6

weeks and was undertaken in groups of four.

The goal of the student project was to design,

implement, and analyse a delay tolerant protocol

using the cnet simulator, to analyse and report on the

observed effectiveness of the protocol developed,

and to report on the differences between the

simulated protocol and the actual, physical one. The

project was designed to assess students'

understanding of important aspects of Media Access

Control (MAC), Data Link and Network Layer

networking protocols, and their ability to analyse

and report on their observations.

In some schools, such a project may appear

under-specified, as no explicit information was

provided as to how students should undertake their

protocol development and evaluation. Students had

already completed four weekly closed laboratory

sessions, which had introduced them to the cnet

environment and, in particular, encouraged them to

experiment with the fundamentals of wireless signal

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transmission and reception, typical transmission

distances, and the handling of frame collisions.

The research topic of delay tolerant networks is

sometimes described as a “solution without a

problem” and, as an artefact of this, there are

extremely few general-purpose protocol

implementations available for consideration or even

described in textbooks. For this reason, we view

delay tolerant networks as a fertile ground for

student projects in which students are expected to

demonstrate initiative in developing and analysing

their own designs. Projects involving delay tolerant

networks hold many opportunities for creativity, as

there are so many protocol characteristics that can be

varied. Another beneficial consequence was that no

internet-sourced plagiarism was observed.

6 STUDENT EXPERIENCES

Students undertook the project in 17 groups of four,

but often chose to co-opt other students to carry

additional iPods to form DTN networks of up to 15

nodes. About ten students also used their own

iPhones and iPods in the groups' experiments.

Student groups typically partitioned the networking

responsibilities into lower and upper layers.

Prompted by lecture materials, one pair of students

developed a carrier-sense multiple-access with

collision-avoidance (CSMA/CA) MAC protocol to

transmit wireless data frames, and a simple mobility

model (both only required on the simulator). The

other pair developed the node beaconing and

message transmission and carrying responsibilities.

cnet protocols are written in ISO-C99, and the C-

preprocessor is thus employed to isolate the

simulator-specific code.

Students typically implemented a simple random

waypoint model of node mobility, in which nodes

would randomly choose their next remote

destination and walking speed, walk toward that

destination at a constant speed and, once there,

pause for a random period of time before setting off

again (Camp, 2002). At first, students anticipated

that they would experience difficulty with both

walking and transmitting and receiving

simultaneously (generation-Y's equivalent of being

able to walk and chew gum?), but cnet's internal

scheduler represents and reports events, such as

periodic movement and wireless frame arrival,

consistently, making the anticipated complexity

disappear.

To provide a strong sense of localization and

realism, our students were provided with a

representation of the floor plan of our Computer

Science building as cnet enables this map to be

displayed on the simulation background. The

random waypoint model was modified to honour the

position of walls in our building, and moving nodes

could only enter corridors and lab areas through the

actual doorways. Similarly, cnet protocols (under

simulation) have the opportunity to define their own

wireless signal propagation models. Earlier

experiments had identified that the Wi-Fi signals of

the iPods could pass through one, but not two, of the

(concrete block) walls in our building, and students

implemented a signal propagation model that

dropped signals attempting to pass through two or

more walls. A representative student DTN

simulation is presented in Figure 1.

As well as developing solutions for mobility and

wireless signal propagation, students had to design

and implement a strategy for message generation

and delivery. The simplest metric for evaluating the

success of a strategy was, while holding constant the

number of nodes and the walking speeds and pause

durations, to measure the average delivery time of

each message (cnet permits all simulation attributes

and required statistics to be specified in an external

topology file). During the project it was very

encouraging to see students using our asynchronous

message forum to independently promote a

competition to find the shortest possible message

delivery time.

From the students' submissions it was clear that

most groups investigated a number of strategies of

increasing complexity, driven by the desire to

minimize message delivery time. The simplest

strategy seen amongst the students' submissions was

one making no attempt to transmit a generated

message until the intended recipient was within

wireless range (such nodes periodically transmitted

beacon frames). While this strategy works, messages

would typically take several minutes to be

exchanged between nodes that appeared within the

same region by chance (although the presence of

social hotspots increased the likelihood of their

meeting). The next strategy attempted was one in

which nodes quickly transmitted their messages to

any close neighbour, attempting to simply “pass-off”

the responsibility for delivery (akin to the basic “hot

potato” routing algorithm often cited in networking

textbooks). Students quickly observed that this

strategy frequently resulted in two adjacent nodes

continually exchanging messages until one of them

commenced walking. The solution to this required

nodes to either mark messages that they had most

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recently held, or for nodes to maintain queues of

recently seen messages (to thwart cyclic exchanges).

The most complex student strategies attempted

involved nodes not only maintaining queues of

outgoing and recently seen messages, but also

maintaining information about recently seen nodes

and, using the signal strength of frames arriving

from those nodes, estimates of the direction and

speeds of those nodes to determine if they would

still be within wireless range. These advanced

solutions involved nodes “bidding” to forward an

offered message in a direction in which the intended

recipient was seen very recently. In later simulations

involving as many as three hundred nodes on our

campus (i.e. high node densities in an environment

in which signals could propagate 70 metres),

message delivery times were often less than a

second.

7 REFLECTING ON

SIMULATION

As recently as 2002, large enrolments in the

discipline of Computer Science resulted in a number

of modifications to our teaching methods to address

the problems associated with teaching large cohorts

of students. Changes included the dropping of

weekly tutorials, an increased use of software

simulations to demonstrate many Computer Science

concepts, and the development and support of

flexible learning approaches facilitating use of home

and laptop computers.

More recently, the dramatic drop in enrolments

in Computer Science has not been matched by a

return of some of our traditional teaching methods,

such as tutorials and in-class and in-laboratory

demonstrations and experiments. Students are still

exposed to high quality teaching materials and

exercises, that are typically undertaken alone, but the

student experience is considerably poorer for the

reduced interaction with other students, and a

reduced focus on experimental techniques involving

physical, error-prone, scenarios. Related concerns

are also raised by Kotz, who identifies a number of

simplistic assumptions made by many wireless

network simulation tools (Kotz, 2004). Excessive

use of simplistic simulations, without accompanying

physical exercises to both support and verify the

observations made from the simulations, should

raise the obvious concerns about what is actually

being taught to, and learnt by, our students.

The dramatic reduction in Computer Science

enrolments is not, of course, specific to our

university, and is being widely experienced in the

western hemisphere. Prospective Computer Science

students, and even those already within our

programmes, have realistic concerns about the

“offshoring” of programming skills, and the

relevance of some Computer Science material to

their future careers. It is not unreasonable for

students to question the value of their university

contact, if they can ``successfully'' undertake all

learning online, and in isolation.

A wealth of recent literature in Computer

Science Education journals and conferences focuses

on teaching practices to revitalize the discipline, and

to increase the on-campus engagement of students.

In particular, many universities are reintroducing

practical, experimental-based laboratory sessions in

which students see an immediate benefit to

undertaking the work at university, and with other

students.

Our simulation-based investigation into delay

tolerant networks, which was successful in achieving

its initial aims, has highlighted a number of

problems that must be addressed in the near future.

The type of project described in this paper is well

supported by the cnet simulator, making it widely

available to a large number of universities and

colleges. However, while students were able to

successfully demonstrate their ability to develop

reasonably complex network protocols, they

experienced genuine difficulties developing

hypotheses, constructing and executing (simulation-

based) experiments, and evaluating the observed

results. Furthermore, it was clear that students had

great difficulty relating the physical dimensions and

properties, represented in their simulation, to an

actual wireless computing environment that they

observed with the iPods, and it is believed that this is

affecting their ability to construct meaningful

experiments that will expose and demonstrate the

strengths and weaknesses of their protocols.

To address the observed shortcomings with

students' understanding we are currently developing

a sequence of additional projects that will combine

the convenience and versatility of network

simulation, with the error-prone physical

characteristics of real wireless networking. New,

smaller, projects will incorporate actual mobile

wireless devices into our course. The motivation

behind this is the belief, supported through our

observations with this reported project, that many

Computer Science students currently have a poor

understanding of the physical constraints of mobile

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and wireless networking, and that this is limiting

their understanding of how, and why, contemporary

wireless protocols have to address these constraints.

In addition, it is believed that knowledge of this

material is not currently developed when only

exposing students to simulated network

environments.

8 SUMMARY

This paper has described a novel student project in

computer networking focusing on the types of

networking with which contemporary Computer

Science students are most familiar - mobile and

wireless networking. Within the project, students are

still exposed to many of the traditional aspects of

networking including the need for error detection

and recovery, the value of layered network

protocols, and the challenges of robust and reliable

message delivery. The project encourages students

to design and implement a sequence of solutions of

increasing complexity, and our experience has been

that students have been enthused by the challenges

that this project has raised.

Interested readers are welcome to contact the

author for copies of the full project materials

described in this paper, including the cnet network

simulator and its support for iPods.

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Figure 1: A representative student DTN simulation.

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