STUDYING MEDIA ACCESS AND CONTROL PROTOCOLS

Alaelddin Fuad Yousif Mohammed

Department of Computer Engineering, Faculty of Engineering and Technology, University of Gezira,Wad Medani, Sudan

Keywords: GNU Radio, USRP, MAC, Bluetooth, IEEE 802.11.

Abstract: The goal of this project is to enable undergraduate students to gain insight into media access and control

protocols based upon carrying out laboratory experiments. The educational goal is to de-mystifying radio

and other link and physical layer communication technologies as the students can follow packets from the

higher layers down through the physical layer and back up again. The project fills the gap between the

existing documentation for the Universal Software Radio Peripheral (USRP) resources and the knowledge

of undergraduate students. The project is targeted at (1) instructors of undergraduates who might use this

work to develop their own lesson plans and course material and (2) students of physical and link layer

protocols who want a practical tool for carrying out experiments in these layers.

1 INTRODUCTION

Software-defined radio (SDR) introduces flexibility

and rapid development to radio communication

systems by using a software-oriented approach. As

software-based approach offers greater flexibility

when developing wireless communication systems,

since the wireless system architecture is not frozen

into the hardware, but can be changed at any time

via changing the software which is loaded into the

device. By delaying the binding of design decisions

until execution time, the designers can incorporate

the latest developments - enabling them to improve

the performance of the systems. This reduces the

difference between the state of the art and the state

of practice for wireless communication systems.

Additionally, this software-oriented approach to

wireless communication devices allows both

flexibility and simpler maintenance, as most

upgrades can be done by loading new software,

rather than changing physical modules.

The Ettus Research Universal Software Radio

Peripheral (USRP) is an example of an SDR. It

provides an “RF front end for a computer running

the GNU Radio software, converting radio waves

picked up by an antenna into digital copies that the

computer software can handle or, conversely,

converting a wave synthesized by the computer into

a radio transmission” (1). In Figure 1, "IF" standards

for intermediate frequency, representing a version of

the signal at a lower frequency that the actual RF.

Figure 1: Introduction to USRP and GNU Radio.

2 SOFTWARE DEFINED RADIO

(SDR) HISTORY

A SDR is a radio in which software defines signals,

frequencies, modulation, and (optionally)

cryptography. SDR design began 1987, when the

United States Air Force’s Rome Laboratory (AFRL)

developed a programmable modem. The modem was

based on the Integrated Communications,

Navigation, and Identification Architecture (ICNIA)

(2). The term software defined radio was introduced

by Joseph Mitola III in 1991 "to signal the shift from

digital radio to multiband multimode software-

defined radios where "80%" of the functionality is

provided in software, versus the "80%" hardware of

the 1990's." (3).

131

Fuad Yousif Mohammed A. (2010).

STUDYING MEDIA ACCESS AND CONTROL PROTOCOLS.

In Proceedings of the International Conference on Data Communication Networking and Optical Communication Systems, pages 131-134

DOI: 10.5220/0002999301310134

 SciTePress

Table 1: Time line with some representation examples.

Prject Size Features

ICNIA, 1978 Fit in a small

room

A collection of several single-

purpose radios in one box

Speakeasy

Phase I, 1992

Six foot (182

cm) rack

Included a programmable

cryptography chip.

Speakeasy

Phase II, 1995

Stack of two

pizza boxes

The first SDR to include a voice

coder and digital signal

processing resources.

Digital

Modular

Radio, 2000

44.45 x 48.90

x 55.9 cm

Implemented four full duplex

channels and could be remotely

controlled using the Simple

Network Management Protocol

via an Ethernet interface.

USRP, 2004 Fit in 21 x 17

x 5.5 cm box

Allows creating a software radio

using any computer with a USB2

port.

3 THE UNIVERSAL SOFTWARE

RADIO PERIPHERAL (USRP)

The USRP has a Cypress FX2 USB 2.0 interface,

four high speeds digital to analog converters, four

high speed analog to digital converters, and a large

Altera Cyclone field programmable gate array

(FPGA) that interconnects all of the aforementioned

devices.

The USRP includes Analog Devices Mixed

Signal Processor (AD9862). Each AD9862 contains

four ADCs. Programmable gain amplifiers, placed

in-front of the ADCs provides input signal level

adjustment. Further details of the AD9862 can be

found at (6). Each of ADCs runs at 64 Million

samples per second (64 Msps) with 12 bits per

sample, the DAC accept as input 14 bits per sample

generating 128 Msps. As the maximum signaling

rate of a USB 2.0 link is 480 Mbps, this means that

we can not simply forward the entire received signal

to an attached processor - nor can we receive a

signal from an attached processor and output it

directly via the DAC. Reducing the sample rate in

the receive path and increasing the sample rate in the

transmit path must be accomplished by the FPGA.

3.1 USRP Daughter Boards

There are four expansion slots on the USRP mother

board. These enable a user to plug in up to two

transmitter daughter boards and two receiver

daughter boards. These daughters implement the

specific radio frequency front end for a given range

of frequencies. Thus the motherboard only performs

baseband (or intermediate frequency) processing of

the signals. On the USRP motherboard the

transmitter expansion slots are labelled TXA and

TXB, while the receiver expansion slots are labeled

RXA and RXB. Each transmitter expansion slot has

access to two high speed DACs; as the motherboard

has four DACs with two connected to TXA and two

to TXB. Each receiver expansion slot has access to

two high speed ADCs, as the motherboard has four

ADCs with two for RXA and two for RXB.

Figure 2: USRP Block Diagram.

4 GNU RADIO

GNU Radio is free Python-based software

architecture implemented to run on a Linux

platform. More specifically, GNU Radio provides a

collection of signal possessing blocks that support

the USRP.

The GNU Radio code is written in both C++ and

Python. The computationally intensive processing

blocks are implemented in C++, while Python is

used for developing applications that sit on top (and

control) these blocks. The GNU radio code assumes

that the FPGA has already been programmed with a

configuration suitable for use by the GNU radio

code.

4.1 Installing the GNU Radio

This section describes how to build GNU Radio

version 3.2.2 - released on July 15, 2009. In this

project we experienced problems installing GNU

Radio as described in this release’s build guide [9].

The basic concepts underlying the GNU Radio

are flow graphs and blocks (nodes of the graph). The

blocks carry out the actual signal processing. The

data passed between these blocks could be of any

kind.

4.2 GNU Radio Signal Processing

Blocks

The GNU Radio project provides many signal

processing blocks (implemented in C++) as a library

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and supports the ability to be establish connections

between these blocks. The programmer develops a

radio by building a flow graph in which the signal

processing blocks are represented as vertices and the

data flow between them is represented as edges.

Blocks’ attributes specify the number of input ports

and/or output ports and the data type (for example:

short, float, and complex) for this port.

5 LABORATORY EXPERIMENTS

This section describes some of the laboratory

exercises that have been designed during this

project.

5.1 Experiment 1: Simplex Data

Transmission

In this exercise we will learn how simplex data

communication can be implemented. In this case

there will not be any feedback from the receiver

packets arrival of the packets at the receiver. The

transmitter sends 5 packets, then waits one second

and sends the next 5 packets. The equipment

required for this exercise is a PC, together with one

USRP, Basic TX, Basic RX, and an RF cable. Based

on the exercise plan, the following objectives were

developed for the simplex data transmission.

Figure 3: Simplex data transmitter and receiver.

5.2 Experiment 2: Voice Transmission

This experiment is similar to experiment 1; but

instead of a file we are sending and receiving a voice

signal. The code uses GSM-FR encoder and decoder

to as a voice CODEC. The code used in this exercise

is part of the GNU Radio examples.

5.3 Experiment 3: Carrier Sense

Multiple Access Protocol

In this experiment we introduce Carrier Sense

Multiple Access (CSMA) (without collision

detection) as a link layer protocol. This experiment

illustrates a common media access and control

protocol (MAC). Its also provides a framework for

students to build their own MACs, by modifying the

code. In this experiment we use the “TUN/TAP”

Linux interface to intercept frames that are being

sent to (or received from) a virtual network

interface. This enables the student to run any

network protocol or higher level protocol of their

choice – while seeng the frames passed to their

MAC and physical layer.

Figure 4: TUN/TAP and GNU Radio.

5.4 Experiment 4: Bluetooth

(or IEEE 802.15.4) Sniffer

It is hard to sniff Bluetooth because of its wide

frequency band and fast random hopping (calculated

by the master device). We need eleven USRPs to

sniff the 83.5 MHz wide band (USRP can work with

8 MHz wide band centred in a frequency), or we can

use four USRP2.

In this experiment we use gr-bluetooth. This

code was developed by Dominic Spill and Michael

Ossmann (10) and they made the code freely

available (11). This experiment is used to listen to

packets exchanged between a cellular phone and a

Bluetooth headset.

5.5 Experiment 5: IEEE 802.11

Implementation

In this experiment we use the BBN 802.11

implementation by the Adaptive Dynamic Radio

Open-source Intelligent Team and funded by

DARPA’s ACERT program. This project used GNU

Radio and implemented an 802.11 receiver and

transmitter (12).

6 EVALUATION AND ANALYSIS

In this section we will evaluate each of the

laboratory experiment from a pedagogical point of

view.

STUDYING MEDIA ACCESS AND CONTROL PROTOCOLS

133

6.1 GNU Radio: Analysis

There is no enghough documentation of how GNU

Radio is implemented, during runig application we

found that there are some messages printed from

diffrernt classes and tracing and understanding these

message takes some time. The GNU Radio

developers did not found acceptable way to provide

unifed documentation for the system (15). The Gnu

Radio has many releases developed. In release

version 3.2.x the higher block of the system is

updated. This will affect applications developed

under old release from running in new releses.

6.2 USRP: Analysis

The USRP is a device we used in this project to

develop undergraduate’s experiment . This device

has various daughterboards which operate on

different radio frequency bands (from DC to 2.9

GHz); you have to plug-in a sutable daughterboard

for you application.

USRP2 was developed and goes to the market on

May 25, 2009. There are some benefits of using

USRP2 than USRP(5).

6.3 Laboratory Exercises: Analysis

The laboratory exercises were designed based upon

the idea of step-by-step learning. The undergraduate

student initialy follow the steps presented in each

experiment to solve a problem and understands

subject terms. These experiments start with simple

communication systems first, a little bit complex

systems, and finaly real world systems. In each

experiment, the student must solve specific problems

and submit a written report to the instructor. The

instructor can choose which experiment are sutable

for the students.

7 CONCLUSIONS

We developed laboratory experiment for

undergraduate students to help them understands

media and access control protocols protocol. The

experiments are designed in a way that easy to

understand experiments first, and the complicated

experiments. Instructors might use these

experiments and add more exercises to develop their

own lessons plan and course material. You can find

the full work and codes in (15).

In conclusion, we can say that it is not easy job

to implement applications using USRP and GNU

Radio because of the weak documentation of the

GNU Radio. And if we started this project again we

would develop a documentations tool for GNU

Radio to help developers to implement their own

applications.

REFERENCES

Susan Karlin,”Tools & Toys: Hardware for your Software

Radio”, IEEE Spectrum, 34(10) , Oct. 2006, pp51-54.

Bruce A. Fette, et al, ”Cognitive Radio Technology”

Newnes, 2006, 656 pages, ISBN-10: 0750679522,

ISBN-13: 978-0750679527.

Greg Colvin and Beman Dawes, Smart Pointers, Web

Page, March 11, 2009, http://

www.boost.org/libs/smart_ptr/smart_ptr.htm.

Walter Tuttlebee, et al, ”Software Defined Radio:

Enabling Technology”, USA, John Wiley & Sons Ltd,

2002.

Ettus Research LLC, web page ” www.ettus.com”, visited

2009-11-11.

Analog Design, AD9862 12-/14-Bit Mixed Signal Front-

End (MxFE®) Processor for Broadband

Communications, Data Sheet, Revision 0, Dec. 2002,

internet site "http://www.analog.com/static/imported-

files/data_sheets/AD9860_9862.pdf"

GNU Radio, web page” www.gnuradio.org “, visited

2009-02-10

Greg Colvin and Beman Dawes, Smart Pointers, Web

Page, March 11, 2009, http://

www.boost.org/libs/smart_ptr/smart_ptr.htm.

Build Guide- GNU Radio, web page,2009-11-05 ”

http://gnuradio.org/trac/wiki/BuildGuide”.

Michael Ossmann and Dominic Spill, "Building an All-

Challe Bluetooth Monitor", ShmooCon 2009, 6

February 2009.

Gr-Bluetooth, web page, Aug 18, 2009,”

http://sourceforge.net/projects/gr-bluetooth/”

Troxel Gregory D, Blossom Eric, et al “Adaptive Dynamic

Radio Open-source Intelligent Team (ADROIT):

Cognitively-controlled Collaboration among SDR

Nodes”, Networking Technologies for Software

Defined Radio Networks, 2006. SDR '06.1st IEEE

Workshop, Sep 2006, pp 8-17, ISBN: 1-4244-0733-8.

BBN80211 - The Comprehensive GNU Radio Archive

Network, web page, “https://128.2.212.19/

wiki/BBN80211”, visited Nov 7, 2009.

GNU Radio 3.2svn C++ API Documentation, web page,

“http://gnuradio.org/doc/doxygen/index.html”, May

22, 2009.

Alaelddin Mohammed, “Studying Media Access and

Control Protocols”, Master Thesis, 2010-02-19,

“http://web.it.kth.se/~maguire/DEGREE-PROJECT-

REPORTS/100119-Alalelddin_Mohammed-with-

cover.pdf”.

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