Problem 1 : General report about WLAN Standard
1. Describe the history of the WLAN and how this standard started.
It was developed in 1971 by Norman Abramson, a professor in the University of Hawaii.
It included 7 computers deployed over 4 islands, communicating with a central
computer.
WLAN initially was expensive so it was only used as an alternative to cabled LAN in
places where cabling was difficult. Early development included industry-specific
solutions and proprietary protocols.
It first appeared in the market in the 1990’s and it became a standard technology for
connecting various devices to the internet, primarily the various versions of IEEE 802.11.
2. Mention some of WLAN applications.
The main application is the transport of IP packets over layer 3 of ISO protocol.
Applications include:
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It can be used for LAN extension to save on installation of LAN cables.
Users can access the internet from WLAN hotspots in restaurants, hotels, and
portable devices.
WLAN infrastructure can be used to work as indoor positioning systems.
3. Describe the protocol stack of WLAN
The WLAN protocol stack consists of:
a) MAC layer:
It is responsible for sending and receiving the data packets. Each packet has a MAC layer
data header containing addressing information, and packet tracking information.
b) Physical layer:
It defines the physical and electrical characteristics of the network, and it is responsible
for the modulation and demodulation of the data.
4. In details describe the WLAN system architecture (configuration), with
the difference between the two mod
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Ad-Hoc mode:
In this mode two or more devices communicate directly with each other, the
packets are sent directly between each two devices.
All the devices share the same medium and the packets are received by all the
stations, but all stations except the intended recipient discard the packet.
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Infrastructure mode:
Access Points (AP) are used as a gateway between all devices of the network.
In a basic service set (BSS) a device would send the packet to the AP, and the AP
sends the packet to the transmission address.
In an ESS (extended service set) several APs communicate with each other to
extend the range.
5. What are the advantages and disadvantages of Infrastructure mode?
Advantages:
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Two devices can communicate with each other over larger distances, with an AP
in the middle.
It offers scalability, additional APs can be added to increase the range.
Disadvantages:
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A packet between 2 devices has to be transmitted at least two times over air.
The additional cost of adding AP hardware.
6. How to configure Ad-Hoc network?
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The network must have an SSID (service set identity)
All users select the same frequency channel number.
All users use the same ciphering key.
An individual IP address has to be configured in every device.
7. Show in detail how the WLAN standard changed through the versions
802.11b/g/a/n/ac/ad.
802.11b: Uses the 2.4 GHz band, with channel bandwidth 22 MHz (20MHz for
information and 2MHz guard band). Up to 3 access points can be used in close range.
Uses DSSS (direct sequence spread spectrum) to reduce interference.
802.11g: Has the same bandwidth and frequency band as 802.11b. Maximum data rate
is 54Mbps. It is backward compatible with 802.11b. It uses OFDM (orthogonal
frequency division multiplexing) to solve multipath fading.
802.11a: The main difference is the usage of the 5GHz band. It was not compatible with
802.11b and 802.11g, so it was not very popular. The maximum data rate is 54Mbps.
802.11n: Fixed the issue of 802.11a by using both 2.4GHz and 5GHz bands. Uses MIMO
(multiple input multiple output).
802.11ac: It uses the 5GHz band, which makes it backwards compatible with 802.11a
and 802.11n. It also uses MIMO for better communication performance.
802.11ad: Uses tri-band, 2.4GHz, 5GHz and 60GHz. The 7 Gbps can be achieved on the
60GHz band, and it requires line-of-sight, so it cannot penetrate through walls.
8. Discuss how the WLAN system improved through the versions 802.11e/f/h/i/w
802.11e: it defines a set of quality of service (QoS) enhancements for wireless LAN
applications through modifications to the media access control (MAC) layer. The standard is
considered of critical importance for delay-sensitive applications, such as Voice over
Wireless LAN and streaming multimedia.
802.11f: it describes an optional extension to IEEE 802.11 that provides wireless access
point communications among multi vendor systems. It was a Trial Use Recommended
Practice. The IEEE 802 Executive Committee approved its withdrawal later on.
802.11h: refers to the amendment added to the IEEE 802.11 standard for Spectrum and
Transmit Power Management Extensions. It solves problems like interference with satellites
and radar using the same 5 GHz frequency band.
802.11i: implemented as Wi-Fi Protected Access II (WPA2). This standard specifies
security mechanisms for wireless networks, replacing the short Authentication and privacy
clause of the original standard with a detailed Security clause.
802.11w: is an approved amendment to the IEEE 802.11 standard to increase the security
of its management frames. Its objective was to increase security by providing data
confidentiality of management frames, mechanisms that enable data integrity, data origin
authenticity, and replay protection.
9. How to limit/decrease the interference between Access Points(AP) within the
Extended service set (ESS)
To avoid signal interference between APs, each of them must operate on a different frequency
channel that does not overlap with the other.
10. Describe the CSMA/CA Protocol and
RTS/CTS messages.
CSMA/CA : Carrier Sense Multiple Access
with Collision Avoidance
Before node transmits data it checks for the
medium:
If free: node sends its signal
If not: wait for random time then try again
If the terminal senses an idle medium it sends
an RTS (Request To Send) signal.
If AP sees that no other terminal is
transmitting, it sends a CTS (Clear To Send)
signal.
If AP is busy then the terminal waits for
random time then sends another RTS signal.
11. what are beacon frames, how they are constructed and what are their
functionality.
Beacon frame is one of the management frames in IEEE 802.11 based WLANs.
Each beacon frame (or probe response) contains a lot of information about the
specific SSID being offered.
Beacon frames are transmitted by the access point (AP) in an infrastructure basic
service set (BSS).
12. Discuss Direct sequence spread spectrum(DSSS) and how it is useful in WLAN.
The DSSS states that: instead of sending a zero in the data we send a sequence of bits that the
receiver translates it to a zero and instead of sending a one in the data we send the
complementary sequence of the same bits that the receiver translates to a one.
For example:
- 0 in data is exchanged with 01001000111.
- 1 in data is exchanged with 10110111000.
This will cause the signal spectrum to spread and lowers so when any interference occurs from
other signals, it causes minimal effect on the spread spectrum then after the receiver receives
the message he reverses the DSSS operation on the received signal which will cause the
interference/nose to spread and the previously spreaded signal to get back to normal and get
extracted with minimal error.
13. Discuss orthogonal frequency division multiplexing (OFDM) and how it is
useful in WLAN
It is used to solve the problem of multipath fading by :
1-Break up signals into multilple low rate streams.
2-Each stream suffer from almost flat narrow bandwidth channel.
3-Each is modulated over orthogonal frequency,then simple equalization.
14. what is (MIMO and how can we make use of it .
MIMO: Multiple Input Multiple Output.
It is a technique to use more than one antenna for transmitting and receiving data and could be
implemented as:
Spatial Multiplexing: split data across antennas. Each antenna sends different parts of the
data.
Diversity: Send multiple copies of the data. Each antenna sends the same data to insure
correct reception and lower the BER (Bit Error Rate)
Problem 2 : Questions related to the experiment :
1. For WLAN module in our experiment what is the type of antenna in it ?
The MRF24WB0MA RF transceiver module contains integrated PCB—printed circuit
board-- antenna it is also called microchip antenna with range up to 400m.
2. How can we set the network name?
char strSSID[13] = "Ahmed50";
We can set the network name by this line
3. How can we change the channel number we use to transmit?
char channels[11] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11};
We write the number of channel we want in the above line
For example if we want the channel to be equal to 5 then ..
char channels[11] = {5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5};
4. How can we change the mode of WLAN in our Experiment from
Infrastructure to Ad-hoc?
By opening the command window we type in
netsh wlan set profileparameter NAMEOFNETWORK connectiontype=ibss
This changes the mode from bss(which is infrastructure) to ibss( which is Ad-hoc)
Problem 3: Mini-Simulations
clc; clear all; close all;
%% Initialization
Frames = 1000; %Number of Frames
fft_size = 128; %FFT Size (Number of subcarriers)
M = 16; %16-QAM Modulation
delta = 312.5*10^(3); %Carrier Separation
delay_spread = 0.2*10^(-6); %Delay Spread
SNRdb = 0:3:30; %SNR Range in dB
delay_spread_max = delay_spread*fft_size*delta; %Max delay spread
msg_size_bits = log2(M)*fft_size; %number of bits
BER = zeros(length(SNRdb),Frames); %initialization for BER
BER_avg = zeros(length(SNRdb),1);
%initialization for BER average values
%%
for i = 1:length(SNRdb)
for k = 1:Frames
%% Message Generation
msg_bits=randi([0,1],msg_size_bits/4,4);
msg = bi2de(msg_bits,'left-msb')';
%% Modulation and Normalization
modulated_signal = qammod(msg, 16);
%% OFDM Modulation
x = 1/sqrt(128)*ifft(modulated_signal);
%% Cyclic Prefix
CP = x(1:32);
msg_CP = [x CP];
%% Channel (fading + noise)
[fadedSamples, gain] = ApplyFading(msg_CP,1,delay_spread_max);
y = awgn(fadedSamples,SNRdb(i),'measured');
%% Cyclix prefix exclusion
received_signal = y(1:128);
%% OFDM Demodulation
x_demod = sqrt(128)*fft(received_signal);
%% Equalization
Z = x_demod./fft(gain, 128);
%% Demodulation and Denormalization
msg_demodulated_symbols = qamdemod(Z,16);
msg_demodullated_bits = de2bi(msg_demodulated_symbols,'left-msb');
%% BER calculation
[number(i,k),BER(i,k)] = biterr(msg_demodulated_symbols,msg);
BER_avg(i) = sum(BER(i,:))./Frames;
end
end
%% Plotting BER vs. SNR
semilogy(SNRdb',BER_avg)
xlabel('SNR(dB)')
ylabel('BER')
title('OFDM BER vs. SNR for 16-QAM')