Wireless Communication Systems

Background of Wireless Communication

Wireless Communication Technology

Wireless Networking and Mobile IP

Wireless Local Area Networks

Wireless Communication Systems

Physical LayerPhysical Layer

Wireless Personal Area Networks

Wireless Metropolitan Area Networks

Wireless Wide Area Networks

Overview Random Contention Access Distributed Coordination Function

(DCF) 802.11 Contention Window 802.11 Adaptive Contention Window DCF Protocol Issues Virtual Carrier Sense Physical Carrier Sense Mechanisms Exposed Terminal Problem Double Exposed Terminal Problem Point Coordination Function (PCF)

Physical Properties of Wireless Frequency & Public Use Bands Free-space Path-loss Path-loss Exponents Multi-path Propagation Multi-Path Effect Digital Modulation Examples of Digital Modulation Multi-transmitter Interference Symbol Rate & Bandwidth Thermal Noise Multi-transmitter Interference

Problem Medium Access Control

CSMA, CSMA-CD, CSMA/CA

Physical Properties of Wireless

Makes wireless network different from wired networks

Should be taken into account by all layers

Wireless = Waves

Electromagnetic radiation Emitted by sinusoidal current running through a wire

(transmitting antenna) Creates propagating sinusoidal magnetic and electric fields

according to Maxwell’s equations:

Fields induce current in receiving antenna

Wave Propagation Example

electricfield

magneticfield

propagation direction

Frequency & Public Use Bands

Propagating sinusoidal wave with some frequency/wavelength

C (speed of light) = 3x108 m/s

Name 900 Mhz 2.4 Ghz 5 Ghz

Range 902 - 928 2.4 - 2.4835 5.15 - 5.35

Bandwidth 26 Mhz 83.5 Mhz 200 Mhz

Wavelength .33m / 13.1” .125m / 4.9” .06 m / 2.4”

cf

Free-space Path-loss

Power of wireless transmission reduces with square of distance (due to surface area increase of sphere)

Reduction also depends on wavelength Long wave length (low frequency) has less loss Short wave length (high frequency) has more loss

24

DPL

Other Path-loss Exponents

Path-Loss Exponent depends on environment:

Free space 2 Urban area cellular2.7 to 3.5 Shadowed urban cell 3 to 5 In building LOS 1.6 to 1.8 Obstructed in building 4 to 6 Obstructed in factories 2 to 3

Multi-path Propagation

Electromagnetic waves bounce off of conductive (metal) objects

Reflected waves received along with direct wave

Multi-Path Effect

Multi-path components are delayed depending on path length (delay spread)

Phase shift causes frequency dependent constructive / destructive interference

Am

plitu

de

Time

Am

plitu

de

Frequency

Digital Modulation

Modulation allows the wave to carry information by adjusting its properties in a time varying way Amplitude Frequency Phase

Digital modulation using discrete “steps” so that information can be recovered despite noise/interference

Examples of Digital Modulation

BPSK QPSK ODFM

Digital modulation Practical examples BFSK - Mote Sensor Networks QPSK - 2 Mbps 802.11 & CMDA(IS-95)

Multi-transmitter Interference

Similar to multi-path Two transmitting stations will

constructively/destructively interfere with each other at the receiver

Receiver will “hear” the sum of the two signals, which usually means garbage

Symbol Rate & Bandwidth

Modulation allows transmission of one of several possible symbols

Data stream is encoded by transmitting several symbols in succession

Symbol rate ≈ bandwidth Throughput (bits/sec) Spectrum usage (Hz)

Inter-symbol interference (ISI) occurs unless delay spread << symbol time

Thermal Noise

Ever-present thermal noise in wireless medium Sums with any wireless transmission Potentially causes errors in reception (digital) or

degradation of quality (analog) Effectively limits transmission range when

transmitting signal strength falls below noise floor

Noise Limits Transmitting Distance

+

+

=

=

Short range transmission (low path loss)

Long range transmission (high path loss)

Signal to Noise Ratio(SNR)

High

Low

Physical Channel Properties Review

Wireless signal strength Transmit power Loss over distance (falls off by d2) Shadowing (e.g. absorption by walls) Multi-path (e.g. bouncing off of metal objects)

Noise Thermal noise floor Environmental noise (e.g. microwave ovens)

Channel quality Related to signal to noise ratio

Multi-transmitter Interference Problem

Similar to multi-path or noise Two transmitting stations will

constructively/destructively interfere with each other at the receiver

Receiver will “hear” the sum of the two signals (which usually means garbage)

Medium Access Methods

The 802.11 standard ensures that all stations, both radio-based network interface cards (NICs) and access points, implement access methods for sharing the air medium.

When installing wireless LANs (WLAN), most people don't give much thought to these mechanisms.

A solid understanding of 802.11's medium access methods, will enable us to deal more effectively with issues such as radio frequency interference, denial of services attacks and throughput issues.

Medium Access Control (MAC)

Protocol required to coordinate access i.e. transmitters must take turns

Similar to talking in a crowded room

Also similar to hub based Ethernet

Carrier Sense Multiple Access (CSMA)

Procedure Listen to medium and wait until it is free (no one

else is talking) Wait a random back off time then start talking

Advantages Fairly simple to implement Functional scheme that works

Disadvantages Can not recover from a collision

(inefficient waste of medium time)

Carrier Sense Multiple Accesswith Collision Detection (CSMA-CD) Procedure

Listen to medium and wait until it is free Then start talking, but listen to see if someone else starts

talking too If a collision occurs, stop and then start talking after a random

back off time This scheme is used for hub based Ethernet Advantages

More efficient than basic CSMA Disadvantages

Requires ability to detect collisions

Collision Detection Problem

Transmit signal is MUCH stronger than received signal

Due to high path loss in the wireless environment

Impossible to “listen” while transmitting because you will drown out anything you hear

Also transmitter may not even have much of a signal to detect due to geometry

Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA)

Procedure Similar to CSMA but instead of sending packets

control frames are exchanged RTS = request to send CTS = clear to send DATA = actual packet ACK = acknowledgement

Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA)

Advantages Small control frames lessen the cost of collisions

(when data is large) RTS + CTS provide “virtual” carrier sense protects

against hidden terminal collisions (where A can’t hear B)

A B

Carrier Sense Multiple Accesswith Collision Avoidance (CSMA-CA)

Disadvantages Not as efficient as CSMA-CD Doesn’t solve all the problems of MAC in wireless

networks

Random Contention Access Slotted contention period

Used by all carrier sense variants Provides random access to the channel

Operation Each node selects a random back off number Waits that number of slots monitoring the channel If channel stays idle and reaches zero then transmit If channel becomes active wait until transmission

is over then start counting again

Distributed Coordination Function (DCF)

The 802.11 standard makes it mandatory that all stations implement the DCF, a form of carrier sense multiple access with collision avoidance (CSMA/CA).

CSMA is a contention-based protocol making certain that all stations first sense the medium before transmitting.

The main goal is to avoid having stations transmit at the same time, which results in collisions and corresponding retransmissions.

Distributed Coordination Function (DCF)

If a station wanting to send a frame senses energy above a specific threshold on the medium (which could mean the transmission of another station), the station wanting access will wait until the medium is idle before transmitting the frame.

The collision avoidance aspect of the protocol pertains to the use of acknowledgements that a receiving station send to the sending station to verify error-free reception.

Think of this process of accessing the medium as a meeting where everyone is polite and each person only speaks when no one else is talking.

In addition, everyone who understands what the person is saying nods their head in agreement.

Distributed Coordination Function (DCF)

The DCF protocol is somewhat more complex than this, though.

For example, an 802.11 station utilizes information it gains from other frames that stations are sending over the wireless network.

In the control field of each frame, there is a duration field that a sending station places a value in, to indicate how long the station will require the medium.

As part of making a decision on whether to transmit a frame, a station must see that the time associated with the duration value of the last frame sent has expired, as well as sense that no physical transmission is taking place.

The duration field enables stations to reserve the medium for subsequent frames of some specific 802.11-defined frame exchanges (e.g. RTS/CTS)

802.11 DCF Example with Backoff

data

waitB1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervalsat nodes 1 and 2cw = 31

B2 = 10

802.11 Contention Window Random number selected from [0,cw] Small value for cw

Less wasted idle slots time Large number of collisions with multiple senders (two or

more stations reach zero at once) Optimal cw for known number of contenders & know packet

size Tricky to implement because number of contenders is

difficult to estimate and can be VERY dynamic

802.11 Adaptive Contention Window

802.11 adaptively sets cw Starts with cw = 31 If no CTS or ACK then increase to 2*cw+1 (63, 127, 255) Reset to 31 on successful transmission

802.11 adaptive scheme is unfair Under contention, unlucky nodes will use larger cw than

lucky nodes (due to straight reset after a success) Lucky nodes may be able to transmit several packets while

unlucky nodes are counting down for access Fair schemes should use same cw for all contending nodes

(better for high congestion too)

802.11 DCF (CSMA-CA)

Full exchange with “virtual” carrier sense(called the Network Allocation Vector)

RTS

CTS

DATA

ACK

Sender

Receiver

Sender Receiver

A B

A

B

NAV (RTS)

NAV (CTS)

802.11 DCF (CSMA-CA)

Because of its nature, DCF supports the transmission of asynchronous signals.

A distinguishing factor of asynchronous signaling is that there are no timing requirements between data carrying frames.

For example, the DCF protocol doesn't make any attempt to deliver a series of data frames within any timeframe or at any instant in time.

As a result, there is a random amount of delay between each data frame transmission.

This form of synchronization is effective for network applications, such as e-mail, Web browsing and VPN access to corporate applications.

DCF Protocol Issues

The DCF protocol is the heart of many WLAN troubles. RF Interference is probably the biggest problem. If a source of RF interference (e.g., cordless phone or other

WLAN) is present, the DCF can block stations from transmitting for as long as the interfering signal is present.

The stations sense enough energy on the medium and wait patiently, in most cases for just a few seconds or minutes.

This causes the throughput of the network to drop significantly.

That's why you should perform an RF site survey in the facility before installing a WLAN.

DCF Protocol Issues

Similar to the impact of typical RF interference, someone could implement a denial of service attack, which is a deliberate action to instill RF interference at a level high enough to block a majority of the stations from transmitting.

Again, all of the stations will not transmit because they respectfully follow the DCF protocol.

DCF Protocol Issues

Similar to the impact of typical RF interference, someone could implement a denial of service attack, which is a deliberate action to instill RF interference at a level high enough to block a majority of the stations from transmitting.

Again, all of the stations will not transmit because they respectfully follow the DCF protocol.

Instead of lasting for only a few seconds, a denial of service attack could be planned in a way to corrupt the network for hours or days until the jamming source is found.

This type of attack will generally cause the network to be useless (i.e., throughput equal to zero).

In order to reduce this impact, maximize the use of directional antennas to minimize the reception of RF signals from outside the facility where someone could attack them with a high-powered jamming device

Virtual Carrier Sense

Provided by RTS & CTS Designed to protect against hidden terminal collisions

(when C can’t receive from A and might start transmitting)

However this is unnecessary most of the time due to physical carrier sense

A B C

RTS CTS

Physical Carrier Sense Mechanisms

Energy detection threshold Monitors channel during “idle” times between packets to

measure the noise floor Energy levels above the noise floor by a threshold trigger carrier

sense DSSS correlation threshold

Monitors the channel for Direct Sequence Spread Spectrum (DSSS) coded signal

Triggers carrier sense if the correlation peak is above a threshold More sensitive than energy detection (but only works for 802.11

transmissions) High BER disrupts transmission but not detection

Physical Carrier Sense Range

Carrier can be sensed at lower levels than packets can be received Results in larger carrier sense

range than transmission range

Long carrier sense range helps protect from interferenceReceive Range

Carrier Sense Range

Hidden Terminal Revisited

Virtual carrier sense no longer needed in this situation

A B C

RTS CTS

Physical Carrier Sense

RTS CTS Still Useful Sometimes

Obstructed hidden terminal situation

Fast collision resolution for long data packets

A B

Exposed Terminal Problem

Hidden terminal is not the only challenge for a distributed wireless MAC protocol

A blocks B, and C doesn’t know what is happening (B is exposed)

A B CD

Double Exposure Problem

If A and C are out of phase, there is NO time D can transmit without causing a collision

A B

D

C

Point Coordination Function (PCF)

As an optional access method, the 802.11 standard defines the PCF, which enables the transmission of time-sensitive information.

With PCF, a point coordinator within the access point controls which stations can transmit during any give period of time.

Within a time period called the contention free period, the point coordinator will step through all stations operating in PCF mode and poll them one at a time.

For example, the point coordinator may first poll station A, and during a specific period of time station A can transmit data frames (and no other station can send anything).

The point coordinator will then poll the next station and continue down the polling list, while letting each station to have a chance to send data.

Point Coordination Function (PCF)

Thus, PCF is a contention-free protocol and enables stations to transmit data frames synchronously, with regular time delays between data frame transmissions.

This makes it possible to more effectively support information flows, such as video and control mechanisms, having stiffer synchronization requirements.

Timing mechanisms within the 802.11 protocol ensure that stations on the WLAN alternate between the use of DCF and PCF.

As a result, the WLAN can support both asynchronous and synchronous information flows.

For a period of time, stations will access channel by using CSMA. For the following time period, the stations will wait for a poll from the point coordinator before sending data frames.

Point Coordination Function (PCF)

The big name vendors, such as Cisco, Proxim, and Symbol, don't include PCF mode in their devices as a standard.

Some chipsets have PCF functionality embedded somewhere in the firmware, but access point vendors seem to be reluctant to activate it, even though PCF has been part of the 802.11 standard since its inception in 1997.

The problem is that the 802.11 standard is fairly vague in defining portions of the PCF protocol.

As a result, you need to use the same vendor for the access points and radio cards to make it work properly.

The Wi-Fi Alliance does not include PCF functionality in their interoperability standard.

Q&A

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Assignment #1

Explain the terms highlighted in GREEN color throughout this lecture slide

Send your assignments in Word Document Format to [email protected] or [email protected]

Last date of submission of assignment is 24th March 2009.