Introduction

It all started like this First telephone (photophone) – Alexander

Bell, 1880 The first car mounted radio

telephone – 1921

Going further 1946 – First commercial mobile radio-telephone

service by Bell and AT&T in Saint Louis, USA. Half duplex(PTT)

1973 – First handheld cellular phone – Motorola.

First cellular net Bahrein 1978

Wireless Comes of Age Galileo Marconi invented the wireless telegraph in

1896 Communication by encoding alphanumeric characters in

analog signal Sent telegraphic signals across the Atlantic Ocean

Communications satellites launched in 1960s Advances in wireless technology

Radio, television, mobile telephone, communication satellites More recently

Satellite communications, wireless networking, cellular technology

Broadband Wireless Technology Higher data rates obtainable with broadband

wireless technology Graphics, video, audio

Shares same advantages of all wireless services: convenience and reduced cost Service can be deployed faster than fixed service No cost of cable plant Service is mobile, deployed almost anywhere

Today’s Wireless Networks

Radio Controller

Gateway

TelephoneNetwork

Internet

RadioEdge

RouterInternet

Telephone network

3G: ITU-Developed IMT-2000

Satellite

MacrocellMicrocell

UrbanIn-Building

Picocell

Global

Suburban

Basic TerminalPDA Terminal

Audio/Visual Terminal

Future of Networks

Limitations and Difficulties of Wireless Technologies Wireless is convenient and less expensive Limitations and political and technical difficulties

inhibit wireless technologies Lack of an industry-wide standard Device limitations

E.g., small LCD on a mobile telephone can only displaying a few lines of text

E.g., browsers of most mobile wireless devices use wireless markup language (WML) instead of HTML

Part One: Background Provides preview and context for rest of

book Covers basic topics

Data Communications TCP/IP

Band name Abbr ITU band

FrequencyWavelengt

h

Example uses

< 3 Hz> 100,000 km

Extremely low frequency

ELF 1 3–30 Hz100,000 km – 10,000 km

Communication with submarines

Super low frequency SLF 2 30–300 Hz10,000 km –

1000 km

Communication with submarines

Ultra low frequency ULF 3 300–3000 Hz1000 km – 100 km

Communication within mines

Very low frequency VLF 4 3–30 kHz100 km – 10

km

Submarine communication, avalanche beacons, wireless heart rate monitors

Low frequency LF 5 30–300 kHz10 km – 1 km

Navigation, time signals, AM longwave broadcasting

Medium frequency MF 6 300–3000 kHz

1 km – 100 m

AM (Medium-wave) broadcasts

High frequency HF 7 3–30 MHz100 m – 10 m

Shortwave broadcasts and amateur radio

Very high frequency VHF 8 30–300 MHz10 m – 1 m

FM and television broadcasts

Ultra high frequency UHF 9 300–3000 MHz

1 m – 100 mm

television broadcasts, mobile phones, wireless LAN, ground-to-air and air-to-air communications

Super high frequency

SHF 10 3–30 GHz100

mm – 10 mm

microwave devices, mobile phones (W-CDMA), WLAN, most modern

Radars

Extremely high frequency

EHF 11 30–300 GHz

10 mm – 1 mm

Radio astronomy, high-speed microwave radio relay

Above 300 GHz< 1 mm

Night vision

Advantages

It would save time and money due to the fact that you would spare the expense of installing a lot of cables

A wireless networking system would rid of the downtime you would normally have in a wired network due to cable problems

client computer needs to relocate to another part of the office then all you need to do is move the machine with the wireless network card.

Characteristics of Wireless Networks

High bit error rate Low bandwidth (Data Rate) Variable delay Inconsistent performance Mobility

Switched Network

Switching Terms Switching Nodes:

Intermediate switching device that moves data Not concerned with content of data

Stations: End devices that wish to communicate Each station is connected to a switching node

Communications Network: A collection of switching nodes

Observations of Figure 3.3 Some nodes connect only to other nodes (e.g., 5

and 7) Some nodes connect to one or more stations Node-station links usually dedicated point-to-

point links Node-node links usually multiplexed links

Frequency-division multiplexing (FDM) Time-division multiplexing (TDM)

Not a direct link between every node pair

Techniques Used in Switched Networks

Circuit switching Dedicated communications path between two

stations E.g., public telephone network

Packet switching Message is broken into a series of packets Each node determines next leg of transmission

for each packet

Phases of Circuit Switching Circuit establishment

An end to end circuit is established through switching nodes

Information Transfer Information transmitted through the network Data may be analog voice, digitized voice, or binary

data Circuit disconnect

Circuit is terminated Each node de-allocates dedicated resources

Characteristics of Circuit Switching

Can be inefficient Channel capacity dedicated for duration of connection Utilization not 100% Delay prior to signal transfer for establishment

Once established, network is transparent to users

Information transmitted at fixed data rate with only propagation delay

How Packet Switching Works Data is transmitted in blocks, called packets

Before sending, the message is broken into a series of packets Typical packet length is 1000 octets (bytes) Packets consists of a portion of data plus a packet

header that includes control information

At each node en route, packet is received, stored briefly and passed to the next node

Packet Switching

Packet Switching

Packet Switching Advantages

Line efficiency is greater Many packets over time can dynamically share the same

node to node link Packet-switching networks can carry out data-rate

conversion Two stations with different data rates can exchange

information Unlike circuit-switching networks that block calls

when traffic is heavy, packet-switching still accepts packets, but with increased delivery delay

Priorities can be used

Disadvantages of Packet Switching Each packet switching node introduces a delay Overall packet delay can vary substantially

This is referred to as jitter Caused by differing packet sizes, routes taken and

varying delay in the switches Each packet requires overhead information

Includes destination and sequencing information Reduces communication capacity

More processing required at each node

Packet Switching Networks - Datagram

Each packet treated independently, without reference to previous packets

Each node chooses next node on packet’s path Packets don’t necessarily follow same route and

may arrive out of sequence Exit node restores packets to original order Responsibility of exit node or destination to detect

loss of packet and how to recover

Packet Switching Networks – Datagram

Advantages: Call setup phase is avoided Because it’s more primitive, it’s more flexible Datagram delivery is more reliable

Packet Switching Networks – Virtual Circuit

Preplanned route established before packets sent

All packets between source and destination follow this route

Routing decision not required by nodes for each packet

Emulates a circuit in a circuit switching network but is not a dedicated path

Packets still buffered at each node and queued for output over a line

Packet Switching Networks – Virtual Circuit

Advantages: Packets arrive in original order Packets arrive correctly Packets transmitted more rapidly without

routing decisions made at each node

Wireless Enhancements

Application Layer

Middleware and OS

Transport Layer

Network Layer

Link Layer & Below (Scheduling, MAC)

(Mobility, routing, QoS)

(Wireless TCP)

(Content adaptation,Consistency, File system)

Wireless Web, Location Services, etc

Physical Layer Communications infrastructure is the air/

frequency band

limited bandwidth shared, public media often regulated re-use of resources is key to providing sufficient

capacity to users

Multiple Access Techniques

Multiple access schemes are used to allow many mobile users to simultaneously share a finite amount of radio spectrum

Sharing should be done in such a way that over all performance of system should not be effected

Duplexing

In conventional telephone systems it is possible to talk and listen simultaneously

Frequency division duplexing(FDD) use two frequency bands for forward and reverse traffic.

Time division duplexing(TDD) uses time instead of frequency for forward and reverse traffic

Multiple Access Techniques

Three major access techniques

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)

Frequency Division Multiple Access

FDMA assigns individual channels to individual users.

Each user is allocated a unique frequency channel which can not be shared by other users

For duplexing pair of channels is provided to user

Frequency Division Multiple Access

30 KHz

30 KHz

30 KHz

30 KHz

30 KHz

30 KHz

30 KHz

30 KHz

Fre

qu

ency

FDMA — Frequency Division Multiple Access

Features of FDMA

The FDMA channel caries only one phone circuit at a time

If FDMA channel is not in use it sits idle and thus a wasted resource

FDMA is usually implemented for narrow band systems like (AMPS)

Due to large symbol time in Narrowband systems no equalization is required

Since FDMA is a continuous transmission scheme fewer over head bits are used as header and framing bits

As FDMA systems use duplexers thus expensive in implementation

Tight RF Filtering to minimize adjacent channel interference

Time Division Multiple Access

Time division multiple access system divide radio spectrum into time slots and allocate these slots to individual user.

Slots are repeated in cyclic manner and N time slots make a frame

Each frame is made up of preamble (address and synchronization info) and tail bit

TDD use half of time slots for forward and half for reverse traffic

Time Division Multiple Access

Fre

qu

ency

Time

200 KHz

200 KHz

200 KHz

200 KHz

One timeslot = 0.577 ms One TDMA frame = 8 timeslots

Features of TDMA TDMA shares a single carrier frequency with many users so maximum sharing

No of slots vary in frame depending upon available bandwidth, modulation techniques and other factors

Low battery consumption as transmitter can be shutdown between slot intervals

TDMA uses different time slots for TX and RX so no duplexer needed thus lower module cost

Wastage due to guard and other control bits

High synchronization over head as compared to FDMA

Different number of slots can be allocated to a user depending upon data requirements

Code Division Multiple Access

All users in CDMA use same carrier frequency and transmit simultaneously

Each user has its own codeword which is orthogonal to all other users codeword

The receiver uses specific user codeword to decode that user signal other code words appear as noise to receiver

Near far problem occurs and power control is used to minimize it in CDMA

Feature of CDMA Many users of CDMA system use same frequency

No limitation for number of users but as number of users increases performance degraded due to increase in noise floor

Multipath fading may be substantially reduced due to large spectrum

Self jamming problem occurs if two users code are not orthogonal

Near far problem occurs if no power control method is used

Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access) unique “code” assigned to each user; ie, code set partitioning

used mostly in wireless broadcast channels (cellular, satellite,etc)

all users share same frequency, but each user has own “chipping” sequence (ie, code) to encode data

encoded signal = (original data) X (chipping sequence)

decoding: inner-product of encoded signal and chipping sequence

allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

CDMA Encode/Decode

CDMA: two-sender interference

Packet Radio In packet radio access technique many subscribers

attempt to access a single channel in an uncoordinated manner.

Transmission is done by using burst of data

Collision due to transmission from multiple stations is detected at base station

ACK and NACK used to provide feed back about received data burst

Frequency Hoping

Frequency Hoping Signal is broadcast over seemingly random series of

radio frequencies A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth of input

signal

Signal hops from frequency to frequency at fixed intervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval, a new carrier frequency is

selected

Frequency Hoping Spread Spectrum

Channel sequence dictated by spreading code

Receiver, hopping between frequencies in synchronization with transmitter, picks up message

Advantages Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed

only at knocking out a few bits

Migration to 3G

CDMA

GSM

TDMA

PHS (IP-Based)

64 Kbps

GPRS

115 Kbps

CDMA 1xRTT

144 Kbps

EDGE

384 Kbps

cdma20001X-EV-DV

Over 2.4 Mbps

W-CDMA (UMTS)

Up to 2 Mbps

2G2.5G

2.75G 3G

1992 - 2000+2001+

2003+

1G

1984 - 1996+

2003 - 2004+

TACS

NMT

AMPS

GSM/GPRS

(Overlay) 115 Kbps

9.6 Kbps

9.6 Kbps

14.4 Kbps/ 64 Kbps

9.6 Kbps

PDC

Analog Voice

Digital Voice

Packet Data

IntermediateMultimedia

Multimedia

PHS

TD-SCDMA

2 Mbps?

9.6 Kbps

iDEN

(Overlay)

iDEN

Source: U.S. Bancorp Piper Jaffray

Wireless Transmission Impairments Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath

Attenuation Strength of signal falls off with distance over

transmission medium

Attenuation factors for unguided media: Received signal must have sufficient strength so that

circuitry in the receiver can interpret the signal Signal must maintain a level sufficiently higher than

noise to be received without error Attenuation is greater at higher frequencies, causing

distortion

Free Space Loss Free space loss, ideal isotropic antenna

Pt = signal power at transmitting antenna

Pr = signal power at receiving antenna = carrier wavelength d = propagation distance between antennas c = speed of light (» 3 ´ 10 8 m/s)

where d and are in the same units (e.g., meters)

2

2

2

2 44

c

fdd

P

P

r

t

Free Space Loss Free space loss equation can be recast:

d

P

PL

r

tdB

4log20log10

dB 98.21log20log20 d

dB 56.147log20log204

log20

df

c

fd

Free Space Loss Free space loss accounting for gain of other

antennas

Gt = gain of transmitting antenna

Gr = gain of receiving antenna

At = effective area of transmitting antenna

Ar = effective area of receiving antenna

trtrtrr

t

AAf

cd

AA

d

GG

d

P

P2

22

2

224

Free Space Loss Free space loss accounting for gain of other

antennas can be recast as

rtdB AAdL log10log20log20

dB54.169log10log20log20 rt AAdf

Physical Impairments: Noise

Unwanted signals added to the message signal

May be due to signals generated by natural phenomena such as lightning or man-made sources, including transmitting and receiving equipment as well as spark plugs in passing cars, wiring in thermostats, etc.

Sometimes modeled in the aggregate as a random signal in which power is distributed uniformly across all frequencies (white noise)

Signal-to-noise ratio (SNR) often used as a metric in the assessment of channel quality

Categories of Noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise

Thermal Noise Thermal noise due to agitation of electrons Present in all electronic devices and

transmission media Cannot be eliminated Function of temperature Particularly significant for satellite

communication

Thermal Noise Amount of thermal noise to be found in a

bandwidth of 1Hz in any device or conductor is:

N0 = noise power density in watts per 1 Hz of bandwidth

k = Boltzmann's constant = 1.3803 ´ 10-23 J/K

T = temperature, in kelvins (absolute temperature)

W/Hz k0 TN

Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz

(in watts):

or, in decibel-watts

TBN k

BTN log10 log 10k log10 BT log10 log 10dBW 6.228

Noise Terminology Intermodulation noise – occurs if signals with different

frequencies share the same medium Interference caused by a signal produced at a frequency that

is the sum or difference of original frequencies

Crosstalk – unwanted coupling between signal paths

Impulse noise – irregular pulses or noise spikes

Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults

and flaws in the communications system

Other Impairments Atmospheric absorption – water vapor and oxygen

contribute to attenuation

Multipath – obstacles reflect signals so that multiple copies with varying delays are received

Refraction – bending of radio waves as they propagate through the atmosphere

Signal degradation

• Multi-path:

• Signal can get severely distorted due to •reflection (objects larger than wavelength), •Scattering (objects smaller than wavelength)•diffraction (shadow fading)

Multipath Propagation Reflection - occurs when signal encounters a surface

that is large relative to the wavelength of the signal

Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave

Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

The Effects of Multipath Propagation

Multiple copies of a signal may arrive at different phases If phases add destructively, the signal level

relative to noise declines, making detection more difficult

Intersymbol interference (ISI) One or more delayed copies of a pulse may

arrive at the same time as the primary pulse for a subsequent bit

Improving Wireless System Performance

Wireless Communication systems require signal processing techniques that improve the link performance in hostile mobile radio environment.

Mobile radio channel impairments cause the signal at receiver to distort or fade significantly.

Techniques used to Improve system performance

Equalization: Equalization is term used in signal processing operation that minimizes ISI.

Equalizer works by tracking and training.

Equalizer

First a fixed length training sequence is sent by the transmitter so that a receiver equalizer properly analyze the Bit Error Rate, and use recursive algorithm evaluate the channel and estimate filter coefficients to compensate for distortion due to multi path in channel.

Immediately after training sequence data is send

When equalizer has been properly trained it said to have converged

Channel Coding Channel coding is used to improve link

performance by adding redundant bits in the transmitted signal.

At the receiver end these redundant bits are used to detect and/or correct errors introduced during transmission.

Turbo codes ,Convolution codes, block codes etc

Security

Safeguards for physical security must be even greater in wireless communications

Encryption: intercepted communications must not be easily interpreted

Authentication: is the node who it claims to be?

Sine Wave Parameters General sine wave

s(t ) = A sin(2ft + ) The effect of varying each of the three parameters

(a) A = 1, f = 1 Hz, = 0; thus T = 1s (b) Reduced peak amplitude; A=0.5 (c) Increased frequency; f = 2, thus T = ½ (d) Phase shift; = /4 radians (45 degrees)

note: 2 radians = 360° = 1 period

Sine Wave Parameters

Basic Encoding Techniques

Amplitude-Shift Keying One binary digit represented by presence of carrier, at

constant amplitude

Other binary digit represented by absence of carrier

where the carrier signal is Acos(2πfct)

ts tfA c2cos0

1binary 0binary

Binary Frequency-Shift Keying (BFSK) Two binary digits represented by two different

frequencies near the carrier frequency

where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts

ts tfA 12cos tfA 22cos

1binary 0binary

Phase-Shift Keying (PSK) Two-level PSK (BPSK)

Uses two phases to represent binary digits

ts tfA c2cos tfA c2cos

1binary 0binary

tfA c2cos

tfA c2cos1binary 0binary

Phase-Shift Keying (PSK) Four-level PSK (QPSK)

Each element represents more than one bit

ts

42cos

tfA c 11

4

32cos

tfA c

4

32cos

tfA c

42cos

tfA c

01

00

10

Effect of Packet Size on Transmission

Effect of Packet Size on Transmission

Breaking up packets decreases transmission time because transmission is allowed to overlap

Figure 3.9a Entire message (40 octets) + header information (3

octets) sent at once Transmission time: 129 octet-times

Figure 3.9b Message broken into 2 packets (20 octets) + header

(3 octets) Transmission time: 92 octet-times

Effect of Packet Size on Transmission

Figure 3.9c Message broken into 5 packets (8 octets) + header (3 octets) Transmission time: 77 octet-times.

Figure 3.9d Making the packets too small, transmission time starts

increases Each packet requires a fixed header; the more packets, the

more headers

Asynchronous Transfer Mode (ATM) Also known as cell relay Operates at high data rates Resembles packet switching

Involves transfer of data in discrete chunks, like packet switching

Allows multiple logical connections to be multiplexed over a single physical interface

Minimal error and flow control capabilities reduces overhead processing and size

Fixed-size cells simplify processing at ATM nodes

ATM Terminology Virtual channel connection (VCC)

Logical connection in ATM Basic unit of switching in ATM network Analogous to a virtual circuit in packet switching

networks Exchanges variable-rate, full-duplex flow of fixed-size

cells

Virtual path connection (VPC) Bundle of VCCs that have the same end points

Advantages of Virtual Paths Simplified network architecture

Increased network performance and reliability

Reduced processing and short connection setup time

Enhanced network services

Call Establishment

Virtual Channel Connection Uses Between end users

Can carry end-to-end user data or control signaling between two users

Between an end user and a network entity Used for user-to-network control signaling

Between two network entities Used for network traffic management and routing

functions

Virtual Path/Virtual Channel Characteristics

Quality of service Specified by parameters such as cell loss ratio and cell delay

variation

Switched and semi permanent virtual channel connections

Cell sequence integrity

Traffic parameter negotiation and usage monitoring

Virtual channel identifier restriction within a VPC

ATM Cell Header Format

Generic flow control (GFC) – 4 bits, used only in user-network interface

Used to alleviate short-term overload conditions in network

Virtual path identifier (VPI) – 8 bits at the user-network interface, 12 bits at network-network interface

Routing field

Virtual channel identifier (VCI) – 8 bits Used for routing to and from end user

ATM Cell Header Format

Payload type (PT) – 3 bits Indicates type of information in information field

Cell loss priority (CLP) – 1 bit Provides guidance to network in the event of

congestion

Header error control (HEC) – 8 bit Error code

ATM Service Categories

Real-time service Constant bit rate (CBR) Real-time variable bit rate (rt-VBR)

Non-real-time service Non-real-time variable bit rate (nrt-VBR) Available bit rate (ABR) Unspecified bit rate (UBR)

Examples of CBR Applications

Videoconferencing

Interactive audio (e.g., telephony)

Audio/video distribution (e.g., television, distance learning, pay-per-view)

Audio/video retrieval (e.g., video-on-demand, audio library)

Examples of UBR applications

Text/data/image transfer, messaging, distribution, retrieval

Remote terminal (e.g., telecommuting)