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Chapter 2

The Physical Layer

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The Physical Layer

It defines the mechanical, electrical, and timing interfaces to the network.

Purpose: To transport a raw bit stream Each one has its own function in terms of

bandwidth, delay, cost, ease of installation and maintenance

Max Data Rate of a Channel Nyquist Theorem – noiseless channel Max Data Rate = 2Hlog2V bits/sec

H : Bandwidth V : V discrete levels of a signal

• For binary = 2 levels• Ex: for 3KHz channel,

• Max data rate – 6000 bps Signal to Noise Ratio (SNR)

SNR = Signal Power / Noise Power Measured in decibels (db) – 10log10 S/N

For Noisy channel – Shanon theorem Maximum number of bits/sec = Hlog2(1+S/N)

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Physical Media Groups

Roughly grouped into Guided media,

copper wire and fiber optics, Unguided media,

radio and lasers

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Guided Transmission Media

Magnetic MediaTwisted PairCo-axial cableFiber optics

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Guided Transmission Media

Magnetic media It is the most common way of transferring data Transmission time is measured in minutes or hours Used

Where very less frequent transportation is needed Where amount of data is very high Cost effective, especially for applications in which

high bandwidth or cost per bit transported is the key factor.

Ex.Ultrium Tape-200 Gigabytes.

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Twisted Pair

Transmission time is measured in milliseconds.

Consisting of two insulated copper wires.

Thickness = 1 mm

Wires are twisted together in a helical form like

DNA molecule.

To remove electro-magnetic effect on data

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Twisted-Pair Cable

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Twisted Pair

The most common application is telephone system Can transfer data for several kilometers without amplificationBut for very long distances amplification is needed

Repeaters are used

Many TP cables grouped together and covered by protected material. They can be used for digital as well as analog transmission.The bandwidth depends on the thickens of the wire and the distance traversed.It is the cheapest solution

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Twisted Pair - Types

Two types Category 3,4 pairs

• 16 MHz

• Gently twisted Category 5,4 pairs

• 100 MHz

• More twists

• Less crosstalk, better signal quality

Category 6 (250 MHz) and 7 (600 MHz) are also coming It is also called UTP (Unshielded Twisted pair) cable.

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Twisted-Pair Cable

Figure 7-8

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A Coaxial Cable

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Coaxial cable

Construction stiff copper wire as the core surrounded by an insulating material The insulator is encased by a cylindrical

conductor • a closely-woven braided mesh

The outer conductor is covered in a protective plastic sheath

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Coaxial Cable

Advantages Better shielding than twisted pairs High bandwidth (1 GHz) [600 MHz] Excellent noise immunity

Use Within the telephone system for long-distance

lines For cable television For metropolitan area networks

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Coaxial cable

Two kinds of coaxial cable 50-ohm cable

• used for digital transmission 75-ohm cable

• used for analog transmission and cable television

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Fiber Optics

It transmit data by pulses of light A pulse of light indicates a 1 bit and the absence

of light indicates a 0 bit Optical transmission system has three components

The light source The transmission medium The detector

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Working of Fiber Optics Light source is either LED or a laser diode. The transmission medium is ultra thin fiber of glass The detector is a Photodiode which emits electric

pulse when light falls on it. Attaching a light source to one end of an optical

fiber and a detector to the other, we have a unidirectional data transmission system that accepts an electrical signal, converts and transmits it by light pulses, and then reconverts the output to an electrical signal at the receiving end.

Data rate = 10 Gbps

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Example

a) Three examples of light rays a) Three examples of light rays from inside a silica fiberfrom inside a silica fiberimpinging on the air/silicaimpinging on the air/silicaboundary at different anglesboundary at different angles

b) light trapped by total internal b) light trapped by total internal

reflectionreflection

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The Single Mode Fiber

Fiber with core diameter less than about ten times the wavelength of light then it is known as single mode fiber

Single mode fiber acts like a wave guide, and the light propagates in a straight line

They require expensive laser diodes but are more efficient and run for longer distances

It can transmit data at 50 Gbps for 100 km without amplification

The most common type of single-mode fiber has a core diameter of 8 to 10 μm

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Single-mode fibers are more expensive but are widely used for longer distances.

Fiber with large (greater than 10 μm  ) core diameter called multimode fiber. A fiber can pass more than one rays at a time, at

different angles then it is known as multimode fiber.

The Single Mode Fiber

(a) Multimode fiber: multiple rays follow different paths

(b) Single mode: only direct path propagates in fiber

direct path

reflected path

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Transmission of Light Through Fiber

Glass used is Very transparent! Attenuation of light passing thru glass

depends on the wavelength of it Attenuation = reduction in power

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Attenuation of Light Thru Fiber in the Infrared Region

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The Spectrum Used Three wavelengths are used for optical

communication. 0.85 micron, 1.30, 1.55 microns are centers Later two have less then 5% loss per km

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Dispersion and Solution Light pulses spread out in length as they propagate,

This spreading is known as chromatic dispersion The amount of dispersion is Wavelength dependent. Dispersion results in overlapping of light waves in

multimode fiber To stop spread out pulses overlapping, is to increase

the distance between them. This can be done only by reducing the signaling rate.

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The Fiber Cable

At the center is the glass core through which the light propagates.

In multimode fiber, the core is about 50 microns thick,( thickness of human hair)

In single mode fiber, the core is 8 to 10 microns wide

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The Fiber Cable

The core is surrounded by a glass cladding with a lower index of refraction

Next comes a thin plastic jacket to protect the cladding

Fibers are typically grouped together in bundles, protected by an outer sheath

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Fiber Cables

a) Side view of a single fibre

b) End view of a sheath with three fibres

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Light SourcesA Comparison of

LED and Semiconductor diodes as Light sources

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Interfaces (With Computers)

The connector is very difficult to make and substantial light is lost

Two type of interfaces are used first one is called the Passive Interface second one is called Active repeater

Both of them, at each computer, serves as a T junction to allow the computer to send and accept messages, and pass data through

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A Fiber optic Ring With Active repeaters

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The Active Repeater In the Active repeater the incoming light is

converted to an electric signal It is regenerated to the full strength and

retransmitted as light connector is a simple copper wire If an active repeater fails, the ring gets broken

and the network goes down There is no virtual limit on the size of ring

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Passive Interfaces:

Passive interface consists of two tapes fused onto main fiber

One tap has LED or Laser diode at the end of it and the other has the Photodiode

It is extremely reliable because a broken LED or photodiode does not break the ring. It just takes one computer off-line.

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A Passive Star Connection

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Comparing Fiber and Copper

High bandwidth with min loss of power Not affected by power line surges,

Electromagnetic interference Repeaters are needed every 50km compared

to 5 km in copper wire They are very thin and light weight

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Comparing Fiber and Copper One thousand twisted pairs 1 km long weigh 8000

kg. Two fibers have more capacity and weigh is only 100 kg.

Fibers do not leak light and are quite difficult to tap Since optical transmission is inherently

unidirectional, two-way communication requires either two fibers or two frequency bands on one fiber

It is an less familiar technology for most Engineers. Can be damaged easily by being bent too much Fiber interfaces cost more than electrical interfaces

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Wireless Transmission

For people who need to be on-line all the time For mobile users. Running a fiber to a building is difficult due

to the terrain (mountains, jungles, etc.)

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The Electromagnetic Waves When electrons move, they create EM waves that

can propagate thru free space Frequency( f )

The number of oscillations per second of wave measured in Hz

Wavelength (λ) The distance between consecutive maxima or minima By attaching an antenna to an Electric Circuit, the EM

Wave can be broadcasted and can be received by receiver some distance away.

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The Properties of EMWs

In Vacuum all EMWs travel at the same speed even though different frequency- 3 * 108 m/sec or 30 cm/nano sec

In copper or fiber, it slows about 2/3rd of above value and become slightly freq dependent.

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The EM Spectrum

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The EM Spectrum

The Radio, microwave, infrared and visible light, all can be used for transmission

Transmission can be done by modulating either amplitude, frequency or phase

UV, X-rays, and gamma rays are hard to produce, do not propagate well thru buildings and are dangerous to living things

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The Capacity of Transmission

The amount of info an EMW can carry is related to its bandwidth

It is possible to encode few bits per Hz at lower freqs but nearly 8 at high freqs

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Transmission Methods Direct Sequence spread spectrum & Frequency hopping spread spectrum The transmitter and receiver hops from

frequency to frequency hundreds of times per second makes transmissions hard to detect which spreads the signal over a wide frequency

band good efficiency high noise immunity Used in military and commercial world

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Radio Waves

They are easy to generate, can travel long distances, penetrate buildings easily

They are omni directional ( can travel in all directions) so transmitter and receiver are not needed to be aligned

Radio waves are frequency dependent At low freq, power falls off sharply with distance

from the source. At higher freq, they tend to travel in straight lines

and bounce of obstacles

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Continue… In VLF, LF and MF bands radio waves

follow the ground Easily pass through buildings These bands offer relative low bandwidth

for data communication

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Continue…

In HF and VHF bands, the ground waves tends to be absorbed by earth

The waves that reach ionosphere, are refracted by it and sent back to earth

military operates on these bands for long distance talks

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Transmission of Radio Waves

In the VLF, LF and MF bands, radio waves follow the Curvature of the earth

In the HF, and VHF they bounce of The ionosphere

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Microwave Transmission

Microwave = Wave above 100 MHz Travel in Straight line

Transmitting and receiving antennas must be accurately aligned with each other

Repeaters are needed • If towers are too far, the earth will get in the way

• Height : distance Ratio = r:r2

• Higher the tower, farther apart they can be.

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Terrestrial Microwave

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Microwave Transmission To achieve high data rate -10GHz, microwave

is in routine use but At about 4 GHz, MW absorbed by water and

generate hit These waves are only a few centimeters long

and are absorbed by rain Soln :- Shut off links where rain is falling &

take another route

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MW use

microwave communication is so widely used for long-distance telephone communication, mobile phones, television distribution

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Comparison with Fiber optics

Inexpensive Putting up two simple towers may be far cheaper

than buying 50 km of fiber through a congested urban area or up over a mountain

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Infrared and Millimeter Waves

Used for short range communication Remote control of electronic products

Can not pass through solid objects due to high freq

infrared system in one room will not interfere with a similar system in adjacent rooms

No need of government license

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Light Wave Transmission

Lasers can connect two LANs Coherent optical signaling using lasers is

inherently unidirectional Each one need its own laser and photo

detector It offers very high bandwidth at very low cost

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Continue…

Advantage : It is relatively easy to install, and no licensing is needed

Disadvantage : It can not penetrate even a rain or thick fog Heat generated by sun can defocus the beam

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A Bi-directional System With Two Lasers

Communication satellites

Contains several transponders which listens to some portion of the spectrum, amplifies the incoming signal, rebroadcasts it at another frequency to avoid interference with the incoming signal.

Communication Satellites – GEO (Geostationary Satellite)

Orbit slots allocation is done by the ITU Min 2 degrees distance between satellites 360/2 = 180 satellites

The effects of solar, lunar, and planetary gravity tend to move them away from their assigned orbit slots and orientations, an effect countered by on-board rocket motors. This fine-tuning activity is called station keeping.

Satellite Bands

Ku = commercial telecommunication carriers

Ka = commercial

Many government and military bands also exists

Communication satellites A modern satellite has around 40 transponders, each with

an 80-MHz bandwidth. Spot Beams - Each downward beam can be focused on a

small geographical area elliptically shaped, and can be as small as a few hundred km

in diameter. VSATs (Very Small Aperture Terminals)

1-meter or smaller antennas (versus 10 m for a standard GEO antenna) and can put out about 1 watt of power.

uplink -19.2 kbps, downlink - 512 kbps. Microstations do not have enough power to communicate

directly with one another (via the satellite). Instead, a special ground station, the hub, with a large, high-gain antenna is needed to relay traffic between VSATs,

VSAT

VSATs have great potential in rural areas.

Communication satellites - Properties Broadcast media Propagation delay – 3 µsec (fibre optic – 5

µsec) Because electromagnetic waves travel faster

in air than in solid materials Satellites are a complete disaster:

everybody can hear everything. Encryption is essential when security is

required. High error rates Cost to any place is same

Satellites versus Fiber High bandwidth via satellite than Fiber Communication on move (mobile) Broadcasting

A message sent by satellite can be received by thousands of ground stations at once.

Launching one satellite was cheaper than stringing thousands of undersea cables

Satellites is to cover areas where laying fiber is difficult or unduly expensive.

Military communication systems in time of war, satellites win easily.

The Public Switched Telephone Network

Structure of the Telephone System

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How TN is used for data communication.

Transmission line suffers from 3 problems

Transmission line suffers from 3 problems Attenuation-it is loss of energy as signal

propagates outward (db/km) Distortion- speed difference leads to

distortion of received signal Noise-unwanted energy from

sources other than transmitter.

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MODEM Aim = To transfer digital data in

existing analog twisted pair cable Achieved through modulation of one

or more properties of analog signal such as Amplitude Frequency Phase

Modulation(b) AM (c) FM (d) PM

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AM, FM, PM

AM

PM

FM

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Speed

Modem can sample 2400 times per second

Each sample is called baud During each baud one symbol is

transmitted Symbol may carry one or more bits If the symbol consists of 0 volts for a

logical 0 and 1 volt for a logical 1(or one bit per symbol), than bit rate is 2400 bps (2.4kbps)

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Baud Rate / Bit Rate voltages 0, 1, 2, and 3 volts are used, every

symbol consists of 2 bits, so a 2400-baud line can transmit 2400 symbols/sec at a data rate of 4800 bps.

Similarly, with four possible phase shifts, there are also 2 bits/symbol, so again here the bit rate is twice the baud rate.

The latter technique is widely used and called QPSK (Quadrature Phase Shift Keying).

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Continue…

To improve the speed from one bit, we can use modulation techniques Using 4 voltage level (2 bit per symbol) Using 4 phase shifts (2 bit per symbol) Combination of both (4 bit per symbol)

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QPSK All advanced modems use a combination

of modulation techniques to transmit multiple bits per baud.

Often multiple amplitudes and multiple phase shifts are combined to transmit several bits/symbol.

Fig- (a) has four valid combinations and can be used to transmit 2 bits per symbol. It is QPSK (Quadrature Phase Shift Keying).

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(a) QPSK. (b) QAM-16. (c) QAM-64.

Constellation Diagrams

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QAM-16 In Fig-(b) we see a different modulation scheme,

in which four amplitudes and four phases are used, for a total of 16 different combinations.

This modulation scheme can be used to transmit 4 bits per symbol.

It is called QAM-16 (Quadrature Amplitude Modulation).

Sometimes the term 16-QAM is used instead. QAM-16 can be used, for example, to transmit 9600 (2400*4) bps over a 2400-baud line.

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QAM-64 Fig- (c) is yet another modulation scheme

involving amplitude and phase. It allows 64 different combinations, so 6

bits can be transmitted per symbol. It is called QAM-64. Higher-order QAMs

also are used. Each high-speed modem standard has its

own constellation pattern and can talk only to other modems that use the same one

Modems types

Extra bit for error detection

V.32 => 4+1 = 5 v.90 => 33.6 kbps

V.32 bis => 6+1 = 7 V.92 => 48 kbps V.34 => 28800 bps V.32 bis => 33600 bps

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Asymmetric Digital Subscriber Lines

The maximum speed possible by modems is 56 kbps

To start Services with more bandwidth than standard telephone service

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ADSL - Two Approaches Dividing the spectrum of 1.1 MHz

into three frequency bands: POTS (Plain Old Telephone Service) Upstream (user to end office) and Downstream (end office to user).

The alternative approach, called DMT (Discrete MultiTone )

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DMT

Divide the available 1.1 MHz spectrum on the local loop into 256 independent channels of 4312.5 Hz each

Channel 0 is used for POTS. Channels 1–5 are not used Of the remaining 250 channels, one is

used for upstream control and one is used for downstream control.

So rest(248) are available for user data.

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Operation of ADSL using discrete multitone modulation.

80%–90% of the bandwidth is allocated to the downstream channel since most users download more data than they upload

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ADSL Arrangement

DSLAM – DSL Access Multiplexer

Consists of DSP

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ADSL Arrangement NID (Network Interface Device)

To interface with Telephone Network At customer premises

Splitter(Analog filter) separates the 0-4000 Hz band (voice from the data) POTS signal routed to telephone network Data signal routed to ADSL modem

Disadvantage: presence of the NID and splitter on customer premises

required ADSL is physical layer standard.

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Wireless Local Loops What is the need of WLL ?

Any company wishing to get into the local phone business in some city must do the following things• First it must buy or lease a building for its

first end office • Second it must fill the end office with

telephone switches and other equipment

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Continue…• Third, it must run a fiber between the end

office and its nearest toll office so the new local customers will have access to its national network.

• Fourth it must acquire customers, typically by advertising better service or lower prices than those of the Existing companies.

• Fifth and hardest path is installing local loop

So WLL, a cheaper solution was discovered

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Expectation of customer

Fixed Wireless Gives high-speed Internet connectivity New customer should not have an

objection with a large directional antenna on his roof

Customer can not be mobile

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Freq Spectrum Allocation The 198 MHz of new spectrum was

allocated for the use of WLL This service called MMDS (Multichannel

Multipoint Distribution Service). Low bandwidth of MMDS limits the no of

users because Allocated spectrum is shared by many users

gave birth to LMDS (high BW service)

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LMDS(Local Multipoint Distribution Service)

1.3 GHz BW At frequencies of 28–31 GHz in the

U.S. and 40 GHz in Europe

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Architecture of an LMDS system

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Architecture of an LMDS system Tower is shown with multiple antennas on it,

each pointing in a different direction. Each antenna defines a sector, independent of

the other ones. Range is 2–5 km, which means that many

towers are needed to cover a city. 36 Gbps downstream and 1 Mbps upstream,

shared among all the users in that sector A single tower with four antennas could serve

100,000 people within a 5-km radius of the tower.

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Few problems with LMDS Waves propagate in straight lines Leaves absorb these waves well Rain also absorbs these waves

Circuit Switching and Packet Switching

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Comparison

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The Mobile Telephone System Wireless telephones come in two

basic varieties: Cordless phones

• consisting of a base station and a handset • sold as a set for use within the home.

Mobile phones (sometimes called cell phones).

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Three Generations Mobile phones Generations:1. Analog voice.2. Digital voice.3. Digital voice and data

(Internet, e-mail, etc.).

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Analog Voice (First-Generation Mobile Phones )

Mobile radio telephones used for maritime and military

communication In 1946, the first system for car-based

telephones was set up in St. Louis Single large transmitter on top of a tall

building and had a single channel, used for both sending and receiving.

To talk, the user had to push a button that enabled the transmitter and disabled the receiver

Known as push-to-talk systems

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IMTS Improved Mobile Telephone System Was installed in the 1960s two frequencies, one for sending

and one for receiving IMTS supported 23 channels spread

out from 150 MHz to 450 MHz

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Disadvantages Small number of channels,

users often had to wait a long time before getting a dial tone.

The large power of the transmitter adjacent systems had to be several

hundred kilometers apart to avoid interference

The limited capacity made the system impractical

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Advanced Mobile Phone System

Invented by Bell Labs first installed in the United States

in 1982 & was also used in England called

TACS in Japan called MCS-L1

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Implementation Geographic region is divided up into

cells Area = 10 to 20 Km Each cell uses some set of

frequencies not used by any of its neighbours

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Same group of Freqs

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Cells are divided in micro cells

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Continue… At the center of each cell is a base station

to which all the telephones in the cell transmit

The base station consists of a computer and transmitter/receiver connected to an antenna

base stations are connected to an MTSO (Mobile Telephone Switching Office) or MSC (Mobile Switching Center)

The MTSOs are connected to at least one telephone system end office

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Handoff When phone moves from one cell to another cell, base station

changes, It takes about 300 ms Two types of Handoff

Soft Handoff :-• Telephone is acquired by the new base station before

the previous one signs off. • In this way there is no loss of continuity. • Telephone needs to be able to tune to two frequencies

at the same time (the old one and the new one). • Neither first nor second generation devices can do this

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Continue…

Hard Handoff• With breaking continuity• Old base station drops the telephone

before the new one acquires it. • If the new one is unable to acquire it

(e.g., because there is no available frequency), the call is disconnected abruptly.

• Users tend to notice this

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Communication Channels Freq Spectrum divided in 832 full-

duplex channels 832 simplex transmission channels

from 824 to 849 MHz 832 simplex receive channels from 869

to 894 MHz

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Channels The 832 channels are divided into

four categories:1. Control (base to mobile) to manage the

system.2. Paging (base to mobile) to alert mobile

users to calls for them.3. Access (bidirectional) for call setup and

channel assignment.4. Data (bidirectional) for voice, fax, or

data.

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Call Management Each mobile has a 32-bit serial number

and a 10-digit(34-bit) telephone number. When a phone is switched on, it scans a

preprogrammed list of 21 control channels to find the most powerful signal.

The phone then broadcasts its 32-bit serial number and 34-bit telephone number

base station hears the announcement, informs the MTSO, and customer's home MTSO

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Call Management To make a call

User dial a no and press send button No. is sent (on access channel) to base station On getting request base station informs MTSO MTSO allot free channel Channel no is sent back (on control channel)

to mobile Mobile switches to that voice channel

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Call Management

Incoming calls all idle phones continuously listen to the

paging channel When a call is placed to a mobile phone a

packet is sent to the callee's home MTSO to find out where it is

Packet sent to callee’s current base station Base station broadcast “are you there” Callee’s phone responses Base station send channel no Callee’s phone switches to that channel and

ringing starts

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Second-Generation Mobile Phones: Digital Voice

Due to the lack of world wide standardization Four systems are in use now D-AMPS, (used in US) GSM, (Everywhere) CDMA, PDC (used in Japan) (almost same as

first)

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D-AMPS The Digital Advanced Mobile Phone

System Frequency Allocation

Upstream: 1850–1910 MHz Downstream: 1930–1990 MHz

Wavelength=16 cm Antenna requirement 4 cm long

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D-AMPS Voice signal picked up by microphone

is digitized and compressed Compression is done through circuit

called vocoder Advantage of digitization &

compression: More than one users can use same

frequency channel

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D-AMPS Each frequency pair supports 25

frames/sec of 40 msec each Each frame is divided into six time slots of

6.67 msec each Each frame holds three users who take

turns using the upstream and downstream links

Using better compression algorithms, in which case six users can be stuffed into a frame

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TDM Frame of D-AMPS

User 1 sending

User 3 receiving

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Hand off In D-AMPS, 1/3 of the time a mobile is

neither sending nor receiving. It uses these idle slots to measure the line

quality. When it discovers that the signal is

waning, it complains to the MTSO, The mobile tuned to a stronger signal

from another base station. Takes about 300 msec to do the handoff. This technique is called MAHO (Mobile

Assisted Hand Off).

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GSM The Global System for Mobile

Communication 124 pairs of simplex channels Each simplex channel is 200 kHz

wide and supports eight separate connections on it

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GSM uses 124 frequency channels, each of which uses an eight-slot TDM system

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A portion of the GSM framing structure

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A portion of the GSM framing structure Eight data slots make up a TDM frame 26 TDM frames = 120msec multiframe. Of the 26 TDM frames in a multiframe, slot 12 is

used for control and slot 25 is reserved for future use, only 24 are available for user traffic. 51-slot multiframe is also used

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A portion of the GSM framing structure TDM slot consists of a 148-bit data frame that

occupies the channel for 577 µsec Each data frame starts and ends with three 0

bits It also contains two 57-bit Information fields,

each one having a control bit that indicates whether the following Information field is for voice or data.

Between the Information fields is a 26-bit Sync (training) field that is used by the receiver to synchronize to the sender's frame boundaries

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CDMA Code Division Multiple Access D-AMPS and GSM are fairly conventional systems.

They use both FDM and TDM to divide the spectrum into channels and the channels into time slots.

CDMA allows each station to transmit over the entire frequency spectrum all the time.

If we have a 1-MHz band available for 100 stations, with FDM each one would have 10 kHz and could send at 10 kbps (assuming 1 bit per Hz). With CDMA, each station uses the full 1 MHz, so the chip rate

is 1 megachip per second. Each bit time is subdivided into m short intervals called

chips. Typically, there are 64 or 128 chips per bit, but in the

example given we will use 8 chips/bit for simplicity.

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Example Each station is assigned a unique m-bit code

called a chip sequence. To transmit a 1 bit To transmit a 0 bit, it sends the one's

complement of its chip sequence.

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Continue… Orthogonal property of chip sequences

A.A = 1 A.A’ = -1 A.B = 0 A.B’ = 0

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Continue… When two or more stations transmit

simultaneously, their bipolar signals add linearly

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(a) Binary chip sequences for four stations.

(b) Bipolar chip sequences.

(c) Six examples of transmissions.

(d) Recovery of station C's signal.

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Continue…

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Third-Generation Mobile Phones: Digital Voice and Data Expectation of Industry Experts

a lightweight, portable device that acts as a telephone, CD player,

DVD player, e-mail terminal, Web interface, gaming machine, word processor, and more,

All with worldwide wireless connectivity to the Internet at high bandwidth.

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To Achieve this dream The single world wide technology

was envisioned (IMT-2000) Basic services

High-quality voice transmission. Messaging Multimedia Internet access

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Several proposals came W-CDMA (Wideband CDMA), was

proposed by Ericsson. Not backward compatibility was there with

GSM UMTS (Universal Mobile

Telecommunications System). Proposed by European Union

CDMA2000, proposed by Qualcomm Handoff was problem

2.5G EDGE (Enhanced Data rates for GSM

Evolution) GSM with more bits per baud

GPRS (General Packet Radio Service) Packet network on top of D-AMPS and GSM Using unused TDMA channels of GSM If SMS over GPRS is used, an SMS

transmission speed of about 30 SMS messages per minute may be achieved.

• This is much faster than using the ordinary SMS over GSM, whose SMS transmission speed is about 6 to 10 SMS messages per minute.

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Cable television was conceived in the late 1940sConsisted of a big antenna on top of a hill to pluck the television signal out of the air, An amplifier, called the head end, to strengthen it, A coaxial cable to deliver it to people's houses,

Cable Television- Community Antenna TV

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HFCHybrid fiber coax system

(Electro optical converter)

Fixed Telephone System

Internet on Cable

a) A single cable is shared by many houses, whereas in the telephone system, every house has its own private local loop

One-way amplifiers have to be replaced by 2 way amplifiers

b) On the other hand, the bandwidth of coax is much higher than that of twisted pairs, but the cable is shared.

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Internet over Cable

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Cable Modems Two interfaces on it: one to the

computer and one to the cable network.

The headend assigns upstream and downstream channels for that modem

Modem scans the downstream channel for system parameters

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Cable Modems Ranging: modem determines its

distance from the headend

Typical details of the upstream and downstream channels

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ADSL versus Cable ADSL providers can give specific

statements about the bandwidth Increasing no of users do not affect speed Availability : You must be near the end

office if you want ADSL More secure due to p to p connection More reliable: work even during a power

outage