More on Propagation

Module B

2More on Propagation

Modulation– Modems translate between digital devices and analog

transmission lines. We will look at the processes used to modulate digital signals

Multiplexing– An important way to reduce costs is to multiplex (mix)

several signals onto a transmission line

Trunk Lines– Trunk lines link the switches of carriers.

Modulation

4Modulation

Modulation converts an digital computer signal into a form that can travel down an ordinary analog telephone line

There are several forms of modulation– Amplitude modulation– Frequency modulation– Phase modulation– Complex modulation

5The Modulation Problem

Modem accepts a digital signal from the computer– Really, binary--ones and zeros– Two voltage levels

Modem converts into waves (analog)

DigitalSignal(1101) Modem

AnalogSignal

6Waves

Frequency of a wave– The number of complete cycles per second– Called Hertz– kHz, MHz, GHz, THz

Frequency (Hz)

Cycles in One Second

7Frequency Modulation (FM)

LowFrequency

(0)

HighFrequency

(1)

FrequencyModulation

(1011)

Wavelength

Wavelength

1

0

1

1

8Wavelength

Physical distance between similar points in adjacent cycles– Not independent of frequency– Frequency * wavelength = speed of light in medium– In a harp, for instance, long strings have low sounds

Wavelength(meters)

9Amplitude Modulation (AM)

Amplitude is the intensity of the signal– Loud or soft

Amplitude(power)

10Amplitude Modulation (AM)

LowAmplitude

(0)

HighAmplitude

(1)

AmplitudeModulation

(1011)

Amplitude (low)

Amplitude (high)

11Phase

Two signals can have the same frequency and amplitude but have different phases--be at different points in their cycles at a given moment

BasicSignal

180 degreesout of phase

12Phase Modulation (PM)

In Phase(0)

180 degreesout of phase

(1)

FrequencyModulation

(1011)

13Phase Modulation (PM)

Human hearing is largely insensitive to phase– So harder to grasp than FM, AM

But equipment is very sensitive to phase changes– PM is used in all recent forms of modulation for

telephone modems

14Complex Modulation

Modern Modems Mix Phase and Amplitude

HighAmplitude

LowAmplitude

90 DegreesOut of Phase,High Amplitude

In Phase

180 Degrees Out of Phase

15Complex Modulation

Baud rate: number of times the state can change per second

– Usually 2,400 to 3,200 baud for telephone modems

Bits sent per possible state change depends on number of possible states

– 2 b=s– b=bits per time period– s=number of possible states– In our example, 2b=8– So b must be 3– 3 bits are sent per time period

16Complex Modulation

Bit rate = baud rate * bits/time period– bits/time period = 3, as just shown

– So if the baud rate = 2,400

– Then the bit rate = 2,400 * 3 = 7,200 bits/second

Multiplexing

18Multiplexing

Multiplexing mixes the signals of different conversations over a single transmission line

– To reduce costs

There are several forms of multiplexing– Time division multiplexing– Statistical time division multiplexing– Frequency division multiplexing– Multiplexing at multiple layers– Inverse multiplexing (bonding)

19Why Multiplex?

Reason 1: Economies of Scale– 64 kbps lines carry a single 64 kbps signal– T1 lines can multiplex 24 such signals– Yet T1 lines cost only about 3-7* times as much as 64 kbps

lines

Example: Suppose you have ten 64 kbps signals– This will require ten 64 kbps lines– But one T1 line will carry them for only 3 to 7* times the cost

of a single 64 kbps line

*Textbook says 3. 3-7 is more realistic

New

20Why Multiplex?

Reason 2: Data transmission tends to be bursty– Uses capacity of a line only a small fraction of the time

Signal A

Signal B

Multiplexing allows several conversations to share a single trunk line, lowering the cost for each

21Economics of Multiplexing

Cost Savings– Economies of scale in transmission lines

– Multiplexing to lower costs for bursty traffic

Cost Increases– Multiplexing costs money for

multiplexers/demultiplexers at the two ends

Net Savings– Usually is very high

22Time Division Multiplexing

Time is divided into short periods– In each period, one frame is sent

Frame times are further divided– Each subdivision is a slot

Slot

Frame

23Simple Time Division Multiplexing (TDM)

Several connections are multiplexed onto a line– In figure, two signals: A and B

Each connection is given one slot per frame– Guaranteed the slot– Slot is wasted if the connection does not use it– Wasteful but still brings economies of scale– Inexpensive to implement

A B

A

Slot not Used

24Statistical Time Division Multiplexing (STDM)

Still Frames and Slots But slots are assigned as needed

– Connections that need more slots get them– More efficient use of line– More expensive to implement– But STDM is now cost-effective– Multiplexers at both ends must follow the same STDM

standard

A B A A

Frame

25Frequency Division Multiplexing (FDM)

Signals are sent in different channels– Signals sent in different channels do not interfere– Brings economies of scale– Used in radio transmission

A

B

Frequency Channel

26Combining TDM and FDM

Use Simple TDM Within a Channel

Frequency ChannelA B

27Spread Spectrum Transmission

Ways to mix signals in a channel statistically– Greater efficiency in the use of the channel– Described in Module C

Frequency ChannelA B A

Carrier Trunk Lines

29Carrier Trunk Lines

Trunk lines are high-speed lines that connect the switches of carriers

There are several kinds of trunk lines– Optical fiber– Radio transmission– Microwave transmission– Satellite transmission

LEOs VSATs

30Optical Fiber

Thin Core of Glass– Surrounded by glass cladding– Inject light in on-off pattern for 1s and 0s– Total reflection at core-cladding boundary– Little loss with distance

LightSource

Cladding

Core

Reflection

31Optical Fiber

Modes– Light entering at different angles will take different

amounts of time to reach the other end– Different ways of traveling are called modes– Light modes from successive bits will begin to overlap

given enough distance, making the bits unreadable

LightSource

Reflection

32Single Mode Fiber

Single Mode Fiber is very thin– Only one mode will propagate even over fairly long

distances– Expensive to produce– Expensive to install (fragile, precise alignments needed)– Used by carriers to link distant switches

33Multimode Fiber

Core is thicker– Modes will appear even over fairly short distances– Must limit distances to a few hundred meters– Inexpensive to purchase and install– Dominates LANs

34Graded Index Multimode Fiber

Index of fraction is not constant in core– Varies from center to edge– Reduces time delays between different modes– Signal can go farther than if core has only a single

index of fraction (step index multimode fiber)– Dominates multimode fiber today

35Multimode Optical Fiber and Frequency

Signal Frequency has Impact on Propagation Distance before Mode Problems Become Serious

Short Wavelength (high frequency) – Signals do not travel as far before mode problems occur– Uses the least expensive light sources– Good for LAN use within buildings

Long Wavelength (low frequency)– More expensive light sources and fiber quality– Within large buildings and between buildings

36Wave Division Multiplexing

Use multiple light sources of different frequencies– Place a separate signal on each– Increases the capacity of the optical fiber

37SONET/SDH

High speed optical fiber trunk system of carriers– Called SONET in the United States– Called SDH in Europe

Arranged in a Dual Ring– If a link is broken, ring is wrapped and still works– Important because broken lines are common because of

construction

WrappedOriginal

38Radio Transmission

Oscillating electron generates electromagnetic waves with the frequency of the oscillation

Many electrons must be excited in an antenna for a strong signal

39Omnidirectional Antennas

Signal is transmitted as a sphere– No need to point at a receiver (or transmitter)– Attenuation with distance is very high– Used in mobile situations where dishes are impossible

40Dish Antenna

Dish captures a (relatively) large amount of signal– Focuses it on a single point (which is the real antenna)– Can deal with weaker signals– You must know where to point the dish– Good in radio trunk lines, some satellite systems

41Frequency Bands

Propagation Characteristics Depend on Frequency

– At lower frequencies, signals bend around objects, pass through walls, and are not attenuated by rain

– At higher frequencies, there is more bandwidth per major band

42Major Bands

Frequency Spectrum is Divided into Major Bands

Ultra High Frequency (UHF)– Signals still bend around objects and pass through walls– Cellular telephony

Super High Frequency (SHF)– Needs line-of-sight view of receiver– Rain attenuation is strong, especially at the higher end– High channel capacity– Used in microwave, satellites

43Microwave Transmission

Terrestrial (Earth-Bound) System– Limited to line-of-sight transmission– Repeaters can relay signals around obstacles

Line-of-SightTransmission

Repeater

44Satellite Transmission

Essentially, places repeaters in sky– Idea thought of by Sir Arthur C. Clarke

Satellite broadcasts to an area called its footprint

Uplink is to satellite; downlink is from satellite

UplinkDownlink

Footprint

45Satellite Frequency Bands

SHF Major Band is Subdivided– Uplink/downlink frequency range (GHz)– Downlink range is always lower

C Ku Ka Q/V

Uplink 6 14 30 Higher

Downlink 4 12 20 Higher

Usage High High Growing Not Yet

46GEOs

Satellite orbits at 36 km (22,300 miles)– Orbital period is 24 hours– Appears stationary in the sky– Easy to aim dishes– Very far for signals to travel– Used in trunking

47LEOs

Low Earth Orbit satellites– Orbits are only 500 to 2,000 km (300 to 500 miles) – Short distance means less attenuation– But 90-minute orbit, so pointing is difficult– Fortunately, close enough for omnidirectional antenna

48MEOs

Medium Earth Orbit satellites– Orbits are 5,000 km to 15,000 km (3,000-9,000 miles)– Longer distance than LEOs means more attenuation– But longer orbit, so fewer hand-offs– Still close enough for omnidirectional antenna

New

49VSATs

Very Small Aperture Terminal satellite system– Small dishes for remote sites (0.25 to 1 meters)– Inexpensive for remote sites– Central hub is powerful and has large dish– Satellite is powerful– Used in direct broadcast for television– Companies use VSATs to bypass carrier networks

50Satellite Limitations

Limited bandwidth, so expensive

Propagation delays for GEOs– Can be bad for data transmission

More expensive than fiber for high-capacity trunk needs

51Specialized Satellite Usage

Thin Routes– Trunks of low volume– Company with several sites

Multipoint Transmission (One-to-Many)– Direct broadcast satellites for television– Distribute cable television channels to local systems– Distribute marketing information to remote sites

Mobile Systems– Cannot have a wire on a truck– Laptop Internet access