More on Propagation Module B. 2 More on Propagation n Modulation – Modems translate between...
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Transcript of More on Propagation Module B. 2 More on Propagation n Modulation – Modems translate between...
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