Transmission Media Chapter 4

1
Transmission Media Chapter 4

• Physically connect transmitter and receiver
  carrying signals in the form electromagnetic
  waves.
• Types of media
• Guided waves guided along solid medium such as
  copper twisted pair, coaxial cable, optical
  fiber.
• Unguided wireless transmission (atmosphere,
  outer space).

2
Guided Media Examples 1

• Twisted Pair
• 2 insulated copper wires arranged in regular
  spiral. Typically, several of these pairs are
  bundled into a cable. (What happens if the twist
  is not regular? Reflection?)
• Cheapest and most widely used limited in
  distance, bandwidth, and data rate.
• Applications telephone system (from home to
  local exchange connection).
• Unshielded and shielded twisted pair.
• What is a differential amplifier?

3
Guided Media Examples 1

• Twisted pair continued
• Category 3 Unshielded twisted pair (UTP) up to
  16MHz.
• Cat 5 UTP to 100 MHz.
• Table 4.2. Suppose Cat 5 at 200m (the limit of
  100Mbps ethernet is 300m).
• The dB attenuation at 100m is 22.0. So at 200m,
  the attenuation is ???. Suppose we transmit at
  80dBW. Then the received signal has energy of
  ????.
• The near-end crosstalk gain is 32dB per 100m. So
  the crosstalk energy is ????
• The SNR is ????? (neglecting thermal noise).

44
124dBW
144dBW
20dB
4
Examples 2

• Coaxial Cable
• Hollow outer cylinder conductor surrounding inner
  wire conductor dielectric (non-conducting)
  material in the middle.
• Less capacitance than twisted pair, so less loss
  at high frequencies. Also, Coaxial has more
  uniform impedance.
• Applications cable TV, long-distance telephone
  system, LANs.
• Repeaters are required every few kilometers at
  500MHz.
• s Higher data rates and frequencies, better
  interference and crosstalk immunity.
• -s Attenuation at high frequency (up to 2 GHz
  is OK) and thermal noise.

5
Examples 3

• Optical Fiber
• Thin, flexible cable that conducts optical waves.
• Applications long-distance telecommunications,
  LANs (repeaters every 40km at 370THz!).
• s greater capacity, smaller and lighter, lower
  attenuation, better isolation,
• -s Not currently installed in subscriber loop.
  Easier to make use to current cables than install
  fiber.

6
Examples 3 types of fiber

• Step-index multimode

lower index of refraction
shorter path
longer path
absorbed
higher index of refraction
total internal reflection
Since the signal can take many different paths,
the arrival the received signal is smeared.
Input Pulse
Output Pulse
7
Examples 3 types of fiber

• Single mode

If the fiber core is on the order of a
wavelength, then only one mode can
pass. Wavelengths are 850nm, 1300nm and 1550nm
(visible spectrum is 400-700nm). 1550nm is the
best for highest and long distances. Attenuation
-0.2dB/km to -0.8dB/km (if the ocean was made of
this glass you could see the floor like you can
see the ground from an airplane)
8
Examples 3 types of fiber
Even for single mode fiber, a pulse gets smeared.
Solitons are a particular wave pulse that does
not disperse.
9
Fiber Repeaters Two Approaches

• Convert the signal to analog. Convert to digital
  and then send a transmit received signal.
• Optical repeater. A nonlinear optical amplifier
  shapes and amplifies the pulse. A single repeater
  works for all data rates! (more about optical
  networks later)

10
Wavelength-division multiplexing (WDM)

• Wavelength-division multiplexing
• Multiple colors are transmitted.
• Each color corresponds to a different channel.
• In 1997, Bell Labs had 100 colors each at 10Gbps
  (1Tbps).
• Commercial products have 80 colors at 10Gbps.

11
Fiber vs. Cable

• Fiber is light and flexible.
• Fiber has very high bandwidth.
• Fiber is difficult to install (I cant do it).
• Fiber interfaces are more expensive than cable
  (?)

12
Wireless Transmission
13
Electromagnetic Spectrum
Cell phones put out 0.6 3 watts. Light bulbs
put out 100 watts.
14
Wireless Transmission

• Omni-directional the signal is transmitted
  uniformly in all directions.
• Directional the signal is transmitted only in
  one direction. This is only possible for high
  frequency signals.

15
Terrestrial Microwave

• Parabolic dish on a tower or top of a building.
• Directional.
• Line of sight.
• With antennas 100m high, they can be 82 km (50
  miles).
• Use 2 40 GHz.
• 2 GHz bandwidth 7MHz, data rate 12 Mbps
• 11 GHz bandwidth 220MHz, data rate 274 Mbps

The M in MCI is for microwave
16
Satellite Microwave

• Satellites are repeaters.
• 1 10 GHz. Above 10 GHz, the atmosphere (like
  rain) attenuates the signal, and below 1 GHz
  there is too much noise.
• Typically, 5.925 to 6.425 GHz for earth to
  satellite and 4.2 to 4.7 GHz for satellite to
  earth. (Why different frequencies?)
• A stationary satellite must be 35,784 km (22000
  miles) above the earth.
• The round-trip delay is about ½ a second.

17
Low-Earth Orbit Satellites (LEO)

• Iridium The idea of some executives wife while
  vacationing in the tropics and her cell phone
  didnt work..
• Cost 5 billion dollars.
• Went out of business in 1999. Sold for 25
  million and is still operational.
• Provides phone, fax, paging, data and navigation
  WORLD WIDE! (jungle, Afghanistan (both sides),
  etc.)
• 66 low orbit satellites. Low Orbit, so they move
  out of range fast
• Cool thing. The calls go hop from satellite to
  satellite before returning to the destination. So
  they have to track every user.
• Globalstar 48 LEOs. The call goes to the ground
  as soon as possible and uses a terrestrial
  network. So they are simpler. Also, the
  satellites relay the analog signal. On the ground
  is a large, sensitive antenna to pick up the weak
  phone signal.
• Teledesic. 30 satellites. Data network 100Mbps to
  720Mbps. Planned for 2005. Bill Gates and Craig
  McCaw founders.

18
Other

• Cell phones Omni-directional. GSM-900 uses
  900MHz, GSM-1800 and GSM-1900 (PCS). Typical data
  rate seems to be around 40kbps. But the protocol
  is specified to 171kbps.
• 802.11 wireless LANs
• Omni-directional
• 802.11b 2.4 GHz (where microwave ovens and
  cordless phones are) up to 11Mbps
• 802.11a 5 GHz up to 54Mbps
• Infrared Line of sight, short distances.

19
Spectrum Allocation

• Some bands are allocated for unlicensed usage
  (ISM)
• 900 MHz cell phones, cordless phones. Is not
  available in all countries. Bandwidth is 26MHz.
• 2.4 GHz cordless phones, 801.11b, Bluetooth,
  microwave ovens. Is available in most countries.
  Bandwidth is 83.5 MHz.
• 5.7 GHz 802.11a. Is new and relatively
  uncrowded (so far) but a bit expensive. Bandwidth
  is 125MHz. (Why can 802.11a transmit at a high
  data rate?)
• These are actually several bands.

20
Spectrum Allocation
21
Types of Connections

• Long-haul about 1500km (1000 miles) undersea,
  between major cites, etc. High capacity
  20000-60000 voice channels. Twisted pair,
  coaxial, fiber and microwave are used here.
  Microwave and fiber are still being installed.
• Metropolitan trunks 12km (7.5 miles) 100,000
  voice channels. Link long-haul to city and within
  a city. Large area of growth. Mostly coaxial,
  twisted pair and fiber are used here.
• Rural exchange trunks 40-160km link towns.
  Twisted pair, coaxial, fiber and microwave are
  used here.
• Subscriber loop run from a central exchange to
  a subscriber. This connection uses twisted pair,
  and will likely stay that way for a long time.
  Cable uses coaxial and is a type of subscriber
  loop (it goes from central office to homes). But
  a large number of people share the same cable.
• Local area networks (LAN) typically under 300m.
  Sizes range from a single floor, a whole
  building, or an entire campus. While some use
  fiber, most use twisted pair as twisted pair is
  already installed in most buildings. Wireless
  (802.11) is also being used for LAN.

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Data Encoding (Chap. 5)

• Transforming original signal just before
  transmission.
• Both analog and digital data can be encoded into
  either analog or digital signals.

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Digital Transmission Terminology

• Data element bit.
• Signaling element encoding of data element for
  transmission.
• Unipolar signaling signaling elements have same
  polarization (all or all -).
• Polar signaling different polarization for
  different elements.

24
More Terminology

• Data rate rate in bps at which data is
  transmitted for data rate of R, bit duration
  (time to emit 1 bit) is 1/R sec.
• Modulation rate baud rate (rate at which signal
  levels change).

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Approach 1 NRZ
But how do you know when to sample? Phase-locked-l
oop (PLL) measures the difference when
transitions occur on the wire and when they occur
on a local adjustable oscillator, and then make
adjustments accordingly. YOU MUST HAVE
TRANSISTIONS TO LOCK ON TO.
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Multilevel Binary
opposite direction
Pros No DC component. Can be used to force
transitions (to help PLL). Cons We are using 3
levels and could send ?? bits instead of 1
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Scrambling to help the PLL

• If there are not enough transitions, the PLL may
  have problems.
• So we force extra transitions when there are not
  enough.
• Approach 1 Use special coding so that long
  strings of zeros (or ones) dont occur.

28
Scrambling to help the PLL

• Approach 2 Use multilevel binary and set
  illegal transitions to long strings of zeros.
• Here, if an octet of zeros occurs, send a special
  illegal sequence.
• The receiver must be able to interpret this
  special sequence.

used in long-distance transmission
29
Biphase Differential Manchester(Self-Clocking)
A transition always occurs in the middle of the
period. A zero is represented by a transition
occurring at the beginning of the period. A one
is represented by no transition at the beginning
of the period.
0
0
1
1
always a transition in the middle
Used in CD players and Ethernet
30
Methods to Encode Digital Signals

• NRZ
• Multilevel binary
• Manchester
• Issues
• DC?
• Self Clocking?
• How big is the spectrum?

31
Sending Digital Signals over Analog (e.g. Modem)

• Amplitude shift keying (ASK) (Amplitude
  Modulation)
• Frequency shift keying (FSK) (Frequency
  modulation)
• Phase shift keying (PK) (Phase Modulation)
• Modems use phase and amplitude of them.

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Modulation Techniques
ASK
FSK
PSK
33
Phase-shift Keying

• Quadrature phase-shift keying (QPSK) - send 2
  bits.

90
0
180
270
34
QAM - Quadrature Amplitude Modulation
constellation diagrams
90
90
0
180
0
180
270
270
QAM-16 (16 levels, how many bits)
QAM - 64
35
V32
128 bits 6 data and 1 parity (error correction)
36
How fast is V32?
The phone system transmits 300 to 3400 Hz
So what bandwidth can we use. How fast can we
send symbols?
So 2400 6 14400 bps
What is the baud rate?
V.34 2400 baud - with 12 data bits/symbol V.34
2400 baud with 14 data bits/symbol Thats the
fastest there is!
To get 56K you send at 4000 baud (if the phone
system can handle it)
37
Digital Subscriber Lines (DSL)

• ADSL A for asymmetric, faster down load speed
  than up.
• The 56kbps or 33kbps is because of a filter
  installed at the end office.
• If this filter is removed, then the full spectrum
  of the twisted pair is available.
• But, if you are far from the office, then you
  cant get a very high data rate because?
• The DSL standard goes up to 8 Mbps down and 1
  Mbps up.

38
DSL
A total of 256 4kHz channels
Upstream
downstream
empty
25kHz (channel 6)
voice (channel 0)
250 parallel channels Each data channel uses QAM
16 (with 1 parity bit).
The quality of each channel is monitored and
adjusted. So channels may transmit at different
speeds
What is the maximum data rate?
39
Digital Transmission Receiver-Side Issues

• Clocking determining the beginning and end of
  each bit.
• Transmitting long sequences of 0s or 1s can
  cause synchronization problems.
• Signal level determining whether the signal
  represents the high (logic 1) or low (logic 0)
  levels.
• S/N ratio is a factor.

40
Comparing Digital Encoding Techniques

• Signal spectrum high frequency means high
  bandwidth required for transmission.
• Clocking transmitted signal should be
  self-clocking.
• Error detection built in the encoding scheme.
• Noise immunity low bit error rate.

41
Digital-to-Analog Encoding

• Transmission of digital data using analog
  signaling.
• Example data transmission of a PTN.
• PTN voice signals ranging from 300Hz to 3400 Hz.
• Modems convert digital data to analog signals
  and back.
• Techniques ASK, FSK, and PSK.

42
Amplitude-Shift Keying

• 2 binary values represented by 2 amplitudes.
• Typically, 0 represented by absence of carrier
  and 1 by presence of carrier.
• Prone to errors caused by amplitude changes.

43
Frequency-Shift Keying

• 2 binary values represented by 2 frequencies.
• Frequencies f1 and f2 are offset from carrier
  frequency by same amount in opposite directions.
• Less error prone than ASK.

44
Phase-Shift Keying

• Phase of carrier is shifted to represent data.
• Example 2-phase system.
• Phase shift of 90o can represent more bits aka,
  quadrature PSK.

45
Analog-to-Digital Encoding

• Analog data transmitted as digital signal, or
  digitization.
• Codec device used to encode and decode analog
  data into digital signal, and back.
• 2 main techniques
• Pulse code modulation (PCM).
• Delta modulation (DM).

46
Pulse Code Modulation 1

• Based on Nyquist (or sampling) theorem if f(t)
  sampled at rate gt 2signals highest frequency,
  then samples contain all the original signals
  information.
• Example if voice data is limited to 4000Hz, 8000
  samples/sec are sufficient to reconstruct
  original signal.

47
PCM 2

• Analog signal -gt PAM -gt PCM.
• PAM pulse amplitude modulation samples of
  original analog signal.
• PCM quantization of PAM pulses amplitude of PAM
  pulses approximated by n-bit integer each pulse
  carries n bits.

48
Delta Modulation (DM)

• Analog signal approximated by staircase function
  moving up or down by 1 quantization level every
  sampling interval.
• Bit stream produced based on derivative of analog
  signal (and not its amplitude) 1 if staircase
  goes up, 0 otherwise.
• Parameters sampling rate and step size.

49
Analog-to-Analog Encoding

• Combines input signal m(t) and carrier at fc
  producing s(t) centered at fc.
• Why modulate analog data?
• Shift signals frequency for effective
  transmission.
• Allows channel multiplexing frequency-division
  multiplexing.
• Modulation techniques AM, FM, and PM.

50
Amplitude Modulation (AM)

• Carrier serves as envelope to signal being
  modulated.
• Signal m(t) is being modulated by carrier cos(2p
  fct).
• Modulation index ratio between amplitude of
  input signal to carrier.

51
Angle Modulation

• FM and PM are special cases of angle modulation.
• FM carriers amplitude kept constant while its
  frequency is varied according to message signal.
• PM carriers phase varies linearly with
  modulating signal m(t).

52
Spread Spectrum 1

• Used to transmit analog or digital data using
  analog signaling.
• Spread information signal over wider spectrum to
  make jamming and eavesdropping more difficult.
• Popular in wireless communications

53
Spread Spectrum 2

• 2 schemes
• Frequency hopping signal broadcast over random
  sequence of frequencies, hoping from one
  frequency to the next rapidly receiver must do
  the same.
• Direct Sequence each bit in original signal
  represented by series of bits in the transmitted
  signal.

54
Transmission Modes

• Assuming serial transmission, ie, one signaling
  element sent at a time.
• Also assuming that 1 signaling element represents
  1 bit.
• Source and receiver must be in sync.
• 2 schemes
• asynchronous and
• synchronous transmission.

55
Asynchronous Xmission 1

• Avoid synchronization problem by including sync
  information explicitly.
• Character consists of a fixed number of bits,
  depending on the code used.
• Synchronization happens for every character
  start (0) and stop (1) bits.
• Line is idle transmits 1.

56
Asynchronous Xmission 2

• Example sending ABC in ASCII
• 0 10000010 1 0 01000010 1 0 110000 1 1111
• Timing requirements are not strict.
• But problems may occur.
• Significant clock drifts high data rate
  reception errors.
• Also, 2 or more bits for synchronization
  overhead!

57
Synchronous Xmission 1

• No start or stop bits.
• Synchronization via
• Separate clock signal provided by transmitter or
  receiver doesnt work well over long distances.
• Embed clocking information in data signal using
  appropriate encoding technique such as Manchester
  or Differential Manchester.

58
Synchronous Xmission 2

• Need to identify start/end of data block.
• Block starts with preamble (8-bit flag) and may
  end with postamble.
• Other control information may be added for data
  link layer.

8 -bit flag
8 -bit flag
Control
Control
Data
59
Data Link Layer

• So far, sending signals over transmission medium.
• Data link layer responsible for error-free
  (reliable) communication between adjacent nodes.
• Functions framing, error control, flow control,
  addressing (in multipoint medium).

60
Flow Control

• What is it?
• Ensures that transmitter does not overrun
  receiver limited receiver buffer space.
• Receiver buffers data to process before passing
  it up.
• If no flow control, receiver buffers may fill up
  and data may get dropped.

61
Stop-and-Wait

• Simplest form of flow control.
• Transmitter sends frame and waits.
• Receiver receives frame and sends ACK.
• Transmitter gets ACK, sends other frame, and
  waits, until no more frames to send.
• Good when few frames.
• Problem inefficient link utilization.
• In the case of high data rates or long
  propagation delays.

62
Sliding Window 1

• Allows multiple frames to be in transit at the
  same time.
• Receiver allocates buffer space for n frames.
• Transmitter is allowed to send n (window size)
  frames without receiving ACK.
• Frame sequence number labels frames.

63
Sliding Window 2

• Receiver acks frame by including sequence number
  of next expected frame.
• Cumulative ACK acks multiple frames.
• Example if receiver receives frames 2,3, and 4,
  it sends an ACK with sequence number 5, which
  acks receipt of 2, 3, and 4.

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Sliding Window 3

• Sender maintains sequence numbers its allowed to
  send receiver maintains sequence number it can
  receive. These lists are sender and receiver
  windows.
• Sequence numbers are bounded if frame reserves
  k-bit field for sequence numbers, then they can
  range from 0 2k -1 and are modulo 2k.

65
Sliding Window 4

• Transmission window shrinks each time frame is
  sent, and grows each time an ACK is received.

66
Example 3-bit sequence number and window size 7

• A B
• 0 1 2 3 4 5 6 7 0 1 2 3 4... 0 1 2 3 4 5
  6 7 0 1 2 3 4

0
1
2
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
RR3
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
3
0 1 2 3 4 5 6 7 0 1 2 3 4
4
5
0 1 2 3 4 5 6 7 0 1 2 3 4
RR4
6
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
67
Digital/Analog Encoding
Encoding
g(t)
g(t)
(D/A)
Encoder
Digital Medium
Decoder
Source
Destination
Source System
Destination System
Modulation
g(t)
g(t)
(D/A)
Modulator
Analog Medium
Demodulator
Source
Destination
Source System
Destination System
68
Encoding Considerations

• Digital signaling can use modern digital
  transmission infrastructure.
• Some media like fiber and unguided media only
  carry analog signals.
• Analog-to-analog conversion used to shift signal
  to use another portion of spectrum for better
  channel utilization (frequency division muxing).