University of Delaware CPEG 4191 Transmission Media Chapter 4 zPhysically connect transmitter and...

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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).


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?


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

–144dBW20dB


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.


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.


Examples 3 – types of fiber

Step-index multimode

higher index of refraction

lower index of refraction

total internal reflection

absorbed

longer path

shorter path

Since the signal can take many different paths, the arrival the received signal is smeared.

0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 1.20

0.5

1

1.5

f( )t

t 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 1.20

20

40

g( )t

t

Input Pulse Output Pulse



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)



Even for single mode fiber, a pulse gets smeared.

0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 1.20

0.5

1

1.5

f( )t

t 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 1.20

20

40

g( )t

t

Input Pulse Output Pulse

Solitons are a particular wave pulse that does not disperse.


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)


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.


Fiber vs. Cable

Fiber is light and flexible.Fiber has very high bandwidth.Fiber is difficult to install (I can’t do

it).Fiber interfaces are more expensive

than cable (?)


Wireless Transmission


Electromagnetic Spectrum

UV

1016 1022

X-ray

Gamma-ray

Cell phones put out 0.6 – 3 watts. Light bulbs put out 100 watts.


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.


Terrestrial MicrowaveParabolic 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 Mbps11 GHz: bandwidth 220MHz, data rate 274

MbpsThe M in MCI is for microwave


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.


Low-Earth Orbit Satellites (LEO)

Iridium The idea of some executive’s wife while vacationing in the tropics and her cell phone didn’t 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.


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.


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.


Spectrum Allocation

Acrobat Document


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.


Data Encoding (Chap. 5)

Transforming original signal just before transmission.

Both analog and digital data can be encoded into either analog or digital signals.


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.


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).


Approach 1: NRZ

Switch when a 1 occurs

But how do you know when to sample?Phase-locked-loop (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.


Multilevel Binary

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

opposite direction


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) don’t occur.


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


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


Methods to Encode Digital Signals

NRZMultilevel binaryManchester

Issues: DC? Self Clocking? How big is the spectrum?


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.


Modulation Techniques

ASK

FSK

PSK

0bit when 0

1bit when2cos tfAts c

0bit when2cos

1bit when2cos

1

1

tfA

tfAts

0bit hen w2cos

1bit when2cos

1tfA

tfAts c


Phase-shift Keying

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

3 sending when4/72cos




tfA

tfA

tfA

tfA

ts

c

c

c

c

0

90

180

270


QAM - Quadrature Amplitude Modulation

0

90

180

270

QAM-16(16 levels, how many bits)

constellation diagrams

0

90

180

270QAM - 64


V32

128 bits: 6 data and 1 parity (error correction)


How fast is V32?The phone system transmits 300 to 3400 Hz

So what bandwidth can we use. How fast can we send symbols?

Use 2400 sample each way - duplex

Definition: a duplex connection means that we can send data in both directions at the same time.A simplex or half-duplex connection only sends data in one direction at a time.

So 2400 * 6 = 14400 bps

What is the baud rate?

V.34 2400 baud - with 12 data bits/symbolV.34 2400 baud – with 14 data bits/symbolThat’s the fastest there is!

To get 56K you send at 4000 baud (if the phone system can handle it)


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 can’t get a very high data rate because…?

The DSL standard goes up to 8 Mbps down and 1 Mbps up.


DSL

voice(channel 0)

empty

A total of 256 4kHz channels

25kHz (channel 6)

Upstream

downstream

250 parallel channels: Each data channel uses QAM 16 (with 1 parity bit).

What is the maximum data rate?

The quality of each channel is monitored and adjusted. So channels may transmit at different speeds


Digital Transmission: Receiver-Side Issues

Clocking: determining the beginning and end of each bit. Transmitting long sequences of 0’s or

1’s 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.


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.


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.


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.


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.

"0"),2cos()(

"1"),2cos()(

2

1

tfAts

tfAts


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.

"0"),2cos()(

"1"),2cos()(

tfAts

tfAts

c

c


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).


Pulse Code Modulation 1

Based on Nyquist (or sampling) theorem: if f(t) sampled at rate > 2*signal’s highest frequency, then samples contain all the original signal’s information.

Example: if voice data is limited to 4000Hz, 8000 samples/sec are sufficient to reconstruct original signal.


PCM 2

Analog signal -> PAM -> 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.


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.


Analog-to-Analog EncodingCombines input signal m(t) and carrier

at fc producing s(t) centered at fc.Why modulate analog data?

Shift signal’s frequency for effective transmission.

Allows channel multiplexing: frequency-division multiplexing.

Modulation techniques: AM, FM, and PM.


Amplitude Modulation (AM)

Carrier serves as envelope to signal being modulated.

Signal m(t) is being modulated by carrier cos(2 fct).

Modulation index: ratio between amplitude of input signal to carrier.

)2cos()](1[)( tftmtS cAM


Angle Modulation

FM and PM are special cases of angle modulation.

FM: carrier’s amplitude kept constant while its frequency is varied according to message signal.

PM: carrier’s phase varies linearly with modulating signal m(t).


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


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.


Transmission ModesAssuming 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.


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”.


Asynchronous Xmission 2

Example: sending “ABC” in ASCII0 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!


Synchronous Xmission 1

No start or stop bits.Synchronization via:

Separate clock signal provided by transmitter or receiver; doesn’t work well over long distances.

Embed clocking information in data signal using appropriate encoding technique such as Manchester or Differential Manchester.


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 -bitflag

8 -bitflag

Control ControlData


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).


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.


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.


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.


Sliding Window 2

Receiver ack’s frame by including sequence number of next expected frame.

Cumulative ACK: ack’s multiple frames.Example: if receiver receives frames

2,3, and 4, it sends an ACK with sequence number 5, which ack’s receipt of 2, 3, and 4.


Sliding Window 3

Sender maintains sequence numbers it’s 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.


Sliding Window 4

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


Example: 3-bit sequence number and window size 7

A B0 1 2 3 4 5 6 7 0 1 2 3 4... 0 1 2 3 4 5 6 7 0 1

2 3 40

1

20 1 2 3 4 5 6 7 0 1 2 3 4

0 1 2 3 4 5 6 7 0 1 2 3 4RR3

0 1 2 3 4 5 6 7 0 1 2 3 4

3456RR40 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 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 3 4 5 6 7 0 1 2 3 4


Digital/Analog Encoding

Source Destination

Encoder Decoder

Source System Destination System

g(t) g(t)

(D/A)

Source Destination

Modulator Demodulator

Source System Destination System

g(t) g(t)

(D/A)

Digital Medium

Analog Medium

Encoding:

Modulation:


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 mux’ing).

University of Delaware CPEG 4191 Transmission Media Chapter 4 zPhysically connect transmitter and...

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