University of Delaware CPEG 4191 CPEG 419 Introduction to Networks [Week 2]

University of Delaware CPEG 419 1

CPEG 419Introduction to Networks

[Week 2]


Administrative Issues

Homework #1 assigned. Due in 2 weeks.


Transmission Impairments

Types of impairments: Attenuation. Delay distortion. Noise. Multi-path Fading (wireless only).


Attenuation

Weakening of the signal’s power as it propagates through medium.

Function of medium type Guided medium (wired): logarithmic

with distance. Unguided medium (wireless): more

complex (function of distance and atmospheric conditions).


AttenuationProblems and solutions:

Insufficient signal strength for receiver to distinguish between the signal and noise: use amplifiers/repeaters to boost/regenerate signal.

Attenuation increases with frequency: special amplifiers to amplify high-frequencies (equalization).


Attenuation

f

ff T

RA log10

Let Rf be the received signal power at frequency fLet Tf be the transmitted signal power at frequency f

The attenuation in dB is:


Delay Distortion

Speed of propagation in guided media varies with frequency. Different frequency components arrive

at receiver at different times (more about this later).

Solution: equalization techniques to equalize

distortion for different frequencies. Use fewer frequencies.


Noise

Noise: undesired signals inserted anywhere in the source/destination path.

Different categories: thermal (white), crosstalk, impulse, etc.

transmitter +attenuation

noise

received signal is an attenuated version of the transmitted signal plus noise.


Thermal Noise

Any conductor and electronic device has noise due to thermal agitation of electrons

The thermal noise found in 1Hz isN = k T (W/Hz)

k = 1.3 e –23 (Boltzmann’s constant)T is the temperature in KelvinN is noise power in watts per 1Hz of bandwidth (dBW)

Total noise is N = k T B

B is total bandwidth.


Crosstalk Wires act as antennas. They broadcast energy when the

signal switches and receive energy for any other source (e.g., other wires, radios, microwave ovens, the big bang, etc.).

Crosstalk can be reduced by careful shielding and using twisted pairs.

The longer the wires, the more significant the crosstalk.

f

ff O

SC log10Crosstalk gain is

power at other wires

power found on the wire of interest

Suppose that –10 dBW is transmitted on other wires.And the crosstalk gain is 3. Then the noise received had power is –7 dBW.


Other noises

Coupling through common impedance (power supply noise). This is a major source at the transmitter and receiver.

Galvanic Action. Dissimilar metals and moisture produce a chemical wet cell (battery).

Triboelectric effect from bends in cable.Shot Noise. Present in semiconductors.Contact noise. Due to imperfect contacts.Popcorn noise. Minor defects in junction in a

semiconductor, often due to metallic impurities.


Decibel and Signal-to-Noise Ratio

Decibel (dB): measures relative strength of 2 signals. Example: S1 and S2 with powers P1 and P2.

NdB = 10 log10 (P1/P2)

Signal-to-noise ratio (S/N): Measures signal quality. S/NdB = 10 log10 (signal power/noise power)


SNR

Crosstalk-nAttenuatio

Crosstalkpower dtransmitte10log-nAttenuatiopower dtransmittelog10

log10Signal Receivedlog10Signal Received

log10

Noise

NoiseSNR

This depends on the cable.Furthermore, it may not be possible to transmit at such a high power that other noises can be neglected.

Suppose that we transmit at a very high power, so thermal and other noises are small compared to crosstalk.


SNR=13

5 4 3 2 1 0 1 2 3 4 5

0

2

0 1 110 0110

0.5 times the bit-rate5 4 3 2 1 0 1 2 3 4 5

0

2

0 1 110 0110

0.75 times the bit-rate

5 4 3 2 1 0 1 2 3 4 5

0

2

0 1 110 0110

1 times the bit-rate5 4 3 2 1 0 1 2 3 4 5

0

2

2 times the bit-rate

0 1 110 0110


Multi-path Fading (wireless)

Because of reflections, a signal may take many paths from transmitter to receiver.

Objects such as buildings, people, etc.

transmitter

receiver

Signals that take alternative paths will arrive later.


Multi-path reflection or delay spread (wireless)

2 1 0 1 2 30.5

0

0.5

1

.f( )t .6 ( ).f( )t D .3 .f( )t .1.5 D .1

f( )t

f( )t D

f( )t .1.5 D

.5

t

received signal

line of sight signal late arriving signals

getting small

At 10Mbs, if the difference in paths is 30 meters, then the alternative signals arrive at exactly the next slot. (Use the fact that light travels a 300000000 m/s.


Channel Capacity 1

Channel Capacity is the rate at which data can be transmitted over communication channel.

We saw earlier that to send a binary data at a rate R, the channel bandwidth must be greater than ½ R.

So, if the bandwidth of the channel is B, it might be possible to transmit at a rate of 2B.


Channel Capacity 2For a fixed bandwidth, the data rate can

be increased by, increasing number of signal levels. However, the signal recognition at receiver is more complex and more noise-prone.

The data rate becomes C = 2B log2V, where V is number voltage levels.

Is it possible to continually increase V to make C arbitrarily large?


Channel Capacity 3

Noisy channel: Shannon’s Theorem Given channel with B (Hz) bandwidth

and S/N (dB) signal-to-noise ratio, C (bps) isC = B log2 (1+S/N)

Theoretical upper bound since assumes white noise (e.g., thermal noise, not impulse noise, etc).


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 1Twisted 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 (home-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 44. Suppose we transmit at –80dBW. Then the received signal has energy of –124dBW.

The near-end crosstalk is 32dB per 100m. So the crosstalk energy is at –144dBW.

The SNR is 20dB (neglecting thermal noise).


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 Signal Output Signal


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.


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.


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 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 Mbps11 GHz: bandwidth 220MHz, data rate 274

Mbps


Satellite Microwave

1 – 10 GHz (Above 10 GHz, the atmosphere 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.


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 up to 11Mbps 802.11a 5 GHz up to 54Mbps

Infrared – Line of sight, short distances.


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 twisted pair and fiber are used here.

Rural exchange trunks – 40-160km link towns. Twisted pair, 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

Transforming original signal just before transmission.

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


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).


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).


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-Digital Encoding Techniques

Nonreturn to Zero (NRZ)Multilevel BinaryBiphaseScrambling


NRZ Techniques

Use of 2 different voltage levels.NRZ-L: positive voltage represents one

binary value; negative voltage, the other.

NRZI (Nonreturn to zero, invert on ones): transition (low-to-high or high-to-low) represents “1”; no transition, “0”.

NRZI is an example of differential encoding: decoding based on comparing polarity of adjacent signal elements.


Multilevel BinaryUse more than 2 signal levels.Bipolar-AMI: “0”: no signal; “1”: positive

and negative pulse; consecutive “1”s alternate in polarity: avoid synchronization loss.

Pseudoternary: opposite representation.Long sequence of 0’s or 1’s still a problem

for bipolar-AMI and pseudoternary respectively.


BiphaseManchester: transition in the middle of bit

period. Carries data and provides clocking. Low-to-high: “1”. High-to-low: “0”.

Differential Manchester: Mid-bit transition only provides clocking. “0”: transition in the beginning of bit interval. “1”: no transition.


Scrambling

Avoid long sequences of 0’s or 1’s.Bipolar with 8-zeros substitution (B8ZS)

Inserts transitions when transmitting 8 consecutive “0”s.

High-density bipolar-3 zeros (HDB3) Inserts pulses when transmitting 4

consecutive “0”s.Receiver must recognize insertions and

re-generate original signal.


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

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University of Delaware CPEG 4191 CPEG 419 Introduction to Networks [Week 2]