cpeg 419 introduction to networks

University of Delaware CPEG 419 1

CPEG 419Introduction to Networks

[Week 2]


Administrative Issues

Homework #1 assigned.Due in 2 weeks.


ReviewGoal: Send a file from a web server (e.g. yahoo.com) to a web client (e.g. your PC).

Applicatione.g. http server

Transport Layere.g. TCP source

Network Layer: IP

Link Layere.g., CSMA/CD

Physical Layere.g., twisted pair

Network Layer

Link Layer

Physical Layer

Link Layer

Physical Layer

Network Layer

Link Layer

Physical Layer

Applicatione.g. http client

Transport Layere.g. TCP receiver

Network Layer: IP

Link Layere.g., CSMA/CD

Physical Layere.g., twisted pair


Review


Review

Physical (as opposed to logical) Layer – Sends bits from one point to another. Error correction and media access is not an issue.

To transmit at a 10Mbps we need at least 5MHz.

Transmitter Receiver

Wire, fiber, wireless?Noisy, how fast,how to encode?


Review: Multi-level Signals Bit Rate and Baud Rate

The number of bits transmitted can be increased by transmitting more than one bit in one time slot

Baud rate: number of times per second signal changes its value (voltage).

Each value might “carry” more than 1 bit. Example: 8 values of voltage (0..7); each value

conveys 3 bits, ie, number of bits = log2V.Thus, bit rate = log2V * baud rate.For 2 levels, bit rate = baud rate.


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:

A signal is sent at –10dBW.The attenuation is 12dB.

Transmission (dBW) – Attenuation= -10 dBW – 12 dB = -22 dBW.

The received signal has power =


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 is

N = 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

Total noise is N = k T B

B is total bandwidth

Example: 20 degrees C and 1Ghz.

N = k (20+273) 109 = 3.8 10-12 in dBW = 10 log(N) = -114 dBW


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 per km is

power at other wirespower found on the wire of interest

Suppose that –10 dBW is transmitted on other wires.And the crosstalk gain is 30.

better?

Suppose that –10 dBW is transmitted on other wires.And the crosstalk gain is 50. Then the noise received has power =

Transmission Power (dBW) - Gain(dB) = -40 dBW.Then the noise received has power =

Transmission Power (dBW) - Gain(dB) = -60 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

CrosstalknAttenuatio

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


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.


Multi-path reflection (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 signal-to-noise

ratio, C (bps) is C = B log2 (1+S/N)

Theoretical upper bound since assumes white noise (e.g., thermal noise, not impulse noise from crosstalk or multipath reflection ).

Suppose that the noise is –30dBW, the signal strength is at –20dBW and the signal is transmitted with 1MHz bandwidth.The channel capacity is 106 log2 (1 + 10(10/10)) = 106 log2 (1+10) = 3.4 106 bps

How many signaling levels would be required? data rate = 2B log2 (V)3.4 106 bps = 2 106 log2(V)V = 3.4 (?)


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 milliwatts. 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

Acrobat Document


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.


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.

cpeg 419 introduction to networks

Documents

transmitted signal power

noise power

received signal power

boostregenerate signal

signal switches

twisted pair eecs

second signal changes

insufficient signal