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FUNDAMENTAL CONCEPTS

Analog versus Digital Bandwidth (or channel capacity) Switched vs. leased lines Transmission Media Transmission mode (parallel/serial, synchronous/asynchronous Transmission method (simplex, half-duplex, full-duplex)

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Digital or analog channel. . . .

Recall there are 2 types of signals:

digital and analog.

Similarly, there are two types of channels...

Digital Channels

- designed to carry digital signals, i.e. 0's and 1's- signal becomes weak every few miles- need BIT REPEATERS to strengthen it

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Analog Channels- designed to carry analog signals, like speech

- signal again becomes weak every few miles

- need AMPLIFIERS (instead of BIT REPEATERS)

Digital versus Analog

•Digital channels are better.

•Increasingly the phone network is becoming digital.

•More than 90% lines in the US telephone network are digital now (e.g., Sprint is 100% digital).

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Bandwidth of channel

Digital Channel

B.W. (or channel capacity) is defined in Bits/sec. that canflow through it. (Also called DATA RATE.)

Analog Channel

B.W. defined in terms of cycles/sec. (or Hertz) e.g. B.W. oftelephone line to carry speech is approx. 3000 Hz.

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Note:

1) Highest frequency component in speech is about 4000 Hz.

2) Bandwidth required to transmit an analog signal acrossan analog channel is determined by the highestfrequency component in the signal.

3) Rapid changes in signal level correspond tohigh frequency.

gradual changes in signal level correspondto low frequency.

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Electromagnetic Spectrum

Source: Tannenbaum, Computer Networks, 3rd Ed., 1996

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Human ear: 20 Hz to 20,000 Hz.

FM radio: 50 Hz to 15 KHz.

Human voice - 100 Hz to 4000 Hz.

Telephone 300 Hz. To 3 KHz.

Color television: 6 MHz.

Bandwidth

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•Amplitude (volume) is the height of a wave

•Frequency (pitch) is measured in cycles per second

•Phase corresponds to the wave’s relative position

•Bandwidth is the difference between the lowest and highest

frequency (measured in cycles per second or Hz.)

• In general, voice contains many different frequencies.

Properties of Analog Signals

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Switched versus leased. . . .

A channel is switched if it is shared among several users.

It is leased if it is reserved exclusively for a single user.Hence, no switching is necessary.

Your home phone line is a switched channel.

Large companies have leased lines between their variousoffices around the US and the world if the volume oftraffic between these offices is large.

Kai Larsen:

When does switching make sense?

When does leasing make sense?

Kai Larsen:

When does switching make sense?

When does leasing make sense?

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Transmission Media - Copper wire

- Pairs of wire from home to local telephone company office- Limited bandwidth- Deteriorates at high frequency- Ubiquitous on the local loop.- Wires are twisted together in helical pattern to minimize noise and interference.

STP - shielded twisted pairUTP - unshielded twisted pair

Kai Larsen:

Try twisting the power-cord on your clock-radio, check receptions at different levels of twisting!

Kai Larsen:

Try twisting the power-cord on your clock-radio, check receptions at different levels of twisting!

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Transmission media - Twisted Pair Copper

Bandwidth related to distance and thickness but Mbps are possible over small distance.

Two Types commonly used in networks are: Category 3 and Category 5 (UTP).

Most buildings wired before 1988 are Cat 3 (2 or 4 pairs).

After 1988 generally use Cat 5 (more twists per inch, less cross-talk).• Cat 5E

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Outer Jacket Insulation

Shielding Insulation

Conductor

Transmission media - Coaxial Cable

Two types:

1) Baseband.

2) Broadband.

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Baseband Single digital channel. Shielding provides high bandwidth and good noise

immunity. ~2 Gbps on a 1 Kilometer cable. Used for original Ethernet.

Broadband Divides bandwidth into multiple channels Can transmit 300 MHz for long distances (100 km) Uses: TV (6 MHz per channel); CD audio; data.

Transmission media - Coaxial

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Satellites…success and failure

Source: http://www.geom.umn.edu/~worfolk/SaVi/interactive.html

Kai Larsen:

Two Norwegians who started skiing towards the North Pole, 255 km into their 1,741 km long journey… relying only on their Iridium satellite telephone receives the final call… Iridium was going down…

Motorola, however, decided to provided mission critical service…June 3rd 2000 they reached the pole. Ever since, the pole has been without telephone coverage…

Kai Larsen:

Two Norwegians who started skiing towards the North Pole, 255 km into their 1,741 km long journey… relying only on their Iridium satellite telephone receives the final call… Iridium was going down…

Motorola, however, decided to provided mission critical service…June 3rd 2000 they reached the pole. Ever since, the pole has been without telephone coverage…

The Iridium story:Rune Gjeldnes and Torry Larsen attempting to be the first explorers to reach the North Pole completely un-supported. Start weight of sledges: 175 kilos, each… 1741 km long journey…Nine expeditions had tried before…all failed…

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- Transmit signal to satellite 22,230 miles in space (GEOs)- 0.5 - 0.6 sec. delay in transmission.- Various bands between 1 and 40 GHz are used for satellite

transmission.- 600 MHz band at 12 GHz => 200,000 telephone

channels or 100 TV channels.

Other types of satellites: LEO, MEO, HALE.

Transmission media - Wireless Satellite

Kai Larsen:Did you know that:

GEO -> Geo-stationary orbitMEO -> Medium Earth OrbitLEO -> Low Earth OrbitHALE -> High Altitude Long Endurance

Kai Larsen:Did you know that:

GEO -> Geo-stationary orbitMEO -> Medium Earth OrbitLEO -> Low Earth OrbitHALE -> High Altitude Long Endurance

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Transmission media - Microwave

No physical medium - i.e., wireless. Based on radio waves above 100 MHz. Signal is beamed from transmission tower. Microwaves travel in straight lines Usually place 100m towers 50 miles apart. Used for long distance telephone Subject to weather interference (e.g., rain)

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Phone Coder Light Source

Regenerator Decoder Phone

- Latest technology, current speeds > 10 Gbps- Hair thin strands of glass, called optical fibers- Light pulses travel through high quality glass.- Need repeaters every 20 km.- Single fiber-optic cable carries several 100,000 calls.

Detector

Transmission media - Fiber Optics

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Transmission media - Fiber Optics

Glass Core

Cladding

Plastic Sheathing

Multi-mode Fiber (50 micron core/125 micron cladding)

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Transmission media - Fiber Optics

Glass Core

Cladding

Plastic Sheathing

Single mode Fiber (8/125 microns)

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Transmission Modes I. Mode of Transmission Serial - bit at a time

orParallel - Byte at a time

Serial is the most common mode of transmission.

II. Types of serial transmissions.

1. Synchronous

Synchronization character synchronizes the two ends- then, entire message is sent

8-bitflag

8-bitflag

Controlfields

ControlfieldsData field

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Asynchronous

Send separate start and stop bits with each character

S

T

A

R

T

8-BIT

CHARACTER

S

T

O

P

BLANK

S

T

A

R

T

S

T

O

P

8-BIT

CHARACTER

This is conceptually easier, but overhead is higher

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T r a n s m i s s i o n m e t h o d o n a c h a n n e l . . . .

3 w a y s o f t r a n s m i s s i o n :

1 ) S I M P L E X

Undi rect i onal

2 ) H A L F - D U P L E X

O ne di rect i on at a t i me

O R

3 ) F U L L - D U P L E X

si mul taneous 2- w ay

AND

Kai Larsen:

Think about how the class situation works between professor and students

Kai Larsen:

Think about how the class situation works between professor and students

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CHANNEL AND SIGNAL TYPE COMBINATIONS

Modulating a signal means to adapt it so that it can be transmitted over the type of transmission media in use.

Digital Analog

Digital

Analog

SIGNAL TYPE

Send with slight modification

Digitize signal (Pulse Code Modulation)

Send as is

Convert signal to analog form - Analog Modulation (modem)

CHANNEL TYPE

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Cable Selection Criteria

Bandwidth: How fast must the network be? Budget: How much money can you spend on cabling? Capacity: How much traffic must the network carry? How will

the traffic flow? Environmental considerations: How noisy is the deployment

environment? How important is data security? Placement: Where will the cables run? How tight are the

spaces? Scope: How many devices must be connected to the network? Span: What kind of distance does the network need to span?

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Assignment

Please find a partner or two (or three)Your task:

Select the home of one group member Design a home network around the seven cable

selection criteria Write up your solution on a transparency

Select one person to present solutionTime?