## The Physical Layer - Chapter 2

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Theoretical Basis for Data Communications
Information can be transmitted by varying some physical property
such as voltage or current.

* See pages 77-80.

The Maximum Data Rate of a Channel

As early as 1924, H. Nyquist realized the existence of the fundamental
limit and derived an equation expressing the maximum data rate for
a finite bandwidth noiseless channel. In 1948, Claude Shannon carried
Nyquist's work further and extended it to the case of a channel subject
to random (that is, thermodynamic) noise.

Nyquist's Theorem

Nyquist proved that if an arbitrary signal has been run through a
low-pass filter of bandwidth H, the filtered signal can be completely
reconstructed by making only 2H (exact) samples per second. Sampling
the line faster than 2H times per second is pointless.

If the signal consists of V discrete levels, Nyquist's theorem states:

Maximum data rate = 2H log2 V bits /sec (log base 2 of V)

For example, a noiseless 3-KHz channel cannot transmit binary
(i.e., two-levels) signals at a rate exceeding 6000 bps.

Shannon's Law

So far we have considered only noiseless channels. If random
noise is present, the situation deteriorates rapidly. The amount
of thermal noise present is measured by the ratio of the signal
power to the noise power, called the signal-to-noise ratio. If we
denote the signal power by S and the noise power by N, the signal-
to-noise ratio is S/N. Usually, the ratio itself is not quoted;
instead, the quantity 10 log10 S/N is given. These units are
called decibels (dB). An S/N ratio of 10 is 10 dB, a ratio of
100 is 20 dB, a ratio of 1000 is 30 dB and so on.

Shannon's major result is that the maximum data rate of a noisy
channel whose bandwidth is H Hz, and whose signal-to-noise ratio
is S/N, is given by

Maximum number of bits /sec = H log2 (1 + S/N)

For example, a channel of 3000-Hz bandwidth, and a signal to
thermal noise ratio of 30 dB (typical parameters of the analog
part of the telephone system) can never transmit much more than
30,000 bps, no matter how many or few signal levels are used
and no matter how often or how infrequent samples are taken.
It should be noted, however, that this is only an upper bound
and real systems rarely achieve it.

Example,

Assume that a communication line has a bandwidth of 3000 Hz and
a typical signal-to-noise ratio of 20 dB and hence the maximum
theoretical information rate that can be obtained is derived as
follows:

20 = 10 log10 (S/N)

Therefore,

S/N = 100

and

C = 3000 log2 (1+100)

that is

C = 19963 bps.

Nyquist's result is rather open ended. We can increase the data rate
indefinitely by increasing the encoding (number of voltage levels).

Shannon's result is independent of the number of voltage levels
and therefore gives us an absolute upper bound.

Transmission Media

The purpose of the physical layer is to transport a raw bit stream
from one machine to another. Various physical media can be used for
the actual transmission. Each one has its own niche in terms of bandwidth,
delay, cost, and ease of installation and maintenance. Media are grouped
into guided media, such as copper wire and fiber optics, and unguided media,
such as radio and lasers through the air.

Magnetic Media

One of the most common ways to transport data from one computer to another
is to write them onto magnetic tape or floppy disks, physically transport
the tape or disks to the destination machine, and read them back again.
While this method is not as sophisticated as using a geosynchronous
communication satellite, it is often much more cost effective, especially
for applications in which high bandwidth or cost per bit is the key
factor.

Example,

Assume an industry standard 8-mm tape can hold 7 gigabytes.
A box 50*50*50 cm can hold about 1000 of these tapes, for a capacity
of 7000 gigabytes. A box of tapes can be delivered anywhere in the
United States in 24 hours by Federal Express. The effective bandwidth
of this transmission is 56 gigabits/86400 sec or 648 Mbps, which is slightly
better than the high-speed version of ATM (622 Mbps). If the destination
is only an hour away by road, the bandwidth is increased to over 15 Gbps.

Twisted Pair (see page 83)

a.	Open Wire: Attached to ceramic or glass insulators.
They are still in use in parts of the US and in other parts
of the world, particularly when traffic is slow.

Picture:

b.	Twisted Wire Pair Cables: Consist of copper wires insulated
by plastic or other meterials and twisted into pairs.
There are many considerations in the construction of such
cables; it is important, for instance, to minimize cross talk
(the undesirable detection of a signal from one pair on another)

Picture:

. The most common application of twisted pair is the telephone
system

. Twisted pairs can run several km without amplification;
but for longer distance, repeaters are needed.

. Twisted pairs acn be used for either analog or digital
transmission.

. The bandwidth depends on the tickness of the wire and the
distance traveled.

. Several megabits/sec can be achieved for a few km.

Coaxial Cables

Picture:

a.	Single Wire Conductor
b.	Cylindrical Conductor
c.	Insulator
d.	protective jacket

Two kinds of Coaxial cables are used:

a. 50-ohm cable is used for digital transmission
b. 75-ohm cable is used for analog transmission

. Current tends to flow on the inside of the outer shell.
This gives more surface area

. Since current flows on the inside of the outer shell,
signal is shielded from noise & crosstalk

. Can transmit at higher frequencies.

. The construction and shielding of the coaxial cable give it a
good combination of high bandwidth and excellent immunity.
The bandwidth possible depends on the cable length. For
1-km cables, a data rate of 1 to 2 Gbps is feasible. Longer
cables can also be used, but only at lower rates or with
periodic amplifiers. Coaxial cables used to be used within
the telephone system but have now largely been replaced
by fiber optics on long-haul routes.

. There are two ways to connect computers to coaxial cables:

a. T junction

b. Vampire Tap

. There are three coding techniques (see pages 279-280):

a. Straight Binary

b. Manchester encoding
A bit period is divided into two equal intervals.
A binary 1 bit is sent by having the voltage set high
during the fist interval and low in the second one.
A binary 0 is just the reverse: first low and then high.
This scheme scheme ensures that every bit period has a
transition in the middle, making it easy for the receiver
to synchronize with the sender. A disadvantage of manchester
encoding is that it requires twice as much bandwidth as
straight binary encoding, because the pulses are half the width.

c. Differential Manchester encoding, is a variation of basic
Manchester encoding. In it, 1 bit is indicated by the absence
of a transition at the start of the interval. In both cases,
there is a transition in the middle as well. The differential
scheme requires more complex equipment but offers better noise
immunity.

a. Moderate cost

b. Low bit error rate

c. High bandwidth

d. Multidrop configuration

e. Security

Baseband Coaxial Cable (see page 84)

. Single square wave digital signal placed directly on the cable.

. Signals propagate in both directions

. Can support data rates of 50 Mbps over distances of a few miles.

. Error rates: 1 bit in 10^7 and 1 bit in 10^8
(in practice appears error free)

Broadband Coaxial Cable (see page 85)

. Directional broadcast transmission using FDM to devide cable
into a number of independent channels

. Supports multiple channels of 12 Mbps with error rate like baseband

. Broadband systems are divided up into multiple channels, frequently
the 6-Mhz channels used for television broadcasting. Each channel
can be used for analog television, CD-quality audio (1.4 Mbps),
or a digital bit stream at, say, 3 Mbps, independent of the others.

. Broadband can reach up to 450 Mhz and can run over 100 km.

. To transmit digital data over a Broadband network, we convert
binary to analog.

. Typically, a 300 MHz cable will support a total data rate of 150 Mbps.

Satellites

. Use Geo-Stationary (Rotating at the same rate as earth) satellites.

. Orbit at about 23,300 miles (36,000 Km). Generally in plane of
the equator

. Each satellite incorporates a number of transponders which receive
earth-origin signals in one frequency range and broadcast back
to earth in a second frequency range.

. Frequencies

a. C-band (most)
3.7   - 4.2 Ghz down
5.925 - 6.425 GHz up
(4/6 GHz)

b. Ku-band
10-15 Ghz
(12/14 GHz)

c. High frequencies: 20/30 GHz

. Most satellites have between 10 and 25 transponders, each having a
bandwidth of 30-50 MHz.

. Propagation delay of 0.24 - 0.27 second
(Most ground links have delays in the micro to msec range)

. Error rates

1 bit in 10^6 to 10^7 bits 95% of the time

1 in 10^4 during poor conditions (e.g., heavy rain)

. Limitations

. Satellites transmitting at same frequencies can not be closer
than 4 degrees (90 satellites) or 8 degrees for TV satellites
(due to higher power). High band (12/14 GHz) can be spaced as
close as 1 degree.

. Longer delay than ground links:
Satellites: 250-300 msec
coaxial cables have a delay of about 5 micro sec / km

. Typically use TDM

. Communication Satellites (page 163)

. Low-Orbit Satellites (page 167)

. Satellites versus Fibers (page 168)

Fiber Optics (see page 87-91)

. Transmitting data by pulses of light.

. A light pulse can be used to signal a 1, the absence of pulse
signal a 0 bit.

. Bandwidth of 10^8 Mhz is possible

. An optical transmission system has three components:

1. The transmission medium: cylinder of glass surrounded
by a substance of low refractive index

. Components: thin fiber of glass or fused silica
Glass of lower refractive index
absorpative jacket

2.  The light source. Converts electrical signal to light pulses

. LED (Light Emitting Diode)

. Laser diode

3. The detector. Is a photodiode, which generates an electrical
pulse when light falls on it.

. Types of fibers

1. Large core fiber

. refractive index is variable accross the cross section of glass,
causing rays to be continually refocused as they travel down the
fiber.

. Rays speed up as they pass from higher to lower refractive index

. All rays arrive at end at approximately the same time.

. It is called mulltimode fiber.

. Problem of dispersion

. Limits transmission capacity

. Sources of the problem

1. Rays can travel different paths.
Rays traveling a straight line will reach end faster
than one experiencing many reflections.

2. Different frequencies have different velocities in glass.

. A Comparison of Laser and LEDs as light sources

ITEM		LED		Semiconductor Laser
--------------------------------------------------------------

Data Rate		Low		High

Mode			Multimode	Multimode or Single mode

Distance		Short		Long

Temperature sensitivity	Minor		Substantial

Cost			Low cost	Expensive

---------------------------------------------------------------

. Techniques to lesson dispersion

1. Thinner Fibers

3. Small core fiber

. Core is made small enough to allow propagation in only
1 electronic mode (traveling as axial rays only). It is
called single mode fiber

. Single mode fiber requires (expensive) laser diodes to derive
them, rather than (inexpensive) LEDs, but they are more
efficient and can be run for longer distances.

. Currently available fiber systems can transmit data at about
1000 Mbps and with powerful lasers a transmission over 100 km
long without repeaters.

. Characteristics of Optical Fibers

1. Small size

2. Light weight

3. Very wide bandwidth (excess of 1 Gbps)

4. Low cost potential

5. Interference and noise immunity

6. Complete electrical isolation

7. Greater repeater spacing

8. Low crosstalk

9. Security

10. Error rates on the order of 1 in 10 ^9 bits

11. Usually used in point-to-point and rings

Fiber Optic Networks (see page 91)

Fiber optics can be used for LANs as well as for long-haul
transmission, although tapping onto it is more complex than
connecting to an Ethernet. One way around the problem is to
realize that a ring network is really just a collection

Comparison of Fiber Optics and Copper Wire(see page 92)

The Electromagnetic Spectrum (pages 94-97)

Microwave Transmission (page 98)

Infrared and Millimeter Waves (page 100)

The Telephone System (Page 102)

The telephone system is organized as a highly redundant, multileveled
hierarchy. Each telephone has two copper wires coming out of it
that go directly to the telephone company's nearest END OFFICE
(also called a local central office). The distance is typically 1
to 10 km.

In the United States alone there are about 19,000 end offices.
The concatenation of the area code and the first three digits of
the telephone number uniquely specify an end office. The two-wire
connecting between each subscriber's telephone and the end office
are known as the local loop. If the world's local loops were stretched
out end to end, they would extend to the moon and back 1000 times.

.Types of nodes (offices)

Class#5 Central Office (End Office)

Class#4 Toll Center

Class#3 Primary Center

Class#2 Sectional Center

Class#1 Regional Center
AT&T has 7 regional centers.

In addition to tandem center in large cities. It is unfeasible
to connect all Central Offices together.

. In summary, the telephone system consists of three major components:

1.	Local loops (twisted pairs, analog signaling)

2.	Trunks (fiber optics or microwave, mostly digital).

3.	Switching offices

Transmission Impairments

Analog signaling consists of varying a voltage with time to represent
an information stream. If transmission media were perfect, the receiver
would receive exactly the same signal that the transmitter sent.
Unfortunately, media are not perfect, so the received signal is not
the same as the transmitted signal. For digital data, this defference

Transmission lines suffer from three major problems:

1.	Attenuation: is the loss of energy as the signal propagates
outward (decibels)

2.	Delay distortion: It is caused by the fact that different
Fourier components travel at different speeds.

3.	Noise: which is unwanted energy from sources other than
the transmitter.

Transmission Techniques (Page 110)

1.	Baseband Signaling:

. Simply a train of pulses

. Problem: Square waveform is lost due to capacitance and
inductance effects

. Good for slow speeds and short distances.

2.	Amplitude Modulation (AM)

. Continous tone in 1000-2000 hz range (sine wave carrier)

. Amplitude of this tone is modulated to introduce information

3.	Frequency Modulation (FM)

4.	Phase Modulation (PM)

5.	Combination of AM, FM, and PM

. Example, 4 phases and 2 amplitudes give 8 levels. 3 bits
encoded per signal

Modulation

.The process used to "imprint" the binary (square-edged) values
on an analog signal is called MODULATION.

.Essentially, modulation is the modufication of some characteristics
of an otherwise continuous carrier signal and is the whistle that is
often heard on a simple acoustical modem.

Signaling Speed

Number of times per second a signal changes its value- measured
in units called BAUD.

If each state represents a 0 or 1, then baud is the same as bits/second.

If there are four possible states, then one line condition change of
state represents a dibit in which case n bauds is equal to 2*n bits/sec.

Signaling Techniques

An electronic signal is used to move information from one computer
system to another or from a computer to a terminal device.
Different states, often two, can be represented by the modification
of a particular signal; these modified signals are then interpreted
as a zero or one.

Analog Signal

. Continuously varying signal

. A sine wave is an analog signal

. The frequency of a signal, is the number of cycles per second.
Example, a frequency of 1100 hertz has 1100 such cycles occuring
in a period of one second.

Digital Signal

. Propagation of information via discrete signal levels.

. Square wave is a digital signal

. Repeaters can restore the original digital signal

. Typically have higher data rates, and provide more multiplexing
potential.

Types of signals and transmission systems

1.	Analog signal over analog system

2.	Analog signal over digital system

3.	Digital signal over digital system

4.	Digital signal over analog system

Analog to Digital (Digitizing)

1. Delta Modulation (page 123)

Output 1 if current signal >= previous signal

Output 0 otherwise.

. Small levels of speech output result in sampling error

. Slope limiting rapid changes of waveform can not be kept up with

. Simple and cheap.

2. Pulse-Code Modulator (PCM)

. CODEC (Coder-Decoder). digitizes the analog signal

. CODEC makes 8000 samples/sec (it samples every 125 micro seconds)

. It uses 128 levels (7 bits are used)

. 7 * 8000 = 56000 bps is required to transmit the digital signal.

. T1 carrier consists of 24 channels multiplexed together. Each of the
24 channels, in turn, gets to insert 8 bits into the output stream.
7 bits are data, and one is for control, yielding 7*800 = 56000bps
of data, and 1 * 8000 = 8000 bps of signaling information per channel.

. A frame consists of 24 * 8 = 192 bits, plus one extra bit for framing,
yielding 193 bits every 125 micro seonds. This gives a gross data rate
of 1.544 Mbps.

Multiplexing Techniques

1.	Frequency Division Multiplexing (FDM)

. Partitions a limited bandwidth channel into a group of independent
lower speed channels, each using a permanently assigned portion of
the total frequency spectrum.

. Guard bands are required between adjacent channels to prevent
electrical overlapping of signals. That means not all the available
bandwidth is used.

. Example, a wide spread standard is 12 4000hz channels (3000 for user
and two guard bands of 500 hz each) multiplexed into the band 60-108
khz. This is called a group.

. Low cost

along a multiplexed channel.

2.	Time Division Multiplexing

.	Creates a permanently dedicated time slot

.	Incoming data from input ports is cyclically scanned,
characters or bits are peeled off, and interleaved into frames
on a single high speed data stream

.	Makes more efficient use of bandwidth than FDM

.	Demultiplexing assumes an implicit relationship between the
output line and the relative position of the time slot
in the arriving frame.

.	Stations need buffers to assemble data.

. Time Division Multiplexing allows multiple T1 carriers to be
multiplexed into higher-order carriers.

4 T1 - T2
(4*1.544 = 6.312 Mbps)

6 T2 - T3 (44.736 Mbps)

7 T3 - T4 (274.176)

3.	Statistical Time Division Multiplexing (STATDM)

.	Unlike TDM, a dedicated time slot is not provided for each
port in the sharing group

.	Time slots are dynamically allocated in a frame to the
currently active users

.	Need buffers

.	Do better when a few users are active.

Switching

Two main switching techniques are used inside the telephone
system: Circuit switching and packet switching

Circuit Switching

When you or your computer places a telephone call, the switching
equipment within the telephone system seeks out a physical path
technique is called circuit switching.

In the early days of the telephone, the connection was made by
having the operator plug a jumper cable into the input and output
sockets.

An important property of circuit switching is the need to set up
an end-to-end path before any data can be sent.

Message Switching

An alternative switching strategy is message switching. No physical
when the sender has a block of data to be sent, it is stored in the
first switching office (i.e., router) and then forwarded later, one
hop at a time. Each block is received in its entirety, inspected
for errors, and then retransmitted. A network using this technique
is called a store-and-forward network.

With message switching, there is no limit on block size, which means
that routers must have disks to buffer long blocks. It also means
that a single block may tie up a router-router line for minutes,
rendering message switching useless for interactive traffic.

Packet Switching

Packet-switching networks place a tight upper limit on block size,
allowing packets to be buffered in router main memory instead
of on disk. Packet switching networks are well suited to handling
interactive traffic. A further advantage of packet switching over
message swicthing is that the first packet can be forwarded before
the second one has fully arrived, reducing delay and improving
throughput.

A comparision of Circuit-switched and packet-switched networks

Item		Circuit_switched	Packet-swicthed
----------------------------------------------------------------
Dedicated path		Yes			No

Bandwidth available	Fixed			Dynamic

Potentially wasted 	Yes			No
bandwidth
Store-and-forward	No			Yes

Each Packet follows
the same route	Yes			No

Call setup		Required		Not needed

When can congestion 	At setup time		On every packet
occur

Charging		Per minute		Per packet

------------------------------------------------------------------

ISDN (Integrated Services Digital Network) (Page 139-144)

ATM (Asynchronous Transfer Mode).(Page 144-148)

.Packet switching
.Original rate is 155.52 Mbps (capable to tranmit HDTV)

Paging Systems (page 155)

Cordless Telephones (page 157)

Analog Cellular Telephones

Advanced Mobile Phone System (page 158)

Problems (Pages 170-172)

.Numbers 2, 3, 4, 5, 14, 23, and 24)

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Last update April 9, 1998.