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Assignment 2
Data Communication
COIT 12170
C01069988
20/5/02

Part C

18. AM- Amplitude modulation changes the amplitude or volume of the signal.
The height of the sine wave varies between a one and a zero. If you were to
listen to the transmission, the frequency or pitch of the signal would not
alter, but a one would sound louder than a zero. The ones and zeros are
generated by shifting the amplitude. AM is never used alone in modems, but
sometimes a combination of AM and PM is used.

FM- Frequency modulation alters the frequency of the sine wave.
The amplitude is constant. The signal would sound high-pitched for ones and
low-pitched for zeros. Data is transmitted by shifting the frequencies.

PM- Phase modulation uses a change in the phase of the sine wave
to transmit data. The phase of a signal is particular point it has reached in
its cycle. A phase shift occurs when the sine wave is broken abruptly and
started several degrees forward or backward of its previous phase. It offers
best of both AM and FM techniques, as only two frequencies are required for
full duplex transmission and it is not affected by small amplitude variations.

19. Differential Phase Shift Keying is a modem modulation technique. It uses
the change of data to change the phase. Change of data controls the phase. A
phase shift no longer indicates a bit or two bits but indicates a data
change. With two-phase a phase shift indicates a data change from a zero to a
one, or a change from a one to a zero It is differential because the phase
transmitted depends on the previous state. Synchronisation is easier but the
initial state must be known. Clever synchronisation techniques let the
receiver know the initial state.



20. AMI violations are exceptions to the AMI encoding which deal explicitly
with the possibility of synchronization being lost when a long string of
consecutive 0s are transmitted.
The purpose of AMI violations is to provide synchronization for the receiver
when long strings of 0s are sent.

To avoid DC buildup, each set of 8 consecutive 0s has the B8ZS Binary Eight
Zero Substitution algorithm applied to it.:

The 4th bit time has the same polarity as the last data 1- bit sent (even if
it belonged to another earlier group of 8 bits).

Immediately following this in the 5th bit time a mark (logical 1) of the
opposite polarity is sent.

In the 7th bit time another mark is sent with the same polarity as Bit5. .

In the 8th bit time another mark is sent but with the opposite polarity to
Bit8.

In each group of consecutive 0s, there are 2 AMI violations.

By having opposite polarity the DC buildup problem can be minimized.

21.B-ISDN stands for broadband ISDN. Generally 2Mbps or greater.

Advantages of B-ISDN:
- greater data rates.
- multiple traffic types (voice, video and data)
- only charged only for data sent



22. The cellular system (such as AMPS) can provide services to many more
customers then the pre-cellular systems (such as MPS and IMPS) due to
following:

- Integrated Circuits(ICs)
-
Capture effect of Frequency Modulation (weak and strong FM signals do
not intefere with each other, instead the strong signal is captured and the
weak signal is ignored. Signal strength falls off as 1/ r4 - minimum S/I ratio
is 17 dB (or 50 times). This allows frequencies to be reused.

Frequency reuse. MTS (Mobile Telephone Service) used a high-power
centralised transmitter/receiver covering a wide area. Cellular systems use
much lower power and cover considerably less area per cell. Having many low-
powered cells allows frequency reuse. Allows many users to employ the same
frequency. While MTS allowed only a limited number of customers, cellular
allows many times more.However cells using the same frequency must be
separated by a sufficient distance.To maximize the number of channels that can
be used by each cell, we should place cells using the same frequencies as
close together as possible although there are limitations on how close that
can be.



23.
Physical Implementation

FSK 2-PSK QPSK QAM
Different audio frequencies are transmitted for logic “0” and different for
logic “1”
Carrier frequency is higher for logic “0” and lower for logic “1” Sine
wave generator output is sent to two buffers, one inverting and the other non-
inverting. The buffer outputs are 180 degrees out of phase. An electronic
switch switches ;between two phases. Which phase is selected depends on
whether a logic “1” or logic “0” is to be sent. If “1” is to be sent, the 180
degree shifted output is selected ; if “0” is to be sent, the zero degree
phase shifted output is selected Splits the signal into four phases,
allowing a single frequency to take any of four values :90,180, 270 or 360. A
single frequency can carry two bits of data. Combines techniques of AM and
PM to enable a signal to carry more than one bit per cycle. Thus we are able
to represents three bits per cycle.
The four phases are”
01 = 0° phase shift
02 = 90° phase shift
03 = 180° phase shift
04 = 270° phase shift
The two amplitudes are:
A1 = low amplitude
A2= high amplitude
Combining the four phase shifts with the two amplitude shifts results in the
following combinations, each representing a unique three bit pattern of data:
A1 01 = 000
A2 01 = 001
A1 02 = 010
A2 02 = 011
A1 03 = 100
A2 03 = 101
A1 04 = 110
A2 04 - 111
By combining two amplitude shifts with eight phase shift e can represent four
bits per cycle.






Achievable Data Rates

FSK 2-PSK QPSK QAM
Bell Standard 103-300 bps

Bell 202- 1,200 bps V.22 – 1.200
V.29 – 4.800
Bell 212 (V.22)-1200bps

Bell 201(V.26)-
2400bps
Bell 209 (CCITV V.32)-9600 bps

V.33 – 14.400 bps



Advantages

FSK 2-PSK QPSK QAM
Bell 103- full duplex

Bell 202- much faster than 103 For 2 PSK the bit rate is equal to the Baud
rate. Narrow bandwidth, it resulted in lower system noise. Good for use in
cellular and Personal Communication Service as well as satellite and deep
space probe applications. For each baud we send two bits. Bit rate is
twice the baud rate (faster than 2-PSK) Without transmitting signals at a
faster rate we are able to carry more data by carrying multiple bits per cycle.


Disadvantages

FSK 2-PSK QPSK QAM
Bell 103- very slow bit rate

Bell 202 – half duplex

Both methods quickly ran into the PSTN bandwidth limiting the data rate. The
data rate is the same as the baud rate.
Too slow for current standards Still to slow High speed modems are
more sensitive to imperfections in the communication circuit they use to carry
their signals


24. • DS-1 Developed by AT&T consists of 24 DS-0 channels. This service can be
set up either as 24 voice lines, or it can be set up to carry 23 channels of
computer data rated at 64,000 bits per second. The twenty-fourth channel is
used for signaling. Hence 24 low-speed channels combined together give 1544
kbps rate for DS 1.

In Europe CCITT was producing its own long-distance digital telephone
service. AT&T and CCITT went their own ways. There are many differences
between the American T1 and the European version of this service, know as E-1.
The equivalent rate for E-1 system is 2048 kbps Although E-1 uses the same
frame frequency—8,000 a second, it has more channels than its American
equivalent. E-1 is based on 32 channels instead of 24 as in T1. Like T1,
information is transmitted in frames, but they are 256 bits long. This gives E-
1 a nominal bandwidth of 2048 kbps per second.



25. IS-95 Interim Standard-95 is a digital cellular CDMA (Code Division
Multiple Access) communications standard used in North America.


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