Characteristics of Communication Systems 1.

nullpitΔίκτυα και Επικοινωνίες

23 Οκτ 2013 (πριν από 4 χρόνια και 21 μέρες)

80 εμφανίσεις

Characteristics of Communication Systems

1.


System
Name

Main purpose

Administrator

Main Audience

Hardware utilised

Software utilised

Rank (based
on need)

Mobile Phone

-
Verbal Communication

-

Text messaging

-

E
-
mail

-
GPS

-

The user

-

The receiver

-

The
phone

-

Speakers

-

Headphones

-

Phone tower

-

Satellite

-

Phone OS

-

Video player

-

Camera

-

MP3 player

-

Game software

1

ATM

-

Money retrieval and
storage

-

The bank

-

Bank customers

-

ATM machine

-

ATM card

-

ATM’s OS

-

Bank’s money
processing system

2

E
-
mail

-

Text
-
based
communication

-

The user

-

The receiver

-

Hard drive

-

Screen

-

Keyboard

-

Mouse

-

Modem

-

Computer OS

-

E
-
mail program

3


2. Exercise 3.1:

1. All communication systems have 5 basic components:



The
data source

produces the data to be sent.



The
transmitter

encodes the data into a signal suitable for a transmission medium.



The
transmission medium

is a channel, such as a cable, in which the signal is
transmitted to the destination. The signal may be changed or distorted during
transmission.



The
receiver

decodes the signal back into the original data or an approximation of the
data.



The
destination

is the receiver of the information.


2. A protocol is a set of rules that governs the transfer of data between computers.
Handshaking is the process of finding a common protocol to use for transferring data.


3. The OSI reference model divides data communi
cation into several layers. Each layer
expresses the standard, using a protocol. The bottom layers are responsible for transfer of
data from one place to another. The top layers examine the exchange of data between
application programs.


4. Handshaking inv
olves sending signals to indicate the type of protocol to be used. The
transmitting device will send a signal and wait for an appropriate response. When two
devices successfully handshake, then a connection is made.


5. Software flow control uses a special

code sent with the data. It is used for long distance
communication. A common software protocol is XON/XOFF. If a break in transmission is
needed, then the XOFF command is sent. When a transmission is to start again, then the
XON command is sent.


6. Band
width is the capacity of the channel, or transmission medium. A transmission medium
with a high bandwidth can transfer more data, e.g. a TV’s fibre
-
optic cable allows many
channels to be transmitted at the same time.


7. Bits per second (bps) is the maximu
m number of bits that can be transmitted in one second.
The baud rate, on the other hand, is the maximum number of data symbols or electrical
signals that can be transmitted in one second. Because a data symbol can contain more than
one bit of data, the ba
ud rate and the bit rate may be different.


9. Cyclic redundancy check is a method of checking for errors in data transmission using a
division process. The data is divided into predetermined lengths and divided by a fixed
divisor. The remainder of the cal
culation is attached and sent with the data. When the data is
received, the remainder is recalculated. If the remainders do not match, an error in
transmission has occurred.


10. Common parameters used to connect 2 devices include:



Bits per second
is the s
peed of transmission, such as 56000



Data bits

are the number of bits in each group of data. Each data group is usually sent
as a byte, such as 7
-
bit ASCII or an 8
-
bit ASCII.



Parity

is whether the data contains a parity bit for error detection. Parity is odd, even
or none.



Stop/start bits

are the number of stop and start bits used in asynchronous transmission.
The parameter is used to identify each byte. The normal range is between 0

and 2.
Some systems only use a stop bit.



Flow control

is the software handshaking protocol, such a XON/XOFF.


Learning Activities:

1a. In the case of William’s letter:



William is the
data source
.



The envelope is the
transmitter
.



The mail system is the
transmission medium
.



Opening the envelope is the
receiver
.



William’s friend is the
destination
.


1b. In the case of Julie’s diagram:



Julia is the
data source
.



Drawing the diagram is the
transmitter
.



The fax machine is the
transmission medium
.



The fax machi
ne is also the
receiver
.



Julia’s partner is the
destination
.


2. In the case of the sent ASCII character 1011011, having an even parity would require the
parity bit to be a 1. If the received character had an ASCII of 1010001, the error would not
have been

found in transmission as the parity would still have been even.


3. Cyclic redundancy check is a method of checking for errors in data transmission using a
division process. The data is divided into predetermined lengths and divided by a fixed
divisor. Th
e remainder of the calculation is attached and sent with the data. When the data is
received, the remainder is recalculated. If the remainders do not match, an error in
transmission has occurred.


Checksum is a method of checking for errors in data transmi
ssion by counting the number of
bits in a data packet. A data packet is created by dividing the total data into smaller groups.
The count of the bits in the data packet is attached to the data packet and used by the receiver
to check whether all the bits h
ave arrived successfully. If the count matches, it is assumed
that a complete transmission was received.


An advantage of the cyclic redundancy checking method over the checksum method is that
the former uses more advanced calculations. Where checksum uses

only addition, CRC
divides the data into predetermined lengths and then divides it. Although this means the
accuracy of CRC depends heavily on the amount of data, it is generally more accurate than
checksum.


3. Protocols of Information Systems:

8. Parity

checking is a method of checking for errors in data transmission using an additional
bit called a parity bit. This bit is used only for the purpose of identifying whether the bits
being moved have arrived successfully. When data is represented using an 8
-
bit ASCII, then
a 9
th

bit is used that holds a 0 or a 1. When an odd parity is chosen, the number of 1’s in the 9
bits must be odd.

Similarly, an even parity must have an even amount on 1’s.

If an error has
occurred in a single bit, then the parity will be

different and an error in transmission has
occurred.


Checksum is a method of checking for errors in data transmission by counting the number of
bits in a data packet
. The

count

is attached to the data packet and used by the receiver to
check whether all
the bits have arrived successfully. If the count matches, it is assumed that a
complete transmission was received.


Both methods of error detection are effective and it is difficult to say if one is superior,
although they have very different methods of wo
rking. Parity checking, whilst generally
successful, will only detect an error if it has affected an odd number of bits. Checksum, on
the other hand, will only detect an error if one or more bits are missing. Put simply, parity
checking detects changed bit
s, while checksum detects missing bits. Thus their individual
success rate really depends on what kind of error is more common.