Download Unit 2 Lesson 2

Lesson 2: Data Transmission
At a Glance
In this lesson, the process of transmitting data is examined. Computers
encode and transmit data, voice, and video over networks via various
transmission media. Encoding is the process of transforming information
into digital and analog signals. This lesson covers the basics of how data is
encoded, decoded, and transmitted. Data packet structure and its
relationship to the OSI layers is also covered.
What You Will learn
After completing this lesson, you will be able to:

Define technical terms associated with data signaling and
transmission.

Describe the characteristics of digital and analog signaling.

Explain how packets and frames are structured, and describe their
relationship to the OSI model.

Convert binary and hexadecimal digits to decimal digits.

Use Sniffer Basic software to capture and analyze packets.
DRAFT
1
LAN Configurations
Unit 2
Student Notes:
2
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
Tech Talk

Amplitude - Characteristic of a wave measuring wave height from the
base to the peak of a waveform. Indicates the strength of the signal.

Analog Signal - Analog signals change continuously as opposed to
digital signals, which are discretely valued. For example, sound is an
analog signal; it is continuous and varies in strength.

ASCII Code - American Standard Code for Information Interchange. A
7-bit coding scheme that assigns unique numeric values to letters,
numbers, punctuation, and control characters.

Baudot Code - A 5-bit coding scheme used for transmitting data.

Binary Numbers - A number system based on two states, 0 and 1.
Computers use combinations of binary numbers to represent and
encode all kinds of data including words, sounds, colors, and pictures.

Connection-Oriented Communication - A form of network
communication, where the transmitting device must establish a
connection with the receiving device before data can be transmitted,
(e.g. telephone). In connection-oriented communication, the receiving
device acknowledges receipt of the data.

Connectionless Communication - A form of communication over
networks where the transmitting device can send a message without
establishing a connection with the receiving device (e.g. radio). Signals
are sent, but there is no mechanism for acknowledging receipt.

Digital Signal - Data transmitted in discrete states, for example, on
and off. These discrete states can be represented by binary numbers,
and vice versa

Full-Duplex - Two-way, simultaneous data transmission. Each device
has a separate communication channel.

EBCDIC Code - Extended Binary Coded Decimal Interchange Code. An
8-bit coding scheme used by IBM for data representation in mainframe
environments.

Logical Address –An OSI model layer 3 address.

Frame - Basic unit of data transfer at OSI layer 2.

Half- Duplex - Two-way data transmission that is not simultaneous.
Only one device can communicate at a time.

Packet - Basic unit of data transfer at OSI layer 3.

Physical Address – A OSI model layer 2 address.
DRAFT
3
LAN Configurations
Unit 2
Data Transmission
Data is transmitted over networks using signals, which are transformed, or
encoded, by computers into the voice, video, graphics, and/or the print we
see on our computer screens. The signals used by computers to transmit
data are either digital or analog.

Analog signals are continuous signals that vary in strength. Sound is
an example of an analog signal. Sound is actually a wave and is quite
similar, or analogous, to electromagnetic waves, hence the name
analog. Telephones have transmitters that encode sound waves into
electromagnetic waves, which then travel over wires toward their
destination. The receiving telephone decodes the electromagnetic
waves back into sound waves. Our brains then decode the sound waves
into the words we hear. Computer modems use the same principle.
Analog signals can be represented digitally. For instance, a high
electromagnetic voltage could be interpreted as 1and low voltage as 0.
Telephone Encoding/Decoding
Encode
Decode
Cable
Source

Destination
Digital signals are discrete rather than continuous. Either there is a
signal or there isn't a signal. Telegraphs transmit data with discrete
signals. You either hear a tap or you do not hear a tap. Discrete
signals can be represented by on and off pulses. The duration of a
discrete signal can be varied, as with dots and dashes in Morse Code.
Telegraph Encoding/Decoding
Encode
Decode
Cable
Source
4
Destination
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
Discrete signals can also be represented digitally. The presence of a signal
could be coded as a 1 and the absence of a signal coded as a 0. The digits 0
and 1 are used because computer circuitry is based on binary digital data.
Codes are used to group a set number of bits together and have a group of
bits represent a letter, number, or other character. The computer’s brain,
the computer processing unit (CPU), transforms these codes of 0s and 1s
into the voice, video and data we see. One coding scheme, ASCII, codes an
“a” as the binary number 0110-0001.
Digital data is based two states, on or off. The binary numbering system,
uses only two digits 0 and 1, so it makes sense to use the binary numbering
system. One digit, 0 represents off, the other digit represents on. A single
0 or 1 is called a bit. One byte is equal to eight bits (also called an octet
when discussing TCP/IP). In ASCII code, one octet is the equivalent of one
alphabetic or numeric character. In order to appreciate how computers
communicate over networks, it is necessary to be aware of how they encode
information.
Data Transmission
Connection-Oriented and Connectionless Transmissions
Data transmission may be:

Connection-oriented

Connectionless
In connection-oriented transmissions, the sending (source) device
establishes a connection with the receiving (destination) device. The
connection is continued until all data packets have been transmitted and
the source device receives notification that the data was received by the
destination device and has been checked for errors.
A telephone conversation is an example of a connection-oriented
transmission. When a call is made, data is transmitted across phone lines,
the receiving party picks up the phone, and a conversation takes place.
The individual making the call knows that it arrived at the correct
destination and that it was understood.
In a connectionless transmission the source device transmits data but the
connection is not maintained. The source device does not wait for
notification that the destination device actually received the information
accurately. This method is faster than connection-oriented, however less
reliable since there is no notification of whether the data is received or not.
It is more common to find connectionless transmissions on LANs.
DRAFT
5
LAN Configurations
Unit 2
To understand a connectionless transmission, think of a radio broadcast: A
radio disc jockey tells his/her friends to be sure to listen to her/his program
at 9:00 p.m. At that time s/he broadcasts a message to them. Did they
receive the message? Although it is quite likely, s/he cannot be sure that
they turned the radio on, listened, or understood the message.
Synchronous and Asynchronous Transmission
Computers need to know when to expect data and where a character
begins and ends. When receiving data, timing on both computer devices
must be coordinated if they are to work together efficiently. This
coordination is called clocking, timing, or framing. There are two protocols
for the timing or coordination of data signals:

Synchronous

Asynchronous
When transferring data, both the transmitting and receiving nodes need to
agree when the signal begins and ends so the signals can be correctly
measured and interpreted. This timing process is called bit
synchronization, framing, or clocking.
Imaginehowdiffidultitwouldbetoreadifyoudidnotknowwhenawordstartedan
dwhenawordendediftherewasnopunctuationandnospacesyoucandoitbecause
thereareseveraldifferentcharactersanditisnotincodewhatifthiswerecodedasz
erosandonesthenyouwouldhaverealproblems As you can see,
synchronization is important.
Clocking is somewhat like timing in music. There are a specific number of
beats expected per bar. When computer devices are synchronized, a
specific number of signals or “beats” are expected within a set amount of
time. Timing is important because it helps you be prepared. In many
schools, every 50 minutes, a new class period starts. Students watch the
clock and expect a signal. Usually, they are already prepared to leave the
classroom. That is because they expected the signal.
Synchronous transmission requires the communicating devices to maintain
synchronous clocks during the entire connection. The sending device
transmits on a specific schedule and the receiving device accepts the data
on that same fixed schedule. The receiving device knows the timing of the
sending device because the timing information is embedded within the
preamble of the frame. Synchronous transmissions are common in
internal computer communications and usually are sent as entire frames.
Synchronous transmission is common when large blocks of data are
transferred.
6
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
Asynchronous data transmission does not involve synchronizing the clocks
of the sending and receiving devices. Instead, start and stop bits are used
for synchronization of data signals. The start and stop bits tell the
receiving device how to interpret the data.
Data transmission may be half-duplex; meaning data is transferred in only
one direction at a time. An example of half-duplex is a CB radio where
only one person can talk at a time. Or, transmission may be full duplex,
transmitted in two directions simultaneously. A telephone conversation
illustrates full-duplex communication.
Check Your Understanding
 Why do the variations in data transmission signals need to be
synchronized?
 Explain how the two binary numbers, 0s and 1s, are used to
interpret data.
 Distinguish between connectionless and connection-oriented data
transmissions. Give an example of when you think a connectionoriented transmission might be useful.
DRAFT
7
LAN Configurations
Unit 2
Analog Signals
Analog signals, which are electromagnetic waves, are continuous and look
like a copy of the original sound wave. Transmission of data is
accomplished by varying one or more the waves’ properties.
Analog Signal
Analog
+
0
-
All waves have three characteristics, amplitude (strength), frequency, and
phase. Variations, called modulations, in wave characteristics are used to
encode analog signals to digital signals. Amplitude-Shift Keying
(variations in strength) and Frequency-Shift Keying are two examples.
Amplitude-Shift Keying uses a change of the voltages for interpretation.
When there is a voltage change from high to low, the binary digit
represented changes. If high voltage were 1 then low voltage would be 0.
Amplitude-Shift Keying
1
1
0
0
0
ASK
Frequency-Shift Keying uses the frequency of the waves for interpretation.
When there is a frequency change from high to low, the binary digit
changes. If high frequency were 1 then low frequency would be 0 .
8
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
Frequency-Shift Keying
1
0 0
1
FSK
Digital Signals
With digital signaling, either there is or there isn’t a signal. There are
various encoding schemes that use the “on” “off” signal to represent data.
Digital
+
0
-
Depending upon the type of network, different digital encoding schemes
are used. For example, Ethernet and Token Ring LANs do not use the
same encoding scheme. For computer devices to interpret the data
correctly, both the transmitter and receiver must agree on the encoding
scheme in order to determine data elements and their values. When new
technologies are invented, new encoding protocols often need to be
established.
Check Yourself
 What are the three characteristics of waves that are used when
transmitting data?
 Why will new technologies need new encoding schemes?
DRAFT
9
LAN Configurations
Unit 2
Data Transmission and the OSI Model
When transmitting data over networks, conforming to the OSI model is
important. As discussed in the previous lesson, data travels vertically
through the seven OSI layers. Data is encapsulated at each layer of the
transmitting device from top to bottom and stripped at the receiving device
in the reverse direction. The protocols of the OSI model are used to
organize the data into packets, with headers and trailers.
OSI Model Original Data with Headers and Trailers
Hp
Hs
Ht
Hn
Hd
Data
Application Layer
Data
Presentation Layer
Data
Session Layer
Data
Transport Layer
Data
Data
Data
T
T
Network Layer
Data Link Layer
Physical Layer
OSI communication is as follows:
10

Each layer communicates with layers both immediately above and
below it.

Each layer from the sending (source) station also communicates with
its peer layer at the receiving (destination) computer.

Data starts at the application layer of the source device and descends
through the remaining layers before being transmitted to the
destination device.

As each layer receives the data from the layer above it and adds, in the
form of headers, its data, which contains various protocols that enable
communication.

The original data, with the new header and the headers from the
previous layers, is then sent to the next layer down.

When the data reaches the Physical Layer, it is transmitted across
various media to the destination device.
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission

The destination device receives the entire frame and sends it up
through the layers, one after the other in sequence.

Each layer strips the header added by its peer layer at the source
device.
Ethernet packets can contain approximately 1,000 bytes. If the data being
transmitted is larger than 1,000 bytes then the computer breaks it down
into packets. Each packet is transmitted and received separately. Packets
are sequentially numbered. This allows the receiving computer to recreate
the data in the correct order. Depending upon the protocols used, packet
size can change.
Transmission of Data through the OSI Layers
Data transfer begins at the application layer of the source device and each
OSI layer adds header and/or trailer information to help ensure efficient,
error free transfer of data. The destination device strips the information
added by its peer layer until the data is returned to its original form at the
application layer.
Application Layer
Data
The application layer serves as an interface between user applications and
network services, such as electronic mail. Data input by the user is sent to
the presentation layer.
Presentation Layer
Hp
Data
Header information added by presentation layer protocols is responsible for
translation, encryption, and compression of data. If necessary, it is this
layer that translates local data, such as ASCII and EBCDIC.
DRAFT
11
LAN Configurations
Unit 2
Session Layer
Hs Hp
Data
At the session layer checkpoints are built in to assure successful data
transmission. If they are proceeding smoothly, the transmission continues.
If not, retransmission begins again. This layer provides the user interface
to the network (passwords and login).
Transport Layer
H t Hs Hp
Data
The transport layer, provides for message segmentation and ensures error
free delivery, without loss or duplication.
Network Layer
Hn H t Hs Hp
Data
At the network layer, header information identifies the source and
destination of the “logical” network address. The logical network address
assists with data routing, from network to network. Factors affecting
routing decisions include cost, speed, network conditions, and priorities.
Data Link Layer
Hd Hn H t Hs Hp
Data
T
Frames are built at the data link layer. The headers and trailers added at
this layer control error handling and synchronization over the local
segment. This is where the “physical” address of the destination and the
source address of the sender are added.
12
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
Physical Layer
The physical layer of the OSI controls the electrical aspects of data
transfer, such as voltage levels, signal timing, and encoding.
Physical Layer Frame with Preamble, Headers, and Trailers
Transport
Header
Data Link
Presentation
Header
Header
Hd Hn H t Hs Hp
Physical
Preamble
Session
Header
Header
Network
Header
Data
T
Trailer
Although real network applications don’t always incorporate protocols from
every layer, or sometimes combine the functions of two layers, it is
important to understand how the OSI model is used as a framework for the
protocols used when transmitting data.
Transmission of data packets occurs in both directions. Peer layers
communicate back and forth when a data packet is being sent. What is
actually taking place is a checking sequence. For example, does the
address match, is there any congestion, which is the best route, are the
destination and source devices synchronized, is it time to terminate the
connection? All of this takes place in a fraction of a second.
DRAFT
13
LAN Configurations
Unit 2
Check Your Understanding
 What is the difference between half-duplex and full duplex
transmission?
 How is sending a registered letter through the mail similar to
sending data over a network?
14
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
U2L2 Supplemental #1
Converting Binary and Hexadecimal to Decimal Numerals
Computers are electrical devices. Electrical pulses can be turned “on” or
“off.” Binary digits 0 and 1 can be used to represent “on” and “off” pulses.
Computers use these 0s and 1s to represent data, which they then
interpret using various codes. In computer language, each 0 or 1 is
considered a bit and eight bits are equal to an octet (byte). One octet
generally represents one character of data.
We use and understand a base-10 numbering system in everyday life.
Base-10 uses the digits 0-9. In order to understand how computers
transmit data, it is necessary to understand two additional numbering
systems, base-2 and base-16. Two digits, 0 and 1, are used for base-2
numbering and 10 digits plus 6 characters from our alphabet are used for
base-16 numbering.
ASCII code uses the base-2, or binary, numbering system and hexadecimal
code, uses the base-16, or hex, numbering system.
In hexadecimal numbering, there are 16 symbols for the decimal numbers
0-15. The numbers 0 to 9 represent the decimal numerals 0 to 9. The
decimal numbers 10 to 16 are represented by the alphabetic characters A
to F, e.g., A=10, B=11, C=12, D=13, E=14, F=15. Hexadecimal numbers
can be used to represent 8 bits as two hexadecimal digits. MAC addresses,
which you will learn about later in this course, use hex numbers for
address identification.
DRAFT
15
LAN Configurations
Unit 2
In this activity, you will attempt to convert decimal, binary, and
hexadecimal numbers. Use the data from the following tables to help with
conversions.
Number Equivalents
Decimal
Binary
Hexadecimal
0
0000
0
1
0001
1
2
0010
2
3
0011
3
4
0100
4
5
0101
5
6
0110
6
7
0111
7
8
1000
8
9
1001
9
10
1010
A
11
1011
B
12
1100
C
13
1101
D
14
1110
E
15
1111
F
When numbering we give each digit column a name or positional value.
We do this for convenience when reading numbers. The column value is
determined by raising the base (decimal, binary, or hexadecimal) to a
power as shown in the charts below.
16
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
When using a base-10 number such as 34,752, each of the numbers has a
decimal positional value based on the powers of 10. 2 is in the 1s position,
5 in the 10s position, 7 in the 100s position, 4 in the 1,000 position, and 3
in the 10,000 position.
Decimal (Base 10) Positional (Column) Values
100 = 1s column/position
101 = 10s column/position
102 = 100s column/position
103 = 1,000s column/position
104 = 10,000s column/position
Or: 3 4, 7 5 2
Ones position
Tens position
Hundreds position
Thousands position
Ten thousands position
Binary (Base 2) Positional (Column) Values
When you use a base-2 number such as 11001, each of the numbers has a
decimal positional value based on the powers of 2. Starting from the
right, 1 is in the 1s position, 0 in the 2s position, 0 in the 4s position, 1 in
the 8s position, and 1 in the 16s position.
20
= 1s column
21
= 2s column
22
= 4s column
23
= 8s column
24
= 16s column
DRAFT
17
LAN Configurations
Unit 2
Or, 1 1 0 0 1
Ones position
Twos position
Fours position
Eights position
Sixteens position
Hexadecimal (Base 16) Positional (Column) Values
When you use a base-16 number such as B620A each of the numbers has
a decimal positional value based on the powers of 16. Starting from the
right, A is in the 1s position, 0 in the 16s position, 2 in the 256s position, 6
in the 4,096 position, and B in the 65,536s position.
160
= 1s column/position
161
= 16s column/position
162
= 256s column/position
163
= 4,096s column/position
164
= 65,536s column/position
Or, B 6 2 0 A
Ones position
Sixteens position
Two hundred fifty-sixes position
Four thousand ninety-sixes position
Sixty-five thousands, five hundred
thirty-sixes position
18
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
Converting a Binary Number to a Decimal Number
Example: Convert the Binary number 101100 to a decimal number.
Step 1: To change from a binary number to a decimal number you must
first determine the binary digit’s positional value, see chart on previous
page. Start at the right: 0=1, 0=2, 1=4, 1=8, 0=16, and 1=32.
Binary Digit
Value
0
0
1
1
0
1
Positional
Value
1
2
4
8
16
32
Step 3: Multiply the binary digit value and the positional value for each
digit. 0x1=0, 0x2=0, 1x4=4, 1x8=8, 1x32=32.
Binary Digit
Value
0
0
1
1
0
1
Positional
Value
1
2
4
8
16
32
Product of Binary
Digit and
Positional Values
0
0
4
8
0
32
Step 4: Add the products together: 0 + 0 + 4 + 8 + 0 + 32 = 44.
Step 5: The sum of the products of the binary digit and positional values is
equal to the decimal number. Binary number 101100 is equal to a decimal
value of 44.
DRAFT
19
LAN Configurations
Unit 2
Convert the following binary numbers to decimals. Follow the steps above.
Show your work.
a. 110011
b. 0011
c. 010101
d. 1111
e. 01010101
Converting a Hexadecimal Number to a Decimal Number
Example: Convert the hex number 5B6A to a decimal number.
Step 1: To change from a hexadecimal number to a decimal number you
must first change the hex value to a decimal value. Look on the number
equivalents chart and change the hex digits to decimal digits. 5 = 5; B= 11;
6 = 6; A = 10.
Hexadecimal
Value
5
B
6
A
20
Decimal
Value
5
11
6
10
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
Step 2: Determine the positional value for each hex digit (see Positional
Value Tables): 5 is in the fourth position so the positional value is 4,096. B
is in the third position so the positional value is 256; 6 is in the second
position so the positional value is 16; and A is in the first position so the
positional value is 1.
Hexadecimal
Value
5
B
6
A
Decimal
Value
Hex Positional
Value
5
11
6
10
4,096
256
16
1
Step 3: Multiply the decimal value and the positional value for each
hexadecimal. 5 x 4,096 = 20,480; 11 x 256 = 2,816; 6 x 16 = 96; 10 x 1 = 10.
Hexadecimal
Value
5
B
6
A
Decimal
Value
5
11
6
10
Hex
Positional
Value
4,096
256
16
1
Product of
Decimal and Hex
Positional Values
20,480
2,816
96
10
Step 4: Add the products together. 20,480 + 2,816 + 96 + 10 = 23,402.
Hexadecimal
Value
5
B
6
A
Decimal
Value
5
11
6
10
Hex
Positional
Value
4,096
256
16
1
Product of
Decimal and Hex
Positional Values
20,480
2,816
96
10
Step 5: The sum of the products of the decimal and hex positional values is
equal to the decimal number. Hexadecimal number 5B6A = decimal
number 23,402.
DRAFT
21
LAN Configurations
Unit 2
Try the following hexadecimal numbers to decimal numbers. Follow the
steps listed above. Show your work.
a. 23,7AF
b. 57
c. 392
d. FFF
e. BB41A
*** For credit on this supplement you must submit answers to each
number you were asked to convert. Make sure you’ve read closely so you
don’t miss any of them.
U2L2 Supplemental #2
Select one of the following topics to research.
1. ASCII, what is it and what does it have to do with Data Transmission?
2. How does ASCII and binary work together and what does OSI have to
do with it
Prepare a one-page summary of your findings. (double spaced, 12 point, 1
inch margins.) Submit your paper with at least 3 reference (works cited)
where you got your info.
U2L2 Supplemental #3 (Unit Review)
Data Transmission
Part A
1. Describe analog signals. How are they used to transmit data?
2. Describe digital signals.
3. Describe synchronous data transmission.
4. Describe asynchronous data transmission.
5. What is the difference between half-duplex and full duplex
transmissions?
22
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
6. There is a timing process that signals the beginning and ending of data
so it can be correctly measured. This process is called what?
a. Digital signaling
b. Analog signaling
c. Bit synchronization
d. Asynchronous
e. Synchronous
1. Which type of signaling scheme represents data sent as discrete
signals?
a. Digital signaling
b. Analog signaling
c. Asynchronous
d. Synchronous
2. In which type of signaling scheme represents continuously changing
data?
a. Digital signaling
b. Analog signaling
c. Asynchronous
d. Synchronous
3. Which type of bit synchronization transmission requires both a start bit
and a stop bit for clocking purposes?
a. Digital signaling
b. Analog signaling
c. Asynchronous
d. Synchronous
4. Group of bits, including data and control signals, arranged in a specific
format and transmitted as a whole, are called what?
a. Clocking
b. Sequencing
c. Synchronization
d. Packets
DRAFT
23
LAN Configurations
Unit 2
Part B
1. Describe the difference between analog and digital signaling
waves/pulses.
2. What is binary notation and how is it used to transfer data signals over
network media?
3. List three characteristics of waves that are used to encode data.
Part C
Use the OSI model as your reference and explain how data packets are
structured. Give several examples of information that may be contained
within headers . Draw a diagram showing packet addition at each layer.
*** This is pre-test quiz is just given to you so you can
prepare for the test. It is NOT to be turned in.
Unit 2 Lesson 2 Pretest Quiz
1.
2.
List the 3 characteristics of analog signal waves?
1–
2–
3What method is used to convert analog signals to digital signals?
3.
Do Ethernet and Token ring LANs use the same encoding scheme?
YES / NO
4.
Ethernet packets contain approx. how many bytes?
5.
Ethernet packets are transmitted together or separately?
together / separately
6.
Diagram a frame with preamble, header and trailer.
7.
Interstate 99 is an example of:
Half-duplex / Full duplex
8. List 3 devices or communication methods that require full-duplex.
1–
2–
3–
9. List 3 connection-oriented communication examples.
1–
2–
324
DRAFT
Internetworking Fundamentals
Unit 2
Lesson 2: Data Transmission
10. List 3 connection-less communication examples.
1–
2–
3Give the binary equivalents to the following base 10 numbers:
62
44
18
10
Give the base 10 equivalent to the following binary numbers:
010101
0010010
11111
DRAFT
0001
25