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.
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.
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.
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.
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.
Connection-Oriented and Connectionless Transmissions
Data transmission may be:
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.
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:
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.
Imaginehowdiffidultitwouldbetoreadifyoudidnotknowwhenawordstartedandwhenawordendediftherewasnopunctuationandnospacesyoucandoitbecausethereareseveraldifferentcharactersanditisnotincodewhatifthiswerecodedaszerosandonesthenyouwouldhaverealproblems 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.
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.
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 connection-oriented transmission might be useful.
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.
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.
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 .
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.
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.
What are the three characteristics of waves that are used when transmitting data?
Why will new technologies need new encoding schemes?
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
OSI communication is as follows:
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.
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.
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.
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.
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).
The transport layer, provides for message segmentation and ensures error free delivery, without loss or duplication.
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
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.
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
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.
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?
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.
In this activity, you will attempt to convert decimal, binary, and hexadecimal numbers. Use the data from the following tables to help with conversions.
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.
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.
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.
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.
= 1s column/position
= 16s column/position
= 256s column/position
= 4,096s column/position
= 65,536s column/position
Or, B 6 2 0 A
Two hundred fifty-sixes position
Four thousand ninety-sixes position
Sixty-five thousands, five hundred thirty-sixes position
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
Step 3: Multiply the binary digit value and the positional value for each digit. 0x1=0, 0x2=0, 1x4=4, 1x8=8, 1x32=32.
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.
Convert the following binary numbers to decimals. Follow the steps above. Show your work.
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.
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.
Hex Positional Value
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.
Which type of signaling scheme represents data sent as discrete signals?
In which type of signaling scheme represents continuously changing data?
Which type of bit synchronization transmission requires both a start bit and a stop bit for clocking purposes?
Group of bits, including data and control signals, arranged in a specific format and transmitted as a whole, are called what?
Describe the difference between analog and digital signaling waves/pulses.
What is binary notation and how is it used to transfer data signals over network media?
List three characteristics of waves that are used to encode data.
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
List the 3 characteristics of analog signal waves?
What method is used to convert analog signals to digital signals?
Do Ethernet and Token ring LANs use the same encoding scheme?
YES / NO
Ethernet packets contain approx. how many bytes?
Ethernet packets are transmitted together or separately?
together / separately
Diagram a frame with preamble, header and trailer.
Interstate 99 is an example of:
Half-duplex / Full duplex
8. List 3 devices or communication methods that require full-duplex.
9. List 3 connection-oriented communication examples.
List 3 connection-less communication examples.
Give 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 0001