Data Communication – Complete Lesson (Section 1.1)
1.1.1 Definition of Data Communication
My dear student, welcome to the very first topic of Data Communication and Computer Networking! Before we go into any technical details, let me ask you a simple question: What do you do when you want to share information with your friend who is sitting next to you? You probably just talk to them, right? That is very easy. But now think about this: What if your friend is in another city, or even another country? That is where data communication becomes important.
When we communicate, we are sharing information. This sharing can be:
- Local communication — happens face to face, when people are close to each other
- Remote communication — happens over a distance, like making a phone call or sending an email
The word telecommunication (which includes telephony, telegraphy, and television) means communication at a distance. The word “tele” comes from Greek and means “far.” So telecommunications literally means “communicating far away.”
The word data here refers to information presented in whatever form is agreed upon by the parties creating and using the data. It is not just numbers — data can be text, images, audio, video, or any other form of information.
Four Fundamental Characteristics of Effective Data Communication
Now, how do we know if a data communication system is good or bad? We judge it by four fundamental characteristics. Let me explain each one carefully with examples.
1. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and ONLY by that device or user.
- Example: When you send an email to your friend Abebe, the email must reach Abebe’s inbox — not someone else’s inbox. If it goes to the wrong person, the delivery has failed, even if everything else worked perfectly.
2. Accuracy: The system must deliver the data accurately. Data that has been altered during transmission and left uncorrected is unusable.
- Example: Imagine you send a bank transfer request of “5000 Birr” but during transmission, it changes to “9000 Birr.” Even though the data was delivered, it is not accurate. Such a system is dangerous and unreliable.
3. Timeliness: The system must deliver data in a timely manner. Data delivered late is useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order, and without significant delay. This kind of delivery is called real-time transmission.
- Example: If you are watching a live football match online, and the video arrives 5 minutes late, you already know the score from your friend who is watching on TV. The delivery was not timely, so the experience is ruined.
4. Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets.
- Example: Suppose video packets are sent every 30 ms. If some packets arrive with 30-ms delay but others arrive with 40-ms delay, the video will appear uneven and jumpy. This uneven quality is caused by jitter. Even if the average delay is acceptable, variation in delay creates problems for real-time applications.
Fill in the Blank: The variation in packet arrival time that causes uneven quality in video or audio is called ________.
Jitter. Jitter is the uneven delay in the delivery of audio or video packets. For example, if video packets are sent every 30 ms but some arrive with 30-ms delay and others with 40-ms delay, the result is uneven video quality.
MCQ: Which of the following is NOT one of the four fundamental characteristics of data communication?
(a) Delivery (b) Accuracy (c) Bandwidth (d) Jitter
Answer: (c) Bandwidth. The four fundamental characteristics are: (1) Delivery, (2) Accuracy, (3) Timeliness, and (4) Jitter. Bandwidth is an important concept in networking, but it is NOT listed as one of these four effectiveness characteristics.
1.1.2 Communication Basics
Data communications include many different ways of exchanging information. Let me list them all for you:
- Writing and talking — the most basic forms of communication between humans
- Nonverbal communication — such as facial expressions, body language, or gestures. For example, a nod means “yes” in many cultures.
- Visual communication — the use of images or pictures such as painting, photography, video, or film
- Electronic communication — such as telephone calls, electronic mail, cable television, satellite broadcasts
In this course, we focus on electronic data communication — where devices (like computers) exchange data through transmission media. But remember, the underlying principles of communication are the same whether you are talking face to face or sending data through a fiber optic cable.
The key requirement is that the communicating devices must be part of a communication system made up of hardware and software working together. Hardware alone is not enough — you need software (programs and protocols) to tell the hardware what to do and how to do it.
List and Explain: List four types of communication and give one example of each.
1. Writing and talking: Direct human communication. Example: A teacher explaining a concept to students in a classroom.
2. Nonverbal communication: Using body language instead of words. Example: A traffic police officer using hand signals to direct vehicles.
3. Visual communication: Using images or pictures. Example: A television news broadcast showing video footage of an event.
4. Electronic communication: Using electronic devices to exchange data. Example: Sending an email from Addis Ababa to a colleague in Bahir Dar.
1.1.3 Data Representation Techniques
My student, here is an important question for you: How does a computer understand a picture, a song, or a paragraph of text? The answer is — it doesn’t! A computer only understands one thing: bits (zeros and ones). So before any information can be stored, processed, or transmitted by a computer, it must first be converted into a pattern of bits. This process is called data representation.
Information today comes in different forms. Let us look at each one and understand how it is represented as bits.
1. Text Representation
Text is represented as a bit pattern — a sequence of bits (0s or 1s). Different sets of bit patterns have been designed to represent text symbols. Each set is called a code, and the process of representing symbols is called coding.
ASCII (American Standard Code for Information Interchange): This was developed in the United States several decades ago. It uses 7 bits (and later extended to 8 bits) to represent each character. It can represent 128 characters (English letters, digits, punctuation marks, and some control characters).
Unicode: This is the prevalent coding system used today. It uses 32 bits to represent a symbol or character used in any language in the world — including Amharic, Tigrinya, Chinese, Arabic, and thousands of other languages. ASCII now constitutes the first 127 characters in Unicode and is also referred to as Basic Latin.
2. Number Representation
Numbers are also represented by bit patterns. However, a code such as ASCII is NOT used to represent numbers. Instead, the number is directly converted to a binary number. This simplifies mathematical operations.
Example: The decimal number 13 is directly converted to binary as 1101. This is different from representing the text character “1” and “3” using ASCII codes.
3. Image Representation
Images are also represented by bit patterns. To understand how, you need to know what a pixel is.
An image is composed of a matrix of pixels (picture elements), where each pixel is a small dot. The size of the pixel depends on the resolution:
- An image divided into 1,000 pixels has low resolution — it looks blocky
- An image divided into 10,000 pixels has better resolution — it looks sharper
- But higher resolution requires MORE memory to store the image
After an image is divided into pixels, each pixel is assigned a bit pattern. The size and value of the pattern depend on the type of image:
Black and white images: Only 1 bit is needed per pixel (0 = black, 1 = white). Think of a chessboard — each square is either black or white.
Grayscale images: If an image is not made of pure black and pure white pixels, you can use more bits to show shades of gray. For example, with 2-bit patterns you can show 4 levels:
With 8 bits per pixel, you can show 256 levels of gray — giving a very smooth gradient.
Color images: There are several methods to represent color images:
- RGB method: Each color is made of a combination of three primary colors: Red, Green, and Blue. The intensity of each color is measured, and a bit pattern is assigned to each. For example, with 8 bits per color channel (24 bits total per pixel), you can represent over 16 million different colors.
- YCM method: A color is made of a combination of Yellow, Cyan, and Magenta. This method is commonly used in color printers.
4. Audio Representation
Audio refers to the recording or broadcasting of sound or music. Audio is by nature different from text, numbers, or images because it is continuous, not discrete. Even when we use a microphone to change voice or music to an electric signal, we create a continuous signal.
To represent audio in digital form, we use a process called sampling — we measure the continuous signal at regular time intervals and convert each measurement into a bit pattern. The more frequently we sample, the better the quality of the digital audio.
5. Video Representation
Video refers to the recording or broadcasting of a picture or movie. Video can either be:
- A continuous entity — produced by a TV camera, which captures continuous motion
- A combination of images — each image is a discrete entity, but when displayed rapidly in sequence (like frames in a film), they convey the idea of motion
This is why video files are so large — they contain many images (frames) per second, and each image contains thousands of pixels, each needing multiple bits!
- All data must be converted to bit patterns (0s and 1s) for digital communication
- Text: Uses coding systems — ASCII (128 chars, 7 bits) or Unicode (32 bits, all languages)
- Numbers: Directly converted to binary (NOT coded like text)
- Images: Matrix of pixels; 1 bit for B/W, more bits for grayscale; RGB or YCM for color
- Audio: Continuous signal converted to digital by sampling
- Video: Sequence of images displayed rapidly to create illusion of motion
MCQ: Which coding system uses 32 bits to represent characters from any language in the world?
(a) ASCII (b) EBCDIC (c) Unicode (d) Binary
Answer: (c) Unicode. Unicode uses 32 bits and can represent characters from any language in the world. ASCII uses only 7 bits and can represent only 128 characters (Basic Latin). Numbers are directly converted to binary — they do not use a coding system like ASCII.
Fill in the Blank: A small dot that makes up part of an image is called a ________.
Pixel (picture element). An image is composed of a matrix of pixels. The more pixels an image has, the higher its resolution and the more memory it requires.
Short Answer: Why are numbers NOT represented using ASCII codes in data communication?
Numbers are directly converted to binary numbers (not ASCII codes) because this simplifies mathematical operations. If we used ASCII, the number 13 would be stored as two separate characters “1” and “3” (16 bits), and performing arithmetic on character codes would be very complicated. By directly converting to binary (1101, only 4 bits), mathematical operations like addition and multiplication become much simpler and faster for the computer.
1.1.4 Digital Data Transmission Formats
Now we come to a very important topic: How exactly do bits travel from one device to another? There are two main ways: serial transmission and parallel transmission. Let me explain both in detail.
Serial Transmission
In serial transmission, the various bits of data are transmitted one after the other — sequentially. Only one bit travels at a time through a single communication channel.
Key characteristics of serial transmission:
- Only ONE communication line is needed (regardless of how many bits per unit)
- Less costly (only one wire)
- Bits are reassembled at the destination
- Mostly used by computer peripherals like printers
- Used for long distance transmission
- Slower than parallel transmission for a single unit, but more practical for long distances
Bit Order — Little-Endian vs Big-Endian: In serial mode, which bit should be sent first? Consider the number 13 in binary (1101). Should we send the Most Significant Bit (MSB) first (1, then 1, then 0, then 1) or the Least Significant Bit (LSB) first (1, then 0, then 1, then 1)?
- Little-endian: Sends the LSB first
- Big-endian: Sends the MSB first
Either form can be used, but the sender and receiver MUST agree on which order is being used. Usually, the Least Significant Bit (LSB) has been transmitted first in most systems.
Synchronous vs. Asynchronous Serial Transmission
Within serial transmission, there are two sub-types that you must understand clearly:
A. Synchronous Transmission: Data is sent in large blocks (multiple bytes at a time).
- No start and stop bits are used
- No gap between data units
- Data transmission speed is FAST
- Cost is HIGH (requires more complex hardware)
- Used for transfer of data between two computers
- Synchronization between sender and receiver is REQUIRED — both must agree on timing
B. Asynchronous Transmission: Sends only one character (one byte) at a time.
- Uses a Start Bit (usually 0) to signal the beginning of data
- Uses one or more Stop Bit(s) to signal the end of data
- No time relation between consecutive characters (gaps can exist)
- During idle condition (no data being sent), signal “1” is transmitted
- Slower than synchronous because of the overhead of start/stop bits
- Simpler and cheaper
Parallel Transmission
In parallel transmission, ALL the bits of data are transmitted simultaneously on separate communication lines.
Key characteristics of parallel transmission:
- Multiple bits are transferred at a time (e.g., 8 bits through 8 wires)
- Needs multiple (parallel) communication channels
- Used for short distance communication only
- More costly (needs n wires for n bits)
- Faster than serial transmission — data can be transmitted in less time
- Serial: One bit at a time, one wire, cheaper, long distance, slower per unit
- Parallel: All bits at once, multiple wires, expensive, short distance, faster per unit
- Synchronous: Large blocks, no start/stop bits, fast, needs synchronization
- Asynchronous: One byte at a time, uses start/stop bits, slower, simpler
- Little-endian = LSB first; Big-endian = MSB first
MCQ: Which type of transmission uses start and stop bits?
(a) Synchronous serial (b) Asynchronous serial (c) Parallel (d) Both a and b
Answer: (b) Asynchronous serial. Asynchronous transmission sends one byte at a time and uses a start bit (usually 0) to signal the beginning and stop bit(s) to signal the end. Synchronous transmission sends large blocks without start/stop bits. Parallel transmission does not use start/stop bits either.
True or False: Parallel transmission is preferred for long-distance communication because it is faster.
False. Although parallel transmission is faster per data unit, it is only used for short distance communication. For long distances, serial transmission is preferred because it requires only one wire (much cheaper and more practical), whereas parallel would need multiple wires over long distances, which is expensive and causes timing problems.
Fill in the Blank: A system that sends the Least Significant Bit first is called ________.
Little-endian. In little-endian systems, the LSB is transmitted first. In big-endian systems, the MSB is transmitted first. The sender and receiver must agree on which order is used.
1.1.5 Transmission Impairments
My student, let me tell you something very important: no transmission medium is perfect. When a signal travels through a wire or through the air, it does not arrive at the destination exactly as it was sent. The signal gets changed — or “impaired.” This means: what is sent is NOT what is received. Understanding these impairments is crucial for anyone studying data communication.
There are three causes of impairment:
1. Attenuation
Attenuation means a loss of energy. When a signal (simple or composite) travels through a medium, it loses some of its energy in overcoming the resistance of the medium.
- This is why a wire carrying electric signals gets warm, or even hot, after a while
- Some of the electrical energy in the signal is converted to heat energy
- To compensate for this loss, amplifiers are used to amplify (strengthen) the signal
Example: Think of shouting to a friend who is far away. Your voice loses energy as it travels through the air, so your friend hears a weaker sound than what you produced. That is attenuation. If you use a megaphone (amplifier), your friend hears you better.
2. Distortion
Distortion means that the signal changes its form or shape. Distortion occurs in a composite signal (a signal made of different frequencies). Here is why:
- Each signal component has its own propagation speed through a medium
- Therefore, each component has its own delay in arriving at the destination
- Differences in delay may create a difference in phase
- The signal components at the receiver have phases different from what they had at the sender
- The shape of the composite signal is therefore not the same
Example: Imagine a group of runners who start together. If each runner has a different speed, they will arrive at different times. At the finish line, the group formation looks different from the start. That is similar to what happens to a composite signal — its different frequency components arrive “out of formation.”
3. Noise
Noise is another cause of impairment. There are several types of noise:
| Type of Noise | Cause | Example |
|---|---|---|
| Thermal noise | Random motion of electrons in a wire creates an extra signal not sent by the transmitter | Always present in any wire; increases with temperature |
| Induced noise | Sources like motors and appliances act as sending antennas; the medium acts as receiving antenna | Interference from a nearby electric motor |
| Crosstalk | Effect of one wire on another; one wire acts as sending antenna, other as receiving antenna | Hearing another phone conversation on your line |
| Impulse noise | A spike (high energy in very short time) from power lines, lightning, etc. | A sudden crackle on a phone line during a thunderstorm |
- Attenuation: Loss of energy; fixed by amplifiers; signal gets weaker
- Distortion: Change in signal shape; affects composite signals; caused by different propagation speeds
- Noise: External signals added to the original; four types: thermal, induced, crosstalk, impulse
- What is sent ≠ What is received (because of impairments)
MCQ: The random motion of electrons in a wire that creates an extra signal is called:
(a) Crosstalk (b) Impulse noise (c) Thermal noise (d) Induced noise
Answer: (c) Thermal noise. Thermal noise is caused by the random motion of electrons in a wire. It creates an extra signal that was not originally sent by the transmitter. Crosstalk is between wires, induced noise comes from external devices like motors, and impulse noise is a sudden spike from lightning or power lines.
Short Answer: What is the difference between attenuation and distortion?
Attenuation is the loss of energy — the signal becomes weaker but keeps its shape. It can be fixed using amplifiers. Distortion is the change in the shape or form of the signal — it occurs because different frequency components travel at different speeds through the medium, arriving with different phases. Amplifiers cannot fix distortion because the shape itself has changed, not just the strength.
1.1.6 Modes of Data Transmission
Now let us talk about the direction in which data can flow between two devices. There are three modes of data transmission. This is a very common exam topic, so please study it carefully!
1. Simplex Mode
In simplex mode, data flows in only ONE direction — from the transmitter to the receiver. The receiver cannot send anything back.
- Useful when data does not need to flow in both directions
- Examples: Computer to printer, mouse to computer, keyboard to computer, television broadcasting
2. Half-Duplex Mode
In half-duplex mode, data flows in one direction OR the other, but NOT both at the same time. Each end transmits in turn.
- Each end of the connection transmits in turn
- The entire capacity of the channel is used by whichever device is transmitting at the time
- Examples: Walkie-talkies, CB (citizens band) radios — one person speaks while the other listens, then they switch
- Used when there is no need for communication in both directions at the same time
3. Full-Duplex Mode
In full-duplex mode (also called duplex), both stations can transmit and receive SIMULTANEOUSLY.
- Like a two-way street with traffic flowing in both directions at the same time
- Signals going in one direction share the capacity of the link with signals going the other direction
- This sharing can happen in two ways: (a) two physically separate transmission paths, or (b) the capacity of the channel is divided between both directions
- Example: Telephone network — both people can talk and listen at the same time
- Used when communication in both directions is required all the time
- However, the capacity of the channel must be divided between the two directions
- Simplex: One direction only (TV, keyboard, mouse)
- Half-duplex: Both directions, but one at a time (walkie-talkie)
- Full-duplex: Both directions simultaneously (telephone)
- Half-duplex uses full channel capacity for one direction at a time
- Full-duplex shares channel capacity between both directions
MCQ: A walkie-talkie is an example of which transmission mode?
(a) Simplex (b) Half-duplex (c) Full-duplex (d) None of the above
Answer: (b) Half-duplex. A walkie-talkie allows communication in both directions, but NOT at the same time. One person speaks while the other listens, then they press the button to switch. This is exactly half-duplex. Simplex would be one-direction only (like a TV broadcast), and full-duplex would allow both to talk simultaneously (like a telephone).
Fill in the Blank: In ________ mode, both stations can transmit and receive simultaneously, but the channel capacity must be divided between both directions.
Full-duplex (also called duplex) mode. Example: telephone network where both parties can talk and listen at the same time. The channel capacity is shared between the two directions — either through two separate physical paths or by dividing the capacity.
1.1.7 Elements of Data Communication
Every data communication system has five components. Think of these as the essential ingredients — without any one of them, data communication cannot happen.
1. Message: The information (data) to be communicated. Popular forms include text, numbers, pictures, audio, and video. The message is the “what” of communication — what are we trying to send?
2. Sender: The device that sends the data message. Examples: computer, workstation, telephone handset, video camera. The sender is the “who” — who is sending the data?
3. Receiver: The device that receives the message. Examples: computer, workstation, telephone handset, television. The receiver is the “to whom” — who is receiving the data?
4. Transmission Medium: The physical path by which a message travels from sender to receiver. Examples: twisted-pair wire, coaxial cable, fiber optic cable, and radio waves. The medium is the “how” — through what path does the data travel?
5. Protocol: A set of rules that govern data communications. It represents an agreement between the communicating devices.
List and Explain: List the five components of data communication and explain the role of each.
1. Message: The actual data/information to be communicated (text, audio, video, etc.).
2. Sender: The device that sends the message (computer, phone, camera, etc.).
3. Receiver: The device that receives the message (computer, TV, phone, etc.).
4. Transmission Medium: The physical path for the message (twisted-pair wire, coaxial cable, fiber optic, radio waves).
5. Protocol: A set of rules governing the communication — like a common language that both devices understand. Without it, devices cannot communicate meaningfully even if physically connected.
1.1.8 Protocols and Standards
What is a Protocol?
A protocol is one of the components of a data communication system. Without a protocol, communication cannot occur. The sending device cannot just send data and expect the receiving device to interpret it correctly.
When the sender sends a message, it may consist of text, numbers, images, etc. These are converted into bits and grouped into blocks to be transmitted. Often, certain additional information called control information is also added to help the receiver interpret the data.
Without protocols, communicating entities are like two persons trying to talk to each other in different languages without knowing the other’s language.
Three Key Elements of a Protocol
Every protocol has three essential elements:
1. Syntax: The structure or format of the data — the arrangement of data in a particular order. It defines HOW the data is organized.
- Example: A protocol might specify that the first 8 bits are the sender’s address, the next 8 bits are the receiver’s address, and the remaining bits are the actual data.
2. Semantics: The meaning of each section of bits. It indicates the interpretation of each section and tells what action or decision is to be taken based on the interpretation.
- Example: If a certain bit pattern means “request to send,” the semantics define what the receiver should do when it sees that pattern.
3. Timing: It tells the sender about the readiness of the receiver to receive data. It tells the sender at what rate the data should be sent to avoid overwhelming the receiver.
- Example: If the receiver is busy processing previous data, the timing rules tell the sender to wait before sending more data.
Standards in Networking
Standards are necessary in networking to ensure interconnectivity and interoperability between various networking hardware and software components. Without standards, we would have proprietary products creating isolated islands of users that cannot interconnect.
There are two categories of standards:
1. De Facto Standard: Standards that have been traditionally used — established “by fact or by convention.” These are NOT approved by any organized body but are adopted by widespread use.
- Example: The QWERTY keyboard layout became a de facto standard because everyone uses it, not because any official body mandated it.
2. De Jure Standard: Standards that are legislated and approved by a body that is officially recognized — established “by law or by regulation.”
- Example: The USB standard is managed by the USB Implementers Forum — an official standards body.
Standards Organizations
| Type | Organization | Abbreviation |
|---|---|---|
| Standards Committees | International Organization for Standardization | ISO |
| International Telecommunications Union – Telecom | ITU-T | |
| American National Standards Institute | ANSI | |
| Institute of Electrical & Electronics Engineers | IEEE | |
| Electronic Industries Association | EIA | |
| Forums | ATM Forum | — |
| MPLS Forum | — | |
| Frame Relay Forum | — | |
| Regulatory Agency | Federal Communications Commission | FCC |
MCQ: Which element of a protocol defines the structure or format of the data?
(a) Semantics (b) Timing (c) Syntax (d) Bandwidth
Answer: (c) Syntax. Syntax defines the structure or format of the data — the arrangement of data in a particular order. Semantics defines the meaning of each section, and Timing defines when and how fast data should be sent.
Fill in the Blank: Standards that are established by widespread use rather than by an official body are called ________ standards.
De facto standards. These are adopted “by fact or by convention” — like the QWERTY keyboard layout. In contrast, de jure standards are formally approved by an officially recognized body “by law or by regulation.”
List and Explain: What are the three key elements of a protocol? Explain each with an example.
1. Syntax: The structure or format of the data. Example: A protocol specifies that the first 8 bits are the sender address, next 8 bits are the receiver address, and the rest is data.
2. Semantics: The meaning of each section of bits and what action to take. Example: If bits read “1100,” it means “request to send” — the receiver knows it should prepare to receive data.
3. Timing: When data should be sent and at what rate. Example: If the receiver sends a “busy” signal, the sender must wait before transmitting more data to avoid overwhelming the receiver.
1.1.9 Circuit Switching and Packet Switching
My student, here is a very important question: When you send data from Addis Ababa to Bahir Dar, does the data travel in one straight line? No! It passes through many intermediate devices along the way. The mechanism by which data is forwarded from source to destination through intermediate devices is called switching.
Networks have interconnecting devices that receive data from directly connected sources, store the data, analyze it, and then forward it to the next interconnecting device closest to the destination. There are three main switching techniques:
A. Circuit Switching
In circuit switching, a complete physical connection between two computers is established FIRST, and THEN data are transmitted.
- A dedicated path is established before any data is sent
- The path remains reserved for the entire duration of communication, even if no data is being sent
- Example: Traditional telephone network — when you make a call, a circuit is established and held until you hang up
- Advantage: No delay once the circuit is set up; constant data rate
- Disadvantage: Wastes resources — the circuit is reserved even during silent periods
B. Message Switching
In message switching, the source sends the entire message to a switching office, which stores the data in its buffers. It then looks for a free link to another switching office and forwards the data. This process continues until the data reaches the destination.
- Also known as “store and forward”
- There is NO dedicated path — each switching office decides the next link independently
- The entire message is stored on disk at each switching office
- No limit on block size
- Example: Old telegraph systems
C. Packet Switching
In packet switching, the message is broken into fixed-size packets before transmission. Each packet is transmitted independently through the network.
- A fixed size of packet is specified — there is a tight upper limit on block size
- Packets are stored in main memory (not disk), which improves performance
- Access time is reduced compared to message switching
- Throughput of the network is improved
- If a packet of L bits is sent over a link with rate R bits/sec, the transmission time is L/R seconds
- Example: The Internet uses packet switching
- Circuit Switching: Dedicated path established first; used in telephone networks; wastes resources during idle periods
- Message Switching: Store and forward; entire message stored on disk; no dedicated path; slow due to disk access
- Packet Switching: Message broken into fixed-size packets; stored in main memory; efficient; used on the Internet; packets may take different routes
- Transmission time for a packet = L/R seconds (where L = packet size in bits, R = link rate in bits/sec)
MCQ: Which switching technique is also known as “store and forward”?
Message Switching. In message switching, the entire message is stored at each switching office in its buffers (on disk) and then forwarded when a free link is available. This is why it is called “store and forward.” Note: Packet switching also uses store-and-forward, but the term is most strongly associated with message switching. The key difference is that message switching stores on disk (slower) while packet switching stores in main memory (faster).
Short Answer: What is the main difference between message switching and packet switching regarding where data is stored?
In message switching, data is stored on disk at each switching office. Disk access is slow, so this reduces the throughput of the network. In packet switching, data (packets) is stored in main memory (RAM) at each switching point. RAM access is much faster than disk access, so the throughput is improved. This is one of the key reasons why packet switching replaced message switching in modern networks.
MCQ: A packet of L bits is sent over a link with transmission rate R bits/sec. What is the transmission time?
(a) L × R (b) L/R (c) R/L (d) L + R
Answer: (b) L/R. The transmission time equals the packet size divided by the transmission rate. For example, if a packet is 1000 bits and the link rate is 10,000 bits/sec, the transmission time is 1000/10000 = 0.1 seconds.
True or False: In circuit switching, the dedicated path is released when no data is being transmitted to save resources.
False. In circuit switching, the dedicated path remains reserved for the ENTIRE duration of communication, even if no data is being sent. This is one of the main disadvantages of circuit switching — it wastes resources during silent periods (like when you are on a phone call but not talking). The path is only released when the communication is completely finished.
Revision Notes: Data Communication (Section 1.1)
1. Definition and Characteristics
- Data Communication = exchange of data between two devices via a transmission medium
- Requires hardware + software
- 4 characteristics: Delivery (correct destination), Accuracy (unaltered data), Timeliness (not late), Jitter (uneven delay variation)
2. Data Representation
- Text: ASCII (7 bits, 128 chars, Basic Latin) or Unicode (32 bits, all languages)
- Numbers: Directly to binary (NOT ASCII)
- Images: Pixels; 1 bit for B/W; more bits for grayscale; RGB or YCM for color
- Audio: Continuous → sampling → digital
- Video: Sequence of images (frames) creating motion illusion
3. Transmission Formats
- Serial: 1 bit at a time, 1 wire, long distance, cheaper, slower
- Parallel: All bits at once, n wires, short distance, expensive, faster
- Synchronous: Large blocks, no start/stop bits, fast, needs sync
- Asynchronous: 1 byte + start/stop bits, slower, simpler
- Little-endian = LSB first; Big-endian = MSB first
4. Transmission Impairments
- Attenuation: Energy loss → fix with amplifiers
- Distortion: Shape change → composite signals, different speeds
- Noise: 4 types — thermal (electrons), induced (motors), crosstalk (wires), impulse (lightning)
5. Transmission Modes
- Simplex: One direction only (keyboard→computer, TV)
- Half-duplex: Both directions, one at a time (walkie-talkie)
- Full-duplex: Both directions simultaneously (telephone)
6. Five Components
- Message, Sender, Receiver, Medium, Protocol
- Protocol = rules (like common language)
7. Protocols and Standards
- Protocol elements: Syntax (format), Semantics (meaning), Timing (when/rate)
- De facto = by convention; De jure = by law
- Organizations: ISO, ITU-T, ANSI, IEEE, EIA, FCC
8. Switching Techniques
- Circuit: Dedicated path first, then data (telephone)
- Message: Store-and-forward on disk, no size limit (telegraph)
- Packet: Fixed-size packets, stored in RAM, efficient (Internet)
- Transmission time = L/R seconds
Mini Exam
Q1: Define data communication and list its four fundamental characteristics.
Data communication is the exchange of data between two devices via a transmission medium using hardware and software. The four characteristics are: (1) Delivery — data reaches the correct destination only, (2) Accuracy — data is not altered, (3) Timeliness — data arrives on time (real-time for audio/video), (4) Jitter — minimal variation in packet arrival time.
Q2: Compare serial and parallel transmission in at least four points.
(1) Wires: Serial uses 1 wire; Parallel uses n wires for n bits. (2) Distance: Serial for long distance; Parallel for short distance. (3) Cost: Serial cheaper; Parallel more expensive. (4) Speed: Parallel faster per unit; Serial slower per unit. (5) Complexity: Serial simpler; Parallel more complex due to wire synchronization.
Q3: Differentiate between circuit switching and packet switching.
Circuit switching: Dedicated path established before data transfer; path reserved for entire session; resources wasted during idle time; used in telephone networks; consistent delay.
Packet switching: No dedicated path; message broken into fixed-size packets; each packet routed independently; resources shared efficiently; used on the Internet; variable delay possible.
Challenge Exam Questions — Mixed Types
Multiple Choice Questions
MCQ 1: Which of the following represents the correct order of bit transmission in a little-endian system for the binary number 1011?
(a) 1011 (b) 1101 (c) 1001 (d) 1110
Answer: (b) 1101. Little-endian sends the Least Significant Bit (LSB) first. In 1011, the LSB is the rightmost bit (1), then next (1), then next (0), then MSB (1). So the transmission order is: 1, 1, 0, 1 = 1101. Big-endian would send 1011 (MSB first).
MCQ 2: Which type of noise is caused by the effect of one wire on another wire in a cable?
(a) Thermal noise (b) Induced noise (c) Crosstalk (d) Impulse noise
Answer: (c) Crosstalk. Crosstalk is the effect of one wire on another — one wire acts as a sending antenna and the other as a receiving antenna. Thermal noise is from electron motion, induced noise is from external devices like motors, and impulse noise is a sudden spike from lightning or power lines.
MCQ 3: In asynchronous serial communication, what signal is transmitted during idle condition?
(a) 0 (b) 1 (c) Alternating 0 and 1 (d) No signal
Answer: (b) 1. During idle condition (when no data is being sent), a signal “1” is transmitted. The start bit is “0” which signals the beginning of data — the transition from idle “1” to start “0” tells the receiver that data is coming.
MCQ 4: The RGB method of color representation uses which three primary colors?
(a) Red, Blue, Yellow (b) Red, Green, Blue (c) Yellow, Cyan, Magenta (d) Red, Green, Yellow
Answer: (b) Red, Green, Blue. RGB stands for Red, Green, Blue. Each color’s intensity is measured and assigned a bit pattern. With 8 bits per channel (24 bits per pixel), you can represent over 16 million colors. The YCM method (Yellow, Cyan, Magenta) is a different color representation system.
MCQ 5: Which switching technique stores data on disk at switching offices?
(a) Circuit switching (b) Packet switching (c) Message switching (d) All of the above
Answer: (c) Message switching. Message switching stores data on disk at each switching office, which makes it slower. Packet switching stores data in main memory (RAM), which is much faster. Circuit switching does not store data at intermediate points — it uses a dedicated path.
MCQ 6: Standards that are formally approved by an official body are called:
(a) De facto standards (b) De jure standards (c) Proprietary standards (d) Open standards
Answer: (b) De jure standards. “De jure” means “by law or by regulation” — these are formally approved by an officially recognized body like ISO or IEEE. “De facto” means “by fact or by convention” — adopted through widespread use without formal approval.
Fill in the Blank Questions
Fill 1: The ________ of a protocol defines when data should be sent and at what rate.
Timing. Timing tells the sender about the receiver’s readiness and the appropriate rate of data transmission to avoid overwhelming the receiver. Syntax defines format, and Semantics defines meaning.
Fill 2: ________ is used to compensate for energy loss in a signal during transmission.
Amplifiers. Attenuation causes loss of energy as a signal travels through a medium. Amplifiers are used to boost the signal strength to compensate for this loss. Note: Amplifiers fix attenuation but NOT distortion (which changes the signal shape).
Fill 3: In ________ transmission mode, both devices can transmit and receive simultaneously, like a two-way street.
Full-duplex (or duplex). Example: telephone network. Simplex is one-way only, half-duplex is both ways but one at a time.
Fill 4: A ________ is a set of rules that governs data communications, defining what, how, and when data is communicated.
Protocol. A protocol defines what is communicated (syntax/format), how it is interpreted (semantics/meaning), and when it is sent (timing/rate). Without protocols, connected devices cannot communicate meaningfully.
Fill 5: ________ noise is a sudden spike of high energy in a very short time, caused by power lines or lightning.
Impulse noise. Unlike thermal noise (continuous, from electrons), induced noise (from motors), or crosstalk (between wires), impulse noise is a sudden, sharp spike that can cause serious data errors — like a crackle on a phone line during a thunderstorm.
List and Explain Questions
List 1: List four types of noise in transmission media and explain each briefly.
1. Thermal noise: Caused by the random motion of electrons in a wire; creates an extra signal not sent by the transmitter; always present; increases with temperature.
2. Induced noise: Comes from external sources like motors and appliances; the source acts as a sending antenna and the medium acts as a receiving antenna.
3. Crosstalk: Caused by the effect of one wire on another in a cable; one wire acts as sending antenna, the other as receiving antenna; like hearing another conversation on your phone line.
4. Impulse noise: A spike (high energy in very short time) from power lines, lightning, etc.; causes sudden errors in data.
List 2: List and briefly explain the five components of a data communication system.
1. Message: The information to be communicated — text, numbers, images, audio, or video.
2. Sender: The device that sends the message — computer, phone, camera, etc.
3. Receiver: The device that receives the message — computer, TV, phone, etc.
4. Transmission Medium: The physical path — twisted-pair wire, coaxial cable, fiber optic, or radio waves.
5. Protocol: Rules governing communication — defines format, meaning, and timing of data exchange.
List 3: Compare synchronous and asynchronous serial transmission in at least four points.
(1) Data unit: Synchronous sends multiple bytes as a block; Asynchronous sends one byte at a time. (2) Start/Stop bits: Synchronous does NOT use them; Asynchronous uses start bit and stop bit(s). (3) Gaps: Synchronous has no gaps between data units; Asynchronous may have gaps. (4) Speed: Synchronous is faster; Asynchronous is slower due to overhead. (5) Cost: Synchronous is more expensive (complex hardware); Asynchronous is cheaper and simpler. (6) Synchronization: Synchronous requires precise sync between sender and receiver; Asynchronous does not need it (start/stop bits provide sync).
Short Answer and Problem Questions
Q1: A packet of 2400 bits is to be sent over a link with a transmission rate of 800,000 bits/sec. Calculate the transmission time.
Q2: Explain why a keyboard-to-computer connection is simplex, while a telephone connection is full-duplex.
A keyboard only SENDS data to the computer — the computer never needs to send data back TO the keyboard through the same connection. Data flows in only one direction, so it is simplex.
A telephone allows BOTH people to talk and listen at the same time. Data flows in both directions simultaneously, so it is full-duplex. This requires either two separate paths or a shared channel with divided capacity.
Q3: Why is parallel transmission not suitable for long-distance communication? Give two reasons.
Reason 1 — Cost: Parallel transmission requires n separate wires for n bits. For long distances, this means n times the cost of cabling, which is extremely expensive. Serial transmission needs only one wire.
Reason 2 — Timing/Synchronization: Over long distances, wires of different lengths cause bits to arrive at slightly different times (skew). This makes it very difficult to reassemble the data correctly at the receiver. Serial transmission avoids this problem because only one bit travels at a time.
Q4: Explain the difference between distortion and noise. Can an amplifier fix either of them?
Distortion changes the SHAPE of the signal — it occurs because different frequency components travel at different speeds through the medium, arriving with different phases. Noise ADDS an external unwanted signal on top of the original signal — it comes from thermal, induced, crosstalk, or impulse sources.
An amplifier can fix attenuation (loss of energy) by boosting the signal strength. However, an amplifier cannot fix distortion because the shape itself has changed — amplifying a distorted signal just gives a stronger distorted signal. An amplifier also cannot remove noise — in fact, it amplifies the noise along with the signal! Special techniques like filtering are needed to reduce noise.
Q5: Why did Unicode replace ASCII as the dominant text coding system? Explain with specific reasons.
ASCII uses only 7 bits (128 characters), which is enough for English but cannot represent characters from most other languages — Amharic, Tigrinya, Chinese, Arabic, Hindi, and thousands of other languages each have their own unique scripts. As computers spread globally, there was a critical need for a universal coding system. Unicode solved this by using 32 bits, allowing representation of over 4 billion characters — enough for every language in the world plus special symbols and emojis. ASCII is now included as just the first 127 characters of Unicode (called Basic Latin), ensuring backward compatibility.
Q6: “Without a protocol, two devices may be connected but not communicating.” Explain this statement with an analogy.
This statement means that having a physical connection (wire, cable) is NOT enough for communication. The devices also need to agree on rules — what format the data is in, what each part means, and when to send data. Without these rules (protocols), the receiver cannot interpret the bits it receives.
Analogy: Imagine a French-speaking person and an Amharic-speaking person connected by a telephone wire. The physical connection exists (the wire), and both can make sounds (hardware works). But no meaningful communication takes place because they don’t share a common language (protocol). Similarly, two computers connected by a cable need a common protocol to communicate meaningfully.
Q7: Compare circuit switching and packet switching. Which one does the Internet use and why?
Circuit switching: Dedicated path established first; resources reserved for entire session; wasteful during idle periods; consistent delay; suitable for voice.
Packet switching: No dedicated path; data divided into packets; each packet routed independently; resources shared among many users; variable delay; suitable for data.
The Internet uses packet switching because: (1) Internet traffic is bursty — users send data in bursts with long idle periods, making circuit switching very wasteful. (2) Packet switching allows efficient sharing of network resources among many users. (3) It is more robust — if one path fails, packets can be routed through alternative paths. (4) It is more cost-effective — no need to reserve a dedicated path for each conversation.
Q8: A black-and-white image is divided into 10,000 pixels. If the same image is instead divided into 1,000 pixels, what are the trade-offs?
10,000 pixels (higher resolution): Better representation of the image — sharper, more detailed. But requires MORE memory to store (10,000 bits for B/W image) and more data to transmit.
1,000 pixels (lower resolution): Poorer representation — blocky, less detailed. But requires LESS memory (1,000 bits for B/W image) and less data to transmit — faster transmission.
This is a fundamental trade-off in digital imaging: quality vs. storage/transmission cost. Higher resolution always requires more resources. This is why image compression techniques (like JPEG) are so important — they try to maintain quality while reducing the number of bits needed.