Computer Science (9645) | Communications

🚀 Study Notes: Communications (9645) 🚀

Hello future Computer Scientists! This chapter is essential because it moves us from understanding what happens *inside* a single computer to how computers talk to each other—the foundational building blocks of the entire internet! Don't worry if terms like 'baud' or 'asynchronous' sound technical; we will break them down using simple, everyday analogies. Let's learn how data travels!

3.14.1 Communications

3.14.1.1 Communication Methods: How Data Moves

Serial vs. Parallel Data Transmission

When a computer sends data (a sequence of bits, like 10110010) to another device, it has two main options:

1. Parallel Transmission: Imagine a multi-lane highway.

• Data is sent across multiple wires simultaneously (e.g., 8 bits sent using 8 separate wires at the same time).
• Advantage: Very fast over short distances because a whole byte can be sent in one clock cycle.
• Disadvantage: Over long distances, timing errors become a major issue. This is called skew, where bits arrive at different times because the wires aren't exactly the same length. It also requires more expensive, bulkier cables.

2. Serial Transmission: Imagine a single-lane road.

• Data is sent one bit at a time over a single wire or channel.
• Advantage: Highly reliable and cheap over long distances. Since all bits travel the same path, there is no chance of skew.
• Disadvantage: Slower than parallel transmission over very short distances.

💡 Real-World Example:
You use Serial transmission every day! USB (Universal Serial Bus) is a serial technology. Ethernet cables are also serial. Even though it seems slower, the reliability over long distances makes it the preferred choice for networking and external devices.

Quick Takeaway: Serial is for distance and reliability; Parallel is for short, internal tasks (like sending data from a CPU to internal RAM).

Synchronous vs. Asynchronous Data Transmission

Once we choose how many wires to use, we need to decide how to time the data transfer.

1. Synchronous Transmission:

• Data is sent in large blocks or streams.
• The sender and receiver share a common clock signal or timing reference. The receiver knows exactly when the next block of data will arrive based on this clock.
• No extra start or stop bits are needed between data chunks, making it efficient for high volumes of data.
• Analogy: A synchronized swimming team. Everyone moves according to one shared metronome.

2. Asynchronous Transmission:

• Data is sent in individual bytes or small packets.
• The sender and receiver do not share a common clock. The timing is established for each individual piece of data.
• This requires extra control bits (start bit and stop bits) to frame the data.
• Analogy: Sending a text message. Each message is independent and is wrapped in its own addressing information.

The Purpose of Start and Stop Bits

In asynchronous communication, these extra bits are vital for the receiver to correctly interpret the data:

• Start Bit: This is a special signal (usually 0) sent just before the actual data byte. Its purpose is to immediately bring the receiver's timing mechanism (its internal clock) into phase with the sender's clock. It tells the receiver: "A new byte of data is arriving NOW!"
• Stop Bit(s): This is a signal (usually 1 or more bits) sent immediately after the data byte.
  • It tells the receiver: "That's the end of the byte."
  • In older or slower systems, it also provides a necessary time interval for the receiver to process the received byte and prepare for the next possible start bit.

3.14.1.2 Communication Basics: Measuring Performance

When we discuss network performance, we use several key terms to measure speed, capacity, and delay.

Key Definitions

It is crucial to know the precise definitions for these terms:

1. Protocol:
The agreed set of rules and standards that govern how data is transmitted and received between devices. Protocols ensure that all devices speak the same "language."
Example: TCP/IP, HTTP.

2. Baud Rate:
The number of signal events (or symbol changes) that occur per second on a transmission medium.
Analogy: The number of times a traffic light changes state per minute.

3. Bit Rate:
The number of bits transmitted per second (bps). This measures the *actual* amount of data being moved.
Analogy: The actual number of cars passing a point per second.

4. Bandwidth:
The maximum rate at which data can be transferred across a network path. It is often visualized as the "size of the pipe."
Note: Usually measured in bits per second (bps), representing the capacity limit.

5. Latency:
The time delay before a transfer of data begins following an instruction for its transfer. It is essentially the time it takes for the first bit of data to travel from source to destination.
Analogy: The delivery time for a package—the delay between clicking 'send' and the recipient starting to receive the data.

Baud Rate vs. Bit Rate: A Critical Distinction

This is where students often get confused.

The core difference is that Bit Rate can be higher than Baud Rate.

• Baud Rate measures how many times the signal *changes* state.
• Bit Rate measures how many *bits* are actually carried.

Why the difference? Advanced encoding techniques (like those used in modern modems) can encode more than one bit of data into a single signal change (symbol).

Example: If a signal change carries 4 bits of data (e.g., by using 16 different voltage levels), and the signal changes 1000 times per second (1000 Baud):

Bit Rate = 1000 (Baud) $\times$ 4 (bits per signal change) = 4000 bps.

Remember: Always focus on the Bit Rate as the measure of true data throughput. If nothing else is mentioned, 1 Baud usually equals 1 bps, but in high-speed communications, the Bit Rate is often much higher than the Baud Rate.

Bit Rate and Bandwidth Relationship

The Bit Rate is directly proportionate to the Bandwidth.

Think of bandwidth as the theoretical maximum capacity of the communication channel. The bit rate is the actual speed achieved. A larger bandwidth allows for a higher potential bit rate (more data per second). If you increase the bandwidth (make the "pipe" wider), you can increase the bit rate (pump more data through).

Did you know?
The term 'baud' comes from Émile Baudot, who invented an early form of telegraph code. The term has existed much longer than modern digital communications!