2.1 Networks including the Internet

Bit Streaming

Bit streaming is a fundamental concept in network communication. It describes the process of transmitting data as a continuous sequence of individual bits (0s and 1s) over a communication channel. In essence, data is broken down into bits and sent sequentially, without any inherent structure or boundaries within the stream. The receiver must then interpret this stream to reconstruct the original data.

Consider a simple example: transmitting the binary number 1011001. In bit streaming, this would be sent as a sequence of bits: 1, 0, 1, 1, 0, 0, 1. The receiving device needs to know how to interpret this sequence as the binary value.

Challenges with Bit Streaming

Direct bit streaming presents several challenges. Without a defined structure, the receiver has no way of knowing where one piece of data ends and another begins. This leads to the need for framing and synchronization techniques.

Key challenges include:

• Start and Stop Bit Detection: The receiver needs to identify the beginning and end of a data transmission.
• Synchronization: The sender and receiver must be synchronized in terms of bit timing.
• Error Detection: Detecting errors that may occur during transmission.

Framing

Framing is a method used to define the boundaries of data within a bit stream. It involves adding special bits (start and stop bits) to indicate the beginning and end of a data unit. This allows the receiver to correctly identify and separate individual data packets.

Common framing methods include:

1. Start Bit: A single bit (usually 0 or 1) indicates the start of a data unit.
2. Stop Bit: A single bit (usually 0 or 1) indicates the end of a data unit.
3. Parity Bit: An extra bit used for error detection.

Example of Bit Streaming with Framing

Let's consider a simple example using a start bit, stop bit, and parity bit. We want to transmit the data '1011'.

Bit Position	Bit	Description
1	1	Start Bit
2	0	Data Bit
3	1	Data Bit
4	1	Data Bit
5	0	Data Bit
6	0	Stop Bit
7	P	Parity Bit (calculated based on the data bits)

In this example, the parity bit is calculated to ensure an even or odd number of 1s in the data bits. The parity bit helps detect single-bit errors during transmission.

Error Detection

Error detection is crucial in network communication to ensure data integrity. Parity bits are a simple form of error detection. More sophisticated techniques, such as checksums and Cyclic Redundancy Checks (CRCs), are used in modern networks.

Checksums: A checksum is a calculated value based on the data being transmitted. The receiver recalculates the checksum and compares it to the received checksum. If they don't match, an error is detected.

CRCs: CRCs are more powerful error detection codes that can detect a wider range of errors. They are commonly used in data transmission and storage.

$$ \text{CRC} = \text{Data} \times \text{Generator Polynomial} \pmod{2^n} $$

Where 'n' is the degree of the generator polynomial.

Conclusion

Bit streaming is a fundamental concept in networking. Understanding the challenges associated with it, and the techniques used for framing and error detection, is essential for comprehending how data is transmitted reliably over networks, including the internet.