Computer Networks Summary

Layered Architecture

• Each layer has its responsibility
• Main purpose: separation of concerns

What each layer does

1. It takes input from the layer above it
2. It communicates with the layer below it to process the input
3. After processing, it returns output to the layer above it.

OSI Model(The Open Systems Interconnection Model)

• A conceptual model that enables data communication
• Consists of 7 layers: physical layer, data-link layer, network layer, transport layer, session layer, presentation layer, and application layer

TCP/IP Protocol Suite

• A set of protocols used on the Internet
• Maintained by IETF(Internet Engineering Task Force)
• Consists of 4 layers: data access layer, internet layer, transport layer, and application layer

Data-Link Layer(Layer 2)

• Responsible for point-to-point reliable data transfer within the same network
• HDLC(High-Level Data Link Control) is the most commonly used protocol at this layer

Key Features

• Error Control: error detection and correction
• Flow Control: the sender transmits data at the rate the receiver can receive

Error Detection Techniques

• Parity bits
• CRC(Cyclic Redundancy Check): FCS(Frame Check Sequence)
• Checksum: get the sum of fixed-sized bytes and end-around carry(adding carry to the least significant bit)

Error Correction Techniques

• Hamming Code: error correction is used when there is a long propagation delay, leading to a long retransmission time

Flow Control Techniques

• Stop-and-wait flow control: the source sends a single frame and waits until it receives an acknowledgment from the destination
• Sliding window flow control: the source sends multiple numbered frames up to the window size simultaneously

Error Control Techniques

• Stop-and-wait ARQ:
• Go-back-N ARQ: When the receiver received out-of-order or corrupted packets, it returns the acknowledgment for the last in-order and valid frame
• Selective-repeat ARQ

Flow Control

Key Features

Transport Layer(Layer 4)

• Responsible for end-to-end reliable data transfer across multiple networks
• TCP(Transport Control Protocol) and UDP(User Datagram Protocol) are most commonly used protocol at this layer

Key Features

• Error Recovery: Error detection and correction
• Flow Control: The sender transmits data at the rate the receiver can receive
• Congestion Control: The sender transmits data at the rate data while being sent

Error Control Techniques

Acknowledgment

• Positive Acknowledgment:
• Negative Acknowledgment:
• Cumulative Acknowledgment: the receiver sends what to the sender an acknowledgment for the highest sequence numbe