Data Transmission Techniques: Hamming Code & Bit Stuffing

Hamming Code

• Hamming code is a method for error detection and correction in data transmission.
• It uses check bits (parity bits) to identify and sometimes correct errors in data.
• Check bits are placed at positions that are powers of 2 (e.g., positions 1, 2, 4, 8 etc.).
• The remaining positions are for the original data bits.
• Parity can be even or odd depending on the system.
• Hamming codes are designed to detect and correct single-bit errors.
• For a two-bit error, it will fail correction and only detect the error.
• With more than two errors, the detection will also fail.

Bit Stuffing

• Bit stuffing is a technique for data transmission that adds extra bits to a data stream to prevent specific bit patterns from being interpreted as control characters.
• It is a way of ensuring that the receiver can distinguish between data and control signals.
• It is usually done to avoid special patterns that might trigger the receiver to start or stop processing data.
• It works on a bit level, making it more flexible than byte stuffing.
• Bit stuffing involves inserting a "0" bit after every five consecutive "1" bits in the data stream.
• The receiver then removes these "0" bits during destuffing.

Error Control

• Error control mechanisms aim to ensure reliable data transmission in the presence of noise and interference.
• Two types of error control services include error detection and error correction.
• Error correction methods involve using redundancy to detect and correct errors.
• Error detection methods identify errors without necessarily correcting them.
• Errors can occur randomly (single-bit) or in bursts (burst error).
• Burst errors are caused by external noise and their impact depends on the data rate and duration of the noise.

HTTP

• Hypertext Transfer Protocol (HTTP) is a communication protocol that defines how web browsers and web servers exchange data.
• It is a request-response protocol, where the client sends a request to the server, and the server responds with the requested data.
• HTTP is a stateless protocol, meaning that it does not maintain any information about past interactions between the client and server.
• HTTP uses TCP for connections.

DNS

• Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, or other resources connected to the internet or a private network.
• DNS translates domain names (e.g., www.google.com) into IP addresses (e.g., 172.217.160.142), which are the numerical addresses used by computers to communicate on the internet.
• DNS is essential for web browsing. Without DNS, users would need to remember or manually look up IP addresses for every website they want to visit, making the internet much more difficult and cumbersome to use.

Cookies

• Cookies are small pieces of data that websites store on a user's computer.
• They are used to store user preferences, login information, and other data that can be used to personalize the user's web experience.
• Cookies are usually used for:
  • User authentication and session management
  • Shopping cart functionality
  • User tracking
  • Web analytics

Sessions

• A session is information about a user's interaction with a website that is stored on the server side.
• Sessions are typically used to maintain user information, such as login status, preferences, and items in a shopping cart, during a specific browsing session.
• When the user closes the website or the session times out, the session information is deleted from the server.

Web Caching

• Web caching is a technique used to improve website performance and reduce server load.
• It involves storing copies of frequently-accessed web content on a cache server.
• When a client requests a web page, the cache server first checks its local cache. If the content is in the cache, it is returned directly to the client, bypassing the origin server.
• If the content is not in the cache, the cache server fetches it from the origin server and stores it in its cache for future requests.
• This process significantly reduces the response time for subsequent requests for the same content, especially for users who are geographically close to the cache server.

Application Layer

• The application layer in the networking model is responsible for providing services to applications, such as email, web browsing, and file transfer.
• It interacts with users and applications to provide network functionality.
• While the application layer is the top layer of the networking model, it interacts with the transport layer to send data to the destination.
• Several application layers exist, including domain name system (DNS), world wide web (WWW), email, and streaming multimedia data.

Congestion Control

• Congestion control is a mechanism that attempts to manage network congestion by regulating traffic flow.
• Network congestion occurs when too many devices try to send data over a shared network path, causing delays and data loss.
• Congestion control aims to prevent this by:
  • Reducing the amount of data being sent
  • Optimizing network routing to reduce congestion points
  • Prioritizing traffic based on importance
• TCP uses congestion control mechanisms to dynamically adjust the transmission rate based on network conditions.
• This involves:
  • Reducing the transmission window size when congestion is detected
  • Increasing the transmission window size gradually when congestion subsides

Flow Control

• Flow control is a mechanism that ensures that a sender does not send data faster than the receiver can handle.
• It is an end-to-end mechanism that operates between the sender and receiver, ensuring a stable and balanced transmission.
• Flow control is implemented through acknowledgments, such as ACKs, which the receiver sends to the sender confirming data receipt.
• If the receiver is overwhelmed, it can slow down the sender by delaying or suppressing ACKs.
• This reduces the sender's transmission rate to match the receiver's processing capacity.

Data Link Layer Protocols

• Point-to-Point Protocol (PPP)
  • Used on various links (fiber optics, telephone lines)
  • Provides data link layer services (framing, error control, flow control)
  • Uses flag and byte stuffing for framing:
    • Flag byte: 01111110 (6 consecutive 0s)
    • Escape mechanism for flag bytes appearing in the payload
  • PPP frame includes fields for payload (network layer IP packet), error control, and type of service information.
• Packet over SONET
  • Uses PPP as a general data link layer service
  • Frames are split into pieces (around 800 bytes) and sent at regular intervals
• ADSL
  • Asymmetric: wider frequency band for downloading, smaller band for upstream
  • Uses protocols like AAL5 and ATM for supporting PPP over ADSL
  • ATM: Asynchronous Transmission Mode
  • PPP frames are passed to AAL5 (adaptation layer) and then to ATM before being sent to ADSL.

Network Layer Services

• Network layer requests different services:
  • Connection-oriented vs. connectionless
  • Acknowledged vs. unacknowledged
• Services provided to the network layer:
  • Transferring data from the network layer on the source host to the network layer on the destination host
  • Factors to consider when selecting a service: reliability, capacity, cost (time and money)
  • In unreliable channels, additional mechanisms are needed to provide high-quality service.

Unacknowledged Connection Service

• No confirmation from the receiver.
• If a frame is lost, the data link layer attempts to recover it.
• The host transmits frames independently without acknowledgement.
• No logical connection establishment or release.
• No lost frame recovery mechanism (or left to higher levels).
• Suitable for applications:
  • Ethernet LANs
  • Real-time traffic (e.g., audio, video data) where speed is prioritized.

Acknowledged Connectionless Service

• Source host transmits independent frames to recipient host with acknowledgement.
• No logical connection establishment or release.
• Each frame is individually acknowledged and retransmitted if lost or errors occur.
• Suitable for applications:
  • Wireless networks, as they are less reliable than Ethernet and require confirmation from the receiver.

HTTP Protocol

• Responsible for communication and transfer of web pages.
• Request-response protocol.
• One server can handle multiple clients.
• Uses TCP for connections.
• Types of HTTP connections:
  • Non-persistent: A new connection is established for every object within the page.
  • Each object requires a connection establishment (three-way handshake), data transfer, and disconnection – not efficient.
  • Persistent: Multiple objects can be sent over a single TCP connection, reducing overhead.
    • Can lead to resource waste if connections remain open for long periods.

Streaming

• Streaming media: Browser is no longer involved once started, and media goes directly to the media player.
• RTSP (Real-Time Streaming Protocol): Application layer protocol used by media players, can use TCP.
• RTP (Real-Time Transport Protocol): Transport layer protocol, UDP-based, supports multicasting and time stamping, also handles reliability.
• MPEG-4: Standard protocol for compressing large media files.

Multimedia Software Challenges

• Handling errors:
  • FEC (Forward Error Correction): Uses redundant data packets (parity bits, checksum, CRC) to detect and correct errors.
  • Limited to correcting one error.
  • Error Resilient Designs: Minimize the impact of errors by using resync markers, error concealment, and retransmission.
• Jitter Management:
  • Buffer: Used to manage delays and prevent gaps in playback.
    • Low water mark: Prevents buffer underflow.
    • High water mark: Prevents buffer overflow.

TCP Congestion Control

• Additive Increase Multiplicative Decrease (AIMD): Technique for managing congestion based on timeout and duplicate ACKs.
  • Additive Increase: Window size increased gradually.
  • Multiplicative Decrease: Window size decreased significantly when congestion is detected.    

TCP Versions

• TCP Tahoe: Slow start with additive increase.
• TCP Reno: Includes TCP Tahoe with fast recovery.

TCP Slow Start

• Window size increases exponentially until a timeout event occurs, then linearly.
• Slow start threshold: Initial threshold defined by the sender. After a timeout, the threshold is halved, and slow start begins again.

TCP Reno

• Faster than Tahoe.
• Duplicate ACKs (Fast Retransmission) signal congestion.
• Upon receiving 3 duplicate ACKs, the packet is retransmitted immediately.
• Fast Recovery occurs after retransmission.

TCP Challenges in Wireless Networks

• Frequent packet loss and environmental factors present significant challenges for TCP in wireless environments.

Explore the methods of error detection and correction in data transmission with Hamming Code, which uses parity bits to detect and correct errors. Additionally, learn about Bit Stuffing, a technique that prevents specific bit patterns from being mistaken for control characters. Understand how these techniques ensure accurate and reliable data communication.