Network Transmission Methods and Error Control Techniques
Transmission Methods
Synchronous Transmission
In this type of transmission, the transmitter and receiver must use the same clock frequency. Transmission occurs in blocks, delimited by specific bits indicating the beginning and end of each block. This method is efficient as information flows evenly, enabling high transmission rates. Synchronization is achieved by the transmitter sending a start signal, activating the receiver’s clock.
Asynchronous Transmission
Asynchronous transmission sends characters at irregular intervals, such as from a keyboard. To manage this, each character is preceded by a START bit and ends with an END bit, signaling the transmission’s start and completion.
Transmission Media
The transmission medium is the channel enabling information transfer between terminals. Electromagnetic waves typically propagate through this channel. The channel can be physical (like a cable) or non-physical (like a vacuum).
Guided Media
Guided media use cables to direct signals. Transmission speed depends on the distance between terminals and whether the link is point-to-point or multipoint. Examples include UTP (twisted pair), fiber optic, and coaxial cables.
Unguided Media
Unguided media transmit signals wirelessly through a non-physical medium like air or vacuum. Antennas transmit and receive signals. Unguided transmissions can be directional (focused beam) or omnidirectional (dispersed). Examples include Bluetooth, Infrared, and Wi-Fi.
Transmission Modes
Simplex
One station always transmits, and the other receives. Information flows in one direction. Example: Television.
Half Duplex
Stations alternate between transmitting and receiving. Information flows in both directions but not simultaneously. Example: Walkie-talkies.
Full Duplex
Both stations transmit and receive simultaneously. Information flows in both directions at the same time. Example: Direct conversation.
Multiplexing Techniques
Frequency Division
Divides the bandwidth into smaller, independent logical channels, each with a specific frequency range. This allows multiple users to communicate simultaneously without interference.
Time Division
Allocates the entire bandwidth to each channel for a fraction of the total time. This allows multiple sources to share the transmission medium.
Transmission Types
Unicast
One-to-one transmission from a single sender to a single receiver. In unicast, the server responds to multiple requests by sending data separately to each user, potentially causing network congestion.
Multicast
One-to-many transmission from a single sender to multiple receivers simultaneously. The router distributes the information. Example: A conference call.
Broadcast
One-to-all transmission from a single sender to all nodes on the network. Example: Radio or TV broadcasts. This can increase network traffic and send information to unintended recipients.
Network Design Requirements
Network design determines the physical structure of the network. A good design prevents data loss, network downtime, slow processing, and security issues.
Key considerations include:
- Hardware specifications for each computer
- Server selection
- Network adapter types
- Hardware requirements (modems, routers, switches, cables)
- Processor and memory capabilities
- Distance measurements between devices
Additional requirements:
- Number of wiring points
- Network location and facility plans
- Use of certified materials and tools
- Wiring certification
Network Interface Card
A specialized card that manages data transfer between the computer and the network.
Security
Multiple hard drives and backup units are essential for data security and backup.
Flow Control
Flow control prevents a sender from overwhelming a receiver with too much data. The receiver reserves temporary memory for the transfer. Without flow control, this memory could overflow.
Sliding Window Flow Control
A software-based flow control mechanism where the receiver communicates its readiness to receive data using control characters or frames. This solves data flow and transmission efficiency issues.
Transmission Window
Allows the sender to transmit multiple packets before waiting for acknowledgment (ACK). The sender stores unacknowledged frames in a buffer for potential retransmission.
Reception Window
Allows the receiver to accept out-of-order frames, storing them temporarily until the entire sequence is received and sorted.
Stop and Wait Flow Control
The sender transmits one frame and waits for acknowledgment before sending the next. The receiver can also stop the flow by withholding acknowledgments.
Error Detection
Techniques to detect errors during transmission by adding redundant information.
Parity
An extra bit (parity bit) is added to the data. Even parity: parity bit is 0 if the number of 1s is even, 1 if odd. Odd parity: the opposite.
Cyclic Redundancy Check (CRC)
A function that generates a fixed-length output from a data stream of any length, used as a checksum to detect data alteration.
Error Control
Mechanisms to detect and correct errors during frame transmission. Two main error types:
- Lost frame: A sent frame doesn’t reach the receiver.
- Damaged frame: A frame arrives with bit errors.
Automatic Repeat Request (ARQ) techniques address these errors. Three types:
- ARQ with Stop and Wait
- ARQ with Go-Back-N
- ARQ with Selective Reject
ARQ with Stop and Wait
The sender transmits one frame and waits for acknowledgment. If no acknowledgment is received before timeout, the frame is retransmitted. The sender keeps a copy of the frame until acknowledged.
ARQ with Go-Back-N
The sender transmits multiple frames. If the receiver detects an error, it sends a negative acknowledgment (REJ). The sender retransmits the erroneous frame and all subsequent frames.
ARQ with Selective Reject
Only the unacknowledged frame is retransmitted if an error occurs. This improves transmission efficiency but requires more receiver memory to store out-of-order frames.