Types and methods of data transmission Revision Notes

When data is sent over a network, it is not transmitted as one large continuous stream. Instead, it is broken down into smaller units called packets. This segmentation allows for more efficient and reliable transmission. Smaller packets reduce the chance of errors affecting large amounts of data and enable multiple communications to share the same network simultaneously.

For example, when sending a large file, the file is split into many packets that travel independently through the network. This approach also allows the network to route packets around busy or faulty parts, improving overall performance and resilience.

• Packets improve transmission efficiency and reliability.
• Smaller units reduce the impact of errors.
• Multiple communications can share the network simultaneously.

Each packet consists of three main parts: the header, the payload, and the trailer. Each part has a specific role in ensuring the packet reaches its destination correctly and can be reassembled into the original data.

The header contains control information needed for routing and reassembly. It includes the destination address (where the packet is going), the originator's address (where it came from), and the packet number (which helps to order packets correctly).

The payload is the actual data being sent, such as part of a file, a message, or a web page content. It is the main content the sender wants to transmit to the receiver. The size of the payload can vary depending on the network protocol and the maximum transmission unit (MTU). The trailer often contains error-checking information, such as a checksum or cyclic redundancy check (CRC), which helps detect if the packet has been corrupted during transmission.

• Header: destination address, originator's address, packet number.
• Payload: the actual data being transmitted.
• Trailer: error-checking information to detect transmission errors.

Packet switching is the method by which packets are sent independently through the network. Each packet can take a different route to reach the destination, depending on network traffic and availability. Routers are devices that direct packets along the best available path at each step.

Because packets may arrive out of order, the receiving device uses the packet numbers in the headers to reorder them correctly before reconstructing the original data. If any packets are missing or corrupted, the receiver can request retransmission.

For example, if you send an email, the message is split into packets that travel through various routers. Some packets may take a direct route, while others take longer paths. Once all packets arrive, they are reordered and combined to display the complete email.

• Packets travel independently and may take different routes.
• Routers direct packets based on destination addresses.
• Packets can arrive out of order and must be reordered.
• Error detection allows for retransmission requests if needed.

Routers are specialised network devices that manage the flow of packets between different networks. They read the destination address in each packet's header and decide the best next hop for the packet to reach its destination efficiently.

Routers maintain routing tables that help them choose paths that avoid congestion or failures. This dynamic routing ensures packets find the quickest or most reliable route. Without routers, packets would not be able to navigate complex networks such as the internet.

For example, when streaming a video, routers help packets find the fastest path from the video server to your device, even if some network links are busy or down.

• Routers read destination addresses to forward packets.
• They use routing tables to select optimal paths.
• Routers enable dynamic routing around network issues.
• They are essential for managing complex network traffic.

Since packets can arrive at the destination in any order, the receiving device must reorder them using the packet numbers found in the headers. This process ensures the original message or file is reconstructed accurately.

If some packets are delayed or lost, the receiver can detect this by missing packet numbers and request retransmission. Once all packets are received and verified, the payloads are combined in the correct sequence to restore the complete data.

For example, when downloading a file, the computer waits until all packets arrive and are correctly ordered before saving the file, ensuring no corruption or missing parts.

• Packets may arrive out of order due to different routes.
• Packet numbers in headers allow correct reordering.
• Missing or corrupted packets trigger retransmission requests.
• Payloads are combined to reconstruct the original data.

Data transmission is the process of sending digital information between devices. The choice of transmission method affects speed, complexity, cost, and reliability. Understanding the differences between methods helps in designing efficient communication systems.

Data can be transmitted in different ways depending on how bits are sent (serial or parallel) and how communication flows (simplex, half-duplex, full-duplex).

• Transmission methods impact data transfer rate and hardware requirements.
• Choosing the right method depends on the application’s needs for speed, directionality, and cost.

Serial transmission sends data bits one after another over a single channel or wire. This method requires fewer wires and is simpler to implement, making it suitable for long-distance communication where signal degradation is a concern.

Parallel transmission sends multiple bits simultaneously over multiple channels or wires. For example, an 8-bit parallel transmission sends 8 bits at the same time, increasing data transfer speed.

However, parallel transmission is more expensive and complex due to the need for multiple wires and synchronization between channels. It is typically used for short distances, such as inside a computer between components.

• Serial: one bit at a time, fewer wires, slower but reliable over long distances.
• Parallel: multiple bits simultaneously, faster but requires more wires and synchronization.
• Example: USB uses serial transmission; old computer buses like the printer port used parallel transmission.

These terms describe the directionality and timing of data flow between devices.

Simplex communication allows data to flow in only one direction. An example is a keyboard sending data to a computer; the keyboard does not receive data back.

Half-duplex allows data to flow in both directions but not at the same time. Walkie-talkies are a practical example: one person speaks while the other listens, then they switch roles, but both cannot talk simultaneously on the same channel without interference. This method requires devices to switch between sending and receiving modes, which can introduce delays but reduces hardware complexity compared to full-duplex systems.

• Simplex: one-way only, simple and cheap, e.g., keyboard to PC.
• Half-duplex: two-way but alternate, e.g., walkie-talkies.
• Full-duplex: two-way simultaneously, e.g., telephone calls.

Full-duplex communication allows data to flow simultaneously in both directions. This method doubles the effective communication capacity compared to half-duplex.

Full-duplex requires separate channels or frequencies for sending and receiving, or sophisticated timing to avoid collisions. This increases hardware complexity and cost but is essential for applications requiring real-time two-way communication, such as telephone networks and modern Ethernet connections.

Example: In a telephone call, both parties can speak and listen at the same time without interruption.

• Full-duplex: simultaneous two-way communication.
• Requires more complex hardware or separate channels.
• Ideal for real-time interactive communication.

Choosing the appropriate transmission method depends on factors such as distance, speed requirements, cost constraints, and the nature of communication.

For long-distance communication where wiring cost and signal integrity matter, serial transmission is preferred despite lower speed.

For short distances requiring high speed, such as inside a computer, parallel transmission is suitable but limited by signal timing issues over longer cables due to skew (timing differences between wires). Simplex is used when only one-way communication is needed, reducing complexity and cost, such as in sensors sending data to a controller without feedback needed. Half-duplex suits situations where two-way communication is needed but simultaneous transmission is not critical, saving cost compared to full-duplex. Full-duplex is essential for applications requiring continuous two-way communication, despite higher cost and complexity.

• Serial for long-distance, cost-effective communication.
• Parallel for short-distance, high-speed data transfer.
• Simplex for one-way data flow.
• Half-duplex for two-way alternate communication.
• Full-duplex for simultaneous two-way communication.

The Universal Serial Bus (USB) is a standard interface used to connect peripheral devices to a computer or other host device for data transmission and power supply. It allows multiple devices such as keyboards, mice, printers, external storage drives, and cameras to communicate with a computer through a single type of port and connector.

USB replaces older interfaces like serial and parallel ports by providing a universal, standardised connection method that supports plug-and-play and hot-swapping, meaning devices can be connected or disconnected without restarting the computer.

• Standard interface for connecting peripherals.
• Supports both data transfer and power supply.
• Enables plug-and-play and hot-swapping.

USB technology has evolved through several versions, each offering different maximum data transfer speeds and capabilities. Understanding these versions helps in selecting the appropriate USB interface for specific devices and data transmission needs.

The main USB versions include: Low-Speed USB (1.5 Mbps), Full-Speed USB (12 Mbps), High-Speed USB (480 Mbps), USB 3.0 or SuperSpeed (5 Gbps), and USB 3.1/3.2 or SuperSpeed+ (up to 10 Gbps or higher). Each version is backward compatible with previous versions but actual transfer speeds depend on both the host and device supporting the same USB standard.

For example, a USB 3.0 external hard drive connected to a USB 2.0 port will operate at USB 2.0 speeds (up to 480 Mbps).

• Low-Speed USB: 1.5 Mbps, for devices like keyboards.
• Full-Speed USB: 12 Mbps, common for mice and audio devices.
• High-Speed USB: 480 Mbps, used for faster devices like flash drives.
• USB 3.0 and above: 5 Gbps or more, for high-bandwidth devices like external SSDs.

USB interfaces offer several benefits that make them widely used in computer systems:

Compatibility: USB ports and connectors are standardised, allowing a wide range of devices to connect without needing special adapters.

Ease of Use: USB supports plug-and-play, so devices are recognised and configured automatically by the operating system without manual driver installation in most cases. Hot-swapping allows devices to be connected or removed while the computer is running without causing damage or data loss (if safely ejected).  - Power Supply: USB cables can supply electrical power to connected devices, eliminating the need for separate power adapters for many peripherals like mice, keyboards, and some external drives. This simplifies setup and reduces cable clutter.

• Standardised connectors ensure broad compatibility.
• Plug-and-play and hot-swapping simplify device management.
• Power delivery through USB reduces need for extra power sources.

Despite its advantages, USB technology has some limitations:

Cable Length: USB cables have maximum recommended lengths to maintain signal integrity. For example, USB 2.0 cables should not exceed 5 metres, and USB 3.0 cables typically have a maximum length of about 3 metres. Longer cables may require active repeaters or hubs to maintain performance.

Speed Limitations: Although newer USB versions offer high speeds, actual data transfer rates can be limited by the device, cable quality, or host controller. Older USB versions have significantly lower speeds, which may not be suitable for large data transfers or high-bandwidth devices like HD webcams or external SSDs without USB 3.0 or higher support.  - Shared Bandwidth: USB ports on a computer often share bandwidth through hubs or controllers, so connecting multiple high-speed devices simultaneously can reduce effective transfer speeds for each device.  - Connector Fragility: Frequent plugging and unplugging can wear out USB connectors or ports over time, potentially causing connection issues.

• Limited cable length affects device placement.
• Data transfer speed depends on USB version and device capabilities.
• Multiple devices may share bandwidth, reducing speed.
• Connectors can wear out with frequent use.