Types and methods of data transmission
When a large file (such as an image or video) needs to be sent over a network, it is not sent as one continuous stream. Instead, the data is broken down into smaller chunks called packets. This process is essential for efficiency and reliability.
Why do we use packets?
- Efficiency: Multiple users can share the same network cable simultaneously. If one user's connection is slow or blocked, other users' packets can take different routes.
- Reliability: If a packet is lost or corrupted during transmission, only that specific small packet needs to be resent, rather than the entire large file.
- Routing: Packets can travel through the most efficient path available at that moment.
A packet consists of two main parts: the header and the payload.
- Payload: The actual data being transmitted (e.g., part of an image, a line of text).
- Header: Control information added to the payload to ensure it arrives correctly. It contains:
- Destination Address/IP: Where the packet needs to go.
- Source (Originator) Address/IP: Where the packet came from (so the receiver knows who sent it).
- Packet Number/Sequence Number: Allows the receiving device to reassemble the data in the correct order.
- Error Detection Data: Information used to check if the packet was corrupted during transit.
Scenario: Device A sends a file to Device B.
- Segmentation: The file is split into packets (e.g., Packet 1, Packet 2, Packet 3).
- Routing: Each packet is sent independently through the network. Routers examine the destination address and choose the best available path for each packet. Therefore, Packet 1 might take Route X, while Packet 2 takes Route Y.
- Transmission: Packets travel across various network links (cables, wireless).
- Arrival: Because they took different routes, packets often arrive at Device B out of order (e.g., Packet 2 arrives before Packet 1).
- Reassembly: Device B uses the sequence numbers in the headers to reorder the packets and reconstruct the original file.
- Error Handling: If a packet is missing or corrupted, Device B requests that specific packet be resent.
Correction: In packet switching, packets take different routes. This is a key advantage because it avoids congestion. If you state they take the same route, you are describing circuit switching (like an old telephone line), which is incorrect for modern internet data transmission.
Mistake: Students forget to mention reordering.
Correction: You must explicitly state that packets arrive out of order and are reordered at the destination. Without this, the explanation of how the file is reconstructed is incomplete.
Examiner Expectation: The markscheme looks for specific keywords. You must mention:
- Data is broken/split into packets.
- Packets take different routes/paths.
- Routers control the routing/path selection.
- Packets arrive out of order.
- Packets are reordered/reassembled at the destination.
Why this works: Examiners award marks for each distinct step in the logical flow. Missing 'different routes' or 'reordering' will cost you marks even if you describe the rest correctly.
- Data is broken/split/divided into packets.
- Each packet takes a different route/path.
- Routers control/select the route/path (often shortest/fastest).
- Packets arrive at the destination out of order.
- Packets are reordered/reassembled in the correct sequence at the destination.
Data transmission methods are defined by how bits move and how many wires are used.
Serial Transmission:
- Bits are sent one at a time (sequentially).
- Uses a single wire (or channel).
- Physical Media: Can use copper cables, fiber optics, or wireless radio waves.
- Suitability: Best for long distances because there is less interference/crosstalk. It is the standard for USB and internet connections.
Parallel Transmission:
- Multiple bits are sent simultaneously (e.g., 8 bits at once).
- Uses multiple wires (a bus).
- Physical Media: Typically copper cables (e.g., old printer cables, internal computer buses).
- Suitability: Fast for short distances. However, over long distances, bits may arrive at slightly different times (skewing), causing errors.
This describes the direction data can flow between two devices.
Simplex:
- Data flows in one direction only.
- Example: Keyboard to computer, Radio broadcast.
- Only one device can transmit; the other can only receive.
Half-Duplex:
- Data can flow in both directions, but not at the same time.
- Devices must take turns transmitting.
- Example: Walkie-talkies (you press a button to speak, release to listen).
Full-Duplex:
- Data can flow in both directions simultaneously.
- Example: Telephone calls, modern Ethernet networks.
Scenario 1: Sending a large video file from a server to a user over the internet.
- Method: Serial Full-Duplex.
- Reasoning: Internet cables/fiber optics are long. Parallel transmission would suffer from skewing over such distances. Serial is reliable for long distances. Full-duplex allows the user to request data while receiving it.
Scenario 2: Connecting a keyboard to a computer.
- Method: Serial Simplex (or Half-Duplex depending on interface, but logically simplex in function).
- Reasoning: Keyboards only send data to the computer; they never receive data from the computer. Simplex is sufficient and simpler/cheaper.
Correction: You must specify that it happens simultaneously. If you just say "both ways," you might be describing Half-Duplex. The word simultaneously is the key differentiator.
Mistake: Saying Parallel transmission is always faster.
Correction: Parallel is only faster for short distances. For long distances, Serial is often faster and more reliable because it avoids skewing and crosstalk issues inherent in parallel cables.
Examiner Expectation: Do not just list benefits. Connect the feature to the scenario.
- Correct Phrasing: "Serial transmission is suitable because it uses a single wire, which reduces crosstalk/interference and avoids bit skewing that occurs in parallel cables over long distances."
Why this works: Examiners accept answers that link the technical limitation (skew/crosstalk) to the environmental constraint (long distance). Simply saying "Serial is faster" is often marked incorrect for long distances.
- Data is less likely to be skewed (bits arrive in order).
- Less chance of interference/crosstalk/error.
- Can transmit data over longer distances.
- Requires fewer wires (cheaper/simpler).
USB is a standardized hardware interface used to connect devices to a computer. It is important to distinguish between the interface (the physical port and standard) and the transmission method (how data moves).
Transmission Method: USB uses Serial transmission.
- Data is sent one bit at a time.
- Data travels down a single wire (within the cable).
Physical Interface (Connectors):
- USB defines specific connector shapes to ensure compatibility.
- Type-A: The standard rectangular plug found on computers/hubs.
- Type-B: The square-ish plug often used for printers/scanners.
- Micro-USB: Small connector for older mobile phones/MP3 players.
- USB-C: Reversible, smaller connector used in modern devices (phones, laptops).
Power Delivery:
- USB cables carry both data and electrical power. This allows devices like mice, keyboards, and phone chargers to be powered directly from the computer without a separate power adapter.
Key Features of USB:
- Plug and Play: Devices are automatically detected and configured by the operating system.
- Hot Swapping: Devices can be connected or disconnected while the computer is running (unlike older parallel ports which often required shutdown).
- Standardization: All USB devices use the same communication protocol, regardless of the manufacturer.
Scenario: Connecting a USB flash drive to a laptop.
- Physical Connection: The USB-C plug is inserted into the laptop's port. The pins in the connector establish electrical contact for power and data lines.
- Handshake: The laptop detects the device and loads the necessary drivers (Plug and Play).
- Data Transfer: When you copy a file:
- The file is broken into packets.
- Each packet is sent via Serial transmission (one bit at a time) over the single data wire inside the USB cable.
- Power is simultaneously supplied to the flash drive to operate its internal circuitry.
Correction: The question asks about the transmission mechanism. You must state: "Data is sent one bit at a time down a single wire." Benefits like speed or ease of use are secondary and do not answer 'how it transmits'.
Mistake: Saying USB sends data in parallel.
Correction: Despite the name 'Universal Serial Bus', the transmission is Serial. The word 'Bus' refers to the communication system, not parallel bit transfer.
Examiner Expectation: The markscheme specifically looks for:
- Data is sent one bit at a time.
- Data is sent down a single wire (or single channel).
Why this works: These are the defining technical characteristics of USB's transmission method. Mentioning connector types (Type-A/C) is good for describing the interface, but to explain data transmission, you must focus on the serial bit flow.
- Data is sent one bit at a time.
- Data is sent down a single wire.