2.1 Types and methods of data transmission

## Types and Methods of Data Transmission

### Data Packets
- #### Packet Structure
  - Header
    - Contains the IP address of the sender and the receiver
    - The sequence number of the packet
    - Size of the packet
  - Payload
    - Contains the actual data
  - Trailer
    - Includes a method of identifying the end of the packet
    - Error-Checking methods
#### Packet Switching
  - Data is broken/split/divided into packets
  - Each packet (could) take a different route
  - A router controls the route/path a packet takes selecting the shortest/fastest available route/path
  - Packets may arrive out of order
  - Once the last packet has arrived, packets are reordered
  - If a packet is missing/corrupted, it is requested again

The benefits of packet switching are:
» there is no need to tie up a single communication line
» it is possible to overcome failed, busy or faulty lines by simply re-routing packets
» it is relatively easy to expand package usage
» a high data transmission rate is possible.

The drawbacks of packet switching include:
» packets can be lost and need to be re-sent
» the method is more prone to errors with real-time streaming (for example, a live sporting event being transmitted over the internet)
» there is a delay at the destination whilst the packets are being re-ordered.
### Data Transmission

- *Simplex data transmission* is in one direction only (e.g. computer to printer)
![image.png](https://doc.tiegg.com/media/202512/3a489221bd114e69be13556c8f9d68dc3451.png)
	- Benefits
		- less data collisions碰撞
	- Drawback
		- Data can only be transmitted in one direction.

- *Half-duplex data transmission* is in both directions but not at the same time (e.g., in a phone conversation where only one person speaks)
	- Benefits 
		- Higher bandwidth than full duplex
	- Drawback
		- May be more data collisions
![截屏2025-12-15 09.13.43.png](https://doc.tiegg.com/media/202512/02ce81c399e0432f8de20b1fe37b37528621.png)
- *Full-duplex data transmission* is in both directions simultaneously同步的 (e.g. broadband connection on the phone line)
	- Benefits
		- It sends data in both directions at the same time
		- … so the devices can send data to each other with no delay
	- Drawback
		- More chance of data collisions as data sent in both directions at the same time
![截屏2025-12-15 09.12.37.png](https://doc.tiegg.com/media/202512/ab21a3995fd04cb1863b11e03ea5620c5014.png)

- *Serial data transmission* is when data is sent one bit at a time over a single wire
	- Benefits
		- Bits will be arrive in sequence so bits less likely to be skewed
		- Less crosstalk/interference so less likely to have errors
		- More accurate/reliable/efficient over long distances
		- Serial is cheaper to purchase/install/maintain
	- Drawback
		- The transmission of data may be relatively slow
		- Data cannot be sent and received at the same time
	- Application
		- sending data from a computer to a printer internal data transfer (buses)
- *Parallel data transmission* is when data of several bits are sent down several wires at the same time.
	- Benefits
		- It sends multiple bits of data simultaneously using multiple wires
		- … so the transmission speed of the data will be faster
	- Drawback
		- More interference/crosstalk due it to multiple wires
		- Data may be skewed due to multiple bits at a tie
	- Application
		- connect computer to a modem
\*skewed (data) – data that arrives at the destination with the bits no longer synchronised
\*crosstalk/interference - Crosstalk is the unwanted interference that occurs when a signal transmitted on one communication channel affects a signal in an adjacent channel.

## Universal Serial Bus (USB)

- USB is an asynchronous serial data transmission method
what is meant by USB
- Universal Serial Bus
- Uses serial transmission // bits of data are sent one at a time 
- Universal standard // common interface

Benefits
	One marks per benefit：
		- It is a universal standard
		- It can’t be inserted the wrong way around
		- Supports different transmission speeds
		- Backward compatible
		- Automatically detects if correct driver installed
		- Can power devices
		- Inexpensive to purchase/manufacture
	Two marks per benefit:
		- It is a universal standard
			- so it is likely to be compatible with the computer
		- It can only be inserted one way
			- so there is less chance of connecting a device incorrectly
		- It is a high-speed connection
			- so data will be transmitted quicker
		- It uses serial transmission
			- so it is cheaper to manufacture/buy
			- less chance of skewing / errors
		- It is backwards compatible
			- so no additional technology is needed
		- It can power the device
			- therefore no separate source of power is needed
		- Device is automatically identified
			- so no need to find them online / install them manually

| Advantages                                       | Disadvantages                             |
| ------------------------------------------------ | ----------------------------------------- |
| Automatically detected                           | Transmission rate is less than 120 MB/sec |
| Only fit one way, prevents incorrect connections | Maximum cable length is about 5 metres    |
| Different data transmission rates                |                                           |
| Backwards compatible                             |                                           |
| Industry-standard                                |                                           |