Unit 2 - Data Transmission Flashcards
What is the structure of a packet?
- The packet header - contains the IP address of both the sending station and the receiving station, the sequence number of the packet (so it can be reassembled if needed), and the packet size (so the receiving station can identify when the whole packet has been received).
- The payload - contains the actual data being transmitted.
- The packet trailer - contains error checking methods (such as cyclic redundancy check) and acts as a way of identifying the end of the packet.
What is packet switching?
Data packets allow data to be transmitted along the most efficient routes (from the sending station to the receiving station). Routers (nodes) control the path that a data packet takes between these two stations. Each data packet is able to take a different route depending on any interference along the original route, this is known as packet switching. This means that packets can arrive in a different order than what they were sent in, so a sequence number in the packet header allows all of the packets to reassemble into their original order at the receiving station. The packet header also contains hop numbers to ensure they don’t ’bounce around’ from router to router and eventually become lost.
What are the main benefits and drawbacks of packet switching?
Benefits:
- No need to tie up a single communication line.
- Possible to overcome failed routes by re-routing packets.
- Relatively easy to expand packet usage.
- A high data transmission rate is possible.
Drawbacks:
- Packets can be lost and need to be re-sent.
- Doesn’t work well with real-time streaming (e.g. a live sporting event).
- There is a delay at the destination whilst the packets are being reassembled.
What are the main features of serial data transmission?
- Less risk of external interference.
- More reliable over longer distances.
- Transmitted bits don’t need to be reassembled.
- Amount of data transmitted is usually smaller due to slower transmission rate.
- Used to send data over long distances.
- Less expensive due to fewer hardware requirements.
What are the main features of parallel data transmission?
- Faster rate of data transmission.
- Works well over shorter distances.
- Transmitted bits may need to be reassembled due to multiple channels being used.
- Easier to program input/output operations.
- Preferred method when transmitting large amounts of data.
- Most appropriate method if data is time-sensitive.
- Requires more hardware, making it more expensive.
What is a USB?
The Universal Serial Bus (USB-A) is a form of serial data transmission. It is the industry standard. When a USB is used to connect a device to a computer:
- the computer automatically detects the device.
- the device is automatically recognised and the appropriate device driver is loaded.
What are the main benefits and drawbacks of a USB?
Benefits:
- Device plugged into a computer is automatically recognised and device drivers are automatically loaded up.
- Connections only fit one way preventing incorrect connections.
- Has become an industry standard.
- Can support different data transmission rates.
- No need for external power source.
- USB protocol notifies transmitter to re-transmit data if any errors are detected.
- Relatively easy to add more USB ports by using USB hubs.
- Backward compatible.
Drawbacks:
- Standard USB supports a maximum cable length of 5m; if exceeded, USB hubs are required.
- Very early USB standards may not always be supported by the latest computers.
- Even the latest versions (V3, V4, and USB-C) have a slower data transmission rate compared to, e.g. Ethernet.
What is USB-C and what are its advantages?
USB-C is the latest version of USB, which uses totally symmetrical 24-pin USB connection, so it will fit into a USB port either way round. USB-C is actually USB 3.1 but has been named USB-C to distinguish it from the old format, USB-A.
The main advantages include:
- much smaller and thinner which suits the current trend in technology.
- offers 100 watt (20 volt) power connectivity, so full-sized devices can now be charged.
- it can carry data 10 Gigabits per second (10Gbps); it can support 4K video delivery.
- connection is backward compatible (to USB 2.0 and 3.0) if a suitable adaptor is used.
What is a parity check?
A parity check is a method which checks whether data has been corrupted following transmission. Parity can be even or odd. The left-most bit is reserved for a parity bit.
Parity blocks can be used to determine exactly which bit has been corrupted following transmission.
What is checksum?
Another error checking method. The sending computer calculates the checksum from the block of data using an agreed algorithm. The checksum is then sent along with the block of data. The sending computer re-calculates the checksum using the same algorithm; any differences in the checksum indicates an error.
What is an echo check?
Data is sent back to the sending computer to be compared with the original data. This is not very reliable as if the two sets of data are different, it is unknown whether the error occurred when the data was originally transmitted or when it was sent back.
What are check digits?
Used to identify any errors following data entry. A check digit value is calculated (by e.g. ISBN-13 or modulo-11). The generated digit is added to the end (right) of the number. They can detect incorrect digits entered, transposition errors, omitted or extra digits in the number, and phonetic errors (e.g. thirteen instead of thirty).
What is an automatic repeat request?
An ARQ checks data following transmission. The receiving device receives an error detection code as part of the data transmitted to detect whether the receiving data contains any transmission errors. If no errors are detected, a positive acknowledgement is sent back to the sending device. However, if an error is detected, a negative acknowledgement is sent back to the sending device and requests re-transmission of the data. If no acknowledgements have been received within a set time limit, it automatically re-sends the data until a positive acknowledgement is received or until a preset number of re-transmissions have taken place.
What is asymmetric encryption?
Matching pairs of private and public keys are generated by an encryption algorithm, these two keys are mathematically linked but cannot be derived from each other. If ‘A’ wants to send confidential information to ‘B’, ‘B’ sends ‘A’ their public key which is used to encrypt the information. ‘B’ now uses their private key to decrypt the information.
