Introduction to Packet Switching

• Data is divided into packets before being sent
• Packets are split into:
  • Packet header: contains the IP address of the sending/receiving station, packet sequence number, packet size
  • Payload: actual data
  • Packet trailer: contains the CRC (error check)

Packet Switching

• Data packets are sent in manageable chunks via efficient routes using routers (nodes)
• Each data packet can take a different, independent route.
• A sequence number in the packet header allows reassembling packets in the correct order at the receiving station.
• Hop numbers are added to packet headers to prevent packets from endlessly bouncing between routers.

Benefits of Packet Switching

• Avoids tying up a single communication line.
• Can overcome failed, busy, or faulty lines by re-routing packets.
• Relatively easy to expand package usage
• A high data transmission rate is possible.

Drawbacks of Packet Switching

• Packets can be lost and need to be re-sent.
• Not ideal for real-time streaming due to potential delays in reordering packets
• A delay at the destination may occur while packets are re-ordered

Data Transmission Modes

• Simplex: data sent in one direction only
  • Sending data to a printer
• Half-duplex: data sent in both directions, but not simultaneously
  • Walkie-talkies
• Full-duplex: data sent in both directions simultaneously
  • Broadband internet

Serial Data Transmission

• Data sent one bit at a time down a single wire/channel
  • Using a USB connection.

Parallel Data Transmission

• Multiple bits of data are sent simultaneously down several wires/channels
  • Data transmission using internal computer circuits.
• Serial and parallel transmission can use serial, half-duplex, or full-duplex methods.

Features of Serial Transmission

• Less susceptible to external interference than parallel transmission.
• More reliable transmission over longer distances.
• Transmitted bits have no risk of being skewed
• Suited for small amounts of data due to slower transmission rate.
• Used to send data over long distances
• Less expensive than parallel because it requires fewer parts

Features of Parallel Transmission

• Faster transmission rate, making it suitable where speed is important
• Works best over shorter distances
• Data skew might happen when there are multiple wires/channels being used
• Easier to program input/output operations
• Preferred when sending large amounts of data or when data is time-sensitive.
• Requires more hardware, making it more expensive

Universal Serial Bus (USB)

• Industry standard serial data transmission.
• When plugging a USB into a computer, it automatically detects the device, recognizes it, and loads the appropriate device driver.

USB Benefits

• Automatic device detection and driver loading.
• Connections fit only one way, preventing incorrect connections.
• Industry standard
• Supports different data transmission rates
• Does not require an external power source
  • Cable provides +5V power
• Notifies the transmitter to re-transmit data if errors are detected
• USB ports can be easily added using USB hubs.
• Compatible with older versions

USB Drawbacks

• Standard USB supports a maximum cable length of 5 meters
  • USB hubs are needed to extend cable length
• Very early USB standards may not be supported by the latest computers.
• The latest USB versions have a slower data transfer rate compared to Ethernet connections.

USB-C

• Latest USB connector with symmetrical 24-pin connection which fits either way round.
• Technically USB 3.1, named USB-C to differentiate from USB-A.

Advantages of USB-C

• Smaller and thinner, suiting trends in thinner devices.
• Offers 100-watt power connectivity
  • Full-sized devices can be charged
• Can carry data at 10 Gigabits per second (10 Gbps), supporting 4K video delivery.
• Backward compatible with USB 2.0 and 3.0 using an adapter.

Need to check for errors

• There is a risk that the data may be corrupted, lost or even gained
• Electrical interference
• Packet switching
• Skewing of data

Parity Checks

• A parity check is a method used to check whether data has been changed or corrupted following data transmission
  • Parity can be even or odd
  • The leftmost big is saved for parity bits
• If two bits have been changed, the parity may stay the same
  • Another error detection method will be needed
• Parity blocks can be used to determine exactly which bit has been corrupted or changed following data transmission

Checksum

• Used to check if data has been changed/corrupted following data transmission
• The checksum is calculated by the sending computer
• The data is re-calculated by the receiving computer
• Differences in the chekcsum indicates an error

Echo Check

• Requires data to be sent back to the sending computer where it is compared with the data originally sent
• Errors of the data is re-sent

Check Digits

• Used to identify any errors following data entry
• Barcodes are used to ensure the barcode is read correctly

Automatic Repeat Requests (ARQ)

_ Commonly used by mobile phone networks

• ARQ uses positive/negative acknowledgements and signals
• The receiving device creates a positive acknowledgement
• A timeout is used by the sending device by waiting a pre-determined amount of time

Encryption

• When data is transmitted over public networks, there is a risk of it being intercepted
• Encryption alters the form which is unreadable
• The original data is called plaintext and the encrypted data is called ciphertext
• Encryption can't stop data being intercepted, but prevents it from making sense*

Symmetric Encryption

• Uses a single encryption key
• The same key is used to encrypt data and to decrypt data
• Modern encryption keys use 256-bit encryption

Asymmetric Encryption

• Uses two keys: a public and private key
• Matching pairs are generated by an encryption algorithm
• Both types of keys are needed to encrypt and decrypt messages

Computer Architecture

• The objectives include Computer Architecture, Data Storage, and Network Hardware

Central Processing Unit (CPU)

• Responsible for executing and processing a computer's data/instructions.
• Consists of the following:
  • CU (Control Unit)
  • ALU (Arithmetic and Logic Unit)
  • Registers and Buses

Microprocessor

• An integrated circuit also referred to as a processor or CPU. Contains:
  • ALU
  • Control Unit to enable instruction interpretation and operation. Also contains:
  • System clock
  • Primary Memory.

Von Neumann Architecture

• The concept of a Central Processing Unit (CPU or Processor)
• The Processor was able to access the memory directly
• Computer memories could store programs and data
• Stored programs were made up of instructions that could be executed in sequential order