RZ Return to zero.	NRZ Non-return to zero.	Bit stuffing
Manchester encoding	Differential Manchester encoding	FSK   Frequency-shift keying.
MFM    Modified Frequency modulation.	Phase-shift keying

Error checking :

Parity bit

• Even or odd.
• Parity bit sent so total 1 bits counted matches parity.
• Detects single bit error
• Cannot identify specific bad bit
• Used in early PCs as a check for main memory.

CRC - cyclic redundancy check

• Effective for quick detection of data corruption when transferring data.
• Uses polynomial math to calculate a short check value.
• Added to data stream. Recalculation results null out on good data transfer.
• Cannot recover data, just detects corruption.
• Useful where re-transmission cheap.
• Used in early hard drive storage.

Hamming code

• Provide error detection and recovery where error rate low.
• Better protection if errors are not close together.
• Used on main memory on high-end servers where accurate data is critical.

Reed-Solomon

• Applied to blocks of data
• Provides check and correction for multiple errors in block.
• Used where re-transmission of bad data not practical.
• Variations used in
  • DVD
  • CD
  • QR code
  • DSL
  • WiMax
  • Digital TV
  • deep space satellite

Turbo-code

• Highly efficient technique for checking and repairing errors.
• Effective where corrupting noise is present.
• Uses a type of cross check on receiving for best-guess fix.
• Used in 3G/4G mobile and newer deep-space satellites.

Low-density parity check code LDPC

• Highly efficient technique for checking and repairing errors.
• Effective where corrupting noise is present.
• Conceived in 60's but implementation only possible with current technology.
• Used in newer satellite communications.
• More efficient than turbo-code for very high speed transmission.

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Bus basics

4 types of bus

• Address - used to identify device/cicuit being contacted.
• Data - used to transfer data between devices/circuits.
• Control - used trigger actions or transmit status of activity.
• Power - used to power circuits/devices.

Daisy-chain
• Grant line linked to masters in serial chain. If a master claims the grant, it doesn't pass the grant on to then next master.
• Lesser masters get bus only if higher master not interested.
• Not fair, but efficient for certain bus designs.
Centralized arbitration
• Uses a dedicated arbiter.
• Can allow for 'programmable' prioritization.
• Masters need only limited additional circuitry (request/grant)
• WiFi uses this, clients wait for a clear channel and requests a reseved time slot to use WiFi. Access point grants or denies(ignores) request. If granted, all other devices see the grant and wait.
• PCI arbitration is performed by the PCI controller.
Distributed arbitration
• Each master needs additional logic (decoder logic) to break ties.
• Each master assigned an ID to establish priority.
• All masters share arbitration lines.
• Some versions all priority to be reassigned dynamically.
• Closely resembles daisy chain, but uses additional logic on each device.
• Examples : SCSI
Distributed using collision detection.
• Any device may use 'bus'.
• Device transmits only on a clear bus
  • As it transmits, it continues to monitor for collision.
  • If detected, transmission pause for short time.
  • Re-transmission attempted again.
  • If collisions continue, transmission abandoned.
• Effective if number of 'masters' not too large.
• Early ethernet used CSMA/CD.