Some digital data streams, especially high-speed serial data streams (such as the raw stream of data from the magnetic head of a disk drive) are sent without an accompanying clock signal

Clock signal

In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...

. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop

Phase-locked loop

A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...

 (PLL). This process is commonly known as clock and data recovery (CDR).

In order for this scheme to work, a data stream must transition frequently enough to correct for any drift in the PLL's oscillator. The limit for how long a clock recovery unit can operate without a transition is known as its maximum consecutive identical digits (CID) specification. To ensure frequent transitions, some sort of encoding is used; 8B/10B encoding

8B/10B encoding

In telecommunications, 8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. This means that the difference between the count of 1s and 0s in a string of at least 20 bits...

 is very common, while Manchester encoding serves the same purpose in old revisions of 802.3

IEEE 802.3

IEEE 802.3 is a working group and a collection of IEEE standards produced by the working group defining the physical layer and data link layer's media access control of wired Ethernet. This is generally a local area network technology with some wide area network applications...

 local area network

Local area network

A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...

s.

See also

• Phase-locked loop
  Phase-locked loop
  A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
  
  
• 8B/10B encoding
  8B/10B encoding
  In telecommunications, 8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. This means that the difference between the count of 1s and 0s in a string of at least 20 bits...
  
  
• Eight-to-Fourteen Modulation
  Eight-to-Fourteen Modulation
  Eight-to-fourteen modulation is a data encoding technique – formally, a channel code – used by compact discs and pre-Hi-MD MiniDiscs. EFMPlus is a related code, used in DVDs and SACDs. EFM and EFMPlus were both invented by Kees A...
  
  
• Manchester code
  Manchester code
  In telecommunication and data storage, Manchester code is a line code in which the encoding of each data bit has at least one transition and occupies the same time...
  
  
• 64B/66B encoding
  64b/66b encoding
  In data networking and transmission, 64b/66b is a line code that transforms 64-bit data to 66-bit line code to provide enough state changes to allow reasonable clock recovery and facilitate alignment of the data stream at the receiver....
  
  
• B8ZS encoding
• HDB3 encoding
• Burst mode clock and data recovery
  Burst mode clock and data recovery
  The passive optical network uses tree-like network topology. Due to the topology of PON, the transmission modes for downstream and upstream are different...
  
  
• Jitter
  Jitter
  Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...