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. Disk storage is a general category of a Computer storage mechanisms in which data is recorded on planar round and rotating surfaces ( disks, discs, or 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 (PLL). A phase-locked loop or phase lock loop (PLL is a Control system that generates a signal that has a fixed relation to the phase of a "reference" 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 is very common, while Manchester encoding serves the same purpose in old revisions of 802.3 local area networks. In Telecommunications 8b/10b is a Line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance (see In Telecommunication, Manchester code (also known as Phase Encoding, or PE) is a Line code in which the encoding of each data Bit IEEE 8023 is a collection of IEEE standards defining the Physical layer, and the media access control (MAC sublayer of the Data link layer,
See also
- Phase-locked loop
- 8B/10B encoding
- Eight-to-Fourteen Modulation
- Manchester code
- 64B/66B encoding
- B8ZS encoding
- HDB3 encoding
- Wikibook Clock and data recovery
- Burst mode clock and data recovery
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