Detecting changes in signal/data stream is easier than detecting levels.

So, any protocol that increases the number of changes in a data stream 
improves detection.

RLL - Run Length Limited 

  Encodes word of data on larger symbol of 'encoding' bits 
    to guarantee changes in signal level with in a limited span.

  Data is encoded so that length of non-changing bit sequence is limited.

  Often also limits the number of transitions in a given time period,
    reduces power use, high frequency attenuation, other issues with
    fast changing signals.

  # Whether a change signals a one or a zero 1 is left to the protocol.
 
  RLL encoding almost always implemented using NRZI encoding.

  Cost - Increases the number of bits needed.

   Generic example 
     
   Data   RLL 
          000 
          001 
   00     010   010
   01     011   011
          100 
   10     101   101
   11     110   110
          111 
   
   When combined as a stream of bits,
     At most 2 zeros or 4 ones next to each other.

   Implementations of real RLL encoding is more complex.

   Once data is encoded as RLL, any of the transmission/storage methods 
     may be used.

FM - Digital Frequency encoding or Delay encoding

  

  Delay encoding - (0,1) RLL
    0  10
    1  11

  2x data stream.
    If minimum detectable width of a pulse is 1 ns.   1 Gbits/sec.
      then a data bit occupies 2 ns.    500 Mbits/sec.
   
       Same data as NRZ 
  

  Always change at beginning of dat bit unit.

  Change in middle if data = 1 (this example)

  No change in middle if data = 0

  Delay encoding is also known differential Manchester encoding.

  Used on floppies and early hard drives.  

Modified Frequency Modulation (MFM) - FM using (1,3) RLL encoding.

  Data is encoded so that there is a zero after any one and no more than
  3 zeros in a row.  Sometimes called (1,3)RLL

  

  Notice there is always a change at the edge of the bit. 
    Like NRZI with a clock signal embedded.

  Produced (~50%) higher bit density that FM.

  Even though there are more 'bits', MFM can represent 50% more bits in
    the same time period.

  Used on
    early hard drives (until mid 1980's)

4B5B ecoding

  Encodes 4 bits of data on a 5 bit encoding value.
    Each of 16 values on 1 of 32 possible values. 
    
  Because of the available number of encoding values,
    a variety of alternative encodings have been implemented.

  Usage 
    Fiber Distributed Data Interface (FDDI) 
      Early to mid 1990s
        Modern Gigibit out performs. 

      Fiber distributed local area network usually configured in a ring,

      Often dual ring to protect against interruption.
 
    Fast Ethernet  100BaseT (copper)

  Group coded recording
     Used in magnetic tape.

RLL(2,7) - encodes n bits of data on 2n bits. Most common on current storage.
     But done in data bit groupings of 2, 3, or 4.

     Guarantees that at least 2 consecutive 0s between bits

     Guarantees that at most 7 consecutive 0s

     RLL vs. MFM.
       The number of actual transitions in the RLL signal allow for
       1/3 smaller foot print when storing bit sequences.
   
       Early hard drives 20 MB (MFM), 30 MB (RLL) - same physical drive.

       Standard coding for hard drives for a long time.

     Produced higher bit density that FM.

     Used on floppies and earliest hard drives.  

8b/10b
  DC balancing. 

    Encodes 8-bit word on a 10-bit symbol.

    Gigabit Fiber Ethernet.  
    SATA (serial ATA)
    SAS (serial SCSI)
    Firewire (Apple - similar to USB)
    DisplayPort

Eight to Fourteen

  Used on CDs, 
    provides pit/land sequences resistant to physical disc defects.
 
  DVDs use EFMPlus (8 to 16) variation.
 
64b/66b
 Also 128b/1xxb 

 DC balancing

 Also offers efficiencies coupled with Hamming code.
   64/66b
     10 GiB Ethernet (most varieties)
     100 GiB Ehternet
     Fibre Channel - alternative to Ethernet.
       Most often used to link nodes in a NAS cloud facility.
     Thunderbolt 
       High performance computer/device interface.  
  
   128/1xxb
     PCIe 3.0
     USB 3.1
     SAS 4