B8ZS
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B8ZS is an abbreviation for bipolar with eight-zero substitution, which is a method of line coding used in the T-carrier system which allows full 64 kbit/s per second per channel.
On a T1, ones are sent by applying voltage to the wire, where a zero is sent by having no voltage on the wire. Sending excessive zeros in a row could cause receiving equipment to lose synchronization with sending equipment, so it is important that such a pattern not be sent.
The original standard of line coding, Alternate Mark Inversion, specifies that there are three states of the line, no voltage is a zero, positive voltage is a one (or mark), and negative voltage is also a one (or mark). Because of the inversion of the voltage for each "mark," or one, sent, the receiving equipment can easily determine the data rate of the line and not lose synchronization.
B8ZS builds upon this, by using violations of this rule to replace a pattern of eight zeros in a row.
| Original signal: | |||||||
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| B8ZS encoded signal (V=bipolar violation) | |||||||
| 0 | 0 | 0 | V | 1 | 0 | V | 1 |
| Signal Polarity (assuming that the previous mark was negative) | |||||||
| 0 | 0 | 0 | - | + | 0 | + | - |
B8ZS is used in the North American hierarchy at the T1 rate. When European E1 was developed much later than T1, it was then common knowledge that forcing 'ones' into a DS0 would corrupt data. E1 uses another method called High Density Bipolar Three (HDB3) code.
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