Biphase
Probably the best existing solution to the problem of synchronization is biphase encoding. In this method, the signal changes at the middle of the bit interval but does not return to zero. Instead, it continues to the opposite pole. As in RZ, these midinterval transitions allow for synchronization.
As mentioned earlier, there are two types of biphase encoding in use on networks today: Manchester and Differential Manchester.
Manchester Manchester encoding uses the inversion at the middle of each bit interval is used for both synchronization and bit representation. A negative-to-positive transition represents binary 1 and a positive-to-negative transition represents binary 0. By using a single transition for a dual purpose, Manchester encoding achieves the same level of synchronization as RZ but with only two levels of amplitude.
In Manchester encoding the transition at the middle of the bit is used for both synchronization and bit representation.
Differential Manchester In Differential Manchester, the inversion at the middle of the bit interval is used for synchronization, but the presence or absence of an additional transition at the beginning of the interval is used to identify the bit. A transition means binary 0 and no transition means binary 1. Differential Manchester requires two signal changes to represent binary 0 but only one to represent binary 1.
Figure 4.1-7 shows the Manchester and Differential Manchester signals for the same bit pattern.
- Fig. 4.1-7 Manchester and Differential Manchester encoding
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