There are various line coding techniques to convert data elements to signal elements. These include Unipolar (NRZ), Polar (NRZ, RZ, Biphase), Bipolar (AMI, Pseudoternary) , Multilevel (2B1Q, 8B6T, 4D-PAM5) and Multitransition (MLT-3). Biphase line coding include Manchester and Differential Manchester encoding.
• Transition is used at the middle of each bit period.
• Transition is used as clock edge and data mapping both.
– Low to High represents 1
– High to Low represents 0
• Used by IEEE 802.3 specification for Ethernet LAN (Short distances)
Differential Manchester Encoding
• Dedicated mid-bit transition used only for clocking
• Data representation is mapped as per bit start time instant.
– No transition at start of a bit period represents as 1
– Transition at start of a bit period represents as 0 (Inverts on 0’s – Opposite of NRZI)
• Example : Differential encoding
• Used by IEEE 802.5 specification for Token Ring LAN
Advantages of Manchester and Differential Manchester (Biphase) Encoding
Both Manchester and differential manchester fall under Biphase encoding type. Following are the advantages of Biphase encoding techniques.
➨It provides guaranteed mid bit transitions. Hence it offers synchronization facility at receiver. It is called self clocking codes.
➨As they are bipolar signals, ideally no DC components are present.
➨It provides error detection by detecting absence of expected transitions.
Disadvantages of Biphase encoding
Following are the disadvantages of Biphase encoding techniques.
➨It maps at least one transition per bit time and possibly two bits. Its modulation or signal rate is two times that of NRZ. Hence it requires more bandwidth.
➨It is used over shorter distances (in LANs).