tailieunhanh - Practical TCP/IP and Ethernet Networking- P4

Practical TCP/IP and Ethernet Networking- P4: One of the great protocols that has been inherited from the Internet is TCP/IP and this is being used as the open standard today for all network and communications systems. The reasons for this popularity are not hard to find. | 12 Practical TCP IP and Ethernet Networking Encoding methods Manchester Manchester is a bi-phase signal-encoding scheme used in Ethernet LANs. The direction of the transition in mid-interval negative to positive or positive to negative indicates the value 1 or 0 respectively and provides the clocking. The Manchester codes have the advantage that they are self-clocking. Even a sequence of one thousand 0s will have a transition in every bit hence the receiver will not lose synchronization. The price paid for this is a bandwidth requirement double that which is required by the RZ-type methods. The Manchester scheme follows these rules V and -V voltage levels are used There is a transition from one to the other voltage level halfway through each bit interval There may or may not be a transition at that start of each bit interval depending on whether the bit value is a 0 or 1 For a 1 bit the transition is always from a -V to V for a 0 bit the transition is always from a V to a -V In Manchester encoding the beginning of a bit interval is used merely to set the stage. The activity in the middle of each bit interval determines the bit value upward transition for a 1 bit downward for a 0 bit. Differential Manchester Differential Manchester is a bi-phase signal-encoding scheme used in token ring LANs. The presence or absence of a transition at the beginning of a bit interval indicates the value the transition in mid-interval just provides the clocking. For electrical signals bit values will generally be represented by one of three possible voltage levels positive V zero 0 V or negative -V . Any two of these levels are needed - for example V and -V. There is a transition in the middle of each bit interval. This makes the encoding method self-clocking and helps avoid signal distortion due to DC signal components. For one of the possible bit values but not the other there will be a transition at the start of any given bit interval. For example in a .