Connecting two hosts in Networking

Connecting two hosts in Networking

Computer Networking : Principles, Protocols and Practice, Release
Host A Physical link Host B
From a Computer Science viewpoint, the physical transmission of information through a wire is often considered as a black box that allows to transmit bits. This black box is often referred to as the physical layer service and is represented by using the DATA.request and DATA.indication primitives introduced earlier. This physical layer service facilitates the sending and receiving of bits. This service abstracts the technological details that are
involved in the actual transmission of the bits as an electromagnetic signal. However, it is important to remember that the physical layer service is imperfect and has the following characteristics :
• the Physical layer service may change, e.g. due to electromagnetic interferences, the value of a bit being
transmitted

  • the Physical layer service may deliver more bits to the receiver than the bits sent by the sender
    • the Physical layer service may deliver fewer bits to the receiver than the bits sent by the sender
    Many other types of encoding have been defined to transmit information over an electrical cable. All physical layers are able to send and receive physical symbols that represent values 0 and 1. However, for various reasons that are outside the scope of this chapter, several physical layers exchange other physical symbols as well. For
    example, the Manchester encoding used in several physical layers can send four different symbols. The Manchester encoding is a differential encoding scheme in which time is divided into fixed-length periods. Each period is divided in two halves and two different voltage levels can be applied. To send a symbol, the sender must set one of these two voltage levels during each half period. To send a 1 (resp. 0), the sender must set a high (resp. low) voltage during the first half of the period and a low (resp. high) voltage during the second half. This encoding ensures that there will be a transition at the middle of each period and allows the receiver to synchronise its clock to the sender’s clock. Apart from the encodings for 0 and 1, the Manchester encoding also supports two additional symbols InvH and InvB where the same voltage level is used for the two half periods. By definition, these two symbols cannot appear inside a frame which is only composed of 0 and 1. Some technologies use these special symbols as markers for the beginning or end of frames.

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