Nevertheless, the OSI model persists as the best general
description of networking services, and is frequently referenced in textbooks and research relating to networks.
For our further discussion of the operation of different network layers and protocols, we will use the
four-layer internet model because it is the model in wide use. Our goal in this chapter is to discuss how
networking is actually implemented today.
SUBNET (DATA-LINK) LAYER
The responsibility of the subnet layer is to send and receive streams of bits between one machine and
another. Computers transmit information by sending signals in the form of electromagnetic energy using some
transmission medium. Computers that are connected using copper wires as the transmission medium will send
signals in the form of electrical signals. Computers that are connected using fiber-optic cable will transmit
signals in the form of light. Computers connected wirelessly broadcast and receive radio signals. Hardware
at the physical layer generates signals of the appropriate type for the medium of transmission.
Because of the layering architecture of the networking protocols, one subnet approach may be substituted
for another. Thus, my laptop computer can use a wired Ethernet connection if one is available, or can instead
use a wireless connection if a Wi-Fi network serves the location where I wish to compute.
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