Layer 3: The Network Layer
The network layer provides connectivity and path selection between two host systems that
might be located on geographically separated networks. The growth of the Internet has
increased the number of users accessing information from sites around the world, and the
network layer manages this connectivity.
Layer 2: The Data Link Layer
The data link layer de?¬?nes how data is formatted for transmission and how access to the
network is controlled. This layer is responsible for de?¬?ning how devices on a common
media communicate with one another, including addressing and control signaling between
devices.
36 Chapter 1: Building a Simple Network
Layer 1: The Physical Layer
The physical layer de?¬?nes the electrical, mechanical, procedural, and functional
speci?¬?cations for activating, maintaining, and deactivating the physical link between end
systems. Characteristics such as voltage levels, timing of voltage changes, physical data
rates, maximum transmission distances, physical connectors, and other similar attributes
are de?¬?ned by physical layer speci?¬?cations.
Data Communications Process
All communications on a network originate at a source and are sent to a destination.
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