Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

74 CHAPTER 5 The OSI Model and Networking Protocols
Layer 7
Application
Layer 6
Presentation
Layer 5
Session
Layer 4
Transport
Layer 3
Network
Layer 2
Data Link
LLC
MAC
FIGURE 5.1
The OSI model
Layer 1
Physical
Layer 1: Physical
The first layer of the OSI model is the physical layer. This layer specifies the
electrical and mechanical requirements for transmitting data bits across the
transmission medium. It involves sending and receiving the data stream on
the carrier – whether that carrier uses electrical (cable), light (fiber optic) or radio,
infrared or laser (wireless) signals. The physical layer specifications include the
following:
■ Voltage changes
■ Timing of voltage changes
■ Data rates
■ Maximum transmission distances
■ Physical connectors to the transmission medium
■ Physical topology or layout of the network
The physical layer addresses digital versus analog signaling, baseband versus
broadband signaling, synchronously or asynchronously transmissions, and how
signals are divided into channels (multiplexing).
EXAM WARNING
Modems translate analog to digital signals and back again. PCs are using digital technology
to communicate, but the phone lines are using analog signaling. Therefore, the signal must
be changed from one signaling method to the other as needed, such as when you want
to connect your PC up to your Internet service provider (ISP) and surf the Internet.
Layer 2: Data Link
The data link layer is responsible for maintaining the data link between two hosts
or nodes. Its characteristics and functions are as follows: