Chapter 8 Notes - Chapter 8 Notes Physical Layer Role To...

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Chapter 8 Notes – Physical Layer Role: To encode the binary digits that represent Data Link layer frames into signals and to transmit and receive these signals across the physical media Communication Signals Purpose – to create the electrical, optical, or microwave signal that represents the bits in each frame Requires: Physical media & associated connectors A representation of bits on the media Encoding of data and control info. Transmitter & receiver circuitry on network devices Operation Media does not carry the frame as a single entity Media carries signals, one at a time, to represent the bits that make up the frame Three basic forms of network media Copper cable Represented as electrical pulses Fiber Patterns of light Wireless Patterns of radio transmission Physical layer may also add its own signals to frame to ID beginning & end Standards Set by: ISO IEEE ANSI ITU EIA/TIA FCC (U.S.A.) Four areas of physical layer standards Physical and electrical properties of media Mechanical properties (materials, dimensions, pinouts) of the connectors Bit representation by the signals (encoding) Definition of control information signals Fundamental Principles Physical components Data encoding Converting a stream into a predefined code (groupings of bits used to provide a predictable pattern used by sender/receiver Signaling Phys. Layer must generate electrical, optical or wireless signals that rep. the 1 and 0 on the media
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Physical Signaling and Encoding: Representing Bits All comm. From the human network becomes binary digits, which are transported individually across the physical media Signaling Bits for the Media Phys. Layer reps each of the bits in the frame as a signal Signal placed on media has a specific amount of time to occupy media (bit time) At phys. Layer of receiving node, signals converted back into bits Signaling Methods Amplitude Frequency Phase NRZ Signaling A low voltage value represents a logical 0 A high voltage value represents a logical 1 Only suited for slow speed data links Uses bandwidth inefficiently & susceptible to EMI Manchester Encoding Bit values represented as voltage transitions Low volt to high volt = 1 High volt to low volt = 0 Not efficient enough to be used at higher signaling speeds Signaling method employed by 10BaseT Ethernet Grouping Bits “Encoding” to represent the symbolic grouping of bits prior to being presented to the media Encoding prior to signals being placed on network – efficiency at higher speed data transmission Higher speeds – possibility data will be corrupted Coding groups – detect errors more efficiently Signal Patterns
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