Physical Layer-Chapter 8
Terms in this set (61)
What does the physical layer provide?
Provides the means to transport across the network media the bits that make up a Data Link layer frame
What are the four elements of delivering frames across the media?
The physical media and associated connectors A representation of bits on the media
Encoding of data and control information Transmitter and receiver circuitry on the network device
What are the three basic forms of media?
What are the three concerns when the physical layer encodes bits?
The Physical layer encodes the bits into the signals for a particular medium, it must also distinguish where one frame ends and the next frame begins
What does the physical layer consist of?
Hardware, developed by engineers, in the form of electronic circuitry, media, and connectors
What are the organizations that are responsible for physical layer standards?
The International Organization for Standardization (ISO)
The Institute of Electrical and Electronics Engineers (IEEE)
The American National Standards Institute (ANSI)
The International Telecommunication Union (ITU)
The Electronics Industry Alliance/ Telecommunications Industry Association (EIA/TIA)
National telecommunications authorities such as the Federal Communication Commission (FCC) in the USA.
The technologies defined by these organizations include what four areas of the Physical layer standards?
Physical and electrical properties of the media
Mechanical properties (materials, dimensions, pinouts) of the connectors
Bit representation by the signals (encoding)
Definition of control information signals
What are the three fundamental functions of the Physical layer?
The physical components
What are the two values that the physical layer encodes?
1 and 0
How are the bits of the frame transmitted by the physical layer?
One bit at a time
What is a bit time?
Specific amount of time each signal has to occupy the media
How are the signaling and receiving ends clocks synchronized in the LAN?
Many signaling methods use predictable transitions in the signal to provide synchronization between the clocks of the transmitting and the receiving devices
What characteristics can be changed to represent bits on the medium?
What will happen if there is no agreement between nodes on the signaling method?
The transmission will fail
What are the three characteristics of NRZ signaling?
Only two states
Voltage jumps between levels
What type of transmission is NRZ suited for?
For slow speed data links
What are represented by in Manchester encoding?
How does the transition help maintain clock synchronization?
One voltage transition must occur in the middle of each bit time
Which medium uses Manchester encoding?
What method is used to signal start and end of a frame in higher speed technologies?
Specific pattern of grouped bits
What is a code group?
Consecutive sequence of code bits that are interpreted and mapped as data bit patterns
What are the advantages of transmitting symbols?
The error detection capabilities and timing synchronization between transmitting and receiving devices are enhanced
What are the four advantages of using code groups?
Reducing bit level error
Limiting the effective energy transmitted into the media
Helping to distinguish data bits from control bits
Better media error detection
What are the three types of code groups?
Data symbols - Symbols that represent the data of the frame as it is passed down to the Physical layer.
Control symbols - Special codes injected by the Physical layer used to control transmission. These include end-of-frame and idle media symbols.
Invalid symbols - Symbols that have patterns not allowed on the media. The receipt of an invalid symbol indicates a frame error.
How does 4B/5B represent a nibble?
As five-bit values known as symbols
What are the three measures for Data Transfer?
What is the difference between throughput and goodput?
Throughput is the measure of bits over a given time and goodput is the measure of usable data transferred over a given period of time
Which measure of data transfer is most important to the end user?
What are the standards for copper media defined for?
Type of copper cabling used
Bandwidth of the communication
Type of connectors used
Pinout and color codes of connections to the media
Maximum distance of the media
What type of connector is commonly used in copper cabling?
What is done to the pairs of wires to minimize signal degradation due to electronic noise?
Shielding and twisting of the wires
What are the other methods listed to limit the susceptibility of copper cables to noise?
Selecting the cable type or category most suited to protect the data signals in a given networking environment
Designing a cable infrastructure to avoid known and potential sources of interference in the building structure
Using cabling techniques that include the proper handling and termination of the cables
What is crosstalk?
The interference caused by the magnetic field around the adjacent pairs of wires in the cable
What is used to limit crosstalk?
Twisting pairs of wires
What changes in the twist from one pair of wires to the next pair of wires to improve the limitation of crosstalk?
The number of twists per meter changes
What are some of the elements defined by the TIA/EIA 568A standard?
Methods of testing cable
Why is it wasteful to install less expensive but lower rated cabling?
If the decision is later made to adopt a faster LAN technology, total replacement of the installed cable infrastructure may be required.
What is UTP cable terminated with?
What are the three types of Ethernet cables?
What is a straight-through used for?
Host to network device such as a hub or switch
What is a crossover used for?
Connecting two hosts
Connecting to network intermediary devices
What is a rollover used for?
Connecting a computers serial port to the console port of a router (with an adapter)
What happens if you use the wrong type of cable when connecting devices?
Communication will not occur
What are the other two type of copper cabling?
2. Shielded Twisted-Pair (STP)
What has replaced the use of coaxial cable in today's Ethernet installations?
What type types of materials can be used for fiber optic cabling?
Glass or plastic
What is the encoding method for fiber optic?
What are the issues with implementing fiber optic cabling?
More expensive (usually) than copper media over the same distance (but for a higher capacity)
Different skills and equipment required to terminate and splice the cable infrastructure
More careful handling than copper media
What areas usually implement fiber optic cabling?
Backbone and between buildings
What are the major advantages of using fiber optic between buildings?
Optical fiber does not conduct electricity and has low signal loss
How many cables are needed for each fiber optic connection? Why?
2, they can only operate in one direction
What are the two types of fiber optic cables?
Single mode and multimode
How do wireless media carry electromagnetic signals?
Radio and microwave frequencies that represent the binary digits of data communications
What is the major concern for wireless signals?
What are the 4 standards for wireless networks?
Global System for Mobile Communications (GSM)
What are the two network devices necessary to implement a wireless network?
Wireless Access Point (AP) - Concentrates the wireless signals from users and connects, usually through a copper cable, to the existing copper-based network infrastructure such as Ethernet.
Wireless NIC adapters - Provides wireless communication capability to each network host.
What are the WLAN Ethernet standards?
IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n
What are some of the CAT5 termination that may performed onsite?
crimped connections to terminate Cat5 media with RJ-45 plugs to make patch cables, and the use of punched down connections on 110 patch panels and RJ-45 jacks
What are the differences in the termination of the two cables pictured?
Insulation cut back to far and wires untwisted too much
What could happen if wires are not terminated correctly?
Damaging voltage levels may be applied between interconnected devices
What are the three main splicing errors when using fiber optic cabling?
Misalignment - the fiber-optic media are not precisely aligned to one another when joined.
End gap - the media do not completely touch at the splice or connection.
End finish - the media ends are not well polished or dirt is present at the termination.
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