business data communications
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Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 1
Business Data Communications
Chapter Two
Physical Layer Fundamentals
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 2
Primary Learning Objectives
Understand the general purpose of the physical layer
Distinguish between analog and digital Signaling
Describe circuit configurations and methods of data flow
Identify characteristics of conducted and radiated Media
Name and differentiate four types of multiplexing
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 3
Physical Layer
Similar in both the OSI and TCP/IP models Specifies the physical characteristics of a network Stacked below the data link layer Transmits an “unformatted” data bit stream Has four key components:
Signaling method Circuit configuration Transmission medium Devices used
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 4
Physical Layer
Pin connector functions are an important physical layer responsibility
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 5
Signaling Methods -- 1st Component of the Physical Layer Analog versus Digital
Analog is continuous Digital is discrete
Analog uses modulation techniques Amplitude, Frequency, Phase, for example Analog is measured in hertz
Digital uses encoding schemes Manchester and Differential Manchester, for example Digital is measured in bps, or bits per second
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 6
Analog Signaling Methods
Can take an infinite form
Modulate a sine wave Can change a wave’s
amplitude, frequency, or phase
Amplitude affects the wave’s height or strength
Frequency measures the waves per second
Phase occurs when a wave changes direction
Modulation is either Simple or Complex
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 7
Analog Signaling Methods
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 8
Analog Signaling Methods
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 9
Analog Signaling Methods
A hertz is a unit of frequency. A period is measured in seconds.
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 10
Analog Signaling Methods
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 11
Analog Signaling Methods
Simple signal methods use: A choice of two amplitudes, or A choice of two frequencies, or A single phase change
A simple signal method has the same symbol and bit rate
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 12
Analog Signaling Methods
A complex signal method occurs when the bit rate and the symbol rate are not the same:
A symbol, or baud, rate can represent more than one bit per time period
When more then one bit is represented within a single symbol, then the symbol and bit rates differ
Complex signal methods require that more than one bit be represented per symbol
Complex signal methods combine different amplitudes, frequencies, or phases, or some combination of these
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 13
Analog Signaling Methods
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 14
Analog Bandwidth
Analog bandwidth is measured in hertz
The bandwidth measurement is the difference between a given analog’s lowest and highest frequencies
The spectrum consists of the entire range of frequencies, from lowest to highest
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 15
Analog Bandwidth andFrequency Spectrum
Example
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 16
Digital Signaling Methods
Are discrete, not continuous
Take the value of a binary 0 or binary 1
Make use of encoding schemes such as:
Manchester Ethernet
Differential Manchester
Token Ring
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 17
Digital Signaling Methods
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 18
Digital Signaling Methods
Make use of a “bit interval”, the time required to send a single bit
The sender and receiver can use this bit interval to clock their transmission with each other
With the bit interval as a clocking mechanism The sender and receiver can synchronize their
transmissions However, self-clocking mechanisms are more efficient
A bit rate is also associated with a digital signaling method
The bit rate is the number of bit intervals per second, or bps
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 19
Digital Bandwidth
Digital bandwidth is typically expressed in bits per second (bps)
Digital bandwidth is determined using the bit interval and the bit rate
Assuming we have a digital signal with a bit interval of 60 microseconds, what is its bandwidth?
The formula is expressed as: bps = 1 / (60 * 10-6), or approximately 16.6 Kbps (thousands of bits per second)
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 20
Digital Signaling Methods
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 21
Circuit Configuration -- 2nd Component of the Physical Layer Two major categories
Point-to-point Multipoint
Point-to-point circuits are dedicated links between two communicating devices
Multipoint circuits are shared among several communicating devices
Either could be appropriate based on network need
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 22
Point-to-point Circuit
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 23
Multipoint or Multidrop Circuit
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 24
Transmission Medium -- 3rd Component of the Physical Layer Two major categories
Conducted Radiated
Conducted – Makes use of cables Twisted wire pair, coaxial cable, fiber optic
Radiated – is “In the Air” Terrestrial microwave, satellite, radio, infrared
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 25
Conducted MediaUse Cable
An example of category 5, unshielded twisted wire pair
An example of a coaxial cable, with its layered sheathing
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 26
Conducted MediaUse Cable
Single mode fiber Cable with Connectors
Fiber cores are measured in microns
Another type of fiber is multimode
Fiber is composed of either glass or plastic strands
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 27
Radiated Media, Signals “in the air”
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 28
Transmission MediumConsiderations
Cost
Bandwidth
Security
Transmission Impairment
Distance
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 29
Transmission MediumImpairment Considerations
Attenuation Cross talk Distortion Environmental factors
Rain, fog, snow, cloud cover, electrical or magnetic storms
Of the various media, fiber optic is the least susceptible to impairment, and the most secure
But also very expensive!
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 30
Comparison ofVarious Media
Type of Medium Security Transmission Distance
Cost Error Potential
Difficulty of Installation
Twisted Wire Pair Moderate Short Low Moderate Low
Coaxial Cable Moderate Short Moderate Low Low
Fiber Optic High Moderate to Long High Very Low High
Radio Low Short Low Moderate Moderate
Terrestrial Microwave Low Long Moderate Low to Moderate
Low to Moderate
Satellite Low Long Moderate Low to Moderate
Low to Moderate
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 31
Devices at the Physical Layer -- 4th Component of the Physical Layer Hubs or Repeaters
Modems
Codecs
Multiplexers
Cabling Tools (Not devices, but still important)
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 32
Devices at the Physical Layer -- 4th Component of the Physical Layer
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 33
Multiplexing Allows slower-speed circuit devices to share a
single high-speed circuit In many cases, individual devices do not need
their own high-speed circuit Type of multiplexing include:
Frequency division Time division Statistical time division Wavelength division
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 34
Frequency Division Multiplexing
A single high-speed circuit with multiple channel frequencies
The circuit is analog Bandwidth is measured in hertz Data transmitted via channels Viewed as horizontal Makes use of guardbands as overhead
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 35
Frequency Division Multiplexing
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 36
Time Division Multiplexing
A single high-speed circuit carrying multiple frames
Time slots may only be used by specifically allocated devices
The circuit is digital Bandwidth is measured in bits per second
(bps) Data transmitted via frames Viewed as vertical Unused time slots create overhead
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 37
Time Division Multiplexing
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 38
Statistical Time Division Multiplexing
A single high-speed circuit carrying multiple frames
Time slots can be allocated to devices as needed
Time slots must carry addressing The circuit is digital Bandwidth is measured in bits per second Data transmitted via frames Viewed as vertical, not horizontal Addressing of time slots creates overhead
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 39
Statistical Time Division Multiplexing
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 40
Wavelength DivisionMultiplexing
Makes use of fiber optics Operates in a manner somewhat similar to the way
that frequency division multiplexing is used with copper
Uses lasers to transmit different frequencies of light through the same fiber optic cable
At the sending end, narrow bands of light are combined into a wider band
The wider band is the high-speed circuit At the receiving end, the signals are separated
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 41
In Summary
The physical layer:
In the OSI and TCP/IP models is similar
Is essential for transporting of data bits from sender to receiver
Has circuits that are Conducted or Radiated
Passes its unformatted data bit stream up to the Data Link Layer
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