business data communications

Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Two 1

Business Data Communications

Chapter Two

Physical Layer Fundamentals


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


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


Physical Layer

Pin connector functions are an important physical layer responsibility


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


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



A hertz is a unit of frequency. A period is measured in seconds.



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



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


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


Analog Bandwidth andFrequency Spectrum

Example


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



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


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)


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


Point-to-point Circuit


Multipoint or Multidrop Circuit


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


Conducted MediaUse Cable

An example of category 5, unshielded twisted wire pair

An example of a coaxial cable, with its layered sheathing


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


Radiated Media, Signals “in the air”


Transmission MediumConsiderations

Cost

Bandwidth

Security

Transmission Impairment

Distance


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!


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


Devices at the Physical Layer -- 4th Component of the Physical Layer Hubs or Repeaters

Modems

Codecs

Multiplexers

Cabling Tools (Not devices, but still important)


Devices at the Physical Layer -- 4th Component of the Physical Layer


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


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


Frequency Division Multiplexing


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


Time Division Multiplexing


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


Statistical Time Division Multiplexing


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


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

business data communications

Documents

symbol rate

single symbol

methodssimple signal

symbolcomplex signal

physical characteristics

methodsa complex signal

unformatted data

physical layerdistinguish