Business Data Communications Chapter Two Physical Layer Fundamentals

Primary Learning Objectives n Understand the general purpose of the physical layer n Distinguish between analog and digital Signaling n Describe circuit configurations and methods of data flow n Identify characteristics of conducted and radiated Media n Name and differentiate four types of multiplexing Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall 2

Physical Layer n n n 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: n n Signaling method Circuit configuration Transmission medium Devices used Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall 3

Physical Layer Pin connector functions are an important physical layer responsibility Business Data Communications,

Signaling Methods -1 st Component of the Physical Layer n Analog versus Digital n n n Analog uses modulation techniques n n n Analog is continuous Digital is discrete Amplitude, Frequency, Phase, for example Analog is measured in hertz Digital uses encoding schemes n n 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 5

Analog Signaling Methods n n n Can take an infinite form Modulate a sine wave Can change a wave’s amplitude, frequency, or phase n n 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 6

Analog Signaling Methods Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall

Analog Signaling Methods A hertz is a unit of frequency. A period is measured

Analog Signaling Methods n Simple signal methods use: n n 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 11

Analog Signaling Methods n A complex signal method occurs when the bit rate and the symbol rate are not the same: n n 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 12

Analog Bandwidth n n n 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 14

Analog Bandwidth and Frequency Spectrum Example Business Data Communications, by Allen Dooley, (c) 2005

Digital Signaling Methods n n n Are discrete, not continuous Take the value of a binary 0 or binary 1 Make use of encoding schemes such as: n Manchester n n Ethernet Differential Manchester n Token Ring Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall 16

Digital Signaling Methods Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall

Digital Signaling Methods n n n 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 n n n 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 n The bit rate is the number of bit intervals per second, or bps Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall 18

Digital Bandwidth n n n 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? n 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 19

Circuit Configuration -2 nd Component of the Physical Layer n Two major categories n n n 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 21

Point-to-point Circuit Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall 22

Multipoint or Multidrop Circuit Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice

Transmission Medium -3 rd Component of the Physical Layer n Two major categories n n n Conducted – Makes use of cables n n Conducted Radiated Twisted wire pair, coaxial cable, fiber optic Radiated – is “In the Air” n Terrestrial microwave, satellite, radio, infrared Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall 24

Conducted Media Use Cable n n 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 25

Conducted Media Use Cable n n 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 26

Radiated Media, Signals “in the air” Business Data Communications, by Allen Dooley, (c) 2005

Transmission Medium Considerations n Cost n Bandwidth n Security n Transmission Impairment n Distance

Transmission Medium Impairment Considerations n n Attenuation Cross talk Distortion Environmental factors n n 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 n But also very expensive! Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall 29

Comparison of Various 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 Fiber Optic High Moderate to Long High Very Low High Radio Low Short Low Moderate Terrestrial Microwave Low Long Moderate Low to Moderate Satellite Low Long Moderate Low to Moderate Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall 30

Devices at the Physical Layer -4 th Component of the Physical Layer n Hubs or Repeaters n Modems n Codecs n Multiplexers n Cabling Tools (Not devices, but still important) Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall 31

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

Multiplexing n n n 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: n n Frequency division Time division Statistical time division Wavelength division Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall 33

Frequency Division Multiplexing n n n 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 34

Frequency Division Multiplexing Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall

Time Division Multiplexing n n n n 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 36

Time Division Multiplexing Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall

Statistical Time Division Multiplexing n n n n 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 38

Statistical Time Division Multiplexing Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice

Wavelength Division Multiplexing n n n 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 40

In Summary n The physical layer: n n 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, by Allen Dooley, (c) 2005 Pearson Prentice Hall 41