Business Data Communications

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

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

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

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

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

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

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

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

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

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

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

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

An example of category 5, unshielded twisted wire pair An example of a coaxial cable, with its layered sheathing Conducted Media Use Cable Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall

Conducted Media Use 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

Transmission Medium Impairment 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

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 Comparison of Various Media Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall

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

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

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

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

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

Wavelength Division Multiplexing • 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

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, by Allen Dooley, (c) 2005 Pearson Prentice Hall

Business Data Communications