© 2009 Pearson Education, Inc. Publishing as Prentice Hall Physical Layer Propagation Chapter 3 Updated January 2009 Raymond Panko’s Business Data Networks and Telecommunications, 7th edition May only be used by adopters of the book

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Signal and Propagation A signal is a disturbance in the media that propagates (travels) down the transmission medium to the receiver If propagation effects are too large, the receiver will not be able to read the received signal

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Types of Data Transmitted Analog data –Produced by telephones –Sound waves –Can take on any value in a wide range of possibilities Digital data –Produced by computers, in binary form, represented as a series of ones and zeros –Can take on only 0 ad 1

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Binary Data Representation

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-5 Binary-Encoded Data Computers store and process data in binary representations –Binary means “two” –There are only ones and zeros –Called bits Non-Binary Data Must Be Encoded into Binary Hello …

© 2009 Pearson Education, Inc. Publishing as Prentice Hall3-6 Binary-Encoded Data Some data are inherently binary –48-bit Ethernet addresses –32-bit IP addresses –Need no further encoding

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Arithmetic with Binary Numbers Binary Arithmetic for Whole Numbers (Integers) (Counting Begins with 0, not 1) Integer Binary

© 2009 Pearson Education, Inc. Publishing as Prentice Hall3-8 Encoding Alternative Bits (N)Alternatives (2 N ) , ,536 An N-bit field can represent 2 N alternatives Each additional bit doubles the number of possibilities Start with one you know and double or halve until you have what you need E.g., if you know 8 is 256, 10 must be 4 times as large or 1,024

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Binary Encoding for a Number of Alternatives Number of Bits in Field Number of Alternatives that Can Be Encoded 1 Specific Bit Sequences Example 12 1 = 20, 1Yes or No, Male or Female, etc = 400, 01, 10, 11North, South, East, West 42 4 = , 0001, 0010, … Top 10 security threats (6 values go unused) 82 8 = , , … ASCII text representation (128 values go unused) 1 There are 2 N alternatives with N bits

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Binary Encoding for a Number of Alternatives Examples: 1. You have N bits. How many alternatives can you represent? 2. You have 4 bits. How many alternatives can your represent? 3. You need to represent 8 things. How many bits must you use? 4. You need to represent 6 things. How many bits must you use?

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : ASCII Purpose –To represent text (A, a, 3, $, etc.) as binary data for transmission ASCII –Traditional code to represent text data in binary –Seven bits per character –2 7 (128) characters possible –Sufficient for all keyboard characters (including shifted values)

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : ASCII ASCII –Sufficient for all keyboard characters CategoryMeaningASCII Capital lettersA Lower-case lettersa Digits Punctuation Special Space Printing controlCarriage Return Printing controlLine feed

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : ASCII Each ASCII Character is Sent in a Byte –8 th Bit in Data Bytes Normally Is Not Used Data Byte ASCII Code for Character Unused. Value does not matter

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : ASCII To send “Hello world!” (without the quotes), how many bytes will you have to transmit?

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Data Encoding and Signals We have just seen this We will now see this Before transmission, two things must happen First, data must be converted into a bit stream We have already seen this Second, the 1s and 0s need to be converted into signals—disturbances that travel down the medium

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Box: Multistate Digital Signaling

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Multistate Digital Signaling Concepts –Bit rate: Number of bits sent per second –Baud rate: Number of clock cycles per second If 1,000 clock cycles per second, 1 kbaud If each clock cycle is 1/1,000 second = 1,000 clock cycles/second = 1 kbaud Box

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Multistate Digital Signaling Computing the Bit Rate Bit rate = Baud rate X Bits sent per clock cycle EX: –If baud rate is 10,000 baud –If two bits per clock cycle –Then bit rate is 2 x 10,000 or 20,000 bps = 20 kbps Box

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Multistate Digital Signaling Computing the Bit Rate –Know the baud rate and the number of states –Compute the number of bits from the number of states –States = 2 Bits per clock cycle Bit rate = Baud rate X Bits sent per clock cycle EX –If baud rate is 10,000 baud (not bauds) –If four states, can send 2 bits per clock cycle –Then bit rate is 2 x 10,000 or 20,000 bps = 20 kbps Box

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Multistate Digital Signaling Computing the Required Number of States –Know the required bit rate and baud rate –Bits sent per clock cycle =Bit rate / Baud rate –Compute the required number of states EX: –Required bit rate is 4 Mbps –Baud rate is 1 Mbaud –Bit rate / baud rate = 4 bits per clock cycle –4 bits per clock cycle are required Box

© 2009 Pearson Education, Inc. Publishing as Prentice Hall21 Bit Rate versus Baud Rate Number of Possible States Bits per Clock Cycle 2 (Binary) If a Baud Rate is 1,200 Baud, Bit Rate is 1,200 bps 2,400 bps 3,600 bps 4,800 bps Each Doubling of States Gives One More Bit per Clock Cycle

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Quiz: 3-22 There are eight states. Each clock cycle is 1/8000 of a second. What is the baud rate? What is the bit rate?

© 2009 Pearson Education, Inc. Publishing as Prentice Hall UTP Propagation Unshielded Twisted Pair wiring

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 24 Two main categories: –wires, cables –wireless transmission, e.g. radio, microwave, infrared, … Wired –Twisted-Pair cables: –Coaxial cables –Fiber-optic cables Transmission Media

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Unshielded Twisted Pair (UTP) Wiring UTP Characteristics –Inexpensive and to purchase and install –Dominates media for access links between computers and the nearest switch

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 4-Pair UTP Cord with RJ45 Connector 3. 8-pin RJ-45 Connector 2. 8 Wires organized as 4 twisted pairs Industry standard pen 1. UTP cord

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Unshielded Twisted Pair (UTP) Wiring Cord Organization –A length of UTP wiring is a cord –Each cord has eight copper wires –The wires are organized as four pairs Each pair’s two wires are twisted around each other several times per inch –There is an outer plastic jacket that encloses the four pairs

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Unshielded Twisted Pair (UTP) Wiring Connector –RJ-45 connector is the standard connector –Plugs into an RJ-45 jack in a NIC, switch, or wall jack RJ-45 Jack RJ-45 Jack 8-pin RJ-45 connectors

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Attenuation and Noise Power Distance 3. Noise Floor (Average Noise level) 2. Noise 4. Noise Spike 1. Signal 6. Signal- to-Noise Ratio (SNR) 5. Error 1.The signal attenuates (falls in power) as it propagates 2.There is noise (random energy) in the wire that adds to the signal 3.The average noise level is called the noise floor 4.Noise is random. Occasionally, there will be large noise spikes 5.Noise spikes as large as the signal cause errors 6.You want to keep the signal-to-noise ratio high

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-30 Limiting UTP Cord Length Limit UTP cord length to 100 meters –This keeps the signal-to-noise ration (SNR) high –This makes attenuation and noise problems negligible –Note that limiting cord lengths limits BOTH noise and attenuation problems 100 Meters Maximum Cord Length

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-31 UTP Wiring Electromagnetic Interference (EMI) –Electromagnetic interference is electromagnetic energy from outside sources that adds to the signal From fluorescent lights, electrical motors, microwave ovens, etc.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Electromagnetic Interference (EMI) and Twisting Interference on the Two Halves of a Twist Cancels Out Twisted Wire Electromagnetic Interference (EMI) UTP is twisted to reduce EMI

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Crosstalk Interference and Terminal Crosstalk Interference Untwisted at Ends Signal Terminal Crosstalk Interference Crosstalk Interference Terminal crosstalk interference normally is the biggest EMI problem for UTP

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-34 UTP Limitations Limit cords to 100 meters –Limits BOTH noise AND attenuation problems to an acceptable level Do not untwist wires more than 1.25 cm (a half inch) when placing them in RJ-45 connectors –Limits terminal crosstalk interference to an acceptable level Neither completely eliminates the problems but they usually reduce the problems to negligible levels 2

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Optical Fiber Transmission Light through Glass Spans Longer Distances than UTP

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Optical Fiber Transceiver and Strand An optical fiber strand has a thin glass core This core is 8.3, 50, or 62.5 microns in diameter This glass core is surrounded by a tubular glass cladding The outer diameter of the cladding is 125 microns, regardless of the core’s diameter The transceiver injects laser light into the core

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Optical Fiber Transceiver and Strand When a light wave ray hits the core/cladding boundary, there is perfect internal reflection. There is no signal loss

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Roles of UTP and Optical Fiber in LANs

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-39 Two-Strand Full-Duplex Optical Fiber Cord with SC and ST Connectors A fiber cord has two-fiber strands for full-duplex (two- way) transmission SC Connectors ST Connectors Two Strands Cord

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Radio Propagation

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-41 Radio Propagation Radio signals also propagate as waves. Radio waves are measured in hertz (Hz), which is a measure of frequency. Radio usually operates in the MHz and GHz range. Hertz (Hz) is the term for cycles per second

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Omnidirectional and Dish Antennas

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Wireless Propagation Problems UTP and optical fiber propagation are fairly predictable. However, radio suffers from many propagation effects. This makes radio transmission difficult to manage. We will look at these problems one at a time.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Wireless Propagation Problems The first propagation problem is electromagnetic interference (EMI) from nearby radio sources This includes other wireless devices It can include microwave ovens an other devices

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Wireless Propagation Problems Another problem is inverse square law attenuation. As a signal propagates, its energy spreads out over the Surface of an ever-expanding sphere.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall46 Laptop Comm. Tower Shadow Zone 3-28: Wireless Propagation Problems No Signal

© 2009 Pearson Education, Inc. Publishing as Prentice Hall47 Multipath Interference Laptop Comm. Tower 3-28: Wireless Propagation Problems Signals Arriving by Different Paths May Cancel Out

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Topology Network topology is the physical arrangement of a network’s computers, switches, routers, and transmission lines It is a physical layer concept

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies The simplest topology is the point-to-point topology

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies Ethernet uses a star topology Note that the switch does not have to be in the middle of the star

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies Larger Ethernet LANs use an extended star topology This is better called a hierarchical topology

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies Mesh (Routers, Frame Relay, ATM) A B C D Path ABD Path ACD In a mesh topology, there are many connections between switches or routers Consequently, there are many alternative routes between hosts

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies In the ring topology, messages travel around a loop

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies The bus topology uses broadcasting. The message receives each host at almost the same time. All wireless transmission uses a bus topology.