Chapter 8 Notes

Chapter 8 Notes – Physical Layer Role: To encode the binary digits that represent Data Link layer frames into signals and to transmit and receive these signals across the physical media  Communication Signals ➢ Purpose – to create the electrical, optical, or microwave signal that represents the bits in each frame  Requires: • Physical media & associated connectors • A representation of bits on the media • Encoding of data and control info. • Transmitter & receiver circuitry on network devices ➢ Operation  Media does not carry the frame as a single entity  Media carries signals, one at a time, to represent the bits that make up the frame  Three basic forms of network media • Copper cable ♦ Represented as electrical pulses • Fiber ♦ Patterns of light • Wireless ♦ Patterns of radio transmission  Physical layer may also add its own signals to frame to ID beginning & end ➢ Standards  Set by: • ISO • IEEE • ANSI • ITU • EIA/TIA • FCC (U.S.A.)  Four areas of physical layer standards • Physical and electrical properties of media • Mechanical properties (materials, dimensions, pinouts) of the connectors • Bit representation by the signals (encoding) • Definition of control information signals ➢ Fundamental Principles  Physical components  Data encoding • Converting a stream into a predefined code (groupings of bits used to provide a predictable pattern used by sender/receiver  Signaling

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Phys. Layer must generate electrical, optical or wireless signals that rep. the 1 and 0 on the media

 Physical Signaling and Encoding: Representing Bits  All comm. From the human network becomes binary digits, which are transported individually across the physical media ➢ Signaling Bits for the Media • Phys. Layer reps each of the bits in the frame as a signal • Signal placed on media has a specific amount of time to occupy media (bit time) • At phys. Layer of receiving node, signals converted back into bits  Signaling Methods • Amplitude • Frequency • Phase  NRZ Signaling • A low voltage value represents a logical 0 • A high voltage value represents a logical 1 • Only suited for slow speed data links • Uses bandwidth inefficiently & susceptible to EMI  Manchester Encoding • Bit values represented as voltage transitions ♦ Low volt to high volt = 1 ♦ High volt to low volt = 0 • Not efficient enough to be used at higher signaling speeds • Signaling method employed by 10BaseT Ethernet ➢ Grouping Bits  “Encoding” to represent the symbolic grouping of bits prior to being presented to the media • Encoding prior to signals being placed on network – efficiency at higher speed data transmission • Higher speeds – possibility data will be corrupted • Coding groups – detect errors more efficiently  Signal Patterns • Begin each frame with a pattern of signals representing bits that the Physical layer recognizes as denoting the start of a frame • Another pattern – end of frame • Valid data bits need to be grouped into frames  Code Groups • A consecutive sequence of code bits that are interpreted and mapped as data bit patterns • Advantages of using code groups: ♦ Reducing bit level error ♦ Limiting the effective energy transmitted into the media ♦ Helping to distinguish data bits from control bits

♦ Better media error detection  Reducing bit level errors • Properly detect an individual bit as 0 or 1, receiver must know how and when to sample the signal on media ♦ Requires timing between receiver and transmitter by synchronized • Code groups designed so that symbols force an ample number of bit transitions to occur on media ♦ Use symbols to insure that not too many 1s or 0s are used in a row  Limiting energy transmitted • DC balancing - Process of balancing number of 1s and 0s transmitted ♦ Prevents excessive amounts of energy from being injected into media during trans. • Transmitting a long series of 1s could overheat transmitting laser and the photo diodes in the receiver – potentially higher error rates  Distinguished data from control • Code groups 3 types of symbols: ♦ Data symbols – symbols that rep the data of the frame as it passed down to the Physical layer ♦ Control symbols – special codes injected by the Physical layer used to control transmission. These include end-of-frame and idle media symbols ♦ Invalid symbols – symbols that have patterns not allowed on the media. Receipt of an invalid symbol indicates a frame error • Symbols representing data being sent have different bit patterns than the symbols used for control – allow physical layer in receiving node to immediately distinguish data from control information  Better media error detection • Code groups contain invalid symbols • Not used by the transmitting node • If a receiving node receives one of these patterns, phys layer can determine an error in data reception  4B/5B • 4bits of data into 5-bit code symbols • Represent data to be trans, as well as a set of codes that help control trans on media • Symbols that indicate beg & end of frame trans • Ensures at least one level change per code to provide synchronization ➢ Data Carrying Capacity  Measured in 3 different ways • Bandwidth ♦ Capacity of a medium to carry data ♦ Digital bandwidth measures the amount of information that can flow from one place to another in a given amount of time

♦ Practical bandwidth determined by ➢ Properties of physical media & technologies chosen for signaling and detecting network signals  Physical media properties  Current technologies  Laws of physics • Throughput ♦ The measure of the transfer of bits across the media over a given period of time ♦ Many factors influence throughput ➢ Amount of traffic ➢ Type of traffic ➢ Number of network devices encountered on the network being measured • Goodput ♦ The measure of usable data after protocol overhead traffic has been removed, therefore the measure that is of most interest to network users ♦ Accounts for bits devoted to protocol overhead  Connecting Communication ➢ Copper Media  Most commonly used for data communication  Consists of series of individual copper wires that form circuits  Data transmitted as electrical pulses  Timing and voltage values susceptible to interference  Cable types with shielding or twisting of the pairs of wires are designed to minimize signal degradation due to electronic noise  Susceptibility to electronic noise can be limited by • Selecting cable type / category most suited to protect the data signals in a given networking environment • Designing a cable infrastructure to avoid known and potential sources of interference in the building structure • Using cabling techniques that include the proper handling and termination of the cables ➢ Unshielded Twisted Pair (UTP) Cable  Twisting cancels unwanted signals  Receiver processes EMI in equal yet opposite ways – cancelling it  Avoid interference from internal sources (crosstalk) • The interference caused by the magnetic field around the adjacent pairs of wires in the cable  UTP Standards: • Cable types • Cable lengths • Connectors • Cable termination • Methods of testing cable  UTP cable types • Ethernet Straight-through

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Ethernet Crossover ♦ 568B patch cables ➢ Wiring pinouts  1 & 2 – Orange  3 & 6 – Green  4 & 5 – Blue  7 & 8 - Brown • Rollover ➢ Other Copper Cable  Coaxial • Copper conductor surrounded by a layer of flexible insulation • Uses: ♦ Wireless and cable access technologies ♦ Carries radio frequency (RF) energy between antennas and radio equipment • Hybrid Fiber Coax – a network which incorporates optical fiber along with coaxial cable to create a broadband network  Shielded Twisted-Pair (STP) • Used to be cabling structure for use in Token Ring network installations ➢ Copper Media Safety  Electrical Hazards • May conduct unwanted voltage • Undesirable voltages & currents can damage network devices and connected computers  Fire Hazards • Cable insulation & sheaths may be flammable or produce toxic fumes when heated or burned ➢ Fiber Media  Uses glass/plastic fiber to guide light impulses from source -> dest  Immune to EMI  Can be operated at much greater lengths  Implementation issues: • More expensive than copper • Different skills & equipment required to terminate and splice cable infrastructure • More careful handling than copper media  Commonly used as backbone cabling  Cable consists of PVC jacket & series of strengthening materials that surround fiber & its cladding  Light can only travel one way -> two fibers are required for duplex operation  Lasers or LEDs generate light pulses  Photodiodes detect the light pulses & convert to voltages to be reconstructed into data frames  2 classificaitons of fiber

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Single-Mode ♦ small core ♦ less dispersion ♦ suited for long distance apps (up to 100km, 62.14mi) ♦ uses lasers as the light source often within campus backbones for distance of several thousand meters • Multimode ♦ Larger core than single-mode cable ♦ Allows greater dispersion -> loss of signal ♦ Used for long dist apps, but shorter than single-mode ♦ Uses LEDs as light source often within LANs or distance of a couple hundred meters within a campus network ➢ Wireless Media  Carry electromagnetic signals at radio & microwave frequencies  Types of Wireless Networks • 802.11 – Wireless LAN (WLAN) ♦ Uses contention / non-deterministic system with a Carrier Sense Multiplier ♦ CSMA/CA media access protection • 802.15 – Wireless Personal Area Network (WPAN) ♦ Bluetooth ♦ Device pairing process (1 – 100 meters) • 802.16 – Worldwide Interoperability for Microwave Access (WiMAX) ♦ Uses a point-to-multipoint topology to provide wireless broadband access • Global System for Mobile Communication (GSM) ➢ The Wireless LAN  802.11a • Operates at 5GHz • Speeds up to 54 Mbps • Smaller coverage & less penetration of building structures  802.11b • 2.4 GHz • Up to 11mbps • Longer range & better able to penetrate building structures  802.11g • 2.4 GHz • 54 Mbps • Same radio frequency of 802.11b, bandwidth of 802.11a  802.11n • 2.5 or 5 GHz • 100 Mbps – 210 Mbps • Range – 70 meters ➢ Common Copper Media Connectors  Correct Connector Termination

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It is essential that all copper media terminations be of high quality to ensure optimum performance with current and future network technologies • Improper cable termination can impact transmission performance Common Optical Fiber Connections • Straight-Tip (ST) – very common bayonet style connector widely used with multimode fiber • Subscriber Connector (SC) – connector that uses a push-pull mechanism to ensure positive insertion. Widely used in single-mode fiber • Lucent Connector (LC) – small connector becoming popular for use with single-mode fiber and also supports multi-mode fiber Common fiber-optic termination and splicing errors: • Misalignment – f-o media are not precisely aligned to on another when joined • End gap – media do not completely touch at the splice or connection • End finish – media ends are not well polished or dirt is present at the termination Recommended that Optical Time Domain Reflectometer (OTDR) be used • Injects a test pulse of light into cable & measures back scatter & reflection • Will calculate approx distance at which faults are detected