Chapter8

Ch 8 - Chapter 8 OSI Physical Layer

Objectives Explain the role of Physical layer protocols and services in supporting communication across data networks Describe the purpose of Physical layer signaling and encoding as they are used in networks Describe the role of signals used to represent bits as a frame is transported across the local media Identify the basic characteristics of copper , fiber , and wireless network media Describe common uses of copper, fiber, and wireless network media Ch 8 -

Role of the Physical Layer Encode the binary digits that represent Data Link layer frames into signals Transmit and receive these signals across the physical media – copper wires, optical fiber and wireless Ch 8 -

Purpose of the Physical Layer The OSI Physical layer provides the means to transport the bits that make up the Data Link frame across the network media Creates the electrical, optical or microwave signal that represents the bits in each frame Retrieve the signals, restore them to their bit representation and pass the bits up to the Data Link layer as a complete frame Ch 8 -

Physical Layer Elements The physical media and associated elements A representation of bits on the media the type of signal depends on the type of media Encoding of data and control information Transmitter and receiver circuitry on the network devices Ch 8 -

Physical Layer Operation The Physical layer may add its own signals to indicate the beginning and end of the frame The signals can be in the form of electrical , light or radio pulses Ch 8 -

Physical Layer Standards The Physical layer consists of hardware in the form of electronic circuitry, media and connectors The standards governing the hardware are defined by the relevant electrical and communications engineering organizations IEEE, EIA/TIA, ISO et al Ch 8 -

Physical Layer Technologies Physical and electrical properties of the media Mechanical properties (materials, dimensions, pin-outs) of the connectors Bit representation by the signals (encoding) Definition of control information signals Ch 8 -

Functions of the Physical Layer Physical components electronic hardware device, media and connectors Encoding a method of converting a stream of data bits into a predefined code using predictable patterns helps to distinguish data bits from control bits and provide better media error detection Signaling generate the electrical, optical or wireless signals that represent the “1” and “0” Ch 8 -

Signals Each signal placed onto the media can occupy a specific amount of time, known as the bit time (or bit interval) A clock signal provides synchronization between the transmitter and receiver for successful delivery of the bits bits must be examined at specific times during the bit time Ch 8 -

Signaling Methods Bits are represented on the medium by changing either the amplitude , frequency or phase of the signal e.g. with Non-Return to Zero, a 0 is represented by one voltage during the bit time and a 1 may be represented by another voltage during the bit time There are also methods of signaling that use transitions to indicate a logic level detect the voltage transitions in the middle of the bit time Manchester coding indicates a 0 by a high to low voltage transition and a 1 by a low to high voltage transition in the middle of the bit time Ch 8 -

Non Return to Zero (NRZ) Signaling The bit stream is transmitted as a series of voltage values a low voltage value represents a logic 0 a high voltage value represents a logic 1 The NRZ signaling is suitable for slow speed data links uses bandwidth inefficiently and is susceptible to electromagnetic interference boundaries between individual bits can be lost when a long strings of 1s or 0s are transmitted Ch 8 -

Manchester Coding The bit values are represented as voltage transitions a low to a high voltage transition represents a logic 1 a high to a low voltage transition represents a logic 0 Voltage transition must occur in the middle of each bit time this transition can be used to ensure the bit times at the receiving nodes are synchronized with the transmitting nodes Signaling method used by 10BaseT Ethernet Ch 8 -

Encoding Encoding is the symbolic grouping of bits prior to being presented on the media improve the efficiency at higher speed data transmission detect errors more efficiently The Physical layer of a device needs to be able to detect legitimate data signals and ignore random non-data signals Signal patterns that provide frame detection start and end of frame valid data bits need to be grouped into a frame random unframed signals due to noise or interference are not decoded Ch 8 -

Signal Patterns The signal patterns, start of frame, end of frame and frame contents , are decoded into bits Ch 8 -

Code Groups A code group is a consecutive sequence of code bits that are interpreted as data bit pattern e.g. the code bits 10101 could represent the data bits 0011 often used as an intermediary encoding technique for higher speed LAN technologies Code groups introduce overhead in the form of extra bits to transmit but improve the robustness of a communication link Ch 8 -

Advantages of Using Code Groups Reducing bit errors to properly detect an individual bit correctly, the receiver must know how and when to sample the signal signal transitions are used for synchronization code groups are designed so that the symbols force an ample number of bit transitions to synchronize the timing ensures that not too many 1s or 0s are used in a row Limiting the effective energy transmitted the number of 1s and 0s in a string of symbols are evenly balanced, known as DC balancing prevents excessive amounts of energy being injected into the media during transmission, thereby reducing the radiated interference transmitting a long series of 1s, presence of energy, could overheat the transmitting laser and the photo diodes in the receiver Ch 8 -

Advantages of Using Code Groups (cont’d) Distinguish data bits from control bits the symbols representing the data have different bit patterns than symbols for control data symbols represent the data of the frame control symbols are special codes injected by the Physical layer invalid symbols are patterns not allowed on the media Better media error detection invalid symbols create long series of 1s or 0s the receipt of an invalid symbol indicates a frame error Ch 8 -

Code Symbol In a 4B/5B code symbol, 4 bits of data are turned into a 5-bit code symbols for transmission Ch 8 -

Data Carrying Capacity Bandwidth the capacity of a medium to carry data is described as the raw data bandwidth digital bandwidth measures the amount of information that can flow from one place to another in a given amount of time typically measured in kbps or Mbps physical media properties, current technologies and the laws of physics are factors that can determine available bandwidth Ch 8 -

Data Carrying Capacity (cont’d) Throughput the measure of the transfer of bits across the media over a given period of time usually does not match the specified bandwidth factors such as the amount of traffic, type of traffic, number of devices and media access methods can influence throughput Goodput the measure of useable data transferred over a given period of time measures the effective transfer of user data between Application layer entities, such as a web server and a client web browser goodput is throughput minus traffic overhead for establishing sessions, acknowledgements and encapsulation Goodput < Throughput < Bandwidth BW – Theoretical, Throughput – Actual, Goodput - Data Ch 8 -

*Data Carrying Capacity (cont’d) Ch 8 -

Physical Media Characteristics The standards for copper media define the type of media, bandwidth, type of connectors, pin-outs and color code connections and maximum cable distance Ch 8 -

Copper Media A coaxial cable has a single conductor that run through the centre of the cable that is encased by, but separated from, a shield Twisted pair cable consists of pairs of color-coded wires twisted together and encased in a plastic sheath Networking media generally make use of modular jacks and plugs provide easy connection and disconnection Ch 8 -

External Signal Interference Data is transmitted on copper wires as electrical pulses The timing and voltages of these data signals are susceptible to interference or “noise” from outside the communication system these unwanted signals can distort and corrupt the data signals Radio waves and electromagnetic devices such as fluorescent lights , electric motors and other devices are potential sources of noise Ch 8 -

Limiting Interference Cable types with shielding or twisting of the pairs of wires are designed to minimize signal degradation due to electronic noise Selecting the cable type or 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 proper handling and termination of the cables Ch 8 -

Unshielded Twisted Pair ( UTP ) UTP cabling consists of four pairs of color-coded wires twisted together and encased in a flexible plastic sheath Twisting has the effect of canceling unwanted signals the unwanted signals present on the twisted pair wires are equal and in opposite magnitude The cancellation effect also helps to avoid interference from internal sources called crosstalk crosstalk is the interference caused by the magnetic field around adjacent pairs of wires Ch 8 -

UTP Cabling Standards UTP cabling conforms to the standards established jointly by the Telecommunications Industry Association (TIA) and Electronics Industries Alliance (EIA) TIA/EIA-568A stipulates the commercial cabling standards for LAN installation cable types, cable lengths, connectors, cable termination and methods of testing cable The electrical characteristics of copper cabling are defined by the Institute of Electrical and Electronic Engineers (IEEE) cables are placed into categories according to their ability to carry higher bandwidth rates Category 5 ( Cat 5 ) cable is commonly used in 100Base-TX Fast Ethernet installations Cat 6 is the recommended type for new building installations Ch 8 -

Coaxial Cables A widely used media in wireless and cable access technologies Coax cables are used to attach antennas to wireless devices Traditional cable TV, transmitting in one direction, was composed completely of coax cable Cable service providers are providing Internet connectivity with the combined use of fiber and coax , referred to hybrid fiber coax ( HFC ) Ch 8 -

Coaxial Cable Connectors There are different types of connectors used with coax cable Ch 8 -

Shielded Twisted-Pair (STP) Cable Each twisted pair is wrapped in a foil shield The entire bundle of shielded twisted pairs are shielded in a braided shield and protected by a plastic jacket STP provides better noise protection than UTP cabling at a higher cost STP was the cabling structure specified for Token Ring network installations Ch 8 -

Copper Media Safety Electrical hazards copper wires could conduct electricity in undesirable ways network cabling could present undesirable voltage levels when used to connect devices that have power sources with different ground potential copper cabling may conduct voltages caused by lightning strikes to network devices Fire hazards cable insulation and sheaths may be flammable or produce toxic fumes when heated or burned Ch 8 -

Fiber Media Fiber optic uses either glass or plastic fibers to guide light impulses bits are encoded as light impulses Capable of providing very large raw bandwidth rates Media is immune to electromagnetic interference and will not conduct unwanted electrical currents due to grounding issues Optical fibers are thin and have relatively low signal loss Can operate at much greater lengths than copper without the need for signal regeneration Ch 8 -

Optical Fiber Implementation Issues More expensive than copper media over the same distance (but for a higher capacity) optical fiber is primarily used as backbone cabling for high-traffic point-to-point connections between data distribution facilities and for the interconnection of buildings in multi-building campus Different skills and equipment required to terminate and splice the cable infrastructure More careful handling than copper media Ch 8 -

Optical Fiber Construction Optical fiber consists of a PVC jacket and a series of strengthening materials that surround the optical fiber and its classing Light can only travel in one direction over optical fiber two fibers are required to support full-duplex operation Fiber-optic patch cables bundle together two optical fiber cables and terminate them with a pair of standard single fiber connectors straight tip (ST) subscriber connector (SC) Ch 8 -

Single Mode Optical Fiber Small core Uses a single ray of light emitted from a laser laser light is uni-directional and travels down the center of the fiber can transmit optical pulses for very long distances Less dispersion Suited for long distance applications (up to 100 km or 62 miles) Ch 8 -

Multimode Optical Fiber Larger core than single mode cable (50 microns or greater) Uses LED emitters that do not create a single coherent light wave LED enters the multimode fiber at different angles Allows greater dispersion and therefore loss of signal long fiber runs may result in pulses becoming blurred on reception Used for long distance application, but shorter than single-mode up to about 2km Ch 8 -

Wireless Media Carry electromagnetic signals at radio and microwave frequencies that represent binary digits Uses air or free space as the medium of data transport Construction materials used in buildings and structures, and the local terrain can limit the effective coverage Wireless is susceptible to interference Network security is a major concern of wireless transmission Ch 8 -

Types of Wireless Networks IEEE 802.11 a wireless LAN technology commonly known as Wi-Fi uses a contention system with CSMA/CA media access process IEEE 802.15 wireless personal area network(PAN) standard commonly known as Bluetooth uses a device pairing process to communicate over distances of up to 100 meters Ch 8 -

Types of Wireless Networks (cont’d) IEEE 802.16 known as WiMAX (Worldwide Interoperability for Microwave Access) uses a point-to-multipoint topology to provide wireless broadband access Global System for Mobile Communications ( GSM ) includes Physical layer specifications for the implementation of the Layer 2 General Packet Radio Service ( GPRS ) protocol to provide data transfer over mobile cellular telephony networks Ch 8 -

Wireless LAN Devices Wireless access point ( AP ) concentrates the wireless signals from users and connects, usually through a copper cable, to the existing copper-based network infrastructure Wireless NIC adaptors provides wireless communication capability to each network host Ch 8 -

Wireless LAN Standards IEEE 802.11a operates in the 5 GHz frequency band and offers speeds of up to 54 Mbps smaller coverage area and is less effective at penetrating building structures not interoperable with 802.11 b and 802.11 g standards IEEE 802.11b operates in the 2.4 GHz frequency band and offers speeds of up to 11 Mbps longer range and are better able to penetrate building structures than devices based on 802.11a standard Ch 8 -

Wireless LAN Standards (cont’d) IEEE 802.11g operates in the 2.4 GHz frequency band and offers speeds of up to 54 Mbps operate at the same radio frequency and range as 802.11b but with the bandwidth of 802.11a IEEE802.11n currently in draft form proposed standard defines frequency of 2.4 GHz or 5 GHz expected data rates are 100 Mbps to 210 Mbps with a distance range of up to 70 meters Ch 8 -

Copper Media Connectors Ch 8 -

Correct Connector Termination It is essential that all copper media terminations be of high quality to ensure optimum performance with current and future network technologies there is the possibility of signal loss and the introduction of noise to the communication circuit each time copper cabling is terminated each cable is a source of Physical layer performance degradation when improperly terminated Ch 8 -

Fiber Media Connectors Straight-Tip (ST) bayonet style connector widely used with multimode fiber Subscriber Connector (SC) a connector that uses a push-pull mechanism to ensure positive insertion widely used with single-mode fiber Ch 8 -

Fiber Media Connectors Lucent Connector (LC) a small connector that can be used with single-mode and multimode fibers Ch 8 -

Type of Cables Ch 8 - OTDR Optical time-domain reflector : Check Fiber Optic cable

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