lecture topic08

8/14/2019 Lecture Topic08

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Physical Layer Protocols & Services Describe the purpose of the Physical layer in the

network and identify the basic elements that enable this

layer to fulfill its function

1 2007 Cisco Systems, Inc. All rights reserved. Cisco Public


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Physical Layer Protocols & Services

The OSI Physical layer provides the means to transportacross the network media the bits that make up a DataLink layer frame.


the following Physical layer elements:

- The physical media and associated connectors

- A representation of bits on the media

- Encoding of data and control information

-Transmitter and receiver circuitry on the network

devices


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The purpose of the Physical layer is to create theelectrical, optical, or microwave signal that representsthe bits in each frame.


individual signals from the media, restore them to theirbit representations, and pass the bits up to the DataLink layer as a complete frame.


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Physical Layer Protocols & Services Describe the role of bits in representing a frame as it is

transported across the local media.


ncapsu

lation

Deca

psulation


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The media does not carry the frame as a single entity.The media carries signals, one at a time, to representthe bits that make up the frame.


data is represented:

- Copper cable

- Fiber

- Wireless


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The representation of the bits - that is, the type of signal- depends on the type of media.

- copper cable media - patterns of electrical pulses.


- er- patterns o g t.- For wireless media - radio transmissions.

(See next slide)


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Physical Layer Protocols & Services Describe the role of signaling in the physical media.



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The physical layer must distinguish where one frameends and the next frame begins - It is often a function ofthe Data Link layer.


add its own signals to indicate the beginning and end ofthe frame


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Physical Layer Protocols & Services Distinguish who establishes and maintains standards

for the Physical layers compared to those for the other

layers of the network



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The Physical layer technologies are defined by organizations such

as:The services and protocols in the TCP/IP suite are defined by theInternet Engineering Task Force (IETF) in RFCs.


The Institute of Electrical and Electronics Engineers (IEEE)

The American National Standards Institute (ANSI)

The International Telecommunication Union (ITU)

The Electronics Industry Alliance/Telecommunications Industry

Association (EIA/TIA)

National telecommunications authorities such as the FederalCommunication Commission (FCC) in the USA or Canadian Radio andTV Commission (CRTC).


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Physical Layer Technologies andHardware

The technologies defined by these organizations(previous slide) include four areas of the Physical layerstandards:


Mechanical properties (materials, dimensions, pinouts)of the connectors

Bit representation by the signals (encoding)

Definition of control information signals


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The 3 fundamental functions of the Physical layer are:

- The physical components - the electronic hardwaredevices, media and connectors other that transmit and


- Data encoding - a method of converting a stream ofdata bits into a predefined code. Codes are groupingsof bits used to provide a predictable pattern that can be

recognized by both the sender and the receiver.


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It may also provide codes for control purposes such asidentifying the beginning and end of a frame.

- Signaling - The Physical layer must generate the


"1" and "0" on the media. .

(See next slide for diagram)


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Physical Layer Protocols & Services Identify hardware components associated with the

Physical layer that are governed by standards



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Physical Layer Signaling and Encoding

The Physical layer represents each of the bits in theframe as a signal.

Each signal placed onto the media has a specific


as its bit time.

Successful delivery of the bits requires some method ofsynchronization between transmitter and receiver.

The signals representing the bits must be examined atspecific times during the bit time to properly determineif the signal represents a "1" or a "0".


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Bits are represented on themedium by changing one or


characteristics of a signal:Amplitude, Frequency, Phase

(See next slide graphic)


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Physical Layer Signaling and Encoding Explain that network communication at this layer

consists of individual bits encoded onto the Physical

layer and describe the basic encoding techniques.



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Signaling Methods

Non-Return to Zero (NRZ), a 0 may be represented byone voltage level on the media during the bit time and a

1 might be represented by a different voltage on themedia during the bit time

This sim le method of si nalin is onl suited for slow


speed data links.

NRZ signaling uses bandwidth inefficiently and issusceptible to electromagnetic interference.

The boundaries between individual bits can be lostwhen long strings of 1s or 0s are transmittedconsecutively.


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Signaling Methods

Manchester Encoding indicates a 0 by a high to low

voltage transition in the middle of the bit time. For a 1there is a low to high voltage transition in the middle ofthe bit time


Although Manchester Encoding is not efficient enoughto be used at higher signaling speeds, it is the signalingmethod employed by 10BaseT Ethernet (Ethernetrunning at 10 Megabits per second).


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The signaling method used must be compatible witha standard so that the receiver can detect the

signals and decode them.


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Encoding Grouping Bits

By using an encoding step before the signals are

placed on the media, we improve the efficiency athigher speed data transmission.


grouping of bits prior to being presented to the media

Using media with higher speed, the possibility that datawill corrupt is higher. By using the coding groups, we

can detect errors more efficiently.


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as the demand for data speeds increase, we seek ways

to represent more data across the media, bytransmitting fewer bits. Coding groups provide amethod of making this data representation.


The Physical layer of a network device needs to beable to detect legitimate data signals and ignorerandom non-data signals that may also be on thephysical medium.


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One way to provide frame detection is to begin each

frame with a pattern of signals representing bits that thePhysical layer recognizes as denoting the start of aframe.


Another pattern of bits will signal the end of the frame.

Signal bits not framed in this manner are ignored by thePhysical layer standard being used.

(See next slide for graphic.)


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Physical Layer Signaling and Encoding Describe the role of encoding as it applies to the

transmission of bits and explain the value of treating a

collection of bits as a code.



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Encoding Group Bits


Encoding techniques use bit patterns called symbols.The Physical layer may use a set of encoded symbols -called code groups - to represent encoded data or control

information.A code group is a consecutive sequence of code bits thatare interpreted and mapped as data bit patterns. Forexample, code bits 10101 could represent the data bits

0011. Ex of Code group: 4B/5B


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Encoding Group Bits Example of code group: 4B/5B

Start & End


InvalidData


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Encoding Group Bits Advantages using code groups include:

a) Reducing bit level errormore later using example

b Limitin the effective ener transmitted into the


mediaThe symbols ensure that the number of 1s and 0s in astring of symbols are evenly balanced. This prevents

excessive amounts of energy from being injected intothe media during transmission, thereby reducing theinterference radiated from the media


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Encoding Group Bits

c) Helping to distinguish data bits from control bits.

The code groups have 3 unique types of symbols:

i) Data symbols - Symbols that represent the data of


t e rame as t s passe own to t e ys ca ayer.

ii) Control symbols - Special codes injected by thePhysical layer used to control transmission.

iii) Invalid symbols - Symbols that have patterns notallowed on the media. The receipt of an invalid symbolindicates a frame error.


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Encoding Group Bits

D) Better media error detection

Code groups can contain invalid symbols. The Physicallayer can determine that there has been an error in



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4B/5B

Curriculum gives a simple code group called 4B/5B.

NOTE: Code groups that are currently used in modernnetworks are generally more complex.

In this technique, 4 bits of data are turned into 5-bit code symbols


for transmission over the media system.

In 4B/5B, each byte to be transmitted is broken into four-bit piecesor nibbles and encoded as five-bit values known as symbols.These symbols represent the data to be transmitted as well as aset of codes that help control transmission on the media.

Among the codes are symbols that indicate the beginning and endof the frame transmission. Although this process adds overhead tothe bit transmissions, it also adds features that aid in the

transmission of data at higher speeds.


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4B/5B 4B/5B ensures that there is at least one level change

per code to provide synchronization.

Most of the codes used in 4B/5B balance the number of1s and 0s used in each symbol.


ync ron za on e ween e rece ver an ransm er s

required and the receiver must know how and when tosample the signal on the media in order to properlydetect an individual bit as a 0 or as a 1. Reduce bitlevel errors.

If the bit patterns being transmitted on the media do notcreate frequent transitions, this synchronization may belost and individual bit error can occur.


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Data Carrying Capacity

Different physical media support the transfer of bits at

different speeds. Data transfer can be measured inthree ways:


Throughput

Goodput


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Bandwidth

Digital bandwidth measures the amount of information that can flowfrom one place to another in a given amount of time.

The practical bandwidth of a network is determined by a combinationof factors: the properties of the physical media and the technologieschosen for signaling and detecting network signals.

Ph sical media ro erties, current technolo ies, and the laws of


physics all play a role in determining available bandwidth.


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Throughput

Throughput is the measure of the transfer of bits across

the media over a given period of time.

Throughput usually does not match the specified


Ethernet. Factors that influence throughput:

amount of traffic, the type of traffic, and the number of network

devices encountered on the network being measured. throughput cannot be faster than the slowest link of the

path from source to destination.


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Goodput

Goodput is the measure of usable data transferred over

a given period of time, and is therefore the measurethat is of most interest to network users.


and not the transfer of usable data, goodput accountsfor bits devoted to protocol overhead.

Goodput is throughput minus traffic overhead for

establishing sessions, acknowledgements, andencapsulation.


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Physical Layer Signaling and Encoding Define the terms bandwidth, throughput, and goodput



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Characteristics & Uses of Network Media

The Physical layer is concerned with network mediaand signaling.

This layer produces the representation and groupingsof bits as voltages, radio frequencies, or light pulses.


Various standards organizations have contributed tothe definition of the physical, electrical, and mechanicalproperties of the media available for different datacommunications.

These specifications guarantee that cables andconnectors will function as anticipated with differentData Link layer implementations.


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Characteristics & Uses of Network Media Standards for copper media are defined for the:

Type of copper cabling used

Bandwidth of the communication

Type of connectors used

Pinout and color codes of connections to the media

Maximum distance of the media



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Characteristics & Uses of Network Media The most commonly used media for data communications is

cabling that uses copper wires to signal data and control bits

between network devices. Cabling used for data communications usually consists of a

series of individual copper wires that form circuits dedicated


.

The copper media type chosen is specified by the Physicallayer standard required to link the Data Link layers of two ormore network devices.

Networking media generally make use of modular jacks andplugs, (Example: RJ 45: RJ = Registered Jack) whichprovide easy connection and disconnection. Also, a singletype of physical connector may be used for multiple types of

connections.


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Type of Media Copper Media Cables that use copper wires to signal data and control

bits between network devices

Ex: Twisted pair, Coaxial cable

Data is transmitted on copper cables as electrical


pu ses.

The timing and voltage values of these signals aresusceptible to interference or "noise" from outside thecommunications system.

Cable types with shielding or twisting of the pairs ofwires are designed to minimize signal degradation dueto electronic noise.


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Type of Media Copper Media

The susceptibility of copper cables to electronic noisecan also be limited by:

Selecting the cable type or category most suited toprotect the data signals in a given networking


env ronmen

Designing a cable infrastructure to avoid known andpotential sources of interference in the buildingstructure

Using cabling techniques that include the properhandling and termination of the cables


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Characteristics & Uses of Network Media Describe the impact interference has on throughput and

the role of proper cabling in reducing interference



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Type of Media Copper Media - UTP UTP Unshielded Twisted Pair

Very common, cheap, easy to install

It consists of 4 pairs of color-coded wires that havebeen twisted together


The twisting has the effect of canceling unwantedsignals.

This cancellation effect also helps avoid interferencefrom internal sources called crosstalk.

Crosstalk is the interference caused by the magneticfield around the adjacent pairs of wires in the cable


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Characteristics & Uses of Network Media Identify the basic characteristics of UTP cable


RJ-45 Connector

The following are main cable types that are obtained by usingspecific wiring conventions:- Ethernet Straight-through,- Ethernet Crossover &

-Rollover


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Type Use

Category 1 (1Mhz) Voice Only (Telephone Wire)

Category 2 (4Mhz) Data to 4 Mbps (LocalTalk)

Category 3 (16Mhz) Data to 10 Mbps (Ethernet)

Category 4 Data to 20 Mbps (16 Mbps Token Ring)


(20Mhz)

Category 5

(100Mhz)

Data to 100 Mbps (Fast Ethernet)

Category 5e

(100Mhz)

Data to 1000Mbps (Full Duplex Fast Ethernet

and Gigabit Ethernet)

Category 6

(250Mhz)

Data to 1000Mbps (more stringentspecifications for crosstalk and system noise)


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Copper Media Shielded TwistedPair (STP)

STP uses two pairs of wires that are wrapped in

an overall metallic braid or foil.

STP provides better noise protection than UTP


ca ng, owever at a s gn cant y g er pr ce


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Media Connector RJ-45 for UTP



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Characteristics & Uses of Network Media Identify the characteristics used to categorize

connectors, describe some common uses for the same

connectors, and identify the consequences formisapplying a connector in a given situation



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Copper Media - Coaxial Cable

Coaxial cable consists of a copper conductor surrounded

by a layer of flexible insulation.

Over this insulating material is a woven copper braid, ormetallic foil that acts as the second wire in the circuit and


as a shield for the inner conductor. The second layer reduces the amount of outside

electromagnetic interference.

Covering the shield is the cable jacket.


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Coax is an important type of cable that is usedin wireless and cable access technologies.

In the past, coaxial cable was used in Ethernet

installations.

Coax cables are used to attach antennas to


wireless devices. The coaxial cable carriesradio frequency (RF) energy between theantennas and the radio equipment.

Hybrid Fiber Coax (HFC) A network whichincorporates optic fiber along with coaxial cableto create broadband networks. Commonly used

by cable TV companies


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Copper Media Safety Electrical Hazards

A potential problem with copper media is that the copper wires couldconduct electricity in undesirable ways.

A defective network device could conduct currents to the chassis ofother network devices.

Additionally, network cabling could present undesirable voltage levels


w en use to connect ev ces t at ave power sources w t erent

ground potentials.Example:

When copper cabling is used to connect networks in differentbuildings or on different floors of buildings that use different powerfacilities.

Copper cabling may conduct voltages caused by lightning strikes tonetwork devices.


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Copper Media Safety Fire Hazards

Cable insulation and sheaths may be flammable or produce toxicfumes when heated or burned. Building authorities or organizations

may stipulate related safety standards for cabling and hardwareinstallations.



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Fiber Media

Fiber-optic cabling uses either glass or plastic fibers toguide light impulses from source to destination.

The bits are encoded on the fiber as light impulses .


bandwidth rates. Most current transmission standards have yet to

approach the potential bandwidth of this media.


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Media Connector for fiber optics



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Fiber Compared to Copper Cabling

Fiber media is immune to electromagnetic interference

Optical fibers are thin and have relatively low signal loss,they can be operated at much greater lengths than


copper me a,

More expensive (usually) than copper media over thesame distance (but for a higher capacity)

Different skills and equipment required to terminate andsplice the cable infrastructure

More careful handling than copper media


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Two fibers are required to support full duplex operation



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Characteristics & Uses of Network Media Describe the role of radio waves when using air as the

media and the increased need for security in wirelesscommunications


Wi l M di


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Wireless Media

Wireless media carry electromagnetic signals at radioand microwave frequencies that represent the binarydigits of data communications.

4 common data communications standards thatapply to wireless media are:


- IEEE 802.11, (Wi-Fi) Wireless LAN technologythat uses CSMA/CA media access process.

- IEEE 802.15, Wireless Personal Area Network

(WPAN) or Bluetooth that uses a device pairingprocess to communicate over distances from 1 to100 meters.


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IEEE 802.16 - Commonly known as WiMAX (WorldwideInteroperability for Microwave Access), uses a point-to-multipoint topology to provide wireless broadband access.

Types of Wireless Networks


Global System for Mobile Communications (GSM) -Includes Physical layer specifications that enable theimplementation of the Layer 2 General Packet RadioService (GPRS) protocol to provide data transfer over

mobile cellular telephony networks.


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Wireless Media Standards and Types

IEEE 802.11


IEEE 802.15

IEEE 802.15


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In general, a wireless LAN requires the following network

devices:

- Wireless Access Point (AP)

The Wireless LAN


Concentrates the wireless signals from users and connects, usually through a

copper cable, to the existing copper-based network infrastructure such asEthernet.

- Wireless NIC

Provides wireless communication capability to each network host.


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Wireless Media - Devices



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