CA Ex S1M08 OSI Physical Layer

8/3/2019 CA Ex S1M08 OSI Physical Layer

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Please purchase a personal

license.

Network Fundamentals Chapter 8


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Objectives

Explain the role of Physical layer protocols andservices in supporting communication across data

networks.

Describe the role of signals used to represent bitsas a frame as the frame is transported across the

Hc vin mng Bach Khoa - Website: www.bkacad.com

oca me a Describe the purpose of Physical layer signaling and

encoding as they are used in networks

Identify the basic characteristics of copper, fiber andwireless network media

Describe common uses of copper, fiber and wirelessnetwork media


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Communication signals



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Physical Layer - Purpose


The Physical layer consists of hardware, developed by engineers, inthe form of electronic circuitry, media, and connectors.

The OSI Physical layer provides the means to transport across thenetwork media the bits that make up a Data Link layer frame.

This layer accepts a complete frame from the Data Link layer andencodes it as a series of signals that are transmitted onto the localmedia.


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1. The physical media and associated connectors

2. A representation of bits on the media

3. Encoding of data and control information

4. Transmitter and receiver circuitry on the network devices

At this stage of the communication process, the user data has beensegmented by the Transport layer, placed into packets by the Networklayer, and further encapsulated as frames by the Data Link layer.


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The purpose of the Physical layer is to create the electrical, optical, ormicrowave signal that represents the bits in each frame. These signals arethen sent on the media one at a time.

It is also the job of the Physical layer to retrieve individual signals from themedia, restore them to their bit representations, and pass the bits up to the

Data Link layer as a complete frame.


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Physical Layer - Operation

There are 3 basic forms

of network media onwhich data isrepresented:

-Copper cable


The media does not carry the frame as a single entity.The media carries signals, one at a time, to represent

the bits that make up the frame.

-

-Wireless


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Physical Layer - Operation


Identifying a Frame Encodes the bits into the signals for a particular medium Distinguish where one frame ends and the next frame begins.

Indicating the beginning of frame is often a function of the Data Link

layer. However, In many technologies, the Physical layer may add its ownsignals to indicate the beginning and end of the frame.

To enable a receiving device to clearly recognize a frame boundary, thetransmitting device adds signals to designate the start and end of a frame.These signals represent particular bit patterns that are only used to denote

the start or end of a frame.


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Physical Layer - Standards


The services and protocols in the TCP/IP suite aredefined by the Internet Engineering Task Force (IETF) in

RFCs.


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The protocols and operations of the upper OSI layers areperformed by software and are designed by software engineers

and computer scientists. The services and protocols in the

TCP/IP suite are defined by the Internet Engineering Task Force(IETF) in RFCs.

The Physical layer technologies are defined by organizations


The International Organization for Standardization (ISO)

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 Federal

Communication Commission (FCC) in the USA.


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Four areas of the Physical layer standards:1. Physical and electrical properties of the media2. Mechanical properties (materials, dimensions, pinouts) of the

connectors

3. Bit representation by the signals (encoding)

4. Definition of control information signals

Hardware components such as network adapters (NICs), interfacesand connectors, cable materials, and cable designs are all specified instandards associated with the Physical layer.


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Physical Layer Fundamental Principles


Three fundamental functions of the Physical layer:1. The physical components

2. Data encoding

3. Signaling


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Encoding - Signaling

Encoding A method of converting a stream of data bits into a predefined "code.

Code: grouping of bits used to provide a predictable pattern, canbe recognized by both the sender and the received.

Predictable patterns: distinguish data bits from control bits;provide better media error detection.


nco ng me o s prov e co es or con ro purposes suc asidentifying the beginning and end of a frame.

Signaling The method of representing the bits is called the signaling method.

The Physical layer standards must define what type of signalrepresents a "1" and a "0 on the media. This can be as simple as achange in the level of an electrical signal or optical pulse or a morecomplex signaling method.


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Physical Signaling and Encoding:Representing Bits



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Signaling Bits for the Media

The transmission of the frame across the media occurs as a stream of bits sentone at a time.

Bit Time ?


The Physical layer represents each of the bits in the frame as a signal. Each signal placed onto the media has a specific amount of time to occupy

the media. This is referred to as its bit time.

At the Physical layer of the receiving node, the signals are converted back intobits.

The bits are then examined for the start of frame and end of frame bitpatterns to determine that a complete frame has been received.

The Physical layer then delivers all the bits of a frame to the Data Linklayer.

Successful delivery of the bits requires some method of synchronization

between transmitter and receiver.


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Bits are represented on the medium by changing oneor more of the following characteristics of a signal:

Amplitude, Frequency, Phase


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Non Return to Zero (NRZ): the bitstream is transmitted as a series ofvoltage values Logical 0: low voltage

Logical 1: high voltage Suite for slow speed data links Inefficient bandwidth, susceptible

to electromagnetic interference.


The boundaries between individualbits can be lost when long stringsof 1s or 0s are transmittedconsecutively. In that case, no voltage

transitions are detectable on

the media. Therefore, thereceiving nodes do not have atransition to use inresynchronizing bit times withthe transmitting node.


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Manchester Encoding: bitvalues are represented asvoltage transitions.

1: low voltage to highvoltage

0: high voltage to lowvoltage


One voltage transition mustoccur in the middle of each bittime.

Manchester Encoding is

employed by 10BaseTEthernet (Ethernet running at10 Megabits per second).


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Extra: Manchester Encoding

All 10 Mbps forms ofEthernet take octets

received from the MAC

sublayer and perform aprocess called line

encoding (describes


signaled on the wire). The direction of the

edge transition in the

middle of the timing

window to determinethe binary value for that

bit period.


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Extra: NRZ,NRZI vs Manchester



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Extra: Bit Rates and Baud Rates



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Encoding Grouping Bits



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One way to provide frame detection is to begin each frame with a pattern ofsignals representing bits that the Physical layer recognizes as denoting thestart of a frame. Another pattern of bits will signal the end of the frame. Signalsbits not framed in this manner are ignored.

Valid data bits need to be grouped into a frame; otherwise, data bits will bereceived without any context to give them meaning to the upper layers of the

networking model. This framing method can be provided by the Data Linklayer, the Physical layer, or by both.

Signal patterns can indicate: start of frame, end of frame, and frame contents.These signal patterns can be decoded into bits. The bits are interpreted ascodes.

The codes indicate where the frames start and stop.


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Code Groups

Code group is a consecutive sequence of code bits that areinterpreted and mapped as data bit patterns.

For example, code bits 10101 could represent the data bits 0011. Code groups are often used as an intermediary encoding technique

for higher speed LAN technologies.


,

synchronization between transmitting and receiving devices areenhanced.

Advantages using code groups include:

1. Reducing bit level error

2. Limiting the effective energy transmitted into the media3. Helping to distinguish data bits from control bits

4. Better media error detection


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Reducing Bit Level Errors

To detect a bit as a 0 or as a 1, the receiver must know how and when tosample the signal on the media. This requires that the timing between thereceiver and transmitter be synchronized.

If too many 1s or 0s being transmitted on the media, the synchronizationmay be lost and individual bit error can occur.

DC balancing ?


Code groups are designed so that the symbols force an ample number of

bit transitions to occur on the media to synchronize this timing. Limiting Energy Transmitted

In many code groups, the symbols ensure that the number of 1s and 0s ina string of symbols are evenly balanced, called DC balancing. Thisprevents excessive amounts of energy from being injected into the mediaduring transmission, thereby reducing the interference radiated from themedia.

In many media signaling methods, a logic level, for example a 1, isrepresented by the presence of energy being sent into the media while theopposite logic level, a 0, is represented as the absence of this energy.Transmitting a long series of 1s could overheat the transmitting laser andthe photo diodes in the receiver, potentially causing higher error rates.


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Distinguish Data from Control

The code groups have 3 types of symbols: Data symbols - Symbols that represent the data of the frame as it is

passed down to the Physical layer.

Control symbols - Special codes injected by the Physical layer used tocontrol transmission. These include end-of-frame and idle media symbols.

Invalid symbols - Symbols that have patterns not allowed on the media.The receipt of an invalid symbol indicates a frame error.


The symbols encoded onto the media are all unique. The symbols representingthe data being sent through the network have different bit patterns than thesymbols used for control. These differences allow the Physical layer in thereceiving node to immediately distinguish data from control information.

Better Media Error Detection

In addition to the data symbols and control symbols, code groups contain

invalid symbols. These are the symbols that could create long series of 1sor 0s on the media; therefore, they are not used by the transmitting node. Ifa receiving node receives one of these patterns, the Physical layer candetermine that there has been an error in data reception.


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In this technique, 4 bits of data are turned into 5-bit code symbols for transmission overthe media system. In 4B/5B, each byte to be transmitted is broken into four-bit pieces or nibblesand

encoded as five-bit values known as symbols. These symbols represent the data tobe transmitted as well as a set of codes that help control transmission on the media.

Among the codes are symbols that indicate the beginning and end of the frametransmission.

Although this process adds overhead to the bit transmissions, it also adds featuresthat aid in the transmission of data at higher speeds.

4B/5B ensures that there is at least one level change per code to providesynchronization.

Most of the codes used in 4B/5B balance the number of 1s and 0s used in each symbol.


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Extra: FastEthernet 100Base-TX

Two separate encodingsteps are used by 100-Mbps Ethernet.

The first part of theencoding uses atechnique called4B/5B


The second part ofthe encoding is theactual line encodingspecific to copper(multi-level transmit-3

levels or MLT-3) orfiber.


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Extra: 4B/5B encoding simulation

http://www.frontiernet.net/~prof_tcarr/4B-5B/



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Data Carrying Capacity

Data transfer can be measured in 3 wa s:


1. Bandwidth2. Throughput

3. GoodputBandwidth The capacity of a medium to carry data is described as the raw data

bandwidth of the media.

Digital bandwidth measures the amount of information that can flow from oneplace to another in a given amount of time. Measured in kbps or Mbps.

Determined by a combination of factors: physical media and technologies Physical media properties, current technologies, and the laws of physics all

play a role in determining available bandwidth.


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Throughput

The measure of the transfer of bits across the media over a given period oftime. Usually does not match the specified bandwidth.

Factors influence throughput: amount of traffic,

type of traffic,

number of network devices encountered on the network.

Throu h ut cannot be faster than the slowest link of the ath from source to


destination.

Goodput

Goodput is the measure of usable data transferred over a given period of time,and is therefore the measure that is of most interest to network users.

Goodput measures the effective transfer of user data between Application

layer entities. Unlike throughput, which measures the transfer of bits and not the transfer ofusable data, goodput accounts for bits devoted to protocol overhead.

Goodput is throughput minus traffic overhead for establishing sessions,acknowledgements, and encapsulation.


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Physical Media

Connectin Communication



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Extra: Cable Specifications



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Type of Physical Media


The Physical layer is concerned with network media andsignaling. This layer produces the representation andgroupings of bits as voltages, radio frequencies, or lightpulses.


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Type of Physical Media



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Extra:



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Extra:


C M di


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Copper Media


Copper: The most common media.

Cables: connect nodes on a LAN to intermediate devices, such asrouters and switches, also connect WAN devices to a data servicesprovider such as a telephone company. Each type of connection andthe accompanying devices have cabling requirements stipulated by

Physical layer standards.

C M di


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Copper Media


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

U hi ld d T i t d P i (UTP) C bl


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Unshielded Twisted Pair (UTP) Cable


UTP: four pairs color-coded wires

Twisting has the effect of canceling unwanted signals. Avoid interference from internal sources called

crosstalk.

Extra Cross Talk


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Extra: Cross Talk




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Extra: UTP Straight through Cable


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Extra: UTP Straight-through Cable

Host or RouterHub or Switch


The cable that connects from the switch port to thecomputer NIC port is called a straight-through cable.

Extra: UTP Cross over Cable


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Extra: UTP Cross-over Cable


The cable that connects from one switch port to anotherswitch port is called a crossover cable.

Extra: UTP Rollover Cable


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The cable that connects the RJ-45 adapter on the com portof the computer to the console port of the router or switchis called a rollover cable.



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Console port

Router


Rollover cable

Com1 or Com2 serial port

PC withterminalemulationsoftware

Coaxial Cable


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Coaxial Cable


Be adapted for different purposes: to attach antennas to wirelessdevices; to carry radio frequency (RF) energy between theantennas and the radio equipment; to transport high RF signals,especially cable television signals.

Shielded Twisted Pair (STP) Cable


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Shielded Twisted Pair (STP) Cable


STP cable shields the entire bundle of wires within the cable as well asthe individual wire pairs. STP provides better noise protection than UTPcabling, however at a significantly higher price.

For many years, STP was the cabling structure specified for use inToken Ring network installations. With the use of Token Ring declining,the demand for shielded twisted-pair cabling has also waned. The new10 GB standard for Ethernet has a provision for the use of STP cabling.This may provide a renewed interest in shielded twisted-pair cabling.

Copper Media Safety


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Copper Media Safety


Fiber Media


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Fiber Media

Fiber-optic cable: uses glass or plastic fibers. The bits are encoded onthe fiber as light impulses. Very large raw data bandwidth rates.

Compared to Copper

Is immune to electromagnetic interference Not grounding issues.

Is thin, low signal loss, so can be operated at much greater lengths


, ,

reach multiple kilometers. More expensive (usually) than copper media over the same

distance (but for a higher capacity)

Different skills and equipment required to terminate and splice the

cable infrastructure More careful handling than copper media

At present, it is primarily used as backbone cabling for high-trafficpoint-to-point connections.

Fiber Media


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Fiber Media


Fiber Media


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Fiber Media


The cladding surrounds the actual glass or plastic fiber andis designed to prevent light loss from the fiber.

Two fibers are required to support full duplex operation.Fiber-optic patch cables bundle together two optical fibercables and terminate them with a pair of standard singlefiber connectors. Some fiber connectors accept both the

transmitting and receiving fibers in a single connector.

Fiber Media


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Dispersion is the technical term for the spreading of pulses of light as they travel down the fiber

Extra: Total internal reflection


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Extra: Total internal reflection


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Extra: Transmitting/Receiving Devices


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g g


Extra: Signals and noise in optical fibers


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Extra: Signals and noise in optical fibers


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Wireless Media


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Wireless Media


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Carry electromagnetic signals at radio and microwavefrequencies that represent the binary digits of datacommunications.

Work well in open environments. However, certainconstruction materials, and the local terrain, will limit theeffective coverage.


common devices as household cordless phones, sometypes of fluorescent lights, microwave ovens, and otherwireless communications.

Further, because no access to a physical strand of media,

devices and users who are not authorized for access to thenetwork can gain access to the transmission, therefore,network security is a major component.

Wireless Media


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Wireless Media


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Four common data communications standards

1. Standard IEEE 802.11 - Commonly referred to as Wi-Fi, is a WirelessLAN (WLAN) technology that uses a contention or non-deterministicsystem with a Carrier Sense Multiple Access/Collision Avoidance(CSMA/CA) media access process.

2. Standard IEEE 802.15 - Wireless Personal Area Network (WPAN)standard, commonly known as "Bluetooth", uses a device pairing


process to communicate over distances from 1 to 100 meters.

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

4. Global System for Mobile Communications (GSM) - Includes Physicallayer specifications that enable the implementation of the Layer 2General Packet Radio Service (GPRS) protocol to provide datatransfer over mobile cellular telephony networks.

Wireless Media


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In general, a wireless LAN requires the following networkdevices:

Wireless Access Point (AP) - Concentrates thewireless signals from users and connects, usually

through a copper cable, to the existing copper-basednetwork infrastructure such as Ethernet.

Wireless NIC adapters - Provides wirelesscommunication capability to each network host.

Wireless Media


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IEEE 802.11a - Operates in the 5 GHz frequency band, speed up to 54Mbps, small coverage area; less effective at penetrating buildingstructures. Not interoperable with the 802.11b and 802.11g standards.

IEEE 802.11b ( Wi-Fi )- Operates in the 2.4 GHz frequency band,

speed up to 11 Mbps. Longer range and better able to penetratebuilding structures than devices based on 802.11a.

IEEE 802.11g - Operates in the 2.4 GHz frequency band, speed up to


same radio frequency and range as 802.11b but with the bandwidth of

802.11a. IEEE 802.11n - Is currently in draft form. The proposed standard

defines frequency of 2.4 Ghz or 5 GHz. The typical expected data ratesare 100 Mbps to 210 Mbps with a distance range of up to 70 meters.

The benefits are evident, especially the savings on costly premiseswiring and the convenience of host mobility. However, network administrators need to develop and apply stringent

security policies and processes to protect WLANs from unauthorizedaccess and damage.

Media Connectors


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Media Connectors


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It is essential that all copper media terminations be of high

quality to ensure optimum performance with current andfuture network technologies.

Improper cable termination can impact transmissionperformance

Media Connectors


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Media Connectors


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Straight-Tip (ST) (trademarked by AT &T) - a very common bayonetstyle connector widely used with multimode fiber.

Subscriber Connector (SC) - a connector that uses a push-pullmechanism to ensure positive insertion. This connector type is widelyused with single-mode fiber.

Lucent Connector (LC) - A small connector becoming popular for usewith single-mode fiber and also supports multi-mode fiber.


Three common types of fiber-optic termination and splicing errors are: Misalignment - the fiber-optic media are not precisely aligned to

one another when joined.

End gap - the media do not completely touch at the splice orconnection.

End finish - the media ends are not well polished or dirt is presentat the termination.

Summary


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CA Ex S1M08 OSI Physical Layer

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