osi physical layer network fundamentals chapter 8

OSI Physical Layer

Network Fundamentals

Chapter 8

Objectives Explain the role of physical layer protocols and

services in supporting communication across data networks.

Describe the role of signals used to represent bits as a frame as the frame is transported across the local media.

Describe the purpose of physical layer signaling and encoding as they are used in networks.

Identify the basic characteristics of copper, fiber and wireless network media.

Describe common uses of copper, fiber and wireless network media.

Outline

Physical layer: Communication signals Purpose of the physical layer Physical layer standards Physical layer fundamental principles

Physical signaling and encoding: Representing bits Signaling bits for the media Encoding: Grouping bits Data-carrying capacity

Physical media: Connecting communication Types of physical media Media connectors

Purpose of the Physical Layer The roles of the OSI physical layer:

To encode the binary digits that represent data link layer frames into signals.

To transmit and receive these signals across the physical media.

There are various types of physical media and they carry different types of signals. Copper cable – electrical voltage. Fiber optic – light pulses. Wireless – electromagnetic waves.

Physical layer will encode the binary data in a frame to the proper type of signal depending on the physical media used.

Purpose of the Physical Layer

Physical Layer Standards

The physical layer technologies are defined by organizations such as: 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


The technologies defined by these organizations include four areas of the physical layer standards: Physical and electrical properties of the media. Mechanical properties (materials, dimensions,

pinouts) of the connectors. Bit representation by the signals (encoding). Definition of control information signals.

Physical Layer Fundamental Principles

Three fundamental functions of the physical layer: The physical components Data encoding Signaling

The physical components refer to the physical media and its connector. Responsible for making sure that signals can

travels reliably from one device to another over the physical media.


Encoding refers to the method of converting a stream of data bits into a predefined “code”. Codes are groupings of bits used to provide a

predictable pattern that can be recognized by both the sender and the receiver.

Encoding is also used for control information such as identifying the beginning and end of a frame. This is normally represented using specific patters of

0’s and 1’s.


Signaling refers to the process of converting the encoded bit streams into signals. The signals generated is dependant on the physical media. The method of representing the bits is called the signaling

method. The processes of encoding and signaling complete

the preparation of data for transmission over the physical media.

The physical layer sends these bits out one at a time onto the medium as a signal and those signals get picked up and decoded at the receiving end.

Signaling Bits for the Media

Bits are represented on the medium by changing one or more of the following signal characteristics: Amplitude Frequency Phase

To make sure that the receiver reads the signals at the right time, the timing for both senders and receivers needs to be synchronized. Done by the use of a clock signal. This ensures that they both have the same bit time (the

time that the signal for one bit stays on the media).

Signaling Bits for the Media

Signaling Method – Nonreturn to Zero (NRZ)

Bits are represented by voltage level: 0 – low voltage value 1 – high voltage value

The simplest signaling method but only suitable for slow speed data link. NRZ is used in communication over serial port.

Disadvantages: Uses bandwidth inefficiently. Susceptible to electromagnetic interference. No inherent clocking capability and therefore easy to lose

synchronization.

Signaling Method – Nonreturn to Zero (NRZ)

Signaling Method – Manchester Encoding

Bits are represented by voltage transition: 0 – change from high to low 1 – change from low to high

Better than NRZ and can be used in faster data links. Provides inherent clocking capability which makes it

possible to transmit signals at faster speed without losing synchronization.

Manchester encoding is used in 10 Mbps Ethernet LAN. Disadvantage: the signal needs to be read twice

during each bit time.

Signaling Method – Manchester Encoding

Encoding: Grouping Bits

In transmitting bits across the transmission media, the bits are normally not transmitted as it is. If we have data bits 0011, normally we do not just

send signals that represent the bits 0011 into the media.

Instead, the bits are first encoded to prepare it for transmission. With encoding, the data bits 0011 may now be

represented by the bits 10101.

Encoding: Grouping Bits

Although encoding may introduce more bits to represent the data, it does provide several advantages, for example: Specifies the start and end of data frame. Provides better error detection. Limiting effective energy transmitted into the media by

making sure that the number of +ve voltage produced is equal to the number of –ve voltage produced.

There are two methods of encoding: Signal patterns Code groups

Encoding – Signal Patterns

Signal patterns can be used to identify the start and end of a frame.

This is done by using a certain pattern of signals. When the receiver “sees” this pattern, it knows that a data

frame will follow afterwards. Enables the receiver to get ready to read the frame.

Any signals that are not followed by the start frame signal pattern will be ignored. This will help the receiver to know which signals to read

and which signals to ignore.

Encoding – Signal Patterns

Encoding – Code Groups

Code group refers to a consecutive sequence of code bits that are interpreted and mapped as data bit patterns. Example: data bits 0011 can be represented by the code

bits 10101. Code groups are normally used in higher speed

LAN technologies. Example: 4B/5B (used in 100 Mbps Ethernet LAN).

4 bits of data are turned into 5-bit code symbols. These 5-bit code symbols may represent data or control

information such as symbols that indicate beginning / end of transmission.


Data Code / Control Information Symbol

0000 11110

0001 01001

0010 10100

… …

1110 11100

1111 11101

Idle 11111

Start of stream 11000

End of stream 00111


Advantages of using code groups include: Reducing bit level error

The receiver read the bits by sampling the signal at certain time interval.

It is important for timing between the sender and receiver to be synchronized.

Timing can be synchronized by having the signal to change its level every so often.

Code groups can help to achieve this by making sure that there are not too many 0’s or 1’s used in a row.


Limiting the energy transmitted into the media It is important to balance the number of high and low signal

levels (this is called DC balancing). Otherwise, excessive energy may be injected into the

media and this may cause interference. Code groups can help to achieve this by balancing the

number of 0’s and 1’s. Helping to distinguish data bits from control bits

In addition to data bits, control bits must also be transmitted to facilitate data transfer.

Code groups specifies special bit sequences for control information (so that it cannot be confused with data codes).


Better media error detection Code groups defined symbols for data and control

information. They are also invalid symbols which are not used to

represent data or control information. These invalid symbols will never be generated by the

sender. If the receiver receives any of the invalid symbols, then it

knows there must be some error in data reception. This will enable the receiver to take an appropriate

corrective action.

Data Carrying Capacity

Different media support the transfer of bits at different speed.

Data transfer can be measured in three ways: Bandwidth

Refer to the capacity of a medium to carry data in a given amount of time.

Commonly measured in kilobits per second (kbps) or megabits per second (Mbps).

Depends on the physical properties of the medium and the signaling method applied.


Throughput Refer to the actual transfer rate over the medium in a

period of time. Influence by multiple factors such as the amount of

traffic, the type of traffic and the number of devices on the network.

Goodput Refer to the transfer rate of actual useable data bits. Goodput = throughput – (overhead for connection

establishment, acknowledgement and packet header).


Example: A 100BaseT Ethernet LAN has a bandwidth of

100 Mbps. However, due to the number of hosts connected

to the LAN and the amount of traffic generated by these hosts, the throughput may only be 60 Mbps.

Out of all the bits transmitted, 1/3 of them may just be control bits. Only the other 2/3 are data bits. Therefore, the goodput is only 40 Mbps.

Types of Physical Media

The physical layer defines the standards for the physical components of a network (copper, fiber cables) and the connectors used on them.

It also defines how bits are represented (signaling method and encoding to be used).

The standards vary depending on the type of physical media used and its applications.

In general, there are three types of media: Copper media Fiber media Wireless media

Copper Media

Copper media is the most widely used media in local networks.

Data travels as small pulses of electrical voltages. However, the voltage is quite low and easily

distorted by outside interference and signal attenuation. Interference (also known as noise): unwanted signals that

can distort or corrupt data signals. Attenuation: the loss of energy in the signal as it travels

longer distance.

Copper Media

There are various types of copper media: Unshielded twisted-pair (UTP) cable Coaxial cable Shielded twisted-pair (STP) cable

For each type, there are standards that specify the following characteristics: Bandwidth of the communication Type of connecters to be used Pinout and color codes of connection to the media Maximum distance of the media

Copper Media

Copper Media – UTP Cable

UTP is the cheapest and the most common type of copper media used.

Consists of eight wires twisted into four color-coded pairs. The colors are used to identify wires for proper connection

at the terminals. These four pair of wires are then bundled together into a

cable jacket. Applications of UTP cable:

Telephone network Local area network (LAN)


The twisting is done to reduce crosstalk interference. When electric current travels a wire, it produces magnetic

fields around it. This magnetic field can cause interference to the data. In a pair, each wire transmit signals in opposite direction. This causes the magnetic fields generated by the two wires

to cancel each other. The rate of twisting (the twist length) in each pair of wires is

different so that each pair self-cancels and reduces crosstalk to a minimum.

Copper Media – UTP Cable There are several categories of UTP cable:

Category 3 (Cat 3) Used in telephone network and 10 Mbps Ethernet LAN.

Category 5 (Cat 5) Used in 100 Mbps Ethernet LAN.

Category 5e (Cat 5e) An improved version of Cat 5 cable with ability to perform

full-duplex transmission. Used in 1 Gbps (1000 Mbps) Ethernet LAN.

Category 6 (Cat 6) Has stricter manufacturing and termination standards. Has higher performance and less crosstalk. Used in 1 Gbps (1000 Mbps) Ethernet LAN.


The most common UTP cable connector in LAN devices is an RJ-45 connector.

There are two standards that specify the cable pinout (the order of wires in the connector): TIA/EIA 568A TIA/EIA 568B

There are three types of UTP cable, each with different pinout configuration. Straight-through cable Crossover cable Rollover cable

Copper Media – Coaxial Cable

Consists of a single, coated copper wire center and an outer metal mesh. The outer metal mesh acts as both a grounding circuit and

an electromagnetic shield to reduce interference.

Applications of coaxial cable: Used in older Ethernet LAN standards such as 10Base2

and 10Base5. Used in wireless implementations to connect antenna to

wireless devices. Used to carry TV signals (cable TV).

Copper Media – Coaxial Cable

Copper Media – STP Consists of four pairs of wires that are wrapped in an

overall metallic braid or foil. The entire bundle of wires as well as the individual

wire are shielded within the cable. STP provides better noise protection than UTP cabling,

however at a significantly higher price. For many years, STP was the cabling structure

specified for use in Token Ring network installations. Token Ring is a LAN technology. It used to rival the Ethernet.

With the use of Token Ring declining, the demand for shielded twisted-pair cabling has also waned.

However, STP is still useful in installations where electromagnetic interference is an issue.

Copper Media – STP

Fiber Media

In fiber optic cable, data bits are encoded as light pulses generated using either laser or LED.

The cable consists of glass or plastic fibers that can guide light pulses. Uses a property of glass called total internal reflection

where the light rays get reflected back and forth along the medium.

Occurs when a ray of light strikes the boundary of a medium that has a higher index of refraction at an angle larger than the critical angle.

On the receiving end, a device called photodiode interprets the light signal and decode it to bits.

Fiber Media

Fiber Media

There are generally two types of fiber optic cable: Multimode

Larger core: 50+ microns, can be glass or plastic Greater dispersion (loss of light) Shortest distance: up to 2 km Uses LEDs as light source for short distances

Single mode Small glass core: 8 – 10 microns Less dispersion of light Longer distance: up to about 100 km Uses lasers as light source

Fiber Media

Fiber Media Advantages of fiber optic cable (as compared to

copper cables): Much greater capacity (bandwidth). Lower attenuation – can run for longer distance. Immunity to electromagnetic interference. Cable has smaller size and weight.

Disadvantages of fiber optic cable: More expensive. More easily damaged.

Fiber optic cable is normally used in backbone connections to connect between floors, buildings or remote sites.

Wireless Media

In wireless media, signal is carried using electromagnetic waves. Electromagnetic waves at different frequencies are called

with different names: radio wave, microwave, etc. The main advantage is that devices no longer need

to use physical cables. However, there are several disadvantages:

The speed is generally slower than cable connection. More susceptible to interference. More susceptible to security breach.

Wireless connections are best used in open areas.

Wireless Media

Four common data communication standards that apply to wireless media: IEEE 802.11 – A wireless LAN standard commonly known

as Wi-Fi. IEEE 802.15 – A wireless personal area network (WPAN)

standard commonly known as Bluetooth. IEEE 802.16 – A wireless WAN network commonly known

as WiMAX. GSM (Global System for Mobile Communication), together

with GRPS, WCDMA or HSDPA – Provide data transfer over mobile cellular network.

Media Connectors

UTP cable RJ-45 connector

Coaxial cable BNC connector N type connector F type connector

STP cable D type connector

Media Connectors

Fiber optic cable Straight Tip (ST) for multimode Subscriber Connector (SC) for single mode Lucent Connector (LC) for both multimode and

single mode MT-RJ Connector for both multimode and single

mode

osi physical layer network fundamentals chapter 8

Documents

purpose of physical

physical layerpurpose

outlinephysical layer

physical layerthe roles

data link layer frames

capacityphysical media

local media

role of signals