1. Network Fundamentals Chapter 8
OSI Physical Layer
Network Fundamentals
Chapter 8

2. 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.

3. 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

4. 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.

5. Purpose of the Physical Layer

6. Purpose of the Physical Layer

7. 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

8. Physical Layer Standards

9. Physical Layer Standards
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.

10. Physical Layer Standards

11. Physical Layer Standards

12. Physical Layer Standards

13. 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.

14. Physical Layer Fundamental Principles
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.

15. Physical Layer Fundamental Principles
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.

16. Physical Layer Fundamental Principles

17. 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).

18. Signaling Bits for the Media

19. 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.

20. Signaling Method – Nonreturn to Zero (NRZ)

21. 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.

22. Signaling Method – Manchester Encoding

23. 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

24. 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

25. 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.

26. Encoding – Signal Patterns

27. 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
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.

28. Data Code / Control Information
Encoding – Code Groups
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

29. Encoding – Code Groups 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.

30. Encoding – Code Groups 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).

31. Encoding – Code Groups 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.

32. 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.

33. Data Carrying Capacity

34. Data Carrying Capacity
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).

35. Data Carrying Capacity

36. Data Carrying Capacity
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.

37. 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

38. 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.

39. 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

40. Copper Media

41. 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)

42. Copper Media – UTP Cable
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.

43. Copper Media – UTP Cable

44. 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.

45. Copper Media – UTP Cable
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

46. Copper Media – UTP Cable

47. 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).

48. Copper Media – Coaxial Cable

49. 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.

50. Copper Media – STP

51. 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.

52. Fiber Media

53. 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

54. Fiber Media

55. 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.

56. 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.

57. Wireless Media
Four common data communication standards that apply to wireless media:
IEEE – A wireless LAN standard commonly known as Wi-Fi.
IEEE – A wireless personal area network (WPAN) standard commonly known as Bluetooth.
IEEE – 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.

58. Media Connectors UTP cable Coaxial cable STP cable RJ-45 connector
BNC connector
N type connector
F type connector
STP cable
D type connector

59. 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