Network Fundamentals: Ch8 - Physical Layer

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy
Network Fundamentals
Abdelkhalik Elsaid Mosa
abdu.elsaid@yahoo.com
http://abdelkhalik.staff.scuegypt.edu.eg/
Last Update: 20-07-2011

Physical layer - Purpose
• Provides the means to transport the bits that make up a Data Link layer frame
across the network media through creating signal that represents each frame.
• The delivery of frames across the local media requires the following:
1. The physical media and associated connectors.
2. Encoding of data and control information.
3. A representation of bits “type of signal” on the media.
4. Transmitter and receiver circuitry on the network devices.

Physical Layer - Standards
• The technologies defined by Standards organizations include four
areas of the Physical layer standards:
1. Physical and electrical properties of the media .
2. Mechanical properties (materials, pinouts) of the connectors .
3. Bit representation by the signals.
4. Definition of control information signals.

Physical Layer – Fundamental Principles
• The three fundamental functions of the Physical layer are:
1.Data encoding
 is a 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.
2.Signaling
 Is the method of representing the bits, what type of signal represents a
"1" and a "0".
 The Physical layer must generate the electrical, optical, or wireless
signals that represent the "1" and "0" on the media.
3.The physical components
 are the electronic hardware devices, media and connectors that transmit
and carry the signals to represent the bits.

Encoding – Grouping Bits
• Symbolic grouping of bits prior to being presented to the media.
• Encoding improves efficiency at higher speed data transmission.

Encoding – Grouping Bits
• A code group: is a consecutive sequence of code bits that are
interpreted and mapped as data bit patterns.
Ex: code bits 10101 could represent the data bits 0011.
• Advantages of using code groups:
 Reducing bit level error “By using symbols to ensure that not too many 1s
or 0s are used in a row”.
 Limiting the effective energy transmitted into the media.
 Helping to distinguish data bits from control bits, “Data, Control & Invalid”.
 Better media error detection, “Using Invalid symbols”.

4B/5B code symbols
• 4 bits of data are turned into 5-bit code symbols for transmission
over the media system.

Signaling bits on the media: NRZ Signaling
• In NRZ, the bit stream is transmitted as a series of voltage values.
• A low voltage value represents a logical 0 and a high voltage
value represents a logical 1.

Signaling bits on the media: Manchester Encoding
• Bit values are represented as voltage transitions.
• Low to High voltage represents 1 and High to Low represents 0.

Data Carrying Capacity
• Data Transfer can be measured in 3 ways:
1. Bandwidth (Theoretical): The capacity of a medium to carry
data in a given amount of time.
Usually measured in kbps or Mbps.
2. Throughput (Practical): is the measure of the transfer of bits
across the media over a given period of time.
Throughput <= Bandwidth.
 Number of devices affect the throughput.
3. Goodput (Qualitative): is the measure of usable data
transferred over a given period of time.
Application level throughput.
Goodput = Throughput - traffic overhead for establishing sessions,
acknowledgements, and encapsulation.

Data Carrying Capacity

Copper media

Copper media - External Signal Interference
• Radio waves and electromagnetic devices such as fluorescent lights, electric
motors, and other devices are potential sources of noise.
• Cable types with shielding or twisting of the pairs of wires are designed to
minimize signal degradation due to electronic noise.

Unshielded twisted-pair (UTP) cable
• Consists of four pairs of color-coded wires that have been
twisted together and then encased in a flexible plastic sheath.
• The twisting cancels unwanted external signals and also helps
avoid interference from crosstalk.
• Crosstalk is the interference caused by the magnetic field around
the adjacent pairs of wires in the cable.

UTP Cable Types

Coaxial Cable
• Coaxial cable consists of a copper conductor surrounded by a
layer of flexible insulation.
• Coax is used in wireless, used to attach antennas to wireless devices, and
cable access technologies.

Shielded Twisted-Pair (STP) Cable
• STP uses two pairs of wires that are wrapped in an overall
metallic braid or foil.
• STP provides better noise protection than UTP cabling, however
at a significantly higher price.
• STP was the cabling structure specified for use in Token Ring
network installations.

Fiber Media
• Fiber-optic cabling uses either glass or plastic fibers to guide light
impulses from source to destination.
• The bits are encoded as light impulses.
• Is immune to electromagnetic interference.
• Can be operated at much greater lengths than copper media.
• Primarily used as backbone cabling for high-traffic point-to-point
connections.

Fiber Media - Cable Construction
• Because 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 fiber connectors.
• Optical fiber media implementation issues include:
1.More expensive than copper but with higher capacity.
2.Different skills and equipment required to terminate and splice
the cable infrastructure.
3.More careful handling than copper media.

Generating and Detecting the Optical Signal
• Either lasers or LEDs generate the light pulses that are used to
represent the transmitted data as bits on the media.
• Photodiodes detect the light pulses and convert them to voltages
that can then be reconstructed into data frames.
• OTDR used to test each fiber-optic cable segment.

Single-mode Fiber Cable
• Single-mode optical fiber carries a single ray of light, usually
emitted from a laser.
• As the laser light is uni-directional and travels down the center of
the fiber, it can transmit optical pulses for very long distances.

Multi-mode Fiber Cable
• Multimode fiber typically uses LED emitters that do not create a
single coherent light wave.
• Modal dispersion, limits the length of multimode fiber segments.

Wireless media
• Carry electromagnetic signals at radio and microwave frequencies
• Is not restricted to pathways, as are copper and fiber.
• Susceptible to interference and can be disrupted by some types
of fluorescent lights, microwave ovens, and other wireless
communications.
• Security is a major component of wireless network admin.

Types of Wireless Networks
• IEEE 802.11 “Wi-Fi”: is a WLAN technology that uses CSMA/CA
media access process.
• IEEE 802.15 "Bluetooth": WPAN standard, uses a device pairing
process to communicate over distances from 1 to 100 m.
• IEEE 802.16 “WiMAX” Worldwide Interoperability for Microwave
Access: provides wireless broadband access.
• GSM: Includes Physical layer specifications that enable the
implementation of the Layer 2 GPRS.

The Wireless LAN
• WLAN requires the following network devices:
1. Wireless Access Point (AP): Concentrates the wireless signals
from users and connects to the existing copper-based network
infrastructure.
2. Wireless NIC: Provides wireless communication capability to
each network host.

WLAN Standards include
1.IEEE 802.11a: Operates in the 5 GHz frequency.
• Speed up to 54 Mbps.
• Smaller coverage area and less effective at penetrating buildings.
• Not interoperable with the 802.11b and 802.11g.
2. IEEE 802.11b: Operates in the 2.4 GHz frequency.
• Have a longer range and are better able to penetrate building structures
than devices based on 802.11a.
3. IEEE 802.11g: Operates in the 2.4 GHz frequency.
4. IEEE 802.11n: The newest, defines frequency of 2.4 GHz or 5 GHz.
• The typical expected data rates are 100 Mbps to 210 Mbps with a
distance range of up to 70 meters.

Media Connectors - Common Copper Media Connectors
• Standards specify the mechanical dimensions of the connectors
and the acceptable electrical properties of each type.
• The ISO 8877 specified RJ-45 being used for Ethernet.
• EIA-TIA 568, describes the wire color codes to pin assignments
(pinouts) for Ethernet straight-through and crossover cables.

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.

Common Optical Fiber Connectors

Appendix: Hubs
• L1, Network access device.
• A hub accepts electronic signals from one port and regenerates
(or repeats) the same message out all of the other ports.
• A shared-bandwidth device.

Appendix: Hubs
• Half duplex device

Appendix: Switch
• L2, Network access device.
• Full duplex

Network Fundamentals: Ch8 - Physical Layer

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