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school:1a_chapter_8_notes

Chapter 8 - The Physical Layer

Where the upper layers prepare human data for transfer, the physical layer actually controls data being sent over the media. It encodes the data link frame into binary digits. The purpose of the physical layer is to create the electrical, optical, or microwave signals that represents the bits on each frame, as well as retrieve these signals, restore them to their bit representations and pass the bits up to the media layer.

Delivering frames on the physical media requires:

  • 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

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

Three basic forms of network media:

  1. Copper
  2. Fiber
  3. Wireless

Standards

Engineering and electrical organizations set the standards for physical media since their institutions created them.

  • ISO - International Organization for Standardization
  • IEEE - The Institute of Electrical and Electronics Engineers
  • ANSOI - The American National Standards Institute
  • EIA/TIA - The Electronics Industry Alliance/Telecommunications Industry Association
  • National Telecommunications Authorities like the FCC,

The technology defined by these organizations include four areas of the physical layer standards:

  1. Physical and electrical properties of the media
  2. Mechanical properties (materials, dimensions, pinouts) of the connectors
  3. Bit representations by the signals (encoding)
  4. Definition of control information signals

Fundamental functions

  1. The physical components
    1. The physical elements are the electronic hardware devices, media and connectors that transmit and carry the signals to represent the bits
  2. Data encoding
    1. Encoding is a method of converting a stream of data bits into a predefined code.
    2. Codes are groupings of bits used to provide a predictable pattern that can be recognized by both the sender and the received.
  3. Signaling
    1. The method of representing bits (electric, optic, radiowaves) is called signaling method.
    2. The physical layer standard must define what type of signal represents a 1 and a 0.

Signaling Bits for the Media

  • Each frame is presented to the physical layer as a unit, but they are transmitted as a stream.
  • Each signal on the media has a specific time to occupy the media called the bit time.
  • Successful delivery of bits requires synchronization because, in order to determine if a signal is a 1 or 0, the signal must be examined at a certain time as designated by a clock.
    • LAN transmission ends have their own clock
    • Many other transmissions decide who will keep time

Signaling Methods for the Media

Bits are represented on the medium by changing on or more of the following:

  • Amplitude
  • Frequency
  • Phase

Types

  • NRZ (Non-return to Zero) - One voltage level represents a 1 and another represents a 0.
    • Only suited for slow-speed data links.
    • Not efficient and susceptible to electromagnetic interference.
  • Manchester Encoding - Indicates a zero by a high-to-low transition in the middle of the bit time. Indicates a one by the opposite.
    • Still not very efficient, but is the standard used at 10BaseT Ethernet.

Encoding

  • Improves efficiency
  • Error detection is more accurate

Signal Patterns

The stream of signals being transmitted must start in such a way that the receiver recognizes the beginning and end of the frame. One way to do this is with signal patterns.

  • It can use a specific pattern of bits to represent “beginning of frame” and another for “end of frame”

Code groups

  • A consecutive sequence of code bits that are interpreted and mapped as data bit patterns.
  • Best used for high-speed transfers.

Advantages

  • Reducing bit-level error
    • Forces an ample number of transitions to occur so the media can adjust its timing.
  • Limiting the effective energy transmitted into the media
    • Ensures the numbers of 1s and 0s are evenly balanced
    • Lots of ones (high energy) could overheat the media, for exampl
  • Helps to distinguish data bits from control bits
    • 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 layer 1 to control transmission, including “end-of-frame” and “idle media” symbols.
    • Invalid symbols - Symbols that have patterns not allowed on the media; indicates a frame error.
  • Better media error detection
    • The use of invalid symbols

Disadvantages

  • Provides more overhead as more bits are sent

4B/5B

  • 4 bits of data are turned into a 5-bit code. These 4 bits are called nibbles.
  • The 5 bits can code the 4 bits of data, as well as code control symbols and invalid symbols

Ethernet encoding

  • 10BaseT uses Manchester encoding
  • 100BaseT uses 4B/5B encoding
  • 1000BaseT uses 8B/10B encoding. Makes sense as you have twice as many pairs as before.

Data carrying capacity

Can be measured in three ways

  1. Bandwidth
    1. The capacity of a medium to carry data
    2. Measured in amount of information over time.
    3. The practical bandwidth is determined by the physical media and technologies the signal and detect signals.
    4. Bandwidth is usually on a smaller scale. Bandwidth of a connection, or of a cable.
  2. Throughput
    1. The measure of transfer bits across the media over a given period of time.
    2. Influenced by number of nodes on the network, processing power of equipment, bottlenecks in network, etc.
    3. Basically, measured by time it takes info to get from one physical endpoint to the other.
  3. Goodput
    1. Measure of usable data transfer over time.
    2. From application to application transfer
    3. Goodput = Throughput minus overhead

Types of physical media

Copper

  • Most common
  • Standards
    • Type of copper cabling
    • Bandwidth of communication
    • Type of connectors
    • Pinouts and color codes of media
    • Maximum distance
  • Ways to reduce interference
    • Twisting and shielding of the media
    • Selecting cable type and category best suited for an environment
    • Design infrastructure to to avoid potential sources of interference
    • Use proper cabling and termination techniques.

UTP

  • Unshielded twisted pair, twisting helps eliminate internal crosstalk
  • The TIA/EIA stipulates the commercial cabling standards for LAN installations. It defines:
    • Cable types
    • Cable lengths
    • Connectors
    • Cable termination
    • Methods of testing
    • Color coding
  • IEEE rates cable according to speed

Straight Through

ethernetcablediagram.jpg

Crossover Cable

crossovercablediagram2.jpg

Roll-over

rollover-cable-wiring.jpg

Coaxial Cable

  • Mostly well suited for one-way transmission of radio frequency (TV, Radio)
  • Cable companies use Hybrid Fiber-Coax (HFC) to turn cable infrastructure into data infrastructure, though terminating media at endpoint is still coax.

STP

  • Shielded twisted pair
  • The pairs are shielded, as is the bundle

Copper Media Safety

It can conduct electricity. Especially dangerous when connecting multiple buildings. The wire jacket can be flammable.

Fiber

  • Most current standards have failed to approach the potential bandwidth of this media.
  • Downsides:
    • More expensive
    • Different skills and equipment to terminate media
    • More careful handling required.
  • Most primarily used as backbone.
  • Usually uses two fibers, one for TX and one for RX
  • Lasers or light-emitting diodes (LEDS) create light source.
  • Single mode
    • Carries single ray of light, usually from laser.
    • Good for very long distances.
  • Multi-mode
    • Usually uses LEDs. Light enters at different angles.
      • This causes different wave lengths of light, which in turn cause modal dispersion.
    • Not as far as single mode.
    • Cheaper than single mode.

Wireless

  • Signals are radio waves.
  • Better for open environments.
  • Subject to more interference by household devices.
  • Types of wireless
    • 802.11 - Wifi
      • Uses a contention or non-deterministic system with a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)
    • 802.15 - Bluetooth or WPAN (Wireless Personal Area Network
      • Uses device pairing process
      • Can communicate from 1 to 100 meters
    • 802.16 - WiMax, Worldwide Interoperability for Microwave Access
      • uses point-to-point topology to provide wireless broadband
    • GSM - Global System for Mobile Communications
      • Allows implementation of Layer 2 General Packet Radio Service
      • Allows data over mobile phone networks
    • GPRS - Communication between earth systems and satellite

Wireless LAN

  • Requires wireless access point and wireless NIC
  • Security is a much bigger issue
  • Standards set by IEEE
    • 802.11a
      • 5GHz spectrum
      • poor building penetration, shorter distance
      • up to 54 Mbps
      • no interoperability with 802.11b/g
    • 802.11.b
      • 2.4 GHz
      • up to 11 Mbps
      • Longer range and better penetration compared to 802.11a
    • 802.11g
      • 2.4 GHz
      • up to 54 Mbps
      • same frequency and range as 802.11b but with more bandwidth
    • 802.11n
      • 2.4 GHz or 5 GHz
      • 100-210 Mbps
      • distance of up to 70 meters

Media Connectors

  • If wires are terminated incorrectly, damage to equipment can occur, especially when using POE.
  • Pitfalls of fiber termination:
    • Misalignment - fiber-optic media are not precisely aligned to one another when joined
    • End gap - the media do not completely touch at the splice or connection
    • End finish - the media ends are not well polished or dirt is present at the termination
  • You can shine a flashlight into one end of the cable to see if it can pass light as a test. Not 100% fool-proof, though.

school/1a_chapter_8_notes.txt · Last modified: 2011/04/02 11:26 by admin