CCNA-1. Chapter 8. OSI Physical Layer

Physical layer elements:

• The 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

Functions of the Physical layer:

• The physical components
• Data encoding (bits -> predefined code. Distinguish data bits from control bits, media error detection, beginning and end of a frame)
• Signaling (method of representing the bits)

Signaling Methods:

• Amplitude
  
  • Non-return to zero (NRZ) (low voltage - logical 0, high voltage - logical 1)
    
    • 
      simple
    • 
      inefficient bandwidth use
    • 
      susceptible to electromagnetic interference
    • 
      boundaries between individual bits can be lost when long strings of 1s or 0s are transmitted
    • 
      not have a transition to use in resynchronizing bit times
• Frequency
• Phase
  
  • Manchester Encoding ( low voltage ->high voltage = logical 1, high voltage -> low voltage = logical 0)
    
    • Used by 10BaseT Ethernet
    • 
      simple
    • 
      have a transition to use in resynchronizing bit times
    • 
      can be used only on slow speed links

Encoding methods:

• Signal Patterns (start of frame, end of frame, and frame contents. Patterns decoded into bits and are interpreted as codes)
• Code Groups (data bits divided to patterns and transmitted as set of code grops(symbols). 4B/5B (100BASE-T) 4D-PAM5 (1000BASE-T); 8B/10B (1000BASE-SX and 1000BASE-LX)
  
  • 
    Reduce bit level error (create more frequent transitions to solve timing synchronization problems)
  • 
    Limit the effective energy transmitted into the media  (long series of 1s changed to another code)
  • 
    Help to distinguish data bits from control bits. 3 Types of symbols used:
    
    • Data symbols - Symbols that represent the data of the frame.
    • Control symbols - Special codes injected by the Physical layer used to control transmission.
    • Invalid symbols - Patterns not allowed on the media. The receipt of an invalid symbol indicates a frame error.
  • 
    Better media error detection (Invalid symbols)
  • 
    Overhead in the form of extra bits to transmit

Data transfer can be measured in:

• Bandwidth (theoretical tecnological network performance)
• Throughput (actual network performance)
• Goodput (throughput minus traffic overhead for establishing sessions, acknowledgements, and encapsulation)

Interferience:

• cooper cables
  
  • fluorescent lights
  • motor venicles
• wireless
  
  • cordless phones
  • some types of fluorescent lights,
  • microwave ovens

Time definitions:

• Bit time (amount of time 1 bit occupies the media)
  
  • 10-Mbps Ethernet - 100 nanoseconds(nS) to transmit a bit
  • 100 Mbps Ethernet - 10 nS to transmit a bit
  • 1 Gbps Ethernet - 1 nS to transmit a bit
  • 10 Gbps Ethernet - 0,1 nS to transmit a bit
• Slot time (maximum theoretical time from first bit sent to time jam signal returned to sender)
-device must learn about the collision before it finishes sending the smallest Ethernet frame size (64byte)
-ensures that if a collision is going to occur, it will be detected within the first 512 bits (4096 for Gigabit Ethernet)
-sets maximum network diameter 2500 meters for CSMA\CD to handle errors and avoid late collisions.
Slot time values:
• 
  
  • 10-Mbps Ethernet - 512 bit time
  • 100 Mbps Ethernet - 512 bit time
  • 1 Gbps Ethernet - 4096 bit time
  • 10 Gbps Ethernet - not applicable
• Interframe spacing (from last bit of one frame to the first bit of next frame)
  
  • 10-Mbps Ethernet - 96 bit time (9.6 nS)
  • 100 Mbps Ethernet - 96 bit time (0,96 nS)
  • 1 Gbps Ethernet - 96 bit time (0,096 nS)
  • 10 Gbps Ethernet - 96 bit time (0,0096 nS)
• Latency (amount of time it takes for data to be sent from sender to reciever)
• Propagation delay (length of cable that signal propagates in 1 nanosecond)
  
  • UTP cable - 20.3 centimeters (8 inches) per nanosecond

Number of bit times to reach the destination = Cable length \ (Cable propagation delay * Bit time)



                                                                Physical media (Ethernet)











Coaxial cable Ethernet:

• 10BASE5 (Thicknet), used a thick coaxial
• 10BASE2 (Thinnet), used a thin coaxial cable that was smaller in diameter and more flexible.

Twisted pair features:

• The twisting cancel interference from external sources (makes common interference on both wires)
• The twisting cancel interference from internal sources - crosstalk (opposite direction causes magnetic field cancellation)
• Different pairs of wires use a different number of twists per meter (protect the cable from crosstalk between pairs)

Utp cable types:  

• Ethernet Straight-through (1+,2-(orange):transmit; 3+,6-(green):recieve)
• Ethernet Crossover
• Rollover 

Coaxial cable usage:

• Carries radio frequency (RF) energy between the antennas and the radio equipment.
• Transporting high radio frequency signals (cable television signals). In hybrid fiber coax (HFC) used only as last mile.
• 
  Ethernet installations - was totally replaced by UTP as a standart

                                                      

Fiber optic:

• Core (glass or optic fiber)
• Cladding (prevent light loss from the fiber)
• Buffer
• Aramid Yarn
• Jacket (polymer)

Fiber optic cables types:

• Single-mode
  
  • Small core (8-10 micron)
  • Less dispersion
  • Distance - up to 100 km (62,14 mi)
  • Uses lasers as the light source often within campus backbones for distance of several thousand meters
• Multimode (cheaper)
  
  • Larger core (50/62.5 microns)
  • Allows greater dispersion (modal dispersion) and therefore, loss of signal
  • Distance - up to ~2km (6560 ft)
  • Uses LEDs as the light source often within LANs or distances of a couple hundred meters within a campus network

Optical Time Domain Reflectometer (OTDR) - device injects a test pulse of light into the cable and measures back scatter and time reflection of light,l calculate the approximate distance at which faults are detected along the length of the cable.