Specifications and Limits

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• Troubleshooting a link:
  • compare the expected performance with the actual current performance
• to do this, assess and distinguish:
  • speed
  • throughput
  • and distance specifications

Speed vs Throughput

• Physical layer:
  • signal transmitted over a communication channel consists of a series of events
  • called symbols
    • E.g.,
      • pulse of higher voltage in electrical current
      • transition between the peak and the trough in electromagnetic wave
  • number of symbols transmitted per second is called the baud rate
    • measured in hertz (Mhz/Ghz)
• Data Link Layer
  • nominal bit rate (bandwidth) of the links is the amount of information that can be transmitted
    • measured in bits per second (bps)
  • to transmit information efficiently, a signaling method can represent more than one bit per symbol
    • helps overcome noise and detect errors
  • these encoding methods means the bit rate is higher than the baud rate
  • in Ethernet terms
    • bit rate is the expected performance of a link that has been installed to operate at 10 Mbps, 100 Mbps, 1 Gbps, or better
• nominal bit rate is not often achieved in practice
• throughput is an average data transfer rate achieved over a period of time
  • excluding encoding schemes, errors, and losses incurred at physical and data link layers
  • typically measured at the Network or Transport layer
  • goodput is the average data transfer rate at the Application layer
  • can be adversely affected by:
    • link distance
    • interference
  • takes account of packet loss
  • sometimes measured as packets per second
• speed is a the speed at which packets are delivered
  • measured as a unit of time, milliseconds (ms)
  • also called latency or delay
  • used to describe how well or badly a link is performing in terms of throughput

Distance Limitations, Attenuation, and Interference

• each type of media can consistently support a given bit rate only over a defined distance
  • vary by media
  • limited by attenuation and interference
• attenuation is the loss of signal strength
  • expressed in decibels (dB)
    • expresses the ratio between signal strength at origin and destination
• Interference (noise) is anything that gets transmitted within or close to the channel that isn’t the intended signal
  • makes the signal difficult to distinguish
  • causes errors in data and forcing retransmissions
  • expressed as signal to noise ratio (SNR)