• A number of important differences between a wired link and a wireless link:
    • Decreasing signal strength:
      • Electromagnetic radiation attenuates as it passes through matter. Even in free space, the signal will disperse, resulting in decreased signal strength as the distance between sender and receiver increases.
    • Interference from other sources:
      • Radio sources transmission in the same frequency band will interfere with each other.
      • In addition to interference from transmitting sources, electromagnetic noise within the environment can result in interference.
    • Multipath propagation:
      • It occurs when portions of the electromagnetic wave reflect off objects and the ground, taking paths of different lengths between a sender and receiver. Moving objects between the sender and receiver can cause multipath propagation to change over time.
  • Wireless links employ powerful CRC error detection codes and link-level reliable-data-transfer protocols that retransmits corrupted frames because bit errors are more common in wireless links.
  • The host receives an electromagnetic signal that is a combination of a degraded form of the original signal transmitted by the sender and background noise in the environment.
    • The Signal-to-noise ratio (SNR) is a relative measure of the strength of the received signal and this noise.
    • The SNR is typically measured in dB.
      • It is 20*the ratio of the base-10 logarithms of the amplitude of the receives signal to the amplitude of the noise.
      • A larger SNR makes it easier for the receiver to extract the transmitted signal from the background noise.
  • BER = Bit error rate
  • Physical-layer characteristics that are important to understand for higher-layer wireless communication protocols:
    • For a given modulation scheme, the higher the SNR, the lower the BER:
      • Since a sender can increase the SNR by increasing its transmission power, a sender can decrease the probability that a frame is received in error by increasing its transmission power.
        • There’s little gain in increasing the power beyond a certain threshold.
      • A disadvantage associated with increasing the transmission power is that it costs more energy for the sender and the sender’s transmissions are more likely to interfere with transmissions of another sender.
    • For a given SNR, a modulation technique with a higher bit transmission rate will have a higher BER:
        • With an SNR of 10 dB, BPSK modulation with a transmission rate of 1 Mbps has a BER of less than $$10^{-7}$$, while with QAM16 modulation with a transmission rate of 4 Mbps, the BER is $$10^{-1}$$ far too high to be practically useful.
        • With an SNR of 20 dB, QAM16 modulation has a transmission rate of 4 Mbps and a BER of $$10^{-7}$$, while BPSK modulation has a transmission rate of only 1 Mbps and a BER that is extremely low.
        • If one can tolerate a BER of $$10^{-7}$$, the higher transmission rate offered by QAM16 would make it the preferred modulation technique in this situation.
    • Dynamic selection of the physical-layer modulation technique can be used to adapt the modulation technique to channel conditions:
      • The SNR many change as a result of mobility or due to changes in the environment.
      • Adaptive modulation and coding are used in cellular data systems and in the 802.11 WiFi and 4G cellular data networks.
        • This allows the selection of a modulation technique that provides the highest transmission rate possible subject to a constraints on the BER, for a given channel characteristics.
  • Suppose that Station A is transmitting to Station B, and that Station C is transmitting to Station B.
    • Hidden Terminal problem:
      • Physical obstructions in the environment may prevent A and C from hearing each other’s transmission, even though A’s and C’s transmissions are indeed interfering at the destination B.
    • A second scenario that also results in undetectable collisions at the receiver:
      • Results from the fading of a signal’s strength as it propagates through the wireless medium. Thus A’s and C’s signals are strong enough to interfere with each other, but not for A and C to detect it.

results matching ""

    No results matching ""