Date Archives September 27, 2017

FDMA, TDMA, and CDMA

fdma

FDMA (Frequency Division Multiple Access)

  • Vary frequencies for each user.
  • If number of users increases, it becomes complicated.

TDMA (Time Division Multiple Access)

  • Using same frequency, divide into many time slots.
  • Allocating different time slot for each user.
  • They can speak only when their turns.

CDMA (Code Division Multiple Access)

  • Each pair communicates in different languages.
  • The mobile and base station are able to distinguish between signals transmitted simultaneously over the same frequency allocation.

Bandwidth and Frequency

bandwidth

Bandwidth (bps – bits per second)

  • Amount of data that can be transmitted in per unit time.
  • Difference between highest frequency and lowest frequency (bandwidth = high – low)

Frequency (Hertz)

  • Number of complete cycles per second.
    • ex) 60Hz = 60 cycles per second

bandwidth_frequency

  • Low Frequency Wave
    • There are many frequencies already being used by other appliances such as microwave ovens(causes interferences).
    • Small bandwidth: There is no much space not being used around it.
    • Too crowded these days.
    • Covers farther distance
    • Low speed
    • ex) Wifi (2.4GHz or 5 GHz)
  • High Frequency Wave
    • Large bandwidth: There is a lot of space not being used around it.
    • Covers short distance
    • High speed
    • ex) Millimeter Wave (60GHz)

 

Latency vs. Jitter

Screen Shot 2017-09-26 at 11.18.26 PM

Latency

  • Amount of time it takes for a packet to travel from point A to point B(or round-trip).
    • ex) Data Latency: time between a query and the result displaying on the screen.

Jitter

  • Variance in latency over time
  • If every packet takes exactly the same amout of time to travle from A to B, there is no jitter.