Algorithms for telecommunications: class #7

Teacher: Galtiere


Wifi networks

Collision detection and avoidance.

CSMA - carrier sense multiple access
CR - collision resolution (non-radio)
CA - collision avoidance (OK for radio)

Timing parameters for 802.11n:

DIFS: 34µs
SIFS: 16µs

Back-off mechanism:

  • contention window w interger: (positive)
  • back off counter l: (non-negative integer)


  • W = CWmin
  • Take k at random on {w, …w - 1)
  • Wait k mini slots before communication.
  • If someone else transmits, resume transmition

After transmission

  • put W = CWmin if successful
  • W = min(2W, CWmin) if collision.
Unclear, need to ask the teachers, since most is obviously derived works.