• Hey All,

    After reading a lot about the CSMA/CA process, I was not able to wrap my head around the slot time.

    What is the slot time (i understand it's a period)?

    But like, what is it?

    1 us, 1 sec, 2 sec ? (I was looking to understand how the slot time is decided).

    Just like other IFS decided for each PHY, does the slot time also depend on PHY?

    Is there a table I can refer to?

    Any help is appreciated.


  • The slot duration depends on the the modulation type, the band (2.4 or 5 GHz), what types of clients are Associated, channel spacing (MHz), and what the STA's Short Time Slot Subfield is set to.  

    It is  measured in micro-seconds (μs).

    Slot time for DSSS and HR is 20 μs - 5μs for turn around time + 15 for energy detect time. 

    9 μs for ERP (with 20 MHz spacing)  - the difference from DSSS being due to the shorter energy detect time .

    Look for CCA_Time in the 802.11 standard (Table 19-.6).

    The short slot time of 9μs is used when only ERP stations are associated to the BSSS.

  • By Howard - edited: April 14, 2016

    Good point risshu.

    That is a really good document, but it is assuming only 20 MHZ channel spacing, so it only mentions 9 and 20 usec durations.

    When 802.11p is ratified (with its 10 MHz spacing), another possibility from the standard will probably kick in.

  • Good to know!

  • Thank you Howard.

  • I went through the doc but missed this table section.

    Thank you Risshuu.

  • Are there values specified for .11 ac?

  • By Howard - edited: April 20, 2016

    /n and /ac are both  9 μsec.

Page 1 of 1
  • 1