What is UL-OFDMA Random Access (UORA)?


As you have learned in previous blogs, an 802.11ax access point uses OFDMA technology to partition a channel into smaller sub-channels called resource units (RUs) so that simultaneous multiple-user transmissions can occur. The AP mandates the RU allocation for multiple clients for both downlink and uplink OFDMA. In simpler words, the AP is in charge of how RU units are assigned to clients within a given channel. The clients can provide feedback to an 802.11ax AP using either solicited or unsolicited buffer status reports, however, the AP still makes the decision in regard to RU allocation for synchronized UL-OFDMA from multiple clients.

Is there a method in which clients can allocate resource units for uplink transmissions instead of the AP making the decision? 802.11ax provides for an optional UL-OFDMA random access (UORA) method. A random-access method is favorable in conditions where the AP is unaware of traffic buffered on the clients. The AP sends a random-access trigger frame to allocate RUs for random access.

Clients that want to transmit will use an OFDMA back-off (OBO) procedure. Initially, a client chooses a random value, with each trigger frame the client decrements the value by the number of RUs specified in the trigger frame until it reaches zero. The client will then randomly select an RU and then transmit its frame.

The process works as follows:

  • AP sends random access trigger frame to allocate RUs for random access.
  • The AID12 subfield of the User Info field within the trigger frame is used by the AP to communicate to client devices about the availability of random-access RUs.
  • A value of 0 indicates that one or more RUs are available for associated clients.
  • A value of 2045 indicates that one or more RUs are available for unassociated clients.
  • The AP indicates a range for an OFDMA contention window (OCW) in the UORA Parameter Set information element.
  • Clients randomly select a number from the OCW and then each client’s OFDMA Back-off (OBO) counter begins to decrement.
  • If the OBO counter of the client is not greater than the number of eligible Random Access-RUs in a trigger frame from that AP, then the client station will set its OBO counter to zero and randomly select one of the eligible RA-RUs.

In our next blog, we will further illustrate the UL-OFDMA random access (UORA) process with some examples.

Portions of this blog have been excerpted from the 5thedition of Sybex Publishing’s Certified Wireless Network Administrator (CWNA) Study Guide:

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