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Enabling Hybrid Coordinator Mobility. John Kowalski Sharp Labs of America November 2001. Why We Need a Mobile HC. Wireless AV applications need Guaranteed Services (meeting objectives for data rate delivery, delay and jitter).

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Enabling hybrid coordinator mobility

Enabling Hybrid Coordinator Mobility

John Kowalski

Sharp Labs of America

November 2001

Why we need a mobile hc
Why We Need a Mobile HC

  • Wireless AV applications need Guaranteed Services (meeting objectives for data rate delivery, delay and jitter).

  • Guaranteed Services require polling, which can only be done with a Hybrid Coordinator (HC) for 802.11e.

  • Thus for mobile applications it is desirable to realize a mobile HC that function with as minimal change to a legacy BSS as possible.

Objectives of this development
Objectives of This Development

  • Determine what is needed to allow any 802.11e station to function as HC if needed and equipped.

  • To allow for minimal changes to the AP to realize this.

Method 1 ap and hc can be separated
Method 1: AP and HC can be separated

  • HC Establishment & Disestablishment

  • QoS State Information

Hc establishment 1
HC Establishment (1)

1. STA associates with (E)AP (may or may not be enhanced access point).

2. IF STA wishes to be HC (or needs Hybrid Coordinator) then

2.1 STA makes query to the Access Point (Request HC Candidacy message,

2.2 STA gets response from (E)AP (“HC ”) indicating: who if anyone is the HC.

Hc establishment 2
HC Establishment (2)

2.3 IF a new HC is chosen, the AP broadcasts that by new HC Status message- new HC is always first in the HC list (see text) The HC status message also includes the QoS state: the list of STAs and TSPECs that define the QoS flows.

3. Stations then renew their “registration” of streams (if they previously have been granted TSPECs for streams), or initiate new TSPEC negotiation with the new HC, and thereby create a new registration with the HC.

Hc disestablishment
HC Disestablishment

1. HC sends Request HC Resignation message

2. AP sends HC Status message.

3. If another STA is in the candidate list, the AP probes the highest candidate on the candidate list, to determine if that STA is still HC-capable. If so, the AP broadcasts the new AP and the procedure of steps 2.3 and 3 of the Establishment process are followed.

Hc disestablishment 2
HC Disestablishment (2)

4. IF another STA wishes to be HC (or needs Hybrid Coordinator) then

the procedure from step 2 on of HC Establishment is followed.

5. If there NO STAs that are candidates for HC or have requested to become HC, polled QoS service is discontinued, and streams are terminated.

Qos table
QoS Table

  • To speed transfer of HCs, a “QoS Table” is transferred, which gives Bandwidth Management information:

    • Stations with streams

    • Their TSPECs.


  • This is not a panacea for ensuring that streams will be kept up and running during HC handover- it merely allows for HC’s to be aware of Guaranteed Services requriments to change if needed.

  • It also allows for AP implementations that do not necessarily have an HC.

  • This scheme ensures that there is at most 1 hybrid coordinator operating at any given time. Naturally if stations cannot hear the HC, no stream is set up. The AP, however, can make determinations as to what the HC ranking is, based on information such as RSSL, etc., so that any station near enough to the AP can act as HC.

Method 2 ap not separate from hc 1
Method 2: AP not separate from HC (1)

Station Assuming Role of AP/HC

1. Through broadcast messages (originating from above the MAC), BSSID’s are broadcast of potential APs.

Stations keep a table of potential APs to associate with in the even they are no longer to communicate with the current AP (which may happen because the STA has moved out of range of the AP for example.) These may be broadcast from the PAL.

Method 2 ap not separate from hc 2
Method 2: AP not separate from HC (2)

When interference becomes great enough, or when a predetermined number of beacons are not detected then:

2. A station wishing to become an AP/HC issues a DISSOCIATION message to the AP it was associated with, and dissociates from the BSS.

3. A station wishing to become an AP/HC issues an Mac Layer Management Entity START (MLME-STARt.request) primitive, instantiating a BSS with itself as AP.

Method 2 ap not separate from hc 21
Method 2: AP not separate from HC (2)

  • Periodically, AP (from PAL Layer) sends QoS Table information to speed up bandwidth allocation and QoS management when new BSSs are created.


  • Either method is sufficient for enabling HC mobility- however, method 1 allows for implementations with AP’s which may not have an HC.

  • Method 1 may - because association and disassociation are not required, be “cleaner.”

  • Method 2- if decided by default- needs to be done in a standard way, and that will be one impetus for approving a PAR for the PAL for AV over 802.11e.

  • Motion: To approve the normative text in document...