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Improving Multicast Reliability. Date: 2008-11-11. Authors:. Slide 1. Multicast Reliability. The reliability of multicast delivery in 802.11 WLANs is notoriously low.

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improving multicast reliability
Improving Multicast Reliability

Date: 2008-11-11


Slide 1

Darwin Engwer, Nortel Networks

multicast reliability
Multicast Reliability

The reliability of multicast delivery in 802.11 WLANs is notoriously low.

The reliability of multicast delivery can be dramatically improved by sending multicast MSDUs from an AP to the associated STAs using the Directed Multicast Service (DMS).

Slide 2

Darwin Engwer, Nortel Networks

directed multicast service benefits
Directed Multicast Service Benefits

Increases the reliability of multicast/broadcast MSDU delivery

Multicast MSDUs are sent as unicast (when channel bandwidth is available)

Unicast is more reliable than multicast

Allows multicast MSDUs to be fragmented, further increasing reliability of delivery

Today, multicast MSDUs cannot be fragmented

Improves Network Discovery Operations

Especially for low duty cycle service advertisement protocols

Improves security of multicast MSDUs

Multicasts sent via unicast can be protected using the client’s negotiated Pairwise cipher with the AP

Today, multicast is protected by the lowest common (or open) authentication

Reduces non-AP STA Power Consumption

Sleeping non-AP STAs can retrieve queued multicast frames along with other individually addressed MSDUs on their own schedule, potentially reducing power consumption at the non-AP STA

Sleeping STA doesn’t need to wake up at every DTIM or FBMS interval to receive multicast frames

Sleeping STA only needs to wake up to check the AID TIM bit at a desirable interval (e.g. listen interval)

Slide 3

Darwin Engwer, Nortel Networks

directed multicast service dms overview
Directed Multicast Service (DMS) Overview

AP advertises Directed Multicast Service (DMS) Capability

Non-AP STA sends a request to specify traffic classes (i.e. multicast address) for the directed multicast traffic delivery

AP may accept or reject the traffic class specified by the non-AP STA

If the service is accepted by the AP,

AP shall transmit the requested group addressed (multicast) traffic as individual addressed (unicast) traffic to the requesting STA in an A-MSDU frame format. The AP may still transmit the group addressed traffic for other STAs within the multicast group in the BSS as normal.

The requesting STA shall discard group addressed frames received fro mthe AP that match a multicast address entry list.

Other STAs will process group addressed traffic as normal.

Slide 4

Darwin Engwer, Nortel Networks

dms request and response frames
DMS Request and Response frames
  • DMS Request frame:



  • DMS Response frame:

DMS Status

Darwin Engwer, Nortel Networks

Slide 5


A-MSDU is used for Directed Multicast

  • Original destination address (DA) is transmitted in A-MSDU subframe header (the DA field)
    • The DA and SA fields of the A-MSDU subframe header contain the values passed in the MA-UNITDATA.request and MA-UNITDATA.indication primitives. (e.g. DA=multicast address)
  • RA (Address 1) in the MAC header is set to the requesting non-AP STA address
  • Support of A-MSDU processing is mandatory according to the IEEE 802.11n draft specification.

Darwin Engwer, Nortel Networks


1. Is DMS Scalable?

2. How does DMS relate to TGaa’s work?

Slide 7

Darwin Engwer, Nortel Networks

question 1 is directed multicast scalable
Question 1: Is Directed Multicast scalable?


DMS improves the reliability of multicast delivery to the same level as unicast, at the cost of wireless medium (WM) bandwidth.

If a large number of non-AP STAs are using DMS the channel capacity could be exceeded.


Limit the DMS service for use by less than x associated STAs.

Enable this feature environments with less than x associated STAs, for example, the home environment.

Allow the AP to disable/reject DMS service when too many STAs are associated.

Allow the AP to disable/reject DMS service when there is insufficient available channel bandwidth to support the current multicast traffic load without unicast traffic effects.

DMS is an optional feature.

Slide 8

Darwin Engwer, Nortel Networks

question 2 how does dms relate to tgaa work
Question 2: How does DMS relate to TGaa work?
  • Considerations:
    • DMS may overlap with TGaa efforts
    • TGaa scope: “Improved link reliability and low jitter characteristics for multicast/broadcast audio/video streams.”
  • DMS should be considered in TGv:
    • DMS extends the existing multicast enhancements in TGv and addresses a TGv objective [Req 2120]
    • Including DMS in TGv can benefit low bandwidth applications sooner
    • Including DMS in TGv will also benefit TGaa
    • DMS is not intended as a general solution to the multicast stream delivery problem (although in an environment of an AP with a single associated STA, DMS is a reasonable, simple approach)
  • Straw poll taken in the May 2008 TGv/TGaa joint meeting:
    • Directed multicast proposal (11-08-0049)should be considered in:
      • TGv:25; TGaa:8; abstain: 8

Darwin Engwer, Nortel Networks

  • IEEE Std. 802.11-2007
  • 802.11n Draft 7.0
  • 11-08-0049-03
  • 11-08-0050-03

Darwin Engwer, Nortel Networks

backup slides
Backup Slides

Darwin Engwer, Nortel Networks

existing multicast enhancements in tgv
Existing Multicast Enhancements in TGv

FBMS – Flexible Broadcast/Multicast Service

Enables flexible delivery interval for broadcast/multicast at STA’s request (at multiples of the DTIM intervals)

Enables group addressed frames to be transmitted at higher data rates, reducing the amount of basic rate traffic sent over the wireless medium.

Multicast Diagnostics Reporting

STA provides the AP an indication of the number of dropped group addressed frames in order to enable reliable service

DMS extends existing multicast enhancements in TGv, naturally fits in TGv

Slide 12

Darwin Engwer, Nortel Networks

802 11aa requirements
802.11aa Requirements
  • Reliable Multicast is a complex problem, and different applications may require different solutions.
  • DMS provides the same level of reliability as unicast for all traffic classes, at the cost of bandwidth
    • Directed multicast can be used while streaming a multicast Standard Definition (SD) stream to 2 or 3 STAs, it does not scale well for High Definition (HD) video streaming.
  • 802.11aa targets multicast robustness (reliability is a part of robustness) for HD video streaming (high bandwidth) applications
    • 802.11n is sufficient for Standard Definition video streaming
  • DMS targets low bandwidth applications, which is not the target of 802.11aa

DMS and 802.11aa target different application spaces

Darwin Engwer, Nortel Networks

dms should be considered in tgv
DMS should be considered in TGv
  • DMS addresses TGv objective [Req 2120]; complements TGv FBMS
    • FBMS provides power saving for TGv devices. However, the legacy device (non-TGv device) within the BSS will experience unexpected multicast delivery delays.
    • DMS provides power saving for TGv devices without introducing latency on the legacy (non-TGv) device. However, its usage is limited to a small number of associated STAs due to bandwidth requirements.
  • Including DMS in TGv can benefit low bandwidth applications sooner
    • TGv is in the process of initial ballots, expected sponsor ballot in 2009
    • TGaa is at the stage of calling for proposals, expected sponsor ballot Nov 2010
  • Including DMS in TGv will also benefit TGaa
    • All features specified in the existing 802.11-2007 standard and draft amendments (e.g. TGv DMS) can be utilized by later amendments (e.g. TGaa)

Darwin Engwer, Nortel Networks