Browse Prior Art Database

Method of providing Reliable Broadcast and Multicast over WLAN

IP.com Disclosure Number: IPCOM000132526D
Original Publication Date: 2005-Dec-20
Included in the Prior Art Database: 2005-Dec-20
Document File: 3 page(s) / 58K

Publishing Venue

Motorola

Related People

Tony Metke: AUTHOR [+2]

Abstract

This paper presents a simple yet effective method for providing reliable Layer 2 broadcast and multicast over a WLAN or other broadcast domain. Although the methods described here are not specific to WLAN, they will have the most benefit on wireless media because these medium have a higher bit error rate than wired LANs. It is assumed that the reader is familiar with typical WLAN operation, beacons, contention free periods, layer 2 multicast and layer 2 broadcast.

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Method of providing Reliable Broadcast and Multicast over WLAN

By Tony Metke and Randy L. Ekl


Abstract

This paper presents a simple yet effective method for providing reliable Layer 2 broadcast and multicast over a WLAN or other broadcast domain.  Although the methods described here are not specific to WLAN, they will have the most benefit on wireless media because these medium have a higher bit error rate than wired LANs. It is assumed that the reader is familiar with typical WLAN operation, beacons, contention free periods, layer 2 multicast and layer 2 broadcast.

The method described achieves reliable broadcast and multicast by introducing several enhancements to standard WLAN operation.  Those enhancements are the utilization of a broadcast CFP, multicast CFPs, and a Restricted Contention Period, as well as ordering of transmitted multicast packets by increasing destination address.  Also, other enhancements to standard WLAN operation are use of sequence numbers on broadcast packets and sequence numbers per layer 2 multicast group on multicast packets, use of NACKs for missed broadcast or multicast packets, and finally the appropriate algorithms in the APs and STAs to utilize the above elements.

Proposed Method

The proposed method to make multicast and broadcast reliable consists of three enhancements to standard WLAN operation.  There are detailed in the next three sections.

Enhancement 1

Every beacon is followed by a new period, a broadcast contention free period (CFP).  Only the Access Point (AP) is allowed to transmit during a broadcast CFP.  At the end of transmitting the broadcast packets, the AP will transmit a CFP_end packet.

If an AP has outbound multicast packets to transmit, that is from the AP on the air interface to STAs, a multicast CFP will follow the broadcast CFP.

Every beacon advertises the number of broadcast packets it will transmit this beacon interval.  If a client station (STA) receives a CFP_end packet for the broadcast CFP, a multicast packet, a unicast packet, or another beacon packet before receiving the advertised number of broadcast packets, it will send a NACK message to request the appropriate broadcast packet.

The question arises: How does the AP know which broadcast packet is missing?  This will be answered below.

Enhancement 2

Independent of Enhancement 1, above, when an AP transmits multicast packets, it will always transmit the packets in order of increasing destination address.  In this way, if the AP indicates in a beacon that multicast packets are to be delivered for a given group and a STA which is a member of that group sees packets with a destination address larger than the destination address for the group to which it belongs, it would immediately know that it missed a packet, and it could transmit a NACK packet at the appropriate time to request the retransmission.  The appropriate time to send a NACK would be at the end of the multicast CFP.

This solution raises the same question as E...