‘Shield’: Protecting High-Priority Channel Access Attempts doc.: IEEE 802.11-00/457 November 2001 November 2001 ‘Shield’: Protecting High-Priority Channel Access Attempts Mathilde Benveniste AT&T Labs, Research M. Benveniste -- AT&T Labs, Research

CFS and PCFS: Definitions November 2001 CFS and PCFS: Definitions Definitions Contention free session (CFS) Any frame exchange sequence that may occur without contention following a successful channel access attempt. A CFS may involve one or more stations. A CFS may be initiated by any station. A Contention-Free Burst and an RTS/CTS exchange are both examples of a CFS Periodic contention free period (PCFS) A CFS that must occur at regular time intervals. A Contention-Free Period is an example of a PCFS M. Benveniste -- AT&T Labs, Research

CFS and PCFS: Definitions November 2001 CFS and PCFS: Definitions Definitions Contention free session (CFS) Any frame exchange sequence that may occur without contention following a successful channel access attempt. A CFS may involve one or more stations. A CFS may be initiated by any station. A Contention-Free Burst and an RTS/CTS exchange are both examples of a CFS Periodic contention free period (PCFS) A CFS that must occur at regular time intervals. A Contention-Free Period is an example of a PCFS CFS/PCFS origin For simplicity, but without loss of generality, we assume that CFSs/PCFSs are initiated by APs only M. Benveniste -- AT&T Labs, Research

November 2001 Overview This is one of 5 independent submissions relating to HCF access and OBSS mitigation. These are: (1) CPMA: An Access Mechanism for Contention-Free Sessions CPMA a protocol for prioritized contention-based access (2) An access mechanism for Periodic Contention-Free Sessions Regularly spaced sessions (3) ‘Shield’: Protecting High-Priority Channel Access Attempts  Prevents CFS/PCFS corruption in case of collision with an (E)STA (4) ‘Neighborhood capture’ in wireless LANs Preventing a capture effect (5) HCF Access Mechanisms: Inter-BSS NAV protection Virtual carrier sense for CFS/PCFSs M. Benveniste -- AT&T Labs, Research

Channel Access for CFSs/PCFSs November 2001 A CFS may collide with (E)DCF transmissions if there are idle gaps longer than PIFS following a transmission Such gaps occur in CPMA: if an AP retires; or at the head of sequence of contiguous CFSs all HCF access methods using random backoff CFS structure protects a CFS in case of a collision with an (E)DCF transmission the CFS is delayed for subsequent transmission, interference-free the channel is reserved by causing a break in the current idle, if one exists M. Benveniste -- AT&T Labs, Research

‘Shield’ - Protection from collision with DCF txs November 2001 ‘Shield’ - Protection from collision with DCF txs DIFS CFS body PIFS AP starts a CFS after its backoff expires Busy channel CFS Shield A CFS is started with a short frame (e.g., PLCP header without MAC data) -- the ‘shield’ The AP will wait for an idle period of PIFS to transmit following the shield If an (E) DCF transmission collides with the shield, the AP will hear the transmission and defer initiation of the CFS body After completion of the (E)DCF transmission, the CFS will start - following a PIFS idle Transmission of the shield before the CFS body is not needed always Not needed if AP knows that the idle gap between the CFS and the previous transmission is equal to PIFS; i.e., when the backoff delay is 1, during the last busy period The algorithms presented here can be combined with a variety of schemes for RF resource allocation. An example is illustrated above. Three channels are available in each cell, each allocated to one of three antenna faces (i.e. to one logical cell each). If all the logical cells to which a channel is allocated transmit simultaneously on that channel, the quality of transmission would be inadequate. In order to increase the signal-to-interference ratio, each frame on the channel must be apportioned so that the different co-channel logical cells use different parts of the frame. In this example, the problem reduces to bandwidth allocation of a single channel among potentially interfering BSSs. We consider only the APs that are allowed to use the same channel. We wish to allocate a portion of each frame of the channel among the logical cells. M. Benveniste -- AT&T Labs, Research

Sample CFS structure November 2001 CFS cycle CFS body PIFS S I F S S I F S S I F S P I F S Other CFSs and Contention Period D 1 + p o l l D2+ack+poll D 3 + a c k D4+poll + p o l U 1 + a c k U2+ack U4+ack Beacon D I F S Shield N o S I F S S I F r e s p o n s e S I F t o C F - P o l l N A V CFS_Max_Duration F M. Benveniste -- AT&T Labs, Research

Benefit of the ‘shield’ November 2001 Benefit of the ‘shield’ Any (E)DCF transmissions colliding with the ‘shield’ will cause postponement of the start of the CFS body until the channel is clear The CFS is thus assured of no (E)DCF conflict because of its shorter AIFS While the colliding (E)DCF transmission is unsuccessful, the CFS body will be transmitted without conflict Channel time is saved this way if CFSs are longer than DCF transmissions This method can be applied also to PCFSs if there is no other mechanism to protect them from collisions with (E)DCF transmissions, as there is in PCF A special ‘shield’ may also be used in Inter-BSS NAV protection M. Benveniste -- AT&T Labs, Research