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‘Shield’: Protecting High-Priority Channel Access Attempts

‘Shield’: Protecting High-Priority Channel Access Attempts. Mathilde Benveniste AT&T Labs, Research. CFS and PCFS: Definitions. Definitions

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‘Shield’: Protecting High-Priority Channel Access Attempts

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  1. ‘Shield’: Protecting High-Priority Channel Access Attempts Mathilde Benveniste AT&T Labs, Research M. Benveniste -- AT&T Labs, Research

  2. 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

  3. 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

  4. 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

  5. Channel Access for CFSs/PCFSs • 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

  6. AP starts a CFS after its backoff expires Shield DIFS CFS PIFS PIFS PIFS CFS body Busy channel Busy channel ‘Shield’ - Protection from collision with DCF txs • 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 M. Benveniste -- AT&T Labs, Research

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

  8. 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

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