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

2. 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

3. 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

4. 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

5. 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

6. ‘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

7. 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

8. 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