1. Concerns Regarding CCF
November 2006
doc.: IEEE /xxxxr0
November 2006
Concerns Regarding CCF
Date:
Authors:
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Michelle Gong, Cisco
Michelle Gong, Cisco

2. Concerns Concerns that can be resolved
November 2006
Concerns
Concerns that can be resolved
Devices have different Channel Switch Delays
Low bandwidth utilization efficiency
Common channel bottleneck
A concern that we don’t know how to resolve
Co-existence problem
Michelle Gong, Cisco

3. November 2006
Operation of CCF
MPs exchange channel information on the common channel
A pair of MPs switch to the agreed-upon data channel for data transmission
RTX
 DIFS
Switching Delay
CTX
SIFS
RTX
 DIFS
CTX
SIFS
CTX
SIFS
DIFS
DATA
Switching Delay
Common Channel
RTX
ACK
SIFS
DATA
Switching Delay
DIFS
Data Channel n
Data Channel m
ACK
SIFS
Michelle Gong, Cisco

4. Concern 1: Different Channel Switch Delay
November 2006
Concern 1: Different Channel Switch Delay
For each data packet exchange, a device needs to switch channel twice. After each channel switch, hardware needs settling time, aka channel switch delay
Channel switch delay is usually implementation dependent and non-negligible
A typical channel switch delay is about ~1ms
Some products can achieve hundreds of us delay
Because devices have different channel switch delays, if a transmitter/receiver pair do not know each others’ channel switch delays, then correct operation is unlikely
An extra field in RTX/CTX 2 6 4
Frame Control
Duration RA TA
Destination Channel Information
FCS
RTX frame 2 6 4
Frame Control
Duration RA
Destination Channel Information
FCS
CTX frame
Michelle Gong, Cisco

5. Concern 2: Low Bandwidth Utilization Efficiency
November 2006
Concern 2: Low Bandwidth Utilization Efficiency
High overhead per data packet exchange
For each packet, CCF needs a RTX/CTX exchange, which takes about 100us at 24Mbps
Channel switch delay ranges from 1ms to hundreds of us
A typical TCP packet is 552 bytes and the packet transmission and ACK takes about ~270us over a 24Mbps wireless link
In this case, overhead is larger than the transmission time. With a channel switch delay of 400us, the CCF efficiency is only 23%
This compares poorly with 2 WLAN networks independently using 1 channel each
Michelle Gong, Cisco

6. Concern 3: Common Channel Bottleneck
November 2006
Concern 3: Common Channel Bottleneck
Common channel bottleneck
All MPs in the neighborhood share the same common channel
The common channel is also shared with non-CCF devices
All multi-cast and broadcast traffic is sent on the common channel (another concern)
TXOP doesn’t solve Concerns 2 & 3 completely and TXOP for data can be harmful to QoS traffic
However, without TXOP, CCF suffers very low efficiency and the common channel becomes a bottleneck
Michelle Gong, Cisco

7. Concern 4: Co-existence Problem
November 2006
Concern 4: Co-existence Problem
When non-CCF devices share the same channel with CCF devices, the channel availability information becomes inaccurate
Collisions and CCA-busy on the data channel are very costly to CCF devices
Because the transmitter/receiver pair stay on the data channel for some time plus channel switch delay, after the pair switch back to the common channel, they no longer have up-to-date channel information
Thus, they have to wait up to multiple ms until the start of the next channel coordination window (CCW)
Michelle Gong, Cisco

8. Concern 4: Co-existence Problem (Cont.)
November 2006
Concern 4: Co-existence Problem (Cont.)
When CTX is not received, the transmitter stays on the common channel and keeps backing off and re-transmitting. But the receiver is on the data channel and will only return after 2 * Channel-switch-delay + DIFS + aSlotTime, which is about 1 to 2ms
This problem also occurs when two CCF networks choose different common channels
Michelle Gong, Cisco

9. Concern 4: Co-existence Problem (Cont.)
November 2006
Concern 4: Co-existence Problem (Cont.)
A CCF device switches from one channel to another frequently. Every time it switches to a new channel, it performs CCA for DIFS time
It is highly likely that a CCF device misses any medium information, such as a preamble or virtual carrier sense, occurring before the CCF device arrives on the data channel
If a packet’s preamble is missed, the CCA sensitivity level is -62dBm as defined in the subclause of ma. Such a high CCA sensitivity level means CCA is unlikely to be triggered in most practical situations
Virtual carrier sensing is an important way of avoiding collisions in , and respecting virtual carrier sensing is mandatory in the standard
Michelle Gong, Cisco

10. Concern 4: Co-existence Problem (Cont.)
November 2006
Concern 4: Co-existence Problem (Cont.)
Missing virtual carrier sense and preambles is damaging to both CCF devices and non-CCF devices
Collisions cost much more for CCF devices than for non-CCF devices
Not being compatible with the protocol means a large number of negative comments during LB, which have to be resolved for us to move forward
Michelle Gong, Cisco

11. Conclusions Concerns that can be resolved
November 2006
Conclusions
Concerns that can be resolved
Different channel switch delays
Low bandwidth utilization efficiency
Common channel bottleneck
A concern that we don’t know how to resolve
Co-existence problem
Other presentation: 11-06/1777r0
The proposed solutions such as lowering the CCA sensitivity level need changes to the PHY and thus changes to the TGs PAR. The group has already decided not to make the change
There is no existing solution to fix the current problems
Michelle Gong, Cisco

12. November 2006
Any Questions? ?
Michelle Gong, Cisco

13. November 2006
Remove or modify the following sections in the Draft P802.11s – D0.04.pdf
Remove all references to CCF from D0.04, including
Section 4: page 4, remove line 15, 17, 34, and 38
Section : remove the section
Section : page 13, Table 1, remove RTX/CTX type
Section : remove the section
Section : remove the section
Section : page 22, Figure s16 and Table s1, remove “Multi-channel capability” element. Page 24, remove line 5 to 13 and Figure s20.
Section : remove the section
Section 11A.1.7: remove the section
Michelle Gong, Cisco

14. November 2006
Motion
Do you think that we should adopt the changes shown in this document 11-06/1716r2?
Moved:
Second:
Yes No
Abstain
Michelle Gong, Cisco