1. Why we need Length Field in VHT SIG
March 2010
doc.: IEEE /0xxxr0
May 2010
Why we need Length Field in VHT SIG
Date:
Authors:
Yuichi Morioka, Sony Corporation

2. March 2010
doc.: IEEE /0xxxr0
May 2010
Abstract
It has been proposed that, contrary to the structure adopted in 11n, the Length Field in VHT SIG is eliminated
And the Length Field in L-SIG is used to signal the length of 11ac PPDU
Some major drawbacks in this method
PPDU Size limitation (3ms)
Blind decoding without knowledge of actual end of packet
TXOP Protection can not be used
We propose that the Length Field is kept within the VHT SIG to alleviate these problems
Yuichi Morioka, Sony Corporation

3. Review of 11n PPDU Structure (1/3)
March 2010
doc.: IEEE /0xxxr0
May 2010
Review of 11n PPDU Structure (1/3)
11n PHY Header consists of L-SIG, HT-SIG and Training
In L-SIG there is a L-SIG Length(L-Length); in HT-SIG there is HT-SIG Length(HT-Length) Field
L-Length indicates the protected duration; HT-Length indicates the length in bytes of the Data
Yuichi Morioka, Sony Corporation

4. Review of 11n PPDU Structure (2/3)
March 2010
doc.: IEEE /0xxxr0
May 2010
Review of 11n PPDU Structure (2/3)
L-Length Field in 11n PHY Header must comply with legacy (non HT) rules so legacy devices correctly defer for L-Length
One important legacy rule is that the L-Length must not exceed 2340[octets]
Max L-Length (2340[octet]) / L-DataRate (6[Mbps]) = (approx) 3[ms]
In n, L-Length endpoint and HT-Length are the same with two exceptions
When the PPDU is over 3[ms]
When using L-SIG TXOP
Yuichi Morioka, Sony Corporation

5. Review of 11n PPDU Structure (3/3)
March 2010
doc.: IEEE /0xxxr0
May 2010
Review of 11n PPDU Structure (3/3)
In n, HT-Length can be longer than L-Length when sending a PPDU longer than approx. 3[ms]
In this case, MAC Level Protection (e.g. RTS/CTS) is used
L-Length can be longer than HT-Length when using L-SIG TXOP
Yuichi Morioka, Sony Corporation

6. March 2010
doc.: IEEE /0xxxr0
May 2010
Losing VHT-SIG Length
Contrary to what we have in 11n, it has been proposed that the Length field from VHT-SIG (VHT-Length) is eliminated
You can easily imagine the drawback with this…
Yuichi Morioka, Sony Corporation

7. March 2010
doc.: IEEE /0xxxr0
May 2010
Problem #1: 3[ms] Limit
There will be no way to signal a PPDU that exceeds 3[ms]
Because in ac, SDMA will be used to transmit simultaneously to multiple STAs, it is expected that more buffering is necessary, which will result in larger PPDUs
This 3[ms] rule will be a big problem in ac
Yuichi Morioka, Sony Corporation

8. Problem #2: No L-SIG TXOP
March 2010
doc.: IEEE /0xxxr0
May 2010
Problem #2: No L-SIG TXOP
L-SIG TXOP is a mechanism where L-Length is used to signal duration of more than one packet
The mechanism is especially useful in a mixture of VHT/HT/Legacy STAs
If there is no VHT-Length, there will be no way to signal the length of the actual PPDU, if it is used along with L-SIG TXOP
Yuichi Morioka, Sony Corporation

9. Problem #3: Blind Decoding
March 2010
doc.: IEEE /0xxxr0
May 2010
Problem #3: Blind Decoding
In 11ac, Padding will be used to adjust the PPDU length per user
It has been proposed that the PPDU length per user is always kept the same
If VHT-Length is lost, there is not way to signal the valid data length per user
This means the addressed STAs must blindly decode until it reaches the Pad
This blind decoding could severely limit implementation (e.g. Pipelining)
Yuichi Morioka, Sony Corporation

10. March 2010
doc.: IEEE /0xxxr0
May 2010
Conclusions
Some major drawbacks in losing the VHT-Length Field have been presented
PPDU Size limitation (3ms)
Blind decoding without knowledge of actual end of packet
TXOP Protection can not be used
We strongly propose that Length Field is kept within the VHT SIG to alleviate these problems with the cost of few bytes
Slide 10
Slide 10
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Yuichi Morioka, Sony Corporation

11. Reference May 2010 March 2010 doc.: IEEE 802.11-10/0xxxr0
Yuichi Morioka, Sony Corporation