Why we need Length Field in VHT SIG March 2010 doc.: IEEE 802.11-10/0xxxr0 May 2010 Why we need Length Field in VHT SIG Date: 2010-05-17 Authors: Yuichi Morioka, Sony Corporation

March 2010 doc.: IEEE 802.11-10/0xxxr0 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

Review of 11n PPDU Structure (1/3) March 2010 doc.: IEEE 802.11-10/0xxxr0 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

Review of 11n PPDU Structure (2/3) March 2010 doc.: IEEE 802.11-10/0xxxr0 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 802.11n, 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

Review of 11n PPDU Structure (3/3) March 2010 doc.: IEEE 802.11-10/0xxxr0 May 2010 Review of 11n PPDU Structure (3/3) In 802.11n, 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

March 2010 doc.: IEEE 802.11-10/0xxxr0 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

March 2010 doc.: IEEE 802.11-10/0xxxr0 May 2010 Problem #1: 3[ms] Limit There will be no way to signal a PPDU that exceeds 3[ms] Because in 802.11ac, 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 802.11ac Yuichi Morioka, Sony Corporation

Problem #2: No L-SIG TXOP March 2010 doc.: IEEE 802.11-10/0xxxr0 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

Problem #3: Blind Decoding March 2010 doc.: IEEE 802.11-10/0xxxr0 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

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

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