Comments on: How does 802.11n get to 600Mbps? http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/ Mobile Unified Communications Fri, 28 Oct 2011 16:47:54 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: Michael http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/comment-page-1/#comment-10710 Michael Mon, 15 Nov 2010 13:39:45 +0000 http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/#comment-10710 4x4 is 4 antennas on the transmit side and 4 antennas on the receive side. So there are 4 antennas on each device, since the same antennas are used for both transmit and receive on a particular device. Each antenna does both TX and RX. The TX and RX take turns. 4×4 is 4 antennas on the transmit side and 4 antennas on the receive side. So there are 4 antennas on each device, since the same antennas are used for both transmit and receive on a particular device. Each antenna does both TX and RX. The TX and RX take turns.

]]> By: Dmitry http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/comment-page-1/#comment-10707 Dmitry Mon, 15 Nov 2010 09:55:23 +0000 http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/#comment-10707 600Mbit/s - "half duplex", "unidirection" ? if you have 4x4 MIMO, is this 2 TX 2 RX antena on the first side and 2TX 2 RX on the second side or 4 TX 4 RX on both sides? if 2, then 300 "full duplex" (minus overhead - 200 and in real life you never have 64QAM 5/6). As result you can garanted to customers 30-50 Mbit/s without QOS (if you use QOS, equipment have low cost processors and "voila" 10-20 Mbit/s). 600Mbit/s – “half duplex”, “unidirection” ?
if you have 4×4 MIMO, is this 2 TX 2 RX antena on the first side and 2TX 2 RX on the second side or 4 TX 4 RX on both sides?
if 2, then 300 “full duplex” (minus overhead – 200 and in real life you never have 64QAM 5/6). As result you can garanted to customers 30-50 Mbit/s without QOS (if you use QOS, equipment have low cost processors and “voila” 10-20 Mbit/s).

]]> By: Michael http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/comment-page-1/#comment-9943 Michael Tue, 31 Aug 2010 14:56:21 +0000 http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/#comment-9943 Here's a good article on the same topic: http://www.eetimes.com/design/communications-design/4206045/How-throughput-enhancements-dramatically-boost-802-11n-MAC-efficiency Here’s a good article on the same topic: http://www.eetimes.com/design/communications-design/4206045/How-throughput-enhancements-dramatically-boost-802-11n-MAC-efficiency

]]> By: DS Nagi http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/comment-page-1/#comment-9017 DS Nagi Thu, 27 May 2010 06:47:25 +0000 http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/#comment-9017 Great overview of 11n improvments Great overview of 11n improvments

]]> By: Michael http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/comment-page-1/#comment-7908 Michael Sun, 24 Jan 2010 13:28:00 +0000 http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/#comment-7908 Here's a discussion of interframe spaces that might help: <a href="http://www.cwnp.com/pdf/802.11_arbitration.pdf" rel="nofollow">http://www.cwnp.com/pdf/802.11_arbitration.pdf</a>. Here’s a discussion of interframe spaces that might help: http://www.cwnp.com/pdf/802.11_arbitration.pdf.

]]> By: Ron http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/comment-page-1/#comment-7819 Ron Wed, 13 Jan 2010 14:53:31 +0000 http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/#comment-7819 I just finished reading "Next Generation Wireless LANs" a Cambridge book. This article was extremely good and helped summarize what I just finished reading. In the book it discussed that the use of RIFs in place of SIFs added significant data throughput (I think Greenfield only). Can someone comment on this? I just finished reading “Next Generation Wireless LANs” a Cambridge book. This article was extremely good and helped summarize what I just finished reading. In the book it discussed that the use of RIFs in place of SIFs added significant data throughput (I think Greenfield only). Can someone comment on this?

]]> By: Michael http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/comment-page-1/#comment-7397 Michael Thu, 10 Dec 2009 15:33:48 +0000 http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/#comment-7397 802.11n doesn't include HARQ because: 1. It's too expensive. 2. And it wouldn't work anyway HARQ works by retaining your record of a packet that was received with error and combining it with a retry packet in such a way as to reduce errors. You can only do this if you retain a sample-by-sample record of the packet. 802.11 PHYs are stateless in the sense that they don't care who spoke to them previously, using what rate or PHY mode; each packet is fully independent. The cost arises in HARQ because the PHY has to keep a copy of previous packets. It doesn't work in 802.11 because it breaks the stateless nature of the PHY, i.e. the processing of the next packet depends on the previous one. In an unlicensed band you don't have full control of this. Furthermore it breaks the layering because now the PHY needs to know which packets came from which STAs before attempting HARQ recombination. So HARQ can work in systems like WiMax because: 1. It's a licensed band - the FCC can shut down anybody who doesn't follow the rules. 2. The base-station controls exactly who talks and when 3. The cost / complexity is loaded into the base-station to improve uplink (where signal is weak), not downlink (where you don't need it due to high tx power). 802.11n doesn’t include HARQ because:
1. It’s too expensive.
2. And it wouldn’t work anyway

HARQ works by retaining your record of a packet that was received with error and combining it with a retry packet in such a way as to reduce errors. You can only do this if you retain a sample-by-sample record of the packet.

802.11 PHYs are stateless in the sense that they don’t care who spoke to them previously, using what rate or PHY mode; each packet is fully independent.

The cost arises in HARQ because the PHY has to keep a copy of previous packets.

It doesn’t work in 802.11 because it breaks the stateless nature of the PHY, i.e. the processing of the next packet depends on the previous one. In an unlicensed band you don’t have full control of this. Furthermore it breaks the layering because now the PHY needs to know which packets came from which STAs before attempting HARQ recombination.

So HARQ can work in systems like WiMax because:
1. It’s a licensed band – the FCC can shut down anybody who doesn’t follow the rules.
2. The base-station controls exactly who talks and when
3. The cost / complexity is loaded into the base-station to improve uplink (where signal is weak), not downlink (where you don’t need it due to high tx power).

]]> By: Wei http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/comment-page-1/#comment-7396 Wei Thu, 10 Dec 2009 14:04:34 +0000 http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/#comment-7396 Is there a reason for not includng Hybrid ARQ in 802.11n ? New standards such as WiMax has included it already, why not 802.11n? Is there a reason for not includng Hybrid ARQ in 802.11n ? New standards such as WiMax has included it already, why not 802.11n?

]]> By: Daniel http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/comment-page-1/#comment-7309 Daniel Fri, 04 Dec 2009 03:28:04 +0000 http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/#comment-7309 Very good review. Quick, to the point, yet quite technical and clever. Thank you Very good review. Quick, to the point, yet quite technical and clever.
Thank you

]]> By: Michael http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/comment-page-1/#comment-7296 Michael Wed, 02 Dec 2009 21:51:30 +0000 http://www.wirevolution.com/2007/09/07/how-does-80211n-get-to-600mbps/#comment-7296 The available spectrum varies from country to country. The Gartner comment probably referred to 2.4 GHz spectrum in the USA, which is usually too crowded to be usable for 40 MHz channels anyway. Here is a diagram of the 5 GHz spectrum in the USA that shows that you can get 23 non-overlapping 20 MHz channels, which equates to 11 non-overlapping 40 MHz channels: <img src="http://www.wirevolution.com/wp-content/uploads/2009/12/80211a-5-ghz-spectrum.png" alt="5 GHz Wi-Fi spectrum in the USA" class="articleimg"/> The available spectrum varies from country to country. The Gartner comment probably referred to 2.4 GHz spectrum in the USA, which is usually too crowded to be usable for 40 MHz channels anyway. Here is a diagram of the 5 GHz spectrum in the USA that shows that you can get 23 non-overlapping 20 MHz channels, which equates to 11 non-overlapping 40 MHz channels:
5 GHz Wi-Fi spectrum in the USA

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