My apologies is this question is stupid. I am in learning phase of IEE 802.11.

The figure below illustrates the single user packet structure of IEEE 802.11ac.

It consists of preamble and data part as you can see. It is known that this packet structure supports bandwidths of 20, 40, 80 , 160 MHz. Therefore as you can see there is a part that is duplicated.

My question is I don't understand why the duplicate portion of the preamble is stacked on top of each other. How does one stack bits on top of each other?

Any thoughts.

1 Answer 1


I believe you are misinterpreting the diagram. This is a rather simple way of trying to show how the RF is used over time to clear the frequency, establish timing, and set any parameters necessary for the information to be transmitted.

802.11ac has to remain backwards compatible with previous 802.11 standards so they can see and avoid transmitting on the RF for the time specified. While 802.11n offered three PHY formats (legacy, mixed and green field modes), 802.11ac was simplified and only offers one.

The legacy training fields and signal fields need to be sent in a format that will be recognized by legacy devices (i.e. 802.11a and 802.11n) which means that they can only occupy a 20MHz channel. As the legacy fields can not set the VHT parameters, the initial VHT fields must follow this as well.

After that point, the rest of the VHT communication can take up the entire channel width if it is larger than the initial 20MHz.

But consider, what happens if you only transmit those initial fields on the first 20MHz but your channel is 40MHz (or larger)? Legacy devices operating on those channels will not receive the information they need to avoid using the channel for the time specified and they could then create a collision outside of that first 20MHz.

So to avoid this potential problem, the same initial fields are duplicated and sent out on the second (and successive) 20MHz channels as well. It is only once these initial fields are sent and the VHT transmission starts that it can utilize the full width of the 40MHz (or larger) channel.

If it helps, think of the diagram as illustrating lanes on a road. If you want to send an "escort vehicle" ahead to clear the route, you would need to send one down all the lanes at the same time to ensure they were clear and that no other vehicle merged into the "gap."

For bonus points, I should point out that 802.11 doesn't really transmit bits, it transmits symbols. The number of bits transmitted per symbol is determined by the modulation and coding rate. So in a sense, bits are always stacked on top of each other. For example, 802.11a BPSK R=1/2 (6 Mbps data rate) can transmit 24 bits per symbol.

  • THANK YOU VERY MUCH! When we say the same initial fields are duplicated and sent out over the successive 20 MHz do we mean there are two packets now (each on one of the 20MHz) instead of one? @Ylearn
    – Tyrone
    May 20, 2015 at 5:08
  • Yes, the frames would be duplicated and sent on the successive channels. However it is within the capabilities of an 802.11ac device to do transmit all of these frame concurrently as they are occupying different frequencies.
    – YLearn
    May 20, 2015 at 5:10
  • Sorry if this question is stupid, when you say frame is it similar to packet?
    – Tyrone
    May 20, 2015 at 5:11
  • Technically, I really should have said symbols as this is really about the PHY or L1 framing. Generally speaking, symbols/bits are used in reference to L1, frames are used for L2 and packets for L3.
    – YLearn
    May 20, 2015 at 5:13
  • its weird because whenever I read a book describing the physical layer they refer to packet structure for example packet structure of IEEE 802.11 ac. Just wondering the frame of the MAC layer would be enbedded in the date part of this packet structure of the PHY?
    – Tyrone
    May 20, 2015 at 5:26

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