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.
802 is the number for the IEEE LAN/MAN Standards Committee, and 802.11 is the Wireless LAN Working Group.
The IEEE 802 committee maintains a web site, which lists the various current working groups within the committee.
Current:
- 802.1 Higher Layer LAN Protocols Working Group
- 802.3 Ethernet Working Group
- 802.11 Wireless LAN Working Group
- 802.15 Wireless Personal Area Network (WPAN) Working Group
- 802.16 Broadband Wireless Access Working Group
- 802.18 Radio Regulatory TAG
- 802.19 Wireless Coexistence Working Group
- 802.21 Media Independent Handover Services Working Group
- 802.22 Wireless Regional Area Networks
- 802.24 Vertical Applications TAG
- 802 5G/IMT-2020 Standing Committee
Hibernating:
- 802.17 Resilient Packet Ring Working Group
- 802.20 Mobile Broadband Wireless Access (MBWA) Working Group
Disbanded:
- 802.2 Logical Link Control Working Group
- 802.4 Token Bus Working Group
- 802.5 Token Ring Working Group
- 802.6 Metropolitan Area Network Working Group
- 802.7 Broadband TAG
- 802.8 Fiber Optic TAG
- 802.9 Integrated Services LAN Working Group
- 802.10 Security Working Group
- 802.12 Demand Priority Working Group
- 802.14 Cable Modem Working Group
- 802.23 Emergency Services Working Group
- QOS/FC Executive Committee Study Group
- ECSG TVWS TV Whitespace study group
- ES-ECSG Emergency Services Executive Committee Study Group
- OmniRAN EC Study Group
- Privacy Recommendation EC Study Group
Best Answer
The reason it is optional is that in 802.11 RTS and CTS are management frames. Management frames are sent at the lowest base/basic/required data rate supported by all clients associated to the ESS, which is typically much lower than the data rates used for normal unicast traffic.
The reason for this is that only the data rates that are configured as base/basic/required for the ESS must be supported by all clients connecting to the BSS (this means that if you want to support older clients the required data rates cannot include data rates from newer standards). Therefore those are the only data rates that can be used that are guaranteed to be able to understood by all clients.
This means that RTS and CTS frames will then use a disproportionate amount of "airtime" and make use of the spectrum much less efficient. As an example (very basic/rough), consider that you have a small data frame to transmit at 600 Mbps. However, the base data rate for the ESS is 12 Mbps (a moderately decent signal for 802.11a/g clients which most networks still support). The RTS and CTS frames may each take as much as 50 times the airtime of the data frame. This takes the total transmit time from one "time slot" to 101 "time slots." Much less efficient.
An alternative solution that has some of the benefits of RTS/CTS but is more efficient is the CTS-to-Self mechanism. This allows a wireless station to send itself a CTS frame, clearing the air for it's transmission. This doesn't alleviate issues such as the hidden node problem, but does decrease the impact on the efficiency.
Most networks do not utilize either RTS/CTS or CTS-to-Self as they can have more of a negative impact on performance than the problems they are trying to alleviate.