Cisco Aironet 2702i AIR-CAP2702I-E-K9 Folheto

 

 

© 2014 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. 

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Compatibility and Coexistence of 802.11a, 802.11n, and 802.11ac Devices 

802.11a 

 

 802.11n device drops down to 802.11a 
PPDUs 

 802.11ac device drops 
down to 802.11a PPDUs 

802.11n 

 

 

 802.11n device drops 
down to 802.11n PPDUs 

802.11ac 

 

 

 

802.11a 

 

 For transmitted HT_MM PPDUs, the third 
party waits for the packet length as indicated 
in the legacy portion of the preamble, then an 
extra EIFS (so no collisions) 
 HT_GF PPDUs, preamble should be 
transmitted only if preceded by MAC 
protection (for example, RTS/CTS or CTS-to-
self) sent using 802.11a format PPDUs (so no 
collisions) 

 The third party waits for 
the packet length indicated 
in the legacy portion of the 
preamble, then an extra 
EIFS (so no collisions) 

802.11n 

 

 

 The third party waits for 
the packet length indicated 
in the legacy portion of the 
preamble, then an extra 
EIFS (so no collisions)  

802.11ac 

 

 

 

Furthermore, the preamble of the 802.11ac formatted packet is identical to an 802.11a formatted packet, so the 

CCA mechanism kicks in for third-party 802.11a and 802.11n devices. As soon as these third-party devices see the 

802.11ac preamble, they know the duration of the packet and know not to transmit during that time. Also, since the 

packet is typically followed by an Ack or Block Ack frame sent in an 802.11a frame, the third-party devices can 

correctly receive the Ack or Block Ack and then can continue to try to transmit as usual. In the worst case, a 

third-party device hears the 802.11ac frame but is out of range of the transmitter of the Ack or Block Ack. But even 

here the third party must wait for an extended duration (called EIFS) to allow time for the Ack or Block Ack to be 

transmitted without fear of collision. 

Because of this preamble-level compatibility, there is no need for 802.11ac devices to precede their 802.11ac 

transmissions by CTS-to-self or RTS/CTS. The kinds of inefficiencies associated with sending 802.11g packets in 

the presence of 802.11b devices are completely avoided at 5 GHz. 

IT administrators are in the fortunate position to be able to pick between two great technologies: (1) 802.11n with 

A-MPDU, MIMO, beamforming, and speeds from 65 to 450 Mbps within 40 MHz, and (2) 802.11ac with A-MPDU, 

MIMO, beamforming, and speeds from 290 to 1300 Mbps within 80 MHz. 

802.11n is available today and is sufficient for many customer use cases. 

802.11ac is the future of wireless LANs, but Wi-Fi-certified 802.11ac APs are not yet available. 802.11ac can 

provide full HD video at range to multiple users, higher client density, greater QoS, and higher power savings from 

getting on and off the network that much more quickly. 

Most IT administrators deploy new APs at the same time as they fit out a building or retrofit a space. For these, we 

recommend installing 802.11n APs today, because of the sheer value of 802.11n. Further, for investment 

protection, it is most desirable to install modular APs that are readily field-upgradable to 802.11ac. As 802.11ac 

APs become available, these users should start installing 802.11ac APs, since the incremental value of 802.11ac 

exceeds any reasonable price differential.