AirTime
Fairness on Mesh Aps feature is very close conceptually to the ATF feature
support release for Local Aps in theprevious releases. We are strongly
recommend to review that feature and deployment steps in the guide at
http://www.cisco.com/c/en/us/td/docs/wireless/technology/mesh/8-2/b_Air_Time_Fairness_Phase1_and_Phase2_Deployment_Guide.html
At the present
time, enterprise class, high density stadium and other major Wi-Fi deployments
with Cisco IOS 11n, 11ac Indoor APs are benefited by "per SSID" based Airtime
Fairness and "per Client within a SSID" based Airtime Fairness through 8.1 MR1
and 8.2 releases.
In a same way,
currently, there is a demand from the Customers with large scale Outdoor
wireless mesh deployments to serve their users by providing fairness among the
Wi-Fi users across the Outdoor wireless mesh network in utilizing the AP radio
Airtime downstream and also provide administrators the key control to enforce
SLA (implied on multiple cellular operator through Wi-Fi hotspot) on the Wi-Fi
users across the Outdoor wireless mesh network. However, since all Wi-Fi users
traffic is bridged between MAPs and RAPs through the wireless backhaul radio
and there is no SSID concept on wireless backhaul radio for backhaul nodes to
enforce policies through SSID's for each backhaul node, there is no easy
solution for Wi-Fi users across the Outdoor wireless mesh network to get
treated fairly in terms of utilizing the Wi-Fi airtime through their Outdoor
Wireless Mesh Aps. As far as the clients on client access radios are concerned,
it's fairly simple to regulate the airtime fairness through SSIDs (w/ or w/o
client fair sharing) in a similar way how it is done for Cisco unified local
mode APs.
Before the
solution overview of supporting ATF on mesh, lets quickly recap ATF - Airtime
Fairness (ATF) is basically a concept which provides an ability to
regulate/enforce the AP radio airtime in downstream direction for the clients
associated through the SSID's. As a result, the Wi-Fi users on wireless network
are fairly treated in terms of utilizing the radio WiFi radio airtime. This
basically provides the key control either to enforce SLA additionally or simply
to avoid certain group or individual from occupying an unfair amount of WiFi
airtime on a particular or on a given AP radio. A service level agreement (SLA)
is a contract between a service provider (either internal or external) and the
end user that defines the level of service expected from the service provider.
SLAs are output-based in that their purpose is specifically to define what the
customer will receive.
In general, in
the Mesh architecture, the Mesh Aps (Parents, child MAPs) in a Mesh Tree will
be accessing the same channel (let's forget about extended sub-backhaul radios
for a minute) on backhaul radio for mesh connectivity between Parents and child
Maps. Whereas, the Root AP will be connected wired to the controller and MAPs
will be connected wireless to the controller. Hence all the CAPWAP, Wi-Fi
traffic will be bridged to the controller through the wireless backhaul radio
and through RAP. In terms of the physical locations, normally the RAPs will be
placed at roof top and the MAPs in multiple hops will be placed some distance
apart within each other based on the Mesh network segmentation guidelines.
Hence each MAP in a Mesh tree can provide 100% of their own radio airtime
downstream to their users though each MAP accessing the same medium. To compare
this in non-mesh scenario, where there can be neighboring local mode unified
Aps in the arena next to each other in different rooms serving to their
respective clients on the same channel with each providing 100% radio airtime
downstream. Therefore, ATF has no control over enforcing clients in two
different neighboring AP's accessing the same medium. Similarly, it's
applicable for MAPs in a Mesh tree. For Outdoor/Indoor Mesh Aps, Airtime
fairness must be supported on client access radios which serve regular clients
as same as how we currently support ATF on non-mesh unified local mode Aps to
serve the clients and additionally it must also be supported on backhaul radios
which bridge the traffic to/from the clients on client access radios to RAPs
(one hop) or through MAPs to RAPs (multiple hops). Its bit tricky to support
ATF on backhaul radio's using the same SSID/Policy/Weight/Client fair sharing
model. Since backhaul radio's doesn't have SSIDs and it always bridges traffic
through their hidden backhaul nodes. Henceforth, on the backhaul radios either
in RAP or MAP, the radio airtime downstream will be fair shared equally based
on the number of backhaul nodes. This approach eliminates the problem and
provides fairness to users across wireless mesh network in the case where the
clients associated to 2nd hop MAP can stall the clients associated to 1st hop MAP where
2nd hop MAP is
connected wireless to 1st hop MAP through backhaul radio though the Wi-Fi users
in the MAPs are separated by a physical location. In the scenario, when a
backhaul radio has an option to serve normal clients through universal client
access feature, ATF considers the regular clients into single node and group
them into it. It enforces the Airtime by equally fair sharing the radio airtime
downstream based on the number of nodes (backhaul nodes + single node for
regular clients). We will see more details how this solution is turned into
design in the next sections.
A bigger mesh
design will looks like this: