Cisco Mesh network, backhaul links, RTS/CTS

[vlad.mihailov]

Guys,

Do you know if there is a need to enable RTS/CTS mechanism for Cisco Mesh links (backhaul) or is there a protection embedded in the protocol itself somehow? Ie in the challenging terrains where hidden mesh nodes can affect communication is there a need (and is there a way) to enable CTS protection or does protocol has a defence mechanism in place?

Thank you!

[Scott Fella · Accepted solution]

There is no configuration to adjust for the backhaul. Interference will always be an issue with mesh or even bridges. You just have to look at how good or bad the air quality is. You can survey the area to see what channels are being utilized and how much of the channel is being used. This way you can adjust the backhaul channel.

Sent from Cisco Technical Support iPhone App

[vlad.mihailov]

True. I was asking more about "self-induced" interference, ie if links are relatively long (2-4km) and hop count is high (say 3-4) (and for the sake of argument distributed in to multiple directions around root AP) that will most certainly create hidden nodes in Mesh deployment. I checked the captures for the backhaul communications and didn't see any CTS packets therefore assumed they either don't exist or are hidden in the protocol. I also couldn't find anywhere in the documentation a reference to address this possible scenario.

Thank you for reply!

[vlad.mihailov]

Add terrain challenges - like RAP up top of the mountain and MAPs in valleys and the hidden node is going to surface :-(

[George Stefanick · Accepted solution]

Hi Vlad,

Here is the 7.3 Mesh Design guide.

http://www.cisco.com/en/US/docs/wireless/technology/mesh/7.3/design/guide/Mesh_chapter_0100.html

As for the CTS, thats a good question. I would expect to CTS frames, you are saying you dont see any in your capture ?

__________________________________________________________________________________________
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
__________________________________________________________________________________________
‎"I'm in a serious relationship with my Wi-Fi. You could say we have a connection."

[vlad.mihailov]

George,

Thank you for the link (quoted below). Somehow I didn't find that statement at my own :-( It is interesting as they are suggesting the RTS/CTS to decrease the impact of hidden node, but I didn't find how to do that. I will do some more digging.

I didn't see RTS/CTS packets in the data captures when I tried to lower the RTS threshold, but I tried this quite a while ago in 7.0.220. Probably I should try again in the newest firmware and run the capture again.

Thank you again for your help.

"If MAP X is the route back to the RAP for MAP Y  and Z, both MAP X and MAP Z might be sending traffic to MAP Y at the  same time. MAP Y can see traffic from both MAP X and Z, but MAP X and Z  cannot see each other because of the RF environment, which means that  the carrier sense multi-access (CSMA) mechanism does not stop MAP X and Z  from transmitting during the same time window; if either of these  frames is destined for a MAP, it is corrupted by the collision between  frames and requires retransmission.

Although all WLANs at some time can  expect some hidden node collisions, the fixed nature of the MAP makes  hidden node collisions a persistent feature of the mesh WLAN backhaul  under some traffic conditions such as heavy loads and large packet  streams.

Both the hidden node problem and the  exposed node problem are inherent to wireless mesh networks because mesh  access points share the same backhaul channel. Because these two  problems can affect the overall network performance, the Cisco mesh  solution seeks to mitigate these two problems as much as possible. For  example, the AP1500s have at least two radios: one for backhaul access  on a 5-GHz channel and the other for 2.4-GHz client access. In addition,  the radio resource management (RRM) feature, which operates on the  2.4-GHz radio, enables cell breathing and automatic channel change,  which can effectively decrease the collision domains in a mesh network.

There is an additional solution that can  help to further mitigate these two problems. To reduce collisions and  to improve stability under high load conditions, the 802.11 MAC uses an  exponential backoff algorithm, where contending nodes back off  exponentially and retransmit packets whenever a perceived collision  occurs. Theoretically, the more retries a node has, the smaller the  collision probability will be. In practice, when there are only two  contending stations and they are not hidden stations, the collision  probability becomes negligible after just three retries. The collision  probability increases when there are more contending stations.  Therefore, when there are many contending stations in the same collision  domain, a higher retry limit and a larger maximum contention window are  necessary. Further, collision probability does not decrease  exponentially when there are hidden nodes in the network. In this case,  an RTS/CTS exchange can be used to mitigate the hidden node problem."