Mesh Network Performance Impact

I've been "noodling" (yes, I scientific term, I assure you) on Wi-Fi mesh networking of late. There exists this "rule-of-thumb" that is often quoted in Wi-Fi circles that goes something like this:

For every mesh hop (after the first hop), throughput degrades by 50% [or more] per hop.

Not being the type to take statements like that at face value, I dug in and did a bit of research. The statement, on its face value, is very plausible. Given a single-radio mesh backhaul where all ingress and egress traffic must share the same frequency, throughput will degrade because each frame transmission must be repeated to the next hop on the same channel.

Note - I will not discuss multi-radio mesh backhaul solutions that use separate radios and frequencies for simultaneous ingress and egress transmission within the mesh network. These solutions are more mature and do not result in such dramatic throughput degradation.

This is very logical, but in my estimation, also a bit misleading.

My line of thought went something like this:

Yes, the 2nd mesh hop results in a 50% throughput degradation because each frame must be transmitted a second time across the backhaul from the 1st hop to the 2nd hop. Hence 50% loss.

But then we get to larger mesh networks, which have 3 or more hops. If a frame must be transmitted across 3 hops with all backhaul radios on the same frequency, then the total throughput should be 1/n, where 'n' represents the number of mesh hops the frame must traverse. The corresponding throughput degradation is 1 - 1/n. This results in 1/3 or 33% of original throughput, resulting in 66% degradation.

However, the rule-of-thumb dicatates that throughput degradation in a 3 hop mesh network would be 50% to 2nd hop and another 50% to the 3rd hop, net 1/4 or 25% of original throughput, resulting in 75% degradation.

This is the gap in analysis that prompted my research. What should engineers be using as planning values for single-radio backhaul mesh networks? 50% loss per hop, or 1 - 1/n loss based on the total number of hops a frame must traverse?

Both Calculations are Valid [based on your assumptions]
After researching it, both throughput models are technically accurate based on assumptions made during the analysis and design.

Single-Radio Mesh Backhaul Bandwidth Degradation
(courtesy of Strix Systems)

The best-case scenario is 1 - 1/n degradation. This assumes the mesh hops are deployed linearly and can only hear their two neighboring mesh nodes, one upstream and one downstream.

The worst-case scenario is 1 - 1/( 2^(n-1) ) degradation, which is 50% loss per hop (our rule-of-thumb). This assumes that the mesh network is designed as more of a web of mesh nodes that can hear three, four, or more neighboring nodes for failover and self-healing capabilities.

Using either method, it's clear that single-radio backhaul mesh networks result in substantial throughput degradation and should only be used when alternatives such as wired networking or traditional wireless access point deployments cannot be used.

Best-Case Mesh Throughput Degradation
(courtesy of Strix Systems)

Conclusion
Although the single-radio mesh backhaul rule-of-thumb gives the desired impression of performance degradation when mesh solutions are discussed, it should be used cautiously for explaining mesh performance issues from a conceptual point of view.

However, for real-world network planning, use either method as a starting point, and then verify through testing in your environment.

Know you know :)

Cheers,
Andrew

Articles Worth Reading: 12/30/2010

Here are a collection of articles from the past week that I have found useful, interesting, or enlightening. As always, for a complete list of articles check out my shared article feed from Google Reader.


Unplugged – Show 4 – Too Many iPads – My Etherealmind: 

"#Content

* 802.11n adoption in the enterprise may be driven by iPads and iPhones today, and Android in the future. This may drive the greater of adoption of wireless networking generally.

* Will the prevalence of personal hotspot devices end up ruining enterprise guest networks? Guest networks are becoming important systems for external consultants and contractors and the wireless networks need to be more reliable. Discussions of some of the challenges around this.

* Fast roaming and why it’s important in an enterprise environment, especially with 802.1x/EAP authentication; forthcoming Wi-Fi Alliance Voice Enterprise certification.

* Keith R Parsons moving to Ruckus."

2010: The Enterprise Wi-Fi Year in Review - www.enterprisenetworkingplanet.com: 

"Thanks to 802.11n, there's a good chance 2010 was the year a wireless LAN became your default network access method. This retrospective from Wi-Fi Planet looks at the major deals, improvements in technology and changes in the market that pushed Wi-Fi ahead in the enterprise."

* I had the honor of contributing to this great article by Lisa Phifer.


Breaking GSM Security With a $15 Phone: 

"Whatever assurances have been given about the security of GSM cellphone calls, forget about them now."

802.11: Safety equipment for outdoor WLAN deployments: 

"Deploying outdoor network is hard. It is hard technology wise. One is bound by the limitations of the technology (the 802.11 standard), regulatory restrictions (FCC and the likes) and the environment itself (buildings, trees, hills, weather changes, etc). And once you have designed the network, you have to deploy the network."

dailywireless.org » AT&T Expands Public WiFi: 

"AT&T will expand Wi-Fi hot spots in New York’s Times Square just ahead of New Year’s celebrations and is also deploying its first hot spots in a public, outdoor area of San Francisco, the Embarcadero waterfront district."

How to Get Your Idea Approved - Amy Gallo - Best Practices - Harvard Business Review: 

"When you have an idea, proposal, or recommendation that you believe in, it's easy to presume that getting it approved will be a breeze. If you see how great the idea is, won't everyone else? However, whether an audience accepts an idea is often less about the idea itself than about how you present it. When you need approval, don't assume that just because it's brilliant, others will see it that way — convince them."

Layer 2 vs Layer 3 in Wireless Mesh: Do You Have to Choose? « Mesh Without Wires: 

"Layer 2 vs Layer 3 benefits and trade-offs have been a topic of discussion for a couple of decades as both approaches have their benefits and drawbacks."

Cheers (and happy reading),
Andrew