In recent times I have come across the question ‘Why do we need m-gig AP backhaul’. I have seen a lot of Wi-Fi experts quickly respond to that question saying ‘You’ll be fine as is and your AP backhaul will not exceed 1Gbps’. I have even heard statements like ‘it is impossible’ to exceed 1Gbps AP backhaul capacity and they back that up with numerous points around 160 MHz channels, 256 QAM etc (some facts, some approximations). I am not challenging anyone’s views but I won’t be so quick to jump to the conclusion that ‘You’ll be ok without m-gig AP backhaul for 802.11ac wave 2’. Here is my personal opinion about the big buzz around Multi-Gigabit for AP backhaul.
I am going to try and use less mathematics or any fancy simulator tools that throw out real/theoretical numbers at you. Frankly speaking its not a 'one size fit all’ model. These numbers (explained later) some people talk about may or may not apply to your deployment. Ok enough of my blabbering, lets dive right into it.
We already see 4SS SOHO APs out in the market with Enterprise grade 4SS 11ac Wave 2 APs ready to roll out very soon. They will indeed support the mystical 160 MHz channels with a theoretical (as most people like to refer
Lets talk more sense; lets not talk about 160 MHz channel at all from this point on. Now coming to the second impractical data around 802.11ac which is 256 QAM. All those who know what 256 QAM means will know that its rather difficult to sustain. You require very clean RF (will avoid going into SNR and receiver sensitivity etc) to achieve and maintain 256-QAM. Lets say the 11ac client does land up achieving (in a RF isolation room, low density area
Lets look at a more real world scenario i.e. 80 MHz channels. MCS 9 for a 4SS 11ac AP / Client using 80 MHz channel approximates to 1.7 Gbps link speed, at about 65-70% throughput efficiency it is 1.1 Gbps (.65 x 1.7 Gbps). Does this warrant m-gig backhaul? Maybe not just yet right? As most of you will point out mixed-client environment and less than optimal RF conditions which make it difficult to reach MCS 9.
Lets rate shift down to MCS 7 at 80 MHz which is 1.3 Gbps. Doing the math again for 65-70% throughput efficiency we arrive at approximately 850 Mbps (hurray!!). We finally arrived to an over the air data rate value which will not over subscribe or stress the 1 Gbps AP backhaul. This is about 85% of your existing AP backhaul capacity.
802.11ac MCS rates table
When most people explain these 'over the air' data rate / link speed numbers they are only talking about ‘upstream’ traffic from the wireless client to the AP. What we don’t consider are things like unwanted downstream traffic hitting the AP port and potential inside the AP packet processing drops. Speaking out of experience (4 plus years in Wi-Fi support) I’ve seen scenarios where downlink traffic other than traffic destined to the wireless clients hits the AP port. Link local broadcasts are very common these days with so many mobile devices invading the Wi-Fi space. These protocols are very chatty and can quickly add up to few tens or hundreds of Mbps. I have personally been involved in troubleshooting issues where the AP port is stressed in terms of traffic ( > 1Gbps) causing packet drops. I do agree that preventive measures can be taken and networks can be designed properly to avoid such cases but more often than not networks are not designed so accurately.
Moving on to MU-MIMO; this is the capability of the 11ac AP to make use of unutilized spatial streams for other clients. What this means is that a 4SS AP can ideally serve
- Two or three 1SS client (usually mobile devices); with 4th radio used for beamforming
- Two 2SS clients
- And more!
This will likely make the AP utilize its top end link speed of 1.3 Gbps more often
Although MCS 8 and 9 may not be achieved 100% of the time, they will come into play some % of time the client remains associated with the 11ac Wave 2 AP.
There are no 4SS 11ac clients in the market at present but it is likely that will change in the near future. We already have 3 SS 11ac clients which can achieve 975 Mbps at MCS 7. This is where MU-MIMO would come in handy to best utilize the AP’s top end link speed.
Above data is only the contribution by 5 GHz radio, what about the 2.4 GHz 802.11n radio? That will add an average wireless traffic of about 100-150 Mbps.
This m-gig dilemma is somewhat similar to the time when 11n hit prime time and was shipped with GigE ports instead of FastEthernet backhaul. How many deployments do you see out in the field where 11n APs are plugged into FastEthernet ports? Probably only a handful. This is why it maybe a good idea to have m-gig switches for 11ac wave 2 APs which will ship with some sort of m-gig capability.
Now lets consider the mixed-client environment. Although older generation (11a/g, 11n etc) Wi-Fi clients are almost always present in the environment but there are a lot of scenarios where you have a mini greenfield deployment if you consider BSSID level coverage. I see deployments like classroom environment which just have latest generation macbook pros or iPads, or office spaces where you only have the latest generation windows / mac laptops which are all 11ac capable.
If you are pondering over the question, ‘will applications exceed the 1 Gbps mark?' There are several use cases where large amounts of data needs to be exchanged over Wi-Fi (in excess of 1 Gbps), and the data demands continue to grow at a rapid pace.
For most of us cost tends to play an important role in decision making. Highlighted are a few areas of investment to be considered for 11ac wave 2 and m-gig technology:
- Cabling cost
- 11ac wave 2 APs
- M-gig capable access switches
A typical refresh cycle for AP is around 4-7 years. When it comes to access switches the refresh cycle is even longer 7-9 years. The point I am trying to make here is if its time for an upgrade or refresh, its a no brainer to be ready for 11 ac wave2 full theoretical capacity (6.9 Gbps) and m-gig capable switches.
For folks who are still on 11n and are thinking of making the transition to 11ac, again the choice is simple, skip 11ac wave 1 and directly adopt 11ac wave 2 as thats a more logical thing to do. This would apply to the SMB markets as well.
Below is a chart showing the proposed channels to the existing 5GHz spectrum which will make the use of 160MHz channels more feasible.
source: FCC
Both these technologies are here to stay and things are just starting to get exciting!
I’d love to see your comments on my views.
1 Comment
