How to Optimize a Canopy network using advanced timing/sync.
Posted by Guruh Mandala, Last modified by Guruh Mandala on 06 August 2009 07:00 PM
How to Optimize a Canopy network using advanced timing/sync.
I often get asked, why does Canopy need a GPS or CMM or timing module?
I also get asked, what is Canopy Sync?
Or I am told repeatedly if you set your AP?s to the same settings everywhere then you are ?synced?.
I swear I type a response to these kinds of questions about once a week and I hope you all don't mind if I use this forum as a repository for this information.
I?m creating this thread so I can refer people to it in the future instead of retyping it over and over again. It will also help to be able to edit and update it so it?s current and correct. If you see something wrong or wish to expand please post to it!
To start this off lets be sure to clarify that Canopy doesn?t need sync to run.
But if you want to take advantage of scaling your canopy network beyond a few units in a cluster or area you need to sync.
Sync is a generic term for saying that Canopy radio units will not broadcast while another (in band) unit is listening. This is how Canopy can set up six AP?s in a cluster inches from each other and not self interfere.
Sync starts by obtaining a pulse from GPS Satellites. But instead of using the coordinate features it creates a simple pulse just like a metronome. Nothing really complicated about it. But it?s the same pulse (generally speaking) that all of the units receive within the area whether they be inches apart or miles apart. In short that?s all the timing device attached to the GPS does, is send the pulse to the AP(s).
It?s what the AP?s do with the metronome pulse that is important.
Three settings affect the timing when the AP stops broadcasting, waits for a while (guard time) and then starts listening to its SMs.
Those settings are Control Slots, Max Distance and Downlink Percentage. Another setting that some may still be using, but that has been deprecated in newer modules is Scheduling (Hardware vs Software). We'll not get into that though.
The Frame Calculator tool (under Tools on most Canopy FSK modules) was originally used to convert old systems from software scheduling to hardware scheduling. But most current networks are hardware scheduled. The Frame Calculator tool can be used to optimize the network instead.
It is common practice and recommended in the manual that all AP?s in a cluster have the same settings for Control Slots, Max Distance and Downlink Percentage (in the same band). This is the easiest way to make sure the whole cluster of AP?s is sending and then stopping and then listening to their respective SM?s at the same time.
If one AP has different settings it is possible that it could be broadcasting while the neighboring AP?s are trying to listen to their SMs. In this case, who does the listening AP hear? Is it the SMs out miles away or the neighboring AP broadcasting inches away? It's going to hear the AP yelling in its ear right next to it of course. This is why it is so important to make sure the APs are synced.
This same problem can occur with multiple AP clusters or sites even miles away, especially in the 900MHz band. So if all APs in a geography that can ?see? each other all have a GPS device and the same three settings then they shouldn?t have problems listening to their respective SMs.
BUT, lets say in a typical cluster of six APs you have one AP servicing clients 10 miles away and the rest of the APs clients are only 2 miles or closer. According to the setup we?ve discussed thus far you would need to set the Max Distance to 10 for ALL APs to assure they are synced and timed correctly.
It turns out that the further out the SMs are, the less throughput you get (speed of light etc). The difference between 10 miles and 2 miles may be significantly less agregate throughput for the 10 mile AP (like mabye 1-2Mbps).
Do you want to regain that throughput for APs with closer clients? YES! But how do we do this?
The Frame Calculator is the answer.
In short, we use the three settings like variables in an algebraic equation; change one and adjust another to compensate.
So if Max Distance is 10 miles on one and 2 miles on another, we can change Control Slots or Downlink Percent to compensate and end up with a synced system.
If you look at the variables, the most granular one appears to be Downlink Percentage with a range of a little shy of 100 units. Control Slots has 15 units and Max distance depends on the Band but around 30-40 units. The variable that might be the easiest to compensate for a change in distance is Downlink Percentage.
The Frame Calculator has all of these settings, but you might have to change Scheduling from Software to Hardware and save changes to get the Control Slot or Control Half Slot setting (instead of three of them).
If you don?t know what the option means in the Frame Calculator then probably leave it at default.
Let's create an example cluster with 3 Control Slots, Downlink Percentage of 75 percent and Max Distance of 10. Input those into the Frame Calculator (don?t worry, it?s just a tool and won?t change anything on the SM/AP/BH that you are on) hit Apply Settings first, then click on Calculate you should see some results on the bottom of the page.
Ignore any warnings or errors in the section when Apply Settings is hit, it doesn?t mean anything. The results might vary a bit depending on the software version. And don?t do this with versions between 8.0 and 8.2.7 because the tool wasn?t working correctly. Y
ou should see something like this:
1. Total Frame Bits : 25000
2. Transmit Details :
3. Total Frame Overhead Bits : 3401
4. Data Slots (Down/UpLow/UpHigh) : 53/17/0
5. AirDelay (Actual/MaxRange) : 0/1080
6. Approximate distance (Actual/MaxRange) : 0.00 miles (0 feet) / 10.02 miles (52920 feet)
7. Uplink Rcv SQ Start : 17027
8. Uplink Rcv SQ End : 24837
The important number here is 17027 next to the Uplink Rcv SQ Start. This is the bits/time that the AP starts to listen to its SMs.
This number must be the same or as close to the same as you can get for all APs in band in a geography (that can see each other).
In our example we want to optimize the other 5 APs in the cluster to 2 miles instead of 10 miles and squeeze out some extra throughput from them!
To do this, and stay in sync/timed with the other AP, we use the Frame Calculator to adjust another variable.
So go back up to the Max Range (Distance) setting and change it to 2 miles. Hit Apply Settings and then Calculate.
You should get a different Uplink Start Time of 17633.
This is quite a bit later in the frame and there is a good chance that the AP is going to be listening while the others are still broadcasting to their SMs!
We need to adjust another variable to get back to 17027 with a miles distance of 2 instead of 10.
Generally I just start changing the Downlink Percentage variable now. Let?s change the Downlink Data (Percentage) to 74 percent and see what happens. Hit Apply Settings and then hit Calculate again.
Now our Uplink Start is at 17339. We are getting closer!
Let?s try 73 percent Downlink. Change it, hit Apply and don?t forget to hit Calculate.
Hey, it did NOT change; it?s still at 17339, why?
You will notice that some changes don?t make a difference in the Frame Timing. Max Range (Distance) sometimes needs to change a few miles and then suddenly it will change the uplink time. It?s the same thing with Control Slots. There is an explanation for this, but I?ll leave that for another write-up.
Let?s try 72 percent Downlink. Don?t forget to Apply and Calculate.
Ah, now we are back to our original Uplink Receive Start time of 17027!
You have successfully calculated a new set of variables that matches the other AP at 10 miles!
So now go to the other 5 APs in that cluster, change their Max Distance to 2 miles instead of 10 and their Downlink to 72 percent instead of 75 percent.
The whole cluster will still be in sync and timed, but you?ve just gained a significant amount of throughput in those 5 APs and their respective SMs.
But what about the downlink percentage changes from 75 to 72? Often you will find that the gain in aggregate throughput compensates for the small shift in less downlink. So maybe before at 75 percent at 10 miles you were doing 8Mbps down by 3Mbps up, and with 72 percent downlink and 2 miles your SMs are able to do 9.5Mbps down by 4Mbps up.
Max Distance (Range) is just one example we use often to optimize. Another would be Control Slots.
If you start out an AP in a cluster or new location you might consider using a smaller control slot like 0 or 1.
Then as customers grow on that sector or start using more upload you might increase your control slots. As you do that, you need to keep the same Uplink Receive Start time (17027 in our example) and adjust the downlink percentage to compensate (again).
The reason being the more control slots used the less aggregate bandwidth the AP has. But sometimes you need to use more control slots to get the traffic flowing and sacrifice a bit of throughput.
I hope this information is useful to those of you running Canopy systems and wish you the best of luck optimizing and making the most of your Canopy networks!
Plug for Motorola Technical Training:
There are a lot more details to this timing stuff and more in the official Motorola Canopy Technical courses CPT200 and CPT300. The courses have nice graphical slides and labs to illustrate these concepts. The classes also go into a lot more detail about the frame structure and optimization. So if you want to learn more cool stuff like this, look up the class schedule on the Motorola web site or contact me and I can point you to more information.
Motorola CPT200: ?www.motorola.com/content.jsp?glo???tId=8922
Motorola CPT300: ?www.motorola.com/content.jsp?glo???tId=9540