When should one be using 50/50 or 75/25 for ptp links?
I assume VS ePTP (no up/down ratio) ? I think the only time you would use a ratio is when you require Sync. Then all your synced radios have to be the same. Initially I assume you pick 50/50 if you expect the synced links to all need as much up as they do down (Pretty rare in my experience) or you set it to 75/25 if you need a lot more going one direction than the other (all our synced PTP is 75/25). I think with the newer firmwares there are options from 75/25 all the way down/up to 30/70 .
Sync has to have a ratio and the ratio has to be the same across all the radios you expect to be in sync with each other.
ePTP (or I guess older firmware/radio TDD Flexible was an option?) non-sync radios just run free, adjusting to whatever the demand is at the moment (not in sync with any other radios).
While in theory it’s nice that you have these options for sync between PtP’s and PtP and PtMP, in practice it’s not that useful because of the different TDD downlink/uplink ratios and the typical flow of WISP traffic.
Ex. You have a site that has a bunch of PtMP all sync’d using a 75/25 ratio, sure you could sync your PtP with that site, but do you really want to feed it with a PtP using a 75/25 split? Most residential serving sites use much more download then upload… so you’d actually want your PtP using 50/50 or even better 25/75… neither of which you can sync with your PtMP.
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I’m sure there are some pretty rare situations where you might want sync between PtP and PtMP on the same tower but I would think it’s very rare. I would think that in most cases the slave would be on the tower with the AP’s and the sync is for the Master back at the NOC so it doesn’t interfere with all the other Masters back at the NOC feeding still more towers.
We don’t even use the same channels for PtP as we do for PtMP. All PtP is DFS and PtMP is 5.1/2 and 5.7/8
In our case sync is mostly due to having, for example, one location with 7 PTP AP/Masters all with 40Mhz+ channels all at one location all with alternate channels set for DFS hits (real or imagined…).
We also have a couple of towers that feed multiple micropops. While the master PtP radios share the tower with several/many PtMP AP’s they do not share frequencies with them but they do share frequencies with each other and must be synced.
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I keep forgetting to reply to the OP’s post instead of just hitting reply to the last post, which was yours. So I wasn’t trying to actually reply to your post.
I guess now re-reading everything my post is actually irrelevant 
Thank you both for your posts… lol I feel like I am still at my original question… In my setup we have our NOC feeding our first tower via 550 link. Then that tower feeds the next with a 550 link and then one more to the last tower again with a 550 link. We are that situation where we need more download than upload for customers so our epmp 3000s are set to 75/25.
Now back to the 550 links, our first link is modulating 99% on both downlink and uplink… so PLEASE correct me if I am wrong but would you not want this link at 50/50? In PtP mode is the downlink and uplink working together to get as much bandwidth to the first site or I the downlink actualy represents download traffic and uplink represents upload traffic?
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TDD ratio’s allocate frame airtime %… so 50/50 would allocate 50% of each frame for download payload and 50% for upload payload. The amount of data you can fit into each payload is determined by the uplink and downlink modulation levels. So even if you had, again, example, a TDD ratio set to 50/50… if your downlink modulation is good, and your uplink modulation is poor, you might be able to achieve a payload maximum bandwidth of 50mbps on the downlink, but due to poor uplink modulation, only 10mbps on the uplink.
Okay, so then 75/25 for ptp lnks it is.
Yeah, if your AP needs more download than upload then (assuming you have to use sync on the PtP) the PtP feeding it needs to provide it more download than upload ( though down/up is relative to which end of the link you are talking about… one ends down is the other ends up). I’m pretty sure the overwhelming number of PTP links that have use sync are probably doing 75/25 (In your case I’m assuming the PtP slave radio is at the 3000 AP end of the PtP link feeding it).
E.G. 550M ---->550S—>3000AP then if another hop <–> 550M---->550S----> next site with 3000AP.
Am I correct you require sync on these backhauls ? If you don’t need sync then ePTP would be all around the best solution unless current firmware breaking it somehow (we don’t have any ePTP links but I know lots of others use it as much as they can).
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This is exactly how I have it setup. As per my RF guy, the way you stated my setup one would not need sync since I have one Master and one slave per tower. I would like to use eptp but I will be utilizing dual radios on my 550 links.
For future reference, if I would chose to branch off one of my towers and add another 550 link, would I just need to sync the 2 masters on that tower and if so what would you suggest using?
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Sync is only to prevent access points (or masters in a PtP) that use the same, overlapping or too near frequency from interfering with each other. So any backhaul masters or APs that can see each other and are using the same or over lapping or too near a channel need to be synced so they become invisible to each other. Normally you only use sync when you are reusing frequencies on AP’s / PTP Masters that can see other and it’s assumed their client radios are not able to see the other AP’s/Masters (except maybe at power levels low enough they don’t interfere).
It does not stop the clients of all these AP/PTP Masters from trashing anything that can see them. If you have two APs/Masters synced but the client from one of them can be seen by another AP/Master running in the same, overlapping, near frequency at high enough RSSI then that client can interfere with that other AP/Master and that AP/Master can interfere with that client.
If you don’t need sync the first advantage of not using sync is that sync locks you to, at most, 75% down (and 25 up). So, (and I’m using 100mbps just to make the math easy) if you have a link that can do 100Mbps total and you split it 75/25 that means your link can never do more than 75mbps down(tx) even if there are only a couple of bits being uploaded because that other 25mbps is reserved for uploading (rx) whether it is being used at the moment or not. If you did 50/50 then you would never be able to do more than 50mbps down regardless of the amount of up being used at any given moment.
If you are using ePTP (or TDD flexible if that’s still a thing) mode there is no ratio, nothing is reserved. The radio can use nearly all 100mbps for upload or download, whatever the moment demands.
Also I believe ePTP should have noticeably less latency since doesn’t have to wait on specific timing to tx or rx.
We only have 1 550 link and it’s not bonded. I don’t know what the rules are on that but I would assume sync doesn’t help here since both client radios at the other end are going to see both Masters and vice versa so I’m guessing different , non-overlapping with possibly larger than normal guard channels are required here ? I’m just guessing though I have no idea what the rules for bonding 550s are.
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