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Author Topic: Building a rack-mount multi-line modem "appliance"  (Read 562 times)

DaveC

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Building a rack-mount multi-line modem "appliance"
« on: July 29, 2018, 09:23:47 PM »

I have two VDSL lines at home, and for a while now, I've been contemplating putting together a rack-mount modem "appliance", using a 200/250mm deep 1U 19" enclosure such as this:



Contents would be:
  • 2 x HG612 PCBs
  • 1 x 5-port managed switch (to combine the PPP and managment LANs over VLANs)
  • Maybe a raspberry Pi (for monitoring purposes)
  • A single PSU to power all these components (preferably internal)
It would just need two RJ45 connectors mounted on the front panel - one to take the DSL lines (I already have the incoming lines connected to a port in my patch panel via a single CAT5e cable from my master sockets, so would split the pairs out internally), and another to present the ethernet to connect to my router (with the management LAN and PPP interfaces on their own VLANs).


The main reason is to tidy up my mess of a comms/data area (the cupboard under the stairs) and reduce the number of individual PSUs, and am wondering what the pros and cons of such a device would be.

I'm assuming putting the modem PCBs in a larger enclosure would help with heatflow, and I could probably further assist this with a fan if needed.

Anyone have any suggestions for how to power this?  I'm worried about introducing more sources of interference, especially if I chose an internal PSU. 

One solution would be a standard "flex ATX" 1U server PSU, which would give me a variety of 12V and 5V outputs.  e.g.

http://www2.seasonic.com/product/ss-250-su-active-pfc-f0/

Would that be a bad idea?

I have a strong VDSL signal (both lines achieving around 80/20), so am not that worried about eeking out the maximum performance.  But I also don't want to make it any worse than it is already.


Any thoughts welcome.
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Weaver

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #1 on: July 30, 2018, 04:37:39 AM »

What a superb idea. I have three hopefully soon to become four modems that are going onto the wall, so the wall plus velcro is my cab. ;-)

That PSU is switch mode; out of interest, see Konrado's recent article about switchmode PSUs potentially knocking out various upstream frequencies. (I would need to dig to find it; it was posted in the last week or two). Your dsl signal is so strong though that you do not have to worry about dc quality like I do, at 65dB d/s attn. For supreme dc quality the answer would be a lead acid battery with a trickle charger, that is what I need.
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roseway

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #2 on: July 30, 2018, 07:39:16 AM »

I don't think I would use an HG612 for this project. It's getting old, and hasn't been manufactured for several years, so I wouldn't want to trust its future reliability.
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DaveC

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #3 on: July 30, 2018, 12:30:32 PM »

I don't think I would use an HG612 for this project. It's getting old, and hasn't been manufactured for several years, so I wouldn't want to trust its future reliability.

I understand your point, but I'm very happy with the HG612 and have no reason to doubt its reliability.  I currently have 3 of them (2 in use, one spare), and intend to buy a 4th so I can build this device without disturbing my existing lines.  So I will then have two spares to swap in in the future.

What would you suggest as an alternative?  I have one VMG1312-B10A laying around (supplied by AAISP with one of my lines), but have never tried it.  Although now a 1500 MTU firmware is available, I may give it a go.  However, this seems much more expensive and harder to find than the HG612s.

Thanks.
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j0hn

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #4 on: July 30, 2018, 12:48:37 PM »

and newer, and better built, and performs considerably better.

I read this yesterday and my initial thought was "good idea but I would use 2 x zyxel 1312-b10a's.
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johnson

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #5 on: July 30, 2018, 01:03:27 PM »

Well the capacitor plague of the 00s is long gone now, so if properly cooled I can see HG612s lasting longer than you care about the appliance... so maybe go with them.

There must be enough space in one of those rack mount cases that you can allow for a decent range of boards, few more stand offs in the right place and others could be drop in.

If I was making this kind of thing right now I'd probably go with 8324/8924 boards with attention paid to heatsinking them, or maybe 3925s, same chipset just a bit more finagling. I guess whatever is cheapest to buy 4 or more on the auction site.

Keep a look out for noctua fans that fit the bill if you want to active cool it as well.
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DaveC

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #6 on: July 30, 2018, 01:37:19 PM »

Thanks both. 

I think my conclusion is that I just need to make sure I build it in such a way I can easily change the modems in the future - the HG612s are currently working well for me (achieving 80/20 on one line, 79/20 on the other, and rock-solid), so there is no immediate benefit to the expense of replacing them - I don't need anything better.

I will have a play with the B10A though.

My main doubt is still the PSU.  Does anyone have any advice regarding powering this device?  It will need 3 x 12V for the modems and switch, and probably a 5V for the Pi.

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johnson

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #7 on: July 30, 2018, 01:56:19 PM »

Well that seasonic you linked would certainly work and probably be really low noise for a switchmode supply of its size, but its massive overkill for the 3/4x5W these devices would need.

Not the most straightforward, but again if I was making it myself, a fairly small toroidal transformer, some rectifier diodes, capacitors and either a linear or switchmode regulator IC and you could make a really low noise supply given the modest power requirements, for cheap. Obviously a ton more work that just buying something off the shelf, but I wouldn't know where to find a super low noise 50W 12V and 5V supply... maybe you could find a good chinese retailer, but its hit and miss, and meanwell supplies are expensive.

Edit: apologies for veering into hobby electronics... maybe a switchmode powerbrick of reputable origin might be better. 
« Last Edit: July 30, 2018, 02:12:45 PM by johnson »
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Weaver

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #8 on: July 30, 2018, 03:56:19 PM »

As the article referred to earlier points out, if you have to have a switchmode psu it just has to be one with a high switching frequency (and is therefore rubbish: hot, inefficient, limited mtbf) so as not to get hit by the odd harmonics of the switching frequency. That article uses the example of 250kHz switching which zaps the fundamental and then really stuffs the 3rd harmonic at 750kHz completely, down to almost zero bits bit-loading.

So linear ones are one less thing to worry about. But Burakkucat et al suggested a battery and that remains my dream.

I would love to know how you get on. If you find a reasonable very high quality brick type PSU then I would love to know about it because I could test one agains the standard ZyXEL article to see if it makes any difference at all using my extreme sensitivity setup (from the otherwise quiet environment combined with ultra-low signal levels).
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burakkucat

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #9 on: July 30, 2018, 05:50:36 PM »

A quick note about the pedigree of the HG612.

When Huawei were approached for a suitable CPE device they decided to take their existing HG610, remove two LAN ports, remove the power switch, add the BBU port and fit it into an Openreach branded white case . . . and so the HG612 was born. Look at the PCB and you will see that it is screen-printed as an "HG610".

If you can find a source of HG610s (no doubt there is an agent, somewhere in China, who could ship you a case of fifty), they would be essentially the same device when deployed in bridging modem mode.
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DaveC

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #10 on: July 30, 2018, 06:45:25 PM »

A quick note about the pedigree of the HG612.

Interesting background, but I'm not sure what you're suggesting - are you arguing against me choosing the HG612 and saying I should instead import 50 HG610s?   ;D

I seem to be the only one on this forum that likes the HG612!

I have to admit to knowing very little about electronics, and even less about power supplies, so am keen to have any other thoughts on that.  As it stands, I may just go with the easiest solution (something like that seasonic I linked to), even though it doesn't sound like the most suitable.
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burakkucat

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #11 on: July 31, 2018, 12:42:19 AM »

If you are happy to use HG612s -- good. If you can source some HG610s they would be just as good.  :)

I have a HG610, a HG612 and a HG622 stored in the grotto. They are used for certain tests & projects but are not routinely used.

For a power supply, what features would you like?

When the public AC mains fails, would you like to keep your Internet access alive? (Remember the DSLAM has back-up batteries, so that won't go down for around eight hours.) If yes, then you need to invest in one (or more) SLA batteries to provide 12V DC. That battery would be floated across the output of a classical transformer, bridge rectifier, capacitative/inductive/capacitative Pi-filter set-up. Normally the AC mains to the transformer powers everything and keeps the battery fully charged. When that supply fails, the battery keeps the equipment energised.
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DaveC

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #12 on: July 31, 2018, 01:53:14 AM »

I've been using a couple of HG8240 BBUs with my pair of HG612s, but loss of power is so rare in my area, they are not really worth it.  I can't remember the last time we lost power.

My network also relies on too much other stuff to work.

So whilst it's nice for the modems to maintain sync, they can't actually be used, so I'm happy to sacrifice that functionality.

I don't want anything complicated, just something that will work as well as the manufacturer-supplied units, but as a single unit I can (preferably) mount inside the enclosure.  Although I'm happy to be convinced an external brick would be better.

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Weaver

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #13 on: July 31, 2018, 03:06:47 AM »

With respect to all contributors, we are getting side-tracked. Any suitable modem power-brick should be fine given your excellent signal. Trying very high quality power sources such as a lead-acid battery or posh linear or switchmode psu would just be an experiment to see if there is any chance at all of seeing a slight improvment. That might work for me, but I would say you are the last person to see any benefit out of trying different power supplies.

You have a great idea, just go with what works. The HG612s are a bit old and old means possibly unreliable, so there are very good modems around such as the ZyXELs which are costly but very strong performers, again though it may be that you cannot possibly gain anything by choosing a stronger performer. But old devices that could prove unreliable could be a bit of a pest so you might consider going for something brand new.
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DaveC

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Re: Building a rack-mount multi-line modem "appliance"
« Reply #14 on: August 07, 2018, 08:43:55 PM »

Thanks Weaver for the encouragement!

I've now bought most of the components, with the rest due to arrive this week.

For PSU, I've gone with a Meanwell LRS-100-12 (12V, 100W) - hopefully it will work OK, but at 17 wasn't a big investment if it proves to be a bad choice.

I've heard the warnings about the HG612, but never suffered with reliability, and now have two spares (and could cheaply buy more).  The main reason I'm sticking with it is the cost (they are now very easily available at 11 on ebay).

BTW, looking at the PCB of one of my HG612s (all 3Bs), it has "HG612BXV VER.B" printed on it, not HG610.  Maybe that was just an earlier revision.

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