Author Topic: Why Does A Cable Modem Take So Long To Boot?  (Read 2429 times)

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Offline TomS_

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Re: Why Does A Cable Modem Take So Long To Boot?
« Reply #25 on: October 13, 2019, 09:20:32 am »
While it very well runs Linux under the hood, I would suggest that Linux comes up very quickly. Many of these broadband devices run a "busybox" style Linux. Its incredibly minimalistic, heavily integrated, no-frills etc because they would typically run on very minimal amounts of CPU power, RAM and internal storage. You simply dont build such mass produced consumer devices with gigabytes of RAM or multi GHz processors capable of booting a full scale Linux distribution.

I would guess that easily within a minute of power on you can access the web interface of the router itself, as they do tend to boot very quickly. But access to the Internet is then reliant on the modem getting itself online, and I think MarkL gave a really good explanation of what is going on there.
 

Offline Berni

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Re: Why Does A Cable Modem Take So Long To Boot?
« Reply #26 on: October 13, 2019, 09:39:21 am »
I don't know about your modems but a lot of the DOCSIS cable modems i had would deinitialize the Ethernet port after about 5-15 seconds into the boot process. This is seen because the router thinks the cable got unplugged (LEDs go out on router and modem, logs show link down etc..) and it comes back a few seconds later. So im pretty sure i couldn't be in the web interface if the ethernet port itself goes down during the process.

My Philips TV is also a good example of it. It takes >30 seconds from cold boot to displaying a image over HDMI, during that process going trough two static boot screens (Likley one being uboot/linux screen and the other being the TV application). Manufacturers simply don't bother.
 

Offline Chig

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Re: Why Does A Cable Modem Take So Long To Boot?
« Reply #27 on: October 21, 2019, 01:56:35 pm »
I have direct knowledge here - i design HFC/DOCSIS networks.

As MarkL Said:

Once the OS is booted, the RF can take a while to come up.  The modem might have to search a large frequency space for a DOCSIS downstream carrier, and once found, perform a process called "ranging" where the modem starts transmitting at low power during specific ranging opportunity slots, while incrementally increasing power until the headend can hear it.  Once the headend hears it, exact timing and power levels are set in another set of exchanges, the modem is authenticated, and then finally configured with and IP address and other provisioning information.

The most likely *actual* cause of this delay is that the firmware loaded on the modem doesn't have a scan list that is matching with atleast one of the frequencies used by downstream signals from the CMTS. In this instance, the modem has no choice but to blind scan for downstream channels after it's checked it's ~30 default scan channels. Depending on how smart the modem is, there's a good chance it'll have to find one of the 16 - 96 channels used by your ISP, exactly. And that's assuming a single ISP cable network - some networks have multiple carriers, and in that instance if your modem finds NOT your ISP first, it'll go through, try to sync, then fail, and start again when it gets booted off (i've done this myself, with 3 x CMTS's on a single cable segment).

The ISP has a couple of options to speed up boot (typically to about 30 seconds once a valid config has been loaded on to the modem once). Firstly - they modify downstream channel list in the CMTS (if they're able to) to one of the defaults in the scan list (typically ~30 channels listed) in the default firmware. Alternatively, they can pre-load a firmware with one of their channels included in the list (normally the firmware is created by the manufacturer of the modem, not the ISP - though larger ISPs could have their own dev's generate the firmware for them).

It only takes the modem to lock on to one of the downstream frequencies, and unless the ISP has configured otherwise, the full frequency list is included within the transmission - after that the modem knows every downstream frequency and upstream frequency, and begins the rest of the process. Bonding typically takes about 5-10 seconds at most, and then you're up to full speed.

Once the modem goes through this process, it *should* save that info for its next boot, and only in the case of a factory reset will it have to go through the blind scan  again. It'll only save that info once it gets accepted into the CMTS.


By ensuring we have one of our channels in the scan list, we managed to drop our first start-up time from anywhere between 5-15 minutes, down to about 30 seconds (not including the config download and subsequent reboot after the config was downloaded (which only happens in case of config changes)).
 

Offline TomS_

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Re: Why Does A Cable Modem Take So Long To Boot?
« Reply #28 on: October 23, 2019, 04:13:34 pm »
I don't know about your modems but a lot of the DOCSIS cable modems i had would deinitialize the Ethernet port after about 5-15 seconds into the boot process. This is seen because the router thinks the cable got unplugged (LEDs go out on router and modem, logs show link down etc..) and it comes back a few seconds later.

Ive seen the exact same thing on other routers and devices in general, so its not just cable modems/routers.

But there is a reasonable explanation for behaviour like this. When the system is reset, everything assumes a default state/configuration. It is reasonable to assume that an ethernet chipset may come up in some kind of workable state without first being configured by the OS. Once the OS loads the driver for that ethernet chipset, it will then reconfigure it with whatever settings it actually wants, or anything defined by the configuration within the OS. That process may cause the chipset to effectively "reboot", thus dropping the link before coming back a moment later.

 

Online Whales

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Re: Why Does A Cable Modem Take So Long To Boot?
« Reply #29 on: October 23, 2019, 09:01:22 pm »
I don't know about your modems but a lot of the DOCSIS cable modems i had would deinitialize the Ethernet port after about 5-15 seconds into the boot process. This is seen because the router thinks the cable got unplugged (LEDs go out on router and modem, logs show link down etc..) and it comes back a few seconds later.

Ive seen the exact same thing on other routers and devices in general, so its not just cable modems/routers.

But there is a reasonable explanation for behaviour like this. When the system is reset, everything assumes a default state/configuration. It is reasonable to assume that an ethernet chipset may come up in some kind of workable state without first being configured by the OS. Once the OS loads the driver for that ethernet chipset, it will then reconfigure it with whatever settings it actually wants, or anything defined by the configuration within the OS. That process may cause the chipset to effectively "reboot", thus dropping the link before coming back a moment later.

Yeah, a lot of SOHO routers/modems' use a bootloader with some form of limited networking support for recovery and uploading new firmware.  This requires bringing up the ethernet switch/ports within the first seconds of power-on.  Then the ports (hopefully) get disabled again a few seconds later and the main linux payloads get booted.

Some of these implementations cause problems when re-purposing routers for other work (eg with OpenWRT) because they provide a small window during boot where various physical ethernet ports on the back of the unit are bridged/switched in unwanted ways.  Some units even leak between ports labelled WAN and LAN :P  see the OpenWRT wiki for more.

Offline TomS_

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Re: Why Does A Cable Modem Take So Long To Boot?
« Reply #30 on: October 24, 2019, 08:56:04 am »
Some units even leak between ports labelled WAN and LAN :P  see the OpenWRT wiki for more.

Oh yeah, Ive seen that, and been victim to it. In a previous job I was a network engineer at a wireless broadband provider (in Australia). The router we supplied to customers had some bug where certain traffic would be bridged from LAN to WAN. But not just leaking traffic out of a customers network, it would then leak into another customers, and they would see IP address conflict messages popping up in Windows, or other customers PCs would show up with browseable shares, printers etc. Absolute nightmare.

I did some investigation in to it, packet captures etc, the only thing I could find was that there was a certain bit pattern in all of the MAC addresses of ethernet frames that had leaked through. I reported that to the manufacturer and they came out with some kind of fix that resolved the issue. I still dont understand why or how, because they wouldnt tell me what the actual issue was. I wasnt a fan of their gear anyway, and that just further cemented my dislike for them.

Our parent company also supplied the same router to DSL broadband customers, but we never saw issues there because the DSL network had better customer isolation such that one customer couldnt talk directly to another via their WAN port.
 


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