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Have 200 MBPS service, but brand new Arris SB8200 modem tops out under 100 MBPS downstream

PhilSchwarz
Frequent Visitor

Have 200 MBPS service, but brand new Arris SB8200 modem tops out under 100 MBPS downstream

I just bought an Arris SB8200 modem (+ Tenda AC5 v3 router) to replace our rented Arris TG1682G xFi gateway.  We have 200 MBPS service, and the TG1682G reported downstream speeds in the 150-200 MBPS range.  The SB8200 is consistently reporting speeds in the 70-100 MBPS range.  

 

I'm not seeing any lapses in Internet connectivity or other instability, at least externally.  (See event log summary below.)

 

I'm wondering whether there might have been a provisioning error -- here's why:

 

We live in a part of town with wretchedly poor cellular coverage.  To deal with that, we run an Ethernet cellular signal extender (a Samsung SLS-BU10B, purchased from our cellular provider, Verizon) that gives us 5 bars of cell signal within our home -- but requires an Internet connection to do so.  "Landline" phone (mainly for occasional faxing) is VoIP, also dependent on the Internet connection.

 

So when I called Comcast Customer Solutions to get the new SB8200 modem set up, what the agent did for me -- so that in case of failure we would not be left without a means of contacting Comcast -- was to temporarily set up the SB8200 as a second device on my account, so that I could get that running, and then de-provision the rented TG1682G gateway.  I got the SB8200, the Tenda AC5 v3 router, and all the devices on our home network running, but I noticed that the SB8200 was testing consistently at just under 100 MBPS.  I assumed that it had been provisioned as a (temporary) second device at a default 100 MBPS.  When I called back to get the rented gateway de-provisioned, I asked that the SB8200 be provisioned for 200 MBPS, since that's what we're paying for.  The agent told me that the speed increase should happen overnight.

 

Next morning I discovered that Comcast had pushed a firmware update to my SB8200.  Out of the box, my SB8200 did not have a login screen on its web interface; now, it did.  Arris implemented the login screen in a firmware update released apparently after my SB8200 was manufactured.

 

But no speed increase. 

 

This could be that it hasn't been provisioned correctly, but I'd like to rule out anything that might be wrong with the SB8200 unit itself or with the RF signal it's seeing.  So here's a summary of channel status and event log entries:

 

I see 32 locked QAM256 downstream channels, ranging in power from -7.0 dBmV to -2.8 dBmV, with SNR ranging from 38.5 to 41.2 dB, ranging from 0 to 5 successful corrections per channel and 0 uncorrectables over the course of 6 hours of observation.  There's also a single DOCSIS 3.1 OFDM channel reporting -5.1 dBmV power, 36.0 dB SNR, and a ridiculously high successful corrections number which I assume is meaningless garbage.

 

There are 4 locked QAM256 upstream channels, ranging in power from 40.0 dBmV to 41.0 dBmV.  According to Arris tech support this is low, but it's within Comcast specs.

 

The event log has the following types of events (and number of occurrences):

"SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing" - 15 times over 4 days.  Timestamp for this event doesn't ever seem to be set -- always displays as 1970-01-01 00:00.

"No Ranging Response received - T3 time-out" - 12 times over 4 days.  Also no timestamp set.

"SYNC Timing Synchronization failure - Loss of Sync" - 4 times over 4 days.

"TLV-11 - Illegal Set operation failed" - 4 times over 4 days.  Always followed by...

"Config File Rejected due to Invalid or Unexpected TLV 11 encoding" (- 4 times over 4 days).

"Honoring MDD; IP provisioning mode = IPv6" - once, during troubleshooting the apparent 100 MBPS max speed with a Comcast tech support rep, who ultimately said she'd "run to the end of her script" and that I needed to pursue the problem with Arris tech support :-P.

"REG-RSP-MP Mismatch Between Calculated Value for P1.6hi Compared to CCAP Provided Value" - once, during that same troubleshooting session

"DBC-REQ Mismatch Between Calculated Value for P1.6hi Compared to CCAP Provided Value" - once, during that same troubleshooting session

 

Does any of this event log activity suggest either (a.) a provisioning error, (b.) a deficiency in the RF signal that is causing the modem to max out at < 100 MBPS, or (c.) a defect in the modem itself?

 

One final thing: I'm running the speed test from a Windows 10 computer plugged into a LAN port on the router.  I tried plugging the computer directly into the cable modem but couldn't get an IPv4 address assigned via DHCP (despite the router being able to do so without fail, when *it* is plugged into the cable modem).  Running ipconfig in an administrator cmd.exe shell when connected directly to the modem, I see a link-local IPv6 address and an fe80:: prefixed IPv6 default gateway address, but nothing else.  Any idea why I'm not getting a DHCP address?  (I disabled APIPA IPv4 169.254.x.x autoconfiguration, which was kicking in before any DHCP response when I tried to connect the computer directly to the modem; do I need to disable IPv6 autoconfiguration too?  If so, how do I do that?)

 

Thanks for any help that can be provided!

 

PhilSchwarz
Frequent Visitor

Re: Have 200 MBPS service, but brand new Arris SB8200 modem tops out under 100 MBPS downstream

Update: now seeing Dynamic Range Window violations spaced about an hour apart:

"RNG-RSP CCAP Commanded Power Exceeds Value Corresponding to the Top of the DRW"

There are now O(100) uncorrectables on about half the downstream channels.

The upstream channels power level is about 40 dBmV on all 4 channels.

Arris claims that that is too low for an SB8200 -- they say minimum 45 dBmV.

But it's within Comcast's specs for minimum upstream power.

Coax coming into the house from overhead drop goes straight into a CommScope amplifier that was installed by Comcast in 2018.  The amplifier feeds 3 TV jacks and 1 jack for the cable modem.  All other ports have terminators on them.  There are no splitters in the coax runs to the 4 jacks, but they're located in various parts of the house.  Currently they're connected to cable TV boxes that are turned off.  (We dropped TV and phone from our Xfinity service in early February.  I was planning to bring all 3 TV boxes and the rented internet gateway to our nearest Xfinity store, which is a long enough drive to want to make it one trip instead of two.)  I haven't disconnected the cable TV boxes because I didn't want to leave those coax runs unterminated.

@EG : I've read a number of your responses and have learned a lot from them (including the above).  Could you take a look at this and possibly refer me to one of the Comcast techs knowledgeable about RF issues who are on this forum?  Trying to determine whether we need a tech to come out to check/correct the RF line conditions, based on what I'm seeing in the modem statuses and logs.

(BTW, is there a way to view cable modem status and logs in the web interface of the Arris TG1682G gateway I'm trying to decommission, for comparison purposes?  It was clocking 150-200 MBPS on speedtest.xfinity.com, whereas the SB8200 is doing 75-95 MBPS.)

PhilSchwarz
Frequent Visitor

Re: Have 200 MBPS service, but brand new Arris SB8200 modem tops out under 100 MBPS downstream

Further update:

From these forums I learned about the SB8200's /main.html page, which shows the filename of the config file.  It's reporting that config file d11_m_sb8200_performancepro_c01.cm was loaded OK.

However, there are 4 instances of "Config File Rejected due to Invalid or Unexpected TLV 11 encoding" in the log, corresponding to times that I was on the phone with either Customer Solutions or tech support.

@EG : Does that tell me anything about whether the modem is properly provisioned?

 

djf55020
Frequent Visitor

Re: Have 200 MBPS service, but brand new Arris SB8200 modem tops out under 100 MBPS downstream

Are you sure the cable between your modem and router (or other device) is connecting at Gig speed?    When I switched to the SB8200 it didn't like my previous ethernet cable and would only connect at 100meg.    Try a new ethernet cable if you can't confirm it's connecting at the higher speed.

PhilSchwarz
Frequent Visitor

Re: Have 200 MBPS service, but brand new Arris SB8200 modem tops out under 100 MBPS downstream

Good catch, but I've already done that.  There was indeed a Cat5 cable between the modem and router, which I replaced with a Cat5e cable (which is rated for gigabit and should not be limiting the modem to 100 MBPS).

Question though: is there some way in which the modem might have "remembered" the router as a 100MBPS device?  Do I need to do a factory reset of the modem to get rid of such a thing?

 

PhilSchwarz
Frequent Visitor

Re: Have 200 MBPS service, but brand new Arris SB8200 modem tops out under 100 MBPS downstream

..aaaand the resolution of this issue: deceptive advertising by the *router* manufacturer.

 

I'd bought a Tenda AC5 v3.0 router to serve as an access point in a part of the house with poor wifi signal but ethernet wiring from where the cable modem and router are located.  I'd had positive experience with Tenda products before, it worked well, and its wifi support looked good, so I figured I'd get one to serve as the firewall/router behind the cable modem.  What *isn't* mentioned in their marketing material, but is on a data sheet I was able to find via Google, is that the wired LAN ports *and the WAN port* on that model are 100 MBPS ports, even though the wifi is rated for much more than that!  (Why would they do that???)

 

I'd been having trouble getting the desktop adjacent to the cable modem and router to play nicely directly hooked up to the cable modem, but I finally bit the bullet and troubleshot that problem in order to get a speedtest.xfinity.com reading directly at the cable modem's ethernet output.  Sure enough -- 170+ to 190+ MBPS.  So the Comcast configuration is copacetic.  I need a router upgrade!

 

The trouble I had getting the desktop to play nicely directly plugged into the cable modem has to do with getting a WAN DHCP lease for whatever is plugged into the cable modem.  It looks like there's a time interval of some sort that has to expire before the DHCP service (in the modem? or upstream?) will respond to a new request for a DHCP lease.  I saw this both with swapping the desktop into the modem and the router out, and vice versa.  @EG, is that true?  It certainly behaves that way!  Even if I power everything off and boot each device one at a time in the correct order, it will take quite a bit of waiting before the DHCP service will honor a new request for a lease.  The router times out and gives up.  If I wait 10-20 minutes and boot everything afresh, it succeeds.  Any idea where that is controlled and whether the waiting time for the DHCP service to honor a new lease request can be adjusted downwards?

EG
Expert

Re: Have 200 MBPS service, but brand new Arris SB8200 modem tops out under 100 MBPS downstream

The 8200 is a plain vanilla cable modem / layer 2 bridge. It does not do DHCP. The DHCP server is the Comcast WAN / default gateway at the CMTS. An end-user can not alter the way it behaves. It should assign a lease right away.



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