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jibbl
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09/24/2026 3:02 AM

Upgrade from Arris S33 to new Arris S34: Frequent "Loss of Sync" and upstream channel flapping

I recently decided to upgrade from my Arris S33 to an Arris S34 to take advantage of the mid-split upload speeds rolled out to our area (Seattle 98102) some time ago.


I'm able to provision the modem, and it sort of works, but I observe frequent, brief disconnects. These correspond to “Loss of Sync” error messages in the Cable Modem event log. Every "Loss of Sync" event corresponds to a missed ping when I run a ping process on the laptop attached to the modem.  These loss of sync messages occur 2-3 times per minute.


In this state the connection is somewhat usable, but very unstable. I can browse the web, but things like streaming audio end up experiencing frequent cuts/drops.


I live in a group of 6 town townhomes, grouped 3 front 3 back. The service to the townhomes in the back comes from a single cable that is split by a BDS103BH 3-way splitter to each of the 3 individual townhomes. To the best of my knowledge, this splitter was installed when the townhomes were built back in 2015. It's a 1000MHz balanced 3-way splitter.


To eliminate building wiring as an issue, I tried connecting my modem directly to the splitter output for my unit. In this configuration, I still observe the problem, although it's slightly less frequent (once every 1-2 minutes, instead of 2-3 times per minute).


Is there anything else I can investigate on my own, here? Is there enough information to warrant a line tech to come check things out?

FOOTNOTES

LOG EXAMPLE (Arris S34 Loss of Sync - just the last few minutes while writing this message)

09/23/2026 18:55:58

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:58

Notice

CM-STATUS message sent. Event Type Code: 2; Chan ID: 2; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:54

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:53

Notice

CM-STATUS message sent. Event Type Code: 2; Chan ID: 7; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:49

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:49

Notice

CM-STATUS message sent. Event Type Code: 4; Chan ID: 4; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:46

Warning

Lost MDD Timeout;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:46

Notice

CM-STATUS message sent. Event Type Code: 16; Chan ID: 193; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: 3.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:46

Warning

Lost MDD Timeout;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:45

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:42

Notice

CM-STATUS message sent. Event Type Code: 2; Chan ID: 1; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:41

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:40

Notice

CM-STATUS message sent. Event Type Code: 2; Chan ID: 3; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:38

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:13

Notice

CM-STATUS message sent. Event Type Code: 2; Chan ID: 1; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:09

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:55:04

Notice

CM-STATUS message sent. Event Type Code: 16; Chan ID: 193; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: 3.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:54:30

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:53:53

Notice

CM-STATUS message sent. Event Type Code: 2; Chan ID: 4; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:53:48

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:53:43

Notice

US profile assignment change. US Chan ID: 41; Previous Profile: 12; New Profile: .;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:53:39

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:53:37

Notice

CM-STATUS message sent. Event Type Code: 2; Chan ID: 3; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:53:23

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:53:18

Notice

CM-STATUS message sent. Event Type Code: 2; Chan ID: 3; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:53:16

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:53:15

Notice

CM-STATUS message sent. Event Type Code: 5; Chan ID: 17; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:53:06

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:52:53

Notice

CM-STATUS message sent. Event Type Code: 16; Chan ID: 193; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: 3.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:52:50

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:52:43

Notice

CM-STATUS message sent. Event Type Code: 2; Chan ID: 3; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:52:15

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:52:12

Notice

CM-STATUS message sent. Event Type Code: 5; Chan ID: 12; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


09/23/2026 18:52:02

Critical

SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;


CHANNEL STAUS EXAMPLE (Arris S34)


Here is an example of the channel status. Note: The Upstream Bonded Channel flaps. When I refresh, sometimes there are 0 channels, 3 channels, 4, or 5 channels. It’s constantly updating. This is an example I caught when 5 were up.


Procedure Status Comment

Acquire Downstream Channel 453000000 Hz Locked

Connectivity State OK Operational

Boot State OK Operational

Configuration File OK

Security Enabled BPI+

DOCSIS Network Access Enabled Allowed

Downstream Bonded Channels

Channel ID Lock Status Modulation Frequency Power SNR/MER Corrected Uncorrectables

12 Locked 256QAM 453000000 Hz -2.4 dBmV 43.4 dB 4982 37869

1 Locked 256QAM 387000000 Hz -3.0 dBmV 43.4 dB 8135 51733

2 Locked 256QAM 393000000 Hz -3.0 dBmV 40.9 dB 8401 47443

3 Locked 256QAM 399000000 Hz -3.1 dBmV 43.4 dB 7742 50308

4 Locked 256QAM 405000000 Hz -2.9 dBmV 43.4 dB 7567 44461

5 Locked 256QAM 411000000 Hz -2.9 dBmV 43.4 dB 7647 47879

6 Locked 256QAM 417000000 Hz -3.0 dBmV 43.4 dB 8887 50734

7 Locked 256QAM 423000000 Hz -2.8 dBmV 40.9 dB 5399 45022

8 Locked 256QAM 429000000 Hz -2.7 dBmV 43.4 dB 6443 39304

9 Locked 256QAM 435000000 Hz -2.7 dBmV 43.4 dB 5684 37211

10 Locked 256QAM 441000000 Hz -2.7 dBmV 44.6 dB 6015 39837

11 Locked 256QAM 447000000 Hz -2.7 dBmV 43.4 dB 6047 37477

13 Locked 256QAM 459000000 Hz -2.5 dBmV 43.4 dB 5988 36836

14 Locked 256QAM 465000000 Hz -2.5 dBmV 43.4 dB 5808 38937

15 Locked 256QAM 471000000 Hz -2.6 dBmV 43.4 dB 5378 37876

16 Locked 256QAM 477000000 Hz -2.6 dBmV 43.4 dB 6372 40011

17 Locked 256QAM 957000000 Hz -5.6 dBmV 40.9 dB 6158 40902

18 Locked 256QAM 963000000 Hz -5.6 dBmV 40.9 dB 5828 42376

19 Locked 256QAM 969000000 Hz -5.5 dBmV 40.9 dB 5851 45135

20 Locked 256QAM 975000000 Hz -5.7 dBmV 40.9 dB 5961 44057

21 Locked 256QAM 981000000 Hz -5.8 dBmV 40.4 dB 5931 43850

22 Locked 256QAM 987000000 Hz -5.7 dBmV 40.4 dB 6201 44288

23 Locked 256QAM 993000000 Hz -5.8 dBmV 40.4 dB 6449 49286

24 Locked 256QAM 999000000 Hz -5.9 dBmV 40.4 dB 6335 52266

193 Locked OFDM PLC 774000000 Hz -3.4 dBmV 44.0 dB 3293192 513

Upstream Bonded Channels

Channel ID Lock Status US Channel Type Frequency Width Power

4 Locked SC-QAM 35600000 Hz 6400000 44.0 dBmV

1 Locked SC-QAM 16400000 Hz 6400000 43.0 dBmV

3 Locked SC-QAM 29200000 Hz 6400000 43.8 dBmV

2 Locked SC-QAM 22800000 Hz 6400000 43.5 dBmV

41 Locked OFDMA 36200000 Hz 44400000 38.2 dBmV

 

Question

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Updated 

1 day ago

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XfinityRay

Official Employee

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4.3K Messages

2 days ago

Hi there, @jibbl! Thanks for all the information you shared about your service. This is sounding like most likely, we need a visit. The loss syncs and splitter are both sticking out and very concerning to me. Our technician would run thorough testing to find the root cause and repair it. Were you getting any disconnects with the S33 or not that you had noticed? 

3

jibbl

Visitor

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3 Messages

Hi, thanks for the response! 

I have also noticed disconnects with the S33; in fact, that's one thing that prompted the decision to try upgrading to S34. We are still running the S33 day-to-day right now, since the disconnects do a occur, but significantly less often (on the order of 1-2 per hour instead of 1-2 per minute).

On the S33, the problem manifests different in the cable modem log involving "UCD invalid" messages:

26/9/2026
06:17:25		 3 UCD invalid or channel unusable;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;
26/9/2026
06:18:01		 6 CM-STATUS message sent. Event Type Code: 16; Chan ID: 193; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: 3.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;
26/9/2026
06:20:03		 6

CM-STATUS message sent. Event Type Code: 20; Chan ID: 193; DSID: N/A; MAC Addr: N/A; OFDM/OFDMA Profile ID: N/A.;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;

It's accompanied with a brief connectivity loss, usually dropping one ping if I am running pings 1-per-second. But often enough to interrupt things like streaming audio sessions.

In just the last day or so, we have very rarely observed a slightly longer drops accompanying a few additional messages, including T3 or T4 timeouts, like:

26/9/2026
00:22:28		 3 UCD invalid or channel unusable;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;
26/9/2026
00:24:51		 3 Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;
26/9/2026
00:26:57		 3 Started Unicast Maintenance Ranging - No Response received - T3 time-out;CM-MAC=[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;
26/9/2026
00:27:48		 3 Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out;CM-[Edited: “Personal Information”];CMTS-MAC=[Edited: “Personal Information”];CM-QOS=1.1;CM-VER=3.1;

S33 STATUS PAGE

Procedure Status Comment
Acquire Downstream Channel 405000000 Hz Locked
Connectivity State OK Operational
Boot State OK Operational
Configuration File OK
Security Enabled BPI+
DOCSIS Network Access Enabled Allowed

Downstream Bonded Channels
Channel ID Lock Status Modulation Frequency Power SNR/MER Corrected Uncorrectables
4 Locked QAM256 405000000 Hz -3 dBmV 44 dB 1970 3594
1 Locked QAM256 387000000 Hz -3 dBmV 43 dB 1692 3588
2 Locked QAM256 393000000 Hz -3 dBmV 44 dB 1818 3569
3 Locked QAM256 399000000 Hz -3 dBmV 44 dB 1979 3578
5 Locked QAM256 411000000 Hz -3 dBmV 44 dB 1954 3633
6 Locked QAM256 417000000 Hz -3 dBmV 44 dB 2065 3572
7 Locked QAM256 423000000 Hz -3 dBmV 44 dB 2113 3597
8 Locked QAM256 429000000 Hz -3 dBmV 44 dB 2104 3562
9 Locked QAM256 435000000 Hz -3 dBmV 44 dB 2043 3579
10 Locked QAM256 441000000 Hz -3 dBmV 44 dB 2078 3563
11 Locked QAM256 447000000 Hz -3 dBmV 43 dB 2115 3599
12 Locked QAM256 453000000 Hz -3 dBmV 44 dB 2006 3395
13 Locked QAM256 459000000 Hz -4 dBmV 44 dB 2025 3409
14 Locked QAM256 465000000 Hz -4 dBmV 43 dB 2072 3445
15 Locked QAM256 471000000 Hz -4 dBmV 44 dB 1949 3409
16 Locked QAM256 477000000 Hz -4 dBmV 44 dB 1827 3356
17 Locked QAM256 957000000 Hz -7 dBmV 41 dB 5980 5631
18 Locked QAM256 963000000 Hz -7 dBmV 41 dB 6708 5506
19 Locked QAM256 969000000 Hz -7 dBmV 40 dB 6420 5150
20 Locked QAM256 975000000 Hz -7 dBmV 40 dB 5448 5132
21 Locked QAM256 981000000 Hz -7 dBmV 40 dB 5127 5259
22 Locked QAM256 987000000 Hz -7 dBmV 40 dB 5215 5971
23 Locked QAM256 993000000 Hz -7 dBmV 40 dB 5420 6978
24 Locked QAM256 999000000 Hz -7 dBmV 40 dB 5718 7785
193 Locked OFDM PLC 774000000 Hz -3 dBmV 41 dB 1717593 1603


Upstream Bonded Channels
Channel ID Lock Status US Channel Type Frequency Width Power
1 Locked SC-QAM 16400000 Hz 6400000 45.0 dBmV
2 Locked SC-QAM 22800000 Hz 6400000 45.5 dBmV
3 Locked SC-QAM 29200000 Hz 6400000 45.3 dBmV
4 Locked SC-QAM 35600000 Hz 6400000 45.0 dBmV

(edited)

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1 day ago

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XfinityRaul

Official Employee

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3.1K Messages

@jibbl Thank you for the additional details. The fact that you're seeing disconnects on both the S33 and S34, but at significantly different frequencies, is very helpful information. I'd like to take a closer look at your account to see what we can do to get this resolved once and for all.

 

To be able to discuss account specifics, please send a direct message by clicking the chat icon in the upper right corner of the page. Click on the pen and paper icon, then enter “Xfinity Support” in the “To” section. Here's a link that includes a guide on how to send a DM https://forums.xfinity.com/conversations/guidelines/how-to-direct-messaging-within-the-forum/697ba1e570934d009185c514.

I am an Official Xfinity Employee.
Official Employees are from multiple teams within Xfinity: CARE, Product, Leadership.
We ask that you post publicly so people with similar questions may benefit from the conversation.
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1 day ago

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jibbl

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Thanks, I have just initiated a conversation.

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