DOCSIS 3.0 Partial Service

DOCSIS 3.0 Partial Service is a new term encountered in the DOCSIS 3.0 MULPI specification and realized in field deployments of DOCSIS 3.0 cable modems using upstream bonding.  This was a topic that I touched on in this years SCTE Cable-Tec Expo, but will explore in greater detail in this article.  Partial service can be considered a feature because the cable modem will stay online even when one or more upstream transmit channels goes offline.  A related topic to partial service is “impaired service”, but I will cover that in a related article.

Defined

A modem is in a partial service mode of operation any time it is operating with a subset of the channels in the Receive Channel Set (RCS) and/or Transmit Channel Set (TCS) because a channel has become unusable, either due to an inability to acquire a channel or because communication on a channel was lost during normal operation.

A typical bonded upstream service group will have up to four channels, that when aggregated together are capable of transmitting over 100 Mbps at 64-QAM modulation in a 6.4 MHz bandwidth.  If only one upstream channel is out of service in the “partial service” scenario, it is possible that the end-user will not notice any impact to performance.  A three channel DOCSIS 3.0 modem would still be capable of over 80 Mbps.  Therefore, in a DOCSIS 3.0 network, partial service is:

• Hard to detect after registration
• Most current test instruments will not identify it
• A throughput test may not identify it on a loaded plant

Two scenarios for partial service:

• The channel was down when the modem came online
• Test equipment will show fewer channels bonded than expected
• Will likely not cause any noticeable performance issues, unless the service tier is very close to the maximum throughput performance with a full channel set
• The Errors for the channels that could not be acquired are reported to the CMTS in the REG-ACK
• The channel becomes un-usable following TCC (Transmit Channel Configuration)
• The channel is already in the bonding group for this modem, and has become unusable.  The CMTS will stop granting transmit opportunities on this channel until communication is reestablished via the ranging process.  CMTS’s may handle re-acquisition differently and may not re-acquire at all until the CM is rebooted!  (it is a SHOULD requirement).
• Will cause temporary performance issues between the time that the channel becomes unusable and the time that the CMTS realizes that it is unusable (Range interval + Range timeout or about one minute). Following that time, the CMTS will refrain from granting timeslots on this channel, and performance degradation will stop.

Troubleshooting

When a technician is troubleshooting, partial service is one of the issues that can be experienced.  It is unlikely that most technicians would go to the field specifically to troubleshoot partial service, as it is not particularly noticeable unless the end-user is bumping up against the max transmit performance of the complete TCS.

The first indication to the technician that a portion of network is experiencing partial service is when the test instrument only bonds a portion of the Transmit Channel Set (TCS).  As seen in the screen captures below, the technician expects to see 4 channels in the TCS (as in the first screen shot), but the instrument only reports that there are 3 channels bonded (as in the second screen shot).

DOCSIS Full Channels 8×4 Configuration

DOCSIS Network in MATLAB

In this situation, the CMTS is aware of the partial service, and would refrain from granting MAP opportunities on the channel that is unavailable.  Therefore, unless the service level of the end-user is above the capabilities of the 3 bonded channels (80+ Mb/s), the end-user would likely not experience any degradation of service. (assuming that the other three channels are not experiencing performance issues)

When the technician determines that a partial service situation exists, the goal is to determine what is causing the partial service.  To accomplish this, the technician should begin traversing the network toward the CMTS, testing at each test point (Ground Block, Tap, Amps, Node) until the point where the missing channel(s) return to service.

When the missing channel(s) have returned to service, it could mean one of two things:

• The gross network issue has been passed, and the previously missing channel is in service and operating at peak performance.
• A minor network issue has been passed, allowing the previously missing channel to return to service, but other network issues continue to impair the performance of the channel.

Because both of these results are possible, it is very important to take a closer look at all of the upstream channels once a partial service scenario is fixed.  The sections that follow will discuss the process of isolating individual upstream channels in a bonded environment to determine performance of each.

Summary

Partial Service can be a tremendous advantage for cable operators because a DOCSIS 3.0 cable modem will stay online even if one or more of the upstream bonded channels goes offline due to RF impairments.  The subscriber will not likely notice any service disruption with the exception of a short disruption of about 15 seconds  when the CMTS and cable are attempting to re-range on the impaired channel.  The subscriber may also notice throughput issues if the DOCSIS upstream is in an over-utilized scenario and they have fewer upstream channels to use based upon their service tier.

A DOCSIS Partial Service outage should not be considered a permanent band-aid to resolve RF plant issues, but a temporary measure to keep the subscriber online while the impairment is identified and resolved.

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