Cable Modems Dropping Offline in a 2-way Cable Network

Document ID: 22543

Contents

Introduction
Before You Begin
      Conventions
      Prerequisites
      Components Used
Why Do Cable Modems Drop Offline?
      RF Plant Quality
      Periodic Ranging (CM View)
      Periodic Ranging (CMTS View)
      Upstream Utilization Too High
      Configuring the Routing Protocol Causes a Reset of the Cable Modems
Related Information

Introduction

This document explains some troubleshooting steps used to determine the cause of Cable Modems dropping offline. Since, in the majority of cases, the cause will be a plant issue or low carrier-to-noise ratio, these issues will be the major emphasis of this document.

Before You Begin

Conventions

For more information on document conventions, see the Cisco Technical Tips Conventions.

Prerequisites

There are no specific prerequisites for this document.

Components Used

The information in this document is based on the software and hardware versions below.

• Cisco hardware uBR7246 VXR (NPE300) processor (revision C)

• Cisco IOS® software (UBR7200-K1P-M), Version 12.1(9)EC

• CVA122 Cisco IOS Software 12.2(2)XA

The information presented in this document was created from devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If you are working in a live network, ensure that you understand the potential impact of any command before using it.

Why Do Cable Modems Drop Offline?

A Cable Modem requires three main things to remain online once it is connected and operational:

• Clean RF plant with a carrier-to-noise consistently above 25 dB in the Upstream, and above 35 in the downstream.

• Unicast polls from the CMTS every 30 seconds (keepalives). These are unicast transmit opportunities for this modem's assigned SID, in which it can send a RNG-REQ to the CMTS. If the cable modem does not receive a unicast transmit opportunity within T4 seconds (30 seconds) it has to time out and re-initialise it's MAC layer. So if there is a problem (RF) in the downstream, the cable modem might not "see" this unicast transmit opportunity, and drop offline.

• If the CMTS does not get a reply from the CM to the unicast transmit opportunity, the CMTS will poll the modem 16 times in short succession in order to try and get an answer. The modem is considered offline by the CMTS if there is no reply after these retries.

RF Plant Quality

According to DOCSIS specifications, the RF plant needs to comply to following requirements for Upstream and Downstream to ensure continued operation:

• The configuration parameters

• The downstream and upstream frequencies used

• The noise measurements in dB. Make certain that they are correct and within the allowed limits. A table of the noise limits is included below:

¹DOCSIS specifications are baseline settings for a DOCSIS-compliant, two-way data-over-cable system.

² QPSK = Quadrature Phase-Shift Keying: a method of modulating digital signals onto a radio-frequency carrier signal using four phase states to code two digital bits.

³ These settings are measured relative to the digital carrier. Add 6 or 10 dB, as determined by your company's policy and derived from the initial cable network setup, relative to the analog video signal.

⁴ QAM = Quadrature Amplitude Modulation: a method of modulating digital signals onto a radio-frequency carrier signal involving both amplitude and phase coding.

⁵ dBc = decibels relative to carrier.

¹DOCSIS specifications are baseline settings for an DOCSIS-compliant, two-way data-over-cable system.

²Transit delay is defined as the "round trip" from the cable headend to the furthest customer and back.

³dBc = decibels relative to carrier.

⁴ns = nanoseconds.

Note:  For a full set of the specifications for the European Standard, please read RF Specifications.

For a document on how to troubleshoot RF issues in your cable plant, go to the Determining RF or Configuration Issues on the CMTS document. For more information on RF measurements using a spectrum analyzer refer to Connecting the Cisco uBR7200 Series Router to the Cable Headend.

Periodic Ranging (CM View)

The CMTS MUST provide each CM a Periodic Ranging opportunity at least once every T4 seconds. The CMTS MUST send out Periodic Ranging opportunities at an interval sufficiently shorter than T4 that a MAP could be missed without the CM timing out. The size of this "subinterval" is CMTS dependent. The CM MUST reinitialize its MAC after T4 seconds have elapsed without receiving a Periodic Ranging opportunity. The default value for T4 is 30 seconds.

T4 is defined as "wait for unicast ranging opportunity". This is the time a modem will wait to get a dedicated transmit opportunity from the CMTS. The value is defined to be minimum 30 seconds, and maximum 35 seconds per SP-RFIv1.1-I03-991105.

If a UBR9xx modem goes offline because of a T4 timeout, you will see following error messages in the debug cable mac log:

router#debug cable mac log verbose
 .... 
 11:05:07: 39907.082 CMAC_LOG_T4_TIMER
 11:05:07: %UBR900-3-RESET_T4_EXPIRED: R04.0 Received Response to
 Broadcast Maintenance Request, But no Unicast Maintenance opportunities received. T4 timeout.
 11:05:07: 39907.090 CMAC_LOG_RESET_T4_EXPIRED
 ....

This usually points to a problem in the RF, so the troubleshooting should focus on that.

Periodic Ranging (CMTS View)

The CMTS will retry polling the CM until it either receives a reply or until the number of retries (default is sixteen) are exhausted. At that time the CM is removed from the poll list and considered offline.

A way to detect if a modem is constantly ranging is to use the show cable flap-list command.

Upstream Utilization Too High

If the upstream utilization is too high, or too many modems are connected to the same upstream, it is possible that some modems will not get the required bandwidth or transmit opportunities to fulfill their periodic ranging requirements, also resulting in a T4 timeout.

Experience teaches us that customers who wish to successfully deploy data over cable networks based upon the DOCSIS standard must take into account many factors for success. One fundamental point that will ensure success is keeping customer return domains within reason. Keeping the homes passed (HHP) per upstream port to a reasonable level can significantly improve deployment success, maintenance costs, and improve customer satisfaction. For best performance it is recommended that 2000 homes passed per fiber node with ~10% penetration yielding 200 subscribing cable modems per upstream port is a highly effective framework by which to deploy.

More on the maximum number of users can be found in the What is the Maximum Number of Users per CMTS? on CCO.

Use the show interface cable slot/port upstream n command as shown below to check for noise within the RF plant. If the uncorrectable errors, noise, and microreflection counters are high and increasing quickly, then this typically indicates there is noise present within the RF plant. You can check the upstream utilization by issuing following command on the CMTS:

VXR# show interfaces cable 6/1 upstream 0
Cable6/1: Upstream 0 is up
     Received 22 broadcasts, 0 multicasts, 247822 unicasts
     0 discards, 1 errors, 0 unknown protocol
     247844 packets input, 1 uncorrectable
     0 noise, 0 microreflections
     Total Modems On This Upstream Channel : 5 (5 active) 
     Default MAC scheduler
     Queue[Rng Polls]  0/64, fifo queueing, 0 drops
     Queue[Cont Mslots]  0/52, FIFO queueing, 0 drops
     Queue[CIR Grants]  0/64, fair queueing, 0 drops
     Queue[BE Grants]  0/64, fair queueing, 0 drops
     Queue[Grant Shpr]   0/64, calendar queueing, 0 drops
     Reserved slot table currently has 0 CBR entries
     Req IEs 360815362, Req/Data IEs 0
     Init Mtn IEs 3060187, Stn Mtn IEs 244636
     Long Grant IEs 7, Short Grant IEs 1609
     Avg upstream channel utilization : 0%
     Avg percent contention slots : 95%
     Avg percent initial ranging slots : 2%
     Avg percent minislots lost on late MAPs : 0%
     Total channel bw reserved 0 bps
     CIR admission control not enforced
     Admission requests rejected 0
     Current minislot count   : 40084    Flag: 0
     Scheduled minislot count : 54974    Flag: 0
VXR#

Note:  Check the noise and microreflection counters. They should be very low values and, in a normal cable plant, increment slowly. If they are at a high value and increment quickly it typically indicates a problem with the RF plant.

Note: Check for uncorrectable errors. These typically indicate a problem with noise within the RF plant. Check the received upstream SNR level.

Note:  Best to keep this around 200 maximum.

Configuring the Routing Protocol Causes a Reset of the Cable Modems

Be aware that when configuring a routing protocol on a Cisco uBR7200 series cable interface on older versions of IOS prior to Cisco IOS Software v12.1, the Cisco IOS software must reset the interface to enable the change. This in turn causes all cable modems on that particular downstream to reinitialize, potentially interfering with data transmission on that downstream. Therefore you should use the interface configuration commands, such as router rip, on a cable interface only when a minimum of subscribers would be affected.

Related Information

• Determining RF or Configuration Issues on the CMTS
• Troubleshooting [uBR7200]
• Troubleshooting uBR Cable Modems Not Coming Online
• Connecting the Cisco uBR7200 Series Router to the Cable Headend
• Flap List Troubleshooting for the Cisco CMTS
• RF Specifications
• Cable Radio Frequency (RF) FAQ
• Technical Support - Cisco Systems

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