Cable Modems Dropping Offline in a 2-way Cable Network - Taken from Cisco's site
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.
Note: The information in this document is based on Cisco hardware uBR7246 VXR (NPE300) processor (revision C) and Cisco IOS® software (UBR7200-K1P-M), Version 12.1(9)EC and a CVA122 Cisco IOS Software 12.2(2)XA.
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:
DOCSIS Cable Upstream RF Specifications
SpecificationsUPSTREAM
DOCSIS Specifications
System/Channel
Frequency range
5 to 42 MHz (North America) 5 to 65 MHz (Europe)
Transit delay from the most distant CM to the nearest CM or CMTS.
< 0.800 millisecond (msec)
Carrier to noise ratio
25 dB
Carrier to ingress power ratio
> 25 dB
Carrier to interference ratio
> 25 dB (QPSK) > 25 dB (16 QAM)
Carrier hum modulation
< -23 dBc (7%)
Burst noise
Not longer than 10 µsec at a 1 kHz average rate for most cases.
1. DOCSIS 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.
DOCSIS Cable Downstream RF Specifications
SpecificationDOWNSTREAM
DOCSIS Specifications
System/Channel
RF channel spacing (bandwidth)
6 MHz
Transit delay
0.800 millisecond (msec)
Carrier to noise ratio
35 dB
Carrier-to-interference ratio for total power (discrete and broadband ingress signals).
Maximum analog video carrier level at CM input, inclusive of above signal level variation.
+17 dBmV
Minimum analog video carrier level at CM input, inclusive of above signal level variation.
-5 dBmV
Digital Signal Levels
Input to cable modem (level range, one channel)
-15 to +15 dBmV
Signal as relative to adjacent video signal
-6 to -10 dBc
1. DOCSIS specifications are baseline settings for an DOCSIS-compliant, two-way data-over-cable system. 2.Transit delay is defined as the "round trip" from the cable headend to the furthest customer and back. 3.dBc = decibels relative to carrier. 4.ns = nanoseconds.
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
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.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.
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