Cisco OL-4015-08 Guide de dépannage télécharger pdf (Page 180)

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide

OL-5169-01

Chapter 5 Troubleshooting DOCSIS Networks

Troubleshooting Slow Peformance

If performance is still slow, then there is a performance problem in the network that is not related to the

CMTS or the cable segment. If performance from the Local CMTS network segment is significantly

better than for users connected to cable modems then you should focus your efforts back on the CMTS

and the cable segment.

Figure 5-9

In the above network, if Server 1, which is connected to the same network segment as the CMTS, is

getting slow performance when browsing the Internet, then the source of the problem is not the CMTS.

Instead, the bottleneck or performance issue will be somewhere else. In order to determine where the

problem is, performance tests would have to be carried out between Server 1 and various other servers

within the Internet Service Provider network and the public Internet.

Noise and Errors on the Cable Plant

If there is an excessive amount of noise or ingress in a cable network then packets between cable modems

and the CMTS can be corrupted and lost. This can lead to a significant degradation in performance.

Aside from a degradation in performance and throughput, some of the prime indicators of noise or RF

issues include

• Cable modems sporadically dropping offline or getting stuck in the init(r1) or init(r2) states.

• A low estimated SNR as seen in the output of a show controller cable X/Y upstream Z, where cable

X/Y is the cable interface being observed and Z is the upstream port being observed. The DOCSIS

specification requires a CNR of at least 25dB for all upstream signals. This equates to an SNR of

approximately 29dB. The Cisco CMTS is able to coherently detect QPSK upstream signals at much

worse SNR levels, however all cable service providers should strive to meet the DOCSIS CNR

requirements in their network. A sample show controller cable X/Y upstream Z output is shown

below.

uBR7246-VXR# show controller cable 6/0 upstream 0

Cable6/0 Upstream 0 is up

Frequency 25.200 MHz, Channel Width 1.600 MHz, QPSK Symbol Rate 1.280 Msps

Spectrum Group is overridden

SNR 28.6280 dB

Nominal Input Power Level 0 dBmV, Tx Timing Offset 6446

Ranging Backoff automatic (Start 0, End 3)

Ranging Insertion Interval automatic (102 ms)

Tx Backoff Start 0, Tx Backoff End 4

Modulation Profile Group 1

Concatenation is enabled

part_id=0x3137, rev_id=0x03, rev2_id=0xFF

nb_agc_thr=0x0000, nb_agc_nom=0x0000

Range Load Reg Size=0x58

Request Load Reg Size=0x0E

Minislot Size in number of Timebase Ticks is = 8

Minislot Size in Symbols = 64

Bandwidth Requests = 0x37EB54

Piggyback Requests = 0x11D75E

1 2 ... 175 176 177 178 179 180 181 182 183 184 185 ... 369 370

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