Cable Modem Troubleshooting Tips

Intro & Index.
Update history.
Installer issues.
Registration issues.
Network setup.
Mobile setup.
Cable Modem LEDs.
IP addresses.
MAC addresses.
Dead connection.
System updates.
Network tweaks.
Ethernet issues.
USB issues.
Surfboard tips.
CM diagnostics.
CM config file.
CM tech info.
DHCP issues.
DNS issues.
Web proxy issues.
Streaming media.
Gaming issues.
P2P file-sharing.
SETI@home issues.
Faster web browsing.
Faster downloads.
Simul down/uploads.
Swapping PCs on CM.
PCs sharing CM.
Simul dialup & CM.
VPN issues.
Time sync.
Virus security.
Firewall security.
Speed testing.
Latency issues.
Traceroute.
Packet loss.
FrontPage tips.
NTL/BY Tech Support.

Introduction and Index

This site covers some of the technical issues which can arise when troubleshooting a cable modem connection (including broadband via digital TV set top box) on the NTLworld and Telewest Blueyonder services in the UK and Ireland. Information specific to NTL has a green background, and information specific to Blueyonder has a blue background. Otherwise, information not on one of those backgrounds should be valid for most cable operators world-wide, although appropriate allowances might need to be made to adjust for local circumstances. Much of the information on this site is also applicable to other forms of broadband connection, such as ADSL, but ADSL-specific issues are not addressed.

Please send feedback to Robin Walker, rdhw@cam.ac.uk

The URL http://homepage.ntlworld.com/robin.d.h.walker/cmtips/index.html points to the most recent update of this site, should you be reading a printed or mirrored version. See the copyright statement.

Scroll down for the complete contents list.

Articles on this site:

Installer issues (BroadJump, Virtual Truck, Correct Connect):

Registration issues:

Modem MAC pre-registration: before first use
PC network interface MAC pre-registration: before first use.
First connection to digital TV set top box: client MAC registration
Post-installation registration for NTLworld account

Network configuration setup for the PC:

Basic Network Settings for network configuration of the PC
Windows 2000 and XP setup details
Internet Options setup for all versions of Windows
Windows XP built-in firewall: try disabling it at first
Mac OS 8.x-9.x setup
Mac OS X setup

Mobile configuration: notebook swapping between work and home
Flashing lights on the cable modem and what they mean

Cable modem data activity light keeps flashing
Surfboard 3100 or 4100 lights
3Com Tailfin lights
3Com CMX lights
Terayon TeraJet 210 lights
ntl:home 100/120 (Ambit) lights
Pace digital STB lights

Finding IP addresses:

What's my IP address? - how to discover your IP address
What's my WAN IP address? - if you are behind a NAT router
Default Gateway address - how to discover
Finding the cable modem's address
Finding the UBR address: how to identify your UBR
Finding the DNS server address(es):
Finding the DHCP server address:
Finding the transparent web proxy cache address.

Finding MAC addresses:

No connectivity at all - what and how to check

System updates and patches important for cable connections.
Networking tweaks that affect performance or latency

Ethernet issues:

Ethernet cable: straight-through or cross-over?
Auto-MDIX ports: take either sort of cable.
Ethernet drivers: updates.
Ethernet card duplex setting: affecting packet-loss and performance.
ntl:home 100/120 (Ambit) duplex setting.
Does semi-duplex mean I can't download and upload at the same time?

USB issues:

Surfboard tips:

The Surfboard Configuration web page.
Frequency Plan setting.
CM Ethernet IP Address setting.
Upstream Channel ID: get possibly better latency.
Downstream Frequency: get faster restarts.
DHCP Server: for use when CATV service is down.
Driver download.
Signal Diagnostics and Event Log are on the CM Diagnostics page.

Cable modem signal levels: how to read, and how they affect performance and packet-loss

Signal levels: a caution
Upstream and Downstream channels
Symptoms of poor signal levels: what to expect
Cable modem spontaneously re-booting: other causes
Desirable signal levels: for acceptable service
Advanced diagnostics: for the techie-minded
How to discover your cable modem signal levels
Inability to connect to cable modem diagnostic address: what to check
DocsDiag diagnostics: typical report
Surfboard diagnostics: how to read
Surfboard event log: what's important and what's not
3Com HomeConnect (Tailfin) diagnostics: how to read
3Com HomeConnect (Tailfin) event log: how to read
3Com CMX diagnostics: how to read
Terayon TeraJet diagnostics: how to read
ntl:home 100/120 (Ambit) diagnostics: how to read
Pace digital TV set-top-box cable modem diagnostics

Cable modem configuration file: quality of service
How the cable modem works: tech info
DHCP issues:

DNS issues:

What is DNS?
Which are my DNS servers?
DNS Lookups: doing them manually
DNS troubleshooting: some tips for common problems
DNS and firewalls: common problems
Dynamic DNS: a fixed name for your broadband PC
Decoding your DNS name: finding your UBR location

Web Proxy issues:

Transparent web proxy caches: what they do Updated 2004 Jan 8
Which is my web cache? - identifying the address of the web proxy
Stale pages in a proxy web cache: how to refresh
Sidestepping the transparent web proxy caches: how to set explicit proxies

NTL web proxy caches: list by centre Updated 2004 Feb 8
Blueyonder web proxy caches: list
Web authoring tips for being cache-aware

Streaming media:

Performance issues
Streaming protocols and Firewalls or NAT routers
Streaming protocols and Inktomi Traffic Servers

Real Player on Windows - work-around
Real Player on Macintosh - work-around
Windows Media Player - work-around
Windows Media Player for Mactintosh - no work-around!

Finding the Streaming Media Proxy

On-line games: settings that affect performance, lag times, and pings
Peer-to-Peer file sharing: settings that affect performance

KaZaA, Grokster configuration
Gnutella, MorpheusPE, BearShare, LimeWire configuration
WinMX configuration
eDonkey2000 configuration

SETI@home issues: work around connection errors
Faster web browsing: settings that affect performance
Faster downloads: strategies to get good speeds
Simultaneous download and upload: how to avoid performance hits

Swapping PCs on the cable modem: how to restore lost connection
Sharing a cable modem between multiple PCs and Home LANs:

Simultaneous dial-up modem and cable modem: setting default route
VPN issues: some problems
Time Synchronisation: keep your clock correct, for ever, from cable time.
Security and viruses: avoiding infection.
Security and firewalls: how to protect your PC(s) from intrusion.

Personal firewalls: the downside
Stealth-mode firewalls considered harmful: avoiding stealth
Personal firewall configuration for cable modem service

Firewall configuration notes on specific setups

Problems with certain applications and firewalls: special settings needed
Network traffic not to get worried about: non-malicious traffic
Why the UBR address is important in firewall configuration: tech info

Speed Tests: limitations and comparisons

KiloBytes-per-sec or kilobits-per-sec: not just times eight
My cable modem is as slow as my dial-up modem!: when and when not

Latency issues: understanding, diagnosing, fixing.

What is Latency?
How can Latency be measured? - the ping command.
Latency in DOCSIS cable modem systems.
Troubleshooting latency problems.

Traceroute: how to perform one, and how they can mislead

How traceroute works
How traceroute can mislead
Misleading traceroutes to certain NTL servers
Traceroute: how to speed it up in Windows
Reverse traceroute: how to get one back to your PC
Partial return route using ping

Packet Loss: understanding, diagnosing, fixing.

FrontPage: using with servers not supporting FrontPage Extensions.
Cable modem technical support: how to obtain Updated 2004 Jan 8

Return to Index.