Cable Modem Troubleshooting Tips


Networking tweaks

Remove dial-up tweaks

If you have previously installed networking tweaks for dial-up modem performance (you might not know you had installed them if your dial-up ISP installation did it for you), you must remove them in order to use the cable modem service. The timings of cable modems are quite different to those of dial-up modems, and certain dial-up tweaks (such as reduced MTU size) will substantially degrade cable modem performance: a virgin Windows ethernet configuration would perform better on cable modem than one which has had dial-up tweaks installed. In extreme cases, or where you have forgotten how to remove the tweaks, re-installation of Windows (a simple re-install, not a re-format) has cured the problem of performance degradation: boot from the Windows system CD-ROM or associated boot diskette. You will need diskettes or CD-ROMs for any special drivers you also need to re-install.

Actually, although a full re-installation of Windows 9x/ME will remove old unwanted tweaks, there is a slightly faster procedure for the confident Windows user. In the control panel Network, in the Properties panel, remove all network adapters (green icons), exit, and allow Windows to restart. It will re-discover all network adapters, and re-install them with fresh settings. You will need your Windows system CD-ROM, and diskettes or CD-ROMs for any drivers required for your network adapters.

Install cable tweaks

A good site for determining whether any tweaking is necessary or desirable is (take the Tweak test: on the results page, declare your connection be be via Normal), but be aware that this site is mainly aimed at xDSL users, and some advice, such as that concerned with PPPoE, does not apply to cable modem users. The only Windows networking tweak which has significant benefit for cable modem users is to increase the TCP Receive Window (RWIN). The RWIN tweak will be more significant to Windows 9x users than to Windows ME/2000/XP or Mac users, who have better default settings.

A site which has good technical information on tweaking cable modem networking for both Windows and Macs (and also debunks many of the myths) is See also (which unfortunately also propagates some of the urban myths), but has a useful collection of pre-packaged downloadable cable tweaks at

The necessary RWIN size for best performance is discussed in the article on Traceroute. The RWIN tweak might make little difference to web browsing performance, because the ISP's transparent caches are very close to your PC, and so the default RWIN setting will be fine. Windows 2000/XP has an adaptive TCP Receive Window, and does not need tweaking.

The optimal MTU size for cable connections is 1500 bytes, leading to an optimal MSS size of 1460 bytes. If the tweak test shows a smaller MTU/MSS, then this might indicate a dial-up tweak that needs to be removed. Smaller than optimal MTU/MSS sizes will significantly damage cable performance.

If your system is Windows 95, 98, or 98SE, you might benefit from the fix downloadable from: This fix is not for any other version of Windows.

Apple Mac users can tweak their network settings with the shareware IPNetTuner, which comes with a pre-defined cable modem tweak configuration. The tweaks last for the current session only: the system reverts to defaults when it restarts or when the TCP/IP stack is deactivated and reactivated, such as on a location switch or re-boot.

I recommend that you make tweaking the last thing you do, after you have corrected any other faults. Get your system working properly first with a standard configuration, before you try the tweaks.

See also the article on Getting Faster Web Browsing for web browser tweaks.

Network configuration tweaks

This section applies primarily if your PC is connected directly to the cable modem: these recommendations are much less important, and maybe even undesirable, if your PC is part of a LAN connected to a NAT router.

Ethernet interface duplex setting

See article Ethernet card: duplex setting.

Disable or uninstall unnecessary network components

Default PC configurations often include network clients, services, and protocols which are of no use on a direct cable network connection. Many of them can generate network traffic, such as search probes, putting needless traffic on the upstream, which can cause latency problems for other more important traffic from your PC. Remember: these changes should only be made if the PC is directly connected to the cable modem: an intervening NAT router will stop this needless traffic anyway.

Windows 2000/XP:

Win 95/98/ME:

Apple Mac OS 8.x/9.x: if the ethernet port is connected directly to the cable modem, disable AppleTalk protocol on the ethernet port as follows:

Apple Mac OS X: if the ethernet port is connected directly to the cable modem, disable AppleTalk protocol on the ethernet port as follows:

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