Is the setup
wall > DSL Modem/router > SMC router > wired/wireless
Update the firmware of the SMC router. More information on the SMC router can be found here
http://www.smc.com/index.cfm?event=promotion&localeCode=EN_USA&pageNum=12
Broadcast SSID
Use WPA security
Do NOT use MAC addresses. WPA security is enough security.
Does the Earthlink DSL modem have a firmware update?
Rick
I recently switched from Brighthouse cable high speed to an Earthlink DSL account at my small office. I am running four PC's through a SMC router into the the cable modem and now into the DSL modem. My problem is that when I connect the SMC router to the Brighthouse cable modem I get very similar bandwidth test results from all four PC's, but when I switch to the new Earthlink DSL modem, three of my PC's consistently perform bandwidth tests up to par, however, one of my PC's (Sony Laptop) tests extremely low. I have tested both wireless and direct ethernet connections with the same poor results on just the one machine. Earthlink will not help as they say the problem resides in my machine, however as soon as I switch back to Brighthouse, the bandwidth on the problem laptop goes back to normal. Does anybody have any troubleshooting suggestions?

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