XPS L702X: cable "not properly plugged in or may be broken" prob when connect cable modem to ethernet port

hi there, has anyone had a similar problem to this? I have a new XPS L702X laptop that I connected to a Scientific Atlantic 2203 cable modem (using modem's RJ-45 cable to connect it to laptop's ethernet port). I followed all steps correctly to set up this modem, (UPC is the ISP), it is 12 megabit fibre broadband. When I start up browser there's no connection, and when I ask Windows 7 to diagnose problem it tells me cable "is not properly plugged in or may be broken". But the cable is plugged in properly. The Realtek network adapter is working properly (according to Windows' device manager), and I installed the Realtek ethernet controller RTL8111EL listed on the Dell euro support site for the L702X. Anyone else had this type of issue connecting to the L702X's ethernet port?