Problem connecting to a Motorola SBG901 cable modem via ethernet

Was wondering if anyone else has had problems connecting a 3rd generation Macbook Air to a Motorola SBG901 cable modem via ethernet? The wireless connection is working fine but the ethernet connection is saying "Cable Unplugged - Either the cable for USB Ethernet is not plugged in or the device at the other end is not responding." Was previously connected to an RCA cable modem via ethernet with no problem. Have searched the net for info but to no avail.