Frame Structure
7 byte preamble: alternating 1/0 combination producing 10Mhz square wave for 5.6 msec, used for recv sync
1 byte SOF (start of frame) 10101011
6 byte dest addr, 6 byte src addr, 2 byte type/length overloaded field
variable sized data portion followed by 4-byte CRC-32
sends low-order bit first for 802.3