#1简单的题，大家进来看看
6) Which of the following is not defined at the physical layer of the OSI reference model (在
O S I参考模型的物理层中没有定义下面的哪一个)？
A. hardware addresses
B. bitstream transmission
C. voltage levels
D. physical interface
6) A。硬件地址在数据链路层的M A C部分上定义。
想问一下C. voltage levels，到底是什么意思
well basically when you refer to voltage level it can meant 'singal'!
computer send signal base on high(say 5 voltage) as 1and low (0.7 voltage) as 0 bit.
The actualy range of voltage is depended on environment and device. So, basically here mean bit 0's and 1's. As bit often refer to physical , its definitely in Layer1.
