What are advantages and disadvantages of random access based multiple access protocols? Why slotted ALOHA protocol has twice channel utilization with respect to pure Aloha?

A datagram network requires two columns; a virtual-circuit network requires four columns. Is there a reason for this? Is there a distinction between the kind of addresses used by each network?

A planet located far from Earth is around 9 x 1010 meters away. If a stop-and-wait protocol is employed for frame transmission on a 64 Mbps point-to-point link, what is the channel utilization? Assume that the speed of light is 3 108 m/s and that the frame size is 32 KB.

Sketch the Bipolar NRZ Voltage encoding on a classic Ethernet for the bit stream0001110101.