He began to realize the inadequacies of the data communications capabilities of the day. Unlike phone calls, where you needed an established circuit with a fixed bandwidth for the duration of the conversation, computer communications were more bursty, with long periods of inactivity. The phone system was so highly centralized that if even a small portion of the main plant were physically damaged, all long-distance communication would be blocked. Two men, two very different projects. But Baran and Davies reached strikingly similar conclusions.
Packet switching breaks big chunks of data into smaller units, called packets.
The packets are transmitted individually using the fastest route available on the network. Each packet holds information about its origin and destination. Once all the packets arrive at the destination, they're recompiled into the original message. With regular telephone service, which is based on circuit switching, transmissions are sent serially over a dedicated line. By comparison, packet-switched networks are more resistant to failure because only problem packets need to be retransmitted, not entire messages.