Unlike "circuit-switched" networks such as the public switched telephone network (PSTN), a packet-switched network is "connectionless." In other words, a dedicated end-to-end transmission path (or circuit) does not need to be opened for each transmission. Rather, each router calculates the best routing for a packet at a particular moment in time, given current traffic patterns, and sends the packet to the next router. Thus, even two packets from the same message may not travel the same physical path through the network. This mechanism is referred to as "dynamic routing."
When packets arrive at the destination point, they must be reassembled, and packets that do not arrive for whatever reason must generally be re-sent. This system allows network resources to be used more efficiently, as many different communications can be routed simultaneously over the same transmission facilities. On the other hand, the inability of the sending computer under such a "best effort" routing system to ensure that sufficient bandwidth will be available between the two points creates difficulties for services that require constant transmission rates, such as streaming video and voice applications.