shment time prior to the Figure 2-4 Network using virtual circuits. Virtual Circuit id: 1 ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER Virtual Circuit id: 1 Packet Switching, IP, and the IETF transfer of user data, whereas virtual circuits have to be requested and established before any user data can transmit. When a terminal wishes to send packets to a number of different destinations, this establishment delay is present every time a new destination address is used. In such situations, datagrams provide more flexibility. On the other hand, once a virtual circuit is established, it usually pro- vides lower overhead than datagrams because headers have less to carry; they just carry a virtual-circuit identifier. Thus, for long transmissions, net- works using virtual circuits are a better choice. Datagrams take advantage of dynamic routing in the network. If a node in the network fails or the network becomes congested, packets can imme- diately take alternative routes to avoid stress points, without needing to establish a new circuit. Load balancing is also enabled; datagrams headed for the same destination can take different paths to distribute the network load more evenly. Virtual circuits can handle some load balancing, but when packets always take the same path, redistribution becomes more complicated. Strengths of Packet Switching Efficiency and suitability are the reasons why packet switching compares favorably to circuit switching. It returns unused resources promptly to the network. Packet switching also provides data rate adaptation between dif- ferent terminals. Routers are designed to buffer incoming packets until they can be sent on the outgoing interface, so the incoming data rate is not necessarily the same as the outgoing data rate. This enables terminals using completely different data rates to communicate. In effect, the network performs data rate conversion. Price is another strength. In general, packet-switching equipment costs less than circuit-switching gear because it's simpler. A router is