ability and congestion. In a national emergency such as the September 11, 2001 attacks in the United States on the Pentagon in Washington, DC, and the World Trade Center in New York City, the Internet still functioned when many portions of the public voice and cellular networks were either out of service or SO overwhelmed with traffic that people could not make calls. If one route on a packet network is unavailable, traffic is rerouted onto other routes. In addition, unlike older voice networks, the Internet does not depend on a few large switches to route traffic. Rather, if one router fails, another router can route traffic in its place. Routing Efficiencies Packet networks are able to handle peak congestion periods better than older types of networks because traffic is balanced between routes. This ensures that one path is not overloaded while a different route carries only a small amount of traffic. Sending data from multiple computers on different routes uses resources efficiently because packets from multiple devices continue to be transmitted without waiting until a single "heavy user" has finished its entire transmission. Thus, if one route is congested, packets are transmitted on other routes that have more availability. Sending Data in Packets Packet Contents: User Data versus Overhead Each packet is made up of user data; data bits, digital voice or video, and specialized header information, such as addressing, billing, sender information, and error correct- ing bits. Error correction might indicate if the packet is damaged, if the receiver is ready to start receiving, or if the packet has been received. The end of the packet con- tains information to let the network know when the end of the packet has been reached. Header, end-of-packet data, and other signaling data are considered overhead. User data (also referred to as the payload) is the actual content of the e-mail message or voice conversation. Throughput Throughput is the amount of user information transmitted, not the actual speed of the line. 