Packet.2 congested route Packet2 4. Verizon putsal packets back order they were sent 5. Sophie 5. Verizon sends email to Sophie'scomputer Figure 1-7 Three of the packets of an e-mail from John to Sophie take different paths through the network. Packet Contents: User Data vs. Overhead Each packet is made up of user data; data bits, digital voice or video, and specialized header information, such www.TechnicalPDF.com as addressing, billing, sender information, and error correction bits. See Figure 1-8 for an example of a data packet. 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 contains 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. A Simplified Example of Fields In a Packet With User Data and Overhead Header: End of packet information: e.g. User bits: email message, sender/receiver error checking and end of address, protocol, text, audio, video message bits Overhead Figure 1-8 User data VS. overhead addressing and end-of-packet bits in packets. Throughput www.TechnicalPDF.com Throughput is the amount of user information transmitted, not the actual speed of the line. The disadvantage of frequent error messages and other protocol-related bits is that overhead bits often consume large amounts of bandwidth. Throughput only measures actual user data transmitted over a fixed period of time. It does not include header bits. Protocols with many bits for error control messages and other types of overhead have lower throughput. Technologies such as Deep Packet Inspection and Traffic Shaping are used to mitigate the effect of delays associated with these protocols. (See the section "Traffic Shaping," later in this chapter, for more information.) So, What Are Carriers and ISPs? At one time, t