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. Protocols with many bits for error control messages and other types of overhead have lower throughput. Technolo- gies such as WAN optimization are used to mitigate the effect of delays associated these protocols. (See the section "Wide Area Network Acceleration and Optimization," later in this chapter, for more information.) So, What Are Carriers, ISPs, and WISPs? At one time, the term "carrier" referred to local telephone companies, such as Verizon Communications, that carried voice and data traffic for consum- ers and commercial organizations. Prior to 1996, cable television companies, such as Comcast, were strictly cable television operators. Now, all companies that provide outside cabling or mobile infrastructure and operate networks are generally referred to as carriers. These include cable television operators, mobile telephone companies, long distance providers and traditional local telephone companies. Cable TV operators, mobile carriers, and traditional local telephone companies transmit voice, data, and television signals as well as providing con- nections to the Internet. To complicate matters further, carriers are also referred to as operators and providers. Continued Computing and Enabling Technologies ISPs such as AOL (America OnLine) primarily supply the connections to the Internet and information services over a carriers' cabling, and sometimes provide the switching infrastructure. ISPs also provide wireless services, e-mail hosting, and other services over a carriers' infrastructure. Wireless ISPs (WISPs) offer a variety of wireless services such as Internet access in areas without broadband landline facilities. See Chapter 7, "Mobile and Wi-Fi Networks," for more infor- mation on WISPs. DEEP PACKET INSPECTION: TRAFFIC MANAGEMENT AND MONITORING Deep Packet Inspection (DPI) is o