ng Carrier Networks TRANSPORTING MOVIES AND TV IN THE CORE Both telephone service providers and cable TV companies have infrastructure in their core networks to receive and transmit entertainment content to customers. They each do this from centralized headends. Using Headends to Receive and Transmit Programming The term "headend" refers to the site where providers transmit content that they receive from satellites. A group of satellite dishes that receives content is referred to as a satel- lite farm. Central office switches and Voice over Internet Protocol (VoIP) equipment to support telephone services are also located at headends. Network Operations Centers (NOCs) capable of monitoring and making programming changes to the network as well as operations support systems can be located here, as well. These centers typically serve seven to ten towns. Multiple hubs that distribute content are connected to the headend, as shown in Figure 4-7. Coaxial cable Amplifier to strengthen signals Transport network Distribution Distribution Fiber ring Head end Coaxial Fiber Fiber cable node* Distribution *Fiber nodes convert light signals carried on fiber-optic cabling to electrical signals for transmission on coaxial cabling, and vice versa. Figure 4-7 A fiber/coaxial network with transport network linking distribution hubs to the headend. Middle-Mile Networks Hub Sites Hubs, also referred to as distribution hubs, are small buildings that are located closer to customers than headends. Local programming or frequently downloaded content might be located at a hub, which serves between 10,000 and 50,000 residences. In addition, cable modem termination systems, which manage traffic to traditional cable TV customers' modems and to the headend, are located at hubs. For more information, see the section "Using CMTS for IP Traffic," later in this chapter. A hub might serve metropolitan area. Large towns would have two hubs. Linking Hubs and Headends via Metropolitan Area Networks Metropolitan Area Network (MANs) are also referred to 