ically, QoS is negotiated (and paid for) by an organization using the Internet as the transport between its vari- ous facilities in a region or around the world, to guarantee a minimum per- formance level between sites; a single ISP is used for all site connections. QoS for general public Internet access is unlikely to arise because the bill- ing and servicing is economically infeasible. (ATM QoS is discussed further in Chapter 10, "Next-Generation Networks.") The ATM switch is front- ended by a DSU. Data service unit (DSU)-The DSU is the data communications equip- ment on which the circuit terminates. It performs signal conversion and provides diagnostic capabilities. The network also includes a physical cir- cuit, which, in a larger higher-tier provider, would generally be in the opti- cal carrier levels. Access concentrator-An access concentrator can be used to create the appearance of a virtual POP. For instance, if you want your subscribers to believe they're accessing a local node-that is, to make it appear that you're The Organization of the Internet in the same neighborhood that they are in-you can use an access concen- trator. The user dials a local number, thinking that you're located down the street in the business park, when in fact, the user's traffic is being hauled over a dedicated high-speed link to a physical POP located elsewhere in the network. Users' lines terminate on a simple access concentrator, where their traffic is multiplexed over the T-1s or T-3s, E-1s or E-3s, or perhaps ISDN PRI. This gives ISPs the appearance of having a local presence when, in fact, they have none. Later text in this chapter discusses the advantages of own- ing the infrastructure versus renting the infrastructure; clearly, if you own your infrastructure, backhauling traffic allows you to more cost-effectively serve remote locations. For an ISP leasing facilities from a telco, these sorts of links to backhaul traffic from more remote locations add cost to the over- all operations. You can see that the architecture o