an be the circuit used least, the most direct, the most reliable, or some other predefined variable. Once again you can see the magic that takes place inside the cloud. Now that the network cloud and the packet exchanges are in place, the next step is to con- nect a user. The User Connection Users sign up with the carrier of choice and let the carrier worry about the physical connection. As shown in Figure 7-3, the user connection into the cloud is through a dedicated or leased line. The carrier notifies the local exchange carrier (LEC) and orders a leased line at the appropriate speed (in the leased line, this can be up to 64 Kbps on digital circuits). The original network connections back in the initial rollout of X.25 services were on ana- log lines at up to 9.6 Kbps. A modem was provided by the carrier at the cus- tomer end, or the customer purchased and provided it. Now that the modem Figure 7-3 The user connection. Terminator User Host Modem Leased @@@@@@@@ Local Exchange Carrier Packet Switching Exchange Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25, Internets, Intranets, and Extranets Chapter 7 is attached at the customer end, the circuit is terminated in a port on the computer (PSE). This is the incoming port that can be used as a permanent virtual connection or as an incoming-only channel. This is part of the addressing mechanism inside the packet where the PSE reads the packets' originating address. The incoming-only channels are the channel numbers assigned to each customer. The next step follows the connection where the customer must initiate the PAD function, as shown in Figure 7-4. Remember that the PAD is the hardware or software installed to break the data into smaller pieces, attach the overhead, and forward the packetized data to the network. In the reverse order, the PAD is responsible for receiving the packets, pe