nal overhead information. This packet passes from node to node within the packet switching network (Fig. 6-14) until it reaches the called port. The device there acknowledges the call request with a second specialized form of packet that is re- turned to the calling device. As these two special packets pass through the network, the nodes that handle them assign a serial number to the call. This is simply a NETWORK STRUCTURES 101 Terminal Modem Packet network Terminal CITY Y CITY X CITY Z Fig. 6-14. Packet switching network connections. large number that is unique to the access link while the call is con- nected; after disconnect, the number is available for use again. The "logical channel number" is listed in temporary memory at each node within the calling station, the called station, and the appro- priate routing (which data links to use). The end-to-end route determined by this logical channel number is the "virtual circuit" between the devices. Each device needs only to include the logical channel number in the packets to be routed to the correct destination. Packet switching networks thus offer the conveniences of variable routes without the need to create a separate private network. Each user "plugs in" a terminal or computer with- out concern for the nature of the network or even what locations he passes through. What the user "plugs in" is the packet assembler/disassembler. The PAD sits between the terminal and the network. The PAD stores input from a typist in a terminal buffer, of say 256 bytes. The PAD then will assemble and create a packet by emptying the buffer when it is full (after almost a minute, at the average rate of five characters 102 TELECOMMUNICATIONS per second of a typist). For faster response a PAD can be pro- grammed to send a smaller packet after a present time. 6.7.3 Packet Switching in Operation Packets move around the network, from switching center to switch- ing center, on a hold-and-forward basis. That is, each switching cen- ter, after receiving a packet, "holds" a copy of it in 