 logical channel number of an- other. 6.7.1 The History of X.25 As packet switching became feasible, an obvious need for a common protocol which would allow users to interface a PDN became evi- Logical channels Packet network Virtual circuit DTE 'N' Fig. 6-13. Virtual circuit service concept. 100 TELECOMMUNICATIONS dent. Various national and international standards groups began working on the problem in 1972. In 1976, the CCITT formally adopted recommendation X.25 with the imposing title: "Interface Between Data Terminal Equipment and Data Circuit Terminating Equipment for Terminals Operating in the Packet Mode on Public Data Networks". The recommendation was revised in 1980 to add additional features and enhancements. CCITT X.25 defines the protocol and procedures necessary for a packet mode terminal to access the services provided by a packet- switched public data network. The services and the related facilities provided include virtual and permanent virtual circuit service. A packet mode terminal generally operates synchronously over full-du- plex circuits at speeds of up to 56,000 bits per second. 6.7.2 How They Work The current interest in the X.25 standard and packet switching has left many people with less than perfect grasp of the concepts in- volved. The concepts are not difficult to understand, but unless stated clearly, many of them easily become confused. A packet switching network provides what appears to be a per- manent connection between devices. On the functional level, a packet network handles blocks of data the way the Post Office passes an envelope through the mail. That is, none of the facilities is dedicated to any one message. The network facilities handle many different messages in quick succession. When the terminal requests a connection to a computer port via a packet switched network, the form of the request is a packet or data block. It contains specific control bits that identify it as a call request, the address of the called port, the address of the calling station, and some additio