flowing in one direction and responses in the other. NETWORK STRUCTURES 93 An example of vertical distribution is an application wherein the inventory of a local warehouse is maintained in a machine at that site. If, during order entry, a particular item is not in stock locally, an inquiry might be sent automatically either to the machine that is servicing another warehouse or to a central location to determine whether the item is available elsewhere. 6.4 MESSAGE SWITCHING NETWORKS Message switching has been in operation since the earliest commu- nications systems were installed. The initial systems, called "torn tape" systems, used paper tape to capture traffic. Operators would then read the message address and manually take the tape to the proper machine for retransmission. Since then dramatic changes have occured in message switching, affecting the size and cost of the re- quired equipment. Microprocessor technology has now made it possible to buy a computer based switch capable of handling a medium-to-large com- pany's traffic for less than $50,000. In general, this approach offers significant cost advantages as well as security and control which many companies desire when transmitting sensitive intra-company infor- mation. Many companies are therefore reassessing their communi- cations strategies with a view towards centering their operations around an in-house message switch. Message switching is a form of data communications where mes- sages are sent along with an address to a central station which then forwards the message to the intended destination. Its major differ- ence from most other forms of data communications is that origi- nator and recipient are not both on-line simultaneously and no two- way communication is possible. Alternatives to message switching include leased-line networks, circuit switching, or some form of manual delivery (see Fig. 6-8). If communication is needed between only a small number of sites, a communication network can be built linking each of those sites with a fixed line. 