In many areas of the world, particularly Europe, the public communication carriers have long offered publicly switched data communications services using the X.25 packet switching system. The X.25 specifications were developed to overcome what were then the common barriers to reliable data communications over telephone lines, namely unpredictable line noise, inconsistent connection times and data stream corruption introduced by what was then primarily analog-based equipment. The X.25 specifications incorporated sequence checking, error correction, retransmission algorithms, network synchronization and flow control. These methods made it possible for business to use publicly switched packet networks to connect geographically diverse locations while at the same time maintaining sufficient data integrity to rely on these networks for critical business activities. A business unit in Marseille could reliably transmit its activity reports whenever needed to a headquarters computer in Antwerp. Both the business enterprise and the public carriers benefited greatly from the X.25 technology.
Since the advent of X.25, the public carriers in both Europe and the United States have replaced much of their analog equipment with digital systems. The newer digital systems offer greater reliability and much lower chance of data corruption by noise or other factors. Nonetheless, the existing investment in X.25 equipment is sufficiently large to drive business to continue to use it, and it still works. For this reason, X.25 remains a solid choice of technology for implementing wide area data communications networks. The same business with offices in Marseille and Antwerp can bring new units in Hamburg and London online without reengineering the entire enterprise network. The savings in cost are compelling.
At the same time, the Transmission Control Protocol/Internet Protocol (TCP/IP) suite has matured into a widely available, widely deployed and highly reliable networking solution. Many businesses and other organizations have built large, complex networks connecting nearly every kind of computer running diverse operating systems using TCP/IP. Applications serving nearly every business need are now on the market at reasonable cost, delivering high value return to the organizations that use them. Like X.25, TCP/IP also provides for error checking, retransmission, packet sequencing, network synchronization and the ability to route packets across wide area links using public communications carrier lines. A business unit in Boston can reliably transmit activity reports whenever needed across the Atlantic to a headquarters computer in Antwerp.
Consider the case in which a business, perhaps an ocean freight forwarding concern, wants to make available to all its locations the advantages offered by powerful database and graphic tracking applications running on Unix workstations at its Boston location. The Boston unit is running TCP/IP, the protocol on which those highly productive applications were originally based. The rest of this company's units, located in Marseille, Antwerp and Hamburg, plus a single agent in London, all interconnect using X.25 and a NetBIOS-based custom system developed specifically for the company. How can this business achieve the desired access?
Two solutions present themselves immediately. The first is to rework the database and tracking applications to use the same system as the rest of the enterprise. This solution is potentially costly and introduces new factors into a stable, reliable and delivering system. The second is to deploy TCP/IP wherever the access is needed, effectively going around the existing and considerable investment in X.25 in the European offices. This potentially requires new hardware on every machine and new hookups with the public carrier systems. In addition, the company must also install TCP/IP routers to connect its disparate LANs. Most public carriers do not switch TCP/IP packets, one of the benefits of X.25.
A third, and more cost-effective, solution is to encapsulate TCP/IP data in the X.25 packet and provide an X.25 carrier hookup at the Boston office. More than one international public carrier offers X.25 service globally. The database and tracking applications do not need to change; the X.25 systems in Europe do not need to change and access is achieved. The company must add the necessary devices to encapsulate the TCP/IP data stream in X.25 packets, and also deploy the TCP/IP end user applications wherever they're needed.
The PC users face the least difficult path for gaining access. PC/TCP, from FTP Software, offers a TCP/IP transport kernel specifically designed for X.25 networks. This kernel (shipped as the executable x25drv in the PC-210 product) communicates directly with the X.25 PAD (Packet Assembler/Disassembler) software running on the PC. The X.25 PAD in turn drives the X.25 hardware needed to connect to the X.25 PSN (Public Switched Network). A PC equipped with this hardware and software acts as a fully independent node, capable of communicating directly with any other similarly equipped PC, or any other computer running an X.25 PAD. The PAD unwraps the TCP/ IP packet and hands it to the TCP/IP stack running on the host. The agent in London, posting financial reports from the markets in England, can easily gain access this way.
The same is true for all the PCs connected to the local area network in Marseille, and, for that matter, all the PCs connected to the network in Antwerp. These machines can now communicate directly with each other using the X.25 PSN. Because the original system deployed used applications based on NetBIOS, these machines retain all of their original capability by running NetBIOS on top of TCP/IP.
This leaves two major hurdles: connecting the Unix workstations to the X.25 network and connecting the older mainframe systems in Hamburg and Antwerp to the TCP/IP system.
The first is the easier problem. Several vendors offer X.25 gateway systems which act both as a TCP/IP router and as an X.25 access unit. A TCP/IP packet destined for London from the Boston-based Unix machine can travel across the Boston LAN to the gateway, where it is encapsulated in X.25 and then sent into the PSN, arriving in London. The London PC, as mentioned, is equipped with the X.25-specific version of PC/TCP. This allows all the PCs in Boston to access any X.25 node without running any new or different software (with the possible exception of NetBIOS, also included in the PC/TCP package).
The second part of the problem, providing access for the users sitting at terminals connected to mainframes in Hamburg and Antwerp, is potentially more difficult. Here TCP/IP modules for the mainframe operating system provide a solution. The mainframe computers, running their own PADs, can now connect as any PC. Another method for achieving access is to replace the dumb terminals with PCs and a LAN. In this case, PC/TCP provides the necessary terminal emulation software to let the PCs communicate with the mainframe as though it were a terminal, protecting the company's investment in mainframe applications. Again, because PC/TCP works on X.25 networks, the replaced terminals now have full access to the global enterprise network.
Using PC/TCP from FTP Software, the global enterprise can take full advantage of its existing investment in X.25 systems, as well as add the considerable power of TCP/IP-based applications. It is the best of both worlds.
(Postscript: Several newer high speed wide are access systems have emerged as alternatives to X.25 on the market. These are ISDN, Frame Relay and SMDS. FTP Software demonstrated the high performance of an X-windows system running over ISDN using PC/TCP as the transport at the 1992 Washington D.C. Interop trade show. Since then, several more paths have become available for connecting PCs directly to ISDN lines using PC/TCP. In some areas of the United States and Europe, ISDN service is available to residential as well as small business customers. The other high speed systems, Frame Relay and SMDS, are compatible with PC/TCP by means of a gateway system described in the text above. FTP software continues to monitor these developments in wide area enterprise networking.)