INTRODUCTION TO X.25 An X.25 PAD (Packet Assembler/Disassembler) in a dial-up, modem creates a novel element in the dial-up X.25 environment. This single port X.25 device may be called a mono-port quad PAD since its single port can support up to four sessions using Hayes AutoStream. Incorporating X.25 capability into a V-series system product extends the usefulness and benefits of an X.25 Packet Switching Network (PSN) to individual workstations and terminals in widely dispersed locations. Growing demands for PC data communications reliability and flexibility make an X.25 PAD an ideal solution. Since access to an X.25 network can be accomplished through the Public Switched Telephone Network (PSTN), use of a mono-port quad PAD opens a variety of communications opportunities not readily or affordably available using previous communications equipment. BACKGROUND - X.25 AND ISO The diversity of the countries, regulatory agencies and equipment used worldwide would create a complex jumble with little chance of interworking if it were not for data communications standards established by the International Telegraph and Telephone Consultative Committee (CCITT). To provide stability and conformity for communications, the CCITT, a United Nations agency, makes recommendations on a broad range of subjects. As these recommendations are implemented, standards are established which greatly benefit the compatibility of data communications equipment and procedures. CCITT Recommendation X.25 defines the interface between data terminal equipment (DTE) and data circuit terminating equipment (DCE) for terminals operating over leased lines in the packet mode while connected to public data networks. First published in 1976 and updated in 1980, 1984 and 1988, X.25 enjoys widespread implementation, a fact that makes certification of X.25 products readily available. The 1984 update, known as X.32, extended X.25's capabilities to include dial-up operation. Future use of dial-up X.25 will greatly benefit from the large body of knowledge residing with users and service providers which has resulted from years of X.25 applications. The International Standards Organization (ISO) has played an integral role in defining the reference model of the structure of data communications networks through Open Systems Interconnection (OSI). The OSI model is divided into seven functional layers to clarify the interfaces between services provided by a network and to avoid confusion in the development of related standards. The goal of the network is to be as transparent as possible, to the extent that two users on separate networks will communicate, unaware that the networks are even involved in the process. The "open" nature of OSI is directly related to the fact that the model is versatile enough to enable any host system, regardless of who made or designed it, to connect with any other host or network. The OSI model provides a means for thinking about how connections ought to be structured so that devices wishing to be compatible can implement the same protocols at every layer. PACKET SWITCHING The technology for packet switching has been in use since ARPANET was developed in 1968. Many technologies are first implemented in military applications and packet switching was initially used by the Advanced Research Projects Agency (ARPA) to enable widely dispersed governmental facilities to share information even though they were using a variety of computer systems, software and data resources. Packet switching's basic concept is to route data from a source to its destination assuring its accurate and in-sequence arrival. In packet switching, the data is divided into packets (blocks of data) of a specific length and tagged with a header which contains addressing and sequencing information. The path that the packets take through the network and the routing information remembered by the switches through which they pass is often called a "virtual circuit." A number of virtual circuits can share the same physical connection because each packet includes information identifying its destination in the network. A sender's Packet Assembler/Disassembler (PAD) assembles individual characters so they can be sent as packets. Once the packets have been transmitted, the receiving PAD then disassembles them into individual asynchronous characters or performs protocol conversion for other types of interfaces such as SNA. The addressing and sequencing information gives each packet switch that the data passes through the necessary information to route the packet and instructs the receiving PAD concerning the order to use in reassembling the data into its original form. The receiving PAD also receives information which enables it to determine if any packets have been lost in transmission or contained transmission errors and to request retransmission of missing or broken packets. Using packet switching, the potential exists to divide a single document into packets which are routed such that each can take a different path to the packets' ultimate destination. Usually, however, most packets moving from one location to another travel over the same path. Data traveling over a packet switching network uses the most efficient route available in going from switch to switch. So, the data may not travel the shortest route to its destination, but it will flow along the most direct route that has the lowest traffic. The network uses dynamic routing to handle the packets and keep a balanced data flow across the switches, and can even react to a broken switch by re-routing packets across the network. X.25 packet switching has been available on public and private networks for a number of years. Unfortunately, X.25 benefits were only enjoyed by users after the network began handling their data. That is, the communications link from a users PC to the network was not an error-control link so data integrity could not be guaranteed. Additionally, the link to the network did not benefit from the cost savings available through X.25 and packetizing information for its entire "journey". Public X.25 networks, such as Telenet, Tymnet, CompuServe, Western Union, MCI and GE Information Services in the U.S. and Datapac in Canada, sell X.25 service to the public. Users are charged based on the number of packets transmitted, and, in most cases, users do not own any part of the network in this configuration. Packet switching will normally be inherently lower in cost than circuit switching when used for a wide range of data communications applications since network resources are shared by many users. Private networks are owned or controlled completely by a corporation or organization for its exclusive use. These custom designed networks meet specific corporate applications needs and entail the corporation owning or having control over all the connecting lines, hardware, PADs and other equipment required to set up the network. This type of network is operated exclusively for the benefit of the organization which established it. In some cases a company may elect to utilize a hybrid network which uses both public and private network equipment or features. X.25 STANDARDS FOR PACKET NETWORK INTERFACE The combination of X.25 and a PAD in a V-series system product provides considerably more functionality than the packet assembly/disassembly associated with a PAD. This results from the benefits inherent in the implementation of multiple CCITT standards available through X.25, X.3, X.28 and X.29. A mono-port quad PAD uses a single RS-232 connection between the PC and the modem to support up to four simultaneous sessions through four PADs which can have different PAD parameter settings. Each of the four PADs supports Triple X-PAD since they conform to three CCITT standards, X.3, X.28 and X.29. X.3 defines a set of PAD parameters which controls the behavior of the PAD it assembles or disassembles packets, and which also provides operating parameters for activities such as flow control and data forwarding conditions. The interface between the DTE and the PAD is defined by X.28. Included in this definition is how data delivery to and from the DTE is controlled as well as a command language to control the virtual circuit. The packet-level control mechanism for the PAD is defined by X.29. It provides the guidelines for remote and local PADs to use when communicating with each other and allows remote host computers to control how data is presented by the PAD to the attached terminal. BENEFITS OF X.25 AND PACKET SWITCHING Using an X.25 packet switching network for data transmission provides users with affordable, reliable, flexible service that can be economical for applications needing short, randomly occurring data transfers. Activities such as a credit card verification in a retail store, cash advances from an Automated Teller Machine (ATM) or access to medical or other records are ideally suited for an X.25 packet switching implementation. The cost benefits of X.25 packet switching result from combining messages from many sources for transmission over the same circuits. With Direct Distance Dialing (DDD) or a leased line, the user pays the same for the circuit whether it is used often or infrequently. By contrast, networks base their charges on the amount of data sent. The economy of scale in the network methodology versus the DDD or leased line approach can be like comparing the U.S. Postal Service with a private courier. With a private courier you pay for the cost of transporting a letter in a vehicle which may only have your letter in it. As a result, you pay a premium for this "customized" delivery of your letter. Using the post office, your message is combined with those of many others, placed in a shared vehicle, and delivered. The cost savings are passed along to you by this more efficient delivery system and the end result, the delivery of your message, is the same as with the custom solution. An X.25 packet switching network maximizes the accuracy of data transferred using special internal error-control protocols, redundant hardware and redundant paths. This means that if the Atlanta network node is down, data going from Miami to San Francisco can be rerouted through Baltimore and St. Louis to speed it on its way. And, this will be accomplished transparent to the user who is only aware that the data has been transferred to San Francisco, not what route the data may have taken. Dynamic routing helps distribute and balance the data flow on a network and provides enhanced network availability by routing data through back-up equipment when necessary. Flexibility over the network lies in several areas, including the ease of connectivity X.25 provides since it is an agreed upon connection standard for the DTE/DCE interface. An additional benefit to the PC user who communicates using a V-series X.25 PAD product is multisession and multipoint capability. This feature, for example, enables a product manager in Chicago to check the corporate mainframe for inventory levels on particular products while maintaining a connection to the engineering library in Dallas to check specifications in a design document, and to conduct an e-mail exchange of advertising copy with the marketing department in San Francisco. Multiple access sessions such as this will enable a user to switch between information sources and destinations to retrieve and provide the most up-to-date information available. This feature will be especially useful in coordinating the various activities necessary for bringing new products to market or for companies whose main asset is information. Having the proper information at the proper place at the proper time is a critical success factor for virtually any company. For example, from his weekend home in the mountains, a product manager could simultaneously view three sessions of different program models t olution required for a research analysis report. X.25 AND DIAL UP MODEMS The ubiquitous nature of the PSTN results in great convenience for users worldwide since it provides both intra- and inter- country, company, government, agency and industry communications. The establishment of various CCITT standards has permitted communications equipment manufactured and used in a variety of countries to transfer data over the PSTN with relative ease using X.25 as the common protocol. While the PSTN was, and is, designed for voice transmission, it has become increasingly utilized for data transmission through the implementation of dial-up modems. Dial-up modems, such as Hayes V-series Smartmodem 9600, incorporating X.25 for communications are taking PC communications to a new level of sophistication. These modems combine the cost effectiveness, flexibility and reliability of packet switching with the wide availability of dial-up service to bring X.25 benefits to the desktop. Without an X.25 PAD in the modem, a user dials an X.25 network asynchronously, data passes into a receiving modem at the network, and a network PAD packetizes the data and sends it out on the network to its destination. Using an X.25 PAD-equipped dial-up V-series system product means that the data leaves the user's modem in packetized form and can go directly onto the network for transmission. The X.25 mono-port quad PAD modem extends X.25's error-control capability from the network to the PC, thus making the entire transmission "protected" by error-control. Without the PAD in the modem, the data is not subject to error-control techniques until it s received by the network for processing and transmission unless the PC and the network are equipped with CCITT V.42 error-control modems. (The V.42 function in the V-series system products is covered in detail later in this document.) By providing error-control capability across the complete scope of the data transmission, the X.25 mono-port quad PAD modem offers extremely high data integrity, reduces data transfer delays, and helps to lower transmission overhead by reducing the steps data must go through after it leaves the PC. Furthermore, this PAD provides the evolving multitasking PC environment with access to multiple sessions where each session will be used by one of the PC tasks. Using X.25 protocols in the modem eliminates concerns over dial-up error-control and allows a user to communicate confidently with a network. Without X.25, users must determine if a modem on the network implements a standard (V.42) or non-standard error-control protocol. Thus, in addition to the performance and cost saving benefits existing with X.25, there is also a high degree of user comfort with this internationally approved, widely used protocol. HAYES IMPLEMENTATION OF X.25 PAD CAPABILITY FOR DIAL-UP MODEMS X.25 technology helps bring multisession and multipoint communications to a personal computer workstation through a Public Data Network (PDN) or the PSTN and provides multiple session connectivity to workstations on an Integrated Services Digital Network (ISDN). The X.25 V-series system products provide the user with error-control from the local DTE, through the X.25 network, to the remote computer without any protocol conversion overhead. Having a local PAD allows the user to set up and establish multiple virtual calls over the physical link and provides greater control over data flow. Functions included in the X.25 communications solution are: % Link Layer Support - The PAD uses CCITT recommended Link Access Procedure Balanced (LAPB). The link layer is responsible for error-control from the local DTE to the network node. Access to link layer protocol parameters allows the user to adjust the protocol to fit specific applications. Parameters which can be adjusted include the mode of operation (X.25,V-series system product or standard asynchronous), and negotiation steps (detection, XID exchange). % Packet Layer Support - After a link has been brought up, the packet layer of X.25 is responsible for establishing virtual circuit calls. The V-series X.25 products implement four PADs to support up to four virtual calls over a single link (X.25 allows for a maximum of up to 4096 virtual calls on a physical connection). CCITT Recommendation X.25 describes several user facilities such as Reverse Charge, Network User ID and Flow Control Parameter Negotiation. These and others which may be required when connecting to X.25 networks supported in the Hayes PAD. % Triple X PAD - The Hayes PAD adheres to CCITT Recommendations X.3, X.28 and X.29 to ensure compatibility with existing and future X.25 equipment. The PAD design is based upon the 1984 specifications for X.25 and all of the mandatory specifications of the 1988 recommendation. % X.32 Support - X.32 defines the functional and procedural aspects for the DTE to dial into an X.25 packet switch network. In order to dial into the network, the Reverse Charging and Network User ID packet layer user facilities supported in the PAD are used to identify the calling DTE and allow the network to accrue charges against it. % Hayes AutoStream - This family of Hayes-developed protocols is included in the AT Command processor to provide enhanced functionality to a single communications link. Smartcom III communications software and X.25 V-series system products support Hayes AutoStream to provide for multiple simultaneous virtual circuits, per channel flow control and device flow control as well as control and setting of PAD parameters over the asynchronous link between the PC and the modem. A mono-port quad PAD's asynchronous dial-up capability addresses a different range of applications compared to a leased line implementation. These new modems make X.25 links affordable for single sites, an important feature for companies which operate a variety of remote offices, stores or branches. Prior to the availability of dial-up X.25, many applications had to be handled through costly leased lines. The availability of those leased circuits was guaranteed, but users paid a high price for that constant access. In the case of low volume or sporadic use, leased lines often become prohibitively expensive and difficult to cost justify. Now, using dial-up X.25, businesses can improve the profitability of existing branches. For instance, it may be advantageous for a car rental company to open small, minimally staffed branch offices in small airports, hotels, travel agencies, etc. These remote sites can use dial-up X.25 for access to the corporate communications network to process orders, handle billing, etc. Affordable, remote sites for a variety of businesses are possible using dial-up X.25. X.25 NETWORK SUPPORT FOR DIAL-UP ACCESS Several industry observers have described the issue of dial-up X.25 capability as a "chicken and egg" situation. Users have been reluctant to use dial-up X.25 until the price is lowered, while service providers did not want to cut prices until the volume of use increases. Both sides recognize the potential held by dial-up X.25 and the users, modem manufacturers and service providers have taken the initiative to help dial-up X.25 realize its potential. Many networks have already implemented dial-up X.25 service and others have indicated they will be implementing the service. Additionally, indications are that dial-up X.25 service will soon be extended into more cities around the world and across the U.S. and trends toward lower costs are being implemented by many packet switch carriers as tariffs are adjusted. THE ROLE OF X.25 AND PACKET SWITCHING IN THE FUTURE As the Integrated Services Digital Network (ISDN) grows in its implementation, X.25 packet switching usage will grow as a result of its use for ISDN. Since ISDN provides packet switching services on both its signaling (D-channel) and voice/data (B-channel) channels, packet switching capability becomes automatic for ISDN users equipped for data transfer. As ISDN islands appear, X.25 can link these islands through X.25 gateways. These gateways result from integrating packet switches into the overall digital switching system. In addition to V-series system products, equipment such as the Hayes ISDN PC Card which offers X.25 support will help to expedite and enhance the growing use of dial-up X.25. ISDN terminal adapters convert a PC to an ISDN terminal capable of accessing ISDN's 2B+D Basic Rate Interface. HAYES SUPPORT FOR X.25 X.25 is a mature standard, widely implemented and supported by a large and growing number of data communications equipment and service companies. This widespread acceptance of X.25 provides a tremendous foundation for use in meeting the data communications needs of the future. Hayes has been an outspoken advocate for the support of recognized CCITT standards by data communications products. The company will continue to be an active participant in helping to establish new standards. The company intends to continue its support of CCITT standards and plans to help develop the data communications promise held by X.25 and packet chnology through working in the standards setting process at the CCITT and through participation in this growing market.