VINES X.25 Guide
Chapter 1 - Overview of VINES X.25
The VINES X.25 option lets a server use the X.25 packet switching protocol to communicate through a private or public data network (PDN), through an Integrated Services Digital Network (ISDN), or directly with other servers and host computers.
This chapter provides instructions and guidelines to help you set up the VINES X.25 option. Setting up the option includes the following major tasks:
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Calculating the number of virtual circuits that you need ![]()
Meeting data networks and ISDN service requirements ![]()
Installing X.25 hardware and software ![]()
Assigning the X.25 communications line
Before you proceed, read through the sections on the topics listed above. This will give you an overview of the process and help you determine the tasks you must perform.
Chapter 2 describes how to install the X.25 option and assign X.25 communications lines.
X.25 consists of a user interface to X.25 and the X.25 software. Figure 1-1 describes how X.25 is implemented in VINES.
VINES X.25 has two categories of "users" : those which use the VINES Applications Toolkit and those which use VINES configuration programs. Third-party applications, X.29 dial-in, and X.29 host communications access X.25 by means of the Toolkit.
X.25 server-to-server connections use the VINES interface. VINES own configuration programs set up and clear an X.25 connection. Once the connection is established, it is seen as another interface to VINES.
There are two separate code paths, as shown in Figure 1-1, into the X.25 protocol driver. The driver handles all the protocol-specific messages to and from the VINES ICATM , ICAmCTM , or ICAplusTM cards. The ICA driver handles the exchange of messages between the ICA and the VINES server. X.25 packet and frame level software resides on the ICA card.
Note: In this document, the term "ICA card" is used to refer to an ICA, ICA mC, or ICAplus card.
For more information on third-party applications, see the VINES X.25 Programming Interface.
Table 1-1 summarizes the facilities that VINES implements.
Appendix A describes the VINES X.25 implementation according to the reference numbers of the 1984 CCITT X.25 recommendations.
This section describes the prerequisites and limitations of the VINES X.25 option.
The VINES X.25 option has the following prerequisites for servers:
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VINES ICA, ICA plus, or ICAmc card ![]()
A synchronous modem for servers connected to a public data network (PDN) or AT&T 7500 Data Module for servers connected to an ISDN network ![]()
VINES X.29 option if X.29 dial-in communications are required ![]()
VINES X.29 option and the VINES Asynchronous Terminal Emulation (ATE) option if terminal emulation to a host computer is required
See the VINES X.29 Dial-In Guide for X.29 Dial-in requirements. For servers and workstations, see the Banyan Asynchronous Terminal Emulation Guide for X.29 host requirements.
The VINES X.25 option has the following limitations:
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A VINES server supports a maximum of 256 virtual circuits. This limit applies to all server platforms. ![]()
An ICA card supports a maximum of 128 virtual circuits. Note that the ICA card is the only serial communications card that the option supports. ![]()
An ICA card has a maximum of two X.25 lines. ![]()
An X.25 line supports a maximum of 32 Permanent Virtual Circuits (PVCs).
Once two servers, a server and a host computer, or DOS workstations and a server are connected through a PDN or ISDN network, communication, such as the delivery of Network Mail messages, asynchronous terminal emulation (ATE), or X.29 dial-in sessions, can take place.
Unlike communications through telephone lines, there is no dedicated point-to-point connection between the two servers, between the server and the host computer, or between the server and the DOS workstation. The PDN or ISDN service routes packets using whatever lines are available in its network. Once a connection to a PDN or ISDN service is made, this process is invisible to the server hardware and software and to the user.
Packets that contain data and control information flow between the two servers, between the server and the host computer, or between the server and the DOS workstation. The control and data packets exchanged by the two servers, or by the server and the host computer, or by the server and workstation use the X.25 communications protocol.
Connections Create a VINES Network
When the X.25 option connects two servers, the servers and any devices connected to them become one VINES network, subject to the internet passwords and access lists defined at each site. Depending on how these controlling elements are set up, users at one site may use services at the other site and the servers may attempt to exchange Network Mail messages.
When you define a connection to a remote server, you can optionally include a schedule for automatic connection. Typically, the schedule reflects how often you believe the two servers should exchange mail messages.
You can initiate a remote connection immediately at any time, whether or not the connection also has a schedule associated with it.
Server-to-Server Communications
Server-to-server communications take place through a PDN or by means of a direct connection.
Servers Connected Through a PDN
Figure 1-2 illustrates connecting two servers through a PDN. Each server must be running the VINES X.25 option. Each server requires a synchronous modem to access the PDN.
Servers Directly Connected
Figure 1-3 illustrates connecting two servers directly over a switched or leased line. Each server requires a synchronous modem.
Workstation-to-Server Communications
X.29 dial-in communications can take place between workstations and servers through PDNs, ISDN networks, or directly over switched point-to-point duplex lines.
To communicate through an X.25 PDN or a switched point-to-point telephone line, workstations require a modem and should have access to a Packet Assembler/Disassembler (PAD).
A PAD is a device that performs the following functions:
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Accepts asynchronous characters from the workstation and assembles them into an X.25 packet for transmission to the server through the PDN or switched telephone line ![]()
Accepts X.25 packets from the PDN or switched telephone company line and disassembles them into asynchronous characters to be passed to the DOS workstation
PADs may be local or remote.
The next sections illustrate these topologies.
Connections Through a PDN
Figure 1-4 shows three workstations that dial in to a PAD at the PDN. The PAD accepts the asynchronous characters from the DOS workstations and assembles them into packets.
Note that workstations could also be connected directly to a multiport PAD with an RS-232 cable. The PAD has an internal modem and dials the PDN directly.
See the VINES X.29 Guide for more information on this topology and the requirements for X.29 dial-in connections.
Connections Through an ISDN Network
Figure 1-5 shows workstations connected to a server through an ISDN network. A workstation requires an AT&T ISDN terminal adapter (TA). TAs are telephone-like devices that accept asynchronous characters from the DOS workstation and assemble them into X.25 packets for transmission to the server. The X.25 line is connected to an AT&T Data Module, which, in turn, is connected to the server.
See the VINES X.29 Guide for more information on this topology and the requirements for X.29 dial-in connections.
Connections over a Switched Line
Figure 1-6 shows three workstations connected to a multiport PAD. The PAD accepts asynchronous characters from the DOS workstations and assembles them into packets for transmission to the server. Even though there is no intermediate PDN or ISDN network between the PAD and the server, X.25 protocols are still used to communicate over the switched line.
See the VINES X.29 Guide for more information on this topology and the requirements for X.29 dial-in connections.
Follow these steps to set up an X.25 connection through a PDN, an ISDN network, or directly:
1. Subscribe to a PDN, an ISDN network, or arrange for lines to connect the two servers or the workstations and server.
2. Install the X.25 hardware and software. The X.25 option uses a serial communications card and attached lines to communicate with remote sites.
3. Assign communications lines at both sites.
4. Make the proper modems and lines available to connect the X.25 hardware to the PDN, ISDN network or to other lines.
Chapter 2 describes how to install the software option and assign lines.
You can connect a server to a host through a PDN. If a PDN is not available, the X.25 option can connect your servers and host computer directly to each other through a switched or a dedicated line. A dedicated line can be either a leased telephone line or a short line (100 feet or less) that uses a modem eliminator.
For server-to-host communications, you must install the X.25 option on a server that has the X.29 option and the Asynchronous Terminal Emulation option installed. Servers equipped with these options let users establish asynchronous terminal emulation sessions with host computers over X.25 lines.
The host must run software that implements the Consultative Committee for International Telephone and Telegraph (CCITT) standards for X.3, X.25, X.28, and X.29 protocols.
Figure 1-7 illustrates a server connected to a host.
Set-up Procedure
Follow these steps to set up a server-to-host connection:
1. Subscribe to a PDN or arrange for lines to connect the server and host.
2. Install the X.25 hardware and software. The X.25 option uses a serial communications card and attached lines to communicate with hosts.
3. Assign a communications line at the server. Chapter 2 describes how you do this. Pay close attention to the configuration. If you have questions about the configuration, contact your PDN.
4. Make the proper modems and lines available to connect the X.25 hardware to the PDN or to other lines.
5. Verify that the X.29 and Asynchronous Terminal Emulation options are installed on the server, and that an asynchronous terminal emulation service is created and properly configured. See the Banyan Asynchronous Terminal Emulation Guide for details on both these options.
Chapter 2 describes how to install the X.25 option and assign lines.
This section explains how you use permanent virtual circuits (PVC) and switched virtual circuits (SVC) to connect two servers, a server and a host computer, or DOS workstations and servers through a PDN.
For information on using the X.25 option directly through switched or dedicated lines without a PDN, see "Using the X.25 Option without a PDN" later in this chapter.
A permanent virtual circuit (PVC) is a permanent logical association between two physically separate DTEs (Data Terminal Equipment). DTEs include servers and hosts.
PVCs are used only for server-to-server communications. They provide you with access to the remote server that is always present as long as the DTEs are running. A PVC reserves a path through the PDN from one server to another so data can be exchanged. A PVC is more expensive than a switched virtual circuit.
The availability of a PVC is analogous to the availability of a leased telephone line in that the user is not required to set up or clear the call. Note that the leased line analogy applies only to the nature of PVCs within a PDN; it does not describe the type of line you use to connect to the PDN.
A PVC is dedicated to a specific remote location. You define a PVC when you subscribe to a PDN and the PDN specifies the channel on which the PVC exists.
Example PVCs Connecting Two Cities
If you have a PVC between a server in city A and a server in city B, that PVC cannot connect either server to one in city C, as shown in Figure 1-8.
A switched virtual circuit (SVC) is a temporary logical connection between two physically separate DTEs. It is set up on one channel that you subscribe for from a PDN. It exists until the call is cleared or the DTEs stop running. An SVC is analogous to a switched telephone line.
SVCs are used for all forms of server-to-server, server-to-host, and workstation-to-server communication. Third-party applications written with the Banyan Application Toolkit also use SVCs.
The availability of SVCs depends on how many users attempt to use the PDN or ISDN network.
Types of SVCs
There are three types of SVCs:
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Incoming virtual circuits (IVCs) - Used to receive an incoming call, but not to place an outgoing call. A host with only IVCs can answer calls, but cannot place any calls. IVCs are used for X.29 dial-in communications, X.25 server-to-server connections, and applications developed with the Banyan Applications Toolkit. When IVCs are used with X.25 server-to-server connections, the remote server places the call. ![]()
Two-way virtual circuits (TVCs) - Used to receive an incoming call and place an outgoing call. A host with only two-way virtual circuits can call remote hosts and answer any calls. TVCs are used for X.25 server-to-server communications, X.29 dial-in communications, X.29 host communications, and applications developed with the Banyan Applications Toolkit. ![]()
Outgoing virtual circuits (OVCs) - Used to place outgoing calls, but cannot receive incoming calls. A host with only outgoing virtual circuits can call other hosts but cannot answer calls from other hosts. OVCs are used for X.29 host communications, X.25 server-to-server connections, and applications developed with the Banyan Applications Toolkit
Using SVCs
You can use an SVC to connect any two servers at any two sites.
You can also use an SVC to connect a user of an ATE service on any server and any host computer. A single outgoing or two-way SVC supports one X.29 asynchronous terminal emulation session between a user and a host.
SVCs connect workstations and servers (X.29 Dial-in). An X.29 Dial-in session uses one Incoming or Two-way Virtual Circuit.
Keep in mind that applications written with the X.25 Programming Interface, X.29 ATE sessions, and X.29 dial-in sessions only use SVCs. Take this into account when you determine the number of virtual circuits you need. For details, see "Subscribing for Virtual Circuits" later in this chapter.
Example SVCs Connecting Two Cities
As shown in Figure 1-9, city A and city B no longer need to use an SVC. That SVC can now connect the server in city B to the server in city C, or any two servers at sites that are connected to the PDN.
Summary
Table 1-2 summarizes the differences between the types of SVCs.
Table 1-3 summarizes how PVCs and SVCs are used.
Refer to Table 1-3 to determine which type of virtual circuits you should subscribe for with the PDN or ISDN provider. For example, if your server is running applications written with the X.25 Programming Interface, you must use SVCs.
VINES makes available 256 X.25 circuits (PVCs and SVCs or both) per server.
You can distribute the 256 VCs among all the cards supported and installed in your server with a maximum of 128 virtual circuits per card.
Figure 1-10 shows a server with two cards and the 256 virtual circuits distributed evenly between the two cards.
Figure 1-11 shows a server with four ICA cards. Each card supports 64 circuits.
Each serial communications card supports two X.25 serial communications lines.
On each card, the virtual circuits can be divided between the two lines as needed.
Example Vines Server With Two ICA Cards
Figure 1-12 shows a server with two cards and 80 and 120 circuits configured on each card. The first card configured for 80 virtual circuits can have one line with 50 circuits and another line with 30 circuits. The 128 virtual circuits on the second card can be divided evenly between the two lines.
Subscribing for Virtual Circuits
You should subscribe to a PDN or telephone company that provides ISDN service for the same number of virtual circuits that you configure. For example, if you have only one ICA card, you can subscribe for 128 virtual circuits and configure all 128.
Configured circuits are subtracted from the server maximum of 256. If you subscribe for and configure 128 circuits on a server, you have 128 extra circuits (256 maximum circuits minus 128 configured circuits). In this case, your server can easily support 128 simultaneous open connections.
There is a problem if you add a second and then a third 128-circuit ICA card to your server, subscribe for more than 256 circuits, and configure all of them. The server now has more than 256 configured circuits, but can use only a maximum of 256 circuits.
If you try to use more than the maximum 256 connections, one of two error conditions can occur:
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If you receive an incoming call, you lose one of your listening circuits. ![]()
If you place an outgoing call, the attempt is unsuccessful and you receive a COM162 error message (no buffer space available).
What Kinds of Virtual Circuits
The type of virtual circuits that you subscribe for depends on the application your server is running or the type of connection you want to establish. Refer to Table 1-3.
To determine the number of virtual circuits for which you should subscribe with the PDN or the ISDN provider, answer the following questions:
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How many servers will your server be communicating with over the PDN or ISDN network? ![]()
How many concurrent X.29 asynchronous terminal emulation sessions with host computers do you need to support? ![]()
How many X.29 dial-in sessions do you need to support between DOS workstations and a server?
How Many SVCs
To determine the number of SVCs you need, use the following formula:
Total SVCs = Total Servers + Total X.29 ATE Sessions + Total X.29 Dial-in Sessions + Other Connections
Total Servers. The number of servers your server communicates with over the X.25 line using SVCs. You need one available virtual circuit for each server with which your server communicates. For example, if your server communicates with six other servers, you need to have six virtual circuits available for the X.25 line for the times when all the connections are active simultaneously.
The SVCs can be used by applications written with the X.25 Programming Interface or by other forms of server-to-server communication such as Network Mail.
Total X.29 ATE Sessions. The number of concurrent X.29 asynchronous terminal emulation sessions with host computers that your users require. See the next section for more information.
Total X.29 Dial-in Sessions. The number of concurrent X.29 dial-in sessions with a VINES server that your DOS workstation users require. Each X.29 dial-in session requires one SVC.
Other Connections. The number of concurrent connections between applications on your server written with the X.25 Programming Interface and applications on non-VINES machines that support the X.25 protocol, such as host computers.
Total SVCs. The total number of VCs to which you need to subscribe.
SVCs for ATE Sessions
Each X.29 asynchronous terminal emulation session uses one two-way or one outgoing SVC for the X.25 line that connects the server and the host computer. Several sessions can run simultaneously. For each session, the asynchronous terminal emulation service allocates an SVC associated with the X.25 line. If none of those SVCs is available, the user must try again later.
Regardless of the number of circuits available, the Asynchronous Terminal Emulation service has a limit of 32 concurrent sessions per service. ATE sessions can be established only when the connection to the PDN or ISDN network is active. Only one ATE service can be started on a server. Keep these limits in mind when you subscribe for SVCs. See the Banyan Asynchronous Terminal Emulation Guide for more information.
Note: Keep in mind that as you reserve more virtual circuits per card for server-to-server communications or X.29 dial-in sessions, you may decrease the number of virtual circuits available for server-to-host communications, and vice versa. You may adversely affect performance as you increase the number of configured SVCs. Performance is affected even when all virtual circuits are not in use. Do not configure virtual circuits unless you need them.
You can use the X.25 Programming Interface to write an application that communicates with an application on a non-VINES machine, such as a host computer. The non-VINES machine must support the X.25 protocol. Each connection to an application on a non-VINES machine uses one SVC.
How Many PVCs
You may want to use PVCs to keep connections with other servers. However, since a PVC is reserved for communications with only one server, one PVC is required for every server you want to connect to. Also remember that only server-to-server connections support PVCs and an ICA line can support only 32 PVCs.
Total Number of Virtual Circuits
Use the following formula to determine the total number of virtual circuits that you must configure:
Total VCs = Total SVCs + Total PVCs
The VINES X.25 option supports the following PDNs as shown in Table 1-4.
Note that the PDN designated "CCITT 84" at the end of the list refers to any PDN or private packet switching network (PSN) that adheres to the CCITT (Consultative Committee for International Telegraphy and Telephony) 1984 version of X.25. A private PSN is a network that a company might set up to connect its own computers and that employs packet-switching technology.
The inclusion of a generic PDN indicates that the VINES X.25 option may work with PDNs that are not listed or with private PSNs that support CCITT 84.
PDN and ISDN Network Requirements
When you subscribe to a PDN or ISDN service provider, you must meet certain requirements.
Before you use the X.25 option through a PDN or ISDN network, you must subscribe to a PDN or a telephone company that provides ISDN services and contract for certain services. Each PDN or ISDN service provider sets its own rate schedule, defines its subscribers' hardware requirements, and provides information that enables your site to participate in the network.
In most cases, the PDN installs a dedicated private line that connects your site to its network. Alternatively, some PDNs provide dial-in service.
The sections that follow describe the requirements you must meet and information you must obtain from the PDN or ISDN service provider to use the X.25 option. Topics include:
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Modem requirements ![]()
Facilities information ![]()
PVC information ![]()
SVC information
The phone line or private line must be connected to modems at both ends - one at the PDN or at the ISDN service provider and one at your site. You must have a modem that meets the requirements of the PDN or ISDN network. The PDN or ISDN network usually offers to lease the appropriate modem or provides you with a list of acceptable modems. An AT&T 7500 Data Module is recommended for ISDN connections. See the VINES X.29 Guide for more information on ISDN modem requirements.
Each modem transmits characters at a certain speed. The PDN, ISDN service provider, or the modem manufacturer will tell you which transmission speed your modem uses.
Some PDNs support what is called a facilities field. This field is transmitted to the network when a connection is made. It specifies the use of special services that the PDN provides, such as reverse billing (the equivalent of a collect call). If you plan to use special facilities, you must find out the hexadecimal codes the PDN uses to specify them. (Table 1-1 lists the facilities that VINES supports.)
Note: Facilities information does not apply to X.29 dial-in sessions.
You specify facilities information at three points:
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When you assign the communications line ![]()
When you define a remote server connection ![]()
When you define an X.29 connection
The facilities information you specify when assigning a line transmits every time the line is used to make a connection. This information therefore acts as a default facilities string for your site.
If you also specify facilities when defining a remote server connection, that data is added onto the facilities string specified when you assign the line.
The X.29 option lets you specify facilities information when you create an X.29 connection, which is part of the asynchronous terminal emulation service configuration. When you create the connection, specify either all of or a subset of the facilities defined when you assigned the X.25 line that the connection uses. See the Banyan Asynchronous Terminal Emulation Guide for details.
In most cases, server-to-server connections do not require any special facilities. Use facilities only if your PDN requires them to establish the connection or to reverse charges.
Note: Invalid facilities information causes a call to fail. Be careful to obtain and supply the correct information.
For each PVC you use, you must obtain information from the PDN provider that you specify when you assign X.25 lines. This information includes:
Packet Size. Each PDN accepts one or more packet sizes used for all connections involving PVCs. The packet size determines how much user data is sent to or received from the network at one time. For each PVC, consult with the PDN for the proper packet size.
You are prompted to select valid receive and transmit packet sizes for each PVC that you specify. If a PDN supports only one packet size, that packet size is used and you are not prompted to select a packet size. You must configure each PVC.
Window Size. This value indicates the maximum number of packets that are sent or received without sending or receiving an acknowledgment. You are prompted to specify a value for all transmit and receive window sizes for each PVC. If a PDN supports only one window size, that size is used and you are not prompted for the size.
Logical Channel Number. Each physical connection (line) to a PDN has a maximum of 4095 possible logical channels, numbered from 1 to 4095. Each number identifies a PVC. Usually, not all of these logical channels are used.
When you purchase services from a PDN, you specify how many PVCs you want. You must obtain the starting number (from 1 to 4095) of the logical channels associated with your PVCs and how many are available under the terms of your contract. Use the logical channel number of that PVC to connect to that server. Since a PVC is a dedicated virtual connection between two specific sites, you do not need to know the DTE address. For each ICA line used for X.25, a total of 32 PVCs are available.
Link Level Window Size.The link level interprets a stream of bits transmitted over a physical link. A bit stream is assembled into frames and then packets at the next level. A window is the maximum number of frames that can be transmitted without receiving an acknowledgment. The PDN tells you what value it uses. The value is usually seven.
Table 1-5 is a worksheet in which you can enter information about PVCs before configuring the X.25 option.
Information about PVCs is given to a PDN when you contract for virtual circuits. All other information is entered when you configure the X.25 option.
For SVCs, you must obtain information for the X.25 configuration program when you assign X.25 lines. This information includes:
DTE Addresses. Each server or host computer that participates in the network has a unique address. These systems are identified by a DTE address. The PDN or ISDN network assigns this address to your server when you contract with it for services.
To connect to a remote server through an SVC, you must know its DTE address. Some PDNs also require that you specify the local DTE address of the server at your site when you attempt to connect to a remote server.
To establish an X.29 asynchronous terminal emulation session with a host computer through an SVC, you must enter the host computer' s DTE address when the asynchronous terminal emulation service is configured. See the Banyan Asynchronous Terminal Emulation Guide for details.
For X.29 dial-in sessions through a PDN, the PAD of the PDN specifies the server' s DTE address. For X.29 dial-in sessions through an ISDN network, the DOS workstation specifies the server' s DTE address. See the VINES X.29 Guide for more information.
Packet Size. The packet size determines how much user data is sent onto the network at one time. This value applies to all types of SVCs. Each PDN accepts one or more packet sizes for all connections involving SVCs. You must find out the acceptable packet sizes from the PDN.
Enter packet size information when you assign the X.25 line. You select valid packet sizes for each PDN you specify, the generic PDN. If a PDN supports only one packet size, that packet size is used and you are not prompted for information.
Window Size. This value applies to all types of SVCs. It indicates the maximum number of packets transmitted without receiving an acknowledgment. You select a valid window size for all PDNs specified. The window size is usually 2, but can be from 1 to 7. If a PDN supports only one window size, that size is used and you are not prompted for information.
Logical Channel Number. Each physical connection (line) to a PDN has a maximum of 4095 possible logical channels, numbered from 1 to 4095. Each number identifies either an outgoing, one-way, or two-way SVC. Usually, not all of these logical channels are used. PDNs or ISDN networks usually begin logical channels with number 1.
When you purchase services from a PDN or ISDN network, specify the types and how many SVCs you want. You must obtain the starting number (from 1 to 4095) of the logical channels associated with your SVCs and how many are available under the terms of your contract. For each ICA card you use for X.25, a total of 128 virtual circuits are available.
Link Layer Window Size. A window is the maximum number of frames transmitted without receiving an acknowledgment. The PDN or ISDN service provider tells you what value it uses. The value is usually seven.
Table 1-6 is a worksheet in which you can enter information about SVCs before configuring the X.25 option.
Information about virtual circuits (IVCs, OVCs, and TVCs) is given to a PDN or the telephone company that provides ISDN service when you contract for virtual circuits. All other information is entered when you configure the X.25 option.
Using the X.25 Option without a PDN
This section describes how to use X.25 to connect servers to hosts and DOS workstations to servers without a PDN.
You can use the X.25 option without a PDN by arranging for a communications line from some other source to connect the two servers. For example, you can use a switched or dedicated line. Dedicated lines include leased telephone lines or lines that use modem eliminators.
Switched or leased telephone lines must have a modem on each end of the line. You need to know the speed of the modem and whether the line you are using is switched or leased
Designating a DTE and DCE
To connect two servers without a PDN, designate one server as the Data Terminal Equipment (DTE) and the other as the Data Communications Equipment (DCE). This distinction is an X.25 protocol requirement that does not affect how the servers behave when connected. Either server plays either role, as long as one server is designated as DTE and the other as DCE.
PVCs or SVCs
Use either PVCs or SVCs to connect two servers unless the servers are running applications written with the X.25 Programming Interface. These applications use SVCs only.
Since only one server is at the other end of the line, you need not specify any detailed connection information about virtual circuits.
Switched and Leased Lines
If you connect to the other server through a switched telephone line, you must:
1. Dial in to the other server to make a physical connection.
2. Establish a logical connection.
See "Adding Connections to Remote Sites with SVCs" in Chapter 2 for the connection procedure.
If you use a leased line, the logical connection is made as soon as you meet all the hardware and software requirements and assign the communications line.
To connect a server and a host computer without a PDN, you must:
1. Assign the X.25 line as if it were a line to a PDN.
2. Supply all the necessary information about SVCs.
Keep in mind that you must configure a sufficient number of SVCs to handle the peak load of X.29 user sessions.
If you connect to the host through a switched telephone line, you must:
1. Dial in to the host to make a physical connection.
2. Establish X.29 asynchronous terminal emulation sessions.
If you use a dedicated line, the connection to the host is active as soon as you meet all the hardware and software requirements and assign the communications line. You can then establish X.29 asynchronous terminal emulation sessions.
Note: With a single X.25 line, you can establish multiple concurrent X.29 asynchronous terminal emulation sessions with more than one host without using a PDN. However, the X.25 line must be connected to special switching equipment that you purchase from a third party.
Connecting DOS Workstations to a Server
To connect DOS workstations and a server without a PDN to X.29 dial-in sessions, follow these steps:
1. Assign the X.25 line as if it were not connected to a PDN.
2. Specify "switched line" when assigning the line. You cannot use a dedicated line to support X.29 dial-in sessions without a PDN or ISDN network.
3. Specify that the server acts as the DCE when you assign the line.
4. Supply the server' s telephone number to the administrator responsible for the PAD that makes the telephone call.
Note that the line can support up to 10 simultaneous sessions. See the VINES X.29 Guide for more information.