This section summarizes requirements for Integrated Service Digital Network (ISDN) hardware.
Under these guidelines, ISDN is recommended, but not required, for high-speed connections. If implemented in a server system, ISDN must meet the requirements defined in this section.
There are two classes of ISDN adapters:
In this section, “internal ISDN device” refers to the ISDN terminal adapter, which exposes raw access to its B channels using NDIS miniports. WDM supported bus classes, such as USB or IEEE 1394, also can be used to attach external devices using NDIS miniports.
“ISDN modem” refers to an internal or external ISDN device that exposes itself as a modem controlled by the AT command set. To the operating system, these devices look like and can be used as modems, provided that the hardware manufacturer has done work to ensure these devices have the following capabilities:
The requirements in this section apply for a serial ISDN modem designed for or included with a server system that complies with Hardware Design Guide Version 2.0 for Windows NT Server.
ISDN modems share the following features:
ISDN modems also share the following differences from wireline PSTN modems:
Required
An ISDN modem must support basic AT commands: TIA-602, which is a subset of ITU V.250. The ISDN modem shall support commands to select the end-to-end protocol used over the ISDN; synchronous PPP, V.110, V.120, and so on. Also, commands must be included to set the switch type, subscriber numbers or directory numbers (where applicable), and SPID or EAZ (where applicable), to allow user selection if auto-detection fails. These can be implemented in the device or in the communications driver.
Recommended
ISDN modems should expose both B channels so that they can leverage the multilink PPP support included in the operating system.
Multilink PPP, as defined in RFC 1717, combines several ISDN B channels to increase the bandwidth of PPP links.
When using ISDN modems connected to the server using a single serial port, the capabilities included in the operating system cannot be leveraged and the users may not be able to fully benefit from the features in the ISDN device, such as supporting two B-channels and combining them into one fast link.
This is because Windows NT cannot see both B channels of the ISDN connection unless each B channel is exposed to the operating system, either as a COM port, or by way of NDIS.
External ISDN modems should be on a port fast enough to expose the full bandwidth of both B channels, such as USB. Providing two separate COM-port cables is not an acceptable solution.
Required
These types of ISDN devices are treated as modems, not as internal ISDN devices supported using NDIS WAN miniports. In the external case, the primary implication is that the operating system will send byte-level PPP, also known as asynchronous PPP. In the NDIS WAN case, the implication is that the operating system will send bit-level PPP, also known as synchronous PPP.
Because ISDN is a synchronous service and an ISDN modem connects to an asynchronous port on the system, the device must provide some means of converting asynchronous data to synchronous data.
Recommended
Because of speed limitations inherent in a server’s COM ports, the connection for ISDN modems should be high speed, such as USB or IEEE 1394. A specification for controlling an ISDN TA over USB is in development by the USB Communications Device Class working group.
Required
Configuration of the dependent parameters, such as SPIDs and switch-type IDs, must be done using the ISDN Configuration Wizard included in the operating system.
This section defines general requirements for ISDN and specific requirements for ISDN terminal adapters.
Required
The driver must support NDIS 5.0. Only NDIS 5.0 miniport drivers and INF files are allowed for complete user-friendly installation and operation of the ISDN adapter. The following network adapter requirements must be met:
Required
High-level data link control (HDLC) framing is a standard for sending synchronous data. Other framing methods are allowed if the miniport driver provides simple HDLC framed synchronous PPP packets to NDIS.
Required
The internal ISDN device and the driver must support raw unframed (non-HDLC) synchronous B channel I/O at 64 Kbps for each B channel, with each B channel individually accessible. This will enable H.320 and voice calls over ISDN without audio breakup.
For these raw interfaces, the direct path to each B channel must support synchronous transmission and reception of H.221 frames, which are of 20 ms duration. To achieve this without additional latency to H.221, there must be support for overlapped I/O buffers at intervals of less than or equal to 20 ms in each direction. As underruns or overruns cause degraded audio, hardware buffering must be adequate to prevent B channel underruns and overruns. For Windows NT 5.0, 20 ms is adequate.
This can be achieved by making buffering software configurable with adequate range to handle foreseeable real-world conditions. The miniport driver should make I/O completion callbacks to NDIS for each I/O buffer as soon as the I/O for that buffer is complete and should not coalesce or delay callbacks.
Required
Configuration of the dependent parameters, such as SPIDs and switch-type IDs, must be done using the ISDN Configuration Wizard included in the operating system.
Recommended
Note: This recommendation applies only in the United States.
A network terminator (NT-1) splits the duplexed transmit and receive signals from the ISDN line into separate transmit and receive components. An ISDN device with a built-in NT-1 can connect directly to the ISDN line. However, doing so prevents other devices from being attached to the ISDN line because only one NT-1 can be connected to an ISDN line. If the ISDN device has a built-in NT-1, it also should have a connector for either analog phone or another ISDN device (S/T-interface), such as an ISDN phone.
Adding an analog (POTS) port or S/T-interface to the ISDN device delivers convenience to the SOHO market, allowing customers to use one ISDN line to meet all their telecommuting needs at minimal cost. Many customers do not want a separate analog or digital phone line for their fax machines, modems, or phone when ISDN can do this with a device that has a POTS port or S/T-interface.
Required
If the ISDN device has an S/T-interface for connecting additional ISDN devices and has configurable terminating resistors, they must be software configurable. The software selectable resistors can be selected on or off. The default value of termination is on in North America, but off in all countries where phone companies unconditionally provide the termination.