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16.4. ISDN

Last modified by Bill Lloyd .

A good resource for information on ISDN technology and hardware is Dan Kegel's ISDN Page.

A quick simple roadmap to ISDN follows:

Cost is a significant factor in determining what solution you will choose. The following options are listed from least expensive to most expensive.

16.4.1. ISDN Cards

Contributed by Hellmuth Michaelis .

This section is really only relevant to ISDN users in countries where the DSS1/Q.931 ISDN standard is supported.

Some growing number of PC ISDN cards are supported under FreeBSD 2.2.x and up by the isdn4bsd driver package. It is still under development but the reports show that it is successfully used all over Europe.

The latest isdn4bsd version is available from ftp://isdn4bsd@ftp.consol.de/pub/, the main isdn4bsd ftp site (you have to log in as user isdn4bsd , give your mail address as the password and change to the pub directory. Anonymous ftp as user ftp or anonymous will not give the desired result).

Isdn4bsd allows you to connect to other ISDN routers using either IP over raw HDLC or by using synchronous PPP. A telephone answering machine application is also available.

Many ISDN PC cards are supported, mostly the ones with a Siemens ISDN chipset (ISAC/HSCX), support for other chipsets (from Motorola, Cologne Chip Designs) is currently under development. For an up-to-date list of supported cards, please have a look at the README file.

In case you are interested in adding support for a different ISDN protocol, a currently unsupported ISDN PC card or otherwise enhancing isdn4bsd, please get in touch with .

A majordomo maintained mailing list is available. To join the list, send mail to and specify:

    subscribe freebsd-isdn

in the body of your message.

16.4.2. ISDN Terminal Adapters

Terminal adapters(TA), are to ISDN what modems are to regular phone lines.

Most TA's use the standard hayes modem AT command set, and can be used as a drop in replacement for a modem.

A TA will operate basically the same as a modem except connection and throughput speeds will be much faster than your old modem. You will need to configure PPP exactly the same as for a modem setup. Make sure you set your serial speed as high as possible.

The main advantage of using a TA to connect to an Internet Provider is that you can do Dynamic PPP. As IP address space becomes more and more scarce, most providers are not willing to provide you with a static IP anymore. Most standalone routers are not able to accommodate dynamic IP allocation.

TA's completely rely on the PPP daemon that you are running for their features and stability of connection. This allows you to upgrade easily from using a modem to ISDN on a FreeBSD machine, if you already have PPP setup. However, at the same time any problems you experienced with the PPP program and are going to persist.

If you want maximum stability, use the kernel PPP option, not the user-land iijPPP.

The following TA's are know to work with FreeBSD.

Most other TA's will probably work as well, TA vendors try to make sure their product can accept most of the standard modem AT command set.

The real problem with external TA's is like modems you need a good serial card in your computer.

You should read the serial ports section in the handbook for a detailed understanding of serial devices, and the differences between asynchronous and synchronous serial ports.

A TA running off a standard PC serial port (asynchronous) limits you to 115.2Kbs, even though you have a 128Kbs connection. To fully utilize the 128Kbs that ISDN is capable of, you must move the TA to a synchronous serial card.

Do not be fooled into buying an internal TA and thinking you have avoided the synchronous/asynchronous issue. Internal TA's simply have a standard PC serial port chip built into them. All this will do, is save you having to buy another serial cable, and find another empty electrical socket.

A synchronous card with a TA is at least as fast as a standalone router, and with a simple 386 FreeBSD box driving it, probably more flexible.

The choice of sync/TA vs standalone router is largely a religious issue. There has been some discussion of this in the mailing lists. I suggest you search the archives for the complete discussion.

16.4.3. Standalone ISDN Bridges/Routers

ISDN bridges or routers are not at all specific to FreeBSD or any other operating system. For a more complete description of routing and bridging technology, please refer to a Networking reference book.

In the context of this page, I will use router and bridge interchangeably.

As the cost of low end ISDN routers/bridges comes down, it will likely become a more and more popular choice. An ISDN router is a small box that plugs directly into your local Ethernet network(or card), and manages its own connection to the other bridge/router. It has all the software to do PPP and other protocols built in.

A router will allow you much faster throughput that a standard TA, since it will be using a full synchronous ISDN connection.

The main problem with ISDN routers and bridges is that interoperability between manufacturers can still be a problem. If you are planning to connect to an Internet provider, I recommend that you discuss your needs with them.

If you are planning to connect two lan segments together, ie: home lan to the office lan, this is the simplest lowest maintenance solution. Since you are buying the equipment for both sides of the connection you can be assured that the link will work.

For example to connect a home computer or branch office network to a head office network the following setup could be used.

Example 16-1. Branch office or Home network

Network is 10 Base T Ethernet. Connect router to network cable with AUI/10BT transceiver, if necessary.

    ---Sun workstation     
    ---FreeBSD box
    ---Windows 95 (Do not admit to owning it)
    Standalone router
    ISDN BRI line

If your home/branch office is only one computer you can use a twisted pair crossover cable to connect to the standalone router directly.

Example 16-2. Head office or other lan

Network is Twisted Pair Ethernet.

        -------Novell Server
        | H |
        |   ---Sun      
        |   |
        | U ---FreeBSD
        |   |
        |   ---Windows 95
        | B |
        |___---Standalone router
            ISDN BRI line

One large advantage of most routers/bridges is that they allow you to have 2 separate independent PPP connections to 2 separate sites at the same time. This is not supported on most TA's, except for specific(expensive) models that have two serial ports. Do not confuse this with channel bonding, MPP etc.

This can be very useful feature, for example if you have an dedicated internet ISDN connection at your office and would like to tap into it, but don't want to get another ISDN line at work. A router at the office location can manage a dedicated B channel connection (64Kbs) to the internet, as well as a use the other B channel for a separate data connection. The second B channel can be used for dialin, dialout or dynamically bond(MPP etc.) with the first B channel for more bandwidth.

An Ethernet bridge will also allow you to transmit more than just IP traffic, you can also send IPX/SPX or whatever other protocols you use.