Workshops

Blazing With Inverse Multiplexed ISDN

We tested inverse multiplexing units that support videoconferencing over ISDN BRI circuits from the "Big Three" fractional T1 imux vendors: Adtran, Ascend Communications and Promptus Communications. Additionally, we tested a U.S. Robotics Sportster ISDN 128K terminal adapter.

Imux box designs are diverse, depending on the application and need for additional functions. One class of inverse multiplexing applications supports LAN internetworking. Routers on two geographically separated LANs can be connected at aggregate speeds that are multiples of 1.54 Mbps by inverse multiplexing a group of dedicated T1 circuits. Ethernet speeds can easily be obtained--seven T1s will aggregate to 10.8 Mbps.

There are imux produ cts on the horizon that will provide fail-soft alternative paths when T1 circuits are cut or broken. The products will have the potential to restore all or fractions of a T1 circuit's bandwidth quickly by using digital dial-up services to connect an ISDN Primary Rate Interface (PRI) or by aggregating a group of ISDN Basic Rate Interface (BRI) or switched-56 circuits.

Inverse multiplexed ISDN BRI is the application area that has the greatest installed base of imux boxes. More than 10,000 standalone boxes for room-system videoconferencing and telemedicine applications have been deployed worldwide since 1991. These boxes typically connect to videoconferencing DTE using RS449 synchronous serial interface cables. But as more rollabout and room videoconferencing systems are designed around Windows PCs, imux devices are showing up as internal plug-in PC Cards. They draw their power from the PC and usually have ISDN U-interfaces, so expensive and bulky network termination unit (NTU) devices are not required.

Chaining BRIs Videoconferencing systems demonstrate the most cost-effective trade-off between acceptable picture quality and affordable communications by running at 384 Kbps. ISDN PRI, which delivers T1 speed (1.54 Mbps) for about $900 per month in many parts of the United States, is overkill. But ISDN BRI, which can deliver 64 Kbps per channel for around $35 per month, is cost-effective--where you can get it.

Additionally, home-based Web surfers are beginning to benefit from another class of imux design: the BRI terminal adapters (TAs). BRI TAs are plug-in PC cards that function like NICs. The internal card design can support full 128-Kbps connections to an Internet service provider (ISP)--data flows through an IP stack and internal bus rather than via a Serial Line IP (SLIP) and serial asynchronous interf ace to a freestanding desktop unit.

Although Ascend once attempted to dominate the market with its Ascend Inverse Multiplexing (AIM) approach, the company has led standardization efforts and now supports the new Bandwidth On Demand Interoperability Network Group (Bonding) protocol. Promptus and Adtran also have good Bonding implementations. Early imux pioneers take note, videoconferencing practitioners report that early glitches have been cleared up: We found that imuxing boxes shipped or field-upgraded during the past year by these manufacturers now interoperate.

In our experience, ISDN BRI service is based on reliable technology. We found that it performs solidly after the necessary connections are completed. Imux devices from the Big Three and U.S. Robotics posed no problems. Each demonstrated clean performance over circuits that were set up identically. Imuxing DS0 or switched-56 circuits, which is an alternative in areas where ISDN is not available, is a different story. Throughout our testing, simple things like line glitches and imprecise clock rates that go undetected or uncorrected led to annoying frame freezes and poor sound quality.

Using ISDN, the setup of that first successful end-to-end connection--between videoconferencing DTEs and their associated IMUX devices, for example--generates lots of horror stories.A variety of provisioning options is available with ISDN, so the imux operator making initial setups at each end must know how the lines are provisioned, and whether the supporting telco switch observes the Custom ISDN or National ISDN standard protocol.

We have found that the DTEs may cloud the issue. A frequent issue in first-time videoconferencing system setups is ensuring that the audio algorithms match. The H.320 standards suite allows three audio options: G.711, G.722 and G.728. If a common choice is not preset, or if autonegotiation does not work between the coder/decoders (codecs), a dialed-up connection may appear to fail, even if the imux layer works. Diagnostic displays of an imux's call progress and codecs' status may have to be examined to determine which settings to adjust.

It's Not Just Bonding Modern imuxes have more to do than just "bond" bitst reams. They also need to make ISDN connections quickly and transparently. Although as many as 16 telephone calls could be required to bond a 1.024-Mbps connection, the most common configuration for videoconferencing is six calls on three ISDN BRI lines for 384 Kbps.