).
The reviewed TAs support V.120, the lingua franca for PC-to-PC ISDN links. It's an international standard for single-channel serial communications that supports rate negotiation. These devices also include the convenience of an RJ-11 jack or two. Thus, you can make outgoing calls with your old analog phone or send faxes from your fax machine. The TAs handle the A/D conversion. In addition to feature sets, we also evaluated performance, interoperability, and--because of the difficulties encountered with ISDN installations--technical support.
The three TAs we tested are ADAK Communications' Model 221, Motorola Information Systems Group's BitSurfr Pro, and 3Com's Impact ISDN External Digital Modem. (These products are not technically modems.) All come with DOS/Windows 3.x software. The TAs from Motorola and 3Com also support the Mac. Under other OSes, you must resort to AT comman
ds. By the time you read this, other TAs will have similar feature sets, including Farallon's Netopia and U.S. Robotics' Courier I-modem.
Interface and Configuration
ISDN has two physical interfaces, U and S/T. The telephone network delivers a two-wire U interface. A device called a network terminator (NT-1) converts the U interface to an eight-wire S/T interface. In a typical single-line installation, you'll want a TA with a built-in NT-1 to convert the U interface. All three reviewed TAs have this feature.
In an environment with a large network, such as the test facility that was used for this review, you may have many incoming ISDN channels arriving via rack-mounted NT-1s located in a building's wire room. In such a case, only the S/T interface is provided at the faceplate. However, because they all have a built-in NT-1, the TAs we tested couldn't connect to an S/T interface. Therefore, we had to rewire the ISDN feeds in the wire room. Ideally, a TA should have a built-in NT
-1 and provide both U and S/T interfaces on the unit.
ISDN line configuration, which is normally a nasty issue, was no problem with any of these devices, once we wired U interfaces. The TAs from Motorola and 3Com support the ISDN Ordering Code (IOC) system. IOCs relate the device's ISDN configuration to predefined code words such as
Capability S
. This spares you from having to provide complex configuration parameters to the telephone company.
The
ADAK 221 TA
has more features than the other two TAs we tested, but it's also the most difficult to set up and use. It handles both switched and packet data, and it operates with many protocols, including clear-channel data, V.120, X.25, SLIP, PPP, and MP. Besides supporting analog devices, such as modems and fax machines, this TA also provides connections for external ISDN devices through its S/T output jack.
The front panel has two RJ-11 jacks for analog devices, two RJ-45 jacks for the ISDN U and S/T interfaces,
and two RS-232 connectors. LEDs indicate power, network activity, and error status. The network activity lights indicate U and S/T interface synchronization but not call status, which sometimes left us wondering whether we had a call active.
The ADAK 221 is the only one of the three with both U and S/T interfaces. However, the S/T jack is deceiving; it handles only external ISDN devices, not an incoming S/T interface. Because the ADAK 221 carries FCC Class A certification, it isn't legal for residential use.
The intimidating 136-page manual is a highly technical document, and configuration options are complex. The documentation devotes nine pages, for example, to flow diagrams that illustrate the unit's many menu options. Clearly, this is a device that's designed for network and communications professionals, not for home or small office use.
You can configure the ADAK 221 through AT commands, the ITU X.28 packet-data command set, its built-in menu system, with a Windows 3.x configuration
utility, or, most interestingly, through a voice interface. The Windows utility had several bugs. With one bug, scroll bars disappeared from a text window, hiding off-screen data. Another bug prevented the saving of configuration files to the host PC's hard drive. Because the utility could properly read and write configuration information to the TA's nonvolatile memory, the configuration file was somewhat redundant. ADAK promised an updated version of the configuration utility (it didn't arrive).
The voice-configuration interface works through a telephone you plug into one of the analog line jacks. Voice commands are supposed to guide you through the configuration of the device, but it didn't work well enough to make it worth using.
One flaw we noted was the ADAK 221's inability to adapt to the terminal speed. The Motorola and 3Com units monitor terminal speed and automatically adapt themselves to speed changes. By comparison, changing speeds with the ADAK 221 requires you to do a series of awkw
ard commands. If you intend to use the unit at a single preset rate, this won't be much of a problem. Otherwise, its lack of autobaud capability is very limiting.
Instead of the usual utilitarian box, Motorola's
BitSurfr Pro
is a sleek, rounded design. With a small footprint, it fits unobtrusively on a desk. Front-panel lights indicate ISDN link status, analog call status, and ISDN call status. LEDs also indicate receive and transmit data. Unfortunately, there are no lights to indicate how many or which B channels are connected. BitSurfr Pro supports both MP and Bandwidth on Demand Interoperability (Bonding) channel aggregation.
The rear panel provides two analog ports with RJ-11 connectors, an RJ-45 connector for the ISDN U interface, and a single RS-232 connector. There is also a four-position DIP switch, but other than making sure that all switches are off, you don't need to bother with it.
An intimidating barrage of errata, addenda, and advisories greets you upon
opening the BitSurfr Pro carton. There are no less than nine separate documents bundled with the unit. Motorola should update its manual and do away with some of the superfluous sheets.
Some of the additional documentation is due to the generous amount of software that Motorola bundles with the unit. Besides the Windows configuration software, Motorola included NetManage's Internet Chameleon, a Windows-based TCP/IP package; Pacific CommWare's TurboCom/2, a Windows communications driver set; and a limited version of Hilgraeve's excellent HyperAccess communications program. Motorola also offers a version of the BitSurfr Pro with Mac software (for the same price).
It's relatively straightforward to configure and use the BitSurfr Pro. You can configure the device with the Windows configuration manager (the most convenient way), AT commands, or a rudimentary menu system invoked by typing AT@MENU. If you have experience with modems, you will feel at home using the BitSurfr Pro.
The boxy
3Com Impact ISDN
External Digital Modem is also fairly simple to use, though it comes with fewer extras than Motorola's offering. Unlike the BitSurfr Pro, front-panel indicator lights do show the call status for both B channels. There are also D channel status, test mode, and power indicators. Strangely, the D channel status light goes off when the line is synchronized. Also, the Impact has no receive or transmit data indicators.
The rear panel includes one RJ-11 phone out jack for an analog device and an RJ-45 for the ISDN U interface. The Impact has one DB-9 serial connector instead of the larger DB-25 connectors used by the other ISDN TAs, but it comes with the appropriate cable and a nine-to-25-pin adapter.
Bundled software is limited to a copy of the TurboCom/2 communications drivers and a Windows-based configuration manager. The brief but well-written user's manual is packed with useful information. A quick start guide is also included.
The Impact's simple configura
tion utility is too Spartan. A single screen provides only five configuration parameters, and there are things it can't do (e.g., switch between PPP and V.120 or use 64-Kbps ISDN connections). These and several other options require setting S registers through AT commands.
Strangely, the Impact is set up for 56-Kbps connections by default. You have to specifically set it to 64 Kbps by typing the command ATS60=64. Also, each time you run the configuration utility, it resets the S60 register back to 56 Kbps. If you don't know this, you will sacrifice 8 Kbps of throughput in end-to-end ISDN calls.
Performance and Interoperability
We asked each vendor to provide two production units for testing. We connected one unit to the serial port of a Dell Latitude 4100CX notebook with a 100-MHz 486 processor and the other to a 120-MHz Dell OptiPlex XMT 5120 Pentium system. Both systems had up-to-date serial ports with buffered 16550 universal asynchronous receiver/transmitters (UARTs).
As a V.120 throughput test, we transferred a 155-KB bit-map file from one system to the other over an ISDN connection using the ZMODEM protocol with Procomm Plus under DOS. At 9.6 and 19.2 Kbps, the transfer performance of all three pairs of TAs was similar. At 57.6 Kbps, we started seeing throughput differences, with the BitSurfr Pro leading slightly (see the figure
"ISDN TA Comparison"
).
When we pushed TA settings to 115.2 Kbps, all the devices experienced frequent cyclic-redundancy-check (CRC) errors as data was corrupted at the higher speeds. The errors and resulting transmission slowdown may have been due, in part, to serial-port limitations on one of the Dell systems. However, some of the CRC errors were due to the inability of the TAs to process asynchronous data at rates of above 57.6 Kbps. As it was, the three TAs reacted differently to the problem. While the ADAK's transmission speed dropped less than 10 percent, the BitSurfr Pro cut to around half-speed, and the Impac
t couldn't complete the file transfer at all.
If you plan to use MP with one of these devices for an Internet hookup, make sure your system's serial port supports a 115.2-Kbps rate. Also, the Microsoft Windows 3.1 communications driver (COMM.DRV) supports data rates of only 38.4 Kbps or below. This is why the Motorola and 3Com TAs include a copy of Pacific CommWare's TurboCom/2.
We found that interoperability was excellent with PC-to-PC communications, proving that V.120 is a well-implemented standard. Each device could place and receive calls with both of the other vendors' devices and complete a Kermit transfer without difficulty.
However, PPP interoperability was a different story. None of the TAs could establish a PPP session with a Cisco Combinet CB-900 Primary Rate Interface (PRI) bridge operating on an enterprise network. In each case, a connection occurred and authentication completed, but then the PPP session would simply die. This is surprising considering the amount of attentio
n given to the "ISDN interoperability bake-offs" sponsored by the California ISDN User's Group. It would seem that vendors have more work to do putting to practice what they learn from testing.
Technical Support
Given that ISDN installation can be tricky and frustrating, a TA vendor's technical support is an important purchase consideration. Although the ADAK 221 has the most complex installation, ADAK also has the most responsive technical support. It's not accessible through an 800 number, and support is limited to regular business hours, but an answer is just a ring or two away. The person we talked to was knowledgeable about the product, understood ISDN well, and was generally helpful.
As might be expected from a networking giant, 3Com provides good technical support through a toll-free number. When we called the company late in the day, 3Com indicated that there were eight calls ahead of us and that its support people would call us back the following business day. Not only
was 3Com true to its word, we also received several follow-up calls from the same support technician on a problem we encountered with high-speed data transfers.
The same efficiency cannot be ascribed to 3Com's customer service. The test units arrived with the wrong power supplies (from marketing). Technical support indicated that this was a packaging problem, assigned a case number, and referred us to customer service. Customer service issued an RMA number and promised to ship replacement supplies to us the following day. It took a week and several follow-up calls before the power supplies finally arrived.
Like 3Com, Motorola offers an 800 support number. Unfortunately, while support is competent, it requires great patience. It takes nearly 3 minutes just to navigate Motorola's voice-mail system and get into the support area. Once you arrive in the support queue, be prepared for a long wait. Our first call took 34 minutes before we got a human on the line. A follow-up call the next day put us i
n the hold queue for 31 minutes. Our third attempt took 46 minutes. Considering the long hold times, one can only marvel at what Motorola's 800 phone bill must look like every month.
Careful Choices
The three TAs we tested worked well for PC-to-PC communications using V.120. Because of its complicated configuration and operation, the ADAK 221 is not a good choice unless you have experience with ISDN. The best choice is a trade-off between the Motorola BitSurfr Pro and 3Com Impact. Both of them get the job done. Because of its unique design, bundled software, and better performance, the BitSurfr Pro is a somewhat better choice for home users.
If your intent is to access an ISP or enterprise network, PPP and MP interoperability is still a problem. Be sure to check with your ISP and the TA vendor before you buy any of these devices. You may also want to look at some of the ISDN bridge/router products. These devices typically implement a more robust and mature implementation of PPP
and MP.
Product Information
ADAK 221.....................................$999
ADAK Communications Corp.
Lansing, MI
Phone: (517) 882-5191
Fax: (517) 882-3194
Internet:
http://www.adak.com
Circle 1074 on Inquiry Card.
Building Bandwidth
