ISDN BRANCHES OUT

ISDN is coming on strong as a means of connecting branch offices and mobile professionals to corporate networks, but choosing the right equipment for your needs is crucial to getting the most out of the service.

To determine the ultimate performance available from an ISDN connection, we tested products from Ascend Communications, Inc., Combinet, Inc., Gandalf Technologies, Inc. and Symplex Communications Corp.

These bridges and routers support compression over both B channels available on an ISDN Basic Rate Interface line. They also support on-demand connection management that makes their operation transparent. Users don't have to explicitly make or break connections; these products do it automatically based on traffic.

Two products stood out in our testing. Ascends Pipeline 50 HX is very attractive for Internet access or telecommuting on IP-based networks, and is an excellent choice for connecting a personal computer or workstation to the Internet. And the Symplex Direct Route RO-1 excels at making it easy to interconnect a number of remote sites to a network where any site may call any other.

The Gandalf and Combinet products are both viable alternatives. The Gandalf 5242i is an excellent choice for work-at-home or small-office networking where bridging rather than routing is preferred. The Combinet 2060 SOHO is a better choice than the Symplex RO-1, where Point-to-Point Protocol support is required for a multivendor network.

Each of these products supports both an ISDN and an Ethernet interface. The Ascend, Combinet and Symplex products are both routers and bridges, while the Gandalf 5242i is a bridge only.

The products we tested are the lowest price U-interface models from each of these manufacturers. The U interface terminates the ISDN line directly and eliminates the need for a separate NT1 device. Ascend, Combinet and Gandalf have other products that support an unlimited number of users for larger networks. They also have less expensive models that support the ISDN S/T interface and require an external NT1.

Three of the products we tested supported only data communications; the 5242i was the sole product that allowed the ISDN line to be used for voice, as well. It was sophisticated enough to support "call bumping," which drops one of the data calls either to make or receive a telephone call when both B channels are being used. This eliminates the need for a separate voice line.

Performance

Our performance tests in-cluded uploading and downloading both uncompressed and precompressed versions of an Excel file using two B channels and the compression implemented in each of these products (see Figure 1, page 42). We created the compressed file by putting the Excel file through PKzip.

For a complete description of our test methodology, see http://www.nww.com/0710how.html.

The throughput of Pipeline 50 HX was excellent, averaging 100K bit/sec for a precompressed file and 236K bit/sec for an uncompressed file. While the performance of the Combinet 2060 SOHO was at the bottom of our group, it was still very good, with average throughput of 92K bit/sec for the precompressed file and 210K bit/sec for the uncompressed file using Combinet's proprietary protocol on the ISDN line. The throughput of the 5242i was also very good, averaging 106K bit/sec for the precompressed file and 224K bit/sec for the uncompressed file.

The RO-1 showed the best performance in our test, with an average of 126K bit/sec on the precompressed file and 238K bit/sec on the uncompressed file. On uploads of the precompressed file, we measured throughput of 148K bit/sec on two 56K bit/sec B-channel connections. This means that it was able to get an additional 30% compression out of a file already compressed with PKzip.

We also compared the time it took for each product to make a connection (see Figure 2, page 43). There was no significant difference between them, indicating that there should be no difference between ISDN connection times when any of these products are used between the same two points.

Setup

Pipeline 50 HX has a menu-oriented interface that users can access via a serial port or telnet. The serial port is required to enter the unit's IP address during the initial setup.

The Combinet 2060 SOHO uses a command-based rather than a menu-based interface. The initial setup uses the serial interface to enter the IP address. After that, most operations can be done via telnet - although some operations such as the Test command, which can be used to test either the ISDN or the LAN connections, must be executed from the serial interface. The 2060 uses SET and SHOW commands to set and view the values of its parameters.

To set up the 5242i, an administrator can use a telephone keypad to enter the telephone number or the Service Profile Identifiers (SPID) of the ISDN BRI and the computer's IP address. This eliminates the need to connect a terminal to the 5242i's serial port except to set up an IP mask that differs from the default.

The 5242i uses a character-based, windowed user interface. At the top level, it shows a number of windows that provide access to menus that control various aspects of the box. It also includes status indicators for each of the two B channels that describe whether the SPIDs have been accepted, the status of an ISDN call on each B channel and the status of the LAN connection. The 5242i also includes an event log that can be useful in identifying problems. This interface is easy to use once you remember that the top-level menus are entered by pressing the Control-o key, while the lower level windows are opened with the Enter key.

The RO-1 has a simple character-based menuing interface. The only irritation is that when you change a number of parameters, such as the connection timers, you have to reinitialize the unit to force it to make synchronization calls with the other units on the network.

The Ascend, Combinet and Symplex products include test commands that can be used to verify the operation of the ISDN connection. In addition, the Combinet 2060 lets you test the error performance of your ISDN local loop. The 2060 and the Symplex RO-1 also provide tests for the LAN connection. All four products will respond to pings, which can be used to verify their operation on the LAN.

The Ascend, Gandalf and Symplex products have transactions logs that administrators can use to review the activity on their ISDN lines. This can be an invaluable tool in resolving problems, particularly since these products all make and break ISDN connections automatically. The RO-1 is the only product in this group that puts date and time stamps in its log entries, which is a real benefit. While the 2060 does not have a log, it provides the best information among these products about the ISDN call status in real time.

We entered incorrect SPIDs into each of these products to see how they would react. Ascends Pipeline 50 HX just presented a blinking light to indicate that there was some kind of initialization problem. The log showed that Layer 2 initialization was completed correctly, but it did not explicitly show the Layer 3 initialization failure. Still, this is a strong clue that the SPIDs were entered incorrectly. The Combinet 2060 reports "SPID Re-jected" every time the unit attempts to initialize the ISDN line, which makes it clear what the problem is. The Gandalf 5242i shows messages in its top-level menu that tell if the SPID has been accepted.

Protocol support

Ascend supports an extended version of the PPP Multilink Protocol (MP) defined by the Internet Engineering Task Force (IETF) in RFC 1717. PPP MP is emerging as the standard for combining B channels for higher speed ISDN applications and promises to be the basis of interoperable, multivendor ISDN applications. Ascend's extensions include data compression, an area for which the IETF has not yet completed its standardization work.

Combinet has implemented PPP MP on the 2060 and it worked well, but it does not support compression; thus, the throughput on the uncompressed file was about 60% lower using PPP MP compared to Combinet's proprietary protocol. PPP was about 15% slower for compressed files than with the firm's proprietary protocols.

Gandalf 5242i uses a proprietary link protocol with proprietary compression. The com- pany's claims to superior compression were not borne out by our tests. This may be due in part to the propagation delay of the link to the company's Ottawa location; however, that delay did not seem to affect the performance of the Symplex Direct Route RO-1 at a comparable distance.

The RO-1 supports only Symplex's proprietary protocol and not PPP. This means that if you plan to use an RO-1, all the sites in the network must also have Symplex products. This can present a significant problem if you want to create a network that includes sites in countries where Symplex's products are not available. Even so, the RO-1 is well designed, and we expect Symplex will add PPP soon.

Management and security

All these products support some form of access control, both for accessing the network as well as for configuring the units themselves. The Ascend Pipeline 50 HX and the Combinet 2060 support PPP's Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP) to control local and remote access security, as well as callback controls. In addition, the 2060 can use the ISDN Calling Line ID to authorize connections.

Each remote site profile in Pipeline 50 HX can be configured separately for PAP, CHAP or callback security. Users can also set separate client and host passwords for each remote site profile.

The Combinet 2060 requires that you set all remote site profiles for PAP or CHAP, and does not permit mixing them, and it requires that you use the same client password for all remote site profiles.

The Galdalf 5242i relies on a list of the serial numbers of the remote products for access security control. It would probably not be that hard for a motivated person to spoof this system.

The first time the Symplex RO-1 is powered up, it leads the administrator through a set of menus to enter the phone numbers, SPIDs, local IP address and other information necessary to complete an initial installation. It even requires administrators to enter a password that protects the unit from unauthorized access to the configuration menus.

The other products in this test do not require a password. The Symplex RO-1 uses its proprietary protocols and its synchronization process to control access.

All the products tested support passwords to control access to their administrative menus. Ascend's password protection restricts its ability to locally or remotely make calls, change the configuration or access field service operations. Ascends Pipeline 50 HX does not provide a way to lock out access to viewing the menus.

The Combinet 2060 allows partial or complete lockout of local or remote access to viewing or changing its menu setting. The Gandalf 5242i supports a password that can completely lock out local or remote access to viewing or changing its menu setting. The Symplex RO-1 is the only product of the group that requires you to enter a password when you first use it. The RO-1 then requires that the password be entered to allow viewing or changing it menu settings.

The Ascend, Combinet and Gandalf products support Simple Network Management Protocol-based management.

Configuring each product was straightforward. We found no major performance discrepancies among the products. The automatic management of the first and second B-channel calls worked well, even without fine-tuning the factory default settings of the timers. We expect that the user interface shortcomings we found will be resolved as the products evolve.