Two routes to ISDN ACCESS

A key to successful implementation of an ISDN telecommuting program is selecting the right ISDN access router for your corporate network to support access from your remote users. Access routers have to support the remote devices that you want to use, provide access security, and produce accounting and other information required to manage a dial-up network.

Ascend Communications, Inc. offers the MAX 1800 and MAX 4000, which differ in configuration but use the same software. We looked at the MAX 4000, which has four ISDN Primary Rate Interface connections with a total of 92 B channels, an Ethernet connection, a V.35 port and six slots that can support option modules. The MAX 4000 we tested included two option modules - one with eight Basic Rate Interface interfaces and the other with eight V.34 modems.

Likewise, Gandalf Technologies, Inc. offers XpressStack and Xpressway, which use the same software. Like the MAX 4000, Xpressway has a modular hardware structure that lets you add hardware option boards. We looked at the XpressStack BRI, which has eight ISDN BRI connections with a total of 16 B channels and one Ethernet connection.

We tested each ISDN access router with both Ascend and Gandalf remote products - in Ascend's case, a Pipeline 25 and a Pipeline 50 ISDN router; and in Gandalf's case, a LANLine 5242i Teleworker Bridge and a LANLine 5242i Edge Router. Because of the proprietary implementations of both multilink and compression, we found better performance using products from the same manufacturer. These four remote products all have an ISDN BRI and an Ethernet interface. We found that both Gandalf and Ascend products were capable of throughput of more than 300K bit/sec.

Ascend's MAX 4000 was able to handle everything that we threw at it (see Figure 1, page 44). When connected to other Ascend products, it supports Stac Electronics, Inc.'s compression and Ascend's MP+ enhancements to the Internet Engineering Task Force's Multilink Protocol (MP). The MAX 4000 also supports two B-channel connections to Gandalf's 5242i Edge Router and 3Com Corp.'s Impact Digital Modem using the PPP MP. And it handles access with U.S. Robotics' Sportster 28,800 Data/Fax Modem.

The MAX 4000 had excellent performance when communicating with Ascend's Pipeline 50. We saw throughput over 300K bit/sec with the compressible file and better than 118K bit/sec with the incompressible file.

Performance with the Pipeline 25 was virtually identical when uploading files to the MAX 4000, but about 20% slower downloading the incompressible file and 40% slower with the compressible file. We measured the performance with Pipeline 50 using IP routing and Pipeline 25 doing both bridging and IP routing.

The MAX 4000 connected to Gandalf 5242i Edge Router doing IP routing gave solid performance with two B channels using the PPP MP. Its times on the incompressible file were at, or close to, the top of the group. The 5242i does not support compression with the MAX 4000; consequently, its performance was less than 40% that of the Pipeline 50 on the compressible files.

The performance of the MAX 4000 with 3Com's Impact under Windows 95 ranged between 58K and 71K bit/sec with two B channels. After we finished our testing, 3Com told us their performance with two B channels on the MAX 4000 would be better if we downloaded new software from Ascend. However, the throughput improvement of less than 25% that we saw in our testing does not justify doubling the connection costs by adding a second B channel.

U.S. Robotics' 28.8K bit/ sec modem with Windows 95 Dial-up Networking worked well with the MAX 4000. The performance of the modem on the compressible file was 44K bit/sec and about 25K bit/sec on the incompressible file.

Gandalf's XpressStack BRI handled connection with LANLine 5242i using both Teleworker Bridge and Edge Router software (see Figure 2). Connections with Edge Router software were made using both bridging and IP routing. XpressStack does not support modem connections.

XpressStack connected to the LANLine 5242i with Edge Router software had performance that was virtually the same as that of MAX 4000 connected to Pipeline 50. In fact, XpressStack connected to Gandalfs 5242i Edge Router configured as a bridge edged out MAX 4000 connected to Ascends Pipeline 50 doing IP routing for the best times in our tests by a hair. But our tests provided no evidence of a decisive advantage for Gandalf's proprietary compression protocol.

XpressStack performed virtually identically when downloading files from the 5242i with Teleworker Bridge software and Edge Router. But with Teleworker Bridge, it was 20% slower than with Edge Router uploading the compressible file and 40% slower uploading the incompressible file.

Since we used the same hardware, the performance difference must have been due to the software implementation of Teleworker Bridge.

We were able to test XpressStack with the Pipeline 25 using one B channel. Ascend's MP+ extensions to the MP are not compatible with XpressStack, so we could not make two B-channel calls between the Pipeline 25 and XpressStack. The performance for this combination ranged between 36K and 58K bit/sec. This is fine for occasional access from a site that already has the Pipeline 25, but it is much too slow for a serious user accessing XpressStack.

The performance of XpressStack with 3Com's Impact under Windows 95 ranged between 49K and 61K bit/sec with one B channel.

Making connections

Connection time is a key measurement of the responsiveness of an ISDN system. The MAX 4000 was significantly faster than XpressStack in most of the connection times measured (see Figure 3, page 45). Connection time is an important measure of the responsiveness of a system since these products make and break connections based on traffic loading. We measured the time for both ISDN connection and PPP link establishment. The Pipeline 25 and 50 took about 3.3 seconds connecting to the MAX 4000. This is about twice as long as the 1.5 seconds we observed with an Ascend Pipeline 400, which is a four-BRI access router that also supports V.32bis modems. The difference is possibly due to the more complex call routing required by the MAX 4000's ability to support a much larger variety of BRI, PRI and modem calls. The 5242i took about 5 to 6 seconds to connect to XpressStack, independent of the software used.

The other interesting observation is the connection times for the Pipeline 25 connection to XpressStack. The Pipeline 25 connects to XpressStack in 2 seconds, compared to 5 to 6 seconds for the Gandalf products. In fact, the Pipeline 25 connects to XpressStack in about 40% less time than it connects to the MAX 4000. This makes it appear that Gandalf's longer connection times are due to the 5242i and not XpressStack itself.

Setup and operation

The MAX 4000 includes four interfaces that can be used for PRI or for T-1/E-1 connections, plus a V.35 interface that can be used for a frame relay connection in the base chassis. With all of this built in, the MAX 4000 can support a large population of remote users without adding modules to its six open chassis slots. Frame relay on the V.35 port permits the interconnection of a network of MAX 4000s on a WAN to support a geographically dispersed set of users. The MAX 4000 has option modules that permit it to support as many as 48 V.34 digital modem connections employing the same pool of PRI B channels used for the ISDN data connections.

Other hardware modules include BRI modules that support eight terminal or network interface ports. The MAX 4000 also supports modules that handle eight connections using the V.110 rate adaptation, which is extensively used outside of the U.S., as well as modules that support RS-232, V.35, RS449 or X.21 inverse multiplexing ports that can be used for data or video applications.

Optional software modules for the MAX 4000 include frame relay, ISDN and conversion of PRI to T-1. The MAX 4000 also has a software option to support Multirate ISDN, which supports switched Nx64K-bit/sec and switched 384K bit/sec services. Other software options include the Ascend Inverse Multiplexing and Bandwidth-On-Demand Interoperability Group specifications for videoconferencing applications; V.120 rate adaptation, used in the U.S.; and both RS366 and V.25bis autodialing.

With the right set of hardware and software op-tions, the MAX 4000 can be used in a large variety of configurations that can include data communications and video. The MAX 4000 also can be used to drop and insert channels so a single T-1 can be used for data and video, as well as providing voice trunks to the private branch exchange.

Xpressway comes with either a four- or an 11-slot chassis. Gandalf offers modules that support BRI or PRI, asynchronous terminal communication using external modems, serial links for leased line, X.25 or frame relay connections.

Xpressway also has LAN modules that support Ethernet or FDDI switching, as well as Ethernet and token-ring wiring hub modules.

Both the MAX 4000 and XpressStack are relatively easy to set up and use. The MAX 4000 uses a hierarchical set of menus displayed along with a set of windows that show the current status of various aspects of its operation. XpressStack uses a character-based windowing system to access the various menus required to set it up.

With both routers, you set up profiles for each remote site to which you want to connect. The MAX 4000 requires you to navigate through subsidiary menus to access all the menu items for a single profile, while XpressStack puts all the menu items for a profile in a single screen.

Both products use these profiles to make calls based on IP addresses and to automatically set up the required dialing groups and static routes.

XpressStack has a mode where it will automatically set up a profile for each LANLine 5242i Teleworker Bridge that calls in, which can simplify initial setup.

Both products have indicators that show whether their ISDN links have initialized correctly. This ISDN line initialization confirms that their basic configuration, including Service Profile Identifiers (SPID), has been entered correctly. TheMAX 4000 has a Test command in its Terminal Server Diagnostic mode that can do a loopback test. This Test command works only with the PRI interfaces and not with the BRI interfaces with which we were testing. XpressStack does not have any capability for making a test call to verify the operation of the ISDN connections.

We ended up using remote products to make the initial connection tests with both XpressStack and the MAX 4000. Employing a remote unit rather than a local loopback to test ISDN connections works, but it is more difficult to determine if the problem is on the local or the remote side. Neither of these products provide all the kinds of information needed to diagnose connection problems.

The MAX 4000 shows the status of ISDN calls in small windows on the screen as they are placed or received. More information can be viewed in a special command-line diagnostic mode. These commands let you look at traffic statistics and at a log of the last 30 ISDN calling events. This is generally adequate for understanding why ISDN calls fail. The MAX 4000 gives very little information about why a PPP connection fails. If you are truly brave, the MAX 4000 has a debug mode that will give you hex dumps of either the ISDN Q.931 messages or the PPP packet exchanges.

XpressStack has a log that flags PPP authentication failures but does not provide information on other kinds of PPP failures. It does, however, provide information on ISDN call failures. In addition, XpressStack logs calls for accounting purposes, with the time and duration of each call, as well as a limited amount of information about why the call terminated.

Management and security

The two products have different management philosophies. XpressStack is self-contained and can store internally 1,000 connection profiles as well as its logs and accounting information. It keeps the 1,000 profiles in a table in the order that they were entered, rather than alphabetizing them, but it does provide a retrieval capability that can use wildcards in profile specifications to aid in finding a particular profile.

The MAX 4000 supports only 32 remote profiles internally, which is not nearly enough for a product that can support 92 B channels on its four PRI interfaces. With the 4000, you would use either the Remote Authentication Dial In User Service (RADIUS) or Terminal Access Controller Access Control System (TACACS) Unix-based remote authentication database servers, which support a database of remote profiles such as name, password, authentication method and routing information. The MAX 4000 refers to the RADIUS or TACACS databases as calls come in and uses information from them to set up the connections. During this process, the database is updated to show the current state of each connection.

Using external servers can make management more complex, especially for a single MAX 4000. On the other hand, TACACS and RADIUS provide a single authentication database that can be used by multiple access routers, which eliminates the redundancy required with Gandalf's approach of maintaining parallel databases in separate XpressStacks.

The MAX 4000 and XpressStack support both the PPP Password Authentication Protocol (PAP) and the Challenge Handshake Authentication Protocol (CHAP) authentication protocols. They also both support the use of Calling Line ID, an ISDN feature that provides the directory number of the caller, to verify the caller. The MAX 4000 also provides callback security. XpressStack authenticates the 5242i with Teleworker Bridge software by verifying the serial number of the calling 5242i.

XpressStack keeps audit records for each call that is attempted, whether it is successful or not. It can then send those records to an FTP server on an external system when the audit record buffers fill up or at a specifically selected time of day. The MAX 4000 supports an external Unix host-based SYSLOG facility for recording information about its operation, including call detail recording that can be used for billing purposes. Ascend told us that the SYSLOG information is useful in identifying and resolving connection problems, but without a Unix server to run it on, we were not able to use it directly ourselves.

Both ISDN routers support the use of Simple Network Management Protocol to monitor or actively manage these units. They both support the Management Information Base (MIB)-2 standard as well as their own MIBs or traps.

After testing, we believe that both Ascend and Gandalf have good product lines. For a network that can be supported by a single XpressStack or Xpressway, Gandalf's products may be preferable since their user profiles are managed within the unit itself. For larger configurations that require multiple units, the external RADIUS and TACACS authentication databases supported by Ascend's MAX products provide a centralized authentication system that eliminates the need to maintain parallel databases.