DirectRoute's ISDN switching technology, inverse multiplexing, and data compression can cut the cost of remote LAN connections
Jeffrey Fritz
Symplex Communications pioneered synchronous data compression for WANs with its Datamizer products. It has now entered the internetworking arena with a family of devices called DirectRoute. Like current Datamizer products, DirectRoute uses data compression and inverse multiplexing with switched digital connections to provide bandwidth on demand for more efficient and economical long-distance network links. But while Datamizers use these capabilities to supplement an existing leased-line connection, DirectRoute products can rely solely on switched services, particularly ISDN, to provide on-demand remote connections.
Although it's tempting to label DirectRoute a bridge or router, it's r
eally a hybrid design. Like a bridge, it operates on the MAC (media access control) layer (layer 2). But it functions like a router when it comes to packet destinations. Routing-like tables, maintained in the form of destination telephone numbers, identify remote devices and their port addresses.
In creating DirectRoute, Symplex has applied matrix-switching technology to WANs. In a sense, this is an electronic version of the crossbar switches that were used in the telephone network for years or are used in hub switches today. Hub switches send packets directly to simultaneous, multiple destinations without sending them through an intermediary network. This topology makes for more efficient WAN interconnectivity than is provided by typical setups using expensive dedicated lines. After all, it doesn't make sense to travel from Nashville to Los Angeles through Atlanta when you can get a direct flight. DirectRoute provides connection-oriented switching technology for ISDN.
Using multiple-channel agg
regation and compression, DirectRoute can accomplish WAN bandwidths of up to 4 Mbps. Currently, it does this using a proprietary protocol. However, Symplex plans to use multilink (PPP over ISDN) so that DirectRoute can interoperate with other vendors' ISDN WAN products.
For sites with digital services other than ISDN, all Direct Route products can be provisioned for frame-relay, T1, Switched-56, or other digital services. Generally, access to non-ISDN services is accomplished through a V.35 serial interface option.
DirectRoute Configuration and Description
The DirectRoute product family has three members. The top-of-the-line DR-5 is intended for central sites with high-volume backbones. It can connect up to 160 sites simultaneously, with 1 Mbps of bandwidth per site (more with compression). It uses a five-slot modular chassis. BRI (Basic Rate Interface)/LAN cards provide four BRI ISDN and one network connection per card. Mixing in PRI (Primary Rate Interface)/Serial cards (which provide tw
o PRI ISDN and two V.35 serial connections per card), the DR-5 can also support frame-relay and leased-line connections. With one BRI/LAN card, the DR-5 has a $7999 base price. Additional BRI/LAN and PRI/Serial cards cost $6000 for Ethernet versions and $6500 for Token Ring versions.
The DR-2 model I reviewed is a scaled-down product capable of connecting up to 64 sites simultaneously and providing 1 Mbps of bandwidth per site. It has a two-slot modular chassis and supports the same cards as the DR-5. The DR-2 has a $6999 base price, which includes one BRI/LAN processor card.
The DR-1, available later this year, is the bottom of the line at $2999. Unlike the DR-2 and DR-5, the DR-1 is slotless. It can connect up to eight remote sites and provides only 512 Kbps of throughput per site. (Not that 512 Kbps is all that bad; most telecommuters do very well with 128 Kbps.) The DR-1 supports four BRI lines plus two serial (V.35) connections for frame relay or leased lines.
Processing power for al
l units comes from AMD 29030 RISC processors, one per card, each with 1 MB of flash RAM for program memory and 1 MB of SRAM for buffering. Both flash RAM and SRAM expand to 4 MB.
I tested a pair of DR-2s, each configured with a single Ethernet BRI/LAN processor card. The sleek, 19-inch, black metal chassis is suitable for stacking or rack mounting. A front-panel door provides access to the hot-swappable processor cards. On the back panel, a BRI/LAN I/O module provides four RJ-45 jacks (labeled A, B, C, and D) for the ISDN lines, and a port E (for Ethernet, no doubt) with an AUI (attachment unit interface) connector. If you plan to use 10Base-T or thinnet (10Base-2) cabling, you'll need transceivers. There is also an RS-232 monitor port and a second AUI port, labeled LAN Command Port, which is not currently used.
Upper Management
You can configure DirectRoute through standard communications programs, SNMP, or Telnet. The DirectRoute system software interface is menu driven but has a nasty t
endency to hide menu options. The reinitialize (reboot) command, for example, is mysteriously hidden on the bottom of the Card Setup screen as part of a status message. Submenu selections show only one item at a time, making it difficult to remember locations of options and features. Several menu items haven't been implemented, although they appear as selections. Clearly, the interface needs work.
A Card Statistics screen provides an array of throughput statistics, including packets per second to and from the LAN and bits per second to and from the WAN. Unfortunately, there is no way to tell if packets are out of sequence, lost, dropped, or corrupted. To help in diagnosing a bad link, the Symplex software should provide this information for each channel.
A PAT (Priority Action Table) determines calling patterns. There are two options, Throughput On Demand and Data Present (referred to as Action Table 1A and 1B, respectively). Despite the names, both modes offer bandwidth on demand. The differenc
e is that Throughput On Demand does not place calls, even when there are packets destined for the other side. It is intended for Internet providers who don't want to issue calls to remote users whenever, say, an E-mail message arrives but would rather wait for the user to call in and then send the E-mail.
For this scheme to operate properly, one side (typically the remote side) has to be set up in Data Present mode. Bringing both sides up in Throughput On Demand mode means that neither side will call and, thus, connections will never be made. Strangely, the default configuration is Throughput on Demand. You must explicitly change that setting to allow connection.
Symplex promises future PAT enhancements, including time-of-day action tables that allow different connectivity based on hour or day parameters. The hooks are in place for these advanced PAT options, but they do not operate yet.
The DirectRoute product line supports SNMP through generic MIBs (management information bases) or thro
ugh a Symplex-developed MIB. The Symplex MIB provides additional management benefits, one of the most important being call-detail reporting. The Call Data Collection and Processing modules in the MIB provide information on call duration, call status, and bandwidth utilization. This provides managers with a good tool to determine if digital lines are sufficient to meet user demand. In addition, the modules allow network managers to charge back usage for WAN connections.
Security and Manuals
DirectRoute provides several security options. Authentication is used whenever a connection is made to ensure that legitimate calls have been placed. Access to the management interface has authority levels and password protection.
The documentation can be summed up in one word--skimpy. The product comes with a 19-page installation guide and a single-sheet quick installation guide. Both guides have screen shots, but the text is sparse, often leaving out important details. There is no operating manual and
no reference documentation, both of which are critical to proper operation of something as complex as DirectRoute. This is an unforgivable oversight. Although Symplex plans to include improved documentation in the future, the product should not have shipped without adequate documentation.
Performance
To test the DR-2, I set up a small 10Base-T Ethernet network to simulate a remote LAN. The host site was a production 10Base-T Ethernet LAN. This Ethernet connected through a Cisco AGS+ router to an FDDI (Fiber Distributed Data Interface) backbone and from there to the Internet. Each DR-2 connected to the phone network with four ISDN lines. (See the figure ``DirectRoute Test Configuration.'')
I tested DR-2 throughput for AppleTalk, Novell IPX, and TCP/IP protocols by transferring a 1.9-MB binary file between LANs using eight- and two-channel configurations, with and without compression (see the figure ``DirectRoute Throughput Performance''). Binary files compress less effectively than text fi
les, so performance with text-only or mixed files should generally be better.
TCP/IP with compression over eight channels was the best performer, with a throughput of 491 Kbps. With 512-Kbps bandwidth, that represents an impressive 95.83 percent link efficiency. As expected, eight channels with compression yielded the best throughput for all the tested protocols. But even the lowest rate, two channels with no compression, was slightly more efficient than measurements made in similar modes with Combinet CB-400 and Digiboard Dual IMAC ISDN bridges. The results weren't shockingly higher, but the difference was measurable.
To test compression, I transferred a 1.9-MB binary file and a 1.1-MB text file with and without compression (see the table ``Compression Results''). The text file did quite well, while the binary file's compression ratio was somewhat less impressive. Compressing the binary file provided a 1.35-to-1 increase in throughput with eight channels and a 1.44-to-1 increase with two channe
ls. On the text file, the DR-2 achieved only a 1.67-to-1 throughput increase when compressing with eight channels. Text-file compression worked much better on two channels, with an increase of 3.13 to 1. Although this is far from the 20-to-1 compression promised in the product literature, compression was good enough that the file actually moved across the two-channel link at almost the same speed as with eight channels and no compression. Like gas mileage figures, compression estimates are only an approximation of what you can expect.
Filtering
DirectRoute can filter transmissions based on device address or network protocol or on a custom basis. Custom filtering works inside the packet, and it filters based on bit patterns. Up to 1000 individual filters can be maintained, which should be more than adequate for most networks.
You can create filters at any time, but they don't become active unless you explicitly turn them on. That makes it simple to prepare filters, but it also makes it easy
to forget to turn them on. Once filters are on, the only way to turn them off is by erasing them from the active filter list. This is an awkward process; it would be better to use check boxes for turning filters on and off.
To set up protocol filters, you must enter the Ethernet type code for the protocol. The documentation does not provide any type codes. I had to ask Symplex to fax a list of codes before I could set up protocol filtering.
Unfortunately, you have to reboot DirectRoute for new filters to take effect, and, obviously, network access is interrupted during the reboot. While the DR-2 reboots quickly (a warm boot typically takes 20 seconds), it is still a nuisance.
Telco Liaison
Symplex provides support directly from its headquarters in Ann Arbor, Michigan, and through its distributors and resellers. Standard telephone support is available, as is an optional 24-by-7 (24 hours a day, seven days a week) support program. Third-party on-site service is provided by G.E. Infor
mation Services.
Generally, I was pleased with the quality of the technical support. The technicians were knowledgeable and were willing to admit when they didn't know the answer to a problem. If it was a new problem, they made note of it and were never defensive. When I had to travel to the remote site, they offered to call me back (good thing, since Symplex support doesn't have a toll-free number). They always called me right on time.
The toughest part of an ISDN installation should be the easiest: ordering the ISDN service. The telephone carriers have done a terrible job of implementing ISDN ordering. You need to understand telco-speak to order even the simplest service. Symplex, sympathizing with the user's plight, has responded by forming a TLT (Telco Liaison Team).
Prior to installation at the desired locations, the TLT determines if ISDN service is available in the local exchanges, and if not, when it will be and what alternative services are available. The TLT then generates a wor
k order on behalf of the user. Symplex claims that it will work with any independent carrier and with the seven RBOCs (Regional Bell Operating Companies).
This is a good start, but the support isn't open ended. Once the ISDN service is operating, Symplex says, ``All line network management is the responsibility of the RBOC providing the carrier service.'' Since RBOCs rarely take responsibility for anything in the ISDN world, it would be better to offer continuing support to users after the ISDN service is installed, even for a fee.
Bugs and Gotchas
The DR-2 has some bugs, particularly in the management interface. For example, Telnet management sessions to the device were unstable. They often failed with cryptic messages like ``Session task creation failure! (15).'' Once this occurred, I could no longer Telnet into DirectRoute. And if a Telnet session aborts, DirectRoute can't time-out the user. As a result, the maximum user limit is quickly reached, and users attempting to log on are greet
ed with a message saying: ``Sorry--Maximum number of logins exceeded!''
Twice during throughput testing, the DR-2s died. ISDN and LAN connections were maintained, but the remote workstations reported the loss of network services. It appeared that the bridging protocol had crashed, because traffic was recorded at the WAN and LAN ports, but nothing was passing between them. The problem stumped the company's technical-support people. The only cure offered was to reboot the dead DR-2.
The DirectRoute product line is new. Features are still being implemented, the documentation is sparse, and the bugs are painfully obvious. Nevertheless, DirectRoute is a good choice for network administrators looking for improved LAN access. Its ability to provide exceptional bandwidth on multiple ISDN channels with decent compression makes it viable for large networks. For diverse networks, it adds value by acting as a matrix switch for multiple sites. These features make DirectRoute a flexible and powerful tool for
remote LAN access.
The Facts
DirectRoute DR-2 $6999
(as reviewed with one
BRI ISDN/Ethernet module)
Symplex Communications Corp.
5 Research Dr.
Ann Arbor, MI 48103
(313) 995-1555
fax: (313) 995-1564
Compression Results
Enabling the DR-2's built-in compression provided a greater increase in throughput with a 1.1-MB text file than with a 1.9-MB binary file. With compression, the text file moved across the two-channel link at almost the same speed as with eight channels and no compression.
Number of ISDN B channels 8 2
Link bandwidth (Kbps) 512 128
Binary-file throughput (Kbps, compressed/uncompressed) 469/346 119/82
Effective compression ratio (binary)* 1.35 1.44
Text-file throughput (Kbps, compressed/uncompressed) 584/350 283/90
Effective compression ratio (text)* 1.67 3.13
*The
effective compression-ratio figures are lower than actual compression because of LAN protocol overhead.
Figure: DirectRoute Test Configuration
DirectRoute DR-2s connected the test remote LAN to a production 10Base-T Ethernet LAN using ISDN services. Four ISDN lines provided eight 64-Kbps ISDN channels between the two LANs. On the remote LAN, a Dell 486 PC, Sun SparcStation LX, and Apple Mac Quadra 660AV allowed testing with Novell IPX, TCP/IP, and AppleTalk protocols, respectively.
Figure: DirectRoute Throughput Performance
In transferring a 1.9-MB binary file, throughput differed for each of the three tested LAN protocols. In each case, using eight 64-bit ISDN channels with inverse multiplexing provided higher throughput than using just two lines, and DirectRoute's data compression enhanced compression somewhat. Performance using compression with text or mixed files will be better.
Photograph: The front of the DR-2's 19-inch-deep, black metal chass
is shows only activity LEDs.
Photograph: The DR-2's front panel is a pull-down door that provides access to the hot-swappable processor cards. If you have two cards installed, you can remove or exchange one while the other continues to function.
Photograph: On the back panel, a BRI/Ethernet I/O module provides four RJ-45 jacks for ISDN and an Ethernet AUI connector. A serial port in the main chassis can handle out-of-band management.
Jeffrey Fritz (Morgantown, WV) is a telecommunications engineer who is responsible for the design and management of data communications for West Virginia University, including its ISDN Applications Lab. He is the author of Sensible ISDN Data Applications (West Virginia University Press, 1992). You can contact him on the Internet at
jfritz@wvnvm.wvnet.deu
or on BIX c/o ``editors.''
Switching ISDN
