ISDN and other digital services provide more ways to connect users to corporate networks
Jeffrey Fritz
Remote access used to be the lifeline you clung to in your hotel room after you realized your presentation was home alone on your hard drive 3000 miles away. But today, remote-access networks are the foundation for companies that don't see corporate headquarters as ground zero anymore. Just as much action is taking place in the field, on the road, or in branch offices.
The trick is to select the right technology or mix of technologies that can make it easy for far-flung workers to reliably connect to your company's main network. More and more, digital-service WAN technologies are becoming part of this mix because they're faster and more dynamic than T-1 or 56-Kbps leased lines.
But e
ven today, no one-size-fits-all solution exists for all companies, and venerable technologies still play a role in WAN connections. Your choices will depend on whether you need full-time point-to-point connections for a branch office, or more temporal connections for mobile workers or telecommuters. Many companies require a combination of these two choices.
Remote Gets Real
The need for efficient remote access has grown in part because work can get done faster if you're closer to a customer site, or because workgroups are better if they're built on expertise rather than on geographical proximity. Either way, employees throughout an enterprise need to make business decisions with the same data that's available at headquarters.
Alternately, people in the home office often need immediate access to the expertise of workers in the field, who may be able to spot business trends faster than those in a central location. In addition, telecommuting is becoming a way of life for mo
re and more people. LINK Resources Corp., a New York-based market research firm, says the U.S. work-at-home market grew to more than 40 million people last year.
Digital technologies shine in those applications because of high throughput. Basic ISDN service, for example, provides for bandwidth of 128 Kbps, while a less commercialized technology such as ATM (asynchronous transfer mode) can scale up to more than 600 Mbps. Although 128 Kbps can seem like a narrow pipe for LANs, WAN devices can make the most of it with compression and filtering. At the same time, network administrators can use protocol filtering to reduce superfluous traffic over the WAN, especially by eliminating multicast and broadcast packets. Address filtering allows only packets addressed to the remote destination to pass across the WAN. If filtering and compression are not enough, digital devices, including those for ISDN, can provide additional bandwidth-on-demand by automatically allocating and binding extra channels based on curre
nt traffic requirements.
Cutting Costs with Contention
Digital dial-up connections make sense in another way: They reduce the amount of line-termination equipment a company must purchase. With leased lines, there is one-to-one correspondence between devices (such as bridges or routers) in the field and allocated ports in the central or hub site. Frequently, network managers use the ratio of devices in the field to available ports at the hub as a way to size up their network connections. This ratio, called
contention
, is based on the assumption that all users will not simultaneously try to connect to the enterprise.
Contention reduces the amount of money you have to spend on lines and hub equipment. For example, a company might have 400 telecommuters, but no more than 50 are expected to be on line at any given time. Rather than installing 400 network ports at the enterprise hub, the company can connect 50 ports in a "roll down" configuration--8:1 contention. User
s dial a common telephone number and connect to the first available port. When the fifty-first caller attempts to connect, he or she will get a busy signal and will need to attempt the connection later. (In some advanced hubs, an administration program keeps track of call attempts. When a hub port opens up, the unit will call the remote user back.)
Finding the optimal contention ratio is an iterative process. Setting contention too low wastes resources and is unnecessarily expensive. Setting contention too high results in call blocking and user frustration. Typically, network administrators will make a conservative guess and monitor access requests to see if the contention threshold is realistic. Once an administrator has some feel for typical usage, contention can be fine-tuned as needed.
Permanent Connections
Despite the performance advantages of digital technologies, it's not always easy to determine the price/performance cut-over point between leased lines or analog
phone lines and digital links. T-1, at 1.544 Mbps, or 56-Kbps leased lines are the traditional ways to connect branch offices to a headquarters' LAN. Leased lines are particularly advantageous if you can count on a stable level of communications activity, which will help you cost-justify this type of connection. That's because leased lines operate at a fixed monthly cost, no matter how much data you're pumping through them. Leased lines are an economical choice if you can find a way to constantly use them; for example, by making them communications links during business hours and data pipelines during the night to update corporate databases.
However, life isn't always predictable enough for leased lines. Your company may need connections to remote offices for only a few hours a day. In this case, high-speed modems (which we'll discuss later) or switched digital networks are better choices than leased lines. Switched networks establish connections only when the need to communicate exists. When there's
no communications, the line shuts down and charges are no longer incurred.
ISDN's Promise
In areas where it's available, ISDN is particularly suited for remote access because it can build upon traditional dial-up strategies where users dial in with modems over analog phone lines. ISDN can handle everything from connecting a single user (at higher data rates than when using a modem) to providing enhanced connections of LANs at different sites (see the figure
"Remote LAN Access Examples"
). In this situation, you establish connections on an as-needed basis rather than having to pay for an idle pipeline, as with a leased line. Also, the call setup time for ISDN can happen in milliseconds, which avoids unnecessary charges.
Traditional modems establish as-needed connections, but modems must convert digital data into analog signals for sending over phone lines. The ISDN difference is that every step of the communication is digital. Basic Rate Interface, o
r BRI, is the baseline ISDN service. It offers two B (bearer) channels of 64 Kbps each for combined throughput of 128 Kbps. (Primary Rate Interface, or PRI, offers 23 B channels.)
ISDN is a multiple-channel service with built-in packet capability, lending itself extremely well to WAN access. Unfortunately, ISDN's benefits don't always come easily or inexpensively. Ordering ISDN service can be a nightmare. Some regional phone companies are still developing expertise with the technology, and you have to make sure the service you receive matches the requirements of your ISDN hardware (for details on these two issues, see "Implementing ISDN," April BYTE).
You may also find that ISDN rates in your area vary widely with what a branch office is quoted in another part of the country. Service prices vary depending on which ISDN service provider you use and if you are charged a flat rate or your usage is metered. You'll also pay different rates for all-day or off-peak service. Expect baseline costs to run
from $20 to about $70 per month for BRI. On top of this, some Regional Bell Operating Companies charge an onerous extra fee just for the privilege of running local data over ISDN. (Some network administrators sarcastically refer to this as the ISDN Data Penalty.)
On the bright side, prices for hardware are going down. Until recently, digital WAN devices were very expensive, costing more than $15,000 per location. But now, stand-alone ISDN bridges, supporting a number of remote workstations, are available for less than $2000. Single-workstation network devices, in the form of stand-alone units, typically sell for less than $750. The price tag for an internal ISDN bus card can be as low as $250. That's comparable to some high-speed modems; however, an ISDN card offers significantly better throughput than even the fastest modem.
ISDN PC cards--from companies such as DiGi International, ISDN*Tek, and others--act like a standard network interface card, which simplifies the network connection to a s
ingle RJ-45 cable plugged into the back of the user's computer. Some external and internal devices even contain the network terminator (also known as NT-1) required with ISDN lines. This can save money, and it can also make installation considerably easier.
Along with single-user and multiuser remote devices, there is a crop of multiple-channel ISDN WAN hubs designed for use on the enterprise backbone. Hub devices support a number of remote users through multiple BRI or PRI connections, or a combination of both. Hub units that can serve 160 simultaneous users cost in the $6000 to $12,000 range. These units are ideal for variable-load WAN environments. A hub site ISDN bridge can be configured to filter packets and compress data so that the link performance is optimized (see the table
"Typical Configuration of a Hub Site ISDN Bridge"
). These bridges can dial back a user for security purposes, and they can save money by consolidating phone charges and getting a volume discount for
calls originating from the central site.
Digital Alternatives
ISDN isn't the only digital game going (see the table
"Comparing Access Technologies"
). ATM handles traditional and multimedia data at throughput speeds that scale from 50 Mbps to 622 Mbps. This cell-switched technology breaks up data into neat 53-byte chunks at the sender. Each chunk carries the destination address and is free to choose the path across the WAN that's quickest to the intended receiver. Once all the chunks arrive, the receiver reassembles the data into its original form.
In time, ATM may be the technology that visually links remote sites via videoconferencing. While promising as a WAN technology, the commercial market for ATM remains nascent. In addition, ATM standards, including those that define how the technology will work with current network protocols, are still being developed. ATM should certainly be on your list of strategic technologies for the future, but to
get actual work done now, rely on more traditional technologies.
Switched 56, another digital WAN option, can provide dial-up connections with up to 24 simultaneous channels. You'll need to install a CSU/DSU (channel service unit/data service unit) or a special modem, but call connection times are only a couple seconds, and throughput rates max out at 1.5 Mbps.
Frame relay is a packet-switched technology related to X.25. The difference is that frame relay jettisons the error-checking capabilities of X.25 to reduce overhead and achieve speeds of 2 Mbps. Frame-relay packets can be of variable sizes to dynamically handle larger files. To transform traditional LAN packetized data into frame-relay packets, you need to connect a router, bridge, or FRAD (frame-relay access device) to your local network. Pricing for frame-relay service varies depending on the number and the line speeds of the access points you set up on your network. Pricing may be a flat rate or it may be based on usage. Costs may run
$500 a month or more.
Don't Dis Modems
Digital technologies may be the flashiest ways of making remote connections, but high prices and availability problems can dull their luster. You may never think of POTS (plain old telephone system) as being flashy, but for ubiquitous service, fast connections for mobile workers, and low costs for modest-size data transfers, it's hard to beat a fast modem (see the figure
"How They Work"
).
Today's V.34 modems offer more than top-end modem speed. The standard makes these modems more efficient than their predecessors ever were. For example, V.34 devices can monitor line conditions throughout the duration of a connection, not just in the beginning. This means that a V.34 modem can slow down or speed up to match changes in line quality. Poor quality can lead to an initial connection of less than 28.8 Kbps, but if quality improves during a transmission, a V.34 unit reacts by boosting throughput. Likewise, it can drop
back to slower speeds if quality dissolves. Earlier modems connected at slower speeds or even broke the connection.
Using protocols such as SLIP and PPP, modems can give remote users access to local networks. By specifying how data is encapsulated before it traverses a WAN, PPP provides a standard way for modems and servers to communicate, no matter if they're running Windows, Unix, the Mac OS, or OS/2.
In addition to traditional single-user modems, some vendors, such as Microcom and Shiva, offer network modems that have been designed with remote access in mind. For example, Shiva's NetModem/E is a V.34 modem designed for dial-in connections and for connecting LANs. The company's LANRover/2E Plus is a router for remote-access applications; it uses a V.34 modem and ISDN module.
While modems provide an economical way to dial into a LAN, even 28.8 Kbps can seem slow if you're transmitting large files. Modem manufacturers have become adept at incorporating compression algorithms into their p
roducts. However, there remains a throughput ceiling above which analog services cannot go.
Nevertheless, modems will simply be the only way to provide enterprise access to a significant number of remote users. Modems are particularly effective if you need a roving link, or if you want to connect remote workers who need only E-mail or who occasionally upload or download files. Modems won't blind you with their speed, but they use time-proven technology and they're readily available.
Take the Long View
As you're evaluating technologies to make your current WAN environment efficient and cost-effective, don't neglect to plan for the future. WAN connections tend to grow exponentially over time as more and more users require enterprise-wide services. As the WAN grows, the job of network management can become complex and time-consuming.
Network operators can minimize management overhead through WAN devices that support SNMP, Telnet, and TFTP (Trivial File Transfer Prot
ocol). These tools allow network technicians to administer and upgrade devices without traveling to the remote site. Using SNMP or Telnet, network managers can monitor and configure remote devices from the network control center. Similarly, TFTP allows managers to install software upgrades over the network. These tools help immensely when the remote device is hundreds of miles away from the enterprise network.
Also keep in mind that bandwidth does not buy you everything. Adequate bandwidth will help remote users feel comfortable accessing their corporate network. However, access does not necessarily translate into efficient or functional usage. Don't confuse wide-area
networks
with wide-area
services
.
For a variety of reasons, WAN users might have restricted access to enterprise services. Certain file servers and printers might be unavailable to WAN users for security reasons. Remote users might have access to TCP/IP-based services, but not to NetWare or AppleTalk. Restricted s
ervices can be a cost, security, or bandwidth-conservation issue. Remote users need to understand that they will not necessarily have all the services they might be used to.
Ad hoc WANs are no longer considered some kind of far-out future computing environment. Telecommuting and remote office connections to enterprise networks are a growing reality in business today. It takes extra resources, planning, and thoughtful deployment to provide secure and efficient WAN services. However, the benefits in getting beyond the LAN outweigh the costs.
TECHNOLOGY/ MONTHLY PROS CONS
SPEED SERVICE
===============================================================================
POTS/V.34 Modem -- * Worldwide network and Slow speed
28.8 Kbps proven technology Poor line quality can
Ready availability for slow transmi
ssions
mobile workers
Relatively low cost for
small transmissions
T-1 $ 16,925 ** Well-established technol- Flat-rate charges mean
1.5 Mbps ogy you pay even when
Widely available you're not using the
High bandwidth for full- connection
time connections Moving or adding con-
nections requires
work orders
Switched 56 $ 1585 ** Relatively fast digital Costs can be uneconom-
56 Kbps service ical for low-traffic
operations
ISDN (BRI) $ 350 *** Fast digital connections Surcharges can be
128 Kbps
can handle data, voice, exorbitant
video Not available every-
Increasing availability where
Frame Relay $ 25,943 ** High bandwidth Service can be expen-
2 Mbps Packets can shrink or grow sive
to match file sizes Connections to LANs
require routers
ATM $ 34,650 ** High, scalable bandwidth Products, services not
622 Mbps Can handle data, voice, yet readily available
video
*
Pricing is too variable to estimate.
**
Prices based on a star topology network used by main headquarters with five branch offices. Source: TeleChoice Inc.
***
Estimate for basic connection; does not include transmission costs or surcharges.
Wher
e To Find
DiGi International Inc.
Eden Prairie, MN
(612) 943-9020
fax: (612) 943-5398
ISDN*Tek
San Gregorio, CA
(415) 712-3000
Microcom Inc.
Norwood, MA
(617) 551-1000
fax: (617) 551-1021
Shiva Corp.
Burlington, MA
(508) 788-3061
fax: (508) 788-1539
illustration_link (14 Kbytes)
Giving remote users access to enterprise networks often involves a combination of dial-up analog and digital connections. Analog offers low-cost, ubiquitous service for transferring modest-size data files. Evolving digital services provide high throughput and, in some cases, scalable bandwidth.
illustration_link (97 Kbytes)
illustration_link (11 Kbytes)
TRANSPORT MECHANISMS:
Analog signal over traditional telephone networks; circuit- switched (a connection is established between two sites for th
e duration of the transmission).
Digital signal over phone network
; circuit-switched.
Digital signal over packet-switched network.
Sender breaks data up into 53-byte cells. Each cell finds fastest route to the receiver. The receiver reassembles the data into its original form.
Jeffrey Fritz is a telecommunications engineer who designs and manages data communications for West Virginia University, including its ISDN applications lab. Fritz chairs the National Information Infrastructure Working Group. You can contact him on the Internet at
jfritz@wvnvm.wvnet.edu
or on BIX c/o editors.
You CAN Take It with You
