Dial-Up Internetworking

Enterprise network occupies a large slot in the industry's lexicon. The term conjures visions of an intercontinental network connecting hundreds of sites and tens of thousands of computers. But enterprise networks of this scale exist in only a small number of companies.

The vast majority of networked companies don't have a multitude of offices flung across the globe, populated by workers who need constant, heavy-duty access to each other's information. Most companies only need remote access on an occasional basis and make relatively minimal demands of many applications.

Look at companies' telephone line purchase patterns. Most phone lines used for data purposes are digital leased lines; to a lesser extent, companies buy fractional T1 and full T1. 56/64Kbps leased lines fill most of the need.

Cost is one reason for the plenitude of 56Kbps lines. Most companies outside the Fortune 1000 can't justify the high price of a T1, unless they piggyback data on their existing private T1 network used for voice traffic. Per month, a 56Kbps line costs in the hundreds of dollars; monthly costs for a T1 fall in the thousands. [This pricing is on the high side in 2003.]

On A Smaller Scale

The term enterprise can and should be applied on a smaller scale. An enterprise network connects every location in your company-even if there are only a few zone office locations. For these smaller, but no less important, networks, internetworking solutions-outside of the usual recommendations of high-speed multiprotocol routers pumping data down multiple T1 pipes-apply. Smaller sites need the same level of connectivity but not the same level of horsepower. If your site is one of these smaller networks, dial-up internetworking is likely to play into your plans.

Most companies don't build private networks for voice, but many build private data networks. Still, for most companies, dial-up lines are the most convenient way to communicate. The public switched telephone network is never more than a phone jack away. It's convenient , it's cheap, and it's reliable.

For data, the drawback has historically been speed. For dial-up, 2,400bps and 9,600bps modems may be the norm, but they provide insufficient speed for more than a single user running an application that requires minimal bandwidth.

Advances in technology have made dial-up a more viable alternative. The majority of the telephone infrastructure is fiber optic , not copper , which reduces the number of transmission errors and translates into higher throughput. With VLSI integration of chips and the use of digital signal processors, modems can operate at higher speeds, at a lower cost. Add some compression software, and a 14.4Kbps modem can achieve data throughput of 20Kbps to 57Kbps. These aren't exactly Ethernet speeds, but neither are they at the 2,400bps modem level.

The bottom line is always the biggest motivator; cost is a reason for using dial-up lines over leased lines. With dial-up lines, you pay for what you use, and only what you use.

With leased lines, you pay a flat rate, no matter how much or how little you use them. For occasional usage, dial-up lines make firm financial sense. According to the calculations of market research firm Infonetics (San Jose, CA), shown in Figure 1, if a company needed one hour per day of connectivity between San Francisco and Pittsburgh, the cost of a leased 56Kbps line would be $1,759 per month, but with dial-up networking, the same connect time would cost $327. That's a savings of $17,000 per year. Think about how many hours of usage you would actually need. An hour a day is enough time to exchange 60MB of information, assuming the connection operates at 25Kbps, according to Infonetics.

Dial-up vs. Leased Line Costs
Distance	Dial-Up Service	Leased 56KBps Line	No. of Hours to Break Even Price
10 miles	$12 per month + $.60 per hour	$137	208
10 miles	$12 per month + $9.60 per hour	$732	75
10 miles	$12 per month + $14.40 per hour	$1,224	84
10 miles	$12 per month + $15.00 per hour	$1,759	116
10 miles	$12 per month + $15.00 per hou	$1,931	128
Source: Infonetics, San Jose, CA

Figure 1: Dial-up internetworking can be more cost effective than leased 56Kpbs lines for users who require a part-time connection between two sites.

The flip side of the raw cost consideration is the throughput cost. Although the cost of using POTS (Plain Old Telephone Service) is cheaper, the time costs are less apparent but are of increased importance if the throughput becomes a problem. After all, there is also a cost factor in the time it takes to transfer files or to run an application.

Spontaneous Internetworks

You could benefit from dial-up networking if your company has any one of the following scenarios:

• You want to connect multiple remote offices that have relatively low traffic demands. Most offices don't need constant, heavy access either back to headquarters or to their branches. Users transfer some files and log into e-mail. The high cost of leased lines has typically prohibited offices with low bandwidth requirements from internetworking, but with the lower costs of dial-up internetworking (and advances in speed), such connectivity can be cost-effective . Small offices can benefit from the increased communication that, until recently, companies with big budgets have been enjoying. For example, a headquarters may allow access to its networks from various branch offices. Another scenario is to let one company access a particular portion of another company's network, say an inventory database or online information service. Many companies have already done so on an informal basis, quite successfully, using standard telephone lines.

• You want to telecommute. As notebooks proliferate and as workers become increasingly mobile, the need to access the home network becomes paramount. Dial-up internetworking provides one solution. Asynchronous communication servers and cellular communication are others. Anyone who has used remote communications software to dial in to a network knows the pitfalls. Remote communications software over a modem can be slow and doesn't always work well. And you can't run any significant applications; at best, you can download or upload a small file (hopefully a compressed one). Cellular holds some promise, but until digital cellular is implemented, the bandwidth offered on the voice network is pitifully low. With dial-up internetworking, the remote user participates as a full-fledged node on the network, entitled to all of his or her access rights and applications. With communications software, you are merely an emulation of your former self.

• Although with dial-up, the speeds can be slow and you wouldn't be eager to restructure a database while remote, you can easily check your e-mail and run similar applications. (Hint: Keep your applications software local. You might also want someone at headquarters to administrate your electronic mailbox for you; it can be dreadfully slow if done remotely.) With higher speed lines such as Switched 56 or ISDN, you could run a CPU- intensive application, such as a database or CAD program.

• You want to manage networks. Remote communications packages are the tried-and-true answer to remote network management, yet these applications provide relatively limited control and functionality. With dial-up internetworking and the capability to run as a full network node, you can do anything remotely that you can do locally. Systems integrators and in-house administrators can take advantage of such products to provide support remotely for their customers.

• You want to back up leased lines. Leased lines are typically quite reliable but nevertheless subject to outages. Dial-up routers can be in place waiting to provide a backup to a leased-line service. When you need the bandwidth, it's there. In this way, you can build a hybrid of a public and private network.

How To Do It

To engage in dial-up internetworking, you don't need much equipment: a dial-up router on both ends of the link and a telephone line. Dial-up routers are new, with products currently or shortly available from Centrum Communications (San Jose), CMC (Santa Barbara, CA), DCA/ICC (Alpharetta, GA), NEC (Sunnyvale, CA), and Telebit (Sunnyvale).

In essence, a dial-up router isn't much different from a "regular" router, except that it handles dialing up the remote site for the user. When the router detects traffic coming in, it checks whether the data is destined for a remote site. If so, the dial-up router uses its built-in modem to dial that site's telephone number, establishes the connection, and transfers the data.

After a certain period of inactivity, the router breaks down the connection. Dial-up routers will most likely end up in locations where the users lack computer expertise, so find a product that requires minimal human intervention (and, of course, supports remote management).

As with ordinary routers, support for multiple protocols, such as NetWare IPX/SPX, TCP/IP, and Apple AFP, is critical. The router must be able to accommodate the different network users who dial in. Also look for standards-compliance. A TCP/IP router that uses Point-to-Point Protocol (PPP) will be able to communicate with other TCP/IP routers using that protocol; a router implementing a vendor's proprietary WAN protocol will be able to communicate only with its own kind.

Look for routers that are designed to survive in a WAN environment. With a dial-up, you pay for every packet sent over the wire; operating in that environment requires you to be packet-wise. NetWare routers should implement Burst Mode and filter Service Advertising Protocols to reduce the sheer numbers of unnecessary packets.

Some dial-up routers locally acknowledge a NetWare server's keepalive packets, a process that also reduces overhead. "Spoofing" the server with local acknowledgements can be tricky business, and it has to be carefully implemented. Whatever the network protocol, compression is key, both for the packet header and for the actual data portion.

Security is of paramount importance. Dial-up lines are inherently less secure than leased lines; you don't know where the line physically goes and who can access it in the process. Passwords are the bare minimum. Call-back, where the user calls in and the router calls back to a predetermined number, is useful. Tighter security, including better authentication schemes, is needed.

This tutorial has focused on low-speed dial-up lines, but other options exist. Switched 56 is a dial-up, circuit-switched service that provides 56Kbps of bandwidth. Available only in the United States, the phone companies have priced Switched 56 rather aggressively. With the impending deployment of National IDSN, IDSN may indeed become a real service. A basic rate ISDN interface can deliver 64Kbps or 128Kbps of bandwidth for the cost of one or two Switched 56 lines

This tutorial, number 53, by Patricia Schnaidt, was originally published in the January 1993 issue of LAN Magazine/Network Magazine.