PENN PRINTOUT
The University of Pennsylvania's Online Computing Magazine

PENN PRINTOUT November 1992 - Volume 9:3

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SLIP: Dial-up access to the Internet

By Randall Couch

If your lab or office computer is connected to PennNet's IP network (either through a direct ethernet connection or a gateway box), you may already be familiar with--and spoiled by--the power and convenience of client/server computing. Perhaps you

  • Initiate high-speed file transfers using FTP clients, such as Fetch or FTP Software

  • Log in directly from your computer to remote Internet hosts using the Telnet protocol

  • Log in to IBM hosts using tn3270 for full-screen IBM terminal emulation

  • Access the network news system directly from your computer

  • Manage your e-mail from your own computer using a POP client such as Eudora

  • Use the point-and-click Macintosh client for PennInfo

  • Work in a windowing environment that allows you to run several of these programs at once

If you are accustomed to any of these capabilities, connecting to PennNet from a home computer can be a real step backward. Wouldn't it be great if you could use IP services from home?

Well, soon you can. Data Communications and Computing Services (DCCS) has completed the necessary work for PennNet to support SLIP, a service that makes IP services available over modems and phone lines. (For a description of PennNet's IP network, see "Visualizing PennNet," Penn Printout, October 1990.)


What SLIP does

SLIP stands for Serial Line Internet Protocol. SLIP is a convention for "framing" IP packets, or bundles of data, so that they can be transmitted over a serial line (such as a cable, or a pair of modems and a telephone circuit). This allows a home machine to dial up and become a part of the Internet. In effect, SLIP turns your computer's serial port into a logical ethernet port. Software that communicates with the outside world via your lab or office computer's direct IP connection will function the same way over SLIP, although somewhat more slowly due to the lower data speed of modem connections.

Remember that IP services make use of the vendor-independent standard TCP/IP protocol suite. SLIP alone will not allow you to participate directly in proprietary networks, such as AppleTalk or Novell Netware, from home.

SLIP employs a client module that runs on your desktop computer, and a server module that runs on the terminal servers connecting the dial-in modem pool to PennNet's IP network. The server module is now in place and ready to accept SLIP traffic. In order to make use of SLIP, you need a client module for your computer and a suitable high-speed modem or PennNet Residential data-over-voice service.


SLIP clients

SLIP clients are available for many computer types, but not all SLIP clients work correctly with PennNet hardware. Through its Technology Release process, DCCS has qualified two SLIP clients: MacSLIP, from Hyde Park Software, for Macintoshes, and FTP Software's PC/TCP version 2.1 for IBM PC/compatibles. Technology Release documents describing specific requirements, installation, configuration, and scripting for these clients are available in the PennNet section of PennInfo.

A Penn site license is currently being negotiated for MacSLIP. In the meantime, software and information are available from the manufacturer, Hyde Park Software, 4003 Avenue H, Austin, TX 78751 (512/454-1170).

FTP Software's PC/TCP includes basic IP functions, such as Telnet and FTP, in addition to SLIP support. With Penn's discount it costs $245 for a single user plus $65 for each additional user. For more information, contact PennNet Services Center at 898-8171 or psc@dccs.

Proper setup and use of SLIP software requires some technical understanding. Newcomers to networking may wish to consult the network administrators in their Schools and centers. DCCS Engineering will provide best-effort support via electronic mail for qualified SLIP clients.


Modem requirements

As mentioned earlier, SLIP provides access to feature-rich IP applications at the cost of slower performance than direct IP connections provide. How much slower? That depends on the capabilities of the modems at both ends of the phone line. PennNet's dial-in modem pool (annex: prompt) now supports hardware flow control and speeds up to 14,400 bps without data compression. It also supports error correction (V.42) and data compression (V.42bis) protocols yielding maximum speeds up to 38,400 bits per second (bps).

How fast does your modem have to be to make SLIP worth using? That is a matter of individual judgement. Experienced SLIP users often recommend a minimum modem speed of 9600 bps to maintain reasonable response time. Higher-speed modems are suggested for network applications that involve many images, such as X-Windows-based graphics applications.

Notwithstanding vendor claims, the technology for 9600-bps and faster modems is still rapidly evolving. Implementation of relevant standards by different makers is not entirely consistent. DCCS has evaluated many low-cost ($400 or less) high-speed modems. All have strengths and weaknesses, including some that may limit their successful use with PennNet's dial-in pool. If you are ready to purchase now, consult the Modem Reports folder in the PennNet section of PennInfo for DCCS test reports. DCCS continues to track this technology and will make an announcement as soon as it can qualify a modem without reservation.

Users of PennNet Residential data-over-voice service enjoy dial-up access to PennNet at 19,200 bps. These connections are ready for SLIP access, but users still need a SLIP client module such as those mentioned earlier.


Campus asynchronous connections

If you have an asynchronous connection in your office or lab, is SLIP service an option for you? Not really. SLIP for campus connections is not a cost-effective service for DCCS to offer, in view of its performance limitations and the declining cost of direct IP connections. Monthly charges for PennNet ethernet connections were reduced 22 percent this fiscal year to $24.50-only $2 more than asynchronous connections. DCCS is committed to similar aggressive reductions next year, to bring high-performance IP services within the reach of the entire campus community.

RANDALL COUCH is a Senior Technical Writer for the ISC Communications Group.