The University of Pennsylvania's Online Computing Magazine

PENN PRINTOUT April 1993 - Volume 9:6

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Making sense of modems

By Mark Litwack

When University offices and services shut down at the end of the day, PennNet, the University's data communications network, keeps on running. For computer users, one of the great advantages of data communications is 24-hour-a-day, 7-day-a-week access to a vast quantity of useful information and services. You can check bibliographic citations on the Franklin online catalog, catch up on your e-mail correspondence, or search for information on remote computers on the worldwide Internet at your convenience. All you need is a properly equipped personal computer, a PennNet ID and password, a modem, and a telephone line.

What is a modem?

The term "modem" is short for "modulator-demodulator," an electronic device used to transmit computer data over ordinary telephone lines. Modems operate in pairs, one for each side of the "conversation." They convert the ones and zeros of computer data into specific tones on the sending (modulating) side, and then convert the tones back into ones and zeros on the receiving (demodulating) side. The specific tones used are described by the transmission protocol standards in use (see sidebar opposite) and can vary greatly in rate of data transmission.

Popular transmission rates in use today range from 1,200 bps (bits per second) to 14,400 bps. These rates correspond to a range of 120 characters (or bytes) per second to 1,440 characters per second, or an average printed page every 20 seconds to 2 seconds. Modem manufacturers have been able to increase these transmission speeds up to four times by the use of data-compression techniques. Newer modems also implement error-correction protocols to ensure virtually error-free transmission of data over telephone lines of marginal quality.

Modem services available on PennNet

PennNet currently supports two dial-in modem pools--the terminal server pool (annex prompt) and the older ISN pool (DIAL prompt).

Using a personal computer equipped with a modem and terminal emulation software, you can dial in to either modem pool to access host computers on PennNet or the Internet. This access mode is functionally equivalent to a direct asynchronous PennNet connection in a campus office.

A new mode of network access that goes beyond the simple terminal- to-host, character-based paradigm has recently become available via the terminal server pool only. Client/server mode computing using SLIP (Serial Line Internet Protocol) can provide functionality equivalent to that of an on-campus Ethernet/IP connection, provided the application in use is based on TCP/IP and there are no strenuous data throughput requirements.

Some typical client functions that can run over SLIP include PennInfo access, direct access to network news, e-mail, high-speed file transfer using FTP, file sharing via NFS, printing, and X Window application access. Applications that transfer large amounts of graphical data are generally not good candidates for SLIP.

Getting started

To access the PennNet modem pools you need the right type of modem for your expected usage, the appropriate cable if the modem is an external model, the appropriate software, and a network ID and password. If you intend to use your modem during much of the day, consider a second phone line so that you can still receive calls while you are online.

Modems. If your communication requirements are modest, consider a low-cost, slower speed (2400 bps) modem. Users in this category are those who occasionally need to access a host to check for mail or perform a few simple tasks. Low-speed modems can be found for less than $100, and are simple to configure and operate. At this speed you needn't worry about data compression and data correction capability.

If your work includes full-screen, character-based applications (such as those on UMIS), or you intend take advantage of the SLIP client/server capability, your best choice is a high-speed modem (9600 bps or greater). Here, too, there are many choices, ranging in price from about $250 to $500. These products almost always include data correction and data compression, which are highly recommended when working at the higher speeds.

The standards associated with high-speed modems, V.32 and V.32bis, are relatively new, however, and there continue to be interoperability problems between modems from different vendors, particularly at the lower end of the price range. Data Communications and Computing Services (DCCS) has tested leading high-speed modems and recommends the Practical Peripherals model PM14400 FXSA external modem for use on PennNet. This modem will operate with Macintoshes, PCs, and other computers.

If you purchase the PM14400, be sure to specify version 1.21 or later of the firmware (built-in software). Earlier versions will introduce performance and interoperability problems. Fax capability is available in this model, but it was not tested. See the sidebar on the back cover for information about distributors.

DCCS does not expect the Practical Peripherals model to remain the only recommended modem. The market is changing rapidly, with new products offering better performance at lower prices. Before you buy, consult PennInfo (keyword "modems") for up-to-date information on recommended modems and distributors. DCCS will support any previously recommended modem to the ex-tent that vendor support is still available for the product.

If you have a laptop computer, you may prefer an internal modem that will reduce extra weight and bulk when traveling. No recommendations are currently available for specific laptops. You are advised to wait until DCCS completes an evaluation before making a purchase.

Cables. To attach an external modem to your computer, you need the proper RS-232 cable. In general, a cable that supports modem control and hardware flow control should be used, and the modem and communications software should be configured to use hardware flow control, especially with higher speed protocols. For an IBM PC/compatible, the right cable is generally not a problem. For a Macintosh, however, a special "hardware flow control cable" cable expressly for modem use is necessary.

Software. In addition, you need the proper communications software for your computer. If you will be accessing hosts on PennNet via character-based terminal emulation, the supported products are ProComm for IBM PC/compatibles (available at the CRC free of charge) and MicroPhone II for Macintoshes ($46 at the Computer Connection). No product is yet supported for Microsoft Windows.

SLIP client/server software is also available for Macintoshes and IBM PC/compatibles running DOS, but not for Windows. The products that have been tested on PennNet are FTP Software for DOS and Hyde Park's MacSLIP. PC users will need to purchase FTP Software for DOS for $245 (single copy). Macintosh users can obtain the site-licensed MacSLIP software from the CRC. Neither of these SLIP packages is fully supported by the ISC, but limited support is available from the DCCS in the form of Technology Releases. Please refer to PennInfo (keyword "SLIP") for more information.

Many public-domain packages are also available for SLIP and terminal emulation. Most will work with PennNet, but no support is available.

ID/password. Finally, you need a PennNet ID and password, which give you authorization to log on to PennNet. (Some resources available via PennNet may require additional passwords.) Password issuing stations are located at the PENNcard ID Center, Suite 323A, 3401 Walnut St.; the Computing Resource Center, Locust Walk opposite the Bookstore; the Biomedical Library, the Johnson Pavilion; and Engineering CETS, Room 164, Moore School Graduate Wing.

Additional remote access methods

If you live in the immediate campus area, you have the option of purchasing PennNet Residential service in lieu of a high-speed modem. PennNet Residential allows you to connect to PennNet at 19,200 bps and provides the same service as a dial-in modem but at a slightly higher rate. The advantages of PennNet Residential are that you do not need to contend with other users for a limited number of slots in the dial-in modem pool, and your phone line will not be busy while you are active on the network. Contact the DCCS Services Center at 898-8171 for more details on PennNet Residential. Network users in the dormitories that are scheduled to be wired with PennNet Ethernet/IP connections will not need either PennNet Residential or a modem.


The high-speed modem market is changing quickly. New V.32bis products that plug directly into portable computers are already being sold by vendors, and ever smaller versions of standalone models are being produced. A new, faster protocol, temporarily called V.FAST, which will support 28,800 bps, will likely be available in a year or less. As these new products appear on the market, DCCS will attempt to select the leaders in the field, test their compatibility and performance in the PennNet environment, and make additional modem recommendations to the computing community.

Sidebar 1: Common dial-up access protocols

Several popular modem protocol standards provide a range of data transmission speeds:

Bell 212A - 200 bps
V.22bis - 2400 bps
V.32 - 9600 bps
V.32bis - 14400 bps

In conjunction with the transmission protocols, data correction and data compression protocols provide error-free data and higher throughput:

MNP 4 - Data correction
MNP 5 - Data compression, used with MNP 4
V.42 - Data correction
V.42bis - Data compression, used with V.42

All of the above protocols are supported by the PennNet terminal server pool (annex prompt), which can be reached at 898-0834. The older ISN service (DIAL prompt) only supports Bell 212A and V.22bis, and can be reached at 898-6184. Settings of 7 data bits and even parity must be used with either pool.

(Note: Often you will hear some of the protocols rated in "baud" rather than "bps." Baud specifically refers to the number of modulation state changes per second and does not necessarily refer to the bps rate. However, the term "baud" is often used interchangeably with "bps," which is incorrect in the context of the higher speed protocols.)

Sidebar 2: Sources for the Practical Peripherals PM14400 FXSA modem

The following distributors have the best known price ($379) at this writing for the Practical Peripherals PM14400 FXSA modem:

111 Sinclair Street
Bristol, PA 19007
Sales: 800/456-7973, Ext. 206
Sales contact: Michael McCafferty

Microcomputer Concepts
15200 Transistor Lane
Huntington Beach, CA 92649
Sales: 800/772-3914
Sales contact: Leslie
Technical support: (800) 336-3914

Macintosh users will need to purchase a special hardware flow control cable to connect the high-speed modem. The appropriate cable and fax software for Macintoshes is available for an additional $20 and can be obtained by requesting the Mac version of the modem. The Mac packaging of the modem can be obtained from the Bookstore for $397.

MARK LITWACK IS Acting Director of Network Engineering for DCCS.