Modems are the most basic networking technology (other than cable itself), but in this era of Internet fever, modems challenge even "glamorous" components like memory and processors for "most favored" status. This chapter begins with a brief review of the theory behind modems. After that, it takes a look at current modem standards and technologies, and at also at such recent outgrowths such as communications servers and shared-access modems. Finally, a set of recommendations for choosing the best modem or comm server for your environment is presented.
In case any of you still don't know its origin, the word modem is an acronym for what a modem does. A modem takes the digital or discrete signal that computers put out, and changes it to an analog signal that can be sent across telephone lines.
At its destination, the analog signal is retranslated to digital, so that the receiving machine can understand it (see fig. 25.1). This conversion and reconversion is known as modulation and demodulation. Hence, modem.
A digital signal is one that exists in either of two states, and only in one of those states. Think of it in terms of numbers óno fractions hereójust the values 0 or 1. An analog signal, on the other hand, can incorporate all possible fractional values.
Figure 25.1 Modulation and demodulation give modems their name.
The methods of signal alteration most frequently used by modems are
If this sounds like detail more appropriate to a discussion of radio, that's because, in converting digital to analog, modems produce audible tones. It's these tones that are sent across phone lines to a receiving system.
Modems, whatever their speed, however they check the data they transmit or receive, and wherever they're located in relation to the computer they serve, share certain basic qualities. Some of these qualities are:
Modems typically communicate with a host through a serial port.
In the PC world, modems converse with the host through its bus by means of a special chip on the motherboard. That chip is called the Universal Asynchronous Receiver/Transmitter (UAR/T).
Regardless of the speed of your modem, data processed by it travels no faster than the top speed that your UAR/T can handle.
ï Modems can be configured to use either software or hardware flow control. Software flow control involves software-generated transmission on (XON) or transmission off (XOFF) signals. Hardware flow control uses the Clear to Send (CTS) and Ready to Send (RTS) channels (pins 5 and 4 respectively) of an RS-232 connector to signal that it's okay to transmit.
Flow control is the regulation of the volume of data a modem sends or receives per unit of time.
Other modem settings can also be customized. Such factors as number of redial attempts, intervals between redials, and what to do with a busy signal are all configurable by the user.
Most modems take their internal commands from the de facto industry standard called the AT command set.
The AT command set got its name because its developer, Hayes Microcomputer Products Inc., chose to use the string AT, literally meaning Attention, as the prefix to its modem commands, and because Hayes modems using this command set became the standard against which so many later communications interfaces were developed.
Modem initialization strings, those portions of the AT command set which get a modem ready to do its job, can, like so many other modem features, be tailored by the user.
A decade ago, modems which met the Bell 103 standard, that is, which operated at a blazing 1,200 baud, were considered top-of-the-line. At the beginning of the 1990s, the great majority of modems in regular use adhered to the 2,400-baud Bell 212 standard. Then, Microcom enhanced the performance of modems by introducing its Microcom Network Protocol, or MNP. MNP made great strides in data communications error detection and correction. For the first time, modems could transmit over noisy lines without seriously degrading data integrity.
In 1990, Microcom defined MNP level 6ñ9, which provided not only error correction and detection, but also data compression along with speeds of 9600 baud. In 1991, U.S. Robotics came up with a then proprietary protocol, High Speed Technology (HST), whose default throughput was 14,400 bits per second (bps).
What's the difference between baud and bits per second? The first, expressed inójust to make things even more confusingóbits per second, has been generally used as a measure of the rate at which data is transferred between any two devices. Baud rate may have been expressed in bits per second, but it did not take into account coding schemes like ASCII or EBCDIC, which use more bits per byte than did Baudot.
The word "baud" is derived from that of its inventor, J.M.E. Baudot, who developed the five-bit code used by teletypes to send and receive data, and whose name was applied to that original five-bit code. Bits per second in an ASCII or EBCDIC context is the more current and correct measurement of modem data transfer, because it takes into account the seven- and eight-bit byte sizes of the modern world. Bits per second also tacitly expresses a fact so often overlookedó contemporary data transfer involves two or three bits per byte which are not used for data as such, but rather for control purposes. So if you want the "net" data transfer rate of a modem, multiply the vendor-stated bps rate by a rule-of-thumb 0.75.
Now, only five years after the groundbreaking MNP 6ñ9, 9,600 baud throughput is considered acceptable at best. As with every other data processing technology, modem standards have evolved not only faster than anyone expected, but faster than we could have imagined.
Table 25.1 summarizes and briefly explains today's standards for modem operations in four categories. The modem standards examined here came about because of the need, recognized by modem manufacturers at the beginning of this decade, to establish and adhere to a common set of communication protocols.
Table 25.1 Modem Standards
Speed
| V.34 | Ratified in September 1994; 28,800 bps |
| V.FC | "Fast Class"; an interim standard used before V.34 was generally approved by the modem industry. V.FC modems are capable of 28,800 bps, but may not be able to connect with V.34/28,800 modems. |
| V.32 bis | 14,400 bps |
| V.32 | 9,600 baud |
| V.22 bis | 2,400 baud |
Error Detection and Correction
| V.42 An effectively error | free transmission |
Data Compression
| V.34 bis | In theory, allows data compression, and therefore an increase in net throughput, by a factor of four. Or, put another way, can (once again, in theoryóthere are a number of factors which affect actual modem performance from moment to moment) help your 28,800 bps modem transmit at 115,200 bps. |
| MNP Class 5 | Compresses data by as much as a factor of two |
FAX Speed
| V.29 | 9,600 baud FAX transmission |
| V.17 | 14,400 bps FAX transmission |
The sections that follow elaborate on some of the speed-related information shown in table 25.1.
What are the differences between V.34 and V.FC? First of all, the latter is a slightly earlier standard. It was developed by Rockwell International and is their proprietary modem technology based on the industry-wide "V.Fast" model.
Both V.34 and V.FC have a maximum modulation bandwidth of 3429 Hertz (Hz); each requires a minimum of 3,200 Hz to support 28,000 bps communications. Each can have its speed lowered by a number of factors, the most common of which is the nature of the "local loop." The local loop is the copper-wire connection from a PC site to the telephone company. Factors like the length and loading ratio of the local loop can lower communications throughput rates.
The differences between V.34 and V.FC are:
The major criticism of V.FC has been that it does not always connect to a V.34 modem, even if both devices are capable of the same speed. This a yes/no situationóit both is and isn't correct. If either end of the conversation is a V.34 modem which can fall back to the V.FC standard, the connection will be made as a V.FC connection, at the highest speed available for that connection. If the V.34 member of the duo has no V.FC capability, the transmission will fall back to a V.32 bis type, and therefore not be able to operate at speeds greater than 14,400 bps.
Finding a roadrunner of a modem is simple. All major modem manufacturers have V.34 models available. The following companies all make a V.34 modem:
V.42 bis and MNP each increase the throughput of "host" modems by factors up to four or two times, respectively. Each delivers virtually error-free transmission. Unfortunately, V.42, the more capable data compressor, carries two types of overhead. It uses a text-based algorithm to accomplish both error correction and data compression. This text-based operation in turn forces other overhead in the form of larger Extended Programmable Read-Only Memory chips (EPROMs) and processors being required.
Rockwell International designed its Rockwell Protocol Interface (RPI) in an attempt to overcome these shortcomings. RPI is a modem protocol that allows a PC to carry out the error correction and data compression tasks that would otherwise be relegated to the modem it houses. This frees that modem to concentrate on signal processing alone. RPI operates between PC and modem, using the same High Level Data Link Control (HDLC) methods employed by both V.42 bis and MNP5. In effect, it permits a V.42 bis conversation to go on between a remote V.42 modem and a less capable modem at the PC end.
Keep in mind that RPI is a Rockwell-proprietary product. However, you can find a number of communications packages that are RPI-compatible. These include:
It should be noted that the standards just described apply not only to both external and internal modems, but also to modems which adhere to the Personal Computer Memory Card Association (PCMCIA) model. The PCMCIA standard defines, among other things, how wireless modems used to handle conversations involving at least one portable PC must be structured.
The most important characteristics of PCMCIA modems are
As is the case with so many other pieces of data processing or communication technology, two factors distinguish an internal modem from an external version of the same model.
The term communications server, like so many terms in the data processing industry, is not consistently applied. Some people use it to indicate any device which accomplishes multiple access to even one communications port. Others identify as communications servers only those devices capable of housing, within a single chassis, the equivalent of several PCs.
Furthermore, there are two categories of remote access which can be accomplished by either of these broad definitions of a communications server.
Remote control, the variety of remote access in which a PC reaches software and data on a network indirectly,by taking control of a station rather than by direct logo to a server, can be accomplished strictly through software. Hardware remote control solutions, on the other hand, can be more costly, depending on the number and special capabilities of the processors on which they rely. As "little" as $4,000 or as much as $20,000 can get you a remote hardware controller like those manufactured by, among others, Cubix and J & L. A remote communications server in this category looks like figure 25.2.
Figure 25.2 A remote control communications server.
A PC that acts as a remote workstation is, by means of its modem and the modem or communications server to which it connects, as much a part of the network, makes the same use of network resources, and accesses those resources in the same way, as a machine which is just down the hall from the server.
Access to a LAN as a remote workstation puts greater demands on the disk and processing capacities of a PC. This being the case, be sure to remember, if you're thinking of configuring your desktop PC at home as a remote client to your office network, to apply the workstation criteria set out in chapter 6, "The Workstation Platform."
Unlike remote control, access to a network by means of a remote workstation requires both software and hardware.
Both Microsoft Windows NT and Windows 95 include remote access server and remote client utilities (RAS).
Many hardware vendors, such as Microcom and U.S. Robotics, offer remote-server hardware, and bundle appropriate server and client software with it.
Remote node hardware ranges from a controller for a single modem or FAX line, through multiple modem or FAX management, to components which create, in effect, a Private Branch Exchange (PBX) running from a single network connection. Pricing for these units goes up in direct relationship to the complexity and variety of the functions they offer. For example, Global Village Inc. listed, on the World Wide Web in August 1995, its OneWorld FAX model 30-3600 at a price of $999. The 30-3600 is a one-line network FAX server for LocalTalk/PhoneNet networks. The 30-3750 from the same vendor offers a two-line network modem and FAX server that runs on either Ethernet or LocalTalk networks; this "oneWorld Combo" listed at $2,099. Even more complex, and more expensive, are the models CD-2, CD-4, and CD-8 Call Directors from Dataprobe. All the Call Directors act as a de facto PBX, switching calls coming in on a single line to two, four, or eight extensions (CD-2, CD-4, and CD-8 models, respectively). At the other end of those extensions, any one of a variety of devices may be installed. Modems, FAXes, phones, answering machines, or dialup services can all be "end users" of the Call Director. Further, the Call Director carries out automatic polling, detection, and routing of incoming FAX or modem connections, and offers the ability to screen or restrict both incoming and outgoing calls.
The remote server/client hardware/software bundles mentioned here as available from vendors like Microcom and U.S. Robotics typically cost about $1,000 per incoming port. For example, the Shared Access LAN modem from U.S. Robotics is an external, two-port unit whose "network end" connects directly to an Ethernet network. Its July 1995 Web page listed it at $1,695. U.S. Robotics' Shared Access Communications Server, also an external unit, priced out at about $2,000.
A remote control communications server such as the one in figure 25.2 can cost anywhere from $15,000 to $20,000.
Any means of remote access management worth considering should support at least TCP/IP and IPX/SPX connections; the addition of AppleTalk would be a plus. Also, look for the ability to manage V.34 modems and their 28,800/115,200 bps speeds.
FAX servers were mentioned in the previous sections. These interfaces are backed up by a different and more extensive set of standards than modems. Table 25.2 summarizes FAX modem/server standards.
Table 25.2 FAX Modem/Server Standards
| Standard | Description |
| Group 3 | Communications over ordinary phone lines |
| Group 4 | Communications over digital telephone networks |
| T.30 | International standard for establishing a connection, negotiating protocols and controlling errors |
| T.4 | International standard for FAX image format, compression, and transmission |
| T.6 | Advanced 2D coding; designed for Group 4, but usable by Group 3 with the addition of an API |
| Class 1 | U.S. standard which adds FAX API commands to the modem AT command set |
| Class 2 | Evolving version of Class 1. Implements T.4 and T.30 on FAX/modems |
| Class 2.0 | Final draft of the Class 2 specification |
| CAS | High-level API for FAX |
| T.611 | Proposed international standard for high-level FAX APIs |
Another way of accomplishing remote access service is to configure a dedicated PC as a modem server. Such a machine typically houses a modem or a multiport serial card, or is host to a similar device, a switch that allows a single RS-232 circuit to be selected by any one of four "sub-channels." In either of these scenarios, only one PC slot is required and as many as eight modem connections can be run from that slot.
An example of a multiport serial card is the SS-554 four-port interface from Synergy Solutions, Inc. The SS-554 offers a number of features any network administrator would consider valuable. These features include:
Multiport cards which offer as many as eight serial connections are available from a number of companies, among them DigiBoard and StarGate.
Any PC you're considering as a server for as many as four modems/serial connections should be at least a 386. Use at least a 486 machine as a modem server for even more ports.
While most multiport serial cards come bundled with their own management software, buying third-party modem-server software is another, just-as-viable, alternative. Any package of this sort that you consider must have the following characteristics:
One such package is WINport from LANSource, Inc. This application allows you to locate a shared modem on a nondedicated Windows PC, a dedicated DOS PC (as old as a 286), or a Windows NT workstation or server running as an NT service. It can be configured to be completely "sharing-transparent," so that users sharing a modem or comm port are unaware of that shared status. WINport uses server and network resources effectively in that it has no TSRs under Windows, and is able to redirect call to the Windows COM port from communications software to the appropriate network device. The application also offers administrative and diagnostics utilities.
A more NOS-specific example of third-party modem server software is NMP2, from Network Products Corporation. NMP2 builds on Windows for Workgroups' ability to allow a network station to use a modem physically connected to another such station. Microsoft implemented this modem-sharing ability as a pooled FAXing capability, but made no provision for the full use of such a shared device as a "virtual" COM port accessible by any Windows-based communications software. NMP2 attempts to correct this problem.
There are even shareware enhancements to the NMP2 product, available by download via FTP or the Web, from a company called Software Technology Service.
Unlike a multiport card, a port switch is a (relatively small) external component. One such device is the model 4P-CAS from Dataprobe. This switch allows a modem to be connected to a "master port" in the switch, which in turn selects from and connects to one of four DTE (Data Terminal) ports it houses. At the other end of those ports, a network component such as a workstation, server, or printer can be connected.
Port switches are simpler technologies than multiport cards, and correspondingly less expensive. The 4P-CAS lists at $325. However, these devices have a hidden cost, which you may have already guessed. Because of their means of operationóswitching between users of what is in reality only one serial connectionóthese components are anything but user-transparent. In other words, if the accountant on PC #1 is "switched on" at the moment, neither the manager on PC #2, the secretary on PC #3, nor the printer on DTE port #4 can send or receive anything.
This section contains example of three environments whose needs have been met by using modem sharing or a communications server. These scenarios are used as the basis for compiling a single, concise "checklist" of what to look for in a network modem or communications server.
In an effort to improve collection management, increase circulation through more efficient servicing of patron requests, and increase revenue through more efficient collection of fines, a small public library takes the plunge. It purchases and installs an Ethernet-based, NetWare-controlled LAN like the one in figure 25.3. While none of the library staff are data-communications literate, and many are downright PC-phobic, they quickly realize the potential benefit to the library and the community of offering "fee for service" features such as online research. Which is to say that within months of the initial network implementation, they're already experiencing a "turnpike effect," that process, named by data processing researchers, wherein users' demands on a system or network increase as they become familiar and comfortable with the tools it offers them. How can this possible additional service be incorporated into their net? (They'd like every PC on the LAN to be Internet-capable.)
Figure 25.3 A small library's LAN.
The seven-station, single-server Novell LAN in the Accounting department wants to exchange information and share all network resources with the five-station Windows NT net in Personnel. The trouble is, the two departments are at separate sites. Figure 25.4 illustrates this networking puzzle.
Figure 25.4 Two departamental LANs.
The Anthropology Department of a university wants not only to become active as a Web server, but also to link to the network of the National Foundation for the Sciences. The department has a five-station LAN made up of three Windows PCs capable of running xterm emulations, and two UNIX workstations used primarily for graphics-based modeling applications, an adjunct to the department's research in the field. Figure 25.5 sketches what the department needs to accomplish.
Figure 25.5 A LAN connecting to a WAN.
Table 25.3 contains data applicable to the three scenarios just presented. It can be used as a checklist for any network administrator who must add a communications or remote access server.
Table 25.3 Communications Service Checklist
| If you haveÖ | And you Need toÖ | We Recommend.. |
| 1 to 4 users | Connect only to such services as online information, during normal
business hours
Share a number of network resources |
A shared-access V.34 modem. The sharing can be accomplished by any of the software packages mentioned in the section on "V.42 by Software Means Alone." |
| Share network resources, and connect to WANs. | Two comminication servers; one a dedicated PC that handles remote access to netowrk resources an the other as stand-alone devide that carries our the link to the WAN. See the earlier section "Remote Access as a Workstation," and "A Dedicated PC as a Remote Access Server," respectively | |
| More than 4 users | Connect only to such services as online information, during normal
business hours.
Share network resources, and connect to WANS. |
A stand-alone communications server like that depicted in figure 25.2
A combination of hardware and software components to accomplish remote access as a workstation. A combination of a "PBX emulator" like that described in the earlier section "Remote Access as a Workstation" and one of the combinations described in the same section. |
In selecting a modem for any networking situation, take the following into consideration:
If the modem in question must also handle FAXing, you must also remember the FAX-specific standards discussed in the section "FAX/Modem Server Standards."
If your task is to implement a communications server, be aware that
When users can connect to a modem but do not get a response to specific commands or application requests, or when they connect but receive only garbage characters, check such configuration details as transmission speed.
