<H1>statistical multiplexer, stat mux, dsu, remote access </H1>

Small Business Series: Data Communications, A Primer


In this paper, we discuss popular data communications solutions... the available technology , performance trade-offs, reliability, equipment needs, and general cost involved. Whether you are connecting a single terminal to a remote host computer or multiple LAN's together, this information should be helpful.

Connecting two offices with data communications is both easier and harder than it used to be. It's easier because of the large number of viable options from which to choose. It's harder for the same reason, selecting the optimum choice requires more thought and analysis. We start by determining actual communication needs in order to select the proper combination of communications methods and equipment. Once a communications technology choice is made, equipment requirements are defined. Equipment configurations must be analyzed to select the proper combination of modems, terminal adapters, or DSU/CSUs. The actual applications in use, what are you doing with the computers and how information is processed, along with the specifics of geography usually determine the best solution. Before any technical decisions, we must examine the business purpose and applications running at the two sites. There are rules of thumb to follow, but you should make sure all applicable options are considered.

In this paper, we won't analyze your business computing needs. Instead, we'll explain the different communications technologies in use and show how they compare to one another on performance, equipment requirements, and cost. You can then compare this analysis to your specific situation to make a more informed choice.... or at least know what your consultant is talking about. The total connection between local equipment and the equipment at the remote location consists of communications facilities provided by one or more telephone companies and equipment at each location. This equipment consists of some of the following:

Communications Equipment
Some conversion hardware to convert between the electrical standards used in local computer environments (such as computers and terminals) and the electrical signals needed in a wide area network (WAN) environment (the world of the telephone company or radios). This could be a modem, DSU/CSU, or ISDN Terminal Adapter. The type of communications line and line speed determine this equipment choice.

Modems are used on regular voice dial-up telephone lines and leased "analog" lines. They are typically limited to speeds of 28.8Kbps (Kbps = Thousand bits per second) or less; although some models use compression techniques to increase the apparent speed above that.
DSU/CSU (Data Service Unit/Channel Service Unit) devices terminate digital telephone lines. They usually operate at higher speeds than modems... 56Kbps, 64Kbps, up to 1.544Mbps (MBPS = Million bits per second). Some have additional functions such as splitting that bandwidth into several paths.
Terminal Adapter
Terminal Adapters (TA) are only used to terminate ISDN lines (discussed below). They are rated at speeds of 64Kbps to 128Kbps.
Protocol conversion equipment
This equipment is used between your computers and the communications equipment. It consists of one or more of the following, depending upon your specific situation:
This device enables multiple terminals (or PCs acting as terminals) to share a single data communications line. It is useful where there is a host computer at one site and two or more terminals at the remote location. Performance, number of ports, and data communications technology used determine its selection.
A router filters the data on one Local Area Network (LAN) and only forwards appropriate portions to the remote site. The selection of a router is usually based upon performance, protocols, price, LAN type, and number of sites. Often, it performs other functions such as security filtering, and protocol spoofing.
Terminal Servers
This equipment allows multiple terminals at one location to connect to a host at the other end. Its operation is similar to a multiplexer, but it requires more communications bandwidth and usually used only with UNIX computers and terminals.
In some cases, you connect a computer or terminal directly to the modem or DSU/CSU. No further conversion equipment is needed.
Since there are several communications circuit technologies in popular use, the best technology choice is always based upon analysis for a specific situation. Before listing the options, we'll describe the technologies involved. This technology choice is usually based upon three items.

Here are the most commonly used technologies...

Dial-up Analog line:
Modems and dial-up telephone lines are commonly used when the connection is described as "casual", that is, it is used for very low volumes of information transfer and for just a few minutes at a time. If the offices are in the same city, the telephone costs are quite low... just purchase a plain telephone line at each office and let one automatically dial the other when needed. Usual speeds are now 9.6Kbps to 28.8Kbps.

Dedicated Leased Analog Line:
This technique uses similar modems as above, but a dedicated telephone line is leased between the two sites. This was the "high performance" method in the 70's and 80's. Now, it is the low to medium speed "full-time" connection. Unfortunately, it is the only economical way to reach some sites in rural areas. Speeds are usually the same as dial-up, but the connection is always there. For long-distance circuits, other methods may be more cost effective, even when used at low speeds.

Dataphone Digital Service (DDS) Line:
This is the current work horse of data communications. A digital circuit between the two sites is leased from the telephone company. Instead of modems, devices called DSU/CSU (Data Service Unit/Channel Service Unit) terminate the line. Higher speeds than the modem method are available, but often at higher costs. Usually offered at 56Kbps through 1.544 Mbps.

Frame Relay Circuit:
This is a relatively new technology that many users are finding much more economical than DDS lines. To the user, it is similar to a DDS line between two sites in this application. It provides a full time connection between the sites. Since the frame relay technology offers some advantages to the phone company, they usually pass cost savings on to the customer. When the circuit is long distance, frame relay is quite cost effective as it isn't priced by the mile like other full time circuits. A frame relay circuit between two nearby cities is often the same cost as one between the two coasts. Frame relay speed is similar to DDS lines; faster than modems and leased lines. Frame relay is usually offered at 56 Kbps, 64 Kbps, or some multiple of these up to 1,544 Kbps (1.544Mbps).

The newest popular service is Integrated Services, Digital Network (ISDN). This is a digital dial-up service... similar to your plain telephone dial service, only digital in nature. Although it is a dial-up service, it dials quickly (2-4 seconds for a connection instead of a minute or two for modems) and disconnects in less than a second. The speeds of ISDN are typically two or more times as fast as normal modem dial-up or leased lines. Pricing for ISDN varies from city to city. There is often a per-minute usage charge, and it is not usually available over long distances. A variation on this service, Centrex ISDN, doesn't have the per-minute charge; but is usually available only where there are multiple sites to connect. You must be located within about four miles of a telephone company central office for ISDN. ISDN is most often sold at the speeds known as BRI or Basic Rate Interface. This is either 64 Kbps (known as one B channel) or 128 Kbps (known as 2B or "bonded channels"). Faster than that, you have to get ISDN at 1.544 Mbps known as the Primary Rate Interface (PRI).

A comparison...

Technology Full-Time Cost Usage Charge Speed
Modem Dial-up NO Low Yes Low
Modem Leased YES Medium No Low
DDS YES Medium to High No Medium to High
Frame Relay YES Medium No Medium to High
ISDN NO Low to Medium Sometimes Medium
One other option should be mentioned. Wireless, radio links are available for distances up to about 10 miles. These have some great applications and compete favorably with telephone lines in many cases. (See our "Small Business Series: Wireless Connectivity Options" paper for more details). However, if you are going more that about 10 miles, the telephone company is still the most popular way to connect the sites.

What it all means....

The connection is either full time or dial-up (part-time). If you need more than an hour per day of connection, it us usually better to install a full-time circuit. Full-time circuits are much easier to maintain and their performance is generally better. When using a dial-up system, you typically get a different set of telephone company equipment (lines internal to the telephone company) each time you dial. Higher speed dial-up modems often have to "step down" to a lower speed to make the connection work. This makes the circuit less stable, and harder to troubleshoot. If there are per-minute useage charges, dial-up costs can quickly exceed the cost of a private line. We recommend full-time connections for most customers.

The speed of the circuit should match your application needs. Circuit speed is referred to as "bandwidth". The higher the bandwidth (speed), the faster information is transferred. Few modern applications can survive with the slower speeds of dial-up modems. Common speeds in the real world are...

  1. Under 56 Kbps. Modems on dial-up and leased circuits typical run between 9.6 Kbps and 28.8 Kbps. This is adequate for a terminal to host computer connection, or in some cases for a workstation to host LAN-type connection. It is often used for workstation to Internet connections. It is normally too slow for LAN-to-LAN connections that are used for anything other than email transfers, system management, or remote control with PCs.

  2. 56 Kbps. Here we move to ISDN, leased DDS lines, and Frame Relay. This is the journeyman speed for most of today's cost effective LAN-to-LAN connections. You typically need a file server or host computer on each end of the circuit since this is too slow to load programs across the line, but it is fast enough for most client-server applications and transferring data. This is the minimum bandwidth required for any "real" multi-media applications such as graphic web browsing, or digital sound transfers. Some applications software is written specifically for remote operation (known as client-server). This type of software usually requires at least 56Kbps bandwidth. .

  3. 128 Kbps to 1.544 Mbps: Serious work here! These speeds are necessary to connect multiple locations that use client-server software, "wordy" network systems (usually known as Netware), electronic commerce, and multi-media applications. Trade journals seem to indicate that all corporate data moves at these speeds; but in reality, the most common speed we run into is 56 Kbps. At these speeds, you can also share some of the bandwidth with voice circuits to cut down on long distance voice charges. The technology is almost always Frame Relay or DDS. The 1.544 Mbps circuit is known as a "T-1" circuit. T-1 bandwidth can be divided into 24 channels of 56 (or 64) Kbps each. "Fractional T-1", is a subset of a full T-1 circuit. For example, you may buy 112 Kbps (or two channels at 56) as one circuit. You can buy what you need, but the economics change at around 4 channels... it's often just more economical to buy a full T-1 at that speed.

How much bandwidth is needed?

An analysis of your specific application is required to answer accurately. Most often, when connecting two LAN-based offices, a 56 Kbps circuit is adequate so long as you are not transferring programs, multi-media, or heavy graphics over the line. Connecting two normal, small business offices such as insurance agents, retail stores, common white collar or clerical work locations is most often done with the 56 Kbps circuit. Where ISDN is priced aggressively, the number is 64 Kbps. Same bandwidth class, slightly different technology. Slower, dial-up circuits are sometimes practical but always more trouble-prone.
Heavy LAN traffic or multi-media transfers can quickly consume bandwidth. More complex installations often require speeds between 56Kbps and 1.544Mbps.
If connecting a remote terminal to a local host computer, much less bandwidth is needed. Many sites successfully use a simple dial-on-demand modem and 19.2Kbps (or slower) connection for these applications. DCB has a large number of customers using multiplexers with 8 or more terminals on 19.2Kbps analog leased lines.

Full-time connectivity or part time?

We seldom recommend dial-up because it causes so many problems over the long term and has such low bandwidth. The main exception to this is ISDN. Connection times are quick enough that you don't notice the dialing; and since the circuit is digital all the way, the quality of transmission circuits is high. Unfortunately, ISDN isn't normally offered for long distance circuits. Look at your application. Is it long distance? How many hours per month will it be used? Is it "mission critical" to your profitability?

Selecting the Optimum Data Communications Solution, A Checksheet

A. Bandwidth

Bandwidth needs are the result of the application requirements. Make sure you analyze the applications in detail. Don't take shortcuts, as a circuit without enough bandwidth is not only a lot of trouble for your users, it may cause you to scrap expensive equipment when you start over. DCB has helped some companies simulate or prototype their proposed networks to verify that anticipated bandwidth is adequate.


Again, the application determines requirements for economical connections. Choices are simple... Dial-up analog, Dial-up ISDN, or full-time dedicated lines.

C. Technology

The technology used can now be reduced to economic analysis. Compare installation cost, recurring costs, contract length, and equipment requirements. Frame Relay, DDS, and ISDN may all offer the bandwidth you need. Simply compare the economics of those that meet your requirements.

Data Comm for Business Inc.

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