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Terminal Servers Vs. Multiplexers...
The True Story

As more companies find themselves replacing outdated terminals with workstations and PCs LANs, a basic architecture question often arises. Most trade journals are touting new terminal server equipment as a replacement for traditional statistical multiplexers. However, they don't analyze the realities of wide area networking in those articles, so many designers are left with insufficient information on how to determine the best architecture for a given application. Since DCB sells both routed network equipment and statistical multiplexers, our customers need an objective way to analyze the systems they are designing and select the optimum equipment architecture. This paper will provide that missing information and a methodology you can use to select the best architecture.

The "traditional" method of connecting remote users to a central UNIX system is shown in figure one. This is termed a "muxed system". The system consists of remote terminals connected through a statistical multiplexer and single communications line to the host computer site. The multiplexer enables many terminals to share the same line by manipulating the data in transit so that each user seems to have a direct connection to the computer. Statistical techniques are built into the multiplexer to optimize the use of that single line. Typical systems serve four to thirty or more terminal users through one data line to the host system. The terminals may be "dumb" terminals, PCs emulating terminals, or workstations emulating terminals. The electrical interface between these devices and the multiplexer is typically RS-232. Each terminal device is connected to a specific port on the host computer. This system supports "nailed" remote printers on host computer ports configured for printers; that is, each remote printer is always tied to the same host port. The system is relatively high performance, quite bandwidth efficient, simple, and has the advantage of nailed remote printers. All configuration can be performed from the host site, and no complex equipment is installed at the remote sites.


Figure One. Multiplexer System.
An equivalent terminal server system is shown in figure two. In this architecture, a terminal server instead of a multiplexer is located at the remote site. The remote site must have a LAN and one or two routers are needed to connect the remote terminal server to the host site. User terminals (dumb terminals, PCs, workstations) are connected to the terminal server's RS-232 interfaces. If desired, some workstations can connect to the host via the routed connection using telnet over the LAN/WAN. Printers are connected to the terminal server which must use the LPR/LPD protocol to print from the host computer. Terminal connections are usually dynamic; a given terminal will connect to various logical TTY ports on the host computer... instead of always connecting to the same TTY port as in a muxed system. The terminal server communicates with the host using ethernet and TCP/IP protocols. This system is not as bandwidth efficient as a muxed system and is more complex. Its main advantage is the use of a routed connection that may be required for other applications such as a graphics UNIX X-terminal system. The terminal server can also connect to more than one host computer, so if a terminal needs to switch between hosts, a terminal server is required.


Figure Two. Terminal Server System.
What are the compromises? What tradeoffs are required and which ones are optimized? Lets compare the two methods on a point-by-point basis. We will compare an 8 port system using each method. Equipment common to both systems is ignored (DSU/CSU, cables)

1. Performance.
With equivalent recurring communications cost (the same data line between the host and remote sites), Muxed systems will always perform better than terminal server systems. Muxed systems optimize data line utilization by analyzing the data with statistical methods and customizing the data throughput on a moment-by-moment basis. There is very little overhead passed through the communications link. Terminal Servers package each keystroke (for normal typists) or small group of keystrokes( if you are blazingly fast typing) into an Ethernet packet. That adds tremendous overhead to the system. For example, when viewing a menu screen, you press the enter key to make a selection. The terminal server will send 64 characters to the host system just to transmit that enter keystroke. That's a 6400% overhead. Obviously, when sending larger blocks of data such as printing to a remote printer, the overhead goes down; but, it is still quite substantial. Overhead on a muxed system is quite low. Multiplexers are tuned for a mix of relatively slow operations such as typing on a keyboard or displaying information on a screen along with printing on a remote high speed printer. For Performance, Muxed systems win hands-down.

2. Bandwidth Requirements.
A terminal server architecture will need about five to ten times the bandwidth of a muxed system for comparable response time performance. That additional bandwidth is a recurring cost each month. For bandwidth cost, Muxed systems win by a large factor.

3. Equipment Requirements.
The Muxed system requires only two multiplexers. Terminal server requirements require a terminal server and a router (possibly two routers). If the applications is character intensive, a second router may be required to support a high-speed WAN link. Per Port cost is about the same for both systems.

4. System Complexity.
Terminal server systems are more complex than muxed systems. Terminal servers require TCP/IP configuration, remote printer LPR/LPD configuration, and on-going administration to support these configurations as well as port configuration. Other than the port configuration, there is no additional configuration for a muxed system. Troubleshooting is much easier on a muxed system. There are no protocol issues involved in the muxed system. Terminal server systems rely upon routed ethernet and TCP/IP protocols with many potential points of failure.

5. Voice and Data integration.
Using a muxed system on a 56 Kbps line allows adequate terminal response when integrated with a channel or two of voice. Routed systems require at least 56 Kbps bandwidth so any voice added to a routed system will noticeably affect performance.

Terminal Server Vs. Multiplexer Comparison

Characteristic Multiplexer Terminal Server
Equipment Requirements and cost. 8 Remote Ports Two Multiplexers
$ 4,200 * 		One Terminal Server, One Router minimum, possibly two.
$ 3,500 - $ 4,500 *
Bandwidth Requirements LOW (Minimum 19.2Kbps) HIGH (Minimum 56 Kbps) for equivalent throughput
Response Times (Performance) FAST SLOW
ComplexityLOW HIGH
Feasibility of adding voice on 56Kbps line POSSIBLE NOT ADVISABLE Due to Bandwidth Requirements
Host switching Not Available Available
Applications Character Mode Applications Well Supported Poor Support of Character Mode Applications
Life Cycle Cost LOWER HIGHER
* Prices are based on DCB multiplexer, router and terminal server solutions.
Checking the chart above shows that most of design compromises favor the muxed system in average applications. So why are terminal server systems installed so often? First, they are trendy. Many people depend on the trade journals that push what their advertisers pay for, and right now, that is ethernet networking gear. Many networking professionals don't have the experience to see just how robust and responsive a medium speed data line can be. Often, those who started in the business within the last few years haven't been exposed to multiplexed systems and don't understand the concepts.

Terminal server systems are appropriate under some circumstances. If a routed network is required for some other application such as X-Window workstations, remote email systems, or other internetworking; then the incremental cost of a terminal server is relatively low. Another factor would be the need for a user to switch between several host computers. Terminal servers have this capability which isn't available on muxed systems. The caveat is still centered around bandwidth and performance. Terminal server systems require much more bandwidth than muxed systems for equivalent performance.

Which method is best for your application? The multiplexer route is normally the most cost effective and highest performing option. If using routed facilities in common with other applications such as X-windows, the terminal server may provide adequate performance. If host switching is needed, a terminal server is required. There is a hybrid method, shown in Figure Three, that works very well for a mixed system of terminal users that need some LAN type access for email. By putting the routed LAN traffic on one port of a statistical multiplexer, LAN traffic such as email is available while maintaining the performance advantage of multiplexing terminal access.


Figure Three. Hybrid Multiplexer/Router System.

Future products will be available that provide the advantages of both systems. These will use a remote multiplexer in conjunction with a device that combines multiplexing and terminal services at the host site. Until this class of products are available, select a method that provides maximum performance with minimum recurring cost.


Data Comm for Business, Inc. is a manufacturer and distributor of a diverse line of data communications and networking products. Their experts can help you connect remote sites with phone (1-800-4-dcbnet), email (info@dcbnet.com), or personal (807 Pioneer, Champaign, Il. 61820) assistance. Check out their Web page at http://www.dcbnet.com .