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Terminal Servers Vs. Multiplexers...
As more companies find themselves replacing outdated terminals with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 True Story
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
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
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
* Prices are based on DCB multiplexer, router and terminal server solutions.
|Characteristic || Multiplexer || Terminal Server
| Equipment Requirements and cost. 8 Remote Ports
|| Two Multiplexers |
$ 4,200 *
| One Terminal Server, One Router minimum,
$ 3,500 - $ 4,500 *
|Bandwidth Requirements ||LOW (Minimum 19.2Kbps) || HIGH (Minimum 56 Kbps) for equivalent throughput
|Response Times (Performance) ||FAST || SLOW
|Complexity||LOW || 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
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.
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