TS and RAS - Applications
Marcio Saito - Revision 1.1March 1999

About this document
    This document gives a brief overview with an historical perspective of Terminal Servers and Remote Access Servers. The focus of this discussion is on the possible applications, not in the technical details.

Multi-user Host Systems
    Once upon a time, computers were big, expensive, complex, and took too much space to be in the desktop or even in the corner of the room.
   They were mainframes or Multi-user Hosts. Because they were big and expensive, they had to be shared by several users.
   Users accessed them using terminals. Those terminals were "dumb", in the sense that they did not do much processing, but only passed keystrokes and screen information back and. Those terminals typically used serial interfaces to connect to the Multi-user host.
   The host needed several serial interfaces to connect the terminals. That required multiport terminal adapters.
   Still today, a lot of people buy Multiport serial ports to install in PC-based UNIX "mainframes" and connect a number of local character-based terminals.

Terminal Servers
    Sometimes, the architecture described above was not very convenient. Because the computers were big, it was not always easy to put the user terminals close to the Host. And it was not very convenient to wire several terminals using serial cables over a long distance.
   To solve that problem, there are terminal concentrators, or Terminal Servers. The function of those boxes is to stay close to the user terminals and concentrate/distribute the information between the Host system and the terminals.
   The Terminal Servers we are familiar with use serial ports to connect to the dumb terminals and a TCP/IP connection to the Host System (usually over Ethernet LAN). Because the terminals are not directly connected to the host system the terminal servers offer telnet (which is an application that emulates character-oriented terminal sessions over a network connection).
   Sometimes, one or another terminal needs to be located far away from the others. Or several terminals are dispersed over a large area. In those cases, the terminal server can have modems connected to its ports and provide access to remote locations.

Client-Server and LAN replacing old host systems
    In the late 80's and early 90's, desktop computers (PCs) became ubiquitous. They were not simply keystroke and screen information forwarders, but had local processing capacity that allowed people to run applications without depending on the mainframe computer.
   The corporate applications still depended on large databases and shared information that had to be at a central location. So, the desktop computers still had to be connected to the mainframe (now re-baptized "Servers"). After some disputes, the winning network protocol was TCP/IP and, in the Local Area Network, the media of choice was Ethernet.

Remote Access Servers
    While Ethernet was very good for LAN, it was not appropriated for WAN connectivity. To connect computers over phone lines, you still need modems and serial connections.
   But, to provide a network connection (instead of a telnet character-based interface), the old Terminal Servers were updated to support the PPP protocol over the serial lines. The terminal server acts as a router between the WAN (dialup lines) and the LAN (Ethernet).
   Because this new terminal server is used mostly for dialup access and the devices accessing the network are not simple terminals (they are computers running TCP/IP over PPP), we use a new expression to designate them: Remote Access Servers.
   The Cyclades-PR3000 is an example of terminal server with PPP capabilities and can be used to provide remote access over analog phone lines.

ISDN PRI Remote Access Servers
    With the Internet, the demand for dialup access grew by several orders of magnitude in a few years.
   The phone companies have used T1/E1 lines as part of the phone network structure for decades, but the last mile between the central office and the customer premises was covered by a twisted pair of copper wire.
   With the increased demand for phone lines for remote access, it became practical to take the T1 lines all the way to the access sites (Internet service providers or corporate). Instead of splitting the T1 back to multiple phone lines before connecting to the access equipment, the new Remote Access Servers now support direct connection to the T1/E1 line and incorporate the modems inside.
   That is what we call ISDN PRI Remote Access Server. The Cyclades-PR4000 is the Cyclades product that performs this function.

2. Other Terminal Server Applications
    Terminal servers with RS-232 interfaces, such as the Cyclades-PR3000 can and are still used in a lot of different applications.
   The use of terminal servers for terminal access tends to decrease with the time (only installations with very small number of dialup ports would keep using analog phone lines). But the widespread use of RS-232 serial interfaces for data interface and the need to connect everything to the TCP/IP network creates several different applications for Terminal Server devices.
   We have seen some of them and we will discuss a few more.

Comments or suggestions to this document can be sent to Marcio Saito marcio@cyclades.com