Terminal server
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A Terminal Server (also referred to as Serial Servers) enable companies to connect devices with an RS232, RS422 or RS485 serial interface to Ethernet LANs. Products marketed as Terminal Servers can be very simple devices that do not offer any security functionality, such as data encryption and user authentication. The primary application scenario is to enable serial devices to access network server applications, or vice versa, where security of the data on the LAN is not generally an issue. There are also many Terminal Servers on the market that have highly advanced security functionality to ensure that only qualified personnel can access various servers and that any data that is transmitted across the LAN, or over the Internet, is encrypted. Usually companies who need to Terminal Server with these advanced functions want to remotely control, monitor, diagnose and troubleshoot equipment over a network or the Internet.
[edit] History
Historically, a terminal server was a device that attaches to serial RS-232 devices, such as "green screen" text terminals or serial printers, and transports this traffic via TCP/IP TELNET, SSH or other vendor-specific network protocol (e.g. LAT) via an Ethernet connection.
Digital Equipment Corporation's DECserver 100 (1985), 200 (1986) and 300 (1991) are early examples of this technology. (An earlier version of this product, known as the DECSA Terminal Server was actually a test-bed or proof-of-concept for using the proprietary LAT protocol in commercial production networks.) With the introduction of inexpensive Flash memory components, Digital's later DECserver 700 (1991) and 900 (1995) no longer shared with their earlier units the need to download their software from a 'load host' (usually a Digital VAX or Alpha) using Digital's proprietary MOP protocol. In fact, these later terminal server products now also included much larger Flash memory and full support for the TELNET part of the TCP/IP protocol suite.
Many other companies entered the terminal server market with terminal servers pre-loaded with software fully compatible with LAT and TELNET. Some manufacturers also stated specifically that they had emulated Digital's command set for terminal server management. Besides retaining the ability of the older terminal servers to obtain their run-time code from a load host, most were able to bootstrap from on-board flash memory or from a floppy disc held in a drive in the terminal server. Some terminal servers could act as load host for each other; one would hold the code on a PCMCIA Flash card and serve it to another.
[edit] Modern usage
A "Terminal Server" is used many ways but from a basic sense if a user has a serial device and they need to move data over the LAN, this is the product they need.
- RAW TCP socket connection: A raw TCP socket connection which can be initiated from the Terminal Server or from the remote host/server. This can either be on a point to point or shared basis where serial devices (like card readers, scanners, bar code readers, weight scales, etc...) can be shared amongst multiple devices. TCP sessions can be initiated either from the TCP server application or from the Terminal Server.
- RAW UDP socket connection: For use with UDP based applications, Terminal Servers can convert serial equipment data for transport across UDP packets either on a point to point basis or shared across multiple devices.
- Console Management - Rev Telnet, Rev SSH: In console management terminology, users can use Reverse Telnet or SSH to connect to a serial device. They would be running Telnet or SSH on their client (PC) and attach to the Terminal Server to then connect to their ultimate serial device. In this application Terminal Servers are also called console servers because they are used to connect to console ports which are found on products like routers, PBX’s, switches and servers (Linux or SUN). The idea is to gain access to these devices via their console port.
- Connect Serial-based Applications with a COM/TTY Port Driver: Many software applications have been written to communicate with devices that are directly connected to a server’s serial COM ports (bottline machines, scanners, card readers, sensors, blood analyzers). Companies may want to network enable these applications because the devices that were directly connected to the servers COM ports need to be moved to a remote location some distance way from the application server. Since the original application was designed to talk directly to a specific COM port a solution, which is seamless to both the application and device, must be implemented to enable communication across an IP network infrastructure. In other words, a solution that makes the application think is talking directly to the COM port is required. In this application, serial ports can be connected to network servers or workstations running COM Port Redirector software operating as a virtual COM port. Many Terminal Server vendors include COM Port Redirector software with their Terminal Servers. This application need is most common in Windows environments, but it does exist in Linux and Unix environments.
- Serial Tunneling between two Serial Devices: Serial Tunneling enables users to establish a link across Ethernet to a serial port on another Terminal Server.
- Back to Back: This application is designed to solve a wiring problem. For example, a users needs to replace RS232, RS42 or RS485 wire and run their data over Ethernet without making any changes to the server or the ultimate serial device, a users wants to replace a parallel leased line modem network with their parallel Ethernet network, or someone has a chip shooter that puts IC’s on boards and they want to move the server off of the shop floor and into a back room where the equipment will be safe from damage. This application is ideal where any kind of device exists with an application written to gather information from that device (common with sensors). This app allows them to get rid of wiring. It can be also be used with industrial devices (Allen Bradly, Siemens, ModBUS) so that those devices can be run transparently across the network.
- Virtual Modem: Virtual Modem is another example of a Back-to-back application. Let's say someone wants to replace modems but still use an AT command set. Instead of typing the phone number of a serial device into the AT command set, they would type in the IP address.
