Terminal server
Encyclopedia
A terminal server enables organizations to connect devices with an RS-232
, RS-422
or RS-485 serial interface to a local area network
(LAN). 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 which need a terminal server with these advanced functions want to remotely control, monitor, diagnose and troubleshoot equipment over a telecommunications network.
devices, such as "green screen" text terminals or serial printers, and transported traffic via TCP/IP, Telnet
, SSH
or other vendor-specific network protocols (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 Maintenance Operations Protocol
(MOP). In fact, these later terminal server products 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 devices pre-loaded with software fully compatible with LAT and Telnet. Some manufacturers stated specifically they had emulated Digital's terminal-server management command-set. Besides retaining the ability of the older terminal-servers to obtain their run-time
code from a load host, most could bootstrap
from on-board flash memory or from a floppy drive in the terminal server. Some terminal servers could act as load hosts for each other; one would hold the code on a PCMCIA flash card and serve it to another.
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
, RS-422
EIA-422
RS-422 is a technical standard that specifies electrical characteristics of a digital signalling circuit. Differential-mode signals can be sent at rates as high as 10 million bits per second, or may be sent on cables as long as 1200 metres. Some systems directly interconnect using RS 422 signals,...
or RS-485 serial interface to a local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
(LAN). 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 which need a terminal server with these advanced functions want to remotely control, monitor, diagnose and troubleshoot equipment over a telecommunications network.
History
Historically, a terminal server was a device that attached to serial RS-232RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
devices, such as "green screen" text terminals or serial printers, and transported traffic via TCP/IP, Telnet
TELNET
Telnet is a network protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communications facility using a virtual terminal connection...
, SSH
Secure Shell
Secure Shell is a network protocol for secure data communication, remote shell services or command execution and other secure network services between two networked computers that it connects via a secure channel over an insecure network: a server and a client...
or other vendor-specific network protocols (e.g. LAT
Local Area Transport
Local Area Transport is a non-routable networking technology developed by Digital Equipment Corporation to provide connection between the DECserver 90, 100, 200, 300, 500, 700 and DECserver 900 terminal servers and Digital's VAX and Alpha and MIPS_architecture host computers via Ethernet, giving...
) via an Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
connection.
Digital Equipment Corporation
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
's DECserver
DECserver
In computer networking, DECserver initially referred to a highly successful family of asynchronous console server / terminal server / print server products introduced by Digital Equipment Corporation and later referred to a class of UNIX-variant application and file server products based upon the...
100 (1985), 200 (1986) and 300 (1991) are early examples of this technology. (An earlier version of this product, known as the DECSA
DECSA
DECSA is a communications platform developed by Digital Equipment Corporation based upon a PDP-11 core chassis, with the provision for user installable I/O cards including asynchronous and synchronous modules...
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
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...
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
VAX
VAX was an instruction set architecture developed by Digital Equipment Corporation in the mid-1970s. A 32-bit complex instruction set computer ISA, it was designed to extend or replace DEC's various Programmed Data Processor ISAs...
or Alpha) using Digital's proprietary Maintenance Operations Protocol
Maintenance Operations Protocol
The Maintenance Operation Protocol is used for utility services such as uploading and downloading system software, remote testing and problem diagnosis...
(MOP). In fact, these later terminal server products 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 devices pre-loaded with software fully compatible with LAT and Telnet. Some manufacturers stated specifically they had emulated Digital's terminal-server management command-set. Besides retaining the ability of the older terminal-servers to obtain their run-time
Run-time system
A run-time system is a software component designed to support the execution of computer programs written in some computer language...
code from a load host, most could bootstrap
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
from on-board flash memory or from a floppy drive in the terminal server. Some terminal servers could act as load hosts for each other; one would hold the code on a PCMCIA flash card and serve it to another.
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 be point-to-point or shared, where serial devices (like card readers, scanners, bar code readers, weight scales, etc.) can be shared amongst multiple devices. TCP sessions can be initiated 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 on a point-to-point basis or shared across multiple devices.
- Console management - reverse Telnet, reverse SSH: In console management terminology, users can use reverse TelnetReverse telnetReverse telnet is a specialized application of telnet, where the server side of the connection reads and writes data to a TTY line , rather than providing a command shell to the host device. Typically, reverse telnet is implemented on an embedded device , which has an Ethernet network interface...
or SSH to connect to a serial device. They run Telnet or SSH on their client (PC) and attach to the terminal server, then connect to the serial device. In this application, terminal servers are also called console serverConsole serverA console server is a device or service that provides access to the system console of a computing device via networking technologies....
s because they are used to connect to console ports which are found on products like routers, PBXes, switches and servers (Linux or Sun). The idea is to gain access to those 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 (robotic assembly machines, scanners, card readers, sensors, blood analyzers, etc.). Companies may want to network these applications because the devices that were directly connected to the server's COM ports need to be moved to a location some distance away from the application server. Since the original application was designed to talk directly to a specific COM port, a solution seamless to both the application and device must be implemented to enable communication across an IP network. I.e. a solution that makes the application think it is talking directly to a COM port. In this application, serial portSerial portIn computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
s can be connected to network servers or workstations running COM port redirectorCOM port redirectorA COM port redirector is a specialized software that includes the underlying network software necessary to access networked device servers that provide remote serial devices or modems.-Overview:...
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 also exists 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 user needs to replace RS-232, RS-422 or RS-485 wire and run their data over Ethernet without making any changes to the server or the ultimate serial device, a user wants to replace a parallel leased line modem network with their parallel Ethernet network, or someone has a pick and place machine that puts ICs on boards, and they want to move the server into a back room where the equipment will be safe from damage. This application is ideal where a device exists with an application written to gather information from that device (common with sensors). This application allows them to eliminate the wiring. It can be also be used with industrial devices (Allen-Bradley, 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. It may be used to replace modems but still use an AT command set. An IP address is typed into the AT command set instead of the phone number of a serial device.
See also
- Modem sharing deviceModem Sharing DeviceA modem sharing device , also commonly known as a line sharing device , modem sharer, or line sharer, allows multiple devices to share a serial connection. Only multipoint serial protocols such as bisync are supported. Both synchronous and asynchronous datastreams can be used...
- Out-of-band managementOut-of-band managementIn computing, out-of-band management involves the use of a dedicated management channel for device maintenance...
- Time-sharingTime-sharingTime-sharing is the sharing of a computing resource among many users by means of multiprogramming and multi-tasking. Its introduction in the 1960s, and emergence as the prominent model of computing in the 1970s, represents a major technological shift in the history of computing.By allowing a large...