Local Area Transport
| Industry | Computer hardware and software |
|---|---|
| Founded | November 7, 1957 |
| Headquarters | Maynard, Massachusetts , United States |
| Products | LAT protocol, DECserver terminal servers, VAX, Alpha systems |
Local Area Transport (LAT) is a data link layer networking protocol designed by Digital Equipment Corporation to facilitate connections between DECserver terminal servers and Digital's VAX, Alpha, and MIPS systems over Ethernet. It supported communication between hosts and serial peripherals like video terminals and printers. The protocol enhanced packet efficiency by aggregating multiple characters from different ports into a single Ethernet frame.
One of LAT's notable advantages was its proficient management of latency-sensitive data transfer. Over time, implementations of LAT broadened to include support for communication with various Unix-based and other non-Digital operating systems.
History
The initial deployment of LAT occurred in 1984, linking a terminal server to a VMS VAX-Cluster located in Nashua, NH. By "virtualizing" terminal ports on the host side, it enabled multiple plug-and-play VT100-class terminals to connect concurrently, while allowing each terminal to establish multiple sessions across different hosts. Subsequent terminal servers incorporated both LAT and early TELNET protocols, enabling connectivity over TCP/IP networks. The capability to establish reverse connections to unconventional RS232 devices expanded terminal server applications, which evolved into what became known as console servers from the mid-1990s onward.
LAT, together with VMS, played a significant role in promoting early adoption of thick Ethernet within the industry. By 1986, networks based on terminal servers accounted for 10% of Digital's $10 billion revenue. These Ethernet LANs were scaled using Ethernet bridges—another innovation from DEC—and DECnet routers. Eventually, Cisco routers supporting both TCP/IP and DECnet protocols facilitated worldwide interconnection of these packet-based LANs.
As the use of physical terminals diminished, terminal emulator software integrated native LAT clients to preserve compatibility.
LAT features
When a system using LAT does not receive an acknowledgment for a sent packet within 80 milliseconds, it retransmits that packet, which can lead to network congestion. No packets are transmitted if there is no data available. Under high traffic conditions, LAT restricts packet transmission to twenty-four per second—split evenly between twelve outgoing and twelve incoming packets. As more characters accumulate, packet size increases rather than transmission frequency. Delays exceeding 80 milliseconds result in noticeable lag in character echo during typing. This 80-millisecond threshold helps reduce network congestion by minimizing the number of packets sent while increasing their size and lowering system interrupt rates.
Early terminal server vendors
- Digital Equipment Corporation – primarily through their DECserver product line.
- Able Computer – one of the pioneering Terminal Server manufacturers.
- Chase Research – an early Terminal Server supplier based in Europe.
- Cisco Systems – provided LAT support on Terminal Servers as early as 1990.
- Emulex Corporation – California-based company offering X.25 and 3270 functionalities; products distributed via Datability and other OEM partners.
- Equinox Systems – Florida-based early Terminal Server vendor.
- Hughes LAN Systems – introduced LAT capabilities in 1989.
- Interlink Computer Sciences – LAT vendor active in the early 1990s.
- Xylogics Corporation – Massachusetts-based provider within the DEC ecosystem including terminal servers.
- Xyplex Corporation – Massachusetts-based Terminal Server manufacturer.
Open Source Solutions
Numerous Linux distributions provide lat packages for both client and server use, which can be installed through their respective package management systems. This functionality allows users to connect to local area network servers even when operating within corporate VPNs that restrict local TCP/IP communications.
See also
- Terminal server – hardware that links serial devices to Local Area Networks
- Console server – equipment that connects LAN users to operator ports on Linux/Unix servers
External links
- http://sourceforge.net/apps/mediawiki/linux-decnet/index.php?title=LAT - Open source LAT initiative for Linux