computer science

Local Area Transport

Local Area Transport is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Local Area Transport rather than just read about it. In short: Local Area Transport (LAT) is a non-routable (data link layer) networking technology developed by Digital Equipment Corporation to provide connection between the DECserver terminal servers and Digital's VAX and Alpha and MIPS host computers via Ethernet, giving communication between those hosts and serial devices such as video terminals and printers. The protocol itself was designed in such a manner as to maximize p…

Key takeaways

Local Area Transport belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Local Area Transport to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Local Area Transport from memory before moving on to harder problems.

Reference excerpt

Local Area Transport (LAT) is a non-routable (data link layer) networking technology developed by Digital Equipment Corporation to provide connection between the DECserver terminal servers and Digital's VAX and Alpha and MIPS host computers via Ethernet, giving communication between those hosts and serial devices such as video terminals and printers. The protocol itself was designed in such a manner as to maximize packet efficiency over Ethernet by bundling multiple characters from multiple ports into a single packet for Ethernet transport. One LAT strength was efficiently handling time-sensitive data transmission. Over time, other host implementations of the LAT protocol appeared allowing communications to a wide range of Unix and other non-Digital operating systems using the LAT protocol.

History In 1984, the first implementation of the LAT protocol connected a terminal server to a VMS VAX-Cluster in Spit Brook Road, Nashua, NH. By "virtualizing" the terminal port at the host end, a very large number of plug-and-play VT100-class terminals could connect to each host computer system. Additionally, a single physical terminal could connect via multiple sessions to multiple hosts simultaneously. Future generations of terminal servers included both LAT and TELNET protocols, one of the earliest protocols created to run on a burgeoning TCP/IP based Internet. Additionally, the ability to create reverse direction pathways from users to non-traditional RS232 devices (i.e. UNIX Host TTYS1 operator ports) created an entirely new market for Terminal Servers, now known as console servers in the mid to late 1990s, year 2000 and beyond through today. LAT and VMS drove the initial surge of adoption of thick Ethernet by the computer industry. By 1986, terminal server networks accounted for 10% of Digital's $10 billion revenue. These early Ethernet LANs scaled using Ethernet bridges (another DEC invention) as well as DECnet routers. Subsequently, Cisco routers, which implemented TCP-IP and DECnet, emerged as a global connection between these packet-based Ethernet LANs. Over time, when terminals became less popular, terminal emulators had a built-in LAT client.

LAT features If a computer communicating via LAT doesn't receive an acknowledgment within 80 milliseconds for a packet it transmitted, it resends that packet; this can clog a network. No data is sent if no data is offered and under heavy loads LAT limits the number of packets sent per second to twenty-four: twelve transmits and twelve receives. As more characters are sent, the packets get bigger but not more numerous. Above 80 milliseconds latency a touch typist will notice the sluggish character echo. The LAT 80 millisecond delay offloads both the network by sending fewer larger packets which also reduces interrupts at each system.

Early terminal server vendors Digital Equipment Corporation - primarily via their DECserver systems. Able Computer - An early provider of Terminal Server products. Chase Research - A Europe-based early provider of Terminal Server products. Cisco Systems - Provided LAT on Terminal Servers as early as 1990. Emulex Corporation - A California-based early provider of Terminal Server products that included X.25 and 3270 features; they sold through Datability and other OEMs. Equinox Systems - A Florida-based early provider of Terminal Server products. Hughes LAN Systems - Provided LAT capability in 1989. Interlink Computer Sciences, an early 1990 LAT vendor. Xylogics Corporation - An early provider of DEC ecosystem products including terminal servers based in Massachusetts. Xyplex Corporation - An early provider of Terminal Server products based in Massachusetts.

Open Source solutions Most Linux distributions offer a client and server lat package, that can easily be installed via a package manager. This allows e.g. to access a local area network server while being connected to a corporate VPN network that would otherwise block local TCP/IP traffic.

See also Terminal server - An active device to connect serial devices to Local Area Networks Console server - An active device to connect LAN attached users to Linux/Unix server operator ports

References

External links https://sourceforge.net/apps/mediawiki/linux-decnet/index.php?title=LAT - Open source LAT project for Linux

Worked examples

Example 1 — a first encounter with Local Area Transport

Start with the simplest possible case. Write down what Local Area Transport claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Local Area Transport before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Local Area Transport ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Local Area Transport

In research: Local Area Transport appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Local Area Transport in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Local Area Transport is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer-related introductions in 1984, Digital Equipment Corporation, History of software, so understanding it makes those chapters shorter.
In everyday life: Look for Local Area Transport outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Local Area Transport in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Local Area Transport means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Local Area Transport out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Local Area Transport in simple terms?

Why does Local Area Transport matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Local Area Transport?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Local Area Transport.