engineering

Via Net Loss

Via Net Loss is a engineering 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 Via Net Loss rather than just read about it. In short: Via Net Loss (VNL) is a network architecture of telephone systems using circuit switching technologies deployed in the 1950s with Direct Distance Dialing and used until the late 1980s. The purpose of the VNL plan and a five-level long-distance switching hierarchy was to minimize the number of trunk circuits used during a call and maximize the voice quality achieved on each circuit.

Key takeaways

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

Reference excerpt

Via Net Loss (VNL) is a network architecture of telephone systems using circuit switching technologies deployed in the 1950s with Direct Distance Dialing and used until the late 1980s. The purpose of the VNL plan and a five-level long-distance switching hierarchy was to minimize the number of trunk circuits used during a call and maximize the voice quality achieved on each circuit. Excessive noise or loss meant that subscribers may have difficulty hearing each other. This was particularly important in the 1960s when dial-up data applications were developed using analog modems. The five levels of PSTN switching systems used with VNL were:

Class 1 - Regional long-distance switching systems Class 2 - Sectional long-distance switching systems Class 3 - Primary long-distance switching systems Class 4 - Toll-access switching systems Class 5 - End-office switching systems Class 5 end-office switches provide local telephone service and dialtone to residential, business, and government subscribers, as well as telephone company payphones. Residential service includes message rate and flat rate local calling plans with extra charges for long-distance calls and supplementary services such as call waiting, 3-way calling, and call forwarding. Business service is mostly message rate local calling plans with extra charges for long distance and supplementary services. Message Rate calling means that subscribers pay for calls based on duration of the call and distance to the called party. Government subscribers include cities, counties, state, and federal agencies and often included Centrex service. Pay phones were traditionally provided exclusively by telephone companies but during the early 1980s Customer-owned coin-operated telephone services were established. Class 4 toll access switches provide long-distance (toll) telephone service including intrastate calling and inter-state calling. Intrastate calls are generally more expensive than inter-state calls due to favorable tariffs with price plans approved by the Public Utilities Commission or Public Service Commission for each state. Inter-state calls are generally less expensive than intrastate calls since tariffs are filed with the Federal Communications Commission because of the inter-state commerce aspect of the service. Class 4 switches provide access to long-distance service in rural areas. In addition, Class 4 switches traditionally provided operator assisted calls such as person-to-person, collect, and calls billed to third parties. However, many operator services are now automated with minimum human intervention. Class 3 primary switches provided the first layer of the AT&T long-distance switching network. VNL routing methods preferred trunk connections between Class 3 switches to minimize class 1 and class 2 connections. Class 3 switches also act as Service Switching Points or SSP's that provide access to Intelligent Network services such as Toll-Free, Virtual Private Network, Calling Card, and Credit Card calls. If circuits to other Class 3 switches were unavailable, the call was routed to the Class 2 (and/or Class 1) switch in the same region. Calls were not routed "up-chain" to Class 2 or Class 1 switches in a different region. Analog circuits between AT&T long-distance switches are known as Inter-Toll trunks while circuits from a long-distance switch to local switches are known as Toll Completing trunks or toll switching trunks. Trunks between long-distance switches in other carrier networks are known as Inter-Machine Trunks or IMT's. Class 2 sectional switches provide the second layer of long-distance switching. VNL routing methods preferred trunk connections between the originating Class 2 switch and a Class 3 or Class 2 switch in a different region. Calls were not routed "up-chain" to a Class 1 switch in a different region. Class 1 regional switches provide the final layer of long-distance switching. VNL routing methods preferred "down-chain" trunk connections between the originating Class 1 switch and a Class 3, Class 2, or Class 1 switch in a different region. Analog trunk connections between Class 1 switches were required to have a loss of zero decibels. The VNL architecture was gradually phased out due to the conversion of network circuits from analog to digital and the related conversion to a non-hierarchical network routing schemes such as AT&T's Dynamic Non-Hierarchical Routing or Nortel's Dynamically Controlled Routing methods. See IEEE publications for details on DNHR and DCR.

Worked examples

Example 1 — a first encounter with Via Net Loss

Start with the simplest possible case. Write down what Via Net Loss claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, 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 Via Net Loss 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 Via Net Loss 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 Via Net Loss

In research: Via Net Loss appears in engineering 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 Via Net Loss 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: Via Net Loss is common in secondary-school and first-year university syllabi. It links to neighbouring topics Communication circuits, so understanding it makes those chapters shorter.
In everyday life: Look for Via Net Loss 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 Via Net Loss in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Via Net Loss 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 Via Net Loss out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Via Net Loss in simple terms?

Why does Via Net Loss matter?

Because it connects several engineering 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 Via Net Loss?

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 Via Net Loss.