Via Net Loss
Encyclopedia
Via Net Loss 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 modem
s. The five levels of PSTN switching systems used with VNL were:
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 Point
s 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 trunk
s. 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.
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
using circuit switching
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
technologies deployed in the 1950s with Direct Distance Dialing
Direct distance dialing
Direct distance dialing or direct dial is a telecommunications term for a network-provided service feature in which a call originator may, without operator assistance, call any other user outside the local calling area. DDD requires more digits in the number dialed than are required for calling...
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 modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
s. The five levels of PSTN switching systems used with VNL were:
- Class 1 - Regional long-distance switching system
- Class 2 - Sectional long-distance switching system
- Class 3 - Primary long-distance switching system
- Class 4 - Toll-access switching system
- Class 5 - End-office switching system
Class 5 end-office switches provide local telephone service and dialtone to residential, business, and government subscribers, as well as telephone company
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
payphones. Residential service includes message rate and flat rate
Flat rate
A flat fee, also referred to as a flat rate or a linear rate, refers to a pricing structure that charges a single fixed fee for a service, regardless of usage. Rarely, it may refer to a rate that does not vary with usage or time of use...
local calling plans with extra charges for long distance calls and supplementary services such as call waiting
Call waiting
Call waiting , in telephony, is a feature on some telephone networks. If a calling party places a call to a called party which is otherwise engaged, and the called party has the call waiting feature enabled, the called party is able to suspend the current telephone call and switch to the new...
, 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
Centrex
Centrex is a portmanteau of central exchange, a kind of telephone exchange.In the United Kingdom, British Telecom markets this service as FeatureLine .-Use of 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
Public Utilities Commission
A Utilities commission, Utility Regulatory Commission , Public Utilities Commission or Public Service Commission is a governing body that regulates the rates and services of a public utility...
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 Point
Service switching point
In telephony, a service switching point is the telephone exchange that initially responds, when a telephone caller dials a number, by sending a query to a central database called a service control point so that the call can be handled...
s or SSP's that provide access to Intelligent Network
Intelligent network
The Intelligent Network , is the standard network architecture specified in the . It is intended for fixed as well as mobile telecom networks...
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 trunk
Toll switching trunk
In telecommunication, a toll switching trunk or toll connecting trunk is a trunk connecting one or more end offices to a toll center as the first stage of concentration for intertoll or long distance traffic....
s. 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.