{{unreferenced, date=April 2019 Via Net Loss (VNL) is a network architecture of telephone systems using

circuit switching Circuit switching is a method of implementing a telecommunications network in which two network nodes establish a dedicated communications channel ( circuit) through the network before the nodes may communicate. The circuit guarantees the full ...

technologies deployed in the 1950s with

Direct Distance Dialing Direct distance dialing (DDD) is a telecommunication service feature in North America by which a caller may, without operator assistance, call any other user outside the local calling area. Direct dialing by subscribers typically requires extra d ...

and used until the late 1980s. The purpose of the VNL plan and a five-level

long-distance Long distance or Long-distance may refer to: *Long-distance calling * Long-distance operator * Long-distance relationship * Long-distance train *Long-distance anchor pylon, see dead-end tower Footpaths * Long-distance trail *European long-d ...

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 A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...

s. The five levels of

PSTN The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...

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 A telephone company, also known as a telco, telephone service provider, or telecommunications operator, is a kind of communications service provider (CSP), more precisely a telecommunications service provider (TSP), that provides telecommunicat ...

payphones A payphone (alternative spelling: pay phone) is typically a coin-operated public telephone, often located in a telephone booth or in high-traffic outdoor areas, with prepayment by inserting money (usually coins) or by billing a credit or debit ...

. Residential service includes message rate and

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. Less commonly, the term may refer to a rate that does not vary with usage or time of us ...

local calling plans with extra charges for long-distance calls and supplementary services such as

call waiting Call waiting is a telephone service where a subscriber can accept a second incoming telephone call by placing an in-progress call on hold—and may also switch between calls. With some providers it can be combined with additional features such a ...

, 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 is a portmanteau of central exchange, a kind of telephone exchange. It provides functions similar to a PBX, but is provisioned with equipment owned by, and located at, the telephone company premises. Centrex service was first installed ...

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 In telephony, a service switching point (SSP) 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 (SCP) so that the call can be handled. ...

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 In telecommunication, a toll switching trunk or toll connecting trunk is a trunk connecting an end office to a toll center as the first stage of concentration for intertoll or long-distance traffic. Operator assistance or participation may b ...

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 The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operat ...

publications for details on DNHR and DCR.

See also