Telephone carrier frequency networks
Gerd D. Wallenstein · Transactions of the American Institute of Electrical Engineers Part I Communication and Electronics · 1957
THE GROWTH of carrier systems in the United States may be likened to the growth of the country's highways. Highways originally were built between the outskirts of cities to cope with the intercity traffic; so were carrier systems. When he traveled through a city, a motorist had to leave the highway and become submerged in local traffic until he could make connection to the next highway heading out of the city. Likewise, if carrier channels were to pass through a city, they had to become local voice drops first before they could be reinserted into another carrier system and passed on to the next city.