Which structure currently operates as a "virtual fence:"  Boarder Wall or Mexico-United States Barrier?
Mexico–United States barrier The Mexico–United States barrier is a series of walls and fences along the Mexico–United States border aimed at preventing illegal crossings from Mexico into the United States. The barrier is not one continuous structure, but a grouping of relatively short physical walls, secured in between with a "virtual fence" which includes a system of sensors and cameras monitored by the United States Border Patrol. As of January 2009, U.S. Customs and Border Protection reported that it had more than 580 mi of barriers in place. The total length of the continental border is 1,989 mi .
Wall box Wall boxes are a type of post box or letter box found in many countries including France, the United Kingdom, the Commonwealth of Nations, Crown dependencies and Ireland. They differ from pillar boxes in that, instead of being a free-standing structure, they are generally set into a wall (hence the name) or supported on a free-standing pole, girder or other stable structure. In the UK, UK Dependent Territories, Colonies and former Colonies and in many former British Empire countries, wall boxes usually bear the initials of the reigning monarch at the time the box was made. The first UK wall boxes were erected in 1857 in Shrewsbury and Market Drayton.