About L-Carrier

Wikipedia has a good general overview of L-carrier. The goal here is to fill in some detailed (and sometimes conjectural) information on L-carrier infrastructure, with a particular eye towards being useful to other amateur historians trying to identify L-carrier infrastructure in the wild.

L-carrier systems required repeater, rebalancer, and power supply stations at regular intervals along the route. The interval depended on the generation of system. Here's what I've been able to figure out of these intervals, all units miles. It might seem counterintuitive that the intervals actually became shorter for later generations, but keep in mind that the main purpose of the upgrades was to increase capacity; later generations had (multiple) order of magnitude higher capacity in exchange for line coding with a lower margin for error - and thus more frequent repeaters.

System Repeater Main Station
L-1 8
L-2 16
L-3 4 ~125 miles
L-4 2
L-5 1

Repeaters typically consist of a small shack, about five or six feet square, set on a small rectangular fenced lot, usually but not always adjacent to a road (one that existed when the cable was laid). In the case of the hardened L-3I system the repeater system itself is underground, but the surface-level shack is usually still present to house maintenance and diagnostic equipment. An exception seems to be L-3 systems which were upgraded to L-4, in which case the additional repeater stations added in between are only a manhole in a concrete pad.

The shack used for these repeater stations is of white metal construction with a door in one side and a small round vent in the other. No utility power connection is required although in some cases one seems to have been added later on (perhaps to make it possible to use an electric air compressor and dryer to pressurize the cable instead of replacing gas bottles). The same shack seems to be used for all L-carrier systems, it becomes quite recognizable after a while.

Note that L-carrier repeater shacks are never made of precast concrete and never have wall air conditioners. It's not unusual to see these shacks on sites originally used for L-carrier repeaters, but they are new construction and usually house fiber optic equipment.

Repeater sites were numbered according to a scheme that I haven't quite worked out - the numbers are “L1E”, “L2E', etc. Speculatively, perhaps the L simply means L-carrier while 1E, 2E, etc indicate the number of stations east of the nearest main station. I have not seen a repeater station numbered with a W, but perhaps they count the other way on some lines. Although the repeater stations are often right off a main highway today, at the time they were in very sparse areas and not easy to find. To assist technicians, small white or yellow arrow signs were mounted on wooden posts to point the direction to the repeater station from the nearest main road.

The term “main station” was used by AT&T proper but it is somewhat overloaded and confusing. Chad Perkins identified several distinct types (archived) of main stations. For this to make sense, it is first important to understand that L-carrier cables included multiple power feeds that carried DC and AC power to equipment along the route. This allowed repeater and even some main stations to operate without a utility power connection, but requires that some main stations have power equipment to provide this voltage.

L-3I Main Stations

Wikipedia reports that L-3I main stations were equipped to serve as fallout shelters for two weeks. It's unclear exactly where this claim comes from, but it seems believable as I can attest from observation that manned main stations at least featured their own water wells, and I have heard old-timers who knew employees at the stations mention stored survival rations. The L-3I main stations were built to withstand a nuclear blast at a distance of about five miles or so.

L–3I main stations are mostly underground, but they can often be identified by a distinctive utility building with large gable vents, a single bay door on the left, and a single pedestrian door on the right. Typically these buildings are white with blue doors, but of course they have sometimes been repainted by later owners. The large gable vents suggest that this building serves as ventilation for the underground space and probably contains a large blower as well as access to the space below. The photo to the right, from Chad Perkins' now defunct website, shows a typical example.

There is also a “humpback” variant of this building, a Chad Perkins photo of an example in Holbrook, seen from the rear, is on the left. These buildings have an additional vent structure added to the back which is taller than the main roof. This is presumably for additional ventilation equipment, but it's unclear what this necessarily corresponds to as far as station function. The Holbrook station has a “humpback” but no telephone switch, the Socorro station has a telephone switch but no “humpback.” Perhaps it simply depends on differences in the underground construction driven by site topography.

The other typical building at L-3I main stations is an all-white utility building of prefab metal construction with two bay doors on the left and a pedestrian door on the right. These buildings look like newer additions but are almost always present. They may simply be maintenance garages added for technician use later on, it's not clear if they're even directly related to L-carrier or possibly to a later upgrade system.

Finally, there are some manifestations of the underground vault that are often visible at the surface. Most stations have a small metal “roof” just inches above the ground and surrounded by a guardrail, this probably opens to a shaft to allow equipment to be raised in and out by crane. There are typically also metal grate vents surrounded by a short (1.5-2') wall to deflect any shockwave, and one or two short metal vent stacks. Finally, the whole rectangular lot is usually surrounded by an 8' chain-link fence which, if you're lucky, is decorated by signs warning that it is a felony to tamper with a defense communications facility.

The most interesting feature of an L-3i main station is the gamma-radiation based nuclear detonation detector, which is a squat metal cylinder, usually painted blue, mounted on a 2.5' or so high concrete pedestal. It's not clear if this detection equipment reported blasts to manned stations for military planning purposes or was only used to trigger automatic closing of blast valves on the air vents - the latter was definitely the main purpose, though.

The following are the types of main stations and some notes on their features and identification. It's important to understand that these types were often combined - many terminal stations were also power supply stations, although some terminal stations operated only off power provided by other power stations.

Terminal Stations

Terminal stations were so called because they had the capability to mux calls on and off of the cable and include protection and diagnostic equipment. Major functions at terminal stations, besides serving as the interface to the L-carrier system, include automatic switching of coaxial cores in the cable to avoid faulty cores and provision of equipment for technicians diagnosing and repairing trouble. This includes the measurement equipment used to diagnose the cable and conveniences like a telephone connected to the order wire in the cable, used by technicians to speak to each other between stations. Terminal stations were usually manned.

Power Stations

Power stations, which include the DC and AC power supplies. Power stations are connected to utility power for regular use but also featured generators for redundancy. You can tell a power station by the generators, but they're mounted underground so you have to look for surface indications instead. At least two main stations that I've seen have cooling radiators and fans for the generators mounted above ground, this arrangement doesn't look at all able to survive blast pressures though so I suspect it was a later addition when the underground cooling capacity was found to be insufficient. A small concrete pad with two metal pipes sticking up to about a human's height appears to be the exhaust outlet for the generators and is probably the most reliable indicator. Finally, and obviously, power stations will have a connection to utility electrical power and this is usually visible above ground. Power stations were usually manned.

Equalizer Stations

The power and twisted pair leads in the cable used a balanced electrical arrangement, and AT&T found that it was possible to extend the useful range of the cable between main stations by adding an “equalizer station” with some equipment to re-balance signals. Equalizer stations were always unmanned and usually sat half-way in between power stations.


A few L-3I main stations also functioned as hardened telephone exchanges for the AUTOVON system. These had a larger underground facility, more members of staff, and often also had microwave towers for path redundancy and connection to AUTOVON sites not directly on the L-3I system. Because they were full telephone exchanges they had a somewhat higher profile and it is not unusual for there to be a faded sign giving the hours of operation, although there really aren't any surface structures above and beyond a normal L-3I main station.

L-carrier cables were some of the first major infrastructure components to be installed underground. The Bell System was understandably nervous about unintentional damage to the cabled and so marked their paths very liberally. Typically, L-carrier rights of way are marked by approx. 7' tall wooden poles every thousand yards or so, sometimes more often, with a metal sign nailed at around 3' height giving survey information on the right of way - its width, the location of the cable within the width of the right of way, and the distance and heading to any nearby direction changes. Originally these signs were painted yellow but most of the time this paint has all chipped off and only the metal-punched numbers remain.

Because the cable was laid with heavy trenching equipment, the soil immediately around it is usually very compacted and this can leave a scar visible in aerial photos to this day, especially in desert areas where vegetation is slow to regrow.

  • history/bellsystem/l-carrier.txt
  • Last modified: 2020/11/16 23:46
  • (external edit)