Field Guide to L-Carrier

Four major L-carrier standards are called L-1, L-3, L-4, and L-5. There also exist variants L-3I (L-3 Improved) and L-5E (L-5 Enhanced?). L-2 did exist but was never installed on any substantial scale, only really on a pilot system between Baltimore and Washington. The different L versions mostly just reflect ongoing improvement of the standard, but there is some degree of purpose-specificity as the higher capacity of L-4 and L-5 made them more practical for major transcontinental lines. L-4 and L-5(E) are thus typically seen on very long (i.e. transcontinental) routes, with L-1 and L-3 (as well as L-2) usually confined to shorter ones (i.e. city to city). There are exceptions.

L-carrier is fundamentally a relatively simple frequency-division scheme, which allowed the combination of multiple analogue calls onto a single coaxial cable. Further, an “L-carrier cable” was a bundle that consisted of multiple coaxial circuits along with order wire and power delivery circuits. The capacity of L-carrier could be very high, especially in later revisions, but because of the simple analogue process the range was actually fairly low, and L-carrier routes required a great deal of supporting infrastructure. Fortunately, this infrastructure makes them easy and interesting to track today.

L-carrier routes were often later described as consisting of a “backbone” and one or more “sidelegs,” reflecting that telephone exchanges were increasingly being connected to long-distance L-carrier routes via a shorter section of L-carrier, rather than some other shorter-range carrier. The sidelegs usually started at a main station and terminated at a major telephone exchange building—the L-carrier main station equipment would then be installed in the conventional telephone exchange building, rather than building an L-carrier station.

L-carrier routes required several levels of supporting infrastructure. First, each cable required repeaters at regular intervals. These repeaters simply amplified the signal to extend range and were quite consistently installed in underground vaults below manholes. For earlier systems (L-1 and L-3), repeaters were located in vaults underneath surface huts. From L-4 on, repeaters were in underground vaults with no surface hut, leaving very little evidence. To simplify installations, repeaters were powered by high-voltage power wires included in the cable, and did not require surface connections. As a useful exception, L-4 lines required a larger equalizing repeater at 50-mile intervals, which was enclosed in a surface hut much like L-3 repeaters.

The power consumed off the cable by the amplifiers, though, introduced another requirement. L-carrier routes require power injection at regular intervals, from “power feed stations.” These stations are substantially larger, usually underground structures in the range of several thousand square feet. They were originally manned by a maintenance and operations crew. Power stations had grid power connections, but also generators available in the case of an outage.

Finally, L-carrier routes incorporated “main stations” which performed line conditioning and what we might now call add-drop. In order to do so these stations required a great deal of equipment with high power demands, and so they were yet larger than the power feed station and would have a fairly substantial staff similar to an exchange office. These stations were also typically underground and very large. Some main stations were additionally equipped with a tandem switch connecting to other long-distance carriers, in which case they might be called “terminal” (even if they are not at the end of the route).

The following table gives the intervals for all of these requirements. Note that “power feed stations” and “main stations” can be a somewhat nebulous concept that depends on the specific L-carrier version in question. It was common for L routes to be “upgraded” via the installation of new power feed stations in between main stations. The “power feed interval” is best understood as a *maximum spacing of power feed points,* which may be either power feed only stations or main stations. In the case of L-5, main stations required more sophisticate line conditioning equipment than power feed stations, so there was an additional requirement for main stations specifically (this seems to have been a compromise resulting from the fact that L-5 was intended as a replacement for L-4, and so made use of the existing main stations). Also note that, over time, the spacing of repeaters and stations became *smaller,* not *larger* as one might intuitively suspect. It seems that AT&T came to value capacity over reduced installation/maintenance cost, so they pushed the muxing ever further at the cost of needing more frequent support stations.

The following table summarizes the maximum spacing of identifiable features of various L-carriers. When known, an average or typical distance is listed as well. Distances are in miles. The source is various BSTJ articles, especially “L-5 Coaxial-Carrier Transmission System” (BSTJ 53:10).

Carrier Introduced Channels per Cable Repeater Interval Hut Interval Power Feed Interval Main Station Interval
L-1 1941 720 or 2160 8 8 50 n/a
L-3 1953 5580 or 9300 4 4 100 200 (<160 typ.)
L-4 1967 32400 2 50 150 150
L-5 1964 108000 1 n/a 75 150
L-5E 1975 132000 1 n/a 75, average 65 150

Terminology note: People can sometimes be confusing/unclear on what kind of “station” they intend, especially since the “power feed” and “main” distinction only existed in L-5. For the avoidance of doubt, the “official” terminology used in BSTJ is usually “terminal station/terminal main station” (main station with tandem switch), “switching power feed main station” (larger power feed station with a “line protection” switch for redundancy), and “power feed main station.” So they are all main stations, really. The difference between what was actually in a “power feed main station” vs a “main station” varied from system to system. On the contrary, in articles on L-3 the equivalent of a repeater is called an “auxiliary repeater” and the power feed main station is called the “main repeater.”

L-1, the oldest L-carrier, used a repeater located inside of a surface-level hut every eight miles. The hut was often constructed of brick and was relatively large. Hut designs varied somewhat and sometimes matched local vernacular style.

Power feed stations are required every 50 miles. Power feed station design varies considerably, but they are surface structures, typically also brick. There is no strong differentiation between a power feed station or other types of main station in the L-1 system.

L-3 and L-3I are essentially identical, except that L-3I routes are nuclear-hardened while L-3 routes were not. L-3 equipment is usually on the surface and much more variable than L-3I, which seems to have been the start of complete standardization of all structures. For that reason, most of this description applies to L-3I and L-3 may be quite a bit different.


Repeaters (also called “auxiliary repeaters”) are required at 4-mile intervals. Repeaters are located underground in concrete vaults, but for maintenance convenience and to house some test and accessory equipment a prefabricated metal hut was installed on top of each vault—leaving the vault manhole in the center of the hut floor. L-3I huts are about 10' square and originally did not have utility power; the repeaters were line-powered but the huts typically contained lights, power sockets for tools, and a ventilation blower for the vault. Originally these would be powered by a portable generator connected to an inlet on the side of the hut, but later, at least in New Mexico, AT&T seems to have paid for utility power to be installed to most of the huts, even on routes which were not modernized to L-4 or fiber. I'm not especially sure why.

L-3I maintenance technicians were aided in finding repeaters by small white signs, with one end pointed, that had the repeater number painted in black and indicated where to turn off the highway (sometimes a series of several would give driving directions, if the repeater station wasn't immediately next to the highway). A surprising number of these are still present and legible, even if the repeater itself has been demolished. Many L-3I repeaters have signs on the fence around them that refer to a “buried transcontinental telephone cable,” unusually specific wording that seems to have been used specifically for L-carrier. L-3I repeaters may also have a “Warning” sign that refers to “communications facilities used or intended to be used for military or civil defense functions.”

Some L-3I routes were upgraded to L-4, which leaves a convenient distinctive pattern: huts every 4 miles and, halfway in between each pair of huts, a manhole with no hut above.


L-3I power feed main stations (also called “main repeaters”) are underground structures of 5k sq ft and sometimes larger. On the surface, there tend to be a couple of structures and two or sometimes more vents, which consist of a metal grate surrounded by a low concrete wall (perhaps 2') intended to deflect blast energy away from the grate. Similar to the vents, there is usually a low sheet metal roof nearby about the same size as the vents that probably covers an emergency exit. The layout and description of the structures seems to depend on when the station was built. There is almost always a small square hut with pyramidal roof (around 22' square). On later sites, the square hut turns into a rectangular building about 25' by 40'. There is often, but not always, a garage/storage building with a man door and two bay doors. Especially on later sites the garage is often painted a distinctive light blue color with ventilation grating all the way around the top of the walls. Most power feed main stations have had some modifications made; common post-construction additions are a triangular lattice tower (especially in the East, possibly for land-mobile radio?), chillers on the surface piped via one of the vents, generator radiators moved to the surface, and added small buildings.

L-3I switching main stations vary to the extent that it's hard to describe them usefully. There will typically be blast-shielded vents and an emergency exit cover somewhere on the site, but they might be covered. There's usually, but not always, a fairly large surface building at switching main stations… many of these were probably later additions. Many but not all switching main stations have a tower for microwave antennas. A few switching stations, probably ones that have not seen any additions since they were originally built, are basically identical to a power feed stations except for possibly the addition of a microwave tower. Lamar, CO is an example. Most have had at least one surface structure added, though.

It's not always easy to tell the difference between a power feed only and a switching main station. Some power feed stations have towers (seemingly added later), some switching main stations have no additional surface features compared to power feeds. An old square AT&T microwave tower is a good pointer that a station is switching, though.

There are a couple of useful local features to identify L-3I and later L-carrier stations, although you won't be able to see them from satellite images. Almost all L-3I and later infrastructure feature a distinctive sign that says “Warning” in large letters at the top and, in the text, mentions “communications facilities used or intended to be used for military or civil defense functions.” These signs are a pretty sure indicator of a facility built by AT&T for a cold-war COG contract. L-3I and later L-carrier underground stations have gamma radiation and sometimes also optical sensors to detect nuclear detonations (these would automatically shut the ventilation blast valves). These tend to look like bolted-together metal cylinders on top of a 2.5' or so tall concrete pier.


L-3I rights of way were typically cleared of brush to around 50' wide, but in some cases the brush may have all grown back. Still, some combination of vegetation disturbance and uneven soil often makes the trench visible in satellite images even many decades later.

Like most later L-carrier ROWs, L-3I ROWs were laid out in perfectly straight sections with hard angle turns (with few exceptions, usually following curves in railroads). L-3I ROWs typically follow highways and occasionally railroad tracks, and there are sections in which they cross spans far from roads (in which case there was a dirt service road for access to the repeaters, but it may not be apparent any more). L-3I ROWs almost always cross roads, railroad tracks, and small streams at right angles, often resulting in a “dogleg” with turns on each side of the road. This seems to have been a result of AT&T's use of early directional boring equipment to avoid having to repave roads at each crossing.

L-3I ROWs sometimes pass under farm fields, when they do they usually remain perfectly straight in the field and then turn at the fence line (this was probably so that it was easy to find the ROW in the field later based on markers at the fence line, since they couldn't put marker posts in the middle of a working field). When L-3I ROWs pass under swampy areas or large rivers, they usually buried a duct, so there may be little to no surface evidence today.

Perhaps most importantly, L-3I ROWs tend to be “in the middle of nowhere.” Not only were main stations usually intentionally located well clear of urban areas, the ROWs tend to stay away from urban areas as well and usually fairly far from the road they are following. A common pattern for L-3I ROWs following a highway in a rural area is to run parallel to the highway about 1000' away, with a sudden angle towards the highway and then away again to meet each repeater, which were installed immediately adjacent to the highway (perhaps to avoid grading a road?).

Ultimately, L-3I and later L-carrier ROWs usually “made sense.” That is, if you identify two main stations and then look at a road map of the area (ignoring roads which did not exist at the time like freeways in the case of L-3I), the route is usually fairly obvious and follows US highways that form the shortest path between the main stations. In areas where following highways unreasonably extended the route, they may follow minor roads, railroads, or rarely just venture out on their own for a time… usually to “cut corners” or jump between two parallel roads. To ease installation, ROWs usually “bypass” towns by skirting around the edge (at the time, which might not be the edge any more).

ROW markers are virtually never visible in satellite images, but they consist of fencepost-type wooden posts at frequent intervals along the ROW and, at each turn and at longer intervals in between, a taller (7' or so) round wooden post with several metal signs nailed to it. If you are fortunate enough to be able to read the metal signs, one of them mounted lower on the pole gives a “surveyor's description” of the ROW both directions from the post in terms of heading, distance, and width. Many L-3I ROWs were later used for other technology, so there may be other signs present such as buried “lightguide” or fiber optic warnings.


Substantial sections of L-3I cable have been abandoned and remediated to BLM requirements. Remediation usually means the removal of the surface huts and filling in or removing the underground vault. In some areas the ROW markers and posts have been removed as well for wildlife protection reasons (birds of prey liked to perch on top of them which ended up making hunting too easy and impacted prairie dog populations). Even in areas where ROW markers have been removed the repeater direction signs are often still present by highways. For L-3I ROWs that have been reused for fiber, it's common for the surface huts to have been removed and the vaults filled, but then a new hut installed next to some of the foundations of the old ones to contain fiber regenerator equipment.

That said, in many rural areas L-3I infrastructure was abandoned in place. Sometimes random individual repeater huts have been removed, probably local responses to attractive nuisance issues, but you can usually tell where they were based on the square graded and cleared area.

A few L-4 routes are upgraded from L-3I, but for the most parts L-4 routes were new construction. The overall ROW and stations are very similar to L-3I, but the repeaters are quite a bit different.


L-4 repeaters are located in concrete vaults with a manhole, and there are no surface features other than the manhole. They can be almost impossible to see from above. Like later L-carriers, L-4 required some repeaters to contain additional line conditioning functionality. A distinctive element of L-4 is that the powered equalizing repeaters required a surface hut that looks very much like an L-3I hut, but a bit bigger and located only every 50 miles.


L-4 power feed stations usually consist of two concrete-block surface buildings, both perhaps 20' square but sometimes larger, and it's common but not universal for the two buildings to be connected by an L-shaped concrete sidewalk. The vents tend to be of a different design, still having low concrete walls but a metal grate located above the walls instead of below.

Switching main stations are more variable but tend to have larger surface structures and microwave towers. The underground components of switching stations are appreciably larger, which you can sometimes tell by the placement of the vents (further away from the surface buildings than at power feed stations).

L-4 stations, perhaps because they were built later and had shorter service lives, seem to have been sold to private ownership more often than L-3I and so it's common for former stations to have been substantially modified by a later owner. On the flip side, they are also more likely to still be in use as tandem switches or fiber facilities, in which case there are often new chillers at surface level and sometimes new buildings. In general, L-4 stations are not as consistent in appearance as L-3I. A few L-4 stations remain major long-distance junctions today and have extensive surface structures.


L-4 ROWs are basically identical to L-3I ROWs, but often use more modern metal ROW markers. Some L-4 ROWs have orange “tent” markers for aerial survey, but I suspect these were later additions on ROWs reused for fiber.


A surprising number of L-4 repeater vaults seem to have been removed, perhaps for safety reasons. Several times on farms I've seen L-4 repeater vaults removed from the ground but then abandoned on the surface nearby… I suspect these are cases where farmers pulled them out to avoid implements snagging on the manhole but didn't want to figure out disposal.

Many L-4 stations have been sold to private owners who have made major changes, sometimes leaving the site totally unrecognizable other than by position on the ROW.

L-5 represents a particularly late stage of L-carrier evolution. Most L-5 routes were upgrades from L-4, so they reused existing L-4 main stations… but L-5 requires a power feed every 75 miles, which results in “filler” power feed stations added between each L-4 station.


L-5 repeaters are contained in manholes. While there are several types of repeaters, all repeaters are in underground vaults with no surface structures. Like L-4, it may not be possible to find the repeaters in aerial imagery.

Main Stations

Many L-5 main stations are reused L-4 main stations and so look similar.

L-5 power feed stations, added in between L-4 main stations to meet the 75 mile power range, are fairly small.

  • history/bellsystem/lcarrier.txt
  • Last modified: 2021/12/02 02:08
  • by jesse