# Nerdy transponder question

Discussion in 'Flight Following' started by francisco collazos, Nov 15, 2022.

1. ### francisco collazosPre-Flight

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So I found out there are 4096 unique transponder combinations because each number location goes from 0-7. Does anyone know why there are no 8 or 9's used?

2. ### NoHeatEn-Route

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The numbers 0 to 7 are used in base 8, which is called octal.

I'm not sure why they chose octal instead of base 10, way back when.

3. ### MBDiagManEn-Route

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No. That’s why they call it OCTAL. That means base 8. 0 through 7 adds up to 8 different characters. 8 to the fourth power is 4096.

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4. ### dmspilotFinal Approach

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Probably because octal is easily converted to and from binary without wasting any bits, while decimal is not. Computers think in powers of 2 while humans think in powers of 10.

5. ### NoHeatEn-Route

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That's certainly true.

I'm just not sure why the bits were so precious, for this application. If they had been willing to waste a few bits, they could have chosen BCD (binary coded decimal), to represent the digits 0 to 9.

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6. ### camortonPre-takeoff checklist

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I guess ATC decided something around 4,000 distinct codes would be sufficient, so 4,096 is stored most efficiently in 12 bits of data. If you went 0-9, you could have 10,000 codes, but that's really more than they needed and it would take 14 bits of storage and even then you're wasting space.

7. ### TCABMEn-Route

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Computers don’t think. They are programmed by humans to operate. Considering transponders are part of the ATC Radar Beacon System which was developed in 1956, I would guess the logic behind the decision probably had something to do with the limits of known things that related to analog computing at that point in time.

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8. ### dmspilotFinal Approach

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Then they "operate" in base 2. We know computers don't "think" like people do, you don't have to be so pedantic.

Last edited: Nov 15, 2022
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9. ### MBDiagManEn-Route

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Way back then? Okay.

in 1974 I was writing assembler code on a 12 bit wide minicomputer, coding in Octal. Base 10 doesn’t work out in digital logic. Things were more commonly hex, base 16, after that. Code was dealt with at a low level, almost bit by bit in those days for many reasons including lack of speed, massive expense of core memory and the fact that higher level languages made for code that was inefficient and required much more of the expensive memory.

At that time 4K of 12 bit wide memory cost thousands of dollars. Code had to be written for most efficient use of memory.

Now, after explaining all that, you might say “but the transponder wasn’t a minicomputer” and you would be correct, but most control logic was discrete logic designed using integrated circuits containing OR gates, AND gates and the like. Again digital logic doesn’t lend itself to base 10. Without researching the timeframe of transponder development I expect it was a holdover from WWII Identification Friend or Foe IFF. It was a base 8 (Octal) code used for radar identification. There was a transponder in aircraft that would transmit an octal code which allowed for 8 different signatures. There was a code of the day that radar operators looked for. If the code of the day was six, then antiaircraft and other systems would look for a six or even more condemning would be no IFF return at all.

I was a radar mechanic in Germany ca. 1969 to 1971 and IFF was still in use. I fully expect that our 4096 transponders are descendants of that technology.

10. ### elvisAteMySandwichPre-Flight

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We've run into this sort of problem with 640 KB RAM and IPV4 computer network addresses where we *thought* those would be more than enough. I wonder if we'll see a day when more than 4096 aircraft are flying within a Center's area. If drones will need individual transponder codes I could see us getting there in my lifetime.

11. ### pdonahuePre-Flight

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Bits were precious. A few kilobytes back then was a lot. And this is more about transmission time than storage. Transmitting 4 BCD digits would be 16 bits but 4 octal digits is 12 bits. That takes 25% less time to send. And if 4096 codes is enough then why not accommodate more transponders in a given area without them stepping on each other's transmissions?

Original area codes were allocated based on how long they would take to dial on a rotary phone. New York is 212 because it was most frequently called and that's the shortest area code you can dial (2 only ties up the line for two ticks as the dial rotates back to neutral). Chicago is 312 which took slightly longer. Places called less frequently would get 8 and 9 in their area codes.

12. ### Baked PotatoPre-takeoff checklist

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Is one of the objectives of ADS-B to replace transponders eventually?

13. ### WDDEn-Route

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Ever heard of Skynet?

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14. ### MBDiagManEn-Route

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I touched on why memory was precious, but I will expound. In 1974, semiconductor memory was in its infancy. At that time “core” memory was the norm. Core memory cards consisted of a loom of intersecting wires. At each intersection, two tiny wires, one vertical and one horizontal went through the same tiny ferrite core which was a tiny ferrous metal ring. If both wires were flowing current it’s core would be magnetized, providing one bit of memory.

The manufacture of these cards was by hand and very time consuming. From time to time one would fail and they would actually go in and repair the card. Maybe wires had shorted together or who knows. The 4K memory cards cost thousands to purchase and were extremely costly to repair.

Although we thought we were in the space age, and we were, the technology of that time as compared to something like an iPhone was so primitive that comparing what we had then to the incredible semiconductor technology of today is probably like comparing using a Water Buffalo for personal transportation as compared to a new Ferrari.

15. ### CraigCleared for Takeoff

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First non mainframe programming I did was on a MMD-1....You wrote in Octal, entered in KEX and read output in binary.....

16. ### murpheyTouchdown! Greaser!

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People think?

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17. ### MauleSkinnerTouchdown! Greaser!

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I don’t think so.

or
I don’t think, so…

18. ### RgbeardEn-Route

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You have to think binary. With only three data bits there are only eight combinations. From 000-111.

@NoHeat had the suggestion of base 10, but that’s not really in play here. Adding a bit would have given sixteen combinations, 0000-1111.

It was easier to build a transponder, and omit the digits eight and nine, than to build a transponder that adds the letters A through F.

to do this in base 10, would omit six (A-F) combinations of data bits, rendering it rather in efficient.

19. ### PineconeCleared for Takeoff

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Bits were precious.

Realize that in the late 70s, mini computers were 16 bits wide. A 32 user system might have 512 KB of memory and 10 MB of drive space.

In the early 70s, mainframes were 12 - 15 bits with KB of drive space.

Transponders go back before that. Early ones on had 2 digits, so only 64 possible codes. Ever wonder with emergency is 7700 and not 7777?

20. ### Jim_RCleared for Takeoff

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21. ### Albany TomPattern Altitude

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All true. And bits still matter. At the low level, many or most of the protocols used for all this Internet stuff are still designed to be fast and efficient for best utilization of network bandwidth. And we're still using protocols developed 30+ years ago.

Bits only don't matter at the OS and app layer, where so much of the time speed and especially reliability aren't important either. It's just get the dancing whatever out the door quickly to keep the marketing people happy.

22. ### asicerFinal Approach

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Bits still are precious. Radio frequencies are a limited resource and we’re still using the same antiquated modulation techniques from way back when. Remember the uproar from the ADS-B mandate? Now imagine if every transponder in the world were to be made obsolete so that we can switch to a better more modern modulation scheme.

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23. ### FPK1Pre-takeoff checklist

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Very interesting! I grew up in 213 area code...

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24. ### PineconeCleared for Takeoff

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Not exactly.

The first area code assigned was New Jersey with 201. Then DC with 202.

Originally area codes were only used on long distance calls placed through an operator. Local calls assumed the local to you area.

Things got crazy in the 80s and early 90s when phones numbers exploded with pagers. Maryland was all 301. Then they split into two areas with 410 added. Then they overlayed those two areas with 240 (same area as 301) and 443 (same area as 410). That was when they dropped the requirement that the middle digit had to be 0 or 1.

When we had our consulting company, and one point we (wife and I) had 14 phone numbers.

1x Home, 2x Business, 1x Distinctive Ring Fax, 4x Pager (one pager each, but two numbers, one local to Baltimore and one local to DC) and 4x cell phones (original 3 mounted in cars, then added a brick phone), plus two lines for ISDN for internet (before cable and fiber).

https://en.wikipedia.org/wiki/North_American_Numbering_Plan

https://en.wikipedia.org/wiki/Original_North_American_area_codes

25. ### CraigCleared for Takeoff

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When us kids started to go to out of town schools, mom and dad got in on a deal with MCI and had an inbound WATTS line tied to the home phone. Really screwed people up when you gave them an 800 number with an additional 6 digit code to call the house line.

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26. ### donjohnstonPattern Altitude

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But who here knows why the middle digit of area codes were 0 or 1?

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27. ### PineconeCleared for Takeoff

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I think part of it was to make them distinct from the prefix. In the early days, prefix (first 3 of the local number) never had a 0 or 1 as the middle digit.

28. ### RgbeardEn-Route

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entirely correct. The prefix is known as the exchange. With seven-digit dialing, an exchange needed to be 100% distinguishable from an area code. I’m on my iPhone which precludes the somewhat longer answer but it’s essentially this.

29. ### SeanaldinhoPattern Altitude

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There are 4096 numbers available, but not really 4096 codes if that makes sense. Numbers are grouped into pools, so there is a few hundred for VFR flight following, some for military flights, IFR, etc. and the code an aircraft is assigned is blocked for a number of hours prior to departure. It can happen that center needs to cancel VFR flight following (at least radar services) to free up codes.

Also very normal to get a new transponder code when you move from one center to another as they are already using a code for someone else when you get to them.

In theory, yes. The FAA moves at a glacial pace but it is happening. ADS-B has higher fidelity and updates more frequently so it provides a better picture (if the radar display the controllers are equipped with can leverage that higher update rate). The real benefit of ADS-B is space-based ADS-B where satellites pick up the ADS-B transponder and send that info to ATC. Now you can provide radar quality coverage over the ocean and drastically reduce separation standards and provide more real time routing for oceanic traffic. Many countries are starting to add space ADS-B to their systems but it will take some time for the rules to change and I am not sure we will ever see traditional transponders entirely replaced.

30. ### MauleSkinnerTouchdown! Greaser!

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You’re typing on a rotary iphone?

Last edited: Nov 16, 2022
31. ### francisco collazosPre-Flight

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Thanks for all the great replies!!!

32. ### PaulMillnerLine Up and Wait

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don't you have to rework TCAS first? And the airlines have effective lobbyists to prevent that expense!

Paul

33. ### FPK1Pre-takeoff checklist

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And the Exchange had a name. For example, my 213 number was 833-xxxx which was Terminal Exchange or TE3-xxxx

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34. ### jonnyjetpropCleared for Takeoff

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Kind of. They wanted to replace the ground radar system. Then 9/11 happened and the powers to be thought having domestic radar coverage was a good thing.

Fun note: The FAA would still like to control aircraft using ADS-B but ATS hasn’t “turned it on”. One reason is too many commercial/airlines crews improperly set their flight numbers into their transponders.

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