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AUTOVON   ISVN
Switched defense telephone network

AUTOVON (Automatic Voice Network) was a military telephone system that was developed in the US in 1963 to survive nuclear attacks. It allowed non-secure voice calls with precedence (piority override). In the late 1960s, the DoD started the roll-out of a secure version of AUTOVON, called AUTOSEVOCOM, but this project was cancelled a few years later due to problems and high cost.

Both systems were later replaced by IVSN (Initial Voice Switched Network), developed in the mid-1970s by NATO for unclassified voice calls. It was intended as replacement for the expensive AUTOSEVOCOM system. Starting with 4 switches in Europe in 1980, the system grew to 24 switches at the peak of its use in the mid-1980s.

The blue telephone set shown in the image on the right was used with this network. It has an extra column of (red) buttons on the keypad, which is typical for AUTOVON and IVSN phones. The red buttons were used for priority override.
  
DNS handset for IVSN

Also typical for this desktop set, is the black push-button on the handset — used as push-to-talk (PTT) in half-duplex conversations — and an indicator lamp close to the cradle. The ISVN network was operational for more than 25 years. When it was officially closed down on 30 November 2005 it still consisted of 18 switches, some of which were still in use in 2011 [1].

DNS handset for IVSN DNS handset for IVSN Handset off-hook Calling indicator light Push-To-Talk switch (PTT) Using the PTT in half-duplex mode Keypad Bottom view
A
×
A
1 / 8
DNS handset for IVSN
A
2 / 8
DNS handset for IVSN
A
3 / 8
Handset off-hook
A
4 / 8
Calling indicator light
A
5 / 8
Push-To-Talk switch (PTT)
A
6 / 8
Using the PTT in half-duplex mode
A
7 / 8
Keypad
A
8 / 8
Bottom view

Priority override
IVSN had four levels of priority override:

  • FO
    Flash override
  • F
    Flash
  • I
    Immediate
  • P
    Priority
The four extra keys generate DTMF-signals in the rarely used 1633Hz column. On some later keyboards, these keys are sometimes called A, B, C and D. After a nuclear attack, it would be very difficult for government officials to obtain a free telephone line, as nearly everyone would try to make a phone call. By pressing the letter P, the user would signal the switch to assign a free line by priority. Higher ranking officials were allowed to press I (Immediate) to get a higher priority.

Military users were allowed to press F (Flash) in order to get a free line nearly instantly. It was thought that only the president and his circle were allowed to use FO (Flash Override) to give them the highest possible priority. Note that not all levels of priority override were available to all subscribers; the required priority level had to be assigned to specific nodes first.

Crypto phones
AUTOVON and IVNS networks did not provide any cryptographic security, but were able to carry calls that were placed via (NATO) crypto phones that were connected to the network. In some cases the call had to be initiated in-clear, after which one of the parties decided to go secure. The following crypto telephones are known to offer support for AUTOVON and IVSN networks:

References
  1. Wikipedia, AUTOVON
    Retrieved November 2012.
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Crypto Museum. Created: Saturday 21 October 2017. Last changed: Saturday, 21 October 2017 - 15:15 CET.
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