Centrex Quirks of '83

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The Centrex Quirks of '83, also known as the 1AESS Centrex Quirks of '83, is a 4-part series of telephone recordings from around 1983 by former phone phreak Evan Doorbell, documenting some bugs in the 1AESS Centrex software that made certain call bridging anomalies possible.

Synopsis

Part 1

By 1983, Evan's interest in the telephone had more or less gone dormant. However, his friend Les, working as a communications consultant for the State of Georgia, had 1AESS Centrex service at his office. Initially, Evan and Ben thought 1ESS would be the "switch of the future", but the 1ESS/1AESS, being an analog switch, was just "the switch of the 1980s".

One day, Les receives a call waiting, flashes to get a recall dial tone, and then dials an external number. When he flashes again, he gets another recall dial tone and is able to dial another number again. This process is allowed to continue indefinitely, building up a "stack" of calls in the process that could later be torn down, similar to classic "tandem stacking".

In playing with Call Hold and Call Waiting on Centrex, Evan and Les find a number of bugs in the 1AESS software. Evan and Les order Prestige for their apartments in order to explore the 1AESS bugs in more depth. Les is assigned intercom code *20 and Evan is assigned *21. They discover the problems are so bad that they decide to record a series of tapes to send to a contact at Bell Labs so the problems can be fixed.

In Part 1, Evan and Les test the Transfer Permissions bug and the Unwanted Bridge bug.

Calling Features

Call Hold

Call Hold allows users to make two separate calls on one phone line, without conferencing them together as is done in three-way calling. Users can dial one call, flash to get a recall dial tone, dial the Call Hold code, and then dial another number. The first call remains on hold while the second call goes through. The user can then swap between calls by flashing to a recall dial tone and dialing the Call Hold code again[1]. The code in Les' office for Call Hold was 111 (feature and tie line codes on Centrex are typically assigned in the 1XX range).

Call Waiting

Call Waiting allows users to receive another incoming call while they are already in an existing stable call. In Les' office, it was available starting in 1983[2].

When Call Waiting is assigned on a line with the Call Hold feature, operation of the feature is modified slightly. Flashing during a call waiting does not answer the call waiting; instead, it provides a recall dial tone. Dialing the Call Hold code is necessary to answer the call waiting[3]. To swap between calls, flashing and dialing the Call Hold code again is necessary.

Because of the presence of the Call Hold code, it is not necessary to immediately dial the Call Hold code during a call waiting. A user can instead dial another number so as to transfer his existing call to the new destination number, before answering the call waiting. Normally, this kind of procedure is not possible.

Multiline Variety Package

Southern Bell debuted a new service called Prestige[4], also known by its generic name Multiline Variety Package or MVP[5]. Customers in the same central office could be in the same customer group. This offered Call Hold and Call Pickup to residences and small businesses.

Call Pickup is assigned the code *8[6]. Call Hold is assigned the code *9[7]. Three-way calling is called Conference, but also has the "Transfer" feature, so calls can be transferred by hanging up after dialing a second number. Speed Calling 6 is present by the name Convenience Dialing, with 6 entries from #2 through #7. Intercom calls (within the system) are dialed using long-list speed calling, from *20 through *49.

Because *8 is assigned for Call Pickup, the usual *8X vertical service codes cannot be dialed in the usual manner. For this reason, they are assigned the *5X range with MVP[8].

Bugs

Throughout their exploration, Evan and Les find that all of the unusual bridging bugs are caused by bugs which cause the switching system to not work as intended. In retrospect, Evan deduces the problems were caused by four distinct bugs.

These bugs were fixed in the 1980s and since then were no longer present on 1ESS/1AESS switches.

Transfer Permissions bug

Call Waiting on Centrex was an afterthought, and when it became available, the extended definitions for calls that could be transferred were not fully updated. When there is a call waiting, answered or unanswered, the original call cannot be transferred to an outside number[9]. (If the transfer-to number is an intercom call, it does work.)

In one scenario, with a call up between two stations, one station receives an intraoffice (but non-intercom) call waiting. This station flashes to a recall dial tone and dials the Call Hold code, so as to answer the call waiting. This station then flashes again to dial another number and then hangs up, so as to transfer the incoming call to an external number. Afterwards, the station hangs up momentarily to allow the original call to ring back again and then answers the original call. After the external call hangs up, the incoming call then triggers another call waiting[10]. Another scenario has the original stable call between an intraoffice call and a Centrex station.

Unwanted Bridge bug

This bug results in a user no longer being able to flash for a recall dial tone[11]. It can also result in not being able to hear a connected call.

With a stable intercom call, one station dials the Call Hold code and then initiates an outgoing interoffice call. He then switches between the two calls a few times using Call Hold, eventually transferring the outgoing call to another intercom station. This number is busy, and the user can no long flash to get a recall dial tone.

If the outgoing call is an intraoffice call, this bug does not occur, and the busy signal can be dropped successfully for another attempt. If the second incoming call is an incoming call, it also does not occur.

In another scenario, if an answered incoming call is transferred to a ringing extension, if another station puts an existing call on hold and answers the ringing call using Call Pickup, the two parties can converse normally, but if the answerer swaps calls and then comes back, the two parties can no longer hear each other[12]. This also occurs if the initial call is an answered outgoing call, with Call Waiting instead of Call Pickup.

References

  1. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 2:20-4:05".
  2. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 4:05".
  3. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 4:20".
  4. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 8:30".
  5. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 11:10".
  6. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 13:05".
  7. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 11:35".
  8. ↑ "Phrack Magazine, Volume 6, Issue 47, File 7 of 22".
  9. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 19:15".
  10. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 14:00-19:00".
  11. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 26:50".
  12. ↑ Doorbell, Evan. "The (1A ESS) Centrex Quirks of '83, part 1, 29:00".
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