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Understanding Call Control


In the development of dial telephone switching systems two fundamentally different architectures have been devised for controlling the operations of the switches. In one arrangement, the switch at each successive stage is directly responsive to the digit that is being dialed. A system using this method of operation is called a progressive direct dial control (PDDC) system.  An example is the Strowger type Step-by-Step (SxS) system as was commonly used worldwide for about  90 years. 

In the second case, the dialed information is stored for a short time by common control equipment and then used to control the switching operations. Systems using the second method are known as common control systems (CC), examples of which are Rotary, Panel and Crossbar (based on Myers, Nov 1952, Bell System Technical Journal). Before considering common control, let's look at PDDC.


Progressive Direct Dial Control 


A good example of PDDC is how a SxS system connects two subscribers. Here is a case using Strowger-type switches.  


Referring to the figure, the steps below outline the process to connect a caller who dials #4688.

  1. The caller lifts the handset and the Line Finder automatically “finds the calling line.”

  2. The LF is permanently wired to the 1st Selector, and this provides dial tone.

  3. The user dials a 4 and the 1st Selector steps to level 4 and then it rapidly and automatically “hunts” along the horizontal terminals looking for a free 2nd Selector.  It finds one at horizontal position 7 for this example. The hunting action is invisible to the caller and must occur before the second digit is dialed.

    No other 2nd selectors are shown on the diagram but there may be 100 others connected to it: 10 per level and 10 levels. Some switches can hunt vertically and horizontally for the next free selector. Lots of wiring!


  4. The user dials a 6 and the 2nd Selector steps to level 6 and then “hunts” along the horizontal terminals looking for a free Connector. It finds one at position 3 for this example. This must occur before the third digit is dialed.

  5. The Connector receives the final two digits. The Connector responds to the digits and the moving arm lands on Level 8 and position 8. Phone #4688 rings (if not busy) and upon answering the talk path is established.   

The dialing progresses along the chain of switches -- step by step – hence, the name. The design relies on switches hard wired together in a manner such that the expected call traffic load can be met. The SxS switch and its corresponding network architecture is a beautiful invention and has served many small (and a few big) cities worldwide for many decades.

Step-by-step exchange image for 4 digits
icon for info

Common Control


 We turn our attention to the second method of call control – common control (CC).  The figure below is a high-level conceptual view of the plan for a generic panel, rotary or crossbar office. The phones on the right are the same as those on the left side. They are shown this way to separate caller from called (from Fundamentals of Switching, Keister, 1948, Bell Labs). 

The Concentrator, Distribution and Expander stages are individual or combined switch fabrics. See the section on fabrics for insight into this switching concept. These are configured by the common control equipment.  The fabrics are scaled to support K calls simultaneously. For example, if the office supports 5,000 subscribers, a reasonable K may be ~750. This is ~15% of all exchange subscribers. Of course, this is a peak value.  The average phone call duration in 2001 was ~2.75 min (Statistica).

CC methods provide flexibility that progressive direct does not offer. Importantly, the dialed digits are stored in a register dedicated to the caller during the call. So, very complex toll switching and routing can occur after all, or some, of the digits are dialed. This is a non-progressive form of control. Using typical SxS, routing must occur between dialed digits. This can be onerous for a large network.   















The control functions are called into action only when they are needed to complete a call, often for less than one second per action. Different exchange types use different common equipment. The CC components are not fully explained here but some examples are provided below. Some are called multiple times per call. 

The important point is that the CC equipment is shared among all exchange subscribers on an as-needed basis.  For this reason, large metro offices are more suited for CC than smaller offices. The CC equipment is a fixed overhead that direct progressive dialing (SxS) does not have.


 Three examples of common control use are;

  • A new caller is provided an Originating Register (OR, #5 Crossbar system) that delivers dial tone and records the dialed digits. A 5K subscriber office may have 25 OR’s. They are called into use only during dialing then released for reuse.

  • After a new caller dials the first 3 digits (7-digit number) a Marker  (crossbar office) is invoked to analyze them for < 1 second. Is the call in the same office? Another local office? An out of state office? The marker "marks the path" through the in-office switches and closes the appropriate contacts to enable the talking path, all or in part. A marker is normally called at least twice during call completion. See Endnote A. 

  • In a panel office the Sender is the brains.  In a rotary office the Register is similar to the Sender in panel. There is no Marker in either as in a Crossbar office. The Sender provides dial tone and records the dialed digits. It also controls switch operations to establish the talking path. When the call is established, the Sender is returned to the pool for reuse.


The block diagram below (#5 Crossbar exchange) shows several pieces of common control equipment needed to complete a call. Note the Marker, Originating Registers and Senders plus other CC gear. The Line Link and Trunk Link Frames are the switch fabrics.

telephpone exchange high level diagram showing switching stages
number 5 crossbar telephone office block diagram

Block Diagram, #5 Crossbar office showing common equipment

       From Bell Laboratories Record, Jan 1953

See a large block diagram of a #5 Crossbar Office here

Below is a picture of a ~1,500 relay Crossbar Marker. There may be eight or more Markers shared among the 10K subscribers. Why? To meet the statistical traffic needs of callers during peak times.  Some exchanges have more, some fewer. When invoked, a marker is called into action for < 1 second. Picture from "A History of Engineering and Science in the Bell System: The Early Years (1875-1925)." 

 #5 crossbar marker exchange relay racks

Regarding common control in the Panel system, I like what H.P. Charlesworth of Bell Labs said in 1925, “I do not know of any electromechanical device that reminds one so much of the functioning of the human brain as does this mechanism (the Sender) for completing calls following the dialing operation.” 

The ideas around common control first germinated circa 1907 when Bell started co-developing the panel and rotary systems. It was heavily debated since it was not a proven concept. Plus, the minimum requirements establish cost barriers which prohibit the economical use of common controls for small, isolated exchanges. Common control was the mainstay for large electromechanical offices for about 60 years and helped set the stage for computer-based CC systems to follow.  

The common control sophistication for Bell's #1 crossbar office (1938) far exceeds that needed for their panel office (1915). In 1912 Nils Palmgren and G.A. Betulander (of Ney Autotelefon Bertulander, Sweden) received a patent for marker techniques to control an "all-relay" exchange. Bertulander's US patent for this is 1,529,419 filed 1923, is specifically for crossbar control. This patent refers to the "marking" process to mark out a path through the switches. The device to do the marking was eventually called a marker.  

Marker in operation

As mentioned, the marker is invoked only for a fraction of a second to analyze a caller's dialed digits, make decisions, and control the crossbar switches that are needed to advance the call towards completion. The video below is a snapshot of about 45 out of ~1,500 marker relays being invoked. Each "kerchunk" of relays indicates another marker operation. So, when you watch this, imagine ~30x more relays working alongside those in the video. This video was taken at the Connections Museum of Seattle (CMoS). 

Marker anchor

In the video, notice the TCHK, LCK and TK relays all located on the second row from top. These relays, and others, are actors in an interesting detective story told in the maintenance article. You may enjoy reading it and see what it takes to "Get to TK."  

Notes on the Director System

In the mid 1920’s, converting London’s manual switching system to automatic required enormous engineering effort. Due to the high calling density in the central part of London, a very high proportion of the calls that originated on one exchange required routing subscribers to other exchanges. This is a complex problem that requires considerable machine intelligence for call routing. 

Serious consideration was given to the adoption of Bell's Panel system for London (Ref1) to meet the needs. Before a decision was made the British-based Automatic Telephone Manufacturing Co. (ATM) offered an alternative scheme, using the Strowger switch. This new method translated a dialed number to a derivative "machine friendly number." This new number was not subscriber friendly, with a variable number of digits, but the format helped expedite call routing among remote offices. The Panel system had a translator means too. In the ATM scheme the translation device for routing calls was termed a "Director."  

The device used a store-and-forward methodology. Based on dialed digits 2 and 3, it would translate these to a new set of numbers (call it a code from 1 to 6 digits) that would be transmitted out of the director and into the switch train. The Director also recorded the final 4 dialed digits and appended these to the just transmitted code. The code portion digits of the translation was designed to be machine friendly. 

The first Director in Europe was implemented in an exchange at 270 High Holborn, London, in 1927.  Here is a picture of that same Director, on display at the London Science Museum. The "translation jumper field" can be seen on the right side as a cross point matrix of wiring. Other components are the Strowger switch with 3 banks, five digit counters using rotary steppers, and other gear including control relays. 

Director common control equipment for SxS office
Director for SxS

Chris Mattingly wrote an instructive article on the Director system; Automatic Electric Director System –Steering Calls Through a Step-by-Step Exchange. It was first circulated in the Switchers' Quarterly journal (Aug 2023) published by Telephone Collectors International,

The Automatic Electric Company of Chicago USA and British ATM Company apparently cross-licensed technology. This included the Strowger switch from AEC and Director idea from ATM. 

Ref1: The Post Office Electrical Engineers' Journal (POEEJ), THE INLAND TELEPHONE EXCHANGE SYSTEM  (pg 190), Vol. 49, October 1956

Endnote A

Markers, as used in Crossbar offices come in different flavors. In the #1 office there are two types: originating and terminating. In the #5 office there are three types; dial tone, completing and the combined variety.  The combined was used in smaller offices and did the work of the dial tone and completing markers. 

Many authors use the term 'marker' when referring to any of these five types. This causes confusion at times. This site sometimes calls out a specific marker type but at other times leaves it generic. 

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