Marker

A partial view of a No. 5 Crossbar completing marker

A marker is a device that selects and establishes a talking path through crossbar switching frames. In the process of setting up a call through electromechanical telephone switches, a circuit must be established for the calling and called party to communicate over. It is the marker's job to determine who the caller wants to speak to, and to establish a path through the network to the called party.

Markers get their name from the process of marking a path before crosspoint closure: a marker seizes control of one or more crossbar switch frames, finds the best path, and then marks it for use. Then, after completing the remainder of its functions, operates the horizontal and vertical magnets of the crossbar switch to actually close the path. Markers are a critical component of a common-control telephone switch, since they have complete control over the various network functions in the switch, and the network itself has no intelligence whatsoever. Although markers are not Turing-complete, they play a similar role to a CPU in modern computers.

History

Markers began being studied simultaneously by Bell Laboratories in the United States and by Gotthilf Betulander in Sweden in conjunction with studies of crossbar or crosspoint networks. Up until that time, almost all telephone switches had employed some variant of the progressive control principle, where connections were established piece by piece as the call was dialed. Using progressive control, there was no way to know if the desired terminal was available for use until the selector arrived at the terminal, so a certain percentage of calls were ultimately set up, and then immediately taken down if the called terminal was busy. Markers circuits could, in theory, be used to "look ahead" to the desired destination for a call and establish whether or not it was available before actually operating any of the actual switch fabric.

A precursor to markers called the decoder was used in the panel switch. The decoder was responsible for translating the first three digits of a dialed call (known as the office code) into information that was then passed back to the sender. The sender used the information from the decoder to drive selector rods upward over the district and office frames to reach the desired central office. Further instructions told the sender how to communicate with the distant office once it was reached, how much line build out (compensating resistance) to add to the circuit, and how the call should ultimately be billed.

Although they laid the groundwork for later development into markers, decoders lacked the ability to directly observe and select paths for calls to take through the switch. They could only translate information and return instructions to the sender, which would ultimately set up the call.

By the early 1930s in the United States, it had become apparent that a successor to the panel switch for large metropolitan exchanges would have to make use of common control principles. Bell Labs engineers were aware of the great advantages that could be gained from employing markers in a new switching system. These included high speed operation, the ability to "look ahead" and determine the best route for a call, alternate routing whereby a second or third trial could be made if the first was unsuccessful, and detailed error reporting, where any failures to complete a call could be stored and displayed to maintenance staff.

The No. 1 Crossbar was the first telephone switch in the United States to use markers extensively. Originally called decoder-markers, because of their close relationship to the panel decoder, their job was to receive information from a sender, decode that information into a routing choice for the call, and then establish a path through the switching frames to the called destination. While doing so, the markers also returned information to the sender regarding how to handle the call once the signaling and talking path was established. No. 1 Crossbar offices made use of two types of markers, one for the originating half of the switch, and one for the terminating half. These were called Originating and Terminating Markers, respectively.

In future crossbar systems, such as the 5XB, the role of markers increased even further. In addition to selecting and establishing a talk path for the call, the 5XB completing marker also attached auxiliary equipment dynamically to the call to allow for its completion. Whereas in the 1XB, there was a distinct marker for originating and terminating traffic, the 5XB had dial tone markers and completing markers. Dial tone markers, as their name implied, handled dial tone requests when a customer went off-hook, and completing markers handled all other jobs, such as inbound calls, outbound calls, intra-office calls, tandem calls, pulse conversion, etc. The markers also handled requests by the Master Test Center (MTC), whereby tests of all equipment could be made.

Construction

Markers usually take the form of one of more bays of relay control circuits, and one or more cross-connecting fields. The cross-connect fields are used to "program" the marker with instructions for carrying out its various tasks. The core logic of a marker is hard-wired, but the switch-specific parameters can be set manually by means of cross connections. Because the telephone network doesn't change too often, these cross connections are sufficiently stable, and sufficiently easy to change when required.

Because markers are quite expensive, there are comparatively few of them in a central office, usually up to a maximum of twelve. And because there are only a few of them, markers must act with great speed, in order to handle as many calls as possible within a given time frame. In order to do this, markers make use of external circuits, such as registers and senders to communicate with distant subscribers and central offices. The holding time of registers and senders is necessarily longer than the holding time of a marker, and there are generally more registers and senders than there are markers.

Markers use parallel busses called connectors to send and receive data. Each connector consists of hundreds of wires, each of which may have ground or battery potential on it. These potentials operate relays within the marker, which cause its wired cascading logic to function.

It can be helpful to consider markers as a series of smaller circuits, since thinking about a marker holistically may be overwhelming. Each type of marker is somewhat unique, but all share common circuits in different formats that are used in the course of their jobs:

Type of Circuit	Purpose of Circuit
Input and storage circuits	Accept and stores numerical data from some external device
Translator circuits	Convert data in one form to another form
Test circuits	Test parts of the switching network for availability, false crosses and grounds, and double connections
Preference and selection chain circuits	Analyze available options and chooses the best one according to a pre-set algorithm, and data regarding the previous selection that was made
State control circuits	Monitors and advances the overall state of the marker
Error checking circuits	Validate input and output, and reports inconsistiencies through the trouble reporting circuits
Timing circuits	Keep track of how long each operation takes, and act accordingly if times are expired
Trouble reporting circuits	Stop progress of execution and dump status reports to a trouble indicating device

Trouble Reporting

Concentrating control functions into just a few fast-acting but complex units meant that means had to be devised for reporting trouble, and reproducing test calls to aid in localizing problems. Since it was nearly impossible to identify a problem with a marker just by looking at it, marker-based common control switches employed trouble indicators or trouble recorders that displayed faults as they occurred. This aided maintenance staff in resolving issues.

Second Trial and Alternate Routing

One of the primary advantages of employing markers is that they have the capability of retrying if an initial attempt to build a connection fails. This is in contrast to earlier progressive control systems like step-by-step and panel, where if the first attempt fails, a busy or reorder is returned to the subscriber, and they must simply hang up and try again later. Because they analyze available paths (or channels) for a call before actually closing the crosspoints for a channel, markers can determine if a route or channel is busy, and fall-back to alternates if needed.

Second trials are used when there is a failure to find an available channel (failure to match), or when there is some problem that prevents the call from being set up as requested. A marker on second trial will usually try to set up the call using a different path than the first trial.

Alternate routes are used when all trunks to a destination are busy. While there may be available links and junctors in the local office, the fact that the trunks (outputs) are busy means that no attempt to build a channel can even be made. In this case, markers can route advance to the next most preferred route, which will direct the call to leave the office over a different trunk group entirely. This is analogous to the main highway out of town being blocked, so a driver would take an alternate route such as a frontage road, or surface streets to leave town.