A class on "Translations"

I have not blogged in a couple of weeks because Nortel put me through a training class these past two weeks so I can perform my job better. It was a nice change of pace. I had been asking for this particular training for literally 10 years but for budgetary reasons I guess, I was never allowed to take the course. I had been exposed to much of the course material over the years, out of necessity. But the course was still well worth the time and expense because it filled in many of the holes in my knowledge and experience in this area. The class also firmed up my self obtained knowledge with "why" things are done or not done, the way they are.

The course was titled "SL100-Translations." Nobody outside the telecommunications arena has much interest in this area, even if they know what "translations" is in industry terms. But since this is my blog I will attempt enlighten the masses with a cursory explanation. Feel free to click on your favorite bookmark any time. My feelings will not be hurt.

The easiest way for me to explain what telephone translations are it to provide a brief historical background of telephone switching systems.

In the early days of telephones, a pair of wires connected each subscribers phone
to the telephone company's "Central Office" Each phone line coming into the central office connected to an operator position. Lets say you lived in Mayberry. When you picked up your handset, you would be connected to one of these operator positions. Sara, the operator, would answer and ask you for the number you would like to be connected to. She would then physically plug your line into a receptacle on her panel corresponding to the phone number you requested and your phone would be connected to your neighbors phone. In small towns you might tell the operator you want to talk to Floyd the barber perhaps. In larger towns you would provided the operator a phone number. In either case the operator would know how to connect you to your neighbor. Sara, in this case, provided the translations. She would translated your verbal request to a physical circuit on her panel and connected you.

What if you wanted to call your Aunt Nellie in a nearby Salem? The operator would not have a slot on her panel for Nellie's phone but she might have a circuit on her board that would connect you to the operator in Salem. Each central office was connected to the office of other geographically nearby towns with phone circuits. These inter-office circuits are called 'Trunks'. When you picked up the phone and the operator asked for a number, you would say. I would like to talk to My Aunt Nellie in Salem. Her number is SAlem83788. The operator would connect herself to the operator in Salem, and ask her to connect the call if Nellie was available. The Salem operator would connect the trunk from Mayberry to Nellie's phone and the Mayberry operator would then connect your phone to the Salem trunk. If you wanted to talk to Uncle Buck in Wichita, your call would be routed through potentially dozens of central offices and operators, in dozens of towns along the way, until you finally were connected to Buck.

The phone system evolved over time. Operators still exist but the majority of phone calls no longer require an operator to make the connection. A subscriber could pick up their handset and using a dial could connect to their neighbor directly without talking to an operator. Dial out of town still required an operator to route.

Local Telephone calls were being routed by mechanical switches instead of operators. "Step-by-Step Switches." These telephone switches were made up of large succession of electrical switches relays and solenoids. When a phone subscriber picked up their handset, this Step switch would sense a change in voltage. This signal would trip a relay that would open a new circuit path to the next set of relays and solenoids. Then, each time the user would dial a digit, the phone would send a series of pulses, voltage drops, across the line for each digit dialed. A dialed '1' would send one pulse. A '2' would send two pulses, etc.. The switch would mechanically count the pulses to determine what digit was sent, It then would open up the appropriate circuit path to the next set of relays to count the next set of pulses; it would send the call to the next 'Step'. It continued like this until a recognizable route was decoded. At that time the originating caller's phone was directly connected to the person's phone he called. Or if the call was not a local call, the call was connected to the operator, or a trunk, that connected two switches together.

This is a simplified explanation but in essence that is how a telephone switch worked. (See THIS LINK if you would like more nitty gritty on how a step type switch works and evolved) The successive series of switches made a decision, based on each digit dialed where to send your call. It 'translated' the pulses it saw (digits) into a specific route through and out the switch. There are other considerations, like what to do when one trunk is already in use, how to decide to go to another trunk etc but suffice it to say it was handled by a large intricate network of switches and relays.

Each town had, in effect, it's own telephone company, The telephone numbers were assigned locally according to the number of lines in that town. Some towns, with only 300 phones would use only three digit phone numbers. Larger towns, serving more telephone subscribers, would use more digits. There was no consistency and therefore no easy way to know what towns required how many digits. In 1947 AT&T, introduced the North American Numbering Plan (NANP). This plan provided a standard for assigning and routing telephone calls based on the dialed digits. This would enable automatic switching of non-local calls. Regions of the country were assigned a unique 3 digit code. Large cities, like New York or Los Angeles received their own codes, while in less populated regions, a code could represent a whole state. (See THIS LINK for more information on today's North American Dialing Plan)


Fast forward again to the 1970s. The switching systems had evolved to be smaller, faster, more efficient and more flexible but basically worked on the same principle but because of a standard dialing plan only a very small percentage of the calls required operator intervention.

In the later half of the 70's Northern Telecom (today known as Nortel Networks) introduced a "Digital" telephone switch; The Digital Multiplexing Switch (DMS). This switch was revolutionary in several ways but, for the sake of this discussion, it allowed the digit to route translations to be handled using a computer rather than by mechanical device. Not only did this make telephone routing faster and more reliable but it introduced flexibility in how calls were routed that were unknown previously. The "Dialing Plan" or routing capabilities could be customized to the need of each central office, or each customer in that central office. Inter-switch calls still needed to adhere to the NANP but special services or service restrictions could be assigned to individual phone subscribers. For example. Some phones could be made to not accept long distance or toll calls. Some phones could be allowed to dial some regions but not others. Business could their calls would route using different rules depending on prefixed dialed digits or what time of day it is etc.

Later on, with the supreme court ruling that broke up AT&T's monopoly of the nation wide phone system, special access codes were introduced that would permit any subscriber to choose what long distance service provider they could use (known as Equal Access)

When dialing flexibility was introduced, the complexity of interpreting those digits dialed became significantly more complex. The AT&T divestiture also sparked the introduction of many new billing options and level of services, each with it's own dialing plan and routing requirements. But because the telephone switching system was now computerized, most anything could be done, if you write the program to do it.

This brings us to present day.

Nortel, my employer, supplies telecommunications companies, like Verizon, Bell South, Sprint..., with telephone switches. In fact Nortel provides telephony equipment world wide. North America has it's Numbering Plan and The rest of the world have their own. Our equipment needs to be able to handle it all.

The class I took over the past 2 weeks taught me how to configure the DMS to provide dialing flexibility to meet customer requirements while still supporting the government mandated services and dialing plan requirements.