Evolution/History of the Public Switched Telephone Network
Christopher J Wilson
American Public University System

Since the beginning of civilized mankind, there has always been the need for humans to communicate over long distances. At first this required couriers to bring letters from destination to another. It then evolved into trained birds, to Morse code. Over time this type of communication quickly became out dated, because of the fact that it took too long for messages to be moved from one point to another point. So the telephone was created. Allowing people to talk to each other over vast distances. The PSTN had been designed to allow instantious two way voice communication between anyone. These first set of telephones had no networks to them, they were all private user based. If someone wished to speak with another person they would pick up their end and whistle into the receiver until someone on the other end answered, that was the very first introduction of the telephone ring. This was of course replaced with the bell sound, which is still being used to this date although it has changed quite a bit from its creation. After the bell was created people wished for a way to turn it off when not in use, thus the switch hook was created, which aloud you to turn the device off while no calls were coming in. With all these new advancements added to it, it was only a matter of time before some combined all the telephone wires into one central hub.

But as this new device took off, it required that each telephone be connected to every other telephone that person wished to speak with. All these lines came together at a central hub, where employees would receive the call phone operator A, be requested to be transferred to operator B’s line. Over the first few years of these new hubs and allowing people to transfer them this way, required many employees to cover the vast amount of calls coming in. This was the creation of the switch board. Seeing that this way of connecting everyone together through verbal commands, quickly became outdated. So that’s when a man by the name of Almon Strowger created the very first automatic telephone switch system in 1891. This man quickly found out that in his town calls where no being appropriately handled in a timely manner, being that some people of higher status were always connected faster than those of lower status.

That’s when he set out to provide a revolutionary system that would automatically connect people to their desired person, while being non – biased, with pinpoint efficient manner, and with little to no human interaction needed. This system is still in use today, known as the PSTN. This new system was designed using electromechanical switches based around Pawls and electromagnets. By the late 1880s, with the help of his nephew. Almon Strowger had his first working model, to be used. In this model, they used a moving wiper with magnetic contacts on the end, weaved and moved around a bank of many other contacts. Only making contact with the one that it was required to connect to.

Although Mr. Strowger was the created of the switch telephone network, he was not the creator of the base device used. That device was created by a group known by Connolly and McTigthe. Strowger was just the first person to put it into application on a semi global use. After showing that his new device was able to handle any number of calls, Mr. Strowger set out to make a name for himself. And with the help of two gentlemen named Joseph B. Harris and Moses A. Meyer, his company was formed under the name of Strowger Automatic Telelphone Exchange October of 1891. Within 20 years the company goes through another merger and was renamed Automatic Electric. Even to this day this company is still up and running, having gone through many changes and owners, it now is known as AG Communication Systems

As the demands and needs for this new system increased over the years the need for more switches and lines required to handle the increase naturally grew with the demand, and is still growing to this day. The first concern for this new technology was the local populace, how could they make it so that everyone locally and country wide could make calls and have very little delay time. This required for the rapid expansion of this new device to grow across the U.S.

The design of this system was and is always changing, being converted to adapt to the ever changing needs of its consumers. The hierarchical type of design, that local switches named exchanges on bottom, up next are the regional ones called access tandems. This continued to the Switching tandems level which are at the top. Each level handled different types of calls and consumers. The bottom level handled anything locally and were directly connected to other bottom level switches. The level above this handled any type of call that wished to be transferred long distance. It did this by using
The Inter-exchange carriers or LECs. All of these are mostly handled by big name companies which include AT&T, Verzion and sprint, are just some of the companies that handle this. The upper most level handles any type of call that has to go between the carriers. Meaning if a consumer of Sprint wishes to speak with a consumer of Verzion, this the level that would handle that call.

You can see in the image above the three different levels that were used.

With consumers wishing to be able to talk to anyone in any state across the U.S it required a vast amount of what are known today as Area Codes. This allowed people to dial a ten digit number and reach the desired person of their liking. Before this it was only seven number and after a short period of time the number of available of telephone numbers quickly emptied. This required Phone companies to devise a plan in which would allow them to reuse any number as many times as they wished, but would also be unique to each persons.

To help in the routing of traffic over the PSTN a new system was created called the North American Numbering plan, Thus a three digit number was assigned to states, some receiving more than one based on the number of consumers in that area. In the state of California there are at least 5 different area codes if not more. New exchanges were created to organize and help different NPAs and allow further deeper understanding of routing. Breaking down a telephone number can be hard to understand but also quite easy to learn. The first three numbers are the area code, which routes where the call to the correct region. The next three digits determine which exchange server the call will be headed into. The final four numbers of any telephone number simply define the end user address within this exchange. When the bell system was finally broken up, the nation was to be divieded into LATA’s or Local Access Transport Areas, these areas would determine the difference between long distances calls that could be handled by the Local Exchange provider or if it would have to be taken over by the inter-exchange carrier. This LEC used the End-Office networks to determine where a call would be handled. Now of course this type of system could never stay around forever. It would eventually have to be put to rest and allow for a better system to take its place. That’s where SS7 comes into play. This evolved from Common Channel Signaling Number 6 or CCIS6 which began in the late 1960s. This was used to separate all calls of the switch. This CCIS6 was used to help solve two different problems that were still relevant in the traditional Public Switched Telephone network. The first being that the CCIS is “out-of-band” it reduced the cause of fraud, meaning that anyone who tried to use certain tones to manipulate the system for fraudulent intentions had a much harder time doing so.

The second reason was because of call attempts vs call completions, which would tie up the switch board operators and then the automatic switch board. Meaning that when a call is made, the circuit that it used was tied up on that one call for the full duration. Meaning, if a person repeatedly tried to make a connection that circuit would be used the entire time, making it more difficult for the system to route calls as one or more of its circuits were already being used for another call, this meant that lines would get backed up until the few number of open circuits were available. By 1977, CCS had been deployed any many toll networks and by the 1980s SS7 had been defined by the International telegraph & telephone consultative committee or the CCITT. AT&T was the first company to have 80% of their intertoll trunks to be severed by the CCIS. It allowed calls to be set faster, torn down faster and signals could be sent in to different directions at the same time, while the call was still being set up. By the year 1987 installation and trial began for many companies across the U.S CCIS6 was being improved in almost every accept possible by the SS7. This included but was not limited to the bit rates that the CCIS6 used, the multiple layers of the SS7 allowed into be changed at will without much trouble for the rest of the system. The greater volume of the signaling allowed for calls to be set up a lot faster. The trunks time and usage became a lot more efficient and it even lowered the fraud rate for the entire system, making it much safer for consumers to use.

With the help of the SS7 the end offices were not only connected to each other but they were also connected to the SS7 allowing the transfer of long distance calls much faster, seeing as it could make it seem like an end officer in Georgia was connected directly to an end office in Maine. This system stayed in use for a long time, but of course people were always looking for ways to expand and improve upon this system. So after the rise of the SS7 and how it seamlessly switch calls and manage them, the switch intelligence could move away from the switch and into many vast networks. It laid the path for the emergence of the AIN or the Advanced Intelligent Network, and its purpose was very simple. Remove the switch control from service provider. Allowing the owner of the switch to find better services and handle logic from more places

There are two types of primary methods of connecting switch nodes. The first approach is a mesh topology, in which all nodes are in connected to each other. The better approach is a hierarchical tree in which nodes are aggregated as the hierarchy traverses from the subscriber access points to the top of the trees. But both methods are used in the PSTN but of course they are driven by cost and usage.

References http://en.wikipedia.org/wiki/History_of_telecommunication - History of telecommunication http://lyle.smu.edu/~levine/ee8304/moseley.pdf - The Evolution of the Public Switched Telephone Network” http://www.internetsociety.org/sites/default/files/The%20Internet%20and%20the%20Public%20Switched%20Telephone%20Network.pdf – The internet and the Public Switched Telephone Network http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2241658 – No Dialtone: The End of the Public Switched Telephone Network http://homepage.ntlworld.com/martin.essam/development_of_BT.htm - The Public Switched Telephone Network