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October 17, 1996

The Effect of Internet Traffic on the Public Voice Network

Part 1: The Nature of the Public Voice Network

In the next several articles, I would like to examine the effect of Internet traffic on the public voice network. Recently, Bell Atlantic has petitioned the Federal Communications Commission (FCC) to change some rules to lessen this impact on the Telephone companies and their customers. There are also other reports on some smaller communities whose customers cannot get dial tone or reach 911 because of the amount of Internet calls going on during part of the day.

Before we look at how all of that is possible, we need to understand a little about the nature of the public voice network. That is the main subject of this article. The next article in this series will deal with the traffic assumptions of a voice network, and how the telephone companies use it to engineer their networks. The third article in the series will describe the Internet traffic, how does it differ from the regular voice traffic, and how does it impact the voice network. The last article in the series will speculate on what can be done to lessen that impact.

The US telephone network is the largest and most reliable network in the world. It has a long history on how it got to this point in time. With 90% of the households with a telephone, the network has become a necessary, and a life line for an increasing number of people. It is hard for us to imagine how life would be without the telephone. The telephone network has matured a great deal in the last decade. It now offers much more than the regular Plain Old Telephone Service (POTS). A few other types of traffic that it carries include: Paging, Fax, Modem, and Internet. Some other types of traffic that start to flow on the same network are: video conferencing, and video entertainment.

How does the telephone network work?

Conceptually, every telephone in the world is connected to every other telephone. These connections are either physically or via some other transmission technologies such as microwave, radiowave, light, etc. One can imagine 2 persons, each with a tin can connected by a string talking to one another. The real picture is not much different than that other than a little more complicated. The complication begins when we try to connect 10 people together. For this to happen, do we have 9 pieces of string from each person to the other 9 persons? Obviously not, for several reasons. One is when a person is already on the phone, other persons cannot reach him/her any more so there is no need for another line to that person. The second reason is simpler: it is not economical to run multiple lines to each phone.

To solve this problem, the switchboard is invented. This is where the single line from each telephone terminates. If person A wants to talk to person B. A just call the switchboard and ask to be connected to person B. The operator will logically tie the two strings together then the model of the 2 tin cans appears again. If too many people call the operator at the same time, someone will have to wait to be served. If the wait is too long we may need several operators. As technologies improve, this operator function is automated and becomes a sophisticated electronic switch.

Another complication is when the 10 people who are connected to the "operator" live in 2 different cities. Let's say that 5 lives in Atlanta, and 5 in Los Angeles. Where should the operator lives? One solution is may be Kansas City. But when the model grows larger, it is not efficient or economical to have 2 persons, both live in Atlanta, talking to each other via Kansas City. This is when we need one set of operators in Atlanta and another one in LA. The people in the same city can talk to each other using their own local lines. Only when they wish to talk to people in the other city that the two operators need to communicate and "tie" the right strings for the communication to happen. The next question is if there are 10 people in Atlanta and 10 people in LA then how many "strings" do we need between the 2 cities? Obviously, the most would be 10 and the least would be 1. Having 1 connection between cities is not that uncommon in many parts of the world. In our travel, many of us may have experienced the wait by the phone to make a call home from our hotel in Europe or Asia until the operator can get the line out of the country for us to talk. This happens for several reasons: The long link connecting the cities is often expensive, and there is not enough information for the planner to know how many links are needed.

We know that not everybody in one city wants to talk to someone in the other city at the same time. Further more if a telephone is busy because someone already using it then nobody can reach that person anymore (we will touch on call waiting, and three way calling, etc., later on). The problem is reduced to a probability problem: How many people on the average want to talk between cities at the same time? What happens if when someone wants to use that link and cannot get it? Will they go away and try again sometime in the future? Will they re-dial right away? Will they go to a competitor?

The situation is much more complicated when we know that there are n cities that need to be connected, the total number of interconnections can be as much as n(n-1)/2. How many of these interconnections need to be really connected? How should the call from Atlanta to LA be routed? One scenario can be as follows: Try to connect Atlanta to LA direct, if a link exists and is free. If not, try connect through Kansas City if possible, otherwise try Chicago. If all links are busy or unavailable then issue a fast busy signal to the caller. Another method of routing calls is based on what is the best chance for the call to go through. For example, a call from Atlanta to New York at 9:00 AM is often best to be completed via Los Angeles. Since at 9:00 AM eastern time, most people in LA are still in bed. The links to and from the "operator" or switch in LA are pretty much free and we would have a very good chance to complete the call. How to route each call is a subject that is being researched for many years by many people.

As the network becomes larger, it outgrows the capability of the human operator. The electronic switch can do better and faster most of what an operator can do. What it lacks is the intelligence and experience of the operator. Many have attempted to program these in the switches with limited success. For example, before the human operator in Atlanta connects a call to LA via Chicago, he/she would call the operator in LA and ask if the destination phone is free or not? If it is busy then there is no need to try to connect to any link. The Atlanta operator can also call the operator in Chicago and ask to see if there is any free connection from there to LA? If there is not then he/she may try Kansas City instead. If nothing is available then there is no point in connecting to any link and ties up that link for no reason.

The telephone network mimics this forward checking capability of the operator by using a special communication protocol between the switches. The switches communicate by sending short messages on a separate specialized Signaling Network using the Signaling System 7 (SS7) protocol. With these messages, the switches communicate the status of the links (free or busy), the status of the destination phone, the best route to take, etc.: The same thing that the operators have done for a long time.

Technologies have caught up and provided some service features that an operator can do naturally. Features such as call waiting, call forwarding, three way calling, caller ID, etc. have just started to appear in the last 10 years. There are still many other features that a human operator can do possibly but a machine still cannot do very efficiently yet. Some examples of these are:

  • Two people speak two different languages and the operator does the translating,
  • Routing based on the caller, and not the calling number. (e.g., If Mr. A calls then route to one number, but if Mrs. B calls even from the same phone then route to another number)

As technologies continue to improve, more features will be added. Each of these features will add another dimension to the traffic and alter the network in new directions. To build the current telephone network, over a hundred years of traffic measurements have been collected and studied. Statistics such as length of calls, how often does people call? what time of day does people call? Is there a difference between business calling and residential calling? what day of the year is the busiest day? what time during that day? As new services, and new features are added, some of the above statistics will change and will affect the total performance of the network.

In the next article we will discuss the traffic assumption of the public voice network and how to engineer various parts of the network based on the traffic measurements. I have obviously omitted many of the technical details out of the discussion to make the article readable for everyone. I welcome any technical question or comment on this article and any other articles in our regular columns. Please send them to us at and we will discuss them on

Luc T. Nguyen, Ph.D.

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Copyright © 1996 by VACETS and Luc T. Nguyen


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