In days of old there was one network. It was called the Public Switched Telephone Network (PSTN). It utilized what we called circuit switching and the hierarchy of switching systems.

A very abbreviated description of circuit switching is: a calling party using a land-line telephone to place a long distance call, upon lifting the handset, would be connected to an End Office, otherwise known as a Class 5 office. The circuit to this Class 5 office is often called The Last Mile or the local loop. Dial tone would be returned to the subscriber, and a telephone number dialed. A circuit would be established from the Class 5 office to a Class 4 office, and, if need be, a Class 3, 2, or even 1 office. The circuit would then extend downward to the end office of the called party, and hence to the called telephone. The telephone would ring. There would now exist a circuit up, and then down, the hierarchy of switching systems. This is circuit switching.

The called party may be the calling party's next door neighbor, in which case only the Class 5 office would be used. Or the calling and called party might be separated by half a continent, in which case several of the higher order offices would be used. In any case the established circuit is "owned" by the two parties. No matter who is talking, or even if no one is talking, the overall circuit is dedicated to this conversation.

Originally the conversation was analog. It was not digitized -- that is, it wasn't broken into a series of 1s and 0s. As time went on this changed. Digital switching was employed, and the "conversation" was digitized so that the transmitted signal was a series of binary digits (bits).

Also, in later years, the cordless telephone made its appearance. The basic land line telephone remained in place -- it was connected to the jack in the wall, and hence to the local loop and the hierarchy of switching systems. But there was a radio link between this land line telephone and a handset (a short range radio link, to be sure), so now a certain amount of mobility was added to the network.

Cellphone
Cellphones changed all this -- a bit. When employing a cellphone there was no longer a telephone connected to a local Class 5 office. Rather the cellphone was connected by radio to a base station at the foot of a cellphone tower. That tower could be a couple of miles from the calling party, but not much farther. This connection is defined as the Air Interface.

Several different technologies are employed for this link. To take two examples: Verizon uses CDMA, or code division multiple access. AT&T uses GSM, or global system for mobile. About 80% of the world's cellphones use GSM. These two systems (both of which are digital) are not compatible. And both are being improved, on a regular basis.

At the present time we are hearing a great deal about 3rd Generation cellular or 3G systems. There are many differences between 3G and 2G, and for most of us the details are not important. The same holds true for 3G and 4G -- 4G being just around the corner. At the present time it looks like there are 2 likely candidates for the 4G technology: WiMAX and LTE (Long Term Evolution). WiMax has a two- or three-year head start; LTE is more akin to the GSM technology, and therefore shows more promise for the majority of the world. Both are very robust, and under good conditions have a range of up to 30 miles. Think of the cellphone towers that won't have to be built!

The system works like this: From the base station at the foot of the cellphone tower a connection is established to a local end office -- that is, the local Class 5 office. We call this process back haul. This transmission is usually via copper wire, using sophisticated coding techniques, or fiber optics. Occasionally it uses a special radio (i.e., wireless) link. At the Class 5 office the call enters the public switched telephone network, and is handled the same way as was a wireline call. (Certainly it gets a lot more complicated. For instance, if the call is to a cellphone, how does the system know where that cellphone is? That, of course, is important -- but does not alter the basic network.) That basic network consists of millions of miles of copper wire and fiber, and can be configured as required by traffic.

Packet Switching and the Internet
Packet switching and the Internet established a second, totally different, network. Let us take the simple case of your computer and your telephone being attached to a wireline local loop, using an ADSL connection. (Note: With ADSL the computer and the telephone are connected to the same local loop, and can be used simultaneously). A request from your computer to be connected to the Internet will travel over that local loop to the nearby Class 5 office. But here the signal is intercepted by a device called a DSLAM (digital subscriber line access multiplexer). And the call for computer service will be directed around the telephone switching equipment, and to special packet switching equipment. The telephone switching equipment in the Class 5 office does not become involved with the connection to the Internet.

Now comes the greatest change in transmission technology. The information being sent from your computer does not travel over a single circuit toward the destination. In fact, it does not even use the hierarchy of switching systems. Rather it is broken up into thousands of tiny packets, each of which carries, in a header, the desired destination. This little packet is sent on its way over whatever circuit is available heading in the proper direction. And then this circuit link is released. The system "hangs up." When this packet arrives at that next office the same thing happens. The header is read again, another circuit link is selected, the packet is moved onward and the circuit link is released. Notice that releasing the circuit after the packet is transmitted saves huge amounts of time. If we think in terms of transmitting digitized voice over this packet network (and please recognize that digitized voice is no different from data) then we no longer have the inefficiency of one speaker or the other -- or both -- remaining silent, and the circuit sitting idle.

The transmission of these packets happens thousands of times; and each packet is considered to be a total message and sent toward the desired destination. Frequently via different routes. When the packets are finally received at the final computer system they are placed in the proper order and delivered to the desired recipient. This entire transmission process takes only a fraction of a second. In fact, latency (the time a packet is held up at an intermediate office) is on the order of a couple of milliseconds.

The country's entire telecommunications system is moving away from circuit switching and toward packet switching. And it is moving fast.

We started this section on packet switching and the Internet by saying that a computer is connected to a local loop which, in turn, is connected to a Class 5 office. In fact, one other link is possible, and it employs radio. We call it a Local Area Network (LAN) and usually think of it as WiFi. A small electronic device, actually a modem, is connected to the local loop, and besides providing a connection by wire to a computer, also transmits the signal using wireless technologies. Computers throughout the coffee shop or the home or the hotel lobby are now connected wirelessly to the telephone line. (It would not be too much of a stretch to say that this LAN does for computers what the cordless telephone does for voice communications.).

Review
Let's review. There are 2 fundamentally different networks making up the country's telecommunications infrastructure: 1) the public switched telephone network, employing circuit switching and the hierarchy of switching systems, and 2) the packet-switched network now finding increasing application.

Subscribers (voice or data) are connected to these networks with wire or fiber, as is the case with wireline telephones or computers, or wirelessly, as is the case with cellphones.

But…
But what happens if the subscriber's instrument is a combination cellphone and a computer? That is exactly what we have in the form of a smartphone. The telephone transmits voice, and we would expect that the cellphone network would be used. But the computer transmits data and computes, and we would expect that the packet switching network would be used. Furthermore, the only wireless data we have spoken of so far traveled only about 30 feet on a Local Area Network (e.g., Wi-Fi). The computer part of a smartphone isn't going to do much good if there is no local area network at hand.

The first widespread use of wireless data began with the RIM BlackBerrry in the mid-1990s, and it employed the cellular network. The trouble is, smartphones generate 30 times the traffic of basic cell phones, and laptops with wireless modems 450 times the traffic. Next came the iPhone, and it simply exacerbated the situation. Cellphone networks (that is, the air interface portion of the network) quickly became overloaded, and we regularly hear about dropped calls and slow download speeds. The service providers are scrambling to update their networks to accommodate this increased traffic.

What's next? There is meter reading, automated cash registers, various book readers, and an endless variety of special applications. That means more large-area cells, and also more small-area cells (called microcells and picocells). And yet an even smaller cell called a femtocell, which can alleviate traffic by plugging into a subscriber's broadband Internet connection. Operators see this as a great way to off-load data onto a wired network.

So What Does This All Mean?
Certainly wireless is king. The wireline business is dying. AT&T is losing about 700,000 landlines per month, and plain old telephone service (POTS) revenue is dropping precipitously.

The air interface of wireless communications is presently the choke point. Mobile data traffic will double every year through 2013. Companies are searching for more spectrum, and technologies that better use that spectrum. The FCC is searching also.

The wireline long distance network is huge and robust, and shows no signs of being overloaded. Millions of miles of copper and fiber are in place, and can be configured to accommodate almost any desired network.

Circuit switching is dead. Packet switching -- for both voice and data -- are taking over. Ivan Seidenberg, chairman of Verizon, said "We can begin to look at eliminating central offices, call centers, and garages."

What’s your take on this subject? Leave a comment and get the conversation going.