Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 10
Circuit and Packet Switching

TCP/IP allowed for open communications to exist and for the proliferation of LAN–to–LAN and LAN–to–WAN connectivity between multiple operating environments. Its topology and architecture, however, were not based on the methods employed by the phone company: circuit switching.

The phone company (AT&T, before the breakup) basically laughed at the idea of a packet switched network and publicly stated that it could never work. A network whose transmitted information can find its own way around the network? Impossible! A network in which every transmitted packet of information has the same chance for forwarding? The phone company maintained its stance that circuit switching was the only method that should be used for voice, video, or data. Circuit switching by definition provided guaranteed bandwidth and, therefore, Quality of Service. At that time, the phone company was correct, but only for voice. Voice and video cannot withstand delay beyond a small time frame (about 150 milliseconds, or 0.150 seconds), but data could! In packet switching, the path is found in real time, and each time the path should be the same, but it may not be. Still, the information will get from point A to point B.

There are many differences between circuit switching and packet switching. One is that in circuit switching, a path is prebuilt before information is sent, whereas packet switching does not predefine or prebuild a path before sending information. For example, when you make a phone call, the phone company physically builds a circuit for that call. You cannot speak (transmit information) until that circuit is built. This circuit is built via hardware. This path is a physical circuit through the telephone network system; however, the phone company is currently employing other technologies to allow for “virtual circuit switching” through technologies such as Asynchronous Transfer Mode, or ATM (beyond the scope of this book). For our comparison, a voice path is prebuilt on hardware before information is passed. No information is contained in the digitized voice signal to indicate to the switches where the destination is located. Each transmitting node has the same chance in getting its information to the receiver.

In packet switching, the information needed to get to the destination station is contained in the header of the information being sent. Stations, known as routers, in the network read this information and forward the information along its path. Thousands of different packets of information may take the exact same path to different destinations.

Today we are proving that not only is packet switching viable, it can be used for voice, video, and data. Newer, faster stations on the network along with faster transmission transports have been invented. Along with this are new Quality of Service protocols that allow priorities to exist on the network. This allows certain packets of information to “leapfrog” over other packets of information to become first in the transmission.