Circuit and Packet Switching Network Options How telecom networks work and how they are evolving for voice, data and video.
By: John Shepler
There have been two major network technologies slugging it out for supremacy over the last few decades. The first is the original telephone architecture that has served us well over the last century of the industrial age. The newcomer is the computer-centric architecture that has been the driver of the information age. You may also know them as circuit switching and packet switching.
While it’s likely that both technologies will continue to coexist for some time to come, we’re clearly at an inflection point. From here on out, packet switching will dominate on every network from copper to fiber to wireless. Let’s see why that is.
What is Circuit Switching?
Circuit switching is a telephone technology. It’s a logical system for that application. Telephone calls require a connection between two devices for a given amount of time. Specialize phones like intercoms can be connected permanently or “nailed up.” For general use it is desirable that any phone can be connected to any other, but needs that connection only for the duration of the call.
That’s the function of the central office switch. It physically ties together the lines running to any two phone sets using electrical switches. At one time multiple phones in a neighborhood would share a line to the central office. This was called a “party line” and allowed anyone to eavesdrop on their neighbor’s calls. As automatic dialing was introduced, the party line fell out of favor and each location had its own phone line with its own phone number. When phones where connected, they had their own unique electrical circuit. This is where the term circuit switching comes from.
Circuit Switching For Digital Lines
When long distance telephone transmission migrated from analog to digital, the concept of circuit switching was kept. Take the T1 telephone trunk, for instance. Each T1 line is segmented into 24 separate channels. Each of those channels carries one phone conversation exactly as if they were on separate wires.
T1 lines found a new application to interconnect computers to let them “talk.” Each telephone channel only supports 64 Kbps of data… dial up speed. A broadband circuit can be created by combining all the channels to make a single 1.5 Mbps transmission line. Like the original analog telephone lines, the circuit from computer to computer needs to be connected or “nailed up” as long as the machines want to talk. Circuit switching connects point to point, but what if you want many machines to be able to communicate at the same, or nearly the same time?
What is Packet Switching?
Packet switching was developed to allow information to be sent among many machines. In a way, it’s the party line re-imagined. Unlike the old analog party line, multiple “conversations” need to occur simultaneously without interference. You can’t do that if all devices simply spew bits onto the line indiscriminately, but you can if each if the bits for each network node are packaged into self-contained units… packets.
Packets contain chunks of data. They also contain address bits that tell where they are coming from and where they should go. This is what makes packet switching possible. Instead of dialing a phone number to determine what circuits should be tied together, network switches and routers direct the packets on their journey according to the addressing contained in the packets. You can have packets from many different devices on the network line at the same time as long as the traffic is well controlled and each packet tells where it needs to go.
Circuit vs Packet
For a long time now, computer based packet networks and telephone based switched circuit networks have coexisted. The largest circuit switched network is the PSTN or Public Switched Telephone Network. The largest packet switched network is the Internet. What’s happening now is that these networks are being combined.
It is more efficient to have one network instead of two to maintain and there are additional services you can have when voice and data share the same network. But, which technology is best, circuit or packet switching?
And the Winner Is….
Packet switching is clearly winning out and has been since computer traffic has exceeded voice traffic worldwide. Computer traffic now includes digitized video which is more dominant and voice and data combined. All of these share the Internet and private networks using packet switched technology. As wireless voice moves from dedicated cellular to packet transmission over 4G networks, the transition from circuit switched networks to packet switched networks will be complete.
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