(the "Switched" network is primarily used for Voice - the PSTN - or Public Switched Network)
see also the North America Numbering Plan Administration, NANPA
Switched vs Data - the major provider backbones are comprised of two types of networks - Switched, and Data. There are normally several Data networks - one private network with point-to-point dedicated transmission lines, and a few shared access packet/cell-based networks such as the older X.25 networks, Frame Relay/ATM networks, IP Intranets, and a large portion of the U.S. IP Internet. There is usually only one switched network, comprised of ESS or DMS250 switches, and switches which act as gateways for international-bound traffic.
Confusion about the name - the name, "switched network" creates confusion, and it is very important to gain an understanding of this. Originally, there were only two networks - the switched network (voice) and the data network (Clearline, "private lines"). The voice network used switches to route voice traffic. The private line (Clearline) data network, had no switches, and instead used multiplexers and DCS units (Digital Cross-connect System), which permanently "nailed up" a connection to transport data..
In todays world, the X.25, Frame Relay, and ATM networks also use "switches" to route the traffic. This is where the confusion comes into play. You can simply memorize not to use the term "switched" for these networks which obviously use switches. However it is best to delve a bit more deeply, and understand the differences between the networks.
How to Distinguish between the two Networks - the "switched network" uses electrical switches that act as relays - it actually sets up real circuits . . . a direct electrical connection through a series of switches from point A to point B. This switched path is a temporarily "nailed up" connection - set up solely for the two parties to communicate through, and it is not shared by anyone else. The "data network", on the other hand, uses "virtual circuits" which communicate by sending packets of data. They share the path across the network with other virtual circuits. In both networks - point A can communicate with point B.
The switched network uses real circuits with continuously flowing data, while the data network uses virtual circuits with sporadically flowing data in the form of packets (actually, packets are used for IP, frames for Frame Relay, and cells for ATM)
To memorize which is which:
Switched Network - uses electrical switches, or "relays" to nail up a connection for the duration of a conversation. The switches allow data to flow directly through - unimpeded. The continuous signal flows directly from point A to Point B. There are transfer points but the data flows right through them.
Data Network - uses digital data switches to create a "virtual" connection which breaks up continuous data into a series of data packets before sending. The switches stop each packet, inspect it, and send it along to the next transfer point. The data units begin their journey at Point A, make stops at a number of transfer points (the digital switches), and are received at Point B and then reassembled into a continuous stream of data.
These virtual circuits sends packets or cells of data across the path, and they often are delayed for a short time - which is usually no problem since data is sent sporadically. The switched network presents almost no delay, since the data flows directly through the switches, unimpeded.
Why Voice generally "requires" a Switched Network - voice, unlike data, is a continual flow of modulating audio tones. The switched network, with it's real electrical circuits, is perfect for this type of information flow, as it presents zero delay (almost zero). The data network has stopping points along the way, where the digital switch inspects the packet before sending it on it's way. Even a moderate delay in the transmission, and the receiver will hear crackling or popping sounds. Unless these stopping points are utilize powerful, high-speed hardware - the receiver will hear such static. The same is true for video - although the "static" comes through as small flecks of snow in the picture and is more easily tolerated.
Today's data networks have come a long way. They now are comprised of high-speed, powerful switches and routers. They also use techniques such as sending the packets on their way before they have fully inspected them. Voice can now be carried across Frame Relay, ATM, and IP networks. However, after years of development and research, voice over data networks still suffers some degradation. In addition, switched network prices have come down dramatically, to the point where voice over data networks is rarely ordered by customers. Many of the voice over IP companies have gone bankrupt during 2001, and the rest have suffered significant layoffs.
Other types of Switched Traffic - The switched network is the original voice network that the IXC companies' businesses were based on. Over the last 10 years, the "voice network" has gone through major technological transformations. It can now carry digital data through modulation of sine waves by modems, and through the use of ISDN (Integrated Switched Digital Network) and more recently, the many flavors of DSL (ADSL, HDSL, VDSL, etc.). In addition, video can be provisioned across ISDN, or muxed switched 56k circuits. Since the network carries both voice and data, it cannot be called either - and the old voice network has been renamed - to the "Switched Network".