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Demystifying Voice & Data -- Part 4


As an IT/telecom professional, you are likely to have a good understanding about the theory and principles of voice and data. However, even if you are an expert, at some point in time, you will need to discuss or explain voice, data and wireless principles to your nontechnical, line-of-business colleagues. The intent of this, and the other posts in this series, is to help you demystify technical concepts for nontechnical colleagues, customers, clients, and so on.

Each article covers a couple of topics, and tries to explain these in nontechnical terms. While the examples and analogies may not always be 100% applicable, they should provide some perspective -- i.e., highlight the differences and advantages.

For my previous posts in my "Demystifying Voice & Data" series, see:

In this article, I will demystify the topic of data communications.

Data Communications
As I explained in my previous post, circuit switches establish a dedicated circuit or channel for the duration of the transmission or call. Once the call ends, the switch relinquishes the dedicated circuit so it is available for use by the next call. Circuit switching serves primarily to carry voice communications. It works well for this, since a carrier can set up a dedicated circuit for a voice call's duration (i.e., two to five minutes), tearing it down when the call ends and freeing up the bandwidth for the next call.

While practical for voice communications, circuit switching does not work well for data communications, which, in the early days, required a dedicated circuit from the terminal to the mainframe computer. In those days, you would have either needed to sign on to the computer for hours at a time, or sign off and on throughout the day.

Using a dedicated circuit for such a use case was cost-prohibitive. Back in the '70s and '80s, the long-distance cost per minute was about $1.25/minute (inflation adjusted). At that rate, keeping your terminal connected for nine hours a day would have cost your company $675. Clearly, this was not an effective way to establish data communications and companies looked for alternatives.

X.25 packet switching, another technology available at the time, sent data over the public switched telephone network. Old timers may remember the Texas Instrument TI 700, a then state-of-the-art terminal that could transmit and receive data at 30 characters a second -- an unheard of rate at the time. But companies gravitated toward two other alternatives for data communications: private networks of dedicated circuits or frame-relay switching.

Dedicated Circuits - With circuit-based switching not economically viable or practical, carriers began offering dedicated point-to-point circuits from branch offices to headquarters. While still pricey, these circuits were less expensive than the alternatives. However, the dedicated route required companies to buy numerous point-to-point circuits from a number of branches to headquarters.

For example, should a company want to connect a branch office in Seattle to its headquarters in New York, it would request an end-to-end circuit from a long-distance carrier like AT&T. That dedicated line would comprise three primary components:

AT&T would order and manage all three circuits. If the company experiences a problem with this dedicated, point-to-point circuit, it would call AT&T for resolution. If the problem did reside within its network, AT&T would coordinate service with the local telephone companies to troubleshoot their circuits.

The only traffic traversing the three circuits comprising the end-to-end connection would be the company's own. These would be dedicated, not shared, pipes.

Frame Relay - This is a packet-switching protocol for transmitting and receiving data. Frame-relay switches chunk data up into variable size frames, or packets, for transport across the carrier's network from one site to another. Frame relay came into play for companies leasing dedicated circuits from long-distance carriers and setting up their own LANs and WANs.

So, data originating at the branch would get sent over a dedicated line from there to AT&T's Seattle PoP, where a frame-relay switch would disassemble and packetize the data for transmission over various routes and circuits to the destination PoP in New York. This type of internal network, on which packets can move along a huge variety of paths, is often referred to as a "cloud." A frame-relay switch at the destination PoP would reassemble these packets (frames) and send them over the dedicated local line to the company's headquarters. Alternatively, depending on the site equipment, the disassembly/reassembly could occur at the customer site.

The cost advantage of frame relay over dedicated circuits is obvious. By sharing resources (packets/cloud) and eliminating dedicated circuits, the cost of transmitting data from Point A to Point B drops dramatically.

Note that there was nothing inherently wrong with the previous technological solution. It's just that the newer technological solution proved much cheaper. This pattern repeats itself over and over in the IT/telecom industry. In fact, frame relay is now considered obsolete. The carriers have and are in the process or dismantling their frame-relay networks.

Also note that packet switching and frame relay were designed for data communications, not voice.

In the next piece in this series, I will go over the current methodology of handling data communications, IP.

"SCTC Perspectives" is written by members of the Society of Communications Technology Consultants, an international organization of independent information and communication technology professionals serving clients in all business sectors and government worldwide.