Telecommunications (the transmission of digital data by computers) is accomplished either by connecting a computer directly to the digital data portion of the telephone network, or by transmitting the data over the voice network using modulation technology.
Modulation involves transmitting a carrier signal, that is, a tone of a particular frequency, and then varying it according to the rules of a particular modulation protocol. Modulation
protocols dictate the use of techniques that vary the carrier signal amplitude, frequency, phase, or other properties, often in combination. Simplex data transmission occurs when there is only one carrier signal on the media stream, so data transmission can take place only in one
direction. The term half-duplex means that the devices at either end of a simplex media stream are able to take turns transmitting and receiving. Full-duplex communication takes place when two different carrier frequencies are used on the same media stream so that data can be both transmitted and received simultaneously. Demodulation involves decoding the modulated carrier signal into the original digital data stream.
A modulator-demodulator, better known as a modem, is a piece of hardware or software that is able to perform the modulation and demodulation of signals, given full access to a media stream. Hardware-based modems are typically packaged along with all of the electronics needed to establish and gain access to the media stream on a call. Software modems, on the other hand, depend on independent hardware and software mechanisms to deliver the required isochronous media stream data.
The modulation protocols and other related standards that allow modems from various vendors to interoperate are established by the ITU-T and are assigned names beginning with "V".
Commonly used V-series standards include V.32, V.32bis, and V.34, which define full-duplex 9600 bps, 14400 bps, and 28800 bps protocols, respectively. The V.42 standard defines error detection used with V.32, and the V.42 bis standard defines the compression algorithm for use with V.42. The use of compression allows data transmission rates that are much higher than the raw data rates of the modulation protocols. The V.17 standard defines the modulation protocol used for the exchange of documents between Group 3 fax products.
3.3.4 Directional Streams
By default, all connections and their associated media stream channels are bidirectional; that is, there is a media stream associated with the call that is flowing to the device, and another media stream flowing away from the device.
Some switching implementations are able to create unidirectional connections. In these
unidirectional connections there is only one media stream, and it flows either toward or away from the device. Figure 3-11 illustrates the unidirectional streams towards and away from a station device.
Figure 3-11 Directional connections
3.3.5 Symmetric and Asymmetric Communication
All connections and associated media stream channels in a particular call are also symmetric.
Symmetric connections involve media streams that deliver data at the same rate in both directions. In an ordinary telephone call, this means that the same amount of sound information is being sent in both directions.
Asymmetric communication, where the data rate in one direction is different from the other, is abstracted as two separate calls consisting of unidirectional connections. In Figure 3-12, the media stream associated with one call carries high-speed data while the other carries low-speed data in the opposite direction.
Figure 3-12
Asymmetric communication
3.3.6 Point-to-Point and Multi-Point Calls
Most calls are point-to-point calls. This means that the call involves associating two devices with two connections. However, most switching implementations support calls with three or more connections. These are referred to as multi-point calls.
A multi-point call is generally referred to as a conference call, although the term bridged call is also used when the implementation is hardwired in some way.
In the basic case of a multi-point call consisting of bidirectional voice connections, all of the media streams coming from the associated devices are combined and the result is sent back out to all the devices. (Some implementations may use signal processing capabilities to eliminate echo effects and send a slightly different media stream to each device.). All devices "hear"
everything on the call.
Multi-point calls are applicable to voice calls only. Audio is a unique data type in that in its analog representation it may simply be mixed with other audio sources and the result is still a useful audio stream. The same cannot be done with digital data streams or modulated data streams. Adding these data streams together would simply corrupt the data. Synthesizing multi-point calls for digital data and large numbers of voice calls is described below.
Of course, every connection in a multi-point call need not be bidirectional. A multi-point call can be made of any combination of bidirectional and unidirectional connections as long as the media streams are all voice. Three special cases of unidirectional connection usage in a multi-point call are discussed below.
Silent Participation: Tapped Calls
One special case of a multi-point call is referred to as silent participation and is illustrated in Figure 3-13. In this case, a unidirectional connection is added to a call such that the new device can hear everything on the call but cannot be heard by the others participating in the call. This also is commonly referred to as a tapped call. Silent participation is very useful in situations where media resources are to be connected to a call for the purpose of recording audio or doing speech recognition, for example.
Figure 3-13
Three-point call with silent participant
Announcement
Another special case of multi-point calls is the announcement. As shown in Figure 3-14, this involves the addition of a unidirectional connection to the call that delivers a media stream from the added device. This is used primarily for adding a media access resource that plays an announcement.
Figure 3-14
Three-point call with announcement
Broadcast
A third special case of multi-point calls is the broadcast. In this case, all of the connections in a call are unidirectional as depicted in Figure 3-15. Exactly one of the connections is
unidirectional away from its device and all the others are unidirectional toward the devices.
This allows one media stream to be delivered to many devices.
Figure 3-15
Three-point call with broadcaster and listeners