Previous | Table of Contents | Next |
Statistical Time Division Multiplexing (STDM) is much improved over TDM, as the MUXs are intelligent. STDMs, or Stat MUXs, offer the advantage of dynamic allocation of available channels and raw bandwidth. In other words, STDMs can allocate bandwidth, in the form of time slots, in consideration of the transmission requirements of individual devices serving specific applications (Figure 2.12). Further, an intelligent STDM can dynamically adapt to the nature and associated requirements of the load placed on it, and in consideration of the available capacity of the network.
Figure 2.12 Statistical Time Division Multiplexing (STDM) in a data communications application.
Stat muxes can recognize active versus inactive devices, as well as priority levels. Further, they can invoke flow control options that cause a transmitting terminal to cease transmission temporarily in the event that the MUXs internal buffer, or temporary memory device, is full. Flow control also can be used to restrain low priority transmissions in favor of higher priority transmissions. Additionally, STDMs may offer the advantages of data compression, error detection and correction, and traffic statistics reporting
T/E-Carrier, which will be discussed in detail in a later chapter, relies on STDMs. The high-speed, four-wire digital circuit typically is divided into multiple time slots to carry multiple conversations. As an example, T-1 (US) provides 24 time slots to carry 24 conversations, each a maximum of 64 Kbps. E-1 (European) provides 30 time slots to carry 30 conversations
Additionally, the individual channels can be grouped to yield higher transmission rates (superrate) for an individual, bandwidth-intensive communication such as a videoconference. The individual channels also can be subdivided into lower speed (subrate) channels to accommodate many more, less bandwidth-intensive communications, such as low speed data. Additionally, many MUXs also allocate bandwidth on a priority basis, providing delay-sensitive traffic (e.g., realtime voice or video) with top priority in order to ensure that the resulting presentation of the data at the receiving end is of high quality.
Although they are not yet in common usage, Wavelength Division Multiplexers (WDMs), allow multiple high speed channels to be supported over a single fiber optic transmission system. This is accomplished through the transmission of multiple frequencies (wave lengths) of light, much as multiple electrical frequencies can support multiple, simultaneous conversations in a FDM transmission system. For instance, multiple 2 Gpbs channels can be accommodated over a 8 Gbps fiber optic system through the use of four different frequencies of light.
Inverse Multiplexers perform just the inverse (opposite or reverse) process as do traditional MUXs. In other words, they accommodate a single, high-bandwidth data stream by transmitting it over multiple, lower-bandwidth channels or circuits (Figure 2.13). The transmitting MUX segments the data stream on a consistent basis, while the receiving MUX reconstitutes the composite datastream. Clearly, the two devices must be synchronized carefully with each other and with the transmission characteristics of the individual paths and channels to minimize errors and delays. An individual communication might be spread over multiple switched circuits, dedicated circuits, or channels on multichannel circuits. For instance, a low-speed video conference at 128 Kbps might be sent over 2 separate 64 Kbps channels on an ISDN circuit.
Figure 2.13 Inverse Multiplexing in a videoconferencing application.
A large number of manufacturers now offer MUXs that allow data to be sent over voice lines and voice to be sent over data lines. For instance, a digital data circuit also can accommodate voice (for which it was not intended) through the use of a special MUX which digitizes the voice signal and transmits it over a data circuit; the reverse process takes place at the receiving end. The voice and data conversations share the same circuit, sequentially, rather than simultaneously. Bandwidth is allocated as appropriate, with priority provided to the delay-sensitive voice traffic.
While such an approach is somewhat unusual, it allows the user to take advantage of excess capacity on a dedicated circuit. It also can be used to support both voice and data communications over a single circuit-switched analog circuit. There is, of course, an investment required in the multiplexing equipment, although such equipment, increasingly, is quite affordable.
In the very recent past, several manufacturers have developed MUXs which allow voice to share excess capacity on a Frame Relay network. While the quality generally is not remarkable due to issues of data compression and delay, the voice conversation essentially is free.
Switches serve to establish transmission paths between terminal devices (transmitters and receivers) on a flexible basis. They effectively serve as contention devices, managing contention between multiple transmit devices for access to shared circuits. In this manner, the usage and cost of expensive circuits can be optimized based on standard traffic engineering principles. Without switches, each device would require a direct, dedicated circuit to every other device. Such a full mesh network clearly is resource-intensive, impractical and even impossible, as early experience proved. This discussion of switches and switching is presented in chronological order of development, beginning with circuit switching and its evolution, and progressing through packet to frame and cell switching.
In the classic sense, circuit switching provides continuous and exclusive access between physical circuits for the duration of the conversation. Contemporary circuit switches provide continuous access to logical channels over high-capacity physical circuits for the duration of the conversation. While circuit switches originally were developed for voice communications, much data traffic currently also is switched in this fashion.
Previous | Table of Contents | Next |