Demand for capacity constantly accelerates. Mobile telephone subscribers insist on more data-rich services, private network operators enhance the information flow of their networks, public safety agencies continue to improve their services to citizens. There is no end in sight to the ever-increasing requirement for capacity. This unabated demand for network capacity necessitates more efficient use of essential wireless links and is fueling the rush to technologies that make more efficient use of this valuable resource. One method is to switch from time-division multiplexing (TDM) to more effective packet-based technology. Multi-carrier adaptive bandwidth control (ABC) represents the next step in the evolution, boosting the efficiency, cost effectiveness, and resilience of wireless links while simultaneously helping operators achieve better quality of service.
Figure 1: Demand for mobile data accelerates.Circuit switching and multiplexing
In circuit-switching technologies, a circuit is set up at the beginning of a connection and is maintained for its duration. Network resources at the sending and receiving end are reserved and dedicated; the source, information payload, capacity, path and destination are pre-determined. Over the circuit, the information that the sending and receiving stations will transmit is always at the pre-determined rate with no capability for change providing no flexibility in the face of fluctuating transmission conditions.
Circuit switching makes sense for traditional voice traffic where people make a connection, talk back and forth, and then close the connection when they are finished. For the purpose of transmitting data traffic, however, this is not an efficient method because the approach ties up network resources even when there is nothing to transmit; people stay online with the Internet, for example, even though they may not be accessing data.
Multiplexing increases the efficiency of circuit switching. In multiplexing, multiple communication streams are combined into one physical connection. Having a single circuit carry more than one stream more efficient use of network resources—if one stream is not transmitting at a given moment, perhaps other ones are.
TDM is the most widely used method of multiplexing for the transmission of digital signals in circuit-based networks. It is extensively deployed in legacy short- and long-haul wireless links.
In TDM, a transmission burst is divided into some pre-determined number of slots whereby each stream is allowed to insert its information into the appropriate slot. For example, consider two transmission streams—file downloads and voice conversations. To multiplex them over a single physical transmission circuit, we first create two fixed-length slots in each burst. Stream 1’s information (file download) will be placed into the first slot and Stream 2’s information (digitized voice) will be placed into the second slot. The ensuing transmission burst will send the information of both slots over the same circuit. At the receiving end, the information in the first slot will be separated from the information in the second slot and each will be sent onto the proper recipient.
Without multiplexing, two circuits would have to be dedicated for these two streams. If a stream had nothing to send in a given burst, the bandwidth of that circuit would be idle. With multiplexing, we can send both streams over the same circuit. If one of the streams has nothing to send while the other does, one of the slots would be wasted, but we would still be making use of the circuit to transmit the information of the other stream. This is highly simplified, of course. In actual multiplexing transmission schemes, eight or more streams can share slots so that bursts seldom go completely wasted.
Although multiplexing improves the overall efficiency, circuit-switching still suffers from inefficiency when one or more streams have nothing to include in their timeslots which remain empty for many transmission bursts (see figure 2). Overall, the circuit is being used, but timeslots are frequently wasted. If this happens often, the operator is not making good use of this critical resource.
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Figure 2: TDM multiplexing example with three transmission streams over one circuit.