Packet-switching technology remedies TDM’s empty-timeslot inefficiency problem and adds capacity. With packet-switching, large amounts of information from multiple sources can be converted into smaller packets to be placed onto the same transmission stream. In contrast to TDM, packet-switching does not require fixed timeslots so any number of connections can use the same channel in any order at any time.
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Figure 3: Packet-switching carries multiple transmission streams over a single circuit to maximize efficiency and eliminate wasted slots.
We can think of packet-switching as a kind of conveyor belt in a warehouse. Just as physical products can be selected from the warehouse and placed on a conveyor belt every time a customer makes an order (e.g., “Send this file,” “I want to receive my emails,” etc.), any data packet that is ready can be placed in the microwave transmission link and transmitted in the next burst. Just as each physical product carries order information so that it can be placed in the shopping cart of the correct customer, each data packet contains information such as the destination address, and sequence number so that it can be mixed with any other packet, transmitted in the next available burst, and then re-constructed at the receiving end and sent to the proper recipient (see figure 3). Packets can enter the stream in any order; as long as a sender has something to send, the information payload can be packetized and sent out as soon as there is available bandwidth to carry it—a place on the conveyor belt. Packets from different end-to-end connections can all share the same wireless link, making the most effective use of the bandwidth.
Wireless networks can deteriorate or fail. Weather conditions can interfere with, or even terminate, communications for periods of time. In order to provide safeguards against failure, network operators employ backup schemes, for example dividing the capacity of a physical wireless link into two separate carriers where one carrier is on stand-by to back up the other automatically in case of failure. In this way, the network operator can keep the link operational virtually all of the time, but at the cost of 50% of the total capacity. Noticing that carriers rarely fail, operators can adopt more efficient backup schemes such as 7+1, in which the capacity of the wireless link is divided into eight carriers. Seven of the carriers are multiplexed circuits used to carry traffic, while the remaining carrier acts as a backup in case of failure of any one of the others. In this way, only one-eighth of the total capacity of the wireless link is sacrificed to backup. If two carriers fail simultaneously, however, there will be no backup available for one of them.
By their nature, packet-switched links are more resilient than circuit-switched links. For example, in a TDM network, if weather impacts a carrier even to a small degree, the carrier will fail completely and will have to rely on an available backup carrier to continue. In the more resilient packet-switched network, however, inclement weather might reduce the capacity of a carrier, but the transmission can step down automatically to a slower speed, boosting signal strength in order to maintain transmission leaving high-priority traffic unaffected.
Weather conditions are not the only differentiator between TDM and packet-based links. In TDM, all transmissions follow in sequence—a large file is sent in order. Packet technology has a better way: The file is broken up into packets in which each packet can follow a different network path and even arrive out of order. Equipment on the receiving end contains logic for re-constituting the packets in proper order to re-create the file as it was originally sent. In a case in which many carriers transmit over one physical wireless link, failure of one carrier does not stop packet transmission that can proceed on any of the other available carriers. In fact, packet technology is so flexible that, just like in our warehouse example, packets from different transmission streams can be placed on any available carrier in any order, maximizing the utilization of the capacity of the wireless link at any given moment.