ntly in calculating the next hop that should be taken. Traditionally, this was sometimes seen as a disadvantage because it meant that the circuits might not be used as efficiently as possible. Silence Establishing Connections: Networking Modes and Switching Modes accounts for around half of most voice calls. Most people breathe and occasionally pause in their speech. So, when voice communications are conducted over a circuit that's being continuously held, and half the time nothing is being transmitted, the circuit is not being used very efficiently. But remember that this issue is important mainly when bandwidth is constrained. And as mentioned earlier in the book, bandwidth is growing at an astounding rate, through the widespread deployment of fiber optics, SO the efficient use of circuits because of bandwidth constraints will not present the same sort of issue in the future that it once did. Hence, the low latencies or delays that circuit switching guarantees are more important than its potential drawbacks in bandwidth efficiency, and therefore, emerging models for optical networking use circuit switching in the core (see Chapter 11). Circuit switching has been optimized for real-time voice traffic for which quality of service (QoS) is needed. Because it involves path calculation at the front end, you know how many switches and cables you're going to go through, SO you can use a pricing mechanism based on distance and time. The more resources you use, either over time or over distance, the greater the cost. Again, developments in fiber economics are changing some of the old rules, and distance is no longer nec- essarily an added cost element. (QoS is discussed in more detail in Chapter 10, "Next-Generation Networks.") Generations of Circuit Switches Circuit switches have been around for quite some time. They have already been through three basic generations, and we're beginning to see a fourth generation. The History of the Strowger Switch The Strowger switch has a rather amusing history. Once upon a time in t