more susceptible to delays inside a packet switch. Packets are received, checked for errors, stored until transport space is avail- able on an appropriate route, and then sent. This notion of keeping the packets inside the machine until there are transport resources to send them is called store-and- forward technology. This gives packet links extraordinary efficiency compared with circuit-switched data services, especially for bursty data such as Web surfing, but it does mean that a packet can wait for a long time to get out of a packet switch. Since data connection can have several packet switches in its path, the store-and-forward de- lays can add up to significant latency. The relationship between the congestion of a packet switch and the latency of its pipes is discussed in Sec. 23.3. The other time issue in data connections is that all the data packets in a single trans- mission do not take the same amount of time to get to their destination. Packets are routed based on available transport, so some packets may follow a less direct route than others. The physical length of the packet pathways varies, and the number and congestion of the packet switches vary as well. Not only might packets arrive with dif- ferent delays, they also may arrive in a different order. Thus the packet receiver has the responsibility not only of receiving all the packets of a transmission but also of putting them back together in order to form the same data message that was sent. The variability of packet delay is called jitter. 6 5Increased capacity per cell can reduce the need to deploy new base stations as demand grows. 6 Jitter is primarily a packet issue, but it does come up in high-performance data circuits as well. We hear a great deal about jitter, equally spaced data bits coming at unequal time intervals, in audio compact disk playback technology. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to