s on a call can talk (or scream) at the same time. This is a full-duplex conversation. If only one person could talk at a time, the conversation would be half duplex. Using walkie-talkies where you have to push a button to talk is an example of a half-duplex conversation. While the button is depressed, typically, you cannot hear the person with whom you are speaking (although some high-end walkie-talkies can use one frequency for transmitting, and another for receiving, thereby allowing full-duplex conversations). T1s are full-duplex links. Voice T1s transmit and receive audio simultaneously, and data is sent and received simultaneously on WAN-link T1s. Still, I've met many peo- ple in the field who don't understand T1 duplex. This may have a lot to do with the way data flow across WAN links is commonly reported. Figure 20-1 shows a T1 Internet link's bandwidth utilization as monitored by the Multi Router Traffic Grapher (MRTG). The numbers on the bottom of the graph are the hours of the day, with 0 being midnight, 2 being 2:00 a.m., and SO on. The solid graph, which looks like an Arizona landscape, is the inbound data flow. The solid line in the foreground is the outbound data flow. This is a typical usage pattern for a heavily used T1 Internet link. The bulk of the data comes from the Internet. The requests to get that data are very small. At about 8:45 a.m., there's a spike in the out- bound traffic-perhaps a large email was sent at that time. 1600.0 k 1200.0 k 800.0 k 400.0 k 0.0 k Figure 20-1. MRTG usage graph The graph does not make obvious the duplex mode of the link. One could easily assume that the T1 was half duplex and switched from transmit to receive very quickly. If someone who didn't understand the technology only saw graphs like this one, he might conclude that a T1 can only send data in one direction at a time. You'd be surprised how common this misconception is. Types of T1 Terminology is important when discussing any technology, and T1 is no exception. Many terms are commonly misused, even by p