tanding T1 Duplex A full-duplex link can send and receive data at the same time. As anyone who's ever had a fight with his or her significant other over the phone can attest, both parties 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 con- versation. 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 people 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 21-1 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 fore- ground 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 outbound traffic-perhaps a large email message was sent at that time. 1600.0 k 1200.0 k 800.0 k 400.0 k 0.0 k Figure 21-1. 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 