to multiplexer. To do this, it adds a set of bytes that allow receiving devices to find the payload inside the SONET frame. As you will learn a bit later, the payload can occasionally wander around inside the frame, due to the vagaries of the network. These bytes allow the system to track that movement. In addition to these "tracking bytes," the Line Overhead includes bytes that monitor the integrity of the network and have the ability to effect a switch to a backup transmission span if a failure in the primary span occurs, as well as another bit-error checking byte, a robust channel for transporting network management information, and a voice communi- cations channel that allows technicians at either end of a the line to plug in with a handset (sometimes called a butt-in, or buttinski) and commu- nicate while troubleshooting. The final step in the process is to add a layer of overhead that allows the intermediate repeaters to find the beginning of, and synchronize, a received frame. This overhead, called the Section Overhead, contains a unique initial framing pattern at the beginning of the frame, an identi- fier for the payload signal being carried, another bit-error check, a voice communications channel, and another dedicated channel for network management information, similar to, but smaller than, the one identified in the Line Overhead. The result of all this overhead, much of which seems like overkill (and in many peoples' minds is), is that the transmission of a SONET frame containing user data can be identified and managed with tremendous granularity from the source all the way to the destination. So, to summarize, the hard little kernel of DS3 traffic is gradually sur- rounded by three layers of overhead information, as shown in Figure 3- 53, that help it achieve its goal of successfully transiting the network. The Section Overhead is used at every device the signal passes through, including multiplexers and repeaters; the Line Overhead is only used between multiplexers; and the information contained in 