In some cases, two or more conversations were taking place over a single radio channel at the same time. Users on Tactical 1 may have been close enough to one another to communicate because signals in proximity to each other would overpower weaker signals. At any incident of this size, there is likely to be some overlapping radio traffic. In the same way that large incidents exhaust all the firefighting vehicles and staff, the radio channel resources may become taxed to their limits. NIST says about one third of the fire department radio transmissions were not complete or not understandable. Some radio users had selected the wrong channels. For example, on the Repeater 7 channel, a unit was heard to call Manhattan dispatch and Citywide. Although the circumstances that led to the user selecting the wrong channel are not known, this can occur when the user is trapped in darkness or smoke and cannot see the radio. Users will typically try to count steps in a rotary switch channel selector starting from one end of the switch's travel. A communications van operated by FDNY responded to the incident. Its radio identifier was, "Field Comm." A backup van was in use on the day of the incident because the primary van was out-of-service. The backup van was destroyed and audio recordings of tactical channels used at the incident site were lost. FDNY radio programming One annoyance with the fire systems was the presence of unit ID data bursts. These constant squawks, heard at the end of transmissions, are decoded at dispatch to identify the calling radio. The annoyance of the data bursts is a trade-off that could help find a firefighter who has been injured or needs help. It also automatically displays the unit ID at the dispatch console. In most systems, it also saves dispatch personnel from typing the unit ID. They press one key and the calling unit's ID is inserted into the current CAD screen or command line. Recordings show radios were programmed to send unit ID on tactical channels. Radios accept unit ID on a per-channel basis. When mobile or hand-held radios are programmed, the unit ID encoders should be disabled on all channels where the feature is not used. This saves air time for about two to three syllables of speech per push-to-talk press. For example, unless the communications van or chief's vehicles had push-to-talk unit