could you tell me how to dissect fp over udp？

You can't dissect FP over UDP and the higer layers without prior knowledge of the content/Configuration hence the FP dissector requires a "per-packet" structure to be filed in in order to do the dissection. The NBAP dissectore has code to set up a conversation and fill parts of the required information should NBAP signaling be present in the trace(Work in progress). There is provissions for a pseudo UDP FP format should your equipment be able to produce it. See the wireshark wiki and simmilar questions asked before. The FP-hint dissector also handles some pseudo format. It might be possible to constuct a heuristic FP dissector but it might requre "three-pass" dissection. 1) Use the header CRC to confirm FP packet. 1b)Use the command code to find the channel type. If it's not a DCH channel there might be enough information to dissect the rest of the packet.