received in the bitstream reordering in accordance with sequence numbers may be called for in some circumstances the filter determines 214 if network layer content e g a message unit carried by the data link packet or frame is unnecessary for example if it is a maintenance message that is a repeat of a preceding message unit if the current data link frame is determined 214 to be unnecessary e g a redundant maintenance message then the filter 160 deletes or omits 216 from the packet sequence the unnecessary packet or frame carrying the repeat maintenance message unit any remaining or undeleted message units are put 218 into hdlc data link packets including 0 bit stuffing the packets are then transmitted 220 as a bitstream the process of fig1 includes steps that may not be essential for filtering but are helpful in understanding the conceptual effect of the filter 160 for example the putting 218 packetizing framing may not actually be necessary because a frame being passed can simply be sent as received the filter looks into mtp level 3 information to prevent certain information from moving up the protocol stack of the downstream signaling device 200 fig1 shows a detailed embodiment of a filter 160 the embodiment in fig1 may be readily implemented for example using a field programmable gate array fpga an input module 230 receives incoming data and has functionality for serial to parallel conversion flag detection zero deletion and sequence number comparison an input shift register 232 and counter module or state machine 234 are provided for maintaining input state information a temporary fifo 236.