# Secure Inter-Frame Space Communications for Wireless LANs
## 3 Secure Inter-Frame Space Communication
### Implementation of Secure Inter-Frame Space Communications  
The periodic interference detection bit is determined and updated every 0.4 \(\upmu\)s at the PHY. This one-bit data does not really determine the existence of the periodic interference signal, but rather the existence of a powered signal. For the determination of the periodic interference signal, this one-bit data stream needs to be saved and the periodicity of pulses should be searched to decide the existence of a periodic interference signal. Since the periodic pulses occur for durations as large as 100 ms, it is difficult for PHY to handle this long period, which is well beyond one PHY packet period. Note that the PHY enable signal is on only during PHY transmit and receive packet periods. The MAC layer usually deals with a longer period enabled continually. The data stream of the single periodic interference detection bit is passed to the MAC layer. This interface bit is added to the existing MAC-PHY interface. The interface change is not easy once the MAC-PHY data interface is fixed. However, the periodic interference detection bit has nothing to do with WLAN packet communication. If they share the same interface, some information can be lost. For example, while WLAN packet data is being transferred to MAC, the periodic interference detection bit may lose the time to smear on the same interface path. The opposite scenario is also possible. The existing data interface is based on the packet structure. When a packet is received, PHY starts to transfer the header information with packet length information and then the received data. Putting the periodic interference detection bit data into this structure may delay the transfer of the data. It should be noted that the periodic interference detection bit does not have the pulse length information. Although the pulse length is not used for deciding the periodic interference signal, it may give additional information about the periodic interference signal. If the pulse length is excessively large, the pulse can be eliminated from the periodic interference signal category. Once the periodic interference detection bit stream is passed to the MAC layer, it should be stored so that the upper layer fetches the stored data. This raw bit stream has one bit per 0.4 us, which provides 2500 bits per millisecond, a typical pulse period. That is, if there is one pulse during 100 ms, 250,000 bits should be stored in the worst case. Of course, there can be some false alarms, such as noise between pulses. The important design criterion in this memory design is to save memory. One-bit memory width is not efficient either. The periodic interference detection information needs the start time and pulse length in the calculation of periodicity. In MAC, the periodic interference detection bit stream is transformed to a 32-bit time stamp format. That is, every time a periodic interference detection bit is on, the corresponding time stamp is saved. The first 22 bits represent the pulse start epoch, and the epoch has a resolution of 0.4 us, as used in radar signal detection. Then 22 bits can represent a linear time frame of 22\({}^{22}\)\(\times\) 0.4 us = 1.6777 s. The remaining 10 bits represent the pulse length, that is, up to 2\({}^{10}\)\(\times\) 0.4 us = 409.6 us, which can cover over 100-us-long radar pulses. It should be noted that the pulse length information is available at the end of the pulse when the periodic interference detection bit goes from on to off. The write timing is at the end of a pulse, although the start time is already saved in the front of the pulse. The memory depth should also be decided in terms of the write and read speeds. The write speed should be slower than the read speed, otherwise the memory will be full and subsequent data would be lost. The size of memory is set as it stores 64-bit time stamp data. This memory depth can store over approximately 1 ms if signals appear 10 times in 100 us.