Frequency sub-bands in US915/AU915:

 US915/AU915 PHYs define channel structures that can support up to 72 channels for upstream.
 The first 64 channels (0-63), occupy 125 kHz and the last 8 channels (64-71) occupy 500 kHz. 
 However, most of the base stations available in the market support 8 or 16 channels. 
 Network acquisition can become costly if the device has no prior knowledge of the active channel plan and it enables 
 all 72 channels to begin with. 
 
 The LoRaWAN 1.0.2 Regional parameters specification refers to a strategy of probing a set of nine channels (8 + 1) for
 the joining process. According to that strategy, the device alternatively selects a channel from a set of 
 8 125 kHz channels and a 500 kHz channel.
 For example, send a join request alternatively on a randomly selected channel from a set of 0-7 channels and
 channel 64, which is the first 500 kHz channel.
 
 Once the device has joined the network (in case of OTAA) or has sent the first uplink (in the case of ABP), the network
 may send a LinkAdrReq MAC command to set the channel mask to be used. Please note that these PHY layers do not
 support CFList, so LinkAdrReq is the way the network tells you what channel plan to use. 
 
 You can configure the Mbed LoRaWAN stack to use a particular frequency sub-band (FSB), which means that you don't have to
 probe all sets of channels. "fsb-mask" in lorawan/mbed_lib.json is the parameter that you can use to tell the 
 system which FSB or set of FSBs to use. By default, the "fsb-mask" is set to "{0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0x00FF}". 
 That means all channels are active. In other words, 64 125 kHz channels and 8 500 kHz channels are active. If you wish
 to use a custom FSB, you need to set an appropriate mask as the value of "fsb-mask". For example, if you wish to use the 
 first FSB, in other words, the first 8 125 kHz channels (0-7) and the first 500 kHz channel:
    "fsb-mask" = "{0x00FF, 0x0000, 0x0000, 0x0000, 0x0001}" 
 Similarly, if you wish to use the second FSB, in other words, the second set of 8 125 kHz channels (8-15) and the 2nd 500 kHz 
 channel:
    "fsb-mask" = "{0xFF00, 0x0000, 0x0000, 0x0000, 0x0002}"
 
 You can also combine FSBs if your base station supports more than 8 channels. For example:
    "fsb-mask" = "{0x00FF, 0x0000, 0x0000, 0xFF00, 0x0081}"
 means use channels 0-7(125 kHz) + channel 64 (500 KHz) and channels 56-63 (125 kHz) + channel 71 (500 kHz).
 
 Please note that for Certification requirements, you need to alternate between 125 kHz and 500 kHz channels, so before joining, 
 do not set a mask that enables only 500 kHz or only 125 kHz channels.  
 
 Frequency sub-bands in CN470 PHY:
 
 The LoRaPHYCN470 class defines 96 channels in total, as per the LoRaWAN Regional Specification. These 96 channels
 are 125 kHz wide each and can be subdivided into 6 sub-bands containing 16 channels each. 
 "fsb-mask-china" is the parameter that lorawan/mbed_lib.json defines. It can be used to enforce an FSB. It is defined
 as a C-style array, and the first element of the array corresponds to first 8 channels (0-7) and so on. By default, all 
 96 channels are enabled, but there may be base stations that do not support all 96 channels. Therefore, network acquisition
 can become cumbersome if the device hops on random channels. The probability of finding a correct channel for a base station 
 that supports 8 channels would be 1/12. 
 
 For example, if your base station supports 16 channels (channels 0-15), set the "fsb-mask-china" as:
  "fsb-mask-china" = "{0xFFFF, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000}" 
 
 Similarly, if your base station supports 8 channels (channels 0-7), set the "fsb-mask-china" as:
  "fsb-mask-china" = "{0x00FF, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000}" 
 
