Specifications
XBee®/XBee‐PRO®SERFModules
©2009DigiInternational,Inc. 54
Encrypted Transmissions
Encrypted transmissions are routed similar to non-encrypted transmissions with one exception. As
an encrypted packet propagates from one device to another, each device decrypts the packet
using the network key, and authenticates the packet by verifying packet integrity. It then re-
encrypts the packet with its own source address and frame counter values, and sends the
message to the next hop. This process adds some overhead latency to unicast transmissions, but
it helps prevent replay attacks. See chapter 5 for details.
Maximum RF Payload Size
XBee SE firmware includes a command (NP) that returns the maximum number of RF payload
bytes that can be sent in a unicast transmission. Querying the NP command, like most other
commands, returns a HEXADECIMAL value. This number will change based on whether security is
enabled or not. If security is enabled (EE command), the maximum number of RF payload bytes
decreases since security requires additional overhead.
After reading the NP value, the following conditions can affect the maximum number of data bytes
in a single RF transmission:
• Broadcast transmissions can support 8 bytes more than unicast transmissions.
• If source routing is used, the 16-bit addresses in the source route are inserted into the RF
payload space. For example, if NP returns 84 bytes, and a source route must traverse 3 inter-
mediate hops (3 16-bit addresses), the total number of bytes that can be sent in one RF
packet is 78.
• Enabling APS encryption (API tx option bit set) will reduce the number of payload bytes by 9.
Throughput
Throughput in a ZigBee network can vary by a number of variables, including: number of hops,
encryption enabled/disabled, sleeping end devices, failures/route discoveries. Our empirical
testing showed the following throughput performance in a robust operating environment (low
interference).