Zigbee, Thread, and 6LoWPAN build on top of it, but the radio frame, channel access, ACK, and beacon timing are handled here.
802.15.4 Protocol Workbench
Connect MAC frame size, channel access, airtime, beacon timing, and GTS capacity in one visual path
animation
802.15.4
mac-layer
protocols
wsn
interactive
Interactive IEEE 802.15.4 protocol workbench showing frame construction, addressing overhead, CSMA-CA access, PHY airtime, beacon superframe timing, GTS allocation limits, formulas, and quick reference material.
802.15.4 Protocol Workbench
802.15.4 protocol workbench
Build one MAC transaction and watch how byte overhead, CSMA-CA access, PHY airtime, beacon timing, and GTS reservations change the same packet journey.
0 B
MAC frame size against the 127-byte PSDU limit
0 ms
Approximate 2.4 GHz PHY burst airtime
0 ms
Active superframe duration from SO
Ready
Frame, CAP, and GTS checks
Addressing, sequence, security, payload, and FCS all compete for the same MAC frame budget.
CSMA-CA tries random backoff and clear channel assessment. A GTS skips contention only during its assigned CFP slot.
BO sets beacon interval, SO sets active duration, and GTS reservations take capacity from the end of the active superframe.
Scenario
Playback
Frame builder
Channel access
CSMA-CA mode
Superframe and GTS
Frame fields
The MAC frame is assembled before the device can attempt channel access.
Frame bytes
CSMA-CA access
GTS / CFP
Limiting condition
Frame: 0 B
MAC PSDU budget check.
CSMA: idle
NB, BE, and CCA outcome.
Beacon: BO 5 / SO 3
Active and inactive timing.
GTS: 0 slots
Descriptor and CAP fit check.
802.15.4 MAC frames must fit inside 127 bytes, including the FCS.
A short ACK helps reliability but adds turnaround and a second PHY burst.
Duty cycle is active superframe duration divided by beacon interval.
GTS allocations cannot shrink CAP below the minimum CAP length.
Extended addresses are useful before association, but two extended addresses consume 16 address bytes before PAN IDs are counted.
This teaching model adds a 5-byte auxiliary security header and 4-byte MIC; other security levels can use different MIC sizes.
Backoff reduces collisions but does not guarantee latency. ACK loss still causes a retry even if the first transmission was heard.
Reserved CFP slots help periodic control traffic, but they reduce time available for ordinary CAP traffic.
Formula trace
MAC frame budget
MHR + payload + FCS <= 127 B
Current frame fits.
2.4 GHz PHY burst
(PSDU + 6 PHY B) * 8 / 250 kbps
Airtime includes preamble, SFD, and PHR.
Beacon timing
BI = 960 * 2^BO symbols
Active duration uses SO; symbol time is 16 us at 2.4 GHz.
GTS capacity
min CAP slots = ceil(440 / slotSymbols)
GTS must also respect the 7-descriptor limit.
Frame fields and byte limits
The MAC PSDU limit is 127 bytes. This includes MAC header fields, MAC payload, security overhead when used, and the 2-byte FCS.
- Legacy ACK is 5 bytes: Frame Control, Sequence Number, and FCS.
- Short address is 2 bytes; extended address is 8 bytes.
- PAN ID compression saves one repeated 2-byte PAN ID when source and destination share a PAN.
CSMA-CA rules shown here
The workbench shows a teaching model of 802.15.4 CSMA-CA rather than a random live radio trace.
- NB starts at 0 and BE starts at macMinBE, usually 3.
- Unslotted CSMA-CA uses random backoff then one CCA attempt.
- Slotted CSMA-CA aligns to backoff periods and needs CW = 2 idle CCAs before transmission.
Beacon, CAP, CFP, and GTS
Beacon-enabled mode divides the active superframe into 16 slots. CAP comes first; GTS allocations sit in the CFP at the end.
- Base superframe duration is 960 symbols.
- Each slot is 60 * 2^SO symbols.
- GTS allocation is limited by a maximum of 7 descriptors and the minimum CAP length of 440 symbols.