802.15.4 GTS Guaranteed Time Slots Animation

Explore how beacon-enabled IEEE 802.15.4 reserves contention-free slots without starving the CAP

Interactive IEEE 802.15.4 GTS animation showing CAP requests, coordinator allocation checks, beacon GTS descriptors, CFP transmission slots, minimum CAP length, descriptor limits, and superframe timing.

802.15.4 Guaranteed Time Slots Animation

IEEE 802.15.4 Beacon Enabled PAN GTS / CFP

GTS turns part of the superframe into scheduled airtime.

Request a Guaranteed Time Slot in the CAP, check the coordinator's limits, then watch the next beacon announce a CFP slot that the device can use without CSMA-CA contention.

4/16 active slots reserved for CFP

92.2 ms contention access period left

245.8 ms beacon interval at 2.4 GHz

Allocated coordinator decision for this request

1 GTS only exists in beacon-enabled mode.

The coordinator sends regular beacons, so devices can wake at known times and find their assigned CFP slot.

2 The request still happens in the CAP.

A device asks for allocation using normal contention access before it gets deterministic airtime.

3 The beacon is the schedule.

The GTS descriptor in a later beacon tells the device the start slot, length, and transmit or receive direction.

4 GTS capacity is intentionally limited.

802.15.4 allows at most 7 GTS descriptors, and the CAP must stay long enough for management traffic.

1 Beacon sync Coordinator starts the superframe and advertises GTS permit.

2 CAP request Device sends a GTS request command during contention access.

3 Budget check Coordinator checks descriptor count, CAP minimum, and slot length.

4 Beacon descriptor The accepted schedule appears in a following beacon.

5 CFP transfer The device uses its slot without CSMA-CA backoff competition.

6 Maintain or release The allocation persists until deallocation or missed-beacon timeout.

Allocation Controls

Pick a scenario or tune the superframe. The visual updates the CAP, CFP, beacon descriptor, and allocation verdict together.

Beacon order (BO)

BO 4, beacon interval 245.8 ms

Superframe order (SO)

SO 3, active duration 122.9 ms

Requested GTS length

2 superframe slots requested

Existing GTS descriptors

1 descriptor already in the beacon

Existing CFP slot budget

2 slots already reserved near the end

GTS direction

TX from device RX to device

Superframe View

Beacon sync: the coordinator announces the superframe timing and whether GTS requests are permitted.

Allocation accepted The requested slots fit while keeping enough CAP for management traffic.

CAP / contention access Requested GTS Existing GTS Denied preview

2 GTS descriptor count after request

12 requested start slot announced in beacon

TX direction from device perspective

Permit coordinator request status

Watch the CAP boundary.

The CFP starts at slot 12, leaving 12 slots for beacon and CAP traffic.

Watch descriptor pressure.

The beacon can advertise at most 7 GTS descriptors; this request would use 2.

Watch timing scale.

SO changes slot duration and active superframe length; BO changes the interval between beacons.

Watch the trade-off.

GTS improves deterministic access for one flow, but every CFP slot removes contention capacity from everyone else.

7.68 ms

duration of one superframe slot at the selected SO.

122.88 ms

active superframe duration before any inactive period.

3 slots

minimum CAP requirement rounded up from 440 symbols.

30.72 ms

reserved CFP time after the request, including existing GTS slots.

Formula Trace

slot = 60 symbols * 2^SO * 16 us CAP slots >= ceil(440 / slotSymbols) accept if descriptors <= 7 and CFP slots <= 16 - minCAPslots

GTS Fields

• GTS specification carries descriptor count and GTS permit.
• Each descriptor includes device short address, starting slot, length, and direction.
• Direction is from the device perspective: transmit to coordinator or receive from coordinator.

Timing Constants

• Base slot duration is 60 symbols; base superframe duration is 960 symbols.
• At 2.4 GHz O-QPSK, one symbol is 16 us, so SO 0 is 15.36 ms active duration.
• Beacon interval uses BO; active superframe duration uses SO, with SO no greater than BO.

Common Traps

• GTS removes contention during the CFP, but the request and many commands still need the CAP.
• GTS is not unlimited bandwidth; it trades shared CAP capacity for scheduled access.
• Missing beacons can make a device lose timing, so synchronization is part of the reliability story.

Related Learning

802.15.4 Beacon Animation

See how beacon interval, superframe order, CAP, CFP, and inactive period fit together.

802.15.4 CSMA-CA

Compare contention-based CAP access with scheduled CFP access.

802.15.4 Data Rate Calculator

Estimate payload overhead, airtime, retries, and practical channel capacity.

802.15.4 Fundamentals

Review the PHY and MAC terms behind the animation.