Fronthaul Timing Window Diagram
Both DL + UL
C-plane window
U-plane window
UL Ta3/Ta4
T12 / Slot boundary
Hover over bars for details
Timing Parameter Values
Live
Slot Structure — 10 ms Radio Frame
μ=1 → 20 slots
DL
UL
Special (S)
FH timing applies
Key Equations
O-RAN WG4
DL U-plane (O-DU send window)
T1a_min_up = T2a_min_up + T12
T1a_max_up = T2a_max_up + T12
O-DU must send DL U-plane data within [T1a_min_up, T1a_max_up] before slot start. T12 is one-way FH latency.
DL C-plane (scheduling message)
T1a_min_cp = T2a_min_cp + T12
T1a_max_cp = T2a_max_cp + T12
O-DU sends DL C-plane (scheduling commands) earlier than U-plane. C-plane window is always wider than U-plane.
UL timing (O-RU send window)
Ta4_min = T2a_min_up + T12
Ta4_max = T2a_max_up + T12
O-RU sends UL data to O-DU within [Ta3_min=0, Ta3_max] after UL slot. O-DU receives within [Ta4_min, Ta4_max].
FH budget constraint
T12 ≤ T2a_min_up
FH budget = T2a_min_up - T12
T12 must not exceed T2a_min_up otherwise O-RU cannot receive data before slot start. Remaining budget is processing margin.
O-RU receive window
T2a_min_up ≤ t_rx ≤ T2a_max_up
All DL data must arrive at O-RU within this window relative to the DL air interface slot start time.
Ta3 processing time
0 ≤ Ta3 ≤ Ta3_max
Time from UL slot end at O-RU to transmission of UL U-plane message on fronthaul. Cat-A/B have the same Ta3 window.
Fiber Distance Calculator
Based on current T12 = 25 μs
| Fiber refractive index n = 1.4682
| Formula: d = T12 × c / n
One-way fiber distance
--
km
Distance in meters
--
m
Round-trip equivalent
--
km (2 × one-way)
FH budget remaining
--
μs (T2a_min_up - T12)
Note: In practice, total latency includes fiber propagation + switching/routing delay + O-RU/O-DU processing.
The standard defines T12 as the one-way transport latency from O-DU to O-RU port.
Typical fronthaul deployments target T12 < 100 μs. eCPRI spec allows up to 160 μs for Cat-A μ=1.