Last time one of my colleagues asked me what was TOAWS, TOAWE and TOA. I knew it related to Frame Protocol and TOA was time of arrival. But I cannot remember any more. So I did some investigation and here are some notes.

These definitions were initiated by Nokia in Tdoc R3-99663 and refined by Ericsson in Tdoc R3-99875 in 1999. Please note, in R3-99875, it mentioned 3GPP TS25.401(V1.2.1) would be updated accordingly. But later “the synchronisation in UTRAN” chapter was seperated as an individual 3GPP TS25.402. So now the official definitions are presented in sub-clause 5 of TS25.402(V11.0.0) while the mechanism is introduced in sub-clause 7 of the same 3GPP technical specification.

These three timers are used for transport channel synchronisation, more specifically, for timing adjustment on Iub/Iur. Figure 10 in TS25.402 illustrates how it works.

toaws toawe toa

TOAWS(Time of Arrival Window Startpoint) and TOAWE(Time of Arrival Window Endpoint) are defined in NBAP messages for Transport bearer Setup/Addition/Reconfiguration.

In the above figure, it’s assumed that the target CFN on air interface is 152 for certain tranport block. Since Layer 1 in Node B needs some processing time(tproc) to encode & transmit etc. Therefore, this tranport block should arrive in Node B before CFN 152. Node B will send control frame to RNC for timing adjustment with TOA which is the time difference between the TOAWE and when a data frame is received.

  • If TBS arrives too early before TOAWS, it will increase the burden of buffer in Node B. Node B will send Timing Adjustment Control frame with positive TOA to RNC;
  • If TBS arrives right between TOAWS and TOAWE, timing is perfect. Thus no Timing Adjustment Control frame needed;
  • If TBS arrives between TOAWE and LTOA, Node B is still able to process such TBS. But it’s a bit late and dangerous that Node B might not be so quick to process it. So Node B will send Timing Adjustment Control frame with negative TOA to RNC;
  • If TBS arrives too late even after LTOA, Node B is unable to process this TBS and it will send Timing Adjustment Control frame with negative TOA to RNC;

After RNC receives Timing Adjustment Control frame with TOA, it will adjust the DL data sending timing accordingly, to synchronize with Node B in this way.

The situation for soft handover is more complicated. Please check Figure 14 in TS25.402 for more details.