SRBs Definition

I taugh it’s good to write a small description of the SRBs as a reminder for whom might be interested, here is the information from the sharetechnote:

SRB (Signaling Radio Bearer)

As it says, SRB stands for Signaling Radio Bearer. In other words, SRB is a type of Radio Bearer that carries signaling message (i.e, RRC or/and NAS message).

According to 38.331 4.2.2 Signalling radio bearers, there are four different types of SRB in NR defined as follows.

  • SRB0 is for RRC messages using the CCCH logical channel;
  • SRB1 is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the establishment of SRB2, all using DCCH logical channel;
  • SRB2 is for NAS messages, all using DCCH logical channel. SRB2 has a lower-priority than SRB1 and is always configured by the network after security activation;
  • SRB3 is for specific RRC messages when UE is in EN-DC, all using DCCH logical channel.

As you may notice, the type and functions of each SRB in NR is same as the SRBs in LTE except that NR has one more SRB called SRB3. Regarding SRB3, 36.331 4.2.2 has a short additional comments as below.