5G EN-DC: Let’s Talk About Signaling – SRB1/2, SRB-3, Split-SRB

This is a follow up post to my original post on 5G EN-DC, the LTE/5G Dual Connectivity (DC) variant of 3GPP 5G. About a year has passed and a lot of progress has been made in 3GPP TS 37.340, which describes the interworking between the LTE eNB and the 5G NR gNB. One thing that has become much clearer to me now is how the mobile device (the UE) communicates with the eNB and gNB to control the radio link. (No) surprise: 3GPP would not be 3GPP if there weren’t at least 3 different options for doing this.

In this post I’m summarizing how radio resource signaling is done in EN-DC on the RRC layer based on the June 2018 version of TS 37.340. In addition to the specification, have a look at this post over at the Ericsson Research Blog for a different angle.

In LTE and 5G NR, RRC signaling is required to setup and release the user data bears, to aggregate component carriers, to perform handovers to other cells, to allocate dedicated channels for VoLTE, etc. etc. In a pure LTE network, RRC signaling messages are transported over SRB-1 and SRB-2 (Signaling Radio Bearers). For EN-DC, where the 4G eNB controls the 5G gNB, the concept has been extended as follows:

In the EN-DC approach, the schedulers in the eNB and gNB are independent from each other and make their own decisions when and how much data to send or receive from a UE. This also applies to RRC, and the UE now has one RRC context to the eNB and another separate and independent RRC context to the gNB. The question now is how RRC messages from the eNB and gNB are sent to and from the UE. The specification has 3 options:

Option 1: The eNB uses SRB-1/2 for its RRC messages. When the gNB wants to send/receive RRC messages (e.g. to receive RF measurement reports from the UE), RRC messages are exchanged over the X2 interface with the eNB, who then embeds the gNBs RRC messages in SRB-1/2 messages and sends/receives them over the eNB air interface.

Option 2: The eNB uses SRB-1/2 for its RRC messages. The gNB, however, can decide to establish its own signaling bearer, SRB-3, to communicate with the UE itself instead of relying on the eNB to do the job. The SRB-3 is great for independence but there are some operations that require coordination between the eNB and the gNB such as adding new component carriers. In such scenarios, SRB-3 can’t be used. Instead, option 1 is used for such messages to prevent race conditions.

Option 3: The eNB establishes a Split Bearer for signaling. SRB-1/2 messages and gNB RRC messages embedded in SRB-1/2 messages can then be sent either over the eNB air interface, the gNB air interface or over both simultaneously. In this case, no SRB-3 is established. This gives the eNB a lot of flexibility when radio conditions change for the better and worse.

So much for that. Before I close let me quickly introduce some additional acronyms for completeness sake. In my description above, I’ve only used the terms eNB for the LTE side and gNB for the 5G NR side of an EN-DC connection. These are used in the specs as well but quite often, the spec refers to the eNB as the Master Node (MN) or the Master Cell Group (MCG), as it could be more than one cell, at least in theory. The gNB is often also referred to as the Secondary Node (SN) or the Secondary Cell Group (SCG).