How do 5G base stations control mobile phones under NSA?

The 5G network architecture is divided into SA and NSA. In the early stages of network deployment, most operators prefer NSA, or at least take NSA as the first-stage deployment target.

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Let's take a look at the NSA network architecture. The following figure shows two ways of drawing the architecture diagram under option 3x. We can see that there is a slight difference in the lower right corner: the right picture has an additional control plane connection from the mobile phone to the 5G base station, while the left picture does not.

What is going on here? Is one of these two ways of drawing wrong? In other words, under the NSA architecture, is there a control plane connection between the mobile phone and the 5G base station? First, let's take a look at what the so-called "user plane" and "control plane" actually mean. These two planes are actually different sets of two series of functions. For this problem, it can be simplified as follows: 1. What data does the user want to send? This is the real user service flow, which is of course handled by the user plane. 2. How does this data reach the recipient through the network? Obviously, these tasks include a lot of control signaling, which is handled by the control plane. In order for the control plane to convey control instructions, it must establish a transmission channel. The signaling channel established through the wireless link between the mobile phone and the base station is called: Signaling Radio Bearer (SRB for short).

Specifically speaking, in the protocol stack between the mobile phone and the base station, the most critical control layer is the RRC (Radio Resource Control) layer. This layer is equivalent to the third layer of the standard communication protocol stack, so it is also called layer 3, and SRB transmits layer 3 signaling on it.

In the above figure, the signaling radio bearer between the mobile phone and the 4G base station is also divided into SRB0, SRB1 and SRB2. What is going on?

SRB0: Transmits RRC signaling on a public channel, generally used when a mobile phone starts accessing;

SRB1: Transmits RRC signaling on a dedicated channel. Before SRB2 is established, it can carry upper-layer signaling for mobile phones to communicate with the core network, and has a higher priority;

SRB2: Transmits RRC signaling on a dedicated channel, carries upper-layer signaling for mobile phone to communicate with the core network, is activated after the security process, and has a lower priority than SRB1. The upper-layer signaling between the mobile phone and the core network mentioned above is also called NAS signaling (non-access layer signaling), which is used for information exchange between the mobile phone and the core network.

For 4G, these three types of wireless signaling bearers can meet the control plane requirements. However, in 5G NSA networking, the 5G base station also has an RRC layer, and it is also necessary to establish an RRC connection with the mobile phone to send control signaling. The 5G base station under NSA is just a vassal of the 4G base station. Through what channels should these 5G control signaling be sent?

There are currently three solutions:

1. Master Cell Group (MCG) SRB (SRB1, SRB2)

5G RRC signaling is transmitted through SRB1 and SRB2 established by 4G base stations and mobile phones. After being aggregated and processed by the 4G base station, those 5G-related signaling are forwarded to the 5G base station.

This method is also called MCG signaling bearer. All air interface signaling is on the 4G base station. There is no control plane link between the 5G base station and the mobile phone, only the user plane.

2. Split SRB (SRB1 + SRB1S, SRB2 + SRB2S)

An MCG separation signaling bearer is established at the 4G base station. That is to say, the 4G base station divides the SRB1 and SRB2 on it into two paths, one of which continues to be sent from the 4G base station, and the other is sent from the 5G base station.

4G and 5G related signaling can be sent on these two routes, but in the end, they must all be aggregated to the 4G base station for unified management. The 4G base station processes its own 4G signaling, and at the same time extracts 5G related signaling, and then forwards these signaling to the 5G base station for processing.

In this way, the air interface control signaling exists on both the 4G base station and the 5G base station. Although logically SRB1 and SRB2 are both based on 4G signaling bearers, some paths are also divided into 5G base stations. It is not wrong to say that there is a control plane connection between the 5G base station and the mobile phone.

This solution is highly complex and the actual gain is not obvious, so most of the industry does not support it yet.

3. Secondary Cell Group (SCG) SRB (SRB3)

5G base stations establish a new wireless signaling bearer: SRB3, which is used to send 5G-related signaling. 4G base stations and 5G base stations are independently controlled and run in parallel, which can effectively reduce the signaling load of 4G base stations. Of course, the signaling that can be sent on SRB3 is limited, mainly signaling that does not require the participation of 4G base stations, such as 5G measurement configuration delivery, measurement report reporting, and modification of auxiliary nodes without changing the primary node.

There is no doubt that under this solution, there is a control plane connection between the 5G base station and the mobile phone!

The following figure corresponds the above three solutions to the protocol stack: the red solid line plus the green solid line corresponds to the above solution 1 (MCG SRB); the red solid line and dotted line plus the green solid line corresponds to the above solution 2 (MCG Split SRB); the blue solid line corresponds to the above solution 3 (SCG SRB).

Therefore, both of the two ways of drawing the NSA architecture diagram at the beginning of this article are correct. Whether there is a control plane connection between the 5G base station and the mobile phone depends on different signaling radio bearer solutions. It is best to use solution 3 to establish SRB3. If not, no matter how tired the 4G base station is, we must make 5G speed very fast!