This is a 4G base station, simple and refreshing.

However, in the 5G era, everything becomes more complicated. In order to solve the problem of coverage, capacity, or 4G in the early stage of construction, 5G has made “dual connection” very popular.

The so-called dual connection means that the mobile phone can be connected to two base stations at the same time. These two base stations can be a 4G base station plus a 5G base station, or they can all be 5G base stations, so they are called MR-DC (Multi-Radio Dual Connectivity) or NR-DC.

This structure does not seem complicated, but there are quite a lot of problems when you think about it. Is there any difference between these two base stations? Can Dual Connectivity and Carrier Aggregation Coexist? Which base station is the control plane on? How to distribute user plane data? Are there any requirements for the core network? First of all, although these two base stations are serving users, there is indeed a distinction between primary and secondary status.

The one with high status is called “Master Node” (MN, Master Node); the one with low status is naturally called “Secondary Node” (SN, Secondary Node). When the mobile phone wants to surf the Internet, it first connects to the primary node, and then adds secondary nodes as needed. The master node is also called “anchor point” and is responsible for interacting with the control plane of the core network. Regardless of whether it is a primary node or a secondary node, multiple carriers can also be supported internally for carrier aggregation. For the master node, once the internal carrier is aggregated, it is equivalent to packaging multiple carriers into a group, which we collectively call the Master Cell Group (MCG, Master Cell Group). Correspondingly, multiple cells inside the secondary node are called a secondary cell group (SCG, Secondary Cell Group). If we continue to drill down, we will think that the carriers in carrier aggregation are also divided into primary and secondary. How can the MCG and SCG be peacefully unified? For MCG, similar to common carrier aggregation, the primary cell is also called a Pcell (Primary cell), and the secondary cell is also called a Scell (Secondary cell). For the SCG, the primary cell is called a PSCell (Primary Secondary cell, primary secondary cell), and the remaining common secondary cells are still called SCells.

Regardless of whether it is MCG or SCG, the main cell is undoubtedly very important and plays a special role as an outline. Therefore, PCell and PSCell are collectively called special cells (Special Cell, spCell for short). Let’s talk about the user plane logical link between the mobile phone and the base station: data bearer.
From the perspective of mobile phones, whoever has a relationship with me to send and receive data is the bearer established with whom. That is to say, the data bearer between the mobile phone and the primary node is called MCG bearer, and the data bearer between the mobile phone and the secondary node is called SCG bearer. If the mobile phone has a bearer with the primary node and the secondary node at the same time, it is called a split bearer (Split Bearer), which means that the data is split at a certain node.
Since the base station is connected to the mobile phone and the core network at the same time, the field of view is naturally much wider than that of the mobile phone. Not only does it know which node the data is finally sent to the mobile phone, but also knows which nodes the data from the core network flows through and whether it is shunted.
The data distribution point, that is, the base station that has a user plane connection with the core network in the dual connectivity, as the termination point of the radio bearer, will choose whether to perform distribution according to the need. If the offloading point is at the main node, but the offloading is not performed, naturally there is only the MN-terminated MCG bearer; if the offloading is performed, the MN-terminated SCG bearer and the MN-terminated split bearer can be formed. If the offloading point is a secondary node, but the offloading has not been performed, naturally there are only SN-terminated SCG bearers; if offloading is performed, SN-terminated MCG bearers and SN-terminated split bearers can be formed.

Just a dual connection, as to make it so complicated?
In fact, considering whether the connection is to the 4G core network or the 5G core network, and whether the base station is 4G or 5G, the real situation is more complicated than this. A series of options for NSA and SA in the initial stage of 5G network deployment are essentially application examples of dual connectivity. Let’s take option 3x of the NSA architecture and option 2 of the SA architecture as examples to see how they implement dual connectivity. Option 3x is essentially dual connectivity between a 4G base station and a 5G base station, also known as EN-DC. The core network adopts 4G EPC, and the 4G base station is the main node, that is, the anchor point of the control plane; the 5G base station is the auxiliary node, which is also the distribution control point of the user plane.

For voice services, option 3x can only use 4G without offloading, which forms the MCG bearer terminated by the MN; for data services, if the 5G base station does not perform offload, it is the SCG bearer terminated by the SN. SN-terminated split bearer. Implementing dual connectivity on option 2 is called NR-DC, that is, dual connectivity between 5G base stations and 5G base stations. The core network uses 5GC, and one 5G base station uses the Sub6G frequency band as the master node and distribution control point; the other 5G base station uses the millimeter wave frequency band as the auxiliary node.

With the deepening of 5G deployment, on the basis of 3.5GHz or 2.6GHz independent networking (option 2) in the mid-band, and then superimposing millimeter waves through NR-DC to achieve ultra-high uplink and downlink speeds, it has become more and more operators s Choice.