Shulou(Shulou.com)11/24 Report--

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

However, in the 5G era, everything becomes complicated. In order to solve the problem of covering, capacity, or hugging 4G thighs at the beginning of construction, 5G makes the "double connection" unbearable.

The so-called double 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 both are 5G base stations, so they are called MR-DC (Multi-Radio Dual Connectivity) or NR-DC.

The architecture doesn't look complicated, but there are a lot of questions to think about.

Are these two base stations primary and secondary? Can dual connection and carrier aggregation coexist? Which base station is the control plane on? How to divert user-side data? Are there any requirements for the core network?

First of all, although these two base stations are for users, there is indeed a distinction between primary and secondary status.

Those with high status are called "MN,Master Node", while those with low status are naturally called "SN,Secondary Node".

When the mobile phone wants to surf the Internet, first connect to the primary node, and then add secondary nodes as needed. The master node, also known as the "anchor point", is responsible for interacting with the control plane of the core network.

No matter it is the primary node or the secondary node, it can also support multiple carriers for carrier aggregation.

For the master node, the aggregation of the internal carrier is equivalent to packaging multiple carriers into a group, which is called the master cell group (MCG,Master Cell Group).

Accordingly, multiple cells within the secondary node are called auxiliary cell groups (SCG,Secondary Cell Group).

If we continue to drill down, we will think that the carriers in carrier aggregation are also primary and secondary, ah, how on earth can we be peacefully unified with this MCG and SCG?

For MCG, similar to ordinary carrier aggregation, the main cell is also called Pcell (Primary cell) and the secondary cell is also called Scell (Secondary cell).

For SCG, the main cell is called PSCell (Primary Secondary cell, primary and auxiliary cell), and the rest of the ordinary auxiliary cell is still called auxiliary cell SCell.

No matter MCG or SCG, the main cell is undoubtedly very important and plays a special role, so PCell and PSCell are collectively referred to as special cell (Special Cell, abbreviated as spCell).

Let's talk about the user-plane logical link between the mobile phone and the base station: data bearer.

From the perspective of mobile phone, who has the relationship between sending and receiving data with me is the bearer established with whom. In other words, 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 Split Bearer with both the primary node and the secondary node, it is called split bearer, which indicates that the data is diverted at a certain node.

Because the base station is connected to the mobile phone and the core network at the same time, the field of vision is naturally much wider than the mobile phone. We not only know from which node the data is finally sent to the mobile phone, but also know which nodes the data from the core network flows through and whether it is diverted.

The data diversion point, that is, the base station with a user plane connection to the core network in the dual connection, as the end point of the wireless bearer, will choose whether to divert or not according to the need.

If the shunt point is on the master node, but there is no shunt, there is only the MCG load of MN termination; if the shunt is done, the SCG bearer of MN termination and the split bearer of MN termination can be formed.

If the shunt point is a secondary node, but the shunt has not been carried out, there is only the SCG load of SN termination; if the shunt point is divided, the MCG load of SN termination and the split load of SN termination can be formed.

There is only one double connection, is it so complicated to do it?

In fact, considering whether the connection is a 4G core network or a 5G core network, and whether the base station is subdivided into 4G or 5G, the real situation is more complicated than this. A series of options for NSA and SA at the beginning of the deployment of the 5G network are essentially application examples of dual-connection of these technology points.

Let's take option 3x of the NSA architecture and option 2 of the SA architecture as examples to see how they achieve dual connectivity.

Option 3x is essentially a dual connection between a 4G base station and a 5G base station, also known as EN-DC. In the core network, the 4G EPC,4G base station is the primary node, that is, the control plane anchor point; the 5G base station is the auxiliary node and the shunt control point of the user plane.

For voice services, option 3x can only go to 4G without diversion, which forms the MCG bearer for MN termination; for data services, if the 5G base station does not divert, it is the SCG bearer for SN termination, and if it is diverted, it will form a split bearer for SN termination.

The dual connection on option 2 is called NR-DC, that is, the dual connection between the 5G base station and the 5G base station. The core network uses 5GC, one 5G base station uses the Sub6G band as the primary node and the shunt control point, and the other 5G base station uses the millimeter wave band as the secondary node.

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

This article comes from the official account of Wechat: wireless Deep Sea (ID:wuxian_shenhai), author: mayfly acquisition

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