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The 4G-5G switching delay is up to 244 milliseconds. The NSA networking is too complicated.
The 4G-5G switching delay is up to 244 milliseconds. The NSA networking is too complicated.
5G can help operators increase revenue, but wireless network management is too complicated.

Recently, from lightreading reports, South Korean operator SKT executives said that since commercial 5G in April, 5G users use about 33.7GB of data per month, 65% more than 4G users. 5G has boosted user traffic consumption and has also prevented SKT's revenue from declining.

The executive said that since the launch of 5G services, SKT's ARPU value has finally stabilized after seven consecutive quarters of decline.

However, the executive admitted that since the early deployment of 5G in the three major operators in South Korea adopted the NSA (non-independent networking) mode, it is necessary to manage 4G and 5G simultaneously under the NSA network, which leads to very complicated wireless network management.

For example, in the NSA networking mode, voice services are carried over LTE because voice services are very sensitive to delay and dropped calls, while high-speed data services are transmitted through LTE and 5G NR, but low-latency services are transmitted only by 5G NRs. Operators must accurately manage and optimize the network based on different business types.

He also said that under the NSA network, network speed and delay cannot be combined. If you need a lower latency service, it means lower bandwidth and lower network speed.

Coincidentally, the complexity of the NSA networking has recently received much attention from the industry. Not long ago, the network rating agency RootMetrics released a South Korean 5G evaluation report, in which the 4G-5G switching delay caused by the complexity of the NSA networking became a hot topic.

Under the NSA network, the 4G-5G switching delay is as high as 244ms.

According to the report of RootMetrics, the 5G rate in Korea is very fast, but if you want to experience ultra-high-speed 5G connection, you must first switch from LTE to 5G NR, but this switching process takes a long time.

RootMetrics measured that the switching delay of LTE to 5G NR reached 244ms among the three major operators in Korea.

There is no way, this is the price that must be paid in the early adoption of NSA networking.

It is well known that under the early 5G NSA networking, the 5G NR control plane is anchored to 4G, and the 4G core network EPC is used. This architecture is called EN-DC, which is the EUTRA-NR dual connectivity architecture.

As shown in the figure above, under the NSA network, the 4G base station (eNB) is the master node, and the 5G base station (gNB) is the secondary node (Secondary Node).

In the user plane, that is, when transmitting data traffic, both the 4G base station and the 5G base station are directly connected to the 4G core network element S-GW; but in the control plane, that is, in order to transmit data for signaling interaction, only the 4G base station directly The 4G core network element MME is connected.

This means that if a mobile phone wants to transmit high-speed data through a 5G base station, it needs to first connect to the 4G base station for signaling interaction. After the 4G base station allocates 5G wireless resources, the mobile phone user can enjoy 5G high-speed Internet access. To put it simply, under the NSA network, the command and dispatching power of the 5G highway is in the hands of 4G.

It is this dual connectivity architecture that greatly increases the complexity of the radio access network signaling and also affects the 4G-5G handover delay.

Why is the switching delay between 4G-5G so high?

Let's take a look at the 5G wireless resource addition process in EN-DC mode...

Under the NSA network (EN-DC dual connection), the mobile phone first registers the 4G network, and then reports the measured 5G signal strength and quality. For example, when the mobile phone moves to the coverage of the 5G cell, the 5G signal strength and quality are detected. Sufficient to support 5G services, the 4G base station will communicate with the 5G base station to allocate 5G resources to the mobile phone.

Next, the 4G base station notifies the mobile phone of the 5G NR allocated resource through the RRC Connection Reconfiguration message, and after completing the RRC connection reconfiguration process, the mobile phone is simultaneously connected to the 4G and 5G networks.

Specific steps are as follows:

Step 1: The primary node 4G base station decides to add the 5G base station as the secondary node, and sends a SgNB Addition Request message to the 5G base station.

The message carries RRC and radio bearer configuration, UE capabilities and security information, and the like.

Step 2-3: The 5G base station sends a SgNB Addition Request Acknowledge (SgNB Addition Request Acknowlele) message to the 4G base station in response to the request.

The message includes an NR RRC Reconfiguration message, which is encapsulated in an LTE RRC Connection Reconfiguration message and sent to the mobile phone through the 4G base station to allocate 5G radio resources to the mobile phone.

Step 4: The mobile phone sends an RRC Connection Reconfiguration Complete message to the 4G base station, where the message carries an NR RRC Reconfiguration Complete message for notifying the 5G base station.

Step 5: The 4G base station informs the 5G base station that the RRC reconfiguration is completed by using the sgNB Reconfiguration Complete message.

The message encapsulates the NR RRC Reconfiguration Complete message.

Next, the mobile phone detects the NR sync block based on the information contained in the NR RRC Connection Configuration, completes the downlink synchronization, acquires the NR physical cell identifier and the broadcast channel, and then initiates a random access procedure to the 5G base station to connect the 5G base station.

After completing the above steps, it means that the mobile phone and the 5G base station have prepared 5G wireless resources for data transmission.

However, the 5G base station needs to connect to the core network to transmit data.

Next, the 4G base station notifies the core network that the MME data bearer is being handed over from 4G-LTE to 5G-NR, the MME notifies the SGW to update the bearer, and the SGW switches the data transmission path from the 4G base station to the 5G base station, and then informs the 4G base station through the End Marker message. I can rest, and now the 5G base station can transmit data.

It can be seen from the above process that the EN-DC dual connection significantly increases the complexity of the signaling process.

This is like having a “transit station” between the 5G base station and the mobile phone. The NR RRC Reconfiguration message that can be directly sent to the mobile phone needs to be encapsulated and forwarded multiple times through 4G.

It is this complexity that causes the switching delay of 4G LTE and 5G NR to be too large. In theory, the whole process takes 100ms, and the actual network can reach 300ms.

In particular, when the handover between the primary node 4G base stations occurs as the user moves, since the 5G neighboring cells belonging to different 4G primary nodes cannot directly switch, the 5G NR resources of the source 4G base station side are first released, and then the source is executed. The handover between the 4G base station and the target 4G base station, after the handover is completed, the 5G NR resource is added to the target 4G base station, which takes a longer time.