Drop Sessions (Part 2 of 2)

Expert Opinion

Expert Opinion
Short LTE articles and opinions by our team of Subject Matter Experts (SMEs)

Drop Sessions (Part 2 of 2)

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In the previous blog, we described the activities that the UE carries out to evaluate the condition of the radio link to determine if it was in-synch or out of synch. Depending on the vendor’s implementation, an out of synch indication may result in a drop session.

In this blog, we will concentrate on the activities carried out by the eNodeB when it detects that the radio link has failed.

The types of failure that the eNodeB may detect (again, these may be vendor specific) are:

a)  DL failure (RLC failures)

b) UL failure (Physical layer failure).

 

DL Failure at the RLC layer:

The RLC Layer has a failure when data or signaling that is sent over the air is unsuccessful and the RLC Layer stops trying. When data is sent over the air, but is received incorrectly, the receiver will send a NACK. Also, the transmitter can send a request for an acknowledgement of all received packets, by setting the poll bit. The receiver will then send a list of all received packets. If a sent packet is not received, it is considered lost. In either case, the transmitter will retransmit. See figure below.

  

 

This procedure can repeat, but at some point the transmitter will give up on the packet. If that happens, the transmitter declares that the radio link has failed and starts the procedures to communicate that to the other side.

The parameter MaxRetxThreshold determines the number of times a packet is retransmitted at the RLC layer in the downlink. If this number is reached, the eNodeB declares a DL RLC failure and “kills” the context as shown in the picture below.

 

 

UL Failure at the Physical layer:

Not all vendor implementation support this type of failure detection. It essentially consists in measuring the power of the sounding reference signals (SRS) sent by the UE in the UL. If the power is below a given SINR threshold, a timer gets started. If the SINR remains under the stated SINR threshold for the entire duration of the timer, then the eNodeB declares the UL as out of synch and proceeds to “kill” the context. If the SINR of the SRS goes above a second specified threshold during the timer duration, the UL is said to be in-synch and no actions are carried out.

Below, the actions carried out by eNodeB are shown when an UL Physical Layer failure is detected.

 

 

Yes, you are right!!! But think about the consequences again!

Yes, increasing the value of maxretxthreshold may result in a decrease in the number of drop sessions due to RLC DL failures.

However, to avoid a large number of drops, the best thing to do is to clean the RF environment in your network.

  • What happens in case of the DL Failure if the eNB does not receive any message from the UE (because of low link quality, etc..) when will the eNB drop the user.