4G/LTE - Test Loopback Mode

  

 

 

 

Test Loopback Mode

 

For conformance test, we have two major procedure to go through. One is "ACTIVATE TEST MODE" and the other one is "CLOSE UE TEST LOOP". In RF Conformance, only "ACTIVATE TEST MODE" would be enough, but in Protocol Conformance we have to go through both "ACTIVATE TEST MODE" and "CLOSE UE TEST LOOP". (For the details refer to TS 36.509 '5 Test Control (TC) protocol procedures and test loop operation').

 

Loopback Mode

Description

Mode A

 Simply put, this is the loopback made at the top of PDCP layer and this is Mandatory for all LTE UE. UE is supposed to loopback whatever PDCP SDU it gets from the network. It doesn't care about the contents or TFT etc.

Mode B

This is also a loopback sitting on top of PDCP layer, but unlike Mode A, in Mode B the loopback applies to a specific TFT associated with a specific EPS. This loopback can not be be used when more than one PDN connection is established or more than one primary PDP context is active. This is Mandatory for all LTE UE.

Mode C

Loopback Mode C is for E-MBMS testing. It provides counting of successfully received MBMS Packets on a given MTCH while UE is operating in E-MBMS/E-UTRA mode. This is mandatory only for the LTE UE supporting E-MBMS.

 

Overal function diagram for each test mode from TS 36.509 is as follows.

 

 

Message structure of CLOSE UE LOOPBACK is as follows.  (For the details refer to TS 36.509 '5 Test Control (TC) protocol procedures and test loop operation').

 

 

Let me give you an example for the message to help you understand the message structure.

 

HEX String : 0F 80 00 03 01 00 01

 

Analysis Result :

    0  : Protocol discriminator

    F  : Skip indicator

    80 : Message type (10000000)

    00 : UE test loop mode (000000AB), where A=B=0 when Loopback mode is mode A

    03 : Length of UE test loop mode A LB setup list in bytes

    01 00 : Uplink PDCP SDU Size = 256 bits

    01 : Data Radio Bearer ID = 1