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GSM-GPRS Loopback tests

This page describes GSM/GPRS Loopback tests. It covers loopback A, loopback B,loopback C,loopback D,loopback E,loopback F,loopback I and loopback G/H used for performance tests of GSM/GPRS Mobile phone.

Loopback A

It is targetted for residual measurements. Test equipment sends the MS a message instructing it to close Loopback A The layer 3 commands are sent on the DCCH (SDCCH/FACCH). On layer 2, SAPI 0 is used in acknowledged mode MS erases frames received with CRC errors and replaces all the bits in those frames with zeros. Type A Measurements include following:
•  BER for data TCH
•  Number of Bits Tested
•  FER for speech TCH
•  RBER for speech TCH
•  EVSIDUR and EVRFR for AMR speech TCH

Loopback B

It is targetted for non residual measurements. Test equipment sends the MS a message instructing it to close Loopback B The mobile station does not indicate if any frames in the downlink were erased. BER measurement on Test equipment is then able to return CRC count. Type B Measurements include following:
•  BER
•  Bits Tested
•  CRC Ratio
•  CRC Count

Loopback C

Burst by Burst Loopback is performed here. 114 info bits of each DL TCH bursts (excluding SB) after encrypting (but before applying channel decoder) are transmitted in an UL burst. RTD: Number of frames between reception of one burst at MS, and the transmission of the same burst in UL. RTD is less than 26 frames. SACCH is not looped back.

Loopback D

It is measurement of signalling of Erased frames and unreliable frames for TCH/HS channel.

If reliable speech frame (UFI=0) is correctly received by MS in DL then output of CHD is input to CHE and transmitted in UL.

If bad speech frame (BFI=0) or unreliable speech frame (UFI=1) is received in DL or FACCH is detected, input frame to CHE is all zeros and transmitted on UL. FACCH processing is performed normally.

SACCH is not looped back.

Loopback E

It is used for TCH/HS channel and measure signalling of erased SID Frames. If valid SID frame (SID=2) or invalid SID frame (SID=1) are received by MS in DL then output of CHD is input to CHE and transmitted in UL. If CHD detects erased SID frame (SID=0) of if FACCH is detected, input frame to CHE is all zeros and transmitted in UL. FACCH processing is performed normally. Loopback F (TCH/HS: Signalling of erased valid SID Frames) If valid SID frame (SID=2 and BFI=0) is received by MS in DL then output of CHD is input to CHE and transmitted in UL. If CHD detects erased valid SID frame (SID=1, or SID=0, or BFI/UFI=1), or if FACCH is detected, input frame to CHE is all zeros and transmitted in UL. FACCH processing is performed normally.

Loopback I

This test is used for AMR Speech Channel measurement. Loop I is only required to operate when DL DTX is NOT used.
When SID_FIRST/SID_FIRST_P1/SID_FIRST_P2/SID_FIRST_INH/SID_UPDATE/
SID_UPDATE_INH/ONSET is detected,
no need for in band signalling loop back.
Loop I is not designed to measure performance of in-band signalling over SID_UPDATE or RATSCCH messages.

Loopback G/H

It is used for multi Slot TCH loop measurements. With the first loop (G) the 114 information bits of each multi-slot TCH burst (excluding stealing flags) prior to applying benefit of the channel decoder, but after decryption (see Figure 1), shall be transmitted in an uplink burst. All that is received shall be re-transmitted. SACCH and idle bursts are not looped back. The second loop (H) includes the signalling of erased frames and is used to determine Frame Erasure Ratio (FER), Residual Bit Error Ratio (RBER) and Bit Error Ratio (BER) for any multi-slot configuration TCH.

RELATED LINKS

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