This clause describes the state variables used in AM and UM entities in order to specify the RLC protocol. The state variables defined in this clause are normative. All state variables and all counters are non-negative integers. All state variables related to AM data transfer can take values from 0 to 4095 for 12 bit SN or from 0 to 262143 for 18 bit SN. All arithmetic operations contained in the present document on state variables related to AM data transfer are affected by the AM modulus (i.e. final value = [value from arithmetic operation] modulo 4096 for 12 bit SN and 262144 for 18 bit SN). All state variables related to UM data transfer can take values from 0 to 63 for 6 bit SN or from 0 to 4095 for 12 bit SN. All arithmetic operations contained in the present document on state variables related to UM data transfer are affected by the UM modulus (i.e. final value = [value from arithmetic operation] modulo 64 for 6 bit SN and 4096 for 12 bit SN). When performing arithmetic comparisons of state variables or SN values, a modulus base shall be used. TX_Next_Ack and RX_Next shall be assumed as the modulus base at the transmitting side and receiving side of an AM RLC entity, respectively. This modulus base is subtracted from all the values involved, and then an absolute comparison is performed (e.g. RX_Next ≤ SN < RX_Next + AM_Window_Size is evaluated as [RX_Next - RX_Next] modulo 2[sn-FieldLength] ≤ [SN - RX_Next] modulo 2[sn-FieldLength] < [RX_Next + AM_Window_Size - RX_Next] modulo 2[sn-FieldLength]), where sn-FieldLength is 12 or 18 for 12 bit SN and 18 bit SN, respectively. RX_Next_Highest- UM_Window_Size shall be assumed as the modulus base at the receiving UM RLC entity. This modulus base is subtracted from all the values involved, and then an absolute comparison is performed (e.g. (RX_Next_Highest- UM_Window_Size) ≤ SN < RX_Next_Highest is evaluated as [(RX_Next_Highest- UM_Window_Size) - (RX_Next_Highest- UM_Window_Size)] modulo 2[sn-FieldLength] ≤ [SN - (RX_Next_Highest- UM_Window_Size)] modulo 2[sn-FieldLength] < [RX_Next_Highest- (RX_Next_Highest- UM_Window_Size)] modulo 2[sn-FieldLength]), where sn-FieldLength is 6 or 12 for 6 bit SN and 12 bit SN, respectively. The transmitting side of each AM RLC entity shall maintain the following state variables:

1. TX_Next_Ack - Acknowledgement state variable
   This state variable holds the value of the SN of the next RLC SDU for which a positive acknowledgment is to be received in-sequence, and it serves as the lower edge of the transmitting window. It is initially set to 0, and is updated whenever the AM RLC entity receives a positive acknowledgment for an RLC SDU with SN = TX_Next_Ack.
2. TX_Next - Send state variable
   This state variable holds the value of the SN to be assigned for the next newly generated AMD PDU. It is initially set to 0, and is updated whenever the AM RLC entity constructs an AMD PDU with SN = TX_Next and contains an RLC SDU or the last segment of a RLC SDU.
3. POLL_SN - Poll send state variable
   This state variable holds the value of the highest SN of the AMD PDU among the AMD PDUs submitted to lower layer when POLL_SN is set according to clause 5.3.3.2. It is initially set to 0.

The transmitting side of each AM RLC entity shall maintain the following counters:

1. PDU_WITHOUT_POLL - Counter
   This counter is initially set to 0. It counts the number of AMD PDUs sent since the most recent poll bit was transmitted.
2. BYTE_WITHOUT_POLL - Counter
   This counter is initially set to 0. It counts the number of data bytes sent since the most recent poll bit was transmitted.
3. RETX_COUNT - Counter
   This counter counts the number of retransmissions of an RLC SDU or RLC SDU segment (see clause 5.3.2). There is one RETX_COUNT counter maintained per RLC SDU. This counter is reset to zero for each RLC SDU when indicated by upper layer.

The receiving side of each AM RLC entity shall maintain the following state variables:

1. RX_Next - Receive state variable
   This state variable holds the value of the SN following the last in-sequence completely received RLC SDU, and it serves as the lower edge of the receiving window. It is initially set to 0, and is updated whenever the AM RLC entity receives an RLC SDU with SN = RX_Next.
2. RX_Next_Status_Trigger - t-Reassembly state variable
   This state variable holds the value of the SN following the SN of the RLC SDU which triggered t-Reassembly.
3. RX_Highest_Status - Maximum STATUS transmit state variable
   This state variable holds the highest possible value of the SN which can be indicated by "ACK_SN" when a STATUS PDU needs to be constructed. It is initially set to 0.
   d) RX_Next_Highest - Highest received state variable
   This state variable holds the value of the SN following the SN of the RLC SDU with the highest SN among received RLC SDUs. It is initially set to 0.

Each transmitting UM RLC entity shall maintain the following state variables:

1. TX_Next - UM send state variable
   This state variable holds the value of the SN to be assigned for the next newly generated UMD PDU with segment. It is initially set to 0, and is updated after the UM RLC entity submits a UMD PDU including the last segment of an RLC SDU to lower layers.

Each receiving UM RLC entity shall maintain the following state variables:

1. RX_Next_Reassembly - UM receive state variable
   This state variable holds the value of the earliest SN that is still considered for reassembly. It is initially set to 0. For groupcast and broadcast of NR sidelink communication or for SL-SRB4 of NR sidelink discovery, it is initially set to the SN of the first received UMD PDU containing an SN. For the receiving UM RLC entity configured for MCCH or MTCH, it is up to UE implementation to set the initial value of RX_Next_Reassembly to a value before RX_Next_Highest.
2. RX_Timer_Trigger - UM t-Reassembly state variable
   This state variable holds the value of the SN following the SN which triggered t-Reassembly.
3. RX_Next_Highest- UM receive state variable
   This state variable holds the value of the SN following the SN of the UMD PDU with the highest SN among received UMD PDUs. It serves as the higher edge of the reassembly window. It is initially set to 0. For groupcast and broadcast of NR sidelink communication or for SL-SRB4 of NR sidelink discovery, it is initially set to the SN of the first received UMD PDU containing an SN. For the receiving UM RLC entity configured for MCCH or MTCH, it is initially set to the SN of the first received UMD PDU containing an SN.

1. AM_Window_Size This constant is used by both the transmitting side and the receiving side of each AM RLC entity. AM_Window_Size = 2048 when a 12 bit SN is used, AM_Window_Size = 131072 when an 18 bit SN is used.
2. UM_Window_Size
   This constant is used by the receiving UM RLC entity to define SNs of those UMD SDUs that can be received without causing an advancement of the receiving window. UM_Window_Size = 32 when a 6 bit SN is configured, UM_Window_Size = 2048 when a 12 bit SN is configured.

The following timers are configured by TS 38.331:

1. t-PollRetransmit
   This timer is used by the transmitting side of an AM RLC entity in order to retransmit a poll (see clause 5.3.3).
2. t-Reassembly
   This timer is used by the receiving side of an AM RLC entity and receiving UM RLC entity in order to detect loss of RLC PDUs at lower layer (see clauses 5.2.2.2 and 5.2.3.2). If t-Reassembly is running, t-Reassembly shall not be started additionally, i.e. only one t-Reassembly per RLC entity is running at a given time.
3. t-StatusProhibit
   This timer is used by the receiving side of an AM RLC entity in order to prohibit transmission of a STATUS PDU (see clause 5.3.4).

The following parameters are configured by TS 38.331:

1. maxRetxThreshold
   This parameter is used by the transmitting side of each AM RLC entity to limit the number of retransmissions corresponding to an RLC SDU, including its segments (see clause 5.3.2).
2. pollPDU
   This parameter is used by the transmitting side of each AM RLC entity to trigger a poll for every pollPDU PDUs (see clause 5.3.3).
3. pollByte
   This parameter is used by the transmitting side of each AM RLC entity to trigger a poll for every pollByte bytes (see clause 5.3.3).