diff --git a/include/osmocom/pcu/pcuif_proto.h b/include/osmocom/pcu/pcuif_proto.h
index aa22447e..cd2a2983 100644
--- a/include/osmocom/pcu/pcuif_proto.h
+++ b/include/osmocom/pcu/pcuif_proto.h
@@ -42,6 +42,7 @@
 #define PCU_IF_SAPI_PRACH	0x06	/* packet random access channel */
 #define PCU_IF_SAPI_PTCCH	0x07	/* packet TA control channel */
 #define PCU_IF_SAPI_PCH_2	0x08	/* assignment on PCH (confirmed using message id) */
+#define PCU_IF_SAPI_AGCH_2	0x09	/* assignment on AGCH (confirmed using message id) */
 
 /* flags */
 #define PCU_IF_FLAG_ACTIVE	(1 << 0)/* BTS is active */
@@ -276,6 +277,18 @@ struct gsm_pcu_if_pch {
 	bool confirm;
 } __attribute__((packed));
 
+/* Struct to send a (confirmed) IMMEDIATE ASSIGNMENT message via AGCH. The struct is sent as a data request
+ * (data_req) under SAPI PCU_IF_SAPI_AGCH_2. */
+struct gsm_pcu_if_agch {
+	/* message id as reference for confirmation */
+	uint32_t msg_id;
+	/* GSM mac-block (with immediate assignment message) */
+	uint8_t data[GSM_MACBLOCK_LEN];
+	/* Set to true in case the receiving end must send a confirmation
+	 * when the MAC block (data) has been sent. */
+	bool confirm;
+} __attribute__((packed));
+
 struct gsm_pcu_if {
 	/* context based information */
 	uint8_t		msg_type;	/* message type */
diff --git a/src/bts.cpp b/src/bts.cpp
index 5c483d5c..c86c3dc5 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -1033,7 +1033,10 @@ int bts_rcv_rach(struct gprs_rlcmac_bts *bts, const struct rach_ind_params *rip)
 		rip->burst_type);
 	bts_do_rate_ctr_inc(bts, CTR_IMMEDIATE_ASSIGN_UL_TBF);
 	if (plen >= 0) {
-		pcu_l1if_tx_agch(bts, bv, plen);
+		if (the_pcu->pcu_if_version >= 0x0b)
+			pcu_l1if_tx_agch2(bts, bv, plen, GSM_RESERVED_TMSI, false);
+		else
+			pcu_l1if_tx_agch(bts, bv, plen);
 		rc = 0;
 	} else {
 		rc = plen;
@@ -1047,8 +1050,12 @@ send_imm_ass_rej:
 		bv, rip->ra, rip->rfn, rip->burst_type,
 		(uint8_t)osmo_tdef_get(bts->T_defs_bts, 3142, OSMO_TDEF_S, -1));
 	bts_do_rate_ctr_inc(bts, CTR_IMMEDIATE_ASSIGN_REJ);
-	if (plen >= 0)
-		pcu_l1if_tx_agch(bts, bv, plen);
+	if (plen >= 0) {
+		if (the_pcu->pcu_if_version >= 0x0b)
+			pcu_l1if_tx_agch2(bts, bv, plen, GSM_RESERVED_TMSI, false);
+		else
+			pcu_l1if_tx_agch(bts, bv, plen);
+	}
 	bitvec_free(bv);
 	/* rc was already properly set before goto */
 	return rc;
@@ -1128,10 +1135,26 @@ void bts_snd_dl_ass(struct gprs_rlcmac_bts *bts, const struct gprs_rlcmac_dl_tbf
 						    GSM_L1_BURST_TYPE_ACCESS_0);
 	if (plen >= 0) {
 		bts_do_rate_ctr_inc(bts, CTR_IMMEDIATE_ASSIGN_DL_TBF);
-		if (the_pcu->pcu_if_version >= 0x0b)
-			pcu_l1if_tx_pch2(bts, immediate_assignment, plen, true, tbf->imsi(), tbf->tlli());
-		else
+
+		if (the_pcu->pcu_if_version >= 0x0b) {
+			if (ms_imsi_is_valid(tbf->ms())) {
+				pcu_l1if_tx_pch2(bts, immediate_assignment, plen, true, tbf->imsi(), tbf->tlli());
+			} else {
+				/* During GMM ATTACH REQUEST, the IMSI is not yet known to the PCU or SGSN. (It is requested
+				 * after the GMM ATTACH REQUEST with the GMM IDENTITY REQUEST.) When the PCU has to assign a
+				 * DL TBF but the IMSI is not known, then the IMMEDIATE ASSIGNMENT is sent on the AGCH. The
+				 * reason for this is that without an IMSI we can not calculate the paging group, which would
+				 * be necessary for transmission on PCH. Since the IMSI is usually only unknown during the GMM
+				 * ATTACH REQUEST, we may assume that the MS is in non-DRX mode and hence it is listening on
+				 * all CCCH blocks, including AGCH.
+				 *
+				 * See also: 3gpp TS 44.060, section 5.5.1.5
+				 *           3gpp TS 45.002, section 6.5.3, 6.5.6 */
+				pcu_l1if_tx_agch2(bts, immediate_assignment, plen, true, tbf->tlli());
+			}
+		} else {
 			pcu_l1if_tx_pch(bts, immediate_assignment, plen, tbf->imsi());
+		}
 	}
 
 	bitvec_free(immediate_assignment);
diff --git a/src/pcu_l1_if.cpp b/src/pcu_l1_if.cpp
index f92f08ea..7bc4cb9c 100644
--- a/src/pcu_l1_if.cpp
+++ b/src/pcu_l1_if.cpp
@@ -251,6 +251,8 @@ void pcu_l1if_tx_ptcch(struct gprs_rlcmac_bts *bts,
 
 void pcu_l1if_tx_agch(struct gprs_rlcmac_bts *bts, bitvec *block, int plen)
 {
+	/* TODO: When PCUIF v.11 has become mainline, we will use pcu_l1if_tx_agch2() exclusively.
+	 * This will make this function obsolote, so we can remove it. */
 	uint8_t data[GSM_MACBLOCK_LEN]; /* prefix PLEN */
 
 	/* FIXME: why does OpenBTS has no PLEN and no fill in message? */
@@ -263,10 +265,31 @@ void pcu_l1if_tx_agch(struct gprs_rlcmac_bts *bts, bitvec *block, int plen)
 	pcu_tx_data_req(bts, 0, 0, PCU_IF_SAPI_AGCH, 0, 0, 0, data, sizeof(data));
 }
 
+/* Send a MAC block via the access grant channel. This will (obviously) only work for MAC blocks that contain
+ * an IMMEDIATE ASSIGNMENT. In case the MAC block contains an IMMEDIATE ASSIGNMENT message, the receiving end
+ * is required to confirm when the IMMEDIATE ASSIGNMENT has been sent. */
+void pcu_l1if_tx_agch2(struct gprs_rlcmac_bts *bts, bitvec *block, int plen, bool confirm, uint32_t msg_id)
+{
+	struct gsm_pcu_if_agch agch = { 0 };
+
+	agch.confirm = confirm;
+	agch.msg_id = msg_id;
+	agch.data[0] = (plen << 2) | 0x01;
+	bitvec_pack(block, agch.data + 1);
+
+	if (the_pcu->gsmtap_categ_mask & (1 << PCU_GSMTAP_C_DL_AGCH))
+		gsmtap_send(the_pcu->gsmtap, 0, 0, GSMTAP_CHANNEL_AGCH, 0, 0, 0, 0, agch.data, GSM_MACBLOCK_LEN);
+
+	pcu_tx_data_req(bts, 0, 0, PCU_IF_SAPI_AGCH_2, 0, 0, 0, (uint8_t*)&agch, sizeof(agch));
+}
+
 #define IMSI_DIGITS_FOR_PAGING 3
 /* Send a MAC block via the paging channel. (See also comment below) */
 void pcu_l1if_tx_pch(struct gprs_rlcmac_bts *bts, bitvec *block, int plen, const char *imsi)
 {
+	/* TODO: When PCUIF v.11 has become mainline, we will use pcu_l1if_tx_pch2() exclusively.
+	 * This will make this function obsolote, so we can remove it. */
+
 	uint8_t data[IMSI_DIGITS_FOR_PAGING + GSM_MACBLOCK_LEN];
 
 	/* prepend last three IMSI digits (if present) from which BTS/BSC will calculate the paging group */
@@ -561,6 +584,7 @@ static int pcu_rx_data_cnf2(struct gprs_rlcmac_bts *bts, struct gsm_pcu_if_data_
 
 	switch (data_cnf->sapi) {
 	case PCU_IF_SAPI_PCH_2:
+	case PCU_IF_SAPI_AGCH_2:
 		bts_rcv_imm_ass_cnf(bts, NULL, data_cnf->msg_id);
 		break;
 	default:
diff --git a/src/pcu_l1_if.h b/src/pcu_l1_if.h
index 138636d3..b6da337b 100644
--- a/src/pcu_l1_if.h
+++ b/src/pcu_l1_if.h
@@ -147,6 +147,7 @@ void pcu_l1if_tx_ptcch(struct gprs_rlcmac_bts *bts,
 		       uint32_t fn, uint8_t block_nr,
 		       uint8_t *data, size_t data_len);
 void pcu_l1if_tx_agch(struct gprs_rlcmac_bts *bts, bitvec *block, int len);
+void pcu_l1if_tx_agch2(struct gprs_rlcmac_bts *bts, bitvec *block, int plen, bool confirm, uint32_t msg_id);
 #endif
 
 #ifdef __cplusplus