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The AS contains radio air interface signaling protocol layers between UE and UTRAN; UE and E‐UTRAN; and UE‐5G NG‐RAN only. The AS signaling protocol messages terminate at the UMTS UTRAN, LTE E‐UTRAN, and 5G NG‐RAN end. As the name implies, the AS contains all the protocols using which a UMTS or LTE or 5G UE communicates with the UMTS UTRAN or LTE E‐UTRAN or 5G NG‐RAN only to transmit/receive signaling messages over their respective radio air interfaces.

Figure 3.9 UMTS, LTE, and 5G air interface protocol layer groups.

Table 3.6 UMTS and LTE access stratum control plane protocol layers.

Protocol Category	System
Air Interface Access Stratum Protocol Layers	UMTS/UTRAN AS Layers	LTE/E‐UTRAN AS Layers
Physical, MAC, RLC, RRC	Physical, MAC, RLC, PDCP, RRC

Figure 3.10 LTE E‐UTRAN: access stratum and non‐access stratum protocol stack.

Source: © 2015. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2015, 3GPP.

The UMTS and LTE AS and its control or signaling plane protocol layers, over the air interface Uu, are shown in Table 3.6. The AS and NAS protocol structures for the LTE system air interface are shown in Figure 3.10, which is reproduced from TS 36.300 [92].

The typical flow of AS signaling messages, i.e. RRC layer, between a UE and the LTE/E‐UTRAN is described in Example 3.7 below.