3.3 Grouping of UMTS, LTE, and 5G Air Interface Protocol Layers
In the previous section, we have mentioned about the classifications of protocol layers into the control plane or signaling and user plane. In the case of UMTS, LTE, and 5G systems, their air interface control plane or signaling protocol layers are further grouped into what is called the Access Stratum (AS)) and NAS layer. The word “Stratum” means a grouping of protocols that is related to one service aspect of the various services provided by one or several network elements. The LTE, 5G, and UMTS system air interface protocol layer groupings are illustrated in Figure 3.9.
3.3.1 Access Stratum (AS): UMTS UE – UTRAN; LTE UE – E‐UTRAN;5G UE ‐ NG‐RAN
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.
Example 3.7 LTE AS Layer: Radio Resource Connection Establishment Procedure
Let us consider the radio resource connection establishment procedure initiated by an LTE UE toward its RAN E‐UTRAN. To initiate this procedure, a UE makes the radio resource requests and sends the RRCConnectionRequest message to the E‐UTRAN, shown in Figure 3.11, which is reproduced from TS 36.331 [94]. The RRCConnectionRequest is an AS RRC layer message that terminates at the E‐UTRAN end; refer to TS 36.331 [94]. This figure shows the RRC layer message names without showing the contents of each message.
Figure 3.11 LTE access stratum RRC connection establishment procedure.
Source: © 2018. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2018, 3GPP.
3.3.2 Non‐Access Stratum: UMTS UE – CN, LTE UE – EPC; 5G UE‐Core
The NAS contains all the mobility, connection, and SM‐related signaling protocols and procedures that terminate at the CN end. Using NAS signaling protocols, a UMTS UE or LTE or 5G UE communicates with the UMTS or LTE or 5G CN. Note that NAS uses the services of the AS protocols. In this case, the UMTS UTRAN or the LTE E‐UTRAN or 5G NG‐RAN simply forwards those transparent NAS‐related signaling messages to their respective CN.
Figure 3.10 in the previous section shows the protocols that are grouped into AS and NAS types in the case of the LTE system only where the NAS layer terminates at the LTE/EPC MME.
NAS signaling protocols are typically used for the following services:
CS CN procedures, such as MM and connection management (CM).
GPRS PS core network procedures, such as mobility, SM bearer management, and security management.
LTE/EPS core network procedures, such as the mobility, connection, and SM, bearer management, security management, EPS ciphering, and integrity protection function.
5G core network procedures, such as the mobility, connection, and PDU SM, security management, ciphering, and integrity protection function.
Table 3.7 shows the various NAS protocol messages exchanged between MS/UE and its concerned core network in GSM/GPRS/UMTS and LTE/EPS networks.
Example 3.8 below illustrates the typical messages flows associated with a MM layer (NAS) ATTACH procedure in the case of the LTE/EPS network.
Table 3.7 NAS protocol messages from GSM to LTE system.
| System | Type of Call | CM Message | SM Message | MM Message |
|---|---|---|---|---|
| GSM/ UMTS | CS | Call Estb., Call Setup | – |
|