5.1 Basic groups of functions

24.0073GPPGeneral AspectsMobile radio interface signalling layer 3Release 17TS

Most functions of layer 3 and its sub‑layers are described by the service specifications and protocol specifications of the (sub‑)layers.

These functions are in the model realized by protocol control entities, see clause 4.3.3.

In addition, routing functions are contained in layer 3 which are related to the transport of messages, e.g. multiplexing and splitting. These routing functions are defined in the Radio Resource Management or in the 5GRR (for NAS over 3GPP access) or non-3GPP access management (for NAS over non-3GPP access) and Mobility Management or 5GMM sub‑layers.

1) They have the task to pass the messages from upper (sub‑)layers to lower (sub‑)layers.

2) They also have the task to pass messages provided by lower (sub‑layers) to the appropriate sub‑layer and, if applicable, entity.

The routing functions with task 2 make use of the protocol discriminator (PD) which is part of the message header.

A CM sublayer protocol may also define a transaction identifier (TI), procedure transaction identity (PTI) or EPS bearer identity as a part of the message header. This is at least the case if there are parallel entities of the same functional block, see clause 4.3.3. If they are a part of a message, the TI, PTI, EPS bearer identity, or both PTI and EPS bearer identity are also used by the routing functions.

– The MM-sublayer routing function passes the messages of the CM entities as well as of the MM, GMM and CTS-MM entities of its own sublayer to the service access point of RR, GRR, LLC or CTS-RR. Furthermore it multiplexes them in case of parallel transactions.

– The routing function of Radio Resource Management distributes the messages to be sent according to their message type and protocol discriminator (PD), to the actual channel configuration, and, if applicable, to further information received from upper sub-layers to the appropriate service access point of layer 2 (identified by SAPI and logical channel). Paging messages received from the PPCH are always routed to GMM, while paging messages received from the PCH are distributed to GMM or MM based on the temporary identifier (TMSI or TLL). For EPS services, the Paging messages received from the PCH are always routed to EMM.

– The messages provided at the different service access points of layer 2 are distributed by the RR sublayer routing function according to their protocol discriminator (PD). Messages with a PD equal to RR are passed to the RR entity of the own sublayer, all other messages are passed to the MM sublayer at the service access point RR-SAP.

– The routing function of MM-sublayer passes Standard L3 messages according to the protocol discriminator (PD) and, if applicable, the transaction identifier (TI) or the PDP address towards the MM entity or towards the CM entities via the various MM-SAP’s. GPRS L3 messages are routed to mobility management or session management according to the protocol discriminator.

– For EPS services, the routing function of EPS NAS passes standard L3 messages according to the protocol discriminator (PD) and, if applicable, the procedure transaction identity (PTI) and/or EPS bearer identity towards the EMM entity or towards the CM (ESM) entities of the various EPS NAS SAP’s.

– The routing function of LLC passes the messages according to the SAPIs to the MM sublayer or to the SNDCP entities.

For 5GS services, the routing functions with task 2 make use of the extended protocol discriminator (EPD) which is part of the message header, or PDU session identity.

A 5GCM sublayer protocol may also define a procedure transaction identity (PTI) as a part of the message header. This is at least the case if there are parallel entities of the same functional block, see clause 6.2.

– The 5GMM-sublayer routing function passes the messages of the 5GCM entities as well as of the 5GMM entities of its own sublayer to the service access point of 5GRR. Furthermore, it multiplexes them in case of parallel transactions.

– For NAS over 3GPP access, the NR or E-UTRA AS sublayer routing function distributes the messages to be sent according to their message type and extended protocol discriminator (EPD), to the actual channel configuration, and, if applicable, to further information received from upper sub-layers to the appropriate service access point of layer 2. Paging messages received from the PCH are always routed to 5GMM.

– For NAS over 3GPP access, the messages provided at the different service access points of layer 2 are distributed by the 5GRR sublayer routing function according to their extended protocol discriminator (EPD). Messages with a EPD equal to RR are passed to the 5GRR entity of the own sublayer, all other messages are passed to the 5GMM sublayer at the service access point 5GRR-SAP.

– For NAS over non-3GPP access:

– for initial registration, EAP-5G is used to transfer NAS messages between the peer entities of the 5GMM sublayers;

– after successful initial registration, IPSec transport mode and GRE will be used to encapsulate and un-encapsulate the NAS messages between the peer entities of the 5GMM sublayers.

– For 5GS services, the routing function of 5GS NAS passes standard L3 messages according to the protocol discriminator (PD) and, if applicable, the PDU session identity towards the 5GMM entity or towards the 5GSM entities of the various 5GS NAS SAPs.

The message (message header or other parts of the message) are neither changed nor removed by the RR routing function or non-3GPP access management or MM routing function or 5GMM routing function before passing it to the appropriate service access point.