3.1 Overview/General

3GPP44.018GSM/EDGE Radio Resource Control (RRC) protocolMobile radio interface Layer 3 specificationRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

3.1.1 General

Radio Resource management procedures include the functions related to the management of the common transmission resources, e.g. the physical channels and the data link connections on control channels.

The general purpose of Radio Resource procedures is to establish, maintain and release RR connections that allow a point-to-point dialogue between the network and a mobile station. This includes the cell selection/reselection and the handover procedures. Moreover, Radio Resource management procedures include the reception of the uni-directional BCCH and CCCH when no RR connection is established. This permits automatic cell selection/reselection.

If VGCS listening or VBS listening are supported, the radio resource management also includes the functions for the reception of the voice group call channel or the voice broadcast channel, respectively, and the automatic cell reselection of the mobile station in Group receive mode.

If VGCS talking is supported, the radio resource management also includes the functions for the seizure and release of the voice group call channel.

If GPRS point-to-point services are supported, the radio resource management procedures includes functions related to the management of transmission resources on packet data physical channels. This includes the broadcast of system information to support a mobile station in packet idle and packet transfer modes, see also 3GPP TS 44.060.

NOTE 1: This chapter includes some procedures used for multislot operation and for the TCH/H + TCH/H configuration which need not be supported by simple mobile stations.

NOTE 2: The procedures and the information content relating to the TCH/H + TCH/H configuration in RR messages is for further study.

If dedicated mode MBMS Notification is supported, the radio resource management procedures includes functions to allow the announcement of the starting MBMS services.

3.1.2 Services provided to upper layers

A RR connection is a physical connection used by the two peer entities to support the upper layers’ exchange of information flows.

3.1.2.1 Idle mode

In idle mode no RR connection exists.

The RR procedures include (on the mobile station side) those for automatic cell selection/reselection. The RR entity indicates to upper layers the unavailability of a BCCH/CCCH and the cell change when decided by the RR entity. Upper layers are advised of the BCCH broadcast information when a new cell has been selected, or when a relevant part of this information changes.

For cell-reselection the BA (list), together with the 3G Cell Reselection list and/or the E-UTRAN Neighbour Cell list for a multi-RAT MS, shall be used.

In Idle mode, upper layers can require the establishment of an RR connection.

3.1.2.2 Dedicated mode

In dedicated mode, the RR connection is a physical point-to-point bi-directional connection, and includes a SAPI 0 data link connection operating in multiframe mode on the main DCCH. If dedicated mode is established, RR procedures provide the following services:

– establishment/release of multiframe mode on data link layer connections other than SAPI 0, on the main DCCH or on the SACCH associated with the channel carrying the main signalling link;

– transfer of messages on any data link layer connection;

– indication of temporary unavailability of transmission (suspension, resuming);

– indication of loss of RR connection;

– automatic cell reselection and handover to maintain the RR connection;

– setting/change of the transmission mode on the physical channels, including change of type of channel, change of the coding/decoding/transcoding mode and setting of ciphering;

– allocation/release of an additional channel (for the TCH/H + TCH/H configuration);

– allocation/release of additional channels for multislot operation;

– release of an RR connection.

3.1.2.3 Group receive mode

Only applicable for mobile stations supporting VGCS listening or VBS listening.

In this mode, the RR procedures on the mobile station side provide the services:

– local connection to the voice broadcast channel or voice group call channel;

– reception of messages in unacknowledged mode;

– automatic cell reselection for the mobile station in Group receive mode;

– uplink reply procedure to indicate that the mobile station is listening to the voice group call or voice broadcast call;

– local disconnection from the received voice group call or broadcast call channels.

For mobile stations supporting both VGCS listening and VGCS transmit, in addition, the RR procedures on the mobile station side provide the service:

– uplink access procedures to establish the RR connection;

– priority uplink request procedures to establish the RR connection, if talker priority is supported by the mobile station.

– Uplink access procedures to establish the RR connection for sending application-specific data to the network

3.1.2.4 Group transmit mode

Only applicable for mobile stations supporting VGCS talking.

In group transmit mode, the RR connection is a physical point-to-point bi-directional connection, and includes a SAPI 0 and possibly a SAPI 3 data link connection(s) operating in multiframe mode on the main DCCH. If the group transmit mode is established, RR procedures provide the following services:

– transfer of messages on the SAPI 0 and optionally on the SAPI 3 of the data link layer connection;

– indication of loss of RR connection;

– automatic cell reselection and handover to maintain the RR connection;

– setting of the transmission mode on the physical channels, change of type of channel and setting of ciphering;

– release of the RR connection.

3.1.2.5 Packet idle mode

Only applicable for mobile stations supporting GPRS.

In packet idle mode, no temporary block flow exists (see 3GPP TS 44.060). Upper layers may require the transfer of a LLC PDU, which implicitly triggers the establishment of a temporary block flow.

3.1.2.6 Packet transfer mode

Only applicable for mobile stations supporting GPRS.

In packet transfer mode, the mobile station is allocated radio resources providing a temporary block flow on one or more packet data physical channels. If the mobile station supports Downlink Dual Carrier, these physical channels may be on different radio frequency channels for a DLMC configuration (see 3GPP TS 24.008). If the mobile station supports Downlink Multi Carrier, these physical channels may be on different radio frequency channels in the same or in different frequency bands for a DLMC configuration (see 3GPP TS 24.008). The RR sublayer provides the following services, see also 3GPP TS 44.060:

– transfer of LLC PDUs in acknowledged mode;

– transfer of LLC PDUs in unacknowledged mode.

Depending on the GPRS mode of operation (class A or B), the mobile station may leave both packet idle mode and packet transfer mode before entering dedicated mode, group receive mode or group transmit mode.

Cell reselection in packet idle and packet transfer modes is specified in 3GPP TS 45.008. The RR entity on the mobile station side indicates to the upper layers the availability of a cell and a cell change when decided by the RR sublayer. Upper layers are advised of system information broadcast in the cell when a new cell has been selected, or when a relevant part of this information changes.

3.1.2.7 Dual transfer mode (DTM)

In dual transfer mode, the mobile station is simultaneously in dedicated mode and in packet transfer mode.This feature is optional for the mobile station and the network. It is only applicable for a mobile station supporting GPRS, EGPRS or EGPRS2. Dual transfer mode is a subset of class A mode of operation, only possible if there is radio resource allocation co-ordination in the network. Simultaneous handover of resources to maintain the RR connection and the temporary block flows (see 3GPP TS 44.060) may occur in dual transfer mode if both the mobile station and the network support DTM handover (see sub-clause 3.7). DLMC configuration is not supported in DTM.

3.1.3 Services required from data link and physical layers

The RR sublayer uses the services provided by the data link layer as defined in 3GPP TS 44.005.

Moreover, the RR sublayer directly uses services provided by the physical layer such as BCCH searching and transfer of RLC/MAC blocks, as defined in 3GPP TS 44.004.

3.1.4 Change of dedicated channels

3.1.4.1 Change of dedicated channels using SAPI = 0

In case a change of dedicated channels is required using a dedicated assignment and handover procedure, respectively, the RR sublayer will request the data link layer to suspend multiple frame operation before the mobile station leaves the old channel. When the channel change has been completed, layer 3 will request the data link layer to resume multiple frame operation again. The layer 2 suspend/resume procedures are described in 3GPP TS 44.005 and 3GPP TS 44.006.

These procedures are specified in such a way that a loss of a layer 3 message cannot occur on the radio interface. However, messages sent from the mobile station to the network may be duplicated by the data link layer if a message has been transmitted but not yet completely acknowledged before the mobile station leaves the old channel (see 3GPP TS 44.006).

As the RR sublayer is controlling the channel change, a duplication of RR messages does not occur. However, there are some procedures for which a duplication is possible, e.g. DTMF procedures. For all upper layer procedures using the transport service of the GSM RR sub-layer (e.g., MM and CM procedures but not GMM or Session Management procedures), the request messages sent by the mobile station contain a sequence number in order to allow the network to detect duplicated messages, which are then ignored by the network. The same sequence number is used to protect against message duplication caused by channel changes between GSM and UTRAN and also by other UTRAN procedures (e.g. hard handover). The procedures for sequenced transmission on layer 3 are described in 3GPP TS 24.007.

3.1.4.2 Change of dedicated channels using other SAPIs than 0

For SAPIs other than 0, the data link procedures described in 3GPP TS 44.006 do not provide any guarantee against message loss or duplication.

Therefore, if an application uses a SAPI other than 0 and if this application is sensitive to message loss or duplication, then it has to define its own protection mechanism. No general protection mechanism is provided by the protocol defined in this Technical Specification.

3.1.5 Procedure for Service Request and Contention Resolution

Upon seizure of the assigned dedicated channel or group channel, the mobile station establishes the main signalling link on this channel by sending a layer 2 SABM frame containing a layer 3 service request message. The data link layer will store this message to perform the contention resolution. The service request message will be returned by the network in the UA frame.

The data link layer in the mobile station compares the content of the information field (i.e. the layer 3 service request message) received in the UA frame with the stored message and leaves the channel in case they do not match. This procedure resolves contentions in the case where several mobile stations have accessed at the same random access slot and with the same random reference and one has succeeded due to capture. The full description of the procedure is given in 3GPP TS 44.006.

NOTE: When this procedure is used to respond to an encapsulated IMMEDIATE ASSIGNMENT message or an encapsulated DTM ASSIGNMENT message (see sub-clause 3.6), random access and contention resolution are not used.

The purpose of the service request message is to indicate to the network which service the mobile station is requesting. This then allows the network to decide how to proceed (e.g. to authenticate or not).

The service request message must contain the identity of the mobile station and may include further information which can be sent without encryption.

The layer 3 service request message is typically one of the following:

– CM SERVICE REQUEST;

– LOCATION UPDATING REQUEST;

– IMSI DETACH;

– PAGING RESPONSE;

– CM RE-ESTABLISHMENT REQUEST;

– NOTIFICATION RESPONSE;

– IMMEDIATE SETUP.

If talker priority is supported, the layer 3 service request message may be:

– PRIORITY UPLINK REQUEST.

If VGCS talking is supported, the layer 3 service request message may be:

– TALKER INDICATION (applicable only to seizure of a group channel)

If sending application-specific data is supported, the layer 3 service request message may be:

– DATA INDICATION

– DATA INDICATION 2

Figure 3.1.5.1: Service request and contention resolution

3.1.6 Preemption

The datalink layer provides the capability to assign a priority to any message transferred in dedicated mode on SAPI 0 with multiframe operation. The available message priorities defined in 3GPP TS 44.006 are "high", "normal" and "low". Messages assigned a "high" priority are enabled to preempt, in the data link layer, all preceeding untransmitted and partially transmitted messages assigned a "low" priority that are using the same data link connection (same SAPI and logical channel). Messages or message portions that are preempted are discarded without notification to higher layers except that the first 2*N201 octets of any partially transmitted message are not discarded. The following priority assignments are defined for those Radio Resource, Mobility Management and Connection Management messages that use SAPI 0.

Table 3.1.6.1: Priority Values of Layer 3 Messages

Priority	Messages
Low	RR Application Information message (see NOTE 1)
Normal	All MM messages All CM messages All GTTP messages All other RR messages using SAPI 0 not listed here
High	ETWS Primary Notification: APPLICATION INFORMATION message containing an APDU ID of type ETWS (see sub-clause 10.5.2.48). RR Channel Establishment: ADDITIONAL ASSIGNMENT RR Configuration Change: CONFIGURATION CHANGE COMMAND RR Handover related ASSIGNMENT COMMAND HANDOVER COMMAND RR Channel release CHANNEL RELEASE PARTIAL RELEASE
NOTE 1: An Application Information message conveying an ETWS Primary Notification message (see sub-clause 10.5.2.48) is treated as a "high" priority message.

Use of the preemption capability by layer 3 is not required in a BSS or MS that does not send any "low" priority message. In this case, all messages may be treated as having "normal" priority.

Preemption capabilities in Layer 3 is not applicable to the Uplink messages, hence all Uplink messages are treated with "normal" priority. Note that the "Suspension and Resumption of Multiple frame operation" (See 3GPP TS 44.006) will affect the order in which the layer 3 messages are delivered on the Uplink.