Call establishment procedures

Call establishment procedures

1.1.1 Mobile originating call establishemnt
Before call establishment can be initiated the peer to peer connection between the Mobility Management sublayers in the Mobile Station and in the Network (MM-connection) must be established.
The Call Control entity of the Mobile Station requests the MM-sublayer to establish a mobile originating MM-connection enters the “MMconnection pending” state. This request shall contain a parameter to specify whether the call is a basic or an emergency call. This will lead to specific qualities of services to be provided by the MMsublayers.
Timer T303 is started when the CM-SERVICE-REQUEST message is sent.
After completion of the establishment of the MM-connection, a confirmation is given to indicate that MM is ready for data transfer and could be used to send the first call control message (SETUP) to the network.
The MM-connection establishment is indicated to call control at the network side when the first call control message has been received.

1.1.2 Basic call establishment
The Call Control entity of the Mobile Station requests the establishment of a MM-connection indicating that the call will be a basic call. This will lead to a specific quality of service required from the MM-sublayer : the originating Mobile Station must be registered and the cipher mode must be set before the MM-connection establishment can be confirmed.

1.1.3 Basic call request
Upon MM-connection establishment, the call control entity of the Mobile Station initiates call establishment by transferring a SETUP message across the radio interface. Following the transmission of the SETUP message, the call shall be considered by the Mobile Station to be in the “call initiated” state. This state is supervised by timer T303, which has already been started when entering the MM-connection pending state.
The SETUP message shall contain all the information required by the network to process the call and in particular, the called party address information.
When the Mobile Station is in the “call initiated” state and if it receives:
i) a CALL PROCEEDING message, the Mobile Station shall proceed as described in section 5.2.1.1.3;
ii) an ALERTING message, the Mobile Station shall proceed as described
in section 5.2.1.1.5;
iii) a CONNECT message, the Mobile Station shall proceed as described
in section 5.2.1.1.6;
iv) a RELEASE COMPLETE message the Mobile Station shall proceed as described in section 5.2.1.1.2.

Abnormal case:
Since timer T303 is used to supervise the two consecutive states MMconnection pending and Call initiated, the expiry of timer T303 leads to different actions depending on the respective state:
If timer elapses in the MM-connection pending state, the MM-connection in progress shall be aborted and the user shall be informed about the rejection of the call
If timer T303 elapses in the “call initiated” state before any of the CALL PROCEEDING, ALERTING, CONNECT or RELEASE COMPLETE messages has been received, the clearing procedure described in section 5.4 is performed.
1.1.4 Invalid call information
If, following the receipt of SETUP message, the network determined that the call information received from the Mobile Station is invalid (e.g. invalid number), then the network shall initiate call clearing as defined in sect. 5.4 with a cause such as one of the following:
# 1 “unassigned (unallocated) number”
# 3 “no route to destination”
# 22 ” number changed”
# 28 “invalid number format (incomplete number)”
MS Network
…———————Ú
à Ã
à SETUP Ã
à ————–> Ã
à Ã
Û———————”
CALL_PROC (i)
< – – – – – – – – – – – –
ALERTING (ii)
< – – – – – – – – – – – –
CONNECT (iii)
< – – – – – – – – – – – –
REL_COM (iv)
< – – – – – – – – – – – –
Fig. 5.2/GSM 04.08

Mobile originated Call initiation and possible subsequent responses.

1.1.5 Call proceeding
Note: Overlap sending is not supported.
As en-bloc sending is used (i.e. the network can determine that the SETUP message contains all the information required from the Mobile Station to establish the call) the network shall: send a CALL PROCEEDING message to the Mobile Station to indicate that the call is being processed; and enter the “mobile originating call proceeding” state.
Note: The possibility of the network immediately sending a CONNECT message or ALERTING message (thereby suppressing the CALL PROCEEDING message) is for further study.
Nevertheless the Mobile Station shall treat these messages as described in section 5.2.1.1.1.
The network may insert bearer capability information element(s) to select options presented by the mobile station in the Bearer Capability information element(s) of the SETUP message. The bearer capability information element(s) shall contain the same parameters as received in the SETUP except those presenting a choice. In this state the network may initiate the assignment of a traffic channel according to section
5.2.1.1.9 (early assignment).
When the Mobile Station receives the CALL PROCEEDING message, the Mobile Station shall stop timer T303; start timer T310; enter the “mobile originating call proceeding” state.
Similarly, if the network determines that a requested service is not authorized or is not available, the network shall initiate call clearing in accordance with sect. 5.2.4 with one of the following causes:
# 57 “bearer capability not authorized”
# 58 “bearer capability not presently available”
# 63 “service or option not available, unspecified” or # 65 “bearer service not implemented”.

Abnormal case:
If timer T310 elapses before any of the ALERTING, CONNECT or DISCONNECT messages has been received, the mobile station shall perform the clearing procedure described in section 5.2.4.
MS Network
…———————Ú
à Ã
à CALL_PROC Ã
à <————– Ã
à Ã
Û———————”
Fig. 5.3/GSM 04.08
Call proceeding sequence at mobile originating call establishment
1.1.6 Notification of interworking in connection with mobile originated call establishment.
Note: In this section, the term “interworking” is used only in the meaning of interworking with a network other than PLMN or ISDN, not as interworking between PLMN and ISDN since this is the normal case. In this sense, PLMN and ISDN are seen within the same environment, called the PLMN/ISDN environment.

During call establishment, the call may leave a PLMN/ISDN environment; e.g., because of interworking with another network, with a non- PLMN/ISDN user, or with non-PLMN/ISDN equipment within the called user’s premises. When such situations occur, a progress indicator information element shall be returned to the calling Mobile Station either:
a) in an appropriate call control message when a state change is required (e.g. ALERTING or CONNECT); or,
b) in the PROGRESS message when no state change is appropriate.

The progress indicator information element sent to the Mobile Station may contain one of the following progress description values:
a) #1 “call is not end-to-end PLMN/ISDN; further call progress information may be available in-band”.
b) #2 “destination address is non-PLMN/ISDN”.
c) #4 “call has returned to PLMN/ISDN.
If the progress indicator information element is included in a call control message other than PROGRESS, the procedures as described in the remainder of sect. 5.2.1 apply. If the progress indicator information element is included in the PROGRESS message, no state change will occur but all call control timers shall be stopped in the Mobile Station. In both cases, if indicated by the progress indicator information element, the Mobile Station attach the user connection for speech and then monitor it for further in-band information.
MS Network
…———————Ú
à Ã
à PROG Ã
à <————– Ã
à Ã
Û———————”
Fig. 5.4/GSM 04.08
Notification sequence at mobile originating call establishment
1.1.7 Call confirmation indication
Upon receiving an indication that user alerting has been initi-ated at the called address, the network shall: send an ALERTING message across the radio interface of the calling mobile station and enter the “call delivered” state.
When the Mobile Station receives the ALERTING message, the Mobile Station: may begin an internally-generated alerting indication; shall stop timer T303 or 310 and shall enter the “call delivered” state.
Abnormal cases:
On the Mobile Station side, if timer T310 expires, the Mobile Station shall initiate call clearing as described in section 5.4.

MS Network
…———————Ú
à Ã
à ALERT Ã
à <————– Ã
à Ã
Û———————”
Fig. 5.5/GSM 04.08
Call confirmation at mobile originating call establishment
1.1.8 Call connected
Upon receiving an indication that the call has been accepted, the network shall: send a CONNECT message across the radio interface to the calling Mobile Station; start timer T313 and enter the “connect indication” state.
This message indicates to the calling Mobile Station that a connection has been established through the network.
On receipt of the CONNECT message, the calling Mobile Station shall:
– attach the user connection to the radio path;
– return a CONNECT ACKNOWLEDGE message;
– stop any locally generated alerting indication (if applied);
– stop timer T303 or T310 (if any of them is running);
– enter the “active” state.

Abnormal cases:
On the Mobile Station side, if timer T303 or T310 expires, the Mobile Station shall initiate call clearing as described in section 5.4.
Note: The Mobile Station may have applied an internal alerting supervision timing function, e.g. an incorporated Unit Call Control, and when it expires initiates a call clearing.
On receipt of the CONNECT ACKNOWLEDGE message the network shall: stop timer T313 and enter the “active” state.

Abnormal cases:
On the network side, if timer T313 elapses before a CONNECT ACKNOWLEDGE
message has been received, the network shall perform the clearing
procedure as described in section 5.4.
MS Network
…———————Ú
à CONNECT Ã
à <————– Ã
à CONNECT ACK Ã
à ————–> Ã
Û———————”
Fig. 5.6/GSM 04.08
Call acceptance sequence at mobile originating call establishment
1.1.9 Call rejection
Upon receiving an indication that the network or the called user is unable to accept the call, the network shall initiate call clearing at the radio interface to the mobile which originated the call, as described in sect. 5.4 using the cause provided by the terminating network or the called user.
1.9 Transit network selection
Note: For further study.

1.1.10 Traffic channel assignment at mobile originating call establishment

It is a network dependent decision when to initiate the assignment of a traffic channel during the mobile originating call establishment phase.
After initiation of the assignment phase, the CC-entity of the network shall: enter the “assignment pending” state and suspend any message to be sent. Upon completion of the assignment phase, the network shall reenter its previous state and resume any stored message for transmission.
The assignment procedure does not affect any call control timer.

1.1.11 Call queuing at mobile originating call establishment
The conditions to apply queuing are described in Rec. GSM 03.01.
If an idle traffic channel is not available at the assignment instant, the network may place the traffic channel request in a queue. Calls arriving when all positions in the queue are occupied shall be cleared by the network using the cause #34 “no circuit/channel available”.
An explicit queuing indicator is not provided to the mobile station.
However, the mobile station can implicitly derive the queuing status from the combination of states of the call control and radio resource management entities.
While the mobile station is queuing for a traffic channel, further mobile originating and mobile terminating call requests, if supported by the mobile station, are released by the network using cause #34 “no circuit/channel available”.
The maximum queuing interval is supervised by the network. The limit is a network dependent choice. In case the network is not able to allocate a traffic channel within the queuing limit, the network will release the call using cause #34 “no circuit/channel available”.
Specific indications provided in the network to the remote user are a network dependent choice.
1.2.1 Emergency call establishment
The call control entity of the mobile station requests the establishment of a MM-connection indicating that the call shall be an emergency call. This may lead to the provision of a MM-connection with specific characteristics.
In case the network has accepted the emergency call request, it may initiate the Authentication procedure and ciphering. The mobile Station initiates the call establishment by transfering an EMERGENCY SETUP message across the radio interface. All further actions at the radio interface are the same as for a normal outgoing call according to section 5.2.1.1.

1.2.2 Mobile terminating call establishment
Before call establishment can be initiated in the Mobile station, the
MM-connection must be established by the network.
5.2.2.1 Call indication
After the arrival of a call from a remote user, the network shall :
initiate the MM-connection establishment according to section 4 and
enter the “MM-connection pending” state. The request to establish the
MM-connection is passed from the CM-sublayer to the MM-sublayer. It
contains the necessary routing information derived from the SETUP
message.
Upon completion of the MM-connection, the network shall : send the
SETUP message to the mobile station, start timer T303 and enter the
“call present” state.
Upon receipt of a SETUP message, the Mobile Station shall perform
compatibility checking as described in 5.2.2.2. If the result of the
compatibility checking was compatibility, the Mobile Station shall
enter the “call present” state otherwise the Mobile Station shall
proceed as described in 5.2.2.2.
If no response to the SETUP message is received by the network before
the expiry of timer T303, the procedures described in section 5.2.2.3.3
shall apply.
GSM 04.08 – v. 3.14.0 – page 113
MS Network
…———————Ú
à Ã
à SETUP Ã
à <————– Ã
à Ã
Û———————”
CALL_CONF (i)
– – – – – – – – – – – – >
REL_COM (ii)
– – – – – – – – – – – – >
Fig. 5.7/GSM 04.08
Mobile terminating call initiation
and possible subsequent responses.
5.2.2.2 Compatibility checking
The Mobile Station receiving a SETUP message shall perform
compatibility checking before responding to that SETUP message. Any
reference to the Mobile Station in sections 5.2.2.3 through 5.2.2.8
implicitly refers to compatible user equipment included in the Mobile
Station. Annex B defines compatibility checking to be performed by user
equipment upon receiving a SETUP message.
An incompatible Mobile Station shall respond with a RELEASE COMPLETE
message with cause #88 “incompatible destination”, and enter the “null”
state. The network shall process this RELEASE COMPLETE message in
accordance with sect. 5.2.2.3.4.
GSM 04.08 – v. 3.14.0 – page 114
5.2.2.3 Call confirmation
5.2.2.3.1 Response to SETUP
Following the compatibility checking the Mobile Station shall – with
the exception of the cases described below – acknowledge the SETUP
message by a CALL CONFIRMED message, and enter the “mobile terminating
call confirmed” state.
The mobile station may include in the CALL CONFIRMED message to the
network one or several bearer capability information elements to the
network, either preselected in the Mobile Station or corresponding to a
service dependent directory number (see Rec. GSM 09.07).
Note: The possibility of alternative responses (e.g., in
connection with supplementary services) is for further
study.
A busy Mobile Station which satisfies the compatibility requirements
indicated in the SETUP message shall respond either with a CALL
CONFIRMED message or a RELEASE COMPLETE message, both with cause #17
“user busy”.
If the mobile user wishes to refuse the call, a RELEASE COMPLETE
message shall be sent with the cause #21 “call rejected” .
In the two cases mentioned above, the Mobile Station shall release the
MM-connection and enter the “null” state after sending the RELEASE
COMPLETE message.
The network shall process the RELEASE COMPLETE message in accordance
with section 5.4.
GSM 04.08 – v. 3.14.0 – page 115
5.2.2.3.2 Receipt of CALL CONFIRMED and ALERTING by the network
Upon receipt of the CALL CONFIRMED message, the network shall : stop
timer T303, start timer T310 and enter the “mobile terminating call
confirmed” state.
The MS having entered the “mobile terminating call confirmed” state, if
the call is accepted at the called user side, the Mobile Station
proceeds as described in 5.2.2.5. Otherwise, if the signal information
element was present in the SETUP message user alerting is initiated at
the Mobile Station side; if the signal information element was not
present in the SETUP message, user alerting is initiated when an
appropriate channel is available.
Here, initiation of user alerting means:
– the generation of an appropriate tone or indication at the Mobile
Station, and
– sending of an ALERTING message by the MS to the network.
Upon receipt of an ALERTING message subsequent to receipt of a CALL
CONFIRMED message, the network shall: send a corresponding ALERTING
indication to the calling user; stop timer T310; start timer T301, and
enter the “call received” state.
5.2.2.3.3 Call failure procedures
In case of abnormal behavior the following call failure procedures
apply:
i. If the network does not receive any response to the SETUP message
prior to the expiration of timer T303, then the network shall:
initiate clearing procedures towards the calling user with cause #18
“no user responding”; and initiate clearing procedures towards the
called Mobile Station in accordance with 5.4.4 using cause #102
“recovery on timer expiry”.
ii. If the network has received a CALL CONFIRMED message, but does not
receive an ALERTING, CONNECT or DISCONNECT message prior to the
expiration of timer T310, then the network shall:
initiate clearing procedures towards the calling user with cause
#18 “no user responding”; and initiate clearing procedures
towards the called Mobile Station in accordance with sect. 5.4.4
using cause #102 “recovery on timer expiry”.
iii. If the network has received an ALERTING message, but does not
receive a CONNECT or DISCONNECT message prior to the expiry of timer
T301 (or a corresponding internal alerting supervision timing
function), then the network shall: initiate clearing procedures
towards the calling user with cause #19 “user alerting, no answer”;
and initiate clearing procedures towards the called Mobile Station
in accordance with sect. 5.4.4, using cause #102 “recovery on timer
expiry”.
GSM 04.08 – v. 3.14.0 – page 116
5.2.2.3.4 Called Mobile Station clearing during mobile terminating call
establishment
If a RELEASE COMPLETE or DISCONNECT message is received before a
CONNECT message has been received from the Mobile Station, the network
shall stop timer T303, T310 or T301 (if running); continue to clear the
Mobile Station as described in sect. 5.4.3; and clear the call to the
calling user with the cause received in the RELEASE COMPLETE or
DISCONNECT message.
5.2.2.4 Notification of interworking in connection with mobile terminating call
establishment
Note: In this section, the term “interworking” is used only in
the meaning of interworking with a network other than PLMN
or ISDN, not as interworking between PLMN and ISDN since
this is the normal case. In this sense, PLMN and ISDN are
seen within the same environment, called the PLMN/ISDN
environment.
During call establishment the call may enter an PLMN/ISDN environment,
e.g., because of interworking with another network, with a non-
PLMN/ISDN user, or with non-PLMN/ISDN equipment within the calling or
called user’s premises. When this occurs, the point at which the call
enters an PLMN/ISDN environment shall cause a progress indicator
information element to be included in the SETUP message to be sent to
the called Mobile Station:
a) #1 “call is not end-to-end PLMN/ISDN; further call progress
information may be available in-band”.
b) #3 “origination address is non-PLMN/ISDN”.
GSM 04.08 – v. 3.14.0 – page 117
5.2.2.5 Call accept
A Mobile Station indicates acceptance of a mobile terminating call by
sending a CONNECT message to the network. Upon sending the CONNECT
message the Mobile Station shall start Timer T313 and enter the
“connect request” state.
5.2.2.6 Active indication
On receipt of the CONNECT message, the network shall: stop timers T310,
T303 or T301 (if running); send a CONNECT ACKNOWLEDGE message to the
called user; initiate procedures to send a CONNECT message towards the
calling user and enter the “active” state.
There is no guarantee of an end-to-end connection until a CONNECT
message is received at the calling user’s side. Upon receipt of the
CONNECT ACKNOWLEDGE message the Mobile Station shall: stop timer T313,
attach the user connection and enter the “active” state.
When timer T313 expires prior to the receipt of a CONNECT ACKNOWLEDGE
message, the mobile station shall initiate clearing in accordance with
section 5.4.3.
MS Network
…—————–Ú
à CONN Ã
à ————–> Ã
à CONN ACK Ã
à <————- Ã
Û—————–”
Fig. 5.8/GSM 04.08
Call acceptance and active indication
at mobile terminating call establishment
GSM 04.08 – v. 3.14.0 – page 118
5.2.2.7 Traffic channel assignment at mobile terminating call establishment
It is a network dependent decision when to initiate the assignment of a
traffic channel during the mobile terminating call establishment phase.
After initiation of the assignment phase, the CC-entity of the network
shall : enter the “assignment pending” state and suspend any message to
be sent. Upon completion of the assignment phase, the network shall reenter
its previous state and resume any stored message for
transmission.
The assignment procedure does not affect any call control timer.
5.2.2.8 Call queuing at mobile terminating call establishment
The principles described in section 5.2.1.1.10 apply accordingly.
Note: the interworking to the fixed network has to fulfill the
network specific requirements.
GSM 04.08 – v. 3.14.0 – page 119
5.3 Signalling procedures during the “active” state
5.3.1 User notification procedure
This procedure allows the network to notify a Mobile Station of any
appropriate call-related event during the “active” state of a call. It
also may allow a Mobile Station to notify the remote user of any
appropriate call-related event during the “active” state of a call by
sending a NOTIFY message containing a notification indicator to the
network; upon receipt of this message, the network sends a NOTIFY
message containing the same notify indicator to the other user involved
in the call. No state change occurs at any of the interface sides
following the sending or the receipt of this message.
5.3.2 Call rearrangements
Call rearrangements on the air interface are not supported by SUSPEND
and RESUME messages. However if a remote non-PLMN user initiates call
rearrangements, the network shall inform the Mobile Station by means of
a NOTIFY message. In a similar way the Mobile Station can inform the
network about rearrangements by sending a NOTIFY message (e.g. change
of user equipment connected to the Mobile Station).
GSM 04.08 – v. 3.14.0 – page 120
5.3.3 DTMF protocol control procedure
Dual Tone Multi Frequency (DTMF) is an inband one out of four plus one
out of four signalling system primarily used from terminal instruments
in telecommumication networks. The support of DTMF in the network is
described in Rec. GSM 03.14.
The use of DTMF is only permitted when the speech teleservice is being
used or during the speech phase of alternate speech/data and alternate
speech/facsimile teleservices and shall be disabled in all other
phases.
Note 1: Since the DTMF protocol messages are sent in a store
and forward mode on the signalling channels the control of
the device at the far end may be delayed dependent on the
load of quality of the channels.
Note 2: The procedures described in this paragraph supports
DTMF only in the direction Mobile Station to network. The
need for DTMF support in the opposite direction is for
further study.
5.3.3.1 Start DTMF request by the Mobile Station
A user may cause a DTMF tone to be generated e.g. by depression of a
key in the MS. The relevant action is interpreted by the Mobile Station
as a requirement for a DTMF digit to be sent in a START DTMF message on
an established FACCH. This message contains the value of the digit to
be transmitted (0, 1, …, O, A, B, C, D, *, #).
Only a single digit will be transferred in each START DTMF message.
GSM 04.08 – v. 3.14.0 – page 121
5.3.3.2 Start DTMF response by the network
Upon receiving the START DTMF message the network will reconvert the
received digit back into a DTMF tone which is applied toward the remote
user and returns a START DTMF ACKNOWLEDGE message to the Mobile
Station. This acknowledgement may be used in the Mobile Station to
generate an indication as a feedback for a successful transmission.
If the network cannot accept the START DTMF message a START DTMF REJECT
message will be sent ot the Mobile Station.
5.3.3.3 Stop DTMF request by the Mobile Station
When the user indicates that the DTMF sending should cease e.g. by
releasing the key the Mobile Station will send a STOP DTMF message to
the network.
5.3.3.4 Stop DTMF response by the network
Upon receiving the STOP DTMF message the network will stop sending the
DTMF tone and return a STOP DTMF ACKNOWLEDGE message to the Mobile
Station.
5.3.3.5 Sequencing of subsequent start DTMF requests by the Mobile Station
The minimum length of tone generated by the switch should be according
to CEPT recommendation T/CS 46-02.
The minimum gap between two subsequent tones should be according to
CEPT recommendation T/CS 46-02.
There is no defined maximum length to the tone, which will normaly
cease when a STOP DTMF message is received from the MS. However, the
operator may choose to put a pre-defined time limit on the duration of
tones sent.
Therefore the Mobile Station shall ensure that messages are not sent
towards the network faster than the minimum times mentioned above will
allow.
The appropriate sequencing of DTMF control messages is achieved by
using the timers Ts and To shown in Fig. 5.9a and 5.9b which cannot
expire before the minimum intervals. Ts and To are specified in CEPT
Rec. T/CS 34-08.
Note: The network may implement the time limit option where the
DTMF tone duration is controlled by the network irrespective
of the receipt of a STOP DTMF message from the Mobile
Station.
GSM 04.08 – v. 3.14.0 – page 122
Mobile Station Network
START DTMF
———————————–>
START DTMF ACK
<———————————–
Ã
Ts>= Tsmin Ã
à STOP DTMF
———————————–>
STOP DTMF ACK
<———————————–
Fig. 5.9a/GSM 04.08
Single DTMF transmission
Mobile Station Network
START DTMF (x)
———————————–>
START DTMF ACK
<———————————–
Ã
Ts >=Tsmin Ã
à STOP DTMF
———————————–>
STOP DTMF ACK
<———————————–
Ã
To >=Tomin Ã
à START DTMF (y)
———————————–>
START DTMF ACK
<———————————-
.
.
.
Fig. 5.9b/GSM 04.08
Multiple DTMF transmission
GSM 04.08 – v. 3.14.0 – page 123
5.3.4 In-call modification
Note: Considering a possible future evolution, in-call
modification is specified as a symmetrical procedure.
The in-call modification procedure is used to realize the following
required services:
a) Alternate Speech/Unrestricted digital – transparent (GSM 02.02 26)
b) Alternate Speech/Unrestricted digital – non transparent (GSM 02.02
27)
c) Speech then unrestricted digital information – transparent (GSM
02.02 31)
d) Speech then unrestricted digital information – non transparent (GSM
02.02 32)
e) Alternate Speech/Group 3 fax (Teleservice Group 3 fax GSM 02.03).
5.3.4.1 Service Description
This circuit switched service allows the two users on a point-to-point
connection to use the connection between them for different information
transfer during the same call, but not at the same time.
The procedure provided for the support of multiple capability mobile
stations only.
In some cases the in-call modification procedure makes it necessary to
change the channel configuration by allocating a new channel and in
other cases to change channel configuration parameters while keeping
the previously allocated channel. This change is determined by the
network, which initiates either the assignment procedure or channel
mode modify procedure (see section 3).
The capability and the initial mode desired must be identified by the
mobile station by identifying each mode of operation with a separate
information element during call establishment. Further the type of
change between the modes must be identified by means of the repeat
indicator:
mode 1 “alternate” mode 2, or
mode 1 “and then” mode 2.