9.1.8 Channel request

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

This message is sent in random mode on the RACH. It does not follow the basic format. The possible formats are presented directly below, without reference to information fields. The order of bit transmission is defined in 3GPP TS 44.004.

The message is only one octet long, coded as shown in figure 9.1.8.1 and table 9.1.8.1.

Figure 9.1.8.1: CHANNEL REQUEST message content

ESTABLISHMENT CAUSE (octet 1)

This information field indicates the reason for requesting the establishment of a connection. This field has a variable length (from 3 bits up to 6 bits).

RANDOM REFERENCE (octet 1)

This is an unformatted field with variable length (from 5 bits down to 2 bits).

The Channel Request message is coded as follows:

(Random Reference field is filled with "x").

Table 9.1.8.1: CHANNEL REQUEST message content

MS codes According to Establishment cause:
bits 8 …. 1
101xxxxx	Emergency call
110xxxxx	Call re-establishment; TCH/F was in use, or TCH/H was in use but the network does not set NECI bit to 1
011010xx	Call re-establishment; TCH/H was in use and the network sets NECI bit to 1
011011xx	Call re-establishment; TCH/H + TCH/H was in use and the network sets NECI bit to 1
100xxxxx 0010xxxx 0011xxxx 0001xxxx	Answer to paging See Table 9.1.8.2.
111xxxxx	Originating call and TCH/F is needed, or originating call and the network does not set NECI bit to 1, or procedures that can be completed with a SDCCH and the network does not set NECI bit to 1. note
0100xxxx	Originating speech call from dual‑rate mobile station when TCH/H is sufficient and supported by the MS for speech calls and the network sets NECI bit to 1. See note 5
0101xxxx	Originating data call from dual‑rate mobile station when TCH/H is sufficient and supported by the MS for data calls and the network sets NECI bit to 1. See note 5
000xxxxx	Location updating and the network does not set NECI bit to 1. See note 2c.
0000xxxx	Location updating and the network sets NECI bit to 1. See note 2c.
0001xxxx	Other procedures which can be completed with an SDCCH and the network sets NECI bit to 1. See note 1
011110xx 01111x0x 01111xx0	One phase packet access with request for single timeslot uplink transmission; one PDCH is needed.
01110xxx	Single block packet access; one block period on a PDCH is needed for two phase packet access or other RR signalling purpose.
01100111	LMU establishment, see note 2
01100xx0	Single block MBMS access; one block period on a PDCH is needed for transfer of MBMS SERVICE REQUEST message.
01100×01 01100011	Reserved for future use note 2a
01111111	Reserved, see note 2b

NOTE 1: Examples of these procedures are: IMSI detach, Short Message Service (SMS), Supplementary Service management, Location Services, sending of application-specific data.

NOTE 2: If such messages are received by a network, an SDCCH shall be allocated.

NOTE 2a: If such messages are received by a network, an SDCCH may be allocated.

NOTE 2b: This value shall not be used by the mobile station on RACH. If such message is received by the network, it may be ignored. The value is used by the network to answer to a 11 bits EGPRS Packet Channel request.

NOTE 2c: If the MS is Location Updating as part of a CS Fallback Mobile Originating call setup then the MS shall instead use the appropriate Originating call Establishment cause value. If the MS is Location Updating as part of a CS Fallback for other Mobile Originating services (e.g. USSD or Location Services) then the MS may either use the Establishment cause for the other service or the Establishment cause for Location Updating. If the MS is Location Updating as part of a Mobile Terminating CS Fallback procedure then the MS shall instead use the appropriate Answer to paging Establishment cause value as specified below Table 9.1.8.2.

Table 9.1.8.2: CHANNEL REQUEST message
(when answering to paging for RR connection establishment)

MS Capability Paging Indication (note 3)	Full rate only	Dual rate (note 5)	SDCCH only
Any channel	100xxxxx	100xxxxx	100xxxxx
SDCCH	0001xxxx	0001xxxx	0001xxxx
TCH/F	100xxxxx	0010xxxx	0001xxxx
TCH/H or TCH/F	100xxxxx	0011xxxx	0001xxxx

In case of a CHANNEL REQUEST triggered within a CS Fallback procedure, see 3GPP TS 23.272, the CHANNEL REQUEST message content shall be based on the relevant information provided in the paging notification received in source RAT, see 3GPP TS 24.301, as for a Paging Indication (see Table 9.1.8.2) set to:

– "TCH/H or TCH/F" if the mobile station is paged for a speech call or for any other CS service requiring a traffic channel,

– "SDCCH" if the mobile station is paged for a supplementary service transaction, a terminating location services request or any other CS service that can be completed with an SDCCH,

– "Any channel" otherwise (including where no relevant information is provided in the paging notification).

NOTE 3: The Paging Indication is provided by the Channel Needed IE (or the Channel Needed field) associated with the page which triggered the sending of the CHANNEL REQUEST message.

NOTE 4: In some cases the established connection will be used only to allow a default rejection mechanism to take place (typically the mobile station will send a RELEASE COMPLETE message with cause #88 "incompatible destination" as an answer to the incoming SETUP message).

NOTE 5: In this sub-clause, "dual rate capability" means that the MS supports both full rate and half-rate channels at least for the signalling channel mode. In addition, it may support either speech channel mode, or data channel modes, or both on half-rate channels.