The first thing UE does after switching on is to
synchronize to each frequency and check whether this frequency is from the
right operator to which it wants to connect to. UE does this by going through
very initial synchronization process. Once synchronized, UE reads the master
information block and System information blocks to check if this is the
right PLMN. Lets assume it finds that PLMN value to be correct and so UE will
proceed with reading System information block 1 and System
information block 2. The next step is known as Random Access Procedure in
which the network for the first time knows that some UE is trying to get access
and the network provides temporary resources to the UE for initial communication.
RRCConnectionRequest
·
RRC connection establishment is used to make the transition from
RRC Idle mode to RRC Connected mode. UE must make the transition to RRC
Connected mode before transferring any application data, or completing any
signaling procedures
·
The RRC connection establishment procedure is always initiated by
the UE but can be triggered by either the UE or the network. For example, the
UE Triggers RRC connection establishment if the end-user starts an
application to browse the internet, or to send an email. Similarly, the UE
triggers RRC connection establishment if the UE moves into a new Tracking Area
and has to complete the Tracking Area Update signaling procedure. The network
triggers the RRC Connection establishment procedure by sending a Paging
message. This could be used to allow the delivery of an incoming SMS or
notification of an incoming voice call.
·
RRC connection establishment for LTE is relatively simple compared
to RRC connection establishment for UMTS. The UMTS procedure requires NBAP and
ALCAP signaling across the Iub interface between the Node B and RNC. These
signaling protocols are used to setup a radio link and new transport
connection. The flat network architecture for LTE removes the requirement for
these signaling procedures.
·
In the case of LTE, the initial Non-Access Stratum (NAS) message
is transferred as part of the RRC connection establishment procedure. In the
case of UMTS, the initial NAS message is transferred after the RRC connection
establishment procedure. The approach used by LTE helps to reduce connection
establishment delay.
·
RRC connection establishment configures Signaling Radio Bearer
(SRB) 1 and allows subsequent signaling to use the Dedicated Control Channel
(DCCH) rather than the Common Control Channel (CCCH) used by SRB 0.
·
The signaling for RRC connection establishment is shown in Figure.
The entire procedure is completed using only RRC signaling. A 3-way handshake
is used to move the UE into RRC connected mode.
·
The content of the RRC Connection Request message is shown in
Table. It includes a UE identity and an establishment cause. There is no scope
for the UE to report any measurements within the RRC Connection Request
message.
UE identity (TMSI or Random
Value)
·
The UE identity is signaled using the SAE Temporary Mobile
Subscriber Identity (S-TMSI) TMSI is used if UE has previously connected to the
same network. With TMSI value, UE is identified in the core network.
·
Otherwise, the UE selects a random number in the range from 0 to
240- 1 to represent the UE identity. Random value is used if UE is connecting
for the very first time to network. Why we need random value or TMSI? Because
there is a possibility that Temp-CRNTI has been assigned to more than one UEs
in previous step, due to multiple requests coming at same time (Collision
scenario explained later)
UE identity By TMSI
RrcConnectionRequest{
CriticalExtensions {
RrcConnectionRequest-r8 {
ue_Identity {
S-TMSI {
mmec = {8, 0x00}
m_TMSI = {32, 0x01 0x02 0x03 0x04}
}
}
EstablishmentCause = highPriorityAccess
Spare = {1, 0xxxxxxx}
}
UE identity By Random Value
The establishment cause within the RRC
Connection Request message is determined by the Non-Access Stratum (NAS) procedure
for which the connection is being established. The relationship between
establishment cause and NAS procedure is specified by 3GPP TS 24.301. This
relationship is presented in below Table. In all cases, the RRC establishment
cause is set to ‘High Priority Access’ if the UE uses Access Class (AC) 11
to 15. The list of establishment causes is significantly shorter than the list
used by UMTS.
·
The UE starts the T300 timer after transmitting the RRC Connection
Request message. The value of T300 is broadcast within SIB 2. UMTS uses T300 in
combination with N300 to manage re-transmissions of the RRC Connection Request
message. LTE does not have an N300 parameter and the RRC Connection Request
message is sent only once per establishment procedure. LTE uses the T300 timer
to define how long the UE waits for a response to the RRC Connection Request
message. The establishment procedure fails if T300 expires before receiving an
RRC Connection Setup message. The procedure also fails if the UE completes a
cell re-selection prior to receiving the RRC Connection Setup message.
·
Assuming that random access contention does not occur, the UE
proceeds to wait for an RRC Connection Setup message from the eNodeB. The UE
has successfully completed the random access procedure so has been allocated a
C-RNTI (signaled within the random access response message). The UE monitors
the PDCCH for a downlink allocation addressed to its C-RNTI. The PDCCH
specifies the set of PDSCH Resource Blocks used to transfer the RRC Connection
Setup message. The RRC Connection Setup message is transferred using SRB 0 on the
CCCH.
execellent
ReplyDelete