Page 188 - From GMS to LTE

174 From GSM to LTE-Advanced Pro and 5G

UE Node-B S-RNC MSC
RANAP: RAB assignment req.
NBAP: radio link
reconfiguration prepare
NBAP: radio link
reconfiguration ready
UP: DL synchronization
UP: DL synchronization
AAL2: ERQ (channel establish-
ment for the speech path)
AAL2: ECF
UP: initialization (speech codec)
UP: initialization ack.
NBAP: radio link
reconfiguration commit
RRC: RB setup
RRC: RB setup complete
RANAP: RAB assignment resp.

Figure 3.36 Radio resource allocation for a voice traffic channel.

The establishment of a packet‐switched connection is referred to in the standards as
PDP context activation. From the user’s point of view, the activation of a PDP context
means getting an IP address to be able to communicate with the Internet or another IP
network. Background information on PDP context activation can be found in Chapter 2.
As shown for a voice call in the previous example, the establishment of a packet‐switched
connection also starts with an RRC connection setup procedure.
Once the signaling connection has been established successfully, the mobile device
continues the process by sending an Activate PDP Context request message via the
RNC to the SGSN as shown in Figure 3.37. As shown in the previous example, this
triggers the authentication of the subscriber and activation of the air interface encryp-
tion. Once encryption is in place, the SGSN continues the process by establishing a
tunnel to the GGSN, which in turn assigns an IP address to the user. Furthermore, the
SGSN requests the establishment of a suitable bearer from the RNC taking into account
QoS parameters (e.g. minimal bandwidth, latency) sent to the SGSN at the beginning of
the procedure in the Activate PDP Context request message. For normal Internet con-
nectivity, no special QoS settings are usually requested by mobile devices but if
requested, these values can be modified by the SGSN or GGSN based on information
contained in the user’s profile in the HLR in the event that the user has not subscribed
to the requested QoS or in case the connection requires a different QoS setting. The
establishment of the RAB is done in the same way as for a circuit‐switched channel, as
shown in Figure 3.36. However, as the bearer for a packet‐switched connection uses
other QoS attributes, the parameters inside the messages will be different.