Page 183 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 169

UE RNC BSC 3G MSC 2G MSC

UE in RANAP relocation required
3G network MAP prepare 3G MSC requests a
handover handover from a
BSSMAP 2G MSC
handover request
Resource reservation Radio resources
BSSMAP are allocated in the
handover request Ack. 2G BSS
MAP prepare
handover Ack.
ISUP IAM
Speech path is
ISUP ACM established between
RANAP relocation command
RR handover the MSCs
command
UE sends SABM frame on TCH
BSSMAP handover detect
2G BSS informs 2G
UE now in MAP process
2G network access signal MSC of successful
BSSMAP handover complete handover
MAP send end 2G MSC informs
signal 3G MSC of successful
ISUP ANM
RANAP Iu release command handover
Release of resources RANAP Iu release complete
MAP send end sig.
Figure 3.32 3G–2G intersystem hard handover message flow.

does not have to listen for incoming Paging messages all the time but only at a certain
interval. At all other times, the receiver can be deactivated and thus battery capacity can
be saved. A slight disadvantage of this approach is, however, that the paging procedure
takes a little bit longer than if the PCH was constantly monitored by the mobile device.
In the event that the subscriber has an active PDP context while the mobile device is
in idle state, the network will also need to send a Paging message in case of an incoming
IP frame. Such a frame could, for example, originate from a messaging application.
When the mobile device receives a Paging message for such an event, it has to reestab-
lish a logical connection with the network before the IP frame can be forwarded.
In idle state, the mobile device is responsible for mobility management, that is, chang-
ing to a more suitable cell when the user is moving. As the network is not involved in
the decision‐making process, the procedure is called cell reselection.
While the mobile device is in idle state, no physical or logical connection exists
between the radio network and the mobile device. Thus, it is necessary to reestablish a
physical connection over the air interface if data needs to be transported again. For the
circuit‐switched part of the network, the RRC idle state therefore implies that no voice
connection is established. For the SGSN, on the other hand the situation is different.
A PDP context can still be established in idle state, even though no data can be sent or
received. To transfer data again, the mobile device needs to reestablish the connection,
and the network then either establishes a DCH or uses the FACH for the data exchange.
In practice, it can be observed that the time it takes to reestablish a channel is about