Page 173 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 159

3.6 Core Network Mobility Management

From the point of view of the MSC and the SGSN, the mobile device can be in any of the
MM or PMM states described below. The MSC knows the following MM states:
MM detached. The mobile device is switched off and the current location of the
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subscriber is unknown. Incoming calls for the subscriber cannot be forwarded to the
subscriber and are either rejected or forwarded to another destination if the Call
Forward Unreachable (CFU) supplementary service is activated.
MM idle. The mobile device is powered on and has successfully attached to the MSC
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(see attach procedure). The subscriber can at any time start an outgoing call. For
incoming calls, the mobile device is paged in its current location area.
MM connected. The mobile device and MSC have an active signaling and communi-
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cation connection. Furthermore, the connection is used for a voice or a video call.
From the point of view of the RNC, the subscriber is in the Cell‐DCH RRC state as
this is the only bearer that supports circuit‐switched connections.

The SGSN implements the following PMM states:
PMM detached. The mobile device is switched off and the location of the
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subscriber is unknown to the SGSN. Furthermore, the mobile device cannot
have an active PDP context, that is, no IP address is currently assigned to the
subscriber.
PMM connected. The mobile device and the SGSN have an active signaling and
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communication connection. The PMM connected state is only maintained while
the subscriber has an active PDP context, which effectively means that the GGSN
has assigned an IP address for the connection. In this state, the SGSN simply for-
wards all incoming data packets to the S‐RNC. In contrast to GSM/GPRS, the
UMTS SGSN is aware only of the S‐RNC of the subscriber and not of the current
cell. This is due not only to the desired separation of radio network and core net-
work functionality, but also to the soft handover mechanism (see Section 3.7). The
SGSN is also not aware of the current RRC state of the mobile device. Depending
on the QoS profile, the network load, the current data‐transfer activity and the
required bandwidth, the mobile device can be in Cell‐DCH, Cell‐FACH, Cell‐PCH
or URA‐PCH state.
PMM idle. In this state, the mobile device is attached to the network but no logical
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signaling connection is established with the SGSN. This can be the case, for example,
if no PDP context is active for the subscriber. If a PDP context is established, the
RNC has the possibility to modify the RRC state of a connection at any time. This
means that the RNC can decide, for example, after a period of inactivity on the
connection to set the mobile device into the RRC idle state. Subsequently, as the
RNC no longer controls the mobility of the subscriber, it requests the SGSN to set
the connection into PMM idle state as well. Therefore, even though the subscriber
no longer has a logical connection to either the RNC or the SGSN, the PDP context
remains active and the subscriber can keep the assigned IP address. For the SGSN,
this means that if new data arrives for the subscriber from the GGSN, a new signal-
ing and user data connection has to be established before the data can be forwarded
to the mobile device.