Page 236 - From GMS to LTE

222 From GSM to LTE-Advanced Pro and 5G

interface can be ciphered. Further details can be found in Section 4.6.2 on the attach
procedure and default bearer activation.
Establishment of bearers. The MME itself is not directly involved in the exchange of
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user data packets between the mobile device and the Internet. Instead, it communi-
cates with other core network components to establish an IP tunnel between the
eNode‐B and the gateway to the Internet. However, it is responsible for selecting a
gateway router to the Internet, if there is more than one gateway available.
NAS mobility management. In case a mobile device is dormant for a prolonged
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period of time (typical values found in practice are 10–30 seconds), the air interface
connection and resources in the radio network are released. The mobile device is
then free to roam between different base stations in the same Tracking Area (TA)
without notifying the network to save battery capacity and signaling overhead in the
network. Should new data packets from the Internet arrive for this device while it is
in this state, the MME has to send Paging messages to all eNode‐Bs that are part of
the current tracking area of the mobile device. Once the device responds to the pag-
ing, the bearer(s) is (are) reestablished.
Handover support. In case no X2 interface is available, the MME helps to forward
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the handover messages between the two eNode‐Bs involved. The MME is also
responsible for the modification of the user data IP tunnel after a handover in case
different core network routers become responsible.
Interworking with other radio networks. When a mobile device reaches the limit of
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the LTE coverage area, the eNode‐B can decide to hand over the mobile device to a
GSM or UMTS network or instruct it to perform a cell change to suitable cell. In both
cases, described in more detail in Section 4.9, the MME is the overall managing entity
and communicates with the GSM or UMTS network components during this operation.
SMS and voice support. Despite LTE being a pure IP network, some functionality is
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required to support traditional services such as voice calls and SMS, which were part
of the GSM and UMTS circuit‐switched core networks and cannot thus simply be
mapped to LTE. This is discussed in more detail in Section 4.13.
For these tasks, a number of different interfaces such as the S5, S6a, S11 and SGs are
used. These are described in the following sections.
When compared to GPRS and UMTS, the tasks of MMEs are the same as those of the
SGSN. The big difference between the two entities is that while the SGSN is also respon-
sible for forwarding user data between the core network and the radio network, the
MME deals only with the signaling tasks described above and leaves the user data to the
Serving Gateway (S‐GW), which is described in the next section.
Owing to this similarity, nodes that combine the functionality of a 2G SGSN, a 3G
SGSN and an MME are used in networks today. As an interesting option, which is,
however, not widely used in practice, the one‐tunnel enhancement described in
Chapter 3 also removes the user‐plane functionality from the SGSN, making a com-
bined node a pure integrated signaling platform that lies between the access networks
and a single core network for all radio technologies in the future.

4.2.4 The Serving Gateway (S‐GW)
The S‐GW is responsible for managing user data tunnels between the eNode‐Bs in the
radio network and the Packet Data Network Gateway (PDN‐GW), which is the gateway