Page 274 - From GMS to LTE

260 From GSM to LTE-Advanced Pro and 5G

Mobile
Device eNode-B MME HSS Serving-GW PDN-GW

Initial Context Setup Request
(Attach Accept)
RRC Con. Reconfig.
(Attach Accept)
(Act. Default Bearer)
RRC Reconfig. Compl.
Initial Context Setup Resp.
Attach Complete
(Act. Default B. Compl.)
Attach Complete
Modify Bearer Request
Measurement Configuration Modify Bearer Response

IP User Data

Figure 4.19 Attach and default bearer activation message flow – part 2.

the user data tunnel with a Modify Bearer Request message. The Serving‐GW stores the
TEID of the eNode‐B for this tunnel and can now forward any incoming IP packets for
the user over the correct tunnel to the eNode‐B.
At this point, the connection is fully established and the mobile device can now send
and receive IP packets to and from the Internet via the eNode‐B, the Serving‐GW and
the PDN‐GW as shown in Figure 4.19. As the MME is only responsible for the overall
session management, it is not part of the user data path. Despite the overall complexity
of the procedure it is usually executed in only a fraction of a second and is thus per-
formed more quickly than similar procedures in GSM and UMTS. This is due to simpli-
fication on all network interfaces and bundling of messages of several protocol layers
into a single message that is then sent over the air interface.
Once the connection is established, the eNode‐B exchanges a number of additional
RRC Reconfiguration messages to configure measurements and reporting of neighbor
cells so that the connection can be handed over to a different cell later on if required.
This is not part of the attach procedure as such but is mentioned here anyway to give a
complete picture of the overall process.
4.6.3 Handover Scenarios

On the basis of the measurement and reporting configuration that the mobile device
has received from the eNode‐B, it starts measuring the signal strength of neighboring
cells. Once a configured reporting criterion has been met, it reports the current values