Page 276 - From GMS to LTE

262 From GSM to LTE-Advanced Pro and 5G

Mobile source target
Device eNodeB eNodeB MME S-GW PDN-GW
X2 S1
Measurement Report
Handover Request
Handover Req. Ack.
RRC Con. Reconfig.
SN Status Transfer

Random Access
(Dedicated resources)
Uplink user data

Downlink user data
Forwarding
Path Switch Request
Modify Bearer Req.
Mod. Bear. Req. Ack
Path Sw. Req. Ack
Release Resource
Up/downlink data

Figure 4.20 X2‐based handover message flow.

direction. Data arriving from the network over the S1 interface after the handover com-
mand has been issued is forwarded over the X2 interface to the target eNode‐B.
In LTE, there is no dedicated handover command. Instead, an RRC Connection
Reconfiguration message that contains all the parameters necessary to connect to the
new cell is used. Upon receiving the reconfiguration message, the mobile device stops
sending data in the uplink direction and reconfigures itself for communication with the
new eNode‐B. At the same time, the source eNode‐B stops accepting uplink data traffic
and sends an SN Status Transfer message to the target eNode‐B with the sequence
number of the last valid uplink data block. This helps the target eNode‐B to request an
uplink retransmission if it detects that there are some data blocks missing and allows for
data transmission continuity.
As the mobile device has already performed measurements, there is no need to search
for the new cell. Hence, the device can immediately transmit a random access preamble
on the PRACH as shown in Figure 4.12. As dedicated resources are used for the RACH
sequence, the device does not have to identify itself and only the first two messages
shown in the figure are required.
The Random Access Response message from the new eNode‐B ends the handover
procedure from the mobile point of view, and it can immediately resume transmitting
data in the uplink direction.
As the eNode‐B was given the Serving‐GW’s IP address and the TEID for the connection
in the initial handover request, it can forward all uplink data directly to the Serving‐GW