Page 273 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 259

creation of a tunnel over which the user’s IP packets can be sent. This is done by
sending a Create Session Request message to the Serving‐GW of its choice. For load
balancing, capacity and redundancy reasons, an MME can communicate with more
than one Serving‐GW. The Serving‐GW in turn forwards the request to a PDN‐gateway
which is located between the LTE core network and the Internet. The PDN‐GW then
selects an IP address from a pool and responds to the Serving‐GW with a Create
Session Response message. The Serving‐GW then returns the message to the MME
and the tunnel for the IP packets of the user between the Serving‐GW and the PDN‐
GW is ready to be used. This tunnel is necessary as the user’s location and hence its
Serving‐GW can change during the lifetime of the connection. By tunneling the user’s
IP data traffic, the routing of the data packets in the LTE network can be changed at
any time without assigning a new IP address to the user. For further details on user
data tunneling, see Chapter 2. Establishing a tunnel is also referred to in the specifica-
tion as establishing a context.

Establishing a Context in the Radio Network
After the context for the user has been established in the core network, the MME
responds to the initial Attach Request with an Initial Context Setup Request message,
which includes the Attach Accept message as shown in Figure 4.19. On the S1 interface
between the MME and eNode‐B, this message starts the establishment procedure for a
user data tunnel between the eNode‐B and the Serving‐GW. It includes the Tunnel
Endpoint Identity (TEID) used on the Serving‐GW for this connection.
The final link that has to be set up now is the bearer for the user’s IP packets on the
air interface. This is done by the eNode‐B by sending an RRC Connection Reconfiguration
message to the mobile device. Earlier during the attach process, a signaling radio bearer
(SRB‐1) was established for the signaling messages. With this connection reconfigura-
tion, a second signaling radio bearer is established for lower‐priority signaling messages
and a Data Radio Bearer (DRB) for the user’s IP packets. The message also includes two
further NAS messages, the Attach Accept message and the Activate Default Bearer
Context Request message. These messages configure the device’s higher protocol layers
on the radio protocol stack. In addition, this step is also used to assign the IP address to
the mobile device for communication with the Internet and other parameters such as
the IP address of the DNS server, which is required to translate URLs (e.g. www.wire
lessmoves.com) into IP addresses. Also, the message includes the QoS profile for the
default bearer context.
Once the RRC part of the protocol stack has been configured, the mobile device
returns an RRC Connection Reconfiguration Complete message to the eNode‐B. This
triggers the confirmation of the session establishment on the S1 interface with an Initial
Context Setup Response message.
After the mobile has also configured the user‐plane part of the protocol stack, it
returns an Attach Complete message to the eNode‐B, which includes the Activate
Default Bearer Complete message. Both messages are destined for the MME.
The final step of the attach procedure is to finalize the user data tunnel establishment
on the S1 interface between the Serving‐GW and the eNode‐B. So far, only the eNode‐B
is aware of the TEID on the Serving‐GW, because it has received this information in the
Initial Context Setup Request message. At this point in the procedure, the MME can
now inform the Serving‐GW of the TEID that the eNode‐B has assigned on its side for