Page 235 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 221

Figure 4.3 Physical routing of the S1
and the X2 interface.
Core network Internet

Aggregation router

X2

be forwarded between the two base stations involved in the process. For this, the GTP
protocol is used. While the X2 interface directly connects base stations with each other
from a logical point of view as shown in Figure 4.1, the practical implementation is
different. Here, the X2 interface is transported over the same backhaul link as the S1
interface up to the first IP aggregation router. From there, the S1 data packets are routed
to the core network while X2 data packets are routed back to the radio network as
shown in Figure 4.3. The main purpose of the aggregation router is to combine the traf-
fic of many base stations into a single traffic flow. This reduces the number of links
required in the field. In addition, the combined traffic flow is lower than the combined
peak capacity of the backhaul links to the base stations, as, in practice, the utilization of
different base stations varies over time.

4.2.3 The Mobility Management Entity (MME)

While the eNode‐Bs autonomously handle users and their radio bearers once they are
established, overall user control is centralized in the core network. This is necessary as
there needs to be a single point over which data flows between the user and the Internet.
Further, a centralized user database is required, which can be accessed from anywhere
in the home network and also from networks abroad in case the user is roaming.
The network node responsible for all signaling exchanges between the base stations
and the core network and between users and the core network is the Mobility
Management Entity (MME). Figure 4.1 shows its location in the overall network archi-
tecture. In large networks, there are usually many MMEs to cope with the amount of
signaling and due to station redundancy. As the MMEs are not involved in air interface
matters, the signaling they exchange with the radio network is referred to as Non‐access
Stratum (NAS) signaling. In particular, the MME is responsible for the following tasks:
Authentication. When a subscriber first attaches to the LTE network, the eNode‐B
●
communicates with the MME over the S1 interface and helps to exchange authentica-
tion information between the mobile device and the MME. The MME then requests
authentication information from Home Subscriber Server (HSS), which is discussed
in more detail below, and authenticates the subscriber. Once done, it forwards encryp-
tion keys to the eNode‐B so that further signaling and data exchanges over the air