Fixed networks were firstly subject to
scalability problems. The evolution of DSL access architecture has shown in the
past that pushing IP routing and other functions from the core to the edge of
the network results in sustainable network infrastructure. The same evolution
has started to happen within the wireless telecommunication and mobile Internet
era.
The 3GPP network architecture
specifications having the numbers 03.02 and 23.002 show the evolution of the
3GPP network from GSM Phase 1 published in 1995 until the Evolved Packet System
(EPS) specified in Release 8 in 2010. The core part of EPS called Evolved
Packet Core (EPC) is continuously extended with new features in Release 10 and
11. The main steps of the architecture evolution are summarized in this
article. The illustration below illustrates the evolution steps of the
packet-switched domain, including the main user plane anchors in the RAN and
the CN.
In Phase 1 (1995) the basic elements of the
GSM architecture have been defined. The reasons behind the hierarchization and
centralization of the GSM architecture were both technical and economical.
Primarily it offloaded the switching equipment (cross-bar switch or MSC). In
parallel, existing ISDN switches could be re-used as MSCs only if special voice
encoding entities were introduced below the MSCs, hence further strengthening
the hierarchical structure of the network. However, with the introduction of
the packet-switched domain (PS) and the expansion of the PS traffic the
drawbacks of this paradigm started to appear very early.
The main driver to introduce
packet-switching was that it allowed multiplexing hence resources could be
utilized in a greater extent. In Phase 2+ (1997) the PS domain is described,
hence centralized General Packet Radio Service (GPRS) support nodes are added
to the network. Release 1999 (2002) describes the well known UMTS architecture
clearly separating the CS and PS domains. Seeing that UMTS was designed to be
the successor of GSM, it is not strange that the central anchors remained in
place in 3G and beyond.
Progress of mobile and wireless
communication systems introduced some fundamental changes. The most drastic
among them is that IP has become the unique access protocol for data networks
and the continuously increasing future wireless traffic is also based on packet
data (i.e., Internet communication). Due to the collateral effects of this
change a convergence procedure started to introduce IP-based transport
technology in the core and backhaul network: Release 4 (2003) specified the
Media gateway function, Release 5 (2003) introduced the IP Multimedia Subsystem
(IMS) core network functions for provision of IP services over the PS domain,
while Release 6 standardized WLAN interworking and Multimedia Broadcast
Multicast Service (MBMS).
With the increasing IP-based data traffic
flattening hierarchical and centralized functions became the main driving force
in the evolution of 3GPP network architectures. Release 7 (also called Internet
HSPA, 2008) supports the integration of the RNC with the NodeB providing a one
node based radio access network. Another architectural enhancement of this
release is the elaboration of Direct Tunnel service.
Direct Tunnel allows to offload user
traffic from SGSN by bypassing it. The Direct Tunnel enabled SGSNs can initiate
the reactivation of the PDP context to tunnel user traffic directly from the
RNC to the GGSN or to the Serving GW introduced in Release 8. This mechanism
tries to reduce the number of user-plane traffic anchors. However it also adds
complexity in charging inter-PS traffic because SGSNs can not account the
traffic passing in direct tunnels. When Direct Tunnel is enabled, SGSNs still
handle signaling traffic, i.e., keep track of the location of mobile devices
and participate in GTP signaling between the GGSN and RNC.
Release 8 (2010) introduces a new PS
domain, i.e., the Evolved Packet Core (EPC). Compared to four main GPRS PS
domain entities of Release 6, i.e. the base station (called NodeB), RNC, SGSN
and GGSN, this architecture has one integrated radio access node, containing
the precious base station and the radio network control functions, and three
main functional entities in the core, i.e. the Mobility Management Entity
(MME), the Serving GW (S-GW) and the Packet data Network GW (PDN GW). Release 9
(2010) introduces the definition of Home (e)NodeB Subsystem. These systems
allow unmanaged deployment of femtocells at indoor sites, providing almost
perfect broadband radio coverage in residential and working areas, and
offloading the managed, pre-panned macro-cell network.
In Release 10 (2010) Selective IP Traffic
Offload (SIPTO) and Local IP Access (LIPA) services have been published. These enable local breakout of certain IP
traffic from the macro-cellular network or the H(e)NodeB subsystems, in order
to offload the network elements in the PS and EPC PS domain. The LIPA function
enables an IP capable UE connected via Home(e)NodeB to access other IP capable
entities in the same residential/enterprise IP network without the user plane
traversing the core network entities. SIPTO enables per APN and/or per IP flow
class based traffic offload towards a defined IP network close to the UE's point
of attachment to the access network. In order to avoid SGSN/S-GW from the path,
Direct Tunnel mode should be used.
The above evolutionary steps resulted in
that radio access networks of 3GPP became flattened to one single serving node
(i.e., the eNodeB), and helped the distribution of previous centralized RNC
functions. However, the flat nature of LTE and LTE-A architectures concerns
only the control plane but not the user plane: LTE is linked to the Evolved
Packet Core (EPC) in the 3GPP system evolution, and in EPC, the main packet
switched core network functional entities are still remaining centralized,
keeping user IP traffic anchored. There are several schemes to eliminate the
residual centralization and further extend 3GPP.
