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Monday, December 12, 2011

Architectural Evolution of 3GPP Mobile Networks: Moving Towards Flat Architectures

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.

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