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Monday, April 18, 2011

Shrinking Networks: Evolution towards flat architecture

3GPP networks will increasingly be used for IP-based packet services. 3GPP Release 6 has four network elements in the user and control plane: base station, RNC, Serving General Packet Radio System (GPRS) Support Node (SGSN) and Gateway GPRS Support Node. The architecture in Release 8 LTE will have only two network elements: base station in the radio network and Access Gateway (a-GW) in the core network. The evolution towards a flat architecture is also considered beneficial for HSPA, especially when a larger part of the traffic is based on the packet-switched domain.

A flat network architecture reduces the network latency and, thus, improves the overall performance of IP-based services. The flat model also improves both user- and control-plane efficiency. The efficiency of the last mile transport is improved, since the UMTS radio protocols, like the RLC protocol, are terminated in Node B and do not need to be carried over the last mile to RNC. The introduction of HSDPA and HSUPA enables the flat architecture, since more functionalities are anyway distributed from RNC to Node B in Releases 5 and 6. It is possible to design a flat HSPA radio network architecture where all the RNC functionalities are located in Node B. Such a network architecture can be implemented already based on 3GPP Release 5 or Release 6 by using the existing open interfaces. There is no immediate need to define the flat radio network architecture in 3GPP. It was still decided to include a few further definitions and clarifications in 3GPP Release 7 for flat architecture. Those definitions are related to the maximum number of RNCs that can be connected to the core network. Release 6 allows a maximum of 4096 RNCs to be connected to each SGSN. This address space is expanded in Release 7, since the SGSN is connected directly to each base station. Release 7 also includes clarifications for treating CS voice calls when the terminal is located in the packet-only flat architecture frequency layer. The packet architecture evolution in HSPA is designed to be backwards compatible: existing Release 5 and Release 6 terminals can operate with the new architecture. In other words, the architecture evolution is not visible to the terminal. The hierarchical and flat architectures are illustrated in the figure below. Release 7 includes flat architecture improvements both in the packet core and in the radio network. The so-called direct tunnel solution allows the user plane to bypass SGSN. The SGSN dimensioning will not be affected by the user-plane volume. The flat radio network architecture integrates RNC functionality as part of the base station. The RNC dimensioning will not be affected by the traffic volume, since no RNC element is required. When the flat radio architecture and direct tunnel SGSN solutions are combined, there are only two network elements in the user plane in the same way as in the LTE architecture.
While 3GPP is working on a new radio system (LTE) in Release 8, there are a number of important enhancements also brought to the existing HSPA standard. HSPA enhancements provide major improvements to the end-user performance in reducing call setup times, channel allocation times, terminal power consumption and in increasing peak bit rates.

3GPP Release 7 allows simplification of the network architecture. The number of network elements for the user plane can be reduced from four in Release 6 down to two in Release 7. The HSPA flat architecture in Release 7 is similar to the architecture agreed for 3GPP LTE, thus enabling later smooth evolution from HSPA to LTE.

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