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From LTE for UMTS: Evolution to LTE-Advanced, Second Edition. Edited by Harri Holma and Antti Toskala.

© 2011 John Wiley & Sons, Ltd.

When the evolution of the radio interface started (E-UTRAN), it soon became clear that the system architecture would also need to be evolved. The general drive towards optimizing the system only for packet switched services is one reason that alone would have set the need for evolution, but some of the radio interface design goals – such as removal of soft handover – opened up new opportunities in the architecture design. Also, since it had been shown by High Speed Packet Access (HSPA) that all radio functionality can be efficiently co-located in the NodeB, the door was left open for discussions of flatter overall architecture.

Discussions for System Architecture Evolution (SAE) then soon followed the radio interface development, and it was agreed to schedule the completion of the work in Release 8. There had been several reasons for starting this work, and there were also many targets.

The following lists some of the targets that possibly shaped the outcome the most:

  • optimization for packet switched services in general, when there to support the circuit switched mode of operation;
  • optimized support for higher throughput required for higher end
  • improvement in the response times for activation and bearer set-
  • improvement in the packet delivery delays;
  • overall simplification of the system compared to the existing 3GPP systems;
  • optimized inter-working with other 3GPP access networks;
  • optimized inter-working with other wireless access networks.

Many of the targets implied that a flat architecture would need to be developed.
Flat architecture with less involved nodes reduces latencies and improves performance.


Development towards this direction had already started in Release 7 where the Direct Tunnel concept allows User Plane (UP) to bypass the SGSN, and the placement of RNC functions to HSPA NodeB was made possible. The Figure here (left) shows these evolution steps and how this aspect was captured at a high level in SAE architecture.

Some of the targets seem to drive the architecture development in completely different directions. For example, optimized inter-working with several wireless access networks (ANs) indicates the need to introduce a set of new functions and maybe even new interfaces to support specific protocols separately for each one of them. This works against the target of keeping the architecture simple. Therefore, since it is likely that that none of the actual deployments of the architecture would need to support all of the potential inter-working scenarios, the 3GPP architecture specifications were split into two tracks:

  • GPRS enhancements for E-UTRAN access [1]: This document describes the architecture and its functions in its native 3GPP environment with E-UTRAN and all the other 3GPP ANs, and defines the inter-working procedures between them. The common nominator for these ANs is the use of GTP (GPRS Tunnelling Protocol) as the network mobility protocol.
  • Architecture enhancements for non-3GPP accesses [2]: This document describes the architecture and functions when inter-working with non-3GPP ANs, such as cdma2000® High Rate Packet Data (HRPD), is needed. The mobility functionality in this document is based on IETF protocols, such as MIP (Mobile Internet Protocol) and PMIP (Proxy MIP), and the document also describes E-UTRAN in that protocol environment.

Extracted from Chapter 3 of "LTE for UMTS: Evolution to LTE-Advanced"...See Wiley.com for further details about the book.

[13GPP TS 23.401, ‘General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial
Radio Access Network (E-UTRAN) access (Release 8)’.

[23GPP TS 23.402, ‘Architecture enhancements for non-3GPP accesses (Release 8)’

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