SAE Review Part 1: Let’s Be Flexible and Redundant

Release 8 of the 3GPP specification is nearing completion and I thought it's the right time to have a closer look at one of the key core network architecture specifications for LTE, or to be precise, the SAE (System Architecture Evolution) in 3GPP TS 23.401. It's title 'GPRS enhancements for E-UTRAN' is a bit misleading as it is an architecture document in itself that shows the full architecture and not only enhancements. It has become a massive document, 219 pages at the moment, so a single blog post won't do to describe the features which are different compared to GSM and UMTS. So I've decided to split the review into several parts and start with the flexibility and redundancy of network elements which is built into the system from day one.

In the initial 3GPP specs for UMTS (Release 99 or Release 3 if you will after the current counting method), the network was pretty hierarchical. One UMTS base station (NodeB) was connected to one radio network controller (RNC) which was in turn connected to one MSC for voice calls and one SGSN (Serving GPRS Support Node) for packet data traffic. In later 3GPP releases the RNC interface has become more flexible (the famous Iu flex) and in theory, a single UMTS RNC can now be connected to several MSCs and SGSNs for redundancy and load sharing purposes. In practice, however, I suspect it is not used a lot (yet).

Splitting the gateway into MME and SGW and assigning several to a single base station

In the LTE/SAE specs, flexibility and redundancy is built in from day one.  A single LTE base station, called eNodeB, can now be connected to several gateway nodes simultaneously. The gateway node itself is split into a Mobility Management Entity (MME) and a Serving Gateway (SGW) and an interface has been defined between the two. So in practice both can be in the same physical device or split into two different devices. There is also no need to have the same number of MMEs and SGWs in the network, so capacity can be independently increased for the management part (MME) or the datapath (SGW) as needed.

Moving subscribers from one gateway to another and creating redundancy

There are even functions foreseen to move subscribers of one MME or SGW to another MME or SGW, for example to upgrade the software and then reboot the device. Another benefit of pools is that in case one device fails, not all users in the area are affected. If one node fails and the connection is interrupted a device can quickly reconnect and be assigned to a different node. Quite a difference to today where the failure of a single SGSN immediately renders a part of the network useless. It happens often enough…

[Updated 7. September 2009] Tracking Area Lists to prevent border hopping

Another piece of flexibility are tracking area lists, which used to be called location areas (LAs) or routing areas (RAs) in UMTS. Like LAs and RAs, a tracking area is a conglomerate of one or more cells. Mobile devices currently not connected to the network only have to report to the network when they change to a tracking area which is not in the list that was assigned to them by the MME during the last tracking area update. This reduces power consumption and reduces mobility management signaling in the network. Tracking area lists in effect blur the tracking area boundaries and prevent scenarios in which a mobile device keeps hopping between two cells in different tracking areas resulting in frequent signaling exchanges and battery drain.

Packet Data Network Gateway flexibility

And of course the packet data gateway (PDN-GW), the gatekeeper between the mobile network and the Internet (or a fixed line IP network in general) is also not fixed but can be chosen from a pool.

Summary

As shown above, the whole LTE/SAE architecture has been defined in a very flexible way for several reasons. Compared with the flexibility added to GSM/GPRS and UMTS over time, this goes one step further and the use of IP for all interfaces helps a great deal to make this much more simple than in 2G and 3G networks.

So much for today. In the next part, I'll look at Mobility- and Connection Management (EMM and CMM) and the differences to UMTS's Packet Mobility Management and Session Management (PMM and SM).