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May 20 2014

LTE signaling: Prevent attach storms

https://www.youtube.com/embed/sSD1K8gXGCg

LTE is hitting its stride. Today close to 300 LTE networks are commercially launched in more than 100 countries. And that number is set to grow as almost 500 network operators have announced their commitment to deploy LTE[1].

With success comes very high expectations though. Users insist that LTE networks be fast, reliable and available; and those users will often go elsewhere when the network does not meet expectations. Since all user traffic passes through the Evolved Packet Core (EPC), it’s at the heart of ensuring high service reliability and availability as LTE traffic increases.

There are some important steps operators should consider as they design an EPC that will keep customer satisfaction high.

LTE Signaling Storms

It’s important that mobile operators design the EPC to prevent equipment or link failures from propagating to other nodes in the network and triggering additional issues such as LTE signaling storms and overload conditions that further disrupt user services.

High reliability and availability is especially important in the LTE signaling and control plane so that subscriber sessions for voice or data services are not affected when the MME/S4-SGSN fails or must restart.   But prior to the 3GPP release 10.5, the restoration procedures were not adequate to ensure reliable service. The impact on the LTE subscriber and the network was significantly different if the user was in “Idle” versus “Active” mode.

Restoration for Idle Users

A subscriber in idle mode is not currently in a voice call or active data session. Prior to 3GPP 10.5, if an LTE subscriber in idle state was connected to the network and assigned to an MME/S4-SGSN that subsequently failed (or restarted), that subscriber might not receive any services for an extended period of time. The network would not have sufficient device information nor the restoration procedures in place to “reattach” the device to the network.  This could have serious subscriber consequences and be a source of major customer dissatisfaction.

Fortunately, members of the3GPP CT4 working group led changes to Network Terminated Service Requests (TS 29.274) and to the Core Network and Terminal Restoration Procedure technical specifications (TS 23.007) that were implemented in Release 10.5 to resolve this deficiency.  These updated procedures ensured that the subscriber and the mobile device were reachable, even after an MME node failure.

Restoration for Active Users

The situation for active users (ECM connected) is equally problematic. In case of MME/S40SGSN failure, the user’s session would immediately drop and the service would terminate.

Before an LTE service could resume, the user must initiate a procedure so that the device can reattach itself to the network (e.g. service request, tracking area update).  This could generate an “attach storm” with potentially thousands of LTE subscribers assigned to the failed node simultaneously signaling the network to reattach.

Session Restoration Server

When an MME in the network fails or the link to the SGW fails, the user equipment (UE) context data on that MME and SGW is assumed to be stale and is purged from the node’s database. Any LTE/IMS service being used by those users is also dropped. Only when the UE reattaches to the network can the user context data be relearned.

To address this issue, we have developed an innovative Session Restoration Server (SRS) solution.  It stores and maintains a database of key parameters of the UE subscriber context data that can be immediately retrieved by the remaining MMEs in the pool.  This means the devices can be paged and the subscriber sessions restored without requiring the UEs to reattach to the network. The benefits of the SRS to the mobile operator and the LTE subscriber are:

Service resiliency

  • provided for either an MME or SGW failure with or without a restart
  • achieves restoration of user session with minimal increase in network signaling
  • eliminates the need for the UE to reattach to the network

Dependability

  • Network originated IMS services (e.g. voice call, video) will not fail
  • A failed MME does not escalate into a potential LTE signaling Attach storm

The SRS is a deployment option of the Alcatel-Lucent 9471 Wireless Mobility Manager, a high-performance, scalable MME/SGSN that is part of our IP Mobile Core solution. To contact the author or request additional information, please send an email to networks.nokia_news@nokia.com.

Related Materials

Product page: 9471 Wireless Mobility Manager Webpage: Alcatel-Lucent IP Mobile Core Application Note: LTE Subscriber Service Restoration Datasheet: Alcatel-Lucent 9471 Wireless Mobility Manager Application Note: The Impact of Small Cells on MME Signaling

Footnote

  1. [1] According to findings from research by the GSA (Global mobile Suppliers Association) and published in its latest Evolution to LTE report – May 6, 2014.
About David Nowoswiat
David Nowoswiat is a Sr. Product and Solutions Marketing Manager at Alcatel-Lucent supporting LTE Evolved Packet Core Marketing.
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