Subscribe Us

Get free daily email updates!

Follow us!

Thursday, June 7, 2012

Call Setup Failures in LTE

Network accessibility in LTE requires different performance counters than those known in 2G and 3G radio access networks. If the definition can be agreed that a “call” is a single radio connection between the UE and the network that is used to transmit payload using multiple bearers and service flows, then all failures that prevent a UE from attaching to the network and enabling Public Data Network (PDN) connectivity can be summarized by the umbrella term “call setup failures.”

The strong link between attach to network and PDN connectivity is also reflected by the standards, especially 3GPP 24.301 “NAS Protocol for Evolved Packet System (EPS).” Here it is defined that in case of an unsuccessful attach an explicit PDP connection reject message should be sent to the UE to ensure proper transitions of EPS Mobility Management (EMM) and ESM (EPS Session Management) states in the UE’s NAS signaling entity.

The illustration below shows an attach rejected by the network due to problems in the EPC. The most common problems are that one of the GPRS Tunneling Protocol (GTP) tunnels for the default EPS bearer cannot be established or the location update procedure between the MME and Home Subscriber Server (HSS) fails. Also, failures in Stream Control Transmission Protocol (SCTP) transport belong to this category. However, it can be expected that errors on the SCTP level will have a more serious impact on the network performance than just the failed attach of a single subscriber.

A more detailed view of a typical update location failure is shown in the illustration below. Such a failed update location does not need to be necessarily a network issue. In fact, there are a couple of cases where it is meaningful that access to the network is restricted to particular subscribers or groups of subscribers. For instance, if the home network operator of the subscriber does not have a roaming agreement with the operator of the visited network then the subscriber cannot be charged for the roaming services and, hence, it is good not to reject the attach request. In such cases the cause value signaled on DIAMETER or MAP from HSS will be the same as in Attach Reject.

Since the initial attach procedure is linked with many other signaling procedures in the E-UTRAN and EPC, failures that occur in a particular network element or on a particular signaling link often trigger a chain reaction. This is also true in case the initial context setup procedure on S1 between the MME and eNB fails, as shown below.

In the example the eNB is not able to set up the initial context, which triggers in turn Attach Reject (with cause “network failure”? – which depends on the implementation in MME software) and PDP Connection Reject. In addition, due to the failed attach, the GTP tunnels on S1-U and S5 that have already been established need to be deleted. For this purpose the GTP-C delete session procedure will be used on the S11 signaling link between the MME and Serving Gateway (S-GW) and on S5 between S-GW and PDN-GW. Since the UE’s new location was already successfully updated in the HSS, it is now necessary to delete this entry and mark the UE in the HSS database as “not reachable,” because a UE that is not attached to the network should not be paged. The signaling procedure that is used to notify the HSS about this new state of the connection is the purge UE procedure.

Even if the UE can successfully attach to the network, there is another potential error with an impact on the PDN connectivity. In this case, after the successful attach the activation of the default bearer fails. In most cases the origin of this failure is expected to be found in the UE itself. Actually all handsets have to undergo various load and stress test scenarios in the lab before they become available on the market. Thus, the likelihood of seeing such failures is rather small. Nevertheless, it may happen and is for good reason defined in the new LTE NAS protocol (3GPP 24.301). the illustration below shows the signaling pattern of such failures.

Activate Default EPS Bearer Request is sent by the MME to the UE together with Attach Accept. While Attach Accept is a mobility management (EMM) message, Activate Default EPS Bearer Request belongs to the category of session management (ESM) messages. If for some reason the UE is not able to confirm the successful activation of the default EPS bearer it will respond with an activate default EPS bearer failure message. As a result the UE will remain attached to the network (NAS state: EMM Registered) while it does not have an active bearer (NAS state in UE: ESM Bearer Context Inactive).

The reaction of the MME when receiving Activate Default EPS Bearer Failure is to repeat sending Activate Default EPS Bearer Request for a maximum of four times. If this does not help to activate the bearer context on the UE side, the MME may detach the UE so that a new initial registration to the network is required.

Google Profile

0 Responses to “ Call Setup Failures in LTE ”

Post a Comment