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.
