CSFB and SMS over
Circuit Switched Fallback (CSFB) provides an interim solution for
enabling telephony and short message service (SMS) for LTE operators that do
not plan to deploy IMS packet switched services at initial service launch.
Fallback (CSFB) enables the UE to camp on an EUTRAN cell and originate or
terminate voice calls through a forced switch over to the circuit switched (CS)
domain or other CS-domain services (e.g., Location Services (LCS) or
supplementary services). Additionally, SMS delivery via the CS core network is
realized without CSFB. Since LTE EPC networks were not meant to directly anchor
CS connections, when any CS voice services are initiated, any PS based data
activities on the E-UTRAN network will be temporarily suspended (either the
data transfer is suspended or the packet switched connection is handed over to
the 2G/3G network).
CSFB provides an
interim solution for enabling telephony and SMS services for LTE operators that
do not plan to deploy IMS packet switched services at initial service launch.
CSFB function is
realized by reusing Gs interface mechanisms, as defined in 3GPP TS 29.018, on
the interface between the MME in the EPS and the VLR. This interface is called
the SGs interface. The SGs interface connects the databases in the VLR and the
requires that a valid license key be installed. Contact your Cisco Account or
Support representative for information on how to obtain a license.
The following CSFB
features are supported:
- Release 8 and Release 9
- SGs-AP Encode/Decode of all messages
- SGs-AP Procedure Support
Originating Voice Call
Terminating Voice Call
Enhanced TAI to LAI Mapping
Basic LAI to VLR
distribution among multiple MMEMGRs
Multi-homing for SGs interface
for SGs interface
Support (Service/VLR association)
- Disallow CSFB
- Reject EPS if
IMSI attach fails
- Reject EPS if
VoIMS and no CSFB
- CSFB Not
RFSP based on UE Usage and and Voice Domain Preference
Suspend/Resume over S11 (Release 8)
Suspend/Resume over S3/S11 (Release 9)
Support for SGs
AP Timers: TS6-1, ts8,
ts9, ts10, ts12-1, ts12-2, ts-13
- Idle mode
Signaling Reduction (ISR)
- SNMP Trap Support
- VLRAssocDown - sent when
an SCTP association to a VLR is down.
- sent when all SCTP
associations to a VLR are down.
- VlrAllAssocDown - sent
when all associations
Passive VLR Offload: See
- Support for
Active VLR Offload: See
- UE Detach on VLR
- UE Detach on VLR
DSCP Marking for SGs
provides the Differentiated Services Code Point (DSCP) marking functionality.
DSCP marking helps in packet traffic management. DSCP marking can be performed
on both IPv4 and IPv6 packets leaving the SGs interface.
pre-defined DSCP values can be used for marking, or any arbitrary value ranging
from 0x01 to 0x3F can be assigned. The default DSCP value is 0x00 or be (Best
Effort). The default DSCP value is automatically set when the configuration is
[no] ip qos-dscp dscp_value
ip defines the Internet Protocol parameters for the
packets leaving through the SGs interface.
qos-dscp designates the Quality of Service -
Differentiated Services Code Point value to the packet leaving through the SGs
is a value assigned to the packet for DSCP marking. The
value can be a pre-defined DSCP value or an arbitrary value ranging from 0x01
How It Works
EPC core networks
are designed for all IP services and as such lack intrinsic support
for circuit switched voice and telephony applications. This
presents challenges for those operators that do not plan to launch
packet switched IMS core networks at initial service deployment. CSFB
represents an interim solution to address this problem by enabling
dual radio mobile devices (LTE/GSM/UMTS
or CDMA1xRTT) to fallback to GSM/UMTS or CDMA1x access
networks to receive incoming or place outgoing voice calls. The
next section presents highlights of the CSFB procedure.
When the GSM/UMTS/LTE
access terminal attaches to the EUTRAN access network, it
uses combined attachment procedures to request assistance from the
MME to register its presence in the 2G/3G network.
The MME uses SGs
signaling to the MSC/VLR to register on behalf of the AT
to the 2G/3G network. The MME represents itself
as an SGSN to the MSC and the MSC performs a location update to
the SGSN in the target 2G/3G network.
The MME uses the
Tracking Area Identity provided by UE to compute the Location Area
Identity it provides to the MSC.
Execution Phase: Mobile
When a call comes
in at the MSC for the user, the MSC signals the incoming
call via the SGs interface to MME.
If the AT is an active
state, the MME forwards the request directly to the mobile. If
the user wishes to receive the call the UE instructs the MME to
hand over the call to the 2G/3G network. The MME
then informs the eNodeB to initiate the handoff.
If the AT is in dormant
state, the MME attempts to page it at every eNodeB within
the Tracking Area list to reestablish the radio connection. As
no data transfer is in progress, there are no IP data sessions
to handover and the mobile switches to its 2G/3G radio
to establish the connection with the target access network.
If the mobile is active
and an IP data transfer is in progress at the time of the handover, the
data transfer can either be suspended or the packet switched connection
can be handed over if the target network supports Dual Transfer
Mode. Note that this is typically only supported on UMTS
Once the access terminal
attaches to the 2G/3G cell, it answers the initial
paging via the target cell.
Execution Phase: Mobile
example in this section creates an SGs interface and an SGs service for
communicating with a Mobile Switching Center/Visitor Location Register
(MSC/VLR) for Circuit- Switched Fallback capability.
Fallback (CSFB) is a licensed feature and requires the purchase of the Circuit
Switched Fallback feature license to enable it.
Use the following
configuration example to enable CSFB capability on the MME:
lai mcc number mnc number lac area_code
tai mcc number mnc number tac area_code
context mme_context_name -noconfirm
ip address ipv4_address
sgs-service name -noconfirm
sctp port port_number
tac-to-lac-mapping tac value map-to lac value +
vlr vlr_name ipv4-address ip_address port port_number
lac area_code +
hash-value non-configured-value use-vlr vlr_name>
hash-value range value to value use-vlr vlr_name
bind ipv4-address sgs-intf_ipv4_address
associate tai-mgmt-db db_name
associate sgs-service sgs_svc_name
- The MME will attempt to map a
TAI to LAI in the following order:
- If a TAI Management Database
is configured, the MME will first use any TAI to LAI mapping defined within the
- If no TAI Management Database
is configured or if no suitable mapping is found within the TAI Management
Database, the MME will next attempt to map a specific TAC to a specific LAC as
defined in the SGs service according to the
- Lastly, the MME will attempt
to use the default LAC value. This is defined using the
tac-to-lac-mapping command with the
In this release, the number of TAC to LAC mappings is
increased from 512 to 1024 entries.
For the SGs
tac-to-lac-mapping command supports the
configuration of multiple TAC-to LAC values in the same configuration line.
The SGs IP
address can also be specified as an IPv6 address. To support this, the
command can be changed to the
command and the
ipv4-address command can be changed to
also allows for the configuration of a secondary IP address in support of SCTP
command) also supports IPv6 addressing and SCTP