CS FALLBACK
TUTORIAL
Contents
› Introduction
› Concept
› Considerations
› Architecture & features
› Standard references
Commercial in confidence | 2011-07-04 | Page 2
introduction
› How to offer voice services with LTE before IMS/MMTel ?
– How to leverage on LTE smart phones (high data rates & hype etc.)
– How to handle international roamers
– How to reuse existing CS infrastructure
› CS Fallback is the answer to the above questions !
– Roaming with preference on LTE for data services
– Falling back to overlapping GSM or WCDMA for voice services
› Standardized from 3GPP Rel 8, offered from 2011
– 3GPP TS 23.272, Circuit Switched Fallback in Evolved Packet
System
– CSFB features offered in LTE, MSS and EPC
Commercial in confidence | 2011-07-04 | Page 3
concept
The CS FallBack Concept
› CSFB subscribers roam with preference on LTE access, no CS-voice
service available (only PS based services)
› Fallback triggered to overlapping CS domain (GSM / WCDMA)
whenever voice services are requested
› Resumed LTE access for PS services after call completion
LTE island
PS
PS
LTE
LTE
GSM / WCDMA
CS (+PS)
LTE
Commercial in confidence | 2011-07-04 | Page 5
LTE
CS Fallback options
1. RRC Connection Release with Redirect
–
–
eNodeB orders UE to move to a pre-configured GSM or WCDMA
frequency, UE may select other frequency/RAT depending on signal
quality
Optional enhancements:
› WCDMA: Deferred Measurement Control (DMCR)
› Instruction from target cell (in SIB3) to not read other time
consuming SIB’s (related to measurement control)
› GSM: RAN Information Management (RIM)
› eNodeB requests System Information from target network via
packet core and provide it to UE during CSFB. No need to read
time consuming SIB’s in target network
2. PS Handover
–
eNodeB orders UE to move to a GSM or WCDMA cell by using PSHO
procedures. eNodeB may decide target cell based on measurements
during the HO preparation phase. Target cell SI provided to UE during the
HO procedure
Commercial in confidence | 2011-07-04 | Page 6
CS Fallback with RRC Connection
Release with Redirect
5. eNodeB orders UE to move to a GSM or WCDMA frequency with
the RRC Connection Release with Redirect message.
1. CSFB subscriber roam in
LTE. CS signaling over Uu-S1SGs (LU, page etc.)
e-Uu
4. UE may optionally
measure signal on
available cells
(otherwise blind cell
change)
eNodeB
S1
MME
3. Page over
SGs, S1, e-Uu
LTE
SGs
Um / Uu
NodeB
RBS
6. UE fall back to
GSM or WCDMA.
System info read
in target cell.
MSC-S
MSC-S
GSM / WCDMA
2. Incoming call to
subscriber in LTE
7. Page response and call
setup over GSM or WCDMA
Commercial in confidence | 2011-07-04 | Page 7
(A / Iu)
considerations
Considerations
› Call setup time
› Simultaneous PS data sessions
› MSC border areas
› Feature introduction
Commercial in confidence | 2011-07-04 | Page 9
call setup time
› CS Fallback prolongs call setup time due to extra interfaces and
procedures
– Key contributor: Reading of system information (SI) in the new cell
Classic CS call setup in GSM
MSC
GSM
UE
Page
CS Fallback call setup
MSC
LTE
MME
UE
GSM
Page
Page
Page response
Page
Page
Extended service request
Release
Release
Cell change
Read SI
Call setup
Page response
Apprx. 1.5s addition in 3G
RAN information mgmnt (RIM)
Call3G
setup
- 1.2s improvement for
- 2.0s improvement for 2G
Apprx. 2.5s addition in 2G
Deferred measurement control (DMCR)
- 0.7s improvement for 3G
Commercial in confidence | 2011-07-04 | Page 10
RAN Information Management (RIM)
2. eNodeB requests System Information
(SI) for the overlapping GSM cells via the
packet core network.
1. eNodeB registers information (id and
signaling strength/quality) about
neighboring cells from measurement
reports from the UE’s.
3. SI request/response
relayed by the MME
S1
eNodeB
MME
LTE
6. eNodeB provides
SI to UE at CSFB.
RBS
NodeB
S3
A-bis
Iub
BSC
RNC
Gb
IuPS
SGSN
GSM / WCDMA
7. UE do not need to read
SI in GSM / WCDMA
=> Faster call setup !
Commercial in confidence | 2011-07-04 | Page 11
5. BSC/RNC provides SI based on
regular intervals (requests) or when
ever the configuration is changed.
4. SI request/response
relayed by the SGSN
Deferred Measurement Control
1. UE fall back
to WCDMA
S1
eNodeB
MME
LTE
SGs
2. UE reads SIB3 =
DMCR on/off
RBS
Iub
RNC
WCDMA
3a. DMCR on = UE awaits
with reading SIB 11, 11bis
and SIB 12
=> Faster call setup!
SIB 11, 11bis and 12 are
read later during the call
Commercial in confidence | 2011-07-04 | Page 12
3b. DMCR off = UE reads
SIB 11, 11bis and 12 at
their regular broadcast
intervals (every 1.4 s). Call
setup continues after SIB’s
are read.
Iu
MSC-S
Simultaneous PS data sessions
› Interrupted PS data sessions
– Typically 5-10 seconds w/o PSHO
– Typically 200 ms with PSHO
› GSM without DTM
Data rate (kbps)
~5-10s w/o PSHO
3) Resume
LTE
WCDMA
– PS interruption lasts for the
reminder of the call
LTE
GSM
(DTM)
Time (s)
CSFB call
› Reduced PS data rate
– PS service performance may be
degraded in WCDMA/GSM
1) Suspend
3) Suspend
2) Resume
Features:
› Delay for returning back to LTE
– “Fast Return” in BSS speeds up
the return to LTE
Commercial in confidence | 2011-07-04 | Page 13
• PS Handover (LTE - WCDMA)
• Dual Transfer Mode (GSM)
• Fast Return (GSM)
4) Resume
Call setup times and PS data
interruption per option
MO: mobile o fixed
MT: fixed to mobile
Total call setup time including
CSFB (s)
CSFB part of the total call setup
time (s)
RRC Connection Release with Redirect to
UTRAN, SysInfo read in UTRAN
MO 3.8
MT 4.2
MO 1.5
MT 1.5
5.8
RRC Connection Release with Redirect to
UTRAN, Deferred measurement control
UTRAN support
MO 3.1
MT 3.5
MO 0.8
MT 0.8
5.1
RRC Connection Release with Redirect to
UTRAN, with UTRAN Sys Info available in
LTE (with RIM)
MO 2.6
MT 3.0
MO 0.3
MT 0.3
4.6
MO 2.8 / 1.9
MT 3.2 / 2.3
MO 0.5 / -0.4
MT 0.5 / -0.4
<0.2
Release with Redirect to GERAN, no DTM
MO 5.2
MT 5.6
MO 2.7
MT 2.4
No DTM
Release with Redirect to GERAN, GERAN
SysInfo available in LTE (with RIM)
MO 3.2
MT 3.6
MO 0.7
MT 0.4
No DTM
PSHO to GERAN, with DTM non-blind/blind
MO 4.4 / 3.1
MT 4.8 / 3.5
MO 1.9 / 0.6
MT 1.6 / 0.3
<0.2*
PS HO to UTRAN, non-blind/blind
Commercial in confidence | 2011-07-04 | Page 14
PS Outage (s)
MSC border areas
› CS Fallback calls in MSC border areas may end up in the wrong MSC, leading
to failed calls
MSC Area 1
MSC Area 2
MSC
CSFB reg. MSC
MSC
MME
MME
TA = 2
TA = 1
TA = 3
UE
CS Fallback
LA = 1
LA = 2
LA = 3
› Two solutions:
– MSC Pool assures page response to correct MSC (by TMSI-NRI)
› Strongly recommended in CSFB implementations
– MT Roaming Forward reroutes the call and subscriber to the new MSC
› Solving the problem but with additional call setup delay, note ~5s addition
Commercial in confidence | 2011-07-04 | Page 15
MT Roaming Forward in CSFB
3. UE fall back to GSM or WCDMA.
Cell 2 (MSC 2) is selected due to
better signal than in cell 1 (MSC 1)
S1
eNodeB
MME
LTE
4. Page response received in
MSC 2, subscriber not
registered triggers MTRF
2. Page over
SGs, S1, e-Uu
e-Uu
8. MSC 2 awaits the LU
procedure to be completed
and then set up the call e2e
SGs
Um / Uu
(A / Iu)
MSC 2
6. MSC 2 sends UL to HLR
which (optionally) can
authorize use of MTRF.
Um / Uu
NodeB
RBS
NodeB
RBS
GSM / WCDMA
GSM / WCDMA
Cell 2
Cell 1
(A / Iu)
MSC-S
MSC 1
1. Incoming call to
subscriber in LTE
E
5. MSC 2 requests identification
(for auth.) from MSC 1, also
indicating support of MTRF
Commercial in confidence | 2011-07-04 | Page 16
7. MSC 1 sends PRN req. to
MSC 2 which provides
MSRN. MSC 1 reroutes the
call to MSC 2 (IAM)
Feature introduction
› CSFB requires SW upgrade and SGs-interface in the MSC
– With MSC Pool only one or a few MSC’s can serve all LA’s
› More CSFB-MSC’s can be upgraded as traffic demand grows
– CSFB can be provided to a pool of MSC-S Blade Cluster nodes
prior to 12B
CSFB capable
MSC
SGs
MME
Non-CSFB
capable MSC
SGs
UE doing combined
EPS/CS registration
MSC
MSC
MSC
MSC
MSC
UE
LTE:
GSM / WCDMA:
Commercial in confidence | 2011-07-04 | Page 17
CSFB enabled to
overlapping CS
with MSC Pool
Architecture &
features
Architecture and FEATURES
1. RIM (O)
2. DMCR (O)
3. Iu-flex/MSC Pool support (O)
4. PS Handover (O)
1. RIM (O)
2. Fast return to LTE (O)
3. A-flex/MSC Pool support (O)
4. PS Handover (O)
5. Dual Transfer Mode (O)
UTRAN
IuPS
1. RIM (O)
2. PS Handover (O)
SGSN
Gs
Gb
Uu
S3
GERAN
IuCS
A
MSC-S
Um
SGs
UE
LTE-Uu
1. CS Fallback (M)
2. DMCR (O)
3. Dual Transfer Mode (O)
Commercial in confidence | 2011-07-04 | Page 19
E-UTRAN
S1
1. CS Fallback to GSM and
WCDMA (M)
2. RIM (O)
MME
1. CS Fallback (M)
2. SMS over SGs (M)
3. RIM (O)
4. MSC Pool support (O)
1. LTE to CS Fallback (M)
2. SMS over SGs (M)
3. MSC Pool (O)
4. MT Roaming Forward (O)
5. Dual Transfer Mode in GSM (O)
E2e-Feature view
UE
SMS
Voice
LTE
SMS over SGs
CS Fallback
CS Fallback
SGSN-MME
MSC
SMS over SGs
SMS over SGs
CS Fallback
CS Fallback
WCDMA
GSM
RIM
RIM
(MME only)
Call setup time
enhancements
RIM
RIM
(MME & SGSN)
DMCR
DMCR
MSC Pool
Call success rate
in border areas
Iu-Flex
A-Flex
MTRF
DTM
DTM
PS data continuity
DTM
(SGSN only)
PSHO
PSHO
(MME & SGSN)
Commercial in confidence | 2011-07-04 | Page 20
DTM
PSHO
Fast Return
Standard references
Standard References
› 3GPP TS 23.272, Circuit Switched Fallback in Evolved Packet System
› 3GPP TS 48.018, GPRS; BSS - SGSN; BSS GPRS protocol (BSSGP)
– Chapter 8c, Signaling procedures between RIM SAP’s
› 3GPP TS 25.331, Radio Resource Control
– Chapter 8.1.1.6 Deferred Measurements
› 3GPP TS 23.401, GPRS enhancements for E-UTRAN access
– Chapter 5.5.2.1 E-UTRAN to UTRAN Iu mode Inter RAT handover
› 3GPP TS 23.018 (v10.1.10), Basic Call Handling
– Chapter 5.2.3, MT Roaming Forwarding
Commercial in confidence | 2011-07-04 | Page 22
Commercial in confidence | 2011-07-04 | Page 23