CSFB Concepts

Definitions :CSFB (Circuit Switched Fallback):-  is a functionality specified by 3GPP to provide legacy circuit switched services (like voice, SMS, call independent supplementary services, location services, ... for !"# $#s when they are attached in !"# coverage.

"his is achieved by falling back to G#%&' or $"%&' in case those services are reuested. "he )S*+ functionality needs to be implemented in the $#, the #$"%&', the #P) (MM#, the MSS-!%, the /!% (in case of M"%%, M"%*. "o support moving back from G#%&'-$"%&' to !"# functionality in G#%&'-$"%&' is reuired.

Circuit Switched Fall Back (CSFB):)ircuit Switched *allback ()S*+ for a $# in !"# coverage towards legacy GSM or 0)1M&.

Figure 1 – CSFB for MT call

2n )S*+, upon a voice call origination attempt or when receiving a page for )S voice (via SGs interface, the $# is moved to $M"S-GSM $M"S-GSM and the voice is sent over one of these access networks. "he page response is sent over the new %&" on the u or & interface. "he $# will return to !"# after call completion if !"# is preferred and coverage e4ists5 "he )S*+ function is only possible to reali6e in areas where #$"%&' coverage is overlapped with GSM and 0)1M& 0 )1M& coverage )S*+ will allow retaining current roaming relationships between operators, since )S voice is still used.

)S*+ mplementation <ernatives "here are 7 main )S*+ procedures8

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RRC Release with Redirect •

+asic )S*+8 without target %&' System nformation (S

•

#nhanced )S*+8 o

o

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1eferred Measurement )ontrol %eading (0)1M& S e4change based on 3GPP %M procedures (GSM and 0)1M&

PSH (Packet Switched Hando!er)

1epending on the )S*+ procedure different network elements are affected and different )S*+ performance can be achieved, which is shown in the following sections.

Basic RRC release with redirect without S" e#chan$e "he basic )S*+ option is %%) %elease with %edirect without S e4change between 0)1M&-GSM 0)1M&-GSM and !"# !"# %&'. "he impacted nodes are indicated by the )S*+ in the circle.

Figure 2 – Basic CSFB with RRC Connection Release & Redirect 

mpacted nodes are $#, e'+, MM# and MS). 'o impact e4ists for GSM or 0 )1M&, which conseuently does not reuire a rollout of new software in the legacy radio network. n an MS) pool environment only few MS)s need to support SGs, as each MS) serve all !&s of the pool. %ain characteristics8 9

Slower call setup time because S needs to be fetched by the $# in the target %&'

9

!onger PS outage time for 0)1M& or 1"M GSM as no PS/: is performed

#7e *unctionality already available. +asic )S*+ supported from8 MS)S ;7&, MM# 7