Circuit Switched FallBack (CSFB)

Circuit Switched FallBack (CSFB) is a technology whereby voice and SMS services are delivered to LTE devices through the use of GSM or another circuit-switched network.
Circuit Switched FallBack is needed because LTE is a packet-based all-IP network that cannot support circuit-switched calls. When an LTE device is used to make or receive a voice call or SMS, the device "falls back" to the 3G or 2G network to complete the call or to deliver the SMS text message.
CSFB was specified in 3rd Generation Partnership Project (3GPP) Release 8. CSFB requires a software upgrade of the operators core and radio network.
CSFB is often seen as an interim solution for LTE operators. Voice over LTE (VoLTE) is considered to be the long-term goal for the delivery of voice services on LTE networks.

Abbreviations in Telecom

IMS IP multimedia system
MME Mobility management identities
RAT Radio Access Technologies

Stream Control Transmission Protocol (SCTP)

High-Speed Downlink Packet Access (HSDPA) 

Universal Mobile Telecommunications System (UMTS)  

Transmission Control Protocol (TCP)
 User Datagram Protocol (UDP)

LINK TYPES

LINK TYPES
North American Signaling Network links are characterized by their
usage and grouped into the following classes:
• A-link (“access”): connects signaling end-point (for example,
local exchange or MSC in GSM) to STP
• C-link (“cross”): interconnects mated STP nodes
• B-link (“bridge”) and D-link (“diagonal”): interconnect peer
STP pairs
• E-link (“extended”): connects signaling end-point to another
STP pair for enhanced reliability
• F-link (“fully associated”): connects two signaling end-points

Circuit identification code (CIC)

In the Quasi-associated Signaling Mode the signaling messages
belonging to one call may take different paths than speech. The
speech goes directly from node A to B. The signals go via node C
which acts as a transit node. Hence node C is called the Signal
Transfer Point (STP).
The call set-up data sent in a message is not used in the STP node
C. The STP simply reroutes the messages to node B. Therefore, it is
required that the MSU carries some sort of label. The label
indicates not only the source and destination of the MSU, but also
the identity of the speech trunk the message is referring to. This
identity is called the Circuit Identity Code (CIC).

The CIC provides information about where the voice part of the call is carried - on which trunk and in which timeslot.

Integrated Services Digital Network

Integrated Services for Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide.

D channel

D channel (delta channel) is a telecommunications term which refers to the ISDN channel in which the control and signalling information is carried.

The bit rate of the D channel of a basic rate interface is 16 kbit/s, whereas it amounts to 64 kbit/s on a primary rate interface.

For DSS1 signalling, the D channel layer 2 protocol is Q.921 also called LAPD and it carries Layer 3 messages according to Q.931 protocol

Dual-tone multi-frequency signaling

Dual-tone multi-frequency signaling (DTMF) is used for telecommunication signaling over analog telephone lines in the voice-frequency band between telephone handsets and other communications devices and the switching center. The version of DTMF that is used in push-button telephones for tone dialing is known as Touch-Tone. It was developed by Western Electric and first used by the Bell System in commerce, using that name as a registered trademark.

Signalling Tones

1. Dial Tone : Indicates that exchange is ready to accept the dialed digits  from subscriber.
2. Ringing Tone  :- The tone send to called party after establishing the call.
3. Busy Tone:-A bursty 400  Hertz signal to inform  the calling party that subscriber is busy.
4. Number unobtainable tone  :- Called party line is out of order
5. Call in Progress tone(Routing tone ):-When a call progress through no. of exchanges subscriber listens a number of tone due to different exchanges involved in the call routing.