Mobile Terminated (MT) SMS Flow

Here is a Simple Explanation of Mobile Terminated (MT) SMS Flow -

SMS MT Call Flow :-

1. The Short Message is transferred from SMSC to SMS-GMSC.
2. SMS-GMSC queries the HLR (SRI) & receives the routing information for the mobile subscriber (SRI-ACK).
3. The SMS-GMSC sends the short message to the MSC using "Forward Short Message" (FSM) operation.
4. The MSC retrieves the subscriber information from the VLR. This operation may include the Authentication Procedure.
5. The MSC transfers short message to the Mobile Station (MS).
6. The MSC returns the outcome of the "Forward Short Message" operation to the SMS-GMSC (FSM-ACK).
7. If requested by the SMSC, it returns a status report indicating Delivery of the Short Message.

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Mobile Originated (MO) SMS Flow

Here is a simple explanation of Mobile Originated SMS flow.

SMS MO Call Flow :-

1. The mobile station transfers the short message to the MSC.
2. The MSC queries the VLR to verify that the message transfer does not violate the supplementary services invoked or the restrictions imposed on the subscriber.
3. The MSC sends the short message to the SMS-IWMSC (Inter-Working MSC for SMS) using the “forward Short Message” operation.
4. The SMS-IWMSC delivers the short message to the SMSC (Short Message Service Centre).
5. The SMSC acknowledges the successful outcome of the “forward Short Message” operation to the MSC.
6. The MSC returns the outcome of the short message operation to the mobile station.
   
   

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GSM Mobile Terminated Call

A Land Line Subsriber calling a Mobile Subscriber.

The main difference between a call to an MS and a call from an MS is that, the exact location of MS is unknown. Hence, the MS must be located using paging before a connection can be established. 

1. The PSTN subscriber dials the MS’s telephone number (MSISDN), the MSISDN is analyzed in the PSTN, which identifies that this is a call to a mobile network subscriber. A connection is established to the MS’s home GMSC. The PSTN sends an Initial Address message (IAM) to the GMSC.
2. The GMSC analyzes the MSISDN to find out which HLR, the MS is registered in, and queries the HLR for information about how to route the call to the serving MSC/VLR. The HLR looks up the MSISDN and determines the IMSI and the SS7 address for the MSC/VLR that is servicing the MS. The HLR also checks if the service, “call forwarding to C-number” is activated, if so, the call is rerouted by the GMSC to that number.
   
3. The HLR then contacts the servicing MSC/VLR and asks it to assign a MSRN to the call. [MSRN - Mobile Station Routing Number].
4. The MSC/VLR returns an MSRN via HLR to the GMSC.
5. The GMSC sends an Initial Addressing message (IAM) to the servicing MSC/VLR and uses the MSRN to route the call to the MSC/VLR. Once the servicing MSC/VLR receives the call, the MSRN can be released and may be made available for reassignment.
6. The MSC/VLR then orders all of its BSCs and BTSs to page the MS. Since the MSC/VLR does not know exactly which BSC and BTS the MS is monitoring, the page will be sent out across the entire Location Area(LA). 
7. When the MS detects the paging message to the BTS’s in the desired LA. The BTS’s transmit the message over the air interface using PCH. To page the MS, the network uses an IMSI or TMSI valid only in the current MSC/VLR service area.
8. When the MS detects the paging message, it sends a request on RACH for a SDCCH.
9. The BSC provides a SDCCH, using AGCH.
10. SDCCH is used for the call set-up procedures. Over SDCCH all signaling preceding a call takes place. This includes: Marking the MS as “active” in the VLR. Authentication procedure (Start ciphering, Equipment identification).
11. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The BTS and MS are told to tune to the TCH. The mobile phone rings. If the subscriber answers, the connection is established.

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STP (Signaling Transfer Point)

Hi,

There are different types of signaling points in SS7 Standards, like
Service Switching Point (SSP) - like MSC, Tandem Switch, etc.
Service Control Points (SCP) - like HLR, etc.
Signaling Transfer Point (STP).

These Signaling points provide access to the SS7 network, databases, & transfer messages to other signaling points.

STP :- It Ÿis a vital element in SS7 network serving as a Signaling hub for the transfer of digital data packets between network nodes.
  
It routes messages throughout the network using, call information & network addressing structured within SS7 data packets.

It serves as dynamic router, controlling traffic flow & access to variety of SS7 nodes & network.

Functions of STP :-

1. Receives the MSU's & direct them to appropriate destination.
2. Network Management.
3. (ANSI to ITU) or (ITU to ANSI) protocol conversion.
4. Global Title Translation (GTT).
5. Measurement of Data.
6. Gateway Function.
7. Gateway Screening (GWS)
8. Local Number Portability (LNP).

 Different Links used with STP :-

• A-Link - It provide STP/SSP or STP/SCP connectivity. Maximum of 16 links can be there in a link set or maximum of 32 links in combined set.

• B-Link - It connects one STP to other STP of same hierarchical level. Maximum of 8 links in quad configuration of link set.

• C-Link - Maximum of 16 links in a link set.
• D-Link - Maximumof 8 links in a link set.

 

• E Links - It connects a STP to other STP other then its Home STP & provides an alternate route for SS7 messages if congestion occurs at home STP.

•  F Links - It provides SSP to SSP connectivity. It provides only Call Setup/TearDown capability & it should be adjacent.
  

 Why STP ???

• It provides Centralized Network Management, facilitating the delivery of Intelligent Services throughout the network.
• Reduction in Signaling Terminal Hardware in SSPs / SCPs. (like MSC, IN, HLR, etc.)
• Central Database for GTT at STP, Minimizes Errors.
• Efficient Routing of Messages.
• Flexible SS7 Network Management.
• Fast Integeration of New Nodes in the network.
• Easy Migration to Next Generation Networks like VOIP, Soft Switch, etc.

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GSM Originating Call Flow

A Mobile User calling a Land Line Subscriber.

1. MS after dialing a number & pressing SEND key, sends Channel Request(Chan_Req) message on RACH to ask for a signaling channel (Radio Resources). [RACH - Random Access Channel]
   
2. The BSC allocates a Traffic Channel(TCH) using AGCH. TCH allocation assigns a specific Frequency & a Timeslot on that frequency. [AGCH - Access Grant Channel]
   
3. The MS sends a call setup request through SDCCH, to the MSC/VLR. [SDCCH - slow dedicated control channel]. Over SDCCH, all signaling takes place. This includes: marking the MS status as active in the VLR
4. Then comes Authentication Procedure which includes Ciphering (The channel is ciphered so as to protect the call), Equipment Identification, etc.
   
5. The MSC/VLR instructs the BSC to allocate an Idle TCH (this message contains the dialed digits and other information needed for call establishment). The BTS and MS are told to tune to the TCH. 
6. The MSC allocates a voice circuit on one the digital trunks between the MSC and the BSS.
7. MSC informs the BSS about the allocated voice circuit. The call is also switched from signaling to voice.
8. The BSS notifies the Mobile about the changeover to voice mode.
9. The MSC routes the call and sends the call towards the called subscriber.
10. The PSTN indicates to the MSC that it has received all the digits and the called subscriber is being rung.
11. The MSC informs the mobile that the called subscriber is being alerted via a ring.
12. The called subscriber answers the call.

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What is E1 / T1

The PDH (plesiochronous Digital Hierarchy) has 2 primary communication systems as its foundation.

These are,
T1 system based on 1544kbit/s that is recommended by ANSI &
E1 system based on 2048kbit/s that is recommended by ITU-T.

Common Characteristics :- 

1. Both are having Same Sampling Frequency i.e. 8kHz.
2. In both (E1 & T1) Number of samples/telephone signal = 8000/sec.
3. In both (E1 & T1) Length of PCM Frame = 1/8000s = 125µs.
4. In both (E1 & T1) Number of Bits in each code word = 8.
5. In both (E1 & T1) Telephone Channel Bit Rate = 8000/s x 8 Bit = 64 kbit/s.

Differing Characteristics :-

1. In E1 Encoding/Decoding is followed by A-Law while in T1 Encoding/Decoding is followed by µ-Law.
2. In E1 - 13 Number of Segments in Characteristics while in T1 - 15 Number of Segments in Characteristics.
3. In E1 - 32 Number of Timeslots / PCM Frame while in T1 - 24 Number of Timeslots / PCM Frame.
4. In E1 - 8 x 32 = 256 number of bits / PCM Frame while in T1 - 8 x 24 + 1* = 193 number of bits / PCM Frame. (* Signifies an additional bit).
5. In E1 - (125µs x  8)/256 = approx 3.9µs is the length of an 8-bit Timeslot while in T1 - (125µs x  8)/193 = approx 5.2µs is the length of an 8-bit Timeslot.
6. In E1 - 8000/s x 256 bits = 2048kbit/s is the Bit Rate of Time-Division Multiplexed Signal while in T1 - 8000/s x 193 bits = 1544kbit/s is the Bit Rate of Time-Division Multiplexed Signal.

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