Roaming | msc | vlr| sgsn | hlr | gtp | sgsn | gprs

MSC, VLR , SGSN, HLL, HSS, PGW and GGSN are main nodes for core signaling.

SS7 Signaling have three types of nodes in telecom network

1. Signaling Switching Point (SSP):
                These are node which gives access to radio , BTS , BSC , RNC etc. on one side and on other side these nodes are connected to wired ss7 network. These node are e.g MSC, VLR, SGSN etc. These nodes provides roaming information to the HPLMN of mobile subscriber and provide MO, MT , SMS and call functionalities.

2.  Signaling Transfer points (STP):
                These are SS7 nodes which provides routing functionality to the SS7 messages. These are like route in IP network.  All routing is done based on global title or point code. The global title is derived from IMSI or MSISDN. For routing the STP provides global title translation for routing of SS7 messages to the correct destinations.

3. Signaling Control Point (SCP):
               These are nodes , having access to database and queried by other network elements for providing services to a mobile subscriber. One of the example is prepaid charging in this there is a node called Camel Gateway , this uses INAP or Camel Protocol for prepaid charging. This SCP is deployed in home network of a mobile subscriber. One a call is initiated the Camel SCP is queried for services ( if have balance etc.) , if SCP allows then a call is made. 

3G Data  Core:

HLR or Home Location Register

       
This is the central data base for a mobile SIM card. Which stores the subscription profile for a IMSI or a Subscriber. Along with subscription profiles it also stores the dynamic information for the roaming network e.g Visiting MSC, SGSN and VLR.
List of Information Stored in HLR:

1. IMSI
2. MSISDN
3. VLR, MSC and SGSN
4. Call Forwarding Information.
5. Call Waiting Information.
6. Caller Id
7. GPRS subscription profile (APN ,Qos).
8. TeleService list (SMS, Telephony etc).


GPRS Attach for data call

• When a mobile phone is switch on it tries to attach on SGSN for data. 
• SGSN send the Send Authenticate Request with parameters (IMIS, SGSN Number), to the HLR. If HLR finds that IMSI is configured HLR send Authentication Vectors to the SGSN. If Authentication Vectors are received correctly by SGSN. The GPRS location update may be initiated from SGSN to the HLR.
• When HLR receives the GPRS location update from SGSN and roaming is allowed. Hlr sends the GRPS subscription (APN, QoS etc.) to the SGSN in Insert Subscriber Data to the SGSN. 
• When GPRS location update successful , sgsn initiates the pdp context setup with GGSN over GTP (GPRS Tunneling Protocol).  

PDP Context setup:
       After GPRS location update , SGSN have APN and QoS for the subscriber.

•   SGSN sends the CreatePDPContextRequest to the GGSN , with QoS and other parameters.
•   If PDP is accepted the GGSN sends back the CreatePDPContextResponse with Accepted else      error is return in Create PDP Response. 
•   During Data session SGSN may initiate (e.g new QoS) the UpdatePDPContextRequest, if GGSN accept the new PDP, then GGSN sends back the UpdatePDPContexResponse with Accepted else error is return in UpdatePDPResponse.  
• UpdatePDPContext may also be initiated by the GGSN, to update bearer parameters (e.g QoS).
• When SGSN terminates the PDP session , SGSN sends DeletePDPContextRequest  to GGSN.
• GGSN clears the PDP , and sends DeletePDPContextResponse to SGSN.      
• GGSN may also initiate the DeletePDPContext procedure.

3G Voice Core:

       3G voice core have MSC, VLR , HLR ,  GGSN and GMSC.

HLR :
 Stores the voice call related subscription information , e.g TeleService as static information.
Stores the Dynamic information (VLR, MSC etc.).

Voice Call Procedure:

• When a subscriber dials a MSISDN , it reaches to the GMSC of the Called Subscriber. 
•  GMSC Queries (SRI) the HLR for MSRN. 
•  Upon getting response of SRI , the GMSC setup the call with called party.   

SMS Core of Mobile Originated Call Flow:

          SMS core contains the SMSC, HLR , MSC and VLR.

MO SMS Call Flow.

         When a subscriber sends the SMS , SMS stores to the SMSC of sending subscriber.  The address of SMSC is derived from the SMSC number stored in sim card/mobile device. When a subscriber sends MO sms , a GSM MAP level operation MO-ForwardsSM , is initiated from visiting MSC to the SMSC. The SMSC replies with MO-ForwadSM Response. If response is successful the SMS is submitted/stored to the SMSC.
 For a stored SMS , smsc do a HLR lookup for a MSISDN to get IMSI and visiting MSC. When on success full response, SMSC creates a MT-ForwardSM , towards the Visiting MSC. If  mobile phone is temporary offline, SMSC keep tries for delivering SMS , till the validity time expires for SMS. 

MT SMS Call Flow 

SMSC , send SRI-SM to the HLR of destination  , upon receiving IMSI and MSC number in SRI-SM response, SMSC sends SMS to the destination number.

USSD Call flow:

 USSD is a service provide by the mobile operators for access real time (transaction based) data/menu transfer from mobile device to operators backed core network. USSD can be run on basic phone, no internet required. So this is useful for the are where no internet access. E,g rular areas , people can npt effort smart phones.

There are two kinds of USSD Call flow.
  Mobile Initiated and Network Initiated.


HLR Lookup:
 This is used by application to query parameters for a MSISDN.
This can be done via SRI-SM MAP operation. In response application receives , IMSI and MSC number or subscriber status.