Location Update

Location Update(UL):

            When ever the MS tries to connect with the network location update happens. Here we gonna see the location update from MSC to the HLR.

             When your mobile station tries to latch with the network for the very first time, UL request will be sent respective operators MSC/VLR after that MSC will request HLR to provide the authentication   information to authenticate the mobile station(send authentication request). HLR will respond to the MSC/VLR with corresponding authentication vectors. Once MSC receives authentication vectors from HLR, it request the same to the MS(for detailed SAI please refer below) and MSC will compare both MS and HLR requests if both matches MSC will send the UL request to HLR else it will fail the UL.

                           Once the UL request reaches the HLR, HLR will send the Insert Subscriber Data request (ISD)to the MSC which carries what are all the services provided by the operators such as telephony services, SMS services, call waiting call forwarding, etc.
                       
                 Followed by the ISD request HLR will send the UL acknowledgment request to MSC to confirm that UL is done and the same will be notified to MS from MSC once it get done MS will be successfully latch with  that corresponding MSC/VLR.

First UL

UL

    The difference between first time UL and non-first UL is, when the HLR receives UL request from a new MSC/VLR, it will initiate cancel location to the previous MSC/VLR where the customer presently latched. So that customer datas will removed from existing MSC/VLR and the same will available on new MSC/VLR.

                         SAI: Send Authentication Information

Authentication Vectors:

                         There are two different types(majority) of authentication vectors used MSC/VLR to authenticate the MS. Those are Triplet or Quintet parameters together forms a authentication vector.

Quintet:

RAND - Random value

XRES – Expected Signed Response

CK - Ciphering Key

IK - Integrity Key

AUTN - Authentication Token

Triplet: 

RAND – Random Number

XRES- Signed Response

Kc - Ciphering Key

Send Authentication Information for GSM:

                         For GSM-SAI, triplet parameter together forms a authentication vector.

The new VLR sends a request to the HLR/AUC (Authentication Center) requesting the “authentication triplets” (RAND, SRES, and Kc) available for the specified IMSI.

The AUC, using the IMSI, extracts the subscriber's authentication key (Ki). The AUC then generates a random number (RAND), applies the Ki and RAND to both the authentication algorithm (A3) and the cipher key generation algorithm (A8) to produce an authentication Signed Response (SRES) and a Cipher Key (Kc). The AUC then returns to the new VLR an authentication triplet: RAND, SRES, and Kc.

The MSC/VLR keeps the two parameters Kc and SRES for later use and then sends a message to the MS. The MS reads its Authentication key (Ki) from the SIM, applies the received random number (RAND) and Ki to both its Authentication Algorithm (A3) and Cipher key generation Algorithm (A8) to produce an authentication Signed Response (SRES) and Cipher Key (Kc). The MS saves Kc for later, and will use Kc when it receives command to cipher the channel. 

The MS returns the generated SRES to the MSC/VLR. The VLR compares the SRES returned from the MS with the expected SRES received earlier from the AUC. If equal, the mobile passes authentication. If unequal, all signaling activities will be aborted.

Update Location for GSM

GSM (2G)

Global System for Mobile Communication (GSM)

 GSM (Global System for Mobile Communications, originally Groupe SpécialMobile), is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation (2G) digital cellular networks used by mobile phones, first deployed in Finland in July 1991. It is a digital cellular technology used for transmitting mobile voice and data services.

GSM is a circuit-switched system that divides each 200 kHz channel into eight 25 kHz time-slots. GSM operates on the mobile communication bands 900 MHz and 1800 MHz in most parts of the world. In the US, GSM operates in the bands 850 MHz and 1900 MHz.

GSM Architecture

The Base Station System (BSS)

All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs).

Base Transceiver Station : The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC.

Base Station Controller : The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.

                The Base Station Controller (BSC) requires a new hardware called the packet control unit (PCU). The PCU directs the data traffic to the GPRS network.

Network switching subsystem (NSS)

Mobile Switching Center (MSC): The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others

.

Visitor Location Register (VLR): The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.

Authentication center (AUC): A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.

Equipment Identity Register (EIR): The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

Home Location Register (HLR): The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the operators, they are registered in the HLR of that operator.

Interfaces:

Um interface

          Um interface connects the MS and BTS (air interface)

Abis-interface

      Its the interface between the BSC (Base Station Controller) and the BTS. The interface comprises traffic and control channels.

Functions implemented at the Abis-interface are:

- Voice-data traffic exchange.

- Signaling exchange between the BSC and the BTS.

- Transporting synchronization information from the BSC to the 

A-interface
Its the interface between the BSS and the MSC. It manages the allocation of suitable radio resources to the MSs and mobility management.

B-interface

This interface is between between the MSC and the VLR

       VLR uses the MAP/B protocol. Most MSCs are associated with a VLR, making the B interface "internal". Whenever the MSC needs access to data regarding a MS located in its area, it interrogates the VLR using the MAP/B protocol over the B interface.

C-interface

It connects  HLR and GMSC. Also between MSC and HLR.

D-interface

It connects HLR and VLR.

E-interface

It connects MSC and another MSC or G-MSC.

F-interface

It connects EIR and MSC and between EIR and G-MSC.

G-interface

It connects VLR and another VLR.


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