GSM Basic Callflows

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Basic Procedures


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Outlines

6.1Numbers-Identities-Codes6.1.1Mobile Subscriber ISDN Number (MSISDN)6.1.2International Mobile Subscriber Identity (IMSI)

6.1.3 HLR Entries6.1.4Mobile Station Roaming Number (MSRN)6.1.5Handover Number (HON)6.1.6Temporary Mobile Subscriber Identity (TMSI) and Location Area Identity (LAI)6.1.7International Mobile Equipment Identity (IMEI)6.1.8Cell Global Identity (CGI)

6.1.9Base Station Identity Code (BSIC)6.2Procedures

6.2.1Registration of the Mobile Station on the Network6.2.2Location Update and Authentication6.2.3Mobile Terminated Call (MTC) and Encoding6.2.4Mobile Originated Call (MOC) and IMEI Checking6.2.5Handover6.2.6Intra Cell - Intra BSC Handover6.2.7Inter Cell - Intra BSC Handover6.2.8Inter Cell - Inter BSC Handover6.2.9Inter MSC Handover


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In GSM there is a range of numbers and identity codes which arerequired for the provision of the basic procedures in a digital mobilenetwork. The following aspects must be considered: The identity of a subscriber

The services required by the subscriber The national or international location of a subscriber The localization of the subscriber in relation to various base stations or

other network elements

The mobile station used by the subscriber There are several numerical identities for these aspects that will beintroduced in the course of this chapter.


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The international telephone number of a mobile GSM customer is his MobileSubscriber ISDN Number, or MSISDN. The number is only stored in theHLR of the home network, and is never intended to be transmitted on the air

interface. How do you dial this number from a Public Switched Telephone Network, or

PSTN? First, dial a prefix to distinguish between national and international calls. For

national calls, the prefix is usually zero. For international calls, it is 2 zeros,

and two or three digits, depending on the country. In GSM networks, it issufficient simply to enter a ''+'' sign instead of the double zero.


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The second part of the MSISDN, the Mobile Country Code MCC,provides the code for the relevant country. The MCC consists of twodigits and does not have to be entered when making national calls.

The third part of the number is the Network Destination Code NDC.It identifies the appropriate national network operator. The MobileSubscriber Identification Number MSIN forms the fourth part of theMSISDN. The first three digits of this number determine the logicalHLR where the GSM customer is stored and which is used to locate

him/her. The remaining digits identify the individual subscriber, withdifferent services like speech, fax and data services requiringdiffering MSINs.


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To identify a GSM subscriber in a mobile network, each subscriber has hisown International Mobile Subscriber Identity (IMSI). The IMSI is of astandard length of 15 digits and is composed of the following elements:

A Mobile Country Code (MCC) with three digits, A Mobile Network Code (MNC) with two digits and A Mobile Subscriber Identification Number (MSIN) with 10 digits.

The IMSI is required for registration in a PLMN, because it is defined as anindividual number code with a uniform string length of digits. The MSISDN,

by comparison, has differing lengths of digits, depending on the country,and therefore cannot be considered for a simple registration procedure.Furthermore, a customer has different MSISDNs for every service, but onlyone IMSI that can uniquely identify him internationally.


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The Mobile Station Roaming Number (MSRN) is a temporary number produced forrouting the call in case of a Mobile Terminated Call.

MSRN is generated within the serving MSC/VLR, that is where the mobile station justhappens to be at that moment. In order for a called subscriber to be identifiable, theMSRN is linked to the International Mobile Subscriber Identity (IMSI). In the event ofan incoming call, the MSRN is transmitted via the HLR to the Gateway MSC (GMSC)to inform it where the call has finally to be routed.

The MSRN has the same structure as the MSISDN. It consists of the Country Code(CC), the National Destination Code (NDC), and a Subscriber Number (SN) that isdifferent to the SN in the MSISDN. While the MSISDN only establishes a connectionto the HLR, the MSRN is used to route the call to its actual destination.


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If a mobile subscriber, while making a call, moves from one MSC area toanother that is covered by a different VLR/MSC, an Inter-MSC-Handover isrequired, that is the transfer of the call from a currently serving MSC, or the

Anchor MSC, to the new MSC, the Target MSC. The Anchor MSC stays responsible for all call control activities, so it has to

know where to route the call. To support this, the Target MSC generates theHandover Number HON, and sends it to the Anchor MSC. Using thisnumber, it can now create a connection to the Target MSC.

Like the Roaming Number, it is made up of the Country Code CC, theNational Destination Code NDC and a Subscriber Number SN (which is alsodifferent from that of the MSISDN).


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The Temporary Mobile Subscriber Identity (TMSI) and the Location AreaIdentity (LAI) are required for the Location Update of a mobile station. TheTMSI is generated in the VLR of the relevant hosting network after the firstLocation Update, and then linked to the IMSI.

From then on, the IMSI is no longer transmitted for data security reasons.Instead, the TMSI is transmitted via the air interface. This is the temporaryidentity of the mobile radio user. It is assigned for the mobile's presencewithin the Location Area.

The Location Area Identity (LAI) is a unique number worldwide. The net

sends it via the control channels of the air interface to the mobile station sothe mobile station recognizes its current position and can store it on the SIMcard.


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The International Mobile Equipment Identity (IMEI) is stored on thehardware of the mobile station, and is used to check the validity ofthe relevant terminal used.

In the Equipment Identity Register (EIR), the same number is storedto check number compliance. This equipment check is optional inGSM, and not every network operator offers this service. The IMEIcontains a check code (the Type Approval Code), a terminal number

(the Final Assembly Code), and the serial number of the mobilestation.


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The Cell Global Identity (CGI) is a number code used to identifyeach cell within a Location Area of a GSM network.

The CGI consists of the relevant Mobile Country Code (MCC) of acountry, the Mobile Network Code (MNC) for a relevant network, theLocation Area Code (LAC) of a Location Area, and the Cell Identity(CI), which indicates the relevant cell within this Location Area.Using this code, the traffic data relevant to a cell, for example, can

be evaluated statistically in the Network Management Center(NMC).


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If a mobile subscriber is at a certain location, his mobile phone usuallyreceives frequencies from different network operators. In order for hismobile phone to synchronize to the frequency of its own network operator,each base station must be able to identify itself.

This identity is the Base Station Identity Code (BSIC). It is broadcast byevery BTS, and compared with the appropriate entry on the SIM card of thesubscriber. Only after verification that the broadcast BSIC is identical withthe entry on the SIM card can a mobile station be registered on the network.

Furthermore, the BSIC has to make a distinction between the serving celland those other cells that use the same frequency as the serving cell. Itconsists of the Network Color Code (NCC) and the Base Station Color Code(BCC).


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In order for a user to be able to use a mobile station, it must be registered on thenetwork. After a subscriber has switched on his mobile phone, he first enters his PIN.The PIN is stored on the SIM card. This step will not access the network.

The mobile station only begins communications with the network after successful PINquery. "Network search" appears on the display of the mobile station. To select theradio cell, the mobile station measures all Broadcast Control Channels that havebeen sent on all frequencies from all network operators.

Then, a suitable network PLMN that is authorized for the subscriber is selected. Inthe home country, this at first is only the network of a subscriber's own network

operator, then the network of a contract partner. Now the mobile can select a suitablecell from several possible cells. After a successful Location Update, the mobile stationis in Idle Mode. Let's have a closer look at the Location Update procedure.


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Irrespective of where a mobile subscriber happens to be, a callermust always be able to contact him/her.

This means the mobile network must always know the subscriber's

current location so it can put through calls. By ''subscriber's currentlocation'' we understand in this context the appropriate LocationArea, that is the area controlled by a certain VLR. A VLR controls atleast one Location Area.

However, in order to define as accurately as possible a subscriber'scurrent location, the VLR area usually consists of several logicalLocation Areas. The process that keeps the network up to date asregards this current location is called Location Update.


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A Location Update is carried out: When a mobile customer switches on his/her mobile

station

When a mobile customer changes his/her LocationArea At regular intervals after a mobile customer has not

moved his/her mobile station for some time.

What information is exchanged between whichnetwork elements during Location Update?


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The mobile station sends, via the BSS and the appropriate servingMSC, a Location Update Request to the controlling VLR.

This then requests the subscriber ID stored on the SIM card of the

mobile station, which is sent immediately to the VLR. Now the VLRcan request the authentication data, also called the authenticationtriplet, from the subscriber's HLR/AUC. After receiving the reply fromthe HLR, the VLR sends the authentication data to the mobilestation.

After successful authentication, the Location Update, in the form ofthe VLR address, can be sent to the HLR of the subscriber as thecurrent location.


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Let's have a closer look at the authentication process. A new servingVLR requests authentication from the SIM card used. This request isforwarded to the Authentication Center (AUC) of the home network,

which will provide the VLR with an "Authentication Triplet". The triplet consists of a random number (RAND), a SignedResponse (SRES), and a key (Kc). SRES is the calculation result ofthe algorithm A3 combining the individual key Ki stored in the AUCand the random number RAND. Kc is generated in the same way by

the algorithm A8 using RAND. Several of these triplets produced thisway are transmitted to the enquiring VLR.


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The VLR first stores both parameters from the triplet Kc and SRES for lateruse, and only transmits the random number RAND to the mobile station.Using this number and the information stored on the SIM card - that is the

algorithms A3 and A8, and the key Ki, the mobile station can produce thesame parameters as the AUC, namely SRES and Kc. For authentication,the mobile station sends the SRES it has produced back to the VLR. Thekey Kc is used later for channel encoding.

Now the VLR can compare both SRES numbers: one from the mobilestation, and one produced by the AUC. If both parameters tally,authentication was successful. Finally, the TMSI is transmitted in encodedform.


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We refer to a Mobile Terminated Call (MTC) when a mobile subscriber isbeing called. Let us look at the MTC using a call from a PSTN as anexample: A PSTN customer dials the number of a mobile phone. The number dialed is the

MSISDN.

The exchange of the Public Switched Telephone Network (PSTN) analyses thisnumber and contacts the Gateway Mobile Services Switching Center (GMSC),that is the link between the PSTN and the mobile network.

The GMSC does not know the current location of the called subscriber, andtherefore starts an appropriate inquiry to the HLR of the called subscriber.

The HLR checks its database entries to find the current location or the currently

serving MSC/VLR of the called subscriber. Now the HLR asks the serving MSC/VLR for the routing information necessary

for the call connection.


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The serving VLR now generates the temporary MobileSubscriber Roaming Number (MSRN) and sends it via theHLR to the GMSC.

The GMSC now has the routing information it needs, and

transmits the call onwards to the serving MSC. The VLR makes the LAI available to the called subscriber

To locate the called subscriber, the MSC initiates a PagingProcess within the Location Area.

Let's have a closer look at this process.


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The MSC initiates the paging in the entire Location Area using theTemporary Mobile Subscriber Identity (TMSI). This is madeavailable by the VLR and is also known to the called mobile station.It's been allocated to a subscriber after the first Location Update ofhis mobile station, and is used instead of the IMSI for data securityreasons.

Within the relevant Location Area, the TMSI of the called mobilestation is broadcast on the air interface via the Paging Channel

(PCH). However, as the TMSI is clearly allocated within the VLRarea, and thus also within the Location Area, only the called mobilestation can identify the Paging Channel, and react accordingly.


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The called mobile station reacts to the Paging Channel with a request to the networkfor a free channel. This request is sent via the Random Access Channel (RACH) inan uplink to the BSS. This allocates, through the Access Grant Channel (AGCH), a

Stand-alone Dedicated Control Channel (SDCCH) to the mobile station. This communicates with the BSS via the SDCCH, which exists both in the uplink and

the downlink, until a traffic channel is allocated to the mobile station. Parameters forauthentication and encoding, as well as the new TMSI, are transmitted via theSDCCH to the mobile station.

If this is concluded successfully, the actual call set-up is initiated. For this purpose, a

voice link is established between the mobile station and the BSS, by allocating atraffic channel on the air interface. Let's have a more detailed look at the varioussteps of call set-up.


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1. First, the mobile station is informed that an incoming call requires a call set-up. Thisset-up message is sent to the mobile station via the SDCCH

2. The mobile station confirms this set-up inquiry in the uplink via the same channel.

3. Now the MSC selects a terrestrial channel of the A interface, also called a trunk, forthe transmission to the BSS. It sends a message, an Assignment Request Message,to the BSS, asking it to allocate a free resource on the air interface. This messagecontains information on the trunk used, so later on the data stream functionsperfectly.

4. After receiving the Assignment Request Message, the BSS allocates a suitable traffic

channel, and transmits this allocation via the SDCCH to the mobile station. This mustnow prepare itself for a configuration of its air channel.


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5. The mobile now takes over the allocated traffic channel and transmits theconfirmation of the allocation to the BSS via the FACCH. Now, thesubscriber is informed for the first time by the Man Machine Interface

(MMI) of the incoming call, in other words: the telephone rings. The BSStransmits a final confirmation message to the MSC, about the successfulallocation of the air and terrestrial resources.

If the channel allocation is unsuccessful, the BSS sends an appropriateerror message to the MSC about the reason for the failure. Possible

reasons for a failure are that either air or terrestrial resources werereserved previously, and are thus unavailable.


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Speech and signalling data are not transmitted in their original stateon the air interface, but are encoded. Only after successful

authentication can the BTS and the mobile station start the encodingprocess. The 114-bit key used for encoding and decoding at both ends of the

air transmission is produced at the BTS and at the mobile stationusing the encoding algorithm A5. For this purpose, A5 uses the 64-

bit key Kc that was generated during the authentication process, anda 22-bit TDMA frame number in order to encode and decode speechand signalling data.


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We refer to a Mobile Originated Call, or MOC, when a mobile telephone subscribercalls a subscriber on the Public Switched Telephone Network (PSTN) or on a PublicLand Mobile Network (PLMN). The mobile caller dials a number. This means hestarts a call service inquiry in the net where he is currently a visitor. Then the netanalyses the caller's subscriber data to do three things: To authorise or refuse the use of the network To activate the requested service To route the call

If the call is addressed to the same mobile network, the MSC turns to the relevantHLR via the dialed MSISDN to send the call to its destination. If the call is to the

PSTN or to a different PLMN, it is routed there via the Gateway MSC (GMSC) andsent on to the target subscriber.


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If a call is made to a PSTN, the following communication procedures take placebetween the network elements. First, the BSS and the mobile station agree onchannel allocation. When this is done, necessary safety checks relating to thesubscriber are carried out between the mobile station and the serving MSC/VLR.

The mobile check is optional, depending on the network operator. The IMEI, which is

transmitted in encoded form through the air by the mobile station, is sent to theEquipment Identity Register (EIR) that checks the authorization of the terminal used.Only after the security checks have been carried out can the mobile station start thecall set-up via the MSC.

This now checks in the connected VLR whether the subscriber is authorized for thedesired service. If the VLR sends back a positive reply, the call set-up can be carried

out, and a traffic channel can be allocated to the connection. The serving MSC sendsthe call via the relevant GMSC to the exchange of the PSTN. If a confirmation ofsuccessful call set-up is sent from there, the caller is informed by his MSC, and hehears the calling signal on his telephone. The conversation can start.


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We know already that the GSM network is of a cellular design. If a subscriber moves from one cellto a neighboring cell, the current connection must be maintained without any interruption. Toensure this, there is the Handover - that is the neighboring cell takes over the coverage tasks in asort of "substitution without stopping", and the connection to the original cell is interrupted.Handovers occur in the following cases:

1. A handover due to transmission measuring results occurs when the signal strength or its quality

is too low, or the distance from the mobile station to the BTS is too great. The call is thentransferred to a neighboring cell.

2. A handover due to high traffic volume within the cell occurs when the capacity of the cell isexhausted and the call is transferred into a neighboring cell which is less busy.

3. When necessary, a handover can be forced by the OMC - due to maintenance work in the cell,for example.

A distinction is made between various handover types. We'll look at them in more detail in thefollowing lessons.


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The smallest of all handover types is thehandover within a cell, the Intra cell - Intra BSChandover. In this case, a subscriber is divertedto a different traffic channel within the same cell.Generally, this channel is generated with adifferent frequency or time slot. The decision

about the handover is made by the BSC thatcontrols the cell.


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The Inter cell - Intra BSC handover takes placewhen a mobile user moves from one cell into aneighboring cell, both controlled by the same

BSC. The traffic connection to the old cell isdiscontinued as soon as the connection set-upto the new cell is successfully completed. Thisprocess is controlled by the BSC.


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We refer to an Inter Cell - Inter BSC handover when amobile user moves from one cell to a neighboring cellthat is controlled by a different BSC, with both BSCs

being supervised by the same MSC. This type of handover is carried out by the operating

MSC, but the decision about the handover is made by

the old BSC. The connection to the old BSC area is onlyinterrupted when the new connection has beensuccessfully established.


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If a mobile phone user changes over to a new cell operated by a differentMSC, we refer to an inter MSC handover. This is a bit more complicated:

Based on the signal measuring results supplied by the mobile station, theold BSS can detect the need for a handover to a neighboring cell. The old BSS informs the MSC that is still serving of the required handover.

This MSC is also called the Anchor MSC. Next, the Anchor MSC requeststhe Handover Number (HON), from the new MSC, or Target MSC. After

receiving this request, the Target MSC asks the new BSS to provide freeresources at the air interface.


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When terrestrial and air resources have been re-allocated, the Target MSCcan make the required HON available to the Anchor MSC. With thisnumber, the Anchor MSC can establish a connection to the Target MSC.

The Anchor MSC now sends the handover command to the mobile station

via the old BSS. When the handover is successful, the mobile station sendsan appropriate success message to the new BSS, which passes themessage on to the Target MSC.

The Target MSC informs the Anchor MSC via the current connection thatthe old connection to the mobile station can now be interrupted. The Anchor

MSC re-releases the channel allocation by notifying the old BSS.


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GSM Basic Callflows

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