Mobile orginating Call
 

once a mobile phone has successfully attached to a gsm network, calls may be made from the phone to any other phone on the global pstn assuming the subscriber has an arrangement with their "home" phone company to allow the call. The user dials the telephone number presses the send or talk key, and the mobile phone sends a call setup request message to the mobile phone network via the mobile phone mast (bts) it is in contact with. The element in the mobile phone network that handles the call request is the visited mobile switching centre (visited msc).

The msc will check against the subscriber's temporary record held in the visitor location registry to see if the outgoing call is allowed. If so, the msc then routes the call in the same way that a telephone exchange does in a fixed network. If the subscriber is on a pay as you go tariff (sometimes known as prepaid (for example, in UK and india)), then an additional check is made to see if the subscriber has enough credit to proceed. If not, the call is rejected.

If the call is allowed to continue, then it is continually monitored and the appropriate amount is decremented from the subscriber's account. When the credit reaches zero, the call is cut off by the network. The systems that monitor and provide the prepaid services are not part of the gsm standard services, but instead an example of intelligent network services that a mobile phone operator may decide to implement in addition to the standard gsm ones.

How incoming calls are made to a mobile

step one: contact the gateway msc when someone places a call to a mobile phone, they dial the telephone number (also called a msisdn) associated with the phone user and the call is routed to the mobile phone operator's gateway mobile switching centre.

The gteway msc, as the name suggests, acts as the "entrance" from exterior portions of the pstn onto the provider's network. As noted above, the phone is free to roam anywhere in the operator's network or on the networks of roaming partners, including in other countries. So the first job of the gateway msc is to determine the current location of the mobile phone in order to connect the call. It does this by consulting the home location register, which, as described above, knows which visitor location register the phone is associated with, if any. Step two: determine how to route the call when the hlr receives this query message, it determines whether the call should be routed to another number (called a divert), or if it is to be routed directly to the mobile.

• if the owner of the phone has previously requested that all incoming calls be diverted to another number, known as the call forward unconditional (cfu) number, then this number is stored in the home location register. If that is the case, then the cfu number is returned to the gateway msc for immediate routing to that destination.

• if the mobile phone is not currently associated with a visited location register (because the phone has been turned off) then the home location register returns a number known as the call forward not reachable (cfnrc) number to the gateway msc, and the call is forwarded there. Many operators may set this value automatically to the phone's voice mail number, so that callers may leave a message. The mobile phone may sometimes override the default setting.

• finally, if the home location register knows that the phone is in the jurisdiction of a particular vlr , then it will request a temporary number (called an msrn) from that vlr. This number is relayed to the gateway msc, which uses it to route the call to another mobile switching center, called the visiting msc. Step three: ringing the phone when the call is received by the visiting msc, the msrn is used to find the phone's record in the visited location register. This record identifies the phone's location area.
Paging occurs to all mobile phone mast in that area. When the subscriber's mobile responds, the exact location of the mobile is returned to the visited msc. The vmsc then forwards the call to the appropriate phone mast, and the phone rings. If the subscriber answers, a speech path is created through the visiting msc and gateway msc back to the network of the person making the call, and a normal telephone call follows. It is also possible that the phone call is not answered.

If the subscriber is busy on another call (and call waiting is not being used) the visited msc routes the call to a pre-determined call forward busy (cfb) number. Similarly, if the subscriber does not answer the call after a period of time (typically 30 seconds) then the visited msc routes the call to a pre-determined call forward no reply (cfnry) number. Once again, the operator may decide to set this value by default to the voice mail of the mobile so that callers can leave a message.

Hope this information is useful