Authentication, Registration and Location Management

Some of the most complicated transactions in ANSI-41 are performed without the user of a phone being aware. When a phone detects a new system it sends a registration message to the Base Station. This is forwarded to the MSC, which then attempts to authenticate the mobile. This is a sophisticated cryptographic process that involves sending a challenge (a large random number) to the mobile, and verifying that the correct response is received. It is virtually impossible for an illegitimate mobile to respond correctly to this challenge. The data required for authentication is obtained with the ANSI-41 AuthenticationRequest message. The data can be used for as long as the mobile is registered in a system.

Once a mobile has been authenticated, the ANSI-41 RegistrationNotification is sent to the HLR. This verifies that the mobile’s MIN or IMSI is valid, and matches the Electronic Serial Number (ESN) of the mobile. It also obtains the subscriber's profile, which details the types of calls that the subscriber is entitled to make (e.g. international calls may be restricted), and special services such as 3-way calling, call waiting, call forward no-answer and call-forward on busy. The subscriber profile also indicates the triggers that are set, that are required to support special services, including WIN (Wireless Intelligent Network).

Another important function of the RegistrationNotification is that it tells the HLR where a mobile is located. This is necessary to allow Call Delivery to operate.

Call Delivery

Call Delivery is the process of routing a call to a mobile that is roaming outside its home system. It requires the following steps:

• Authentication, Registration and Location Management must already have been performed.
• A call is received by the Home MSC for the mobile.
• The MSC sends a LocationRequest message to the HLR.
• The HLR may return a call forwarding number, if applicable, or deny access to the mobile. Otherwise, it sends a RoutingRequest message to the Serving MSC.
• The Serving MSC assigns a TLDN to this call. This is simply a local phone number that is reserved for this purpose.
• The TLDN is sent back to the HLR in the RoutingRequest RETURN RESULT (response) message.
• The HLR sends the TLDN back to the Home MSC in the LocationRequest RETURN RESULT.
• The Home MSC routes a call using the TLDN. This will cause the call to be routed to the Serving MSC through the PSTN.
• The Serving MSC pages the mobile, and connects the incoming TLDN call to it. The TLDN is now released for use in another call.
• If the mobile does not answer the call or is busy, the call can be redirected to the call forward number, releasing the trunks connected to the Serving MSC.

Short Message Service (SMS)

Short Message Service (SMS) allows the delivery of short text messages to wireless phones. Newer models of phones also support SMS message origination (i.e. from the phone to another phone or an email address). The process of receiving a short message in ANSI-41 is:

• An incoming short message is received by an MC, perhaps through an email gateway.
  
• The MC sends an ANSI-41 SMSRequest message to the HLR to find out where the mobile is, and whether it is eligible to receive the short message.
  
• The MC then sends the short message in an ANSI-41 SMSDeliveryPointToPoint (SMDPP) message to the Serving MSC, which forwards it to the mobile.
  
• If the mobile is not able to receive a short message, it can be retained by the MC, and forwarded when the MSC and HLR inform it that the mobile is again available.

Mobile originated short messages use the reverse of this process.

Mobile Originations

In ANSI-41 systems, mobile originations are very simple because the work is already done! Only in the case when a mobile has just been turned on is the Authentication, Registration and Location Management process necessary. Once the system has determined which services, if any, that a mobile is entitled to, the mobile origination can proceed in exactly the same way as for a home subscriber.