ored into SIM, not the same as the telephone number]. With the help of this, the MSC/VLR knows in which country the home network of this mobile is found. The MSC/VLR sends a signaling message via the international CCS7 signaling network toward the home country of this cellular subscriber. The message includes country code, net- work code, and subscriber identity. The message also includes the address of this new VLR to inform the HLR about the new location of the MS. Mobile Communications 3. When the HLR receives the message it requests the former "old" VLR, where this subscriber was previously located, to remove infor- mation about the subscriber. 4. The VLR (old) acknowledges and removes the subscriber informa- tion from its database. 5. The HLR updates the location information and sends the sub- scriber information, including security codes, to the new VLR. The (new) MSC/VLR stores the subscriber information, performs authentication of the MS, and acknowledges location update. The MS will now show the name of network Y on its screen. 5.6.2 Mobile Call Figure 5.12 illustrates how the GSM network routes a call to a subscriber who has roamed to another network. We assume here that both the calling and called subscribers are originally registered in the same home network, network X. Called subscriber B has traveled to another network Y and switched on her MS. Then the location update, which was illustrated in the previous section, takes place. Then mobile user A calls MS B from the home network. We can identify the following main phases when the call is established from MS A in the home network to MS B located in another network (Figure 5.12). Mobile originated (MO) and mobile terminated (MT) call. Network X Network Y AC/EIR MSC/VLR MSC/VLR PSTN or ISDN Mobile A Mobile B Signaling transfer Speech connection Figure 5.12 Mobile call in a cellular network. Introduction to Telecommunications Network Engineering 1. MS A initiates a call to MS B, which is currently located in another network. The call connection request