The WIN mirrors the wireline IN mode. But the distinction between the wireless and wireline network is that many of the wireless call activities are associated with movement, not just the actual phone call. In the WIN, more call-associated pieces of information are communicated between the MSC and the SCP or HLR. The WIN moves service control away from the MSC and up to a higher element in the network, usually the SCP (see Figure 1).

Figure 1. Components of a WIN

• MSC as service switching point (SSP)—In the IN, the SSP is the switching function portion of the network. The mobile switching center (MSC) provides this function in the WIN.
• service control point (SCP)—This device provides a centralized element in the network that controls service delivery to subscribers. High-level services can be moved away from the MSC and controlled at this higher level in the network. It is cost-effective because the MSC becomes more efficient, does not waste cycles processing new services, and simplifies new service development.
• intelligent peripheral (IP)—The IP gets information directly from the subscriber, be it credit-card information, a PIN, or voice-activated information. The peripheral gets information, translates it to data, and hands it off to another element in the network—like the SCP—for analysis and control.
• signal transfer point (STP)—This is a packet switch in the signaling network that handles distribution of control signals between different elements in the network such as MSCs and HLRs or MSCs and SCPs. The advantage of an STP is that it concentrates link traffic for the network. It can also provide advanced address capabilities such as global title translation and gateway screening.
• location registers—These are used to supplement MSCs with information about the subscriber. The number of subscribers that the switch supports changes as roamers move in and subscribers move to other switches. The database of active subscribers changes very dynamically. Each MSC cannot have the database for all potential users of that switch. The following location registers help to get around that problem:
  • visitor location register (VLR)—Within an MSC there is a VLR that maintains the subscriber information for visitors or roamers to that MSC. Every MSC or group of MSCs will have a VLR.
  • home location register (HLR)—Information on roamers is obtained from that subscriber's HLR. Each subscriber is associated with a single HLR, which retains the subscriber's record. When the subscriber roams to another switch, the VLR queries the subscriber's home HLR to get information about that subscriber. When a phone call goes to a subscriber's MSC, the MSC recognizes that the subscriber is roaming and asks the HLR for the subscriber's location. The HLR will communicate that information to the VLR and relay a temporary location number received from the visited system. In the WIN architecture, the HLR is usually a network element such as an SCP.
• WIN call model—The WIN call model enables the network to handle new triggers (which are decision points in a call) and new transaction capability application part (TCAP) messages.

The evolution to WIN will be a major step forward for North American wireless networks. It involves an industry-wide consensus among equipment vendors and service providers to incorporate IN concepts into existing wireless networks.

The following steps will need to occur before WIN will be a reality:

• incorporation of SCP, IP, and SN into the wireless network architecture
• evolution of the MSC to a SSP
• separation of call control and transport from service control
• development of generic call models, events, and trigger points