6.1 Architecture Description

The IS-95-A/IS-95-B wireless data network is an adjunct to the IS-41-based voice network. The wireless data network is composed of Mobile Switching Center (MSC), Inter-Working Function (IWF), Public Packet Data Network (PPDN), Public Switched Telephone Network (PSTN), Home Location Register (HLR), Visitor Location Register (VLR), Data Communication Equipment (DCE), Data Termination Equipment (DTE), and modems. The network architectures for CS data and PS data are illustrated in Figure 6-2 and Figure 6-3. The subsequent subsections describe the various nodes that constitute the aforementioned architectures. However, nodes such as the HLR and VLR that are common to wireless technologies are not described.

Figure 6-2. Wireless circuit-switched data network architecture.

Figure 6-3. Wireless packet-switched data network architecture.

6.1.1 Data Terminal Equipment

The terminal equipment acts as a DTE in the standard DCE/DTE relationship. The communication between the terminal equipment and its associated DCE is via standard asynchronous serial input. The terminal equipment uses a set of AT commands (The "AT" sequence is called the " AT TENTION COMMAND" sequence) to control the services provided by the DCE. In case of WAP-enabled mobile stations , the MS acts as both the DCE and DTE.

6.1.2 Data Communication Equipment

On the radio side, the mobile station acts as the DCE in the standard DCE/DTE relationship. The data stream received from the DTE is packetized and sent over the air to the cellular infrastructure.

6.1.3 Mobile Switching Center

The MSC for the most part treats the CS data call as a regular voice call. However, it plays a very marginal role in support packet data services, where its primary role is to route the call to the IWF. In addition, the MSC is responsible for controlling intersystem handovers, roaming, and call delivery capabilities that support seamless operation of wireless data services.

6.1.4 Inter-Working Function

The IWF plays the role of a translator between the digital traffic in the CDMA system and the traffic in the connecting wireline voice and IP networks. It translates the protocols, signaling, and transmitted data into suitable forms that are compatible with both CS and PS networks. For a CS or fax data calls, the IWF terminates the air-link protocol suite and converts the data into standard landline modem modulations. In case of PS data for a connection into the PPDN, the IWF terminates the air-link protocol suite and routes the packetized data in the PPDN.

The IWF also plays a role in anchoring a PSTN call for the duration of the data call. This ensures service continuity in the event that a wireless data call is handed off to another MSC. As an example, data modems may drop calls and fax modems may drop calls or cause page errors if a carrier interruption occurs while transferring data through the PSTN. The IWF is located at the anchor MSC for the duration of the call.

6.1.5 Base Station

The Base Station functionality defined in the standard is usually split into two distinct entities: the Base Station Controlle (BSC) and the Base Station Transceiver Subsystem (BTS).

BSC

The BSC contains the radio-link management, cellular mobility management, and data handling (RLP functionalities). In addition, it contains some power control functionality required to provide feedback to the BTS with respect to power control thresholds.

BTS

The BTS provides the TIA/EIA/IS-95 air interface between the MS and the BSS. It is responsible for decoding/encoding the air interface frames and transferring them to the BSS or MS. In addition, it performs power control functionalities by controlling the allocation of power to each channel.

6.1.6 Modem

The modem represents a standard landline modem of the V.x series flavor. Support for V.21, V.22, V.22bis, V.32, V.32bis, V.42bis, V.17, V.27ter, and V.29 is required, whereas support for V.34, Bell-103, and Bell-212A is optional. The modem represents the standard analog and digital fax machines as well as the modem pools that allow users to connect to Internet Service Providers (ISPs).