6.3 Data Services

6.3.1 Wireless Internet Access

CDMA-based digital wireless networks provide a ubiquitous and robust wireless data connectivity for browsing the Internet and sending or receiving e-mail. Users simply have to dial up their ISPs and log-on to the network. This is analogous to the wireline dial-up connection via a modem. In a CDMA network, the MS acts as a wireless modem providing connectivity to the data networks. Once on the network, a user can browse the Internet from anywhere and access all Internet applications. A few Internet applications are as follows :

• Electronic mail

• World Wide Web (text, graphics, sound, links, etc.)

• File Transfer Protocol (FTP)

• Remote File Access

• Multimedia Access (sound and video)

For the most part, existing deployments that support wireless Internet access rely on circuit switched data. For the purposes of explaining the process of setting up a call, we have chosen to describe the BSC-MSC messaging as an open interface called "A-Interface," defined in the TIA/EIA/IS-634 standard. However, it is prudent to note that most vendors have proprietary implementations for the BSC/MSC interface. Figure 6-6 illustrates the call flow for a CS async data call.

Figure 6-6. Mobile-initiated async data call setup message flow.

The user initiates a CS async data call by typing in the appropriate AT commands on the DTE (computer). The DTE forwards the AT command to the DCE (MS) on the Rm interface. Upon receiving the ATD command, the MS initializes the transport layer by issuing an OPEN call with the modem server port (380) as the destination port, leaving the source and destination IP addresses unspecified. The MS then sends an Origination Message with a data service option requested to the base station in order to set up up the CS data call.

The BSC then constructs a CM_Service_Request Message and sends it to the MSC with an async data call service indication. The MSC, recognizing that the request is for a data service, responds with a SCCP_Connection_ Confirmed Message to complete the establishment of the SCCP connection without allocating any terrestrial PSTN circuit. It then initiates the radio link setup by sending an Assignment_Request Message to the BSC.

The BSC and MS then exchange the appropriate messages over the Um interface specified by TIA/EIA/IS-95-A/B in order to complete the establishment of a radio link. Once the mobile successfully arrives on the traffic channel and the appropriate service negotiation is completed, the BSC sends an Assignment_Complete Message to the MSC. During this process the RLP layer is initialized to support CS data service option.

The successful receipt of the Assignment_Complete Message is the trigger for the MSC to set up the IWF-Mobile data path . In order to do so, it exchanges messages over the L-interface specified by TIA/EIA/IS-658. It constructs a SETUP Message in accordance with ANSI T1.617 and sends it to the IWF.

On receiving the SETUP Message, the IWF responds with a CONNECT Message to indicate allocation of a trunk. The MSC responds with a CONNECT_ ACK Message and completes the opening and allocation of an L-interface virtual circuit. The IWF relay layer signals this event to the IWF PPP layer. The PPP layer and the IPCP layer in the IWF then carry out self-configuration as per TIA/EIA/IS-707-A.

The IWF then sends an IPCP Configure-Request Message to the MS with the IP address parameter set to the address of the IWF. The MS stores this IP address in the destination IP-address field of the transport layer connection (which was left empty earlier). The mobile responds with an IPCP Configure-Request Message of its own, with all zeros in the IP address field. This is an indication to the IWF to assign an IP-address to the MS, which it does and includes in the IPCP Configure-Nak Message sent to the MS. It then initializes the transport layer by issuing an OPEN call with the modem server port (380) as the local source port, its own IP-address as the local source address, leaving the foreign network address and port number unspecified.

The MS extracts the IP address from the received Configure-Nak Message and stores it in the source IP-address field of the transport layer connection (which was left empty earlier), thereby completing its transport layer connection and the PPP connection establishment. The establishment of the PPP layer is the trigger for the MS to forward the ATD command to the IWF.

The receipt of the ATD command triggers the IWF into establishing the connection to the data call destination via the PSTN by constructing a SETUP Message in accordance with ANSI T1.607 and sending it to the MSC. The MSC responds with a CALL_PROCEEDING Message while it initiates the process of establishing a PSTN connection. Once the PSTN circuit is allocated, the MSC sends a CONNECT Message to the IWF. This completes the establishment of an end-to-end circuit connection and enables the applications to start communicating with each other.

The connectivity offered by the existing TIA/EIA/IS-95-B CDMA networks is by no means comparable to that offered by the telephone network. The call setup for PS data services mirrors that of CS data services but with enhancements for allocating and deallocating supplemental code channels that facilitate higher data rates. Although the standard allows for speeds up to 115 Kbps, very few manufacturers have chosen to implement that functionality, and data rates for the most part are limited to 14.4 Kbps. The primary reason for this is the quick evolution of the CDMA standard into the third generation, providing true PS capability. The details of this standard, called cdma2000, are provided in Chapter 10.

6.3.2 Short Message Service

Prior to the introduction of wireless data, Short Message Service (SMS) was the only mechanism for transferring data. SMS is the ability to send and receive messages with a limited number of characters directly to your MS's display. SMS allows your phone to behave as an alphanumeric pager. The messages can be transmitted over a common channel, called the paging channel in TIA/EIA/IS-95 networks, or over a dedicated traffic channel.

The SMS network comprises of the SMS Center (SMSC), the MSC, and the access network, consisting of the BSC and BTS. Message entry features, administration features, and message transmission capabilities are distributed between an MSC and the SMS Message Center (SMC). Figure 6-7 shows the topology of a SMS network.

Figure 6-7. SMS- related network components .

CDMA SMS is defined by the TIA/EIA/IS-637 standard, which is based on the TIA/EIA/IS-95 standards suite. SMS service comprises two types of services: SMS Bearer Services and SMS Teleservices. SMS Bearer Services refer to basic functionality, such as paging, text messaging, and message waiting notification. SMS Teleservices are advanced services, such as broadcast services, scheduled delivery, autonomous delivery, and database information services.

The SMS Bearer Services are provided by the SMS Relay and SMS Transport layers, whereas the SMS Teleservices are provided by the SMS Teleservice Layer. These layers reside on the TIA/EIA/IS-95A link layer for over-the-air transmission and reside on the TIA/EIA/IS-41 link layer for intersystem connectivity. An example of a SMS transaction is illustrated in Figure 6-8.

Figure 6-8. Example of an SMS transaction.

6.3.3 Secure Corporate LAN/Intranet Access

In wireless networks based on the CDMA technology, the air link is encrypted, providing security for confidential data. This enables solutions for securely accessing corporate LANs. Some CDMA-based service providers leverage this capability to provide virtual private networks (VPNs) to corporate LANs.

6.3.4 Digital and Analog Fax

Digital or Group 3 fax service uses the standard CCITT fax protocols to determine the condition of the PSTN connection and capabilities of the endpoint fax unit. The data is carried over a CS connection digitally. Data is packetized over the air interface and then converted into digital PCM before transmission into the PSTN to the destination fax unit. Analog fax service works in a similar fashion.

6.3.5 Wireless Application Protocol (WAP) Applications

One shortcoming of wireless data applications is that a computer or laptop is required for accessing them due to the dearth of processing capability and display real estate on the MS. Moreover, there are problems caused by low bandwidth, high latency, unpredictable availability, and dearth of connection stability. In order to overcome these obstacles, a set of standards was developed that are collectively called Wireless Application Protocol, or WAP.

A constituent of the WAP protocol suite is Wireless Markup Language (WML). This is similar to Hyper-Text Markup Language (HTML) used to lay out Web pages on the Internet but is optimized for wireless networks. Procedural and computational logic is enabled in WML-based Web pages using a scripting language called WMLScript.

Using these and other WAP components, information services similar to Web browsing (micro browser), small downloadable programs, telephony functionality combined with browser, and push services can be provided.

6.3.6 Looking Ahead

In spite of the high data rates made possible by the introduction of the TIA/EIA/IS-95-B standard, most vendors have chosen not to implement this capability. The primary reason is the fixed data rate of the supplemental code channel. Consequently, up to eight separate channels have to be allocated and managed in order to provide a high data rate link. This significantly increases the complexity of implementation. The cdma2000 standard alleviates this by allowing a variable-rate supplemental code channel. A single variable rate supplemental code channel can scale the data rate from 9.6 Kbps to 153.6 Kbps, thereby reducing the complexity of maintaining multiple channels.

In addition, there were other enhancements introduced by the cdma2000 standard that facilitated true "always-on" packet data session capability, discussed later in the book. These enhancements, along with the short time frame between the releases of the standards, have resulted in most vendors and carriers choosing to leapfrog from TIA/EIA/IS-95-A to cdma2000.