5.5 Circuit Switched Data Services


IS-136 provides analog and digital circuit switched data services. Digital circuit switched data services are an enhancement of the analog version and provide higher radio interface capacity and optional encryption over the radio link. In the following subsections, analog circuit switched services are briefly described since their application is very limited. Digital circuit switched services are described more in detail, and examples are provided of how these network services can be used to provide end- user services.

5.5.1 Model for Data Services

IS-136 data services are based on the model described in Figure 5-7. This model foresees that there is a splitting between the terminal equipment (TE) and the mobile terminal (MT). In particular, the assumption is that in the provisioning of data services the MT is used as access equipment that allows the applications residing in the TE to communicate to peers in the network. In other words, the MT is used similar to the way a modem is used by a personal computer to connect to the Internet. Data services in IS-136 have been developed considering that the application using the services resides in a TE such as a laptop computer.

5.5.2 Analog Circuit Switched Data Services

Analog circuit switched data services are provided through the architecture shown in Figure 5-9.

Figure 5-9. Architecture of IS-136 analog circuit switched data services.

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In order to add extra error control to cope with errors and packet loss over the radio interface, a cellular modem is used in the network through pools of modems connected to the MSC. Cellular modems provide error control and recovery not available in a landline modem, but due to the extra overhead they cannot provide the same data rates as traditional landline modems. Since there isn't a single common cellular analog modem protocol, the MSC has a pool of different cellular modems.

Analog circuit switched data services are provided through a two-step dial-up process. First, the user dials the access number for the modem pool, requesting a traffic channel (analog voice channel) for circuit switched data; second, when a cellular modem in the network answers, the user dials the number of the destination computer.

Billing for analog circuit switched data services is traditionally the same as for voice calls; since there is no special code used by the user to set up the communication; this allows one to discriminate between a data call and a voice call.

5.5.3 Digital Circuit Switched Data Services

IS-136 digital circuit switched data services are transported over the radio interface through a digital traffic channel (DTC). Cellular modems used for analog circuit switched data cannot be used for digital data services since they do not provide enough efficiency due to the fact they carry data in voice- band tones that are not modeled to a sufficient level of reliability. The data rate that can be achieved over a DTC is 9.6 Kbps uncompressed. It is also possible to have double or triple operations that allow one to achieve uncompressed user data rates up to 28.8 Kbps.

Figure 5-10 shows the architecture of IS-136 digital circuit switched data services. The mobile station is logically divided in a TE where applications reside, and an MT where cellular-specific protocol layers are implemented. Physical implementations in a single unit are possible. The interworking function (IWF) replaces the modem pool used for analog circuit switched data and performs protocol translation between the digital protocols over the radio interface and the modem protocols used in the PSTN. The IWF can be devoted uniquely to a single MSC or shared by multiple MSCs. The IS-136 DTC is implemented in the base station, whereas RLP1 is commonly distributed between the base station and the IWF. Although in Figure 5-11 the IWF is connected to PSTN through the MSC, IWF can be connected directly to the PSTN, ISDN, or packet data networks.

Figure 5-10. Architecture of IS-136 digital circuit switched data services.

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Figure 5-11. Protocol stack for IS-136 digital circuit switched data services.

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PROTOCOL STACK

The protocol stack for IS-136 digital circuit switched data services is shown in Figure 5-11. The data services use the IS-136 DTC L1 protocol described previously. Layer 2 is provided by the Radio Link Protocol 1 (RLP1) defined in IS-130, whereas layer 3 is provided by the IS-135 data part and by the IS-136 layer for the data call setup.

IS-130 "Radio Link Protocol 1" defines the link layer used in digital PCS to support asynchronous data and fax services defined by IS-135 over an IS-136 DTC. IS-135 "Asynchronous Data and Fax" defines a network protocol for asynchronous data and fax transport that describes call setup, monitoring, release, user data transport, and other features. IS-135 requires IS-130 as radio link and uses IS-136 for call setup, monitoring, and release.

The Radio Link Protocol (RLP1) provides link establishment, link supervision, acknowledged and unacknowledged data transport, data qualification, compression and encryption, and flow control. RLP1 uses a half-rate, full-rate, double-rate, or triple-rate IS-136 DTC to allow for a range of data rates based on user need and mobile and system capabilities. RLP1 supports two multiplexed data links, provided to layer 3 for transport of user data and IS-135 control information, respectively.

IS-135 provides a set of services to the DTE user applications:

  1. Call setup, monitoring, and release

  2. AT command handling

  3. User data transport

  4. On-line command signaling, break signaling, and signal leads

IS-135 uses IS-136 call control to perform call setup, monitoring, and release upon request from the user application or the IWF.

In the following we describe how data calls are set up.

MOBILE ORIGINATION OF DATA CALLS

When an asynchronous data call or group 3 fax call needs to be generated from the MS to the PSTN, the TE sends an AT command to the MT. The MT in turn originates a data call on the DCCH and provides the correct service code (for asynchronous data call or group 3 fax call). The MSC, based on the service code, assigns a DTC and connects the call to the appropriate port in the IWF, thus completing the logical links between the MT and the IWF required by IS-130. The MT then forwards the AT commands to the IWF, and the number in the AT command is used to connect the call to the desired destination (Figure 5-12).

Figure 5-12. Mobile originated asynchronous data call or group 3 fax call.

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MOBILE TERMINATION OF DATA CALLS

For mobile terminated calls, several options are available to deliver data calls:

  • The MS has one phone number and the call is "prearranged": In this case, the cellular network cannot detect that the incoming call is a data call and pages the MS with a service code for speech call. The MS needs to know in advance that the call is going to be a data call, and replies to the paging by requesting a data service code. The MSC therefore assigns a DTC and routes the call through the IWF (Figure 5-13).

    Figure 5-13. Prearranged mobile terminated asynchronous data call or group 3 fax call.

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  • The MS has one phone number for each service: This implies that the user has three phone numbers (voice, data, and fax) (Figure 5-14).

    Figure 5-14. Mobile terminated asynchronous data call or group 3 fax call with multiple numbers.

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  • Two-stage dialing is used: In this case, the cellular operator or service provider has dedicated numbers for each data service, and the calling party dials first the appropriate number to indicate it is a data call. The calling party then waits for a second dial tone and dials the called party specific number. In this way, the MSC knows in advance it needs to page the MS with a data service code and to route the call through the IWF (Figure 5-15).

    Figure 5-15. Two-stage dialing mobile terminated asynchronous data call or group 3 fax call.

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APPLICATIONS OVER DATA CALLS

Figure 5-16 describes how Internet access (e.g., for e-mail access and Web browsing) can be supported with IS-136 networks. Two scenarios are depicted: In one case, access to Internet is through the packet data network owned by an Internet service provider (ISP), whereas in the other case access is through the packet data network owned by the IS-136 cellular operator (i.e., the cellular operator acts as ISP).

Figure 5-16. Internet access with IS-136.

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In this application scenario, the SMTP application is considered for e-mail service for an IS-136 MS, and HTTP is used for Web browsing. In order to run such applications over IP, the MS needs first to set up a connection to the IP network (PDN) by setting up a data call as described in previous sections. This typically allows one to set up a PPP connection between the MN and an access point to the PDN (i.e., a modem) over which IP packets can be carried.

Once the IP connection is available, the user can start the e-mail or Web browsing application in the TE. SMTP and HTTP packets will be carried transparently through the PPP connection between the TE and the modem thanks to the data call, and the MS mobility will not be visible to this connection (i.e., if the MS moves between different BS or even to a different MSC, the connection will not be aware of such mobility).



IP in Wireless Networks
IP in Wireless Networks
ISBN: 0130666483
EAN: 2147483647
Year: 2003
Pages: 164

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