4.3 Data Application Examples


We have looked at the details of different functions for carrying data on traffic channels. Now let's look at some of the data applications that can be connected using GSM PLMN.

4.3.1 Fax Transmission

GSM technology provides two types of teleservices for fax: alternate speech/facsimile group 3 (TS 61) and automatic facsimile group 3 (TS 62). TS 61 is used when the user wants to switch between the voice call and the fax machine. It uses an in-call modification procedure via the user interface. The fax service can be provided on both transparent and nontransparent connections. Both mobile originated and terminated fax calls are supported. The bearer service information transfer mode for fax is circuit, duplex, synchronous, and symmetric.

For GSM, the group 3 facsimile service is adapted from the fixed network specification defined in ITU-T F.160. This service specification is comprised of two parts : the control protocol described in ITU-T T.30, and the document transmission coding described in ITU-T T.4.

GSM terminal equipment can be connected to a two-wire basic fax machine with the use of the fax adapter function. The fax adapter function converts the analog signals coming from the two-wire fax machine into a serial digital stream having the ISDN-specific R interface as the output. This R interface output needs a standard synchronous TAF (GSM 07.03) to connect to the GSM MS. A personal computer (PC), emulating a fax machine, can be connected directly to a GSM phone with a commercially available PCMCIA fax card supporting GSM phones. In the PCMCIA fax card, the TAF and fax adapter functions are combined.

As shown in Figure 4-13, a fax, either originated from a PC or two-wire fax machine, is sent through the BSS to the MSC. The MSC transfers the data link to the proper IWF. Different types of IWF are possible for the different type of transit networks. The transit network subsequently transports the fax information to the receiving fax machine. Both fax machines communicate with each other using the standard fax protocol defined in ITU-T T.30.

Figure 4-13. Fax transmission using GSM PLMN.

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Figure 4-14 depicts an expanded view of the protocols and rate adaptation used for transmitting nontransparent fax through a PLMN. It should be noted that depending on the implementation, the R reference point might not exist explicitly. In this case the LAPB protocol and consequently the LAPB entities operating across this interface may be omitted. L2RBOP and RLP protocol stacks are used at the radio interface. As the fax data is synchronous, RA1' is used to transform the data rates supported on the air interface while adding some control bits. The air interface rate could be 3.6, 6, or 12 Kbps, depending on the bearer service subscribed by the fax user. The data is sent over the air with FEC. In the BSS, fax data is transformed into the intermediate rates of 8 or 16 Kbps by the RA1 function. The intermediate rate is transformed into an A interface data stream of 64 Kbps by the RA2 function. A similar view can be established for the transparent fax with the absence of L2R and RLP functions.

Figure 4-14. Data protocol and functional view for fax transmission.

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4.3.2 Internet Connection

A GSM PLMN can provide a circuit switched connection to an IP network or Internet (Figure 4-15). The GSM specifications don't specify a teleservice or bearer service for Internet connection. But the MS can access the Internet by subscribing to any of the data bearer services (e.g., asynchronous PAD access at 9.6 Kbps). Both transparent and nontransparent connections can be utilized. The nontransparent connection provides better protection over the air interface compared to the transparent connection. In this description, we have assumed that an asynchronous nontransparent connection is established to a PAD in the IP network. We will describe both basic and dedicated PAD access to the IP network. In both cases, the GSM PLMN provides a circuit switched bearer to the PAD, and the IP protocol executes transparently on top of this bearer.

Figure 4-15. Data protocol and functional view for Internet connection.

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Just like any other data terminal of a GSM PLMN, an IP host (e.g., a laptop) needs to be connected to the TAF function of the MS. The TAF function is available on a PCMCIA card, which provides a direct connection from an IP host to the MS.

In the basic PAD access case, the IWF is composed of modem pools to allow modem-based communication across the network. The modem pools allow the users to make calls to the Internet service provider (ISP). The cellular network can be thought of as a big serial cable providing access to the modem in the IWF. The user must first dial the E.164 address of the subscribed port. This address can be configured in the dial-in program so a user doesn't have to input the address with every access. The user sends to the PAD the address of the entity in the IP network to which it needs correspondence (e.g., e-mail server address). The PAD reaches the IP network entity on the user's behalf . This case was represented in Figure 4-9 with only one modem.

Some service providers have dedicated trunks connecting GSM PLMN to an IP network. This is the case of dedicated PAD access. For our example of e-mail service access, the user only needs to send the e-mail server addresses for establishing connection to the server. The user doesn't need to dial in a port address in the IP network. This case is also represented in Figure 4-10, the modem in that figure is the digital modem used to access the digital line.

For the protocol and functional view, let's assume the most involved case of nontransparent data, as shown in Figure 4-15. The GSM PLMN provides L2R and RLP functions to carry the IP datagrams over the air interface. A simpler view of transparent data can be visualized with the absence of L2R and RLP.



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

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