All Mobitex networks use the same protocols and operate under the same specifications. Mobitex is a worldwide standard, introduced and controlled by Ericsson of Sweden and administered by the Mobitex Operators Association (MOA). Bell South Wireless Data in the U.S. and Cantel in Canada are two such Mobitex operators. Under the MOA’s leadership, European Mobitex operators are now introducing international roaming, a service that will allow Mobitex users to continue to send and receive messages and access data outside their home countries. The Mobitex specification administered by the MOA also ensures that all operators follow the same standard.

The Elements of Mobitex — Base Stations and Cells
Like mobile telephone systems, the Mobitex network is based on radio cells, with radio links replacing the wiring that connect telephones or data terminals to networks. Intelligent base stations provide the links by allocating channels to active terminals within limited geographic areas. The size of a radio cell is determined by such factors as the number of channels available, the maximum number of units that must be served and the output (transmission) power of both the terminals and the base station.

Base stations serve several essential functions in a mobile communications system. First and foremost, they function as connection points so that calls or messages can be switched from one base station or local exchange in the telecommunications network to another.

A second function of base stations is the ability— in conjunction with mobile user devices — to hand off calls as users change locations. This is the function that makes roaming possible. This concept is similar to circuit-switched cellular networks as described earlier.

Acting as network nodes is a third function of base stations. By fulfilling this role, base stations enable the forwarding of traffic statistics to the network NCC for subscriber billing. This function also makes possible the downloading of control data and software to base stations for the purpose of altering operating characteristics.

Packet Switching for Efficiency
Virtually all Mobitex’s components employ digital technology, which means that information is transferred across the network as data, not analog voice signals. The critical system parameter for data communications is the method of transmission employed. Two transmission techniques are available: circuit switching and packet switching.

In circuit-switched networks, such as those used for mobile telephone systems, a physical connection must be maintained between the sending and receiving nodes for the duration of a call, connection or session. Setting up the connection also takes some time, which is a disadvantage when the user needs to transfer only a small amount of data.

In packet-switched networks, data is divided into small packets that can be transmitted individually as traffic permits. Set-up time is eliminated and network connections are instantaneous. In action, packet-switching make more efficient use of channel capacity, typically allowing 10 to 50 times more subscribers over a radio channel, compared with circuit switching. This point is very important, because the radio spectrum is a scarce resource.

A Mobitex network is organized as a hierarchy, with nodes at three levels: base stations (BRS), area exchanges (MOX), and main exchanges (MHX). At the top level there is the NCC.

As we have seen, coverage is provided by overlapping radio cells, each served by an intelligent base station. Mobitex users communicate with the closest base station. Because intelligence is distributed throughout the network, data packets need only be forwarded to the lowest network node common to the sender and receiver. The base station can handle all local traffic between mobile terminals. Only billing information is passed to the higher levels.

The area and main exchanges handle switching and routing in the Mobitex network and provide connection points for fixed terminals. A number of interfaces (including several ISO and IBM standards) are supported, but X.24 — the ISO and CCITT international standard for public packet-switched data networks — has become the most widely used standard for Mobitex, though TCP/IP support is also becoming common. Subscriber information for billing of services is also processed at this level before being forwarded to the NCC.

Mobitex Connectivity Hardware and Software
Ericsson and its business partners have developed a number of connectivity products based on several Mobitex proprietary and some industry-standard interfaces available on all major network platforms. These products include radio modems that allow mobile users to access the network, software drivers, and MCSS (switches/gateways) that provide links to other data networks and computer systems.

The Mobidem line of radio modems supports several data protocols, including an extended AT command set. This makes them compatible with existing communications software that runs on standard PCs. Mobidem models include the M1000 series, which are portable, battery-operated units; the M2000 series, which now includes a radio modem on a PCMCIA card; and the new M4060, intended for vehicle mounting or fixed installations.

On host systems (commonly called the fixed side) many earlier Mobitex applications used the X.25 protocol, the international standard for packet-switched data networks. Now TCP/IP support is becoming more common. Gateways are available, however, for most major network standards, including IBM SNA (Systems Network Architecture).

These connectivity solutions make Mobitex applications adaptable to different customer configurations so that any standard PC or hand-held computer with Mobitex communications software drivers can be used to access the Mobitex network. Mobile communication server switches/gateways installed at the area-exchange level or on a customer’s premises allow mobile users to access mainframe-hosted databases or global messaging systems such as the Internet.

Key Features of Mobitex
Transparent roaming throughout the country or even internationally in Europe, store-and-forward capability, fault-tolerance, shortest path, interoperability and security are the key features of Mobitex, according to Ericsson and Bell South Wireless Data. We shall describe these features briefly.

Transparent Roaming
The Bell South Wireless Data network provides nationwide, seamless, transparent roaming within the U.S. It is transparent because no manual intervention is required by a subscriber or mobile user once a radio modem has automatically registered with the network. It is seamless because the networks know which base station a mobile is linked to at any given moment. Frequency-agile Mobitex-compatible radio modems are able to operate on all 200 Bell South Wireless Data system frequencies, roaming from one to another automatically, switching to the best currently available channel as needed and thus allowing Bell South Wireless Data mobile users to stay connected as they move around within Bell South Wireless Data’s coverage area. International roaming is also now available in certain markets.

Store-and-Forward
Bell South Wireless Data offers store-and-forward capabilities within its networks to ensure that messages are delivered. If a mobile user turns the modem off or drives through a tunnel, crosses a steel bridge, or leaves the Bell South Wireless Data coverage area, the message is retained within the NCC’s store-and-forward facility. The Bell South Wireless Data network will try to re-transmit the message automatically, provided other Mobitex OSI layers or third-party-developed software have been programmed to handle the packet in transit and have not timed out.

Reliability and Shortest Path
According to Bell South Wireless Data, their network engineering design features 99.99 percent reliability, ensuring transmissions with a bit-error rate equivalent to that of wired line modems. Numerous fault-tolerant components are employed in the system, including link level data checking between adjacent network nodes and forward error correction. Base stations and other equipment have uninterrutible power supply (UPS) and/or generator backups. Messages are routed through the shortest path when they traverse multiple MOX nodes.

All facilities and communications links are remotely monitored by network management staff located at Bell South's  NCC in Woodbridge, NJ.

Interoperability and Interfaces Supported
Ericsson provides lower level APIs and other technical specifications to third parties who have developed various higher level application interfaces to multiple network transport protocols and information servers. These software interfaces are discussed in Chapter 11. Chapter 13 lists several third party products that support the Bell South Wireless Data network. Briefly, the following hardware and software protocols are supported:

Modems : Ericsson, GE, and Motorola radio modems used with Bell South Wireless Data’s service.

Protocols : Mobitex supports SNA 3270, LU2 and LU3, X.25, TCP/PI and asynchronous protocol, as well as the MTP/1 transport protocol.

Security
Unlike the vulnerability of cellular voice communications, it is generally more difficult to tap into and decipher wireless data networks such as the Bell South Wireless Data network with its frequency-agile modems moving from one channel to another. However, no network is considered absolutely secure. Users should investigate, therefore, the appropriateness of end-to-end encryption for their applications. Certainly it is required for public safety and banking transactions.