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Mobile-End Systems

Although the physical location of a mobile-end system, or mobile unit, may change as the user’s location changes, continuous network access is maintained. The CDPD specification stipulates that there will be no changes to protocols above the network layer of the seven-layer OSI model, so that applications software will operate in the CDPD environment. At the network sublayer and below, mobile units and backbone routers cooperate to allow the equipment of mobile subscribers to move transparently from cell to cell, or roam from network to network. This mobility is accomplished transparently to the network layer and above.

OSI Protocols. The recommendations of the CDPD consortium were designed using the OSI reference model (see Exhibit 3). The model not only provides a structure to the standardization process, it offers recommendations regarding protocols available for use in the CDPD network.

Network Layer Protocols. The CDPD overlay network may use either the OSI CLNP or IP at the network layer. These protocols have virtually the same functionality: They both interpret device names to route packets to remote locations.

IP has been used for more than 10 years and is one of the most popular protocols today. Its inclusion in the CDPD specification is intended to accommodate the vast number of networked devices already using it.


Exhibit 3.  The OSI Reference Model.

Application Layer Protocols. Applications required to administer and control CDPD networks use OSI-defined protocols. OSI-defined application-layer protocols are widely accepted and have been tested to ensure robust, open communications among CDPD service providers. The use of these protocols provides a level playing field for manufacturers of the CDPD infrastructure equipment. Therefore, service providers can be confident that the various network elements will communicate together and that no single manufacturer can exert undue influence on the market.

Examples of OSI protocols that operate at the application layer and can be implemented for CDPD network administration and control are explained as follows:

  The CMIP is the object-oriented management standard for OSI networks developed by the ISO.
  The X.400 message handling system is a global messaging standard recommended by the ITU-TSS, formerly known as the ITTCC that defines an envelope, routing, and data format for sending E-mail between dissimilar systems.
  X.500 directory services are a standard for directory services recommended by the ITU/ISO that operate across multiple networks used to convey E-mail. It allows users to look up the E-mail addresses of other users they wish to communicate with.

MOBILITY MANAGEMENT

Traditionally, the network address of the end system has been used to determine the route used to reach that end system. CDPD is unique in allowing mobile units to roam freely, changing their subnetwork point of attachment at any time — even in midsession.

To find the best route for transmitting data to an end system, CDPD mobility management definitions describe the creation and maintenance of a location information database suitable for real-time discovery of mobile unit locations. Three network entities — the mobile units, the home backbone router, and the serving backbone router — participate in mobility management.

Mobile units are responsible for identifying their unique NEIs or network layer addresses to the CDPD network. As the mobile unit moves from cell to cell, it registers itself with the new serving backbone router. Each NEI is permanently associated with a home backbone router. The serving backbone router notifies the home backbone router of a mobile unit when it registers itself in the new serving area. Mobility management makes use of two protocols: the MNRP and the MNLP.

MOBILE NETWORK REGISTRATION PROTOCOL

MNRP is the method mobile units use to identify themselves to the network. This information is used to notify the network of the availability of one or more NEIs at a mobile unit. The registration procedure includes the information required by the network for authenticating the user’s access rights.

The MNRP is used whenever a mobile unit is initially powered up and when the mobile unit roams from cell to cell. In either case, the mobile unit automatically identifies itself to the backbone router so its location can be known at all times.

MOBILE NETWORK LOCATION PROTOCOL

MNLP is the protocol communicated between the mobile serving function and mobile home function of the backbone routers for the support of network layer mobility. MNLP uses the information exchanged in MNRP to facilitate the exchange of location and redirection information between backbone routers, as well as the forwarding and routing of messages to roaming mobile units.

INFORMATION PROTECTION

To facilitate the widespread acceptance of CDPD by cellular service providers, the specifications define methods for ensuring the security of customer information, while still providing an open environment for mobile users. Cellular service providers are legitimately concerned about protecting information about their subscriber base from each other, yet the nature of the service dictates that carriers exchange information with one another to provide subscribers with full mobility.

For example, when a user who is usually served by Carrier X in Chicago roams to the Carrier Z service area in Boston, Carrier Z must be able to find out whether that user is authorized to use the network. To do that, Carrier Z queries the Carrier X database about the user’s access rights using the network equipment identifier. Carrier X provides a simple yes or no response. The details concerning the identity of the user, types of service the user has signed up for, rates being charged, and amount of network usage are all protected.

CDPD NETWORK BACKBONE

The internal network connecting the backbone routers (i.e., MD-ISs) must be capable of supporting CLNP and IP. The backbone routers terminate all CDPD-specific communications with mobile units and MDBS, producing only generic IP and CLNP packets for transmission through the backbone network.

Mobile-End Systems Protocols

As noted, the requirement that mobile units support IP is meant to ensure that existing applications software can be used in CDPD networks with little or no modification. However, new protocols below the network layer have also been designed for CDPD. These protocols fall into two categories: those required to allow the mobile unit to connect locally to an MDBS, and those required to allow the mobile unit to connect to a serving backbone router and the network at large.


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