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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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As part of the development of the CDPD System Specification, the companies agreed upon these design objectives:

  Maintain compatibility with existing data networks, technology, and applications
  Support a wide range of present and future data network services and facilities
  Provide maximal use of existing data network technologies and minimize the impact on existing end-user appliances
  Allow a phased deployment strategy in terms of basic connectivity, security and accounting, network management, and application services
  Provide services that relate to mobile and portable situations
  Support equipment from multiple vendors
  Provide seamless service to all subscribers
  Protect the subscriber’s identity
  Protect the subscriber’s data from eavesdropping
  Protect the CDPD network against fraudulent use
  Support conservative use of the airlink interface
  Support use of a wide variety of mobile situations

The CDPD Airlink Interface specification defines all procedures and protocols necessary to allow effective use of existing analog cellular channels for data communications. The initial CDPD System Specification was published in July 1993. Release 1.1 provided some updates and was published in January 1995.

CDPD Architecture

Figure 4.6 illustrates the architecture of the CDPD system. The Mobile Data Base Station (MDBS) defines a radio cell that interfaces the Mobile End System (M-ES), such as a portable computer with a CDPD modem, with the Mobile Data Intermediate System (MD-IS). The MD-ISs provide mobility management services for the CDPD network. The MDBS acts as a bridge between the wireless protocols of the M-ES and the landline protocols of the MD-IS. Therefore, the MDBS decodes the data received from mobile devices, reconstructs the data frames, and transfers them to the MD-IS. At most cellular telephone base sites, digital communications lines tie the MD-ISs back to the cellular telephone system’s MTSO. The CDPD architecture includes mobility management and internetworking between separate CDPD network providers, resulting in seamless operations as you move between different cells and providers.


Figure 4.6  The architecture of the CDPD system.

An M-ES wishing to send data uses the Digital Sense Multiple Access (DSMA) protocol to share radio channels with other M-ESs. This protocol is similar to the common ethernet protocol used in LANs. DSMA, though, has both a forward and reverse channel that supports full-duplex operation. MDBSs transmit on the forward band, and M-ESs transmit on the reverse band.

The forward channel includes transmission of Busy/Idle and Decode Status channel indicators. If an MDBS detects a transmission on the reverse band (coming from a mobile device), it transmits a signal with the Busy/Idle channel status indicator set. An M-ES wishing to send data will first check for this transmission and indicator before transmitting. The M-ES will not transmit until the indicator is set to Idle. The MDBS sends a Decode Status indicator to indicate whether it successfully decoded the received data transmission. If the M-ES does not receive a positive acknowledgment, it will attempt to retransmit the data.

To utilize CDPD, you will need to lease the service through your local provider and purchase a CDPD modem. An example of a CDPD modem is IBM’s Cellular Modem, which is housed in two packages—a PCMCIA Type II card and an external transceiver that you connect to the card via a cable. IBM also offers the modem in a tray-mounted version that is installed inside the IBM ThinkPad’s floppy drive. The Cellular Modem supports three modes of operation: analog voice, analog data, and CDPD.

WAN-Related Paging Services

Paging has been around for years. You often see doctors, chaplains, and service personnel with pagers on their belts, making it possible for someone to call the beeper number and alert the user to call a particular number. Traditionally, the paged person then responds by finding a phone and calling the person initiating the page. Today, companies are beginning to introduce two-way pager networks that offer networking capabilities.

The use of paging systems, especially those that support two-way transmissions, is an effective method for providing WAN services. Pagers have been very successful because of their light weight, long battery life, and ability to work indoors. These are strong selling points, especially if you require highly mobile communications that must work practically anywhere.

Two-Way Pager Networks

Companies have been working hard to enhance their paging infrastructures to two-way data services. SkyTel, for example, recently launched the first two-way paging and messaging service. The SkyTel 2-Way was the first narrow band Personal Communications Service (PCS), which enabled people to respond to and automatically acknowledge pages.

SkyTel’s two-way paging network, illustrated in figure 4.7, was designed for rapid messaging. Outbound messages from the Network Operations Center (NOC) use a different path to reach subscribers than messages use on the return path. Mtel (SkyTel’s parent company) owns nationwide narrow band licenses consisting of three 50 KHz forward paths and five 12.5 KHz reverse channels. These channels are within the 901 and 940 MHz frequency band.


Figure 4.7  SkyTel’s two-way paging network.

SkyTel’s network uses Motorola’s ReFLEX 50 narrow band PCS protocol that provides 25.6 Kbps throughput for numeric, alphanumeric, and binary data types. Messages are sent to the individual pagers over a 50 Khz channel using a satellite-based system and high-powered transmitters. Responses from the pager are sent at a lower rate (9,600 Kbps) to conserve the pager’s battery power. A series of small receivers deployed around the high-power transmitters collect these responses and send them back to the originator via the telephone system.

The NOC is an intelligent control site and tracks all network activity. The NOC receives all incoming messages originating from sources such as touch-tone phones, operator assistance, computers, the Internet, and network providers. The NOC then distributes the messages, collects confirmations and responses, and transmits them back to the message origination. The NOC can also route responses to one-way paging devices.


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