Click here for ObjectSpace: Business- to- Business Integration Company
home account info subscribe login search My ITKnowledge FAQ/help site map contact us


 
Brief Full
 Advanced
      Search
 Search Tips
To access the contents, click the chapter and section titles.

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

Bookmark It

Search this book:
 
Previous Table of Contents Next


Packet Radio Operation

To carry packets from source to destination, a packet radio network must do the following:

1.  Transmit data packets
2.  Update routing tables at the relay nodes

Transmitting Data Packets

When the application software at a user’s appliance requests the transfer of data through the network, communications software prepares the data for transmission by wrapping it with a header that primarily contains the destination address and some trailer bits that represent a checksum. The relay nodes use the address to determine whether to forward the packet to the next relay node or send it to the final destination. A receiving node utilizes the checksum to detect if the packet encountered any transmission errors. If errors are not present, the receiver sends back an acknowledgment; otherwise, the source retransmits the packet.

Each station, whether it is the user’s access device or a relay node, uses a carrier sense protocol to access the shared air and radio medium. Ethernet and radio-based wireless LANs operate in a very similar pattern. The primary difference is that a packet radio network operates in a partially connected instead of fully connected topology (see fig. 4.3). The propagation boundary of a particular node defines that node’s operating range. Nodes A and C, for example, are within each other’s propagation boundary; therefore, they can communicate directly with each other.

A packet radio station wishing to send a data packet must first listen to determine if another station is transmitting. If no other transmission is heard, then the sending station will transmit the packet in a broadcast mode using its omnidirectional antenna. With most packet radio networks, the first station to receive the packet will be the neighboring relay node. This relay will look in its routing table to determine which node to send the packet to next, based on the final destination address. If the destination is located within range, the relay node will broadcast the packet again and the destination will receive it. If the final destination is not close by, the relay node will obtain the address and broadcast the packet to the next relay node closer to the destination. This process will continue until the packet reaches the destination.


Figure 4.3  The topology of a packet radio network.

Updating Routing Tables

As with wireline WANs, a packet radio topology may change over time. Relay nodes might become inoperative, new relay nodes might appear, and atmospheric conditions, such as rain and sunspots, might affect radio connectivity between stations. These connectivity changes can alter the topology; therefore, the efficient operation of a packet radio network depends on an effective routing protocol capable of updating the routing tables at each relay node.

There are several approaches to updating routing tables; however, the distributed protocol is most common with the Internet and packet radio networks. Wireline WANs utilize routing protocols such as Routing Information Protocol (RIP) and Open Shortest Path First (OSPF). These protocols enable each router within the network to gain a complete picture of the network’s topology. Packet radio routing protocols have the same goal.

Specifically, distributed routing protocols make it possible for each relay node to determine the next path to send a packet by operating as follows: each relay node periodically sends a status packet that announces its presence to all neighboring relay nodes within range. Each node, then, periodically learns the presence of its immediate neighbors. A router can use this information to update its routing table. When a relay node sends its status message, it also sends a copy of its routing table. Each relay node also sets timers for each neighbor, and if the relay node does not receive a status message within a certain time period, the relay node will delete the neighbor from the routing table. Other relay nodes will hear of the deletion via the periodic status messages. Through this process, each relay node will eventually obtain a complete picture of the network in terms of connectivity.

Packet Radio Service Providers

Several packet radio service providers have constructed networks that implement the radio relay nodes. The process of establishing packet radio networking is to lease the service from one of several packet radio access providers. This service is very economical, costing pennies to send small e-mail messages. These companies usually supply the software for no charge; however, users must purchase a radio modem.

The following sections provide an overview of several packet radio network providers.

ARDIS

ARDIS is a company that leases access to their wireless WAN, which is based on packet radio technology. The ARDIS network covers 410 top metropolitan areas in the United States, Puerto Rico, and the U.S. Virgin Islands. This network encompasses more than 80 percent of the population and 90 percent of the business areas. ARDIS uses two different protocols—MDC4800 at a data speed of 4,800 bps, and Radio Data-Link Access Protocol (RDLAP) at a speed of 19,200 bps. ARDIS was originally developed for IBM service technicians who worked indoors. As a result, ARDIS was designed to have good in-building coverage.

In addition to basic wireless WAN interconnectivity, ARDIS offers the following wireless WAN applications that enable you to communicate with other ARDIS and Internet users, as well as implement some specific applications:

  ARDIS PersonalMessaging. Enables users to send and receive messages to ARDIS and Internet users.
  ServiceExpress. A wireless communications solution that makes it easier for field service organizations to incorporate wireless communications. ServiceExpress provides field service engineers with access to corporate information systems using handheld computers. ARDIS offers this service with a low monthly payment per field engineer. The payment covers the cost of all hardware, ARDIS air time, training, equipment repair, maintenance, and a 24-hour help desk. In most cases, ServiceExpress will work with a company’s existing field service management software.
  TransportationExpress. Combines all the components of a wireless dispatch solution including dispatch hardware and software, ARDIS wireless air time, handheld wireless computers for pickup and delivery drivers, project management, installation, training, and a 24-hour help desk.


Previous Table of Contents Next


Products |  Contact Us |  About Us |  Privacy  |  Ad Info  |  Home

Use of this site is subject to certain Terms & Conditions, Copyright © 1996-2000 EarthWeb Inc.
All rights reserved. Reproduction whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Read EarthWeb's privacy statement.