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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
CHAPTER 4 Wireless Wide Area Networks (WANs)
Do your professionals on the road need to have access to e-mail and other computing resources at their home office? The traditional solution is to equip the persons portable computer with a wireline modem and access on-line services and other resources via the Plain Old Telephone System (POTS). The user can interface his modem to the telephone line and dial into the services and resources the user wishes to utilize. This solution works well, assuming the professional has access to a telephone line. Most hotels and office facilities can accommodate a temporary POTS connection; however, other places do not. For instance, many travelers spend a great deal of time in airports waiting for plane connections that are often delayed. Unfortunately, there is no place to plug your computer into the POTS at an airport. In addition, you dont typically find POTS connections at archeological dig sites or environmental survey sites. Some hotels and office buildings also might not have a telephone line that you can use. For these situations, a wireless WAN might be the solution for effectively connecting people to the computing resources they need.
Its important to understand the various wireless WAN technologies and services before deciding on a solution. This chapter describes the following technologies, services, and products that provide wireless WAN connectivity:
- Packet Radio WANs
- Analog Cellular WANs
- Cellular Digital Packet Data WANs
- WAN-Related Paging Services
- Satellite Communications
- Meteor Burst Communications
- Combining Location Devices with Wireless WANs
- Wireless WAN Case Studies
Packet Radio WANs
A packet radio WAN uses packet switching to move data from one location to another. In general, a user wishing to utilize packet radio networking purchases a radio modem for his portable computer and leases access to a packet-based wireless network from a service provider such as ARDIS or RAM Mobile Data. The main advantage of packet radio is its ability to economically and efficiently transfer short bursts of data that you might find in systems such as short messaging, dispatch, data entry, and remote monitoring. However, packet radio systems do not yet have worldwide coverage; today, coverage is limited to large cities. In the next few years, this coverage should be near 100%.
Packet Radio Architecture
A packet radio network performs functions relating to the physical, data link, and network layers of the OSI reference model. Therefore, this type of network performs routing and provides a physical medium, synchronization, and error control on links residing between nodes or routers. If more than one hop is necessary to transfer data packets from source to destination, then the intermediate packet radio nodes relay the data packets closer to the destination, very much like a router does in a traditional wire-based WAN.
Packet Radio Components
To utilize a packet radio network, the user must equip his notebook or palmtop computer with a radio modem, applicable communications and application software, and lease a packet radio network service from one of several service providers (see fig. 4.1).
Figure 4.1 Packet radio network components.
Packet Radio Modems
A radio modem provides an interface between end-user devices and radio relays, using air as the medium. These modems are capable of transmitting and receiving radio waves at user throughput rates up to 20 Kbps. They usually do not require licensing, making it easy to move them from one location to another. Most radio modems transmit omnidirectional radiation patterns.
As long as connectivity exists, a pair of radio modems establishes a channel for data transmission between sites. The main condition for proper connectivity is that the destination must be able to correctly receive data from the source at a specified minimum data rate. For example, if the reception of data on a particular channel results in a number of bit-errors exceeding the maximum error rate for that link, connectivity is lost.
Due to node separation, transmit power, and irregular terrain, most packet radio networks are not able to maintain full connectivity. That is, not every user access device and radio relay node have connectivity with each other. Node separation affects the connectivity of a radio network because the power of a radio signal decreases exponentially as the distance between the nodes increases. If the distance becomes too great, the signal-to-noise ratio decreases and produces too many transmission errors, causing the two stations to become disconnected. The transmit power of the source node also affects link connectivity because higher transmit powers will keep the signal-to-noise ratio higher, resulting in fewer errors and connectivity. Certain types of terrain, such as mountains and buildings, can affect connectivity because they will attenuate and sometimes completely block radio waves. The attenuation will decrease the signal power, resulting in shorter transmission distances. A packet radio network, therefore, must perform routing to move data packets from the source user device, through a number of intermediate radio relays, to the destination user device or network.
Several companies sell radio modems that can interface with packet radio network services provided by various companies. These modems are service provider-specific and use different frequencies. A later section in this chapter describes several packet radio network service providers, as well as the modems that work with those services.
Relay Nodes
The radio relay nodes, which implement a routing protocol that maintains the optimum routes for the routing tables, forward packets closer to the destination. The routing table contains an entry for each possible destination relay node (see fig. 4.2).
Figure 4.2 A relay node routing table.
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