Our discussion thus far has introduced many issues of mobility which are independent of the media used to access a network. This is not an original idea-groups such as the Mobile IP Task Force have worked on mobility for some time without any media-specific considerations.
From this it is obvious thatmobility is not equivalent to wirelessness. Mobility is the ability to communicate anytime anywhere; this is a topological capability-always being able to "connect with" another party. Ideally, this connectivity can be maintained regardless of the location or motion of the mobile entity. This location independence should be available over an area which is physically too large for any single medium, such as an Ethernet cable or an RF channel.
A wireless host is a communications end-point1.19 which is physically untethered by a communications link or cable; this is a capability of the physical media in use. Obviously, wirelessness enables greater mobility than is possible with wired media, especially in-motion correspondence.
Mobility management is closely associated in wireless systems with radio resource management. Radio resource management is typically concerned with assuring proper (effective and efficient) use of the RF medium and is part of accessing the mobile network. In cellular-type systems such as CDPD, radio resource management could actually be considered to be a highly granular form of mobility management. However, in this book, we shall be very specific about mobility management as a Layer 3 activity.
However, WAN mobility does not require a wireless medium. We can easily conceive of ubiquitous Internet taps sometime in the future which would support mobility but not require wireless access. A user of such a system could take a portable computer from place to place, connecting via readily-available universal network taps to send and receive data; this would comprise a mobile capability which doesn't involve any wireless technology.
Conversely, a wireless capability in a host does not imply mobility. There are a number of wireless LANs and campus networks which, although free from the physical constraints of cables, cannot be considered to be WANs because of their limited range of operation. Many telemetry applications such as building security systems or utility meters entail wirelessness, but not mobility.
Wireless systems are limited by constraints such as the availability of radio frequencies, licensing from the Federal Communications Commission (FCC) and other regulatory authorities1.20 , and the expense associated with whatever technology is used to transport data "over the air." Radio Frequency spectrum is depicted in Figure 1.15.
The easiest-to-use (and therefore least expensive) radio spectrum has already been assigned to commercial broadcast applications such as television and radio; much of the previously-allocated spectrum is reserved for government use. Higher radio frequencies are available, but require more complex (more expensive) technology and suffer from greater attenuation (and thus, limited range). As we shall see, cost and coverage are key issues for wireless systems.
Some radio spectrum is freely available for use without requiring FCC licensing. This unregulated spectrum is typified by the 902-928 MHz region, commonly referred to as the ISM (Industrial, Scientific and Medical) band. The ISM band is used for applications such as garage door openers, remote controls, home security systems, microwave ovens, etc. Effective use of this unregulated spectrum for data communications requires the use of jam-proof radio technology such as spread spectrum (which is not an inexpensive technology).
The U.S. government has now reassigned and held public auctions for radio spectrum to be used for two-way Personal Communications Systems (PCS). This newly-available spectrum will encourage further advances in the use of wireless technology for data communications. As a result, we can expect continued growth in (wireless) mobile data applications.
Other factors currently limit the ubiquity of wireless data communications, including power control and signal propagation. Power control is a significant issue because the wireless host can operate only as long as its battery will allow. Although battery technology continues to make great strides, battery life is still a critical design factor for wireless systems; techniques such as sleep mode operation have been designed as key system functions which support wireless hosts by extending their battery life.
Signal propagation is always a factor in wireless systems because an RF signal at a given frequency and power level can be reliably received within a limited range of the transmitter. Increasing power levels tends to extend the range of wireless communication, limited by the battery capabilities of the wireless host. Having smaller RF coverage areas helps reduce wireless host battery requirements, but increases the network deployment costs because of an increased number of base stations providing landline connectivity.
All of these considerations are important to the design and deployment of wireless systems. However, they have little to do with mobility.