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xG networks target to use the spectrum in a dynamic manner by allowing the radio terminals, known as the cognitive radio, to operate in the best available frequency band. This enables ‘‘Get the Best Available Channel’’ concept for communication purposes. To realize the ‘‘Get the Best Available Channel’’ concept, an xG radio has to capture the best available spectrum. Spectrum mobility is defined as the process when an xG user changes its frequency of operation. In the following sections, we describe the spectrum handoff concept in xG networks and discuss open research issues in this new area.

In xG networks, spectrum mobility arises when current channel conditions become worse or a primary user appears. Spectrum mobility gives rise to a new type of handoff in xG networks that we refer to as spectrum handoff. The protocols for different layers of the network stack must adapt to the channel parameters of the operating frequency. Moreover, they should be transparent to the spectrum handoff and the associated latency.
As pointed out in earlier sections, a cognitive radio can adapt to the frequency of operation. Therefore, each time an xG user changes its frequency of operation, the network protocols are going to shift from one mode of operation to another. The purpose of spectrum mobility management in xG networks is to make sure that such transitions are made smoothly and as soon as possible such that the applications running on an xG user perceive minimum performance degradation during a spectrum handoff. It is essential for the mobility management protocols to learn in advance about the duration of a spectrum handoff. This information should be provided by the sensing algorithm. Once the mobility management protocols learn about this latency, their job is to make sure that the ongoing communications of an xG user undergo only minimum performance degradation.
Consequently, multi-layer mobility management protocols are required to accomplish the spectrum mobility functionalities. These protocols support mobility management adaptive to different types of applications. For example, a TCP connection can be put to a wait state until the spectrum hanoff is over. Moreover, since the TCP parameters will change after a spectrum handoff, it is essential to learn the new parameters and ensure that the transition from the old parameters to new parameters are carried out rapidly. For a data communication e.g., FTP, the mobility management protocols should implement mechanisms to store the packets that are transmitted during a spectrum handoff, whereasfor a real-time application there is no need to store the packets as the stored packets, if delivered later, will be stale packets and can not be used by the corresponding application.