Comparative Study 1st, 2nd, 3rd, 4th, Generations from Handoff Aspects

Mobility is the most important feature of a wireless cellular communication system usually, Continuous service is achieved by supporting handoff (or handover) from one cell to another. to enhance the quality of service, its failure can result in ongoing call termination. So handoffs are necessary for providing an enhanced QoS to users and provide a ubiquitous coverage. handoffs have deferent criteria and Its effected on System performance so chosen one criteria of handoff is precise task, and There are numerous methods for performing handoff, as well as the kinds of network entities, The decision-making process of handoff may be centralized or decentralized (i. e. the decision may be made at the MS or network). at least there are three different kinds of handoff decisions. Network-Controlled or Mobile-Assisted, Mobile-Controlled Handoffs, also handoff can be Failures if no channel available on selected BS or Handoff is denied by the network for reasons such as lack of resources. For example, no bridge or no suitable channel card, the MS has exceeded some limit on the number of handoffs that may be attempted in some period of time. It takes the network too long to set up the handoff after it has been initiated. The target link fails in some way during the execution of handoff. Aim of a good handover strategy includes, The number of drop-outs should be minimum, The number of handovers should be minimum, Quick switch over of the call without any disturbance to the call. [2]. For mentioned above use certain criteria of handoff and making decision to depended on a certain method to control handoff are complicated task, so the mobile cellular networks used a deferent types of handoff (hard, soft and softer) through a deferent generations. In this paper we flowing the generations of mobile networks from 1st generation up to4th generation networks, we investigated theses generations from handover point because each type has own benefits and drawback as we see for example with the GSM, then finally we compared theoretically this generations network standard in two tables.