# Primary User Traffic Pattern Based Opportunistic Spectrum Handoff in Cognitive Radio Networks
## 3 PUTPOSH Model  
### Overview of the Model  
The PUTPOSH approach contains the dual handoff modules i.e., proactive and reactive. The PUTPOSH scheme permits CRU to detect patterns of PU's traffic and select a reactive or proactive scheme accordingly. On the arrival of PU, CRU may intelligently decide whether to do handoff or wait for an ongoing channel by considering the overall service time into account.  
The PUTPOSH approach has the following properties:  
1. The cognitive radio networks (CRNs) is assumed to be a time divided system and every CRU performs spectrum sensing in the first half of every time slot. The actual transmission of the CRU is performed in the second part of the time slot where the target channel is sensed as idle.
2. When the target channel is consumed by a PU, the CRU will perform the actual mobility management function by considering the waiting on an up-to-date channel or shifting its communication to the new channel.
3. CRNs comprise of autonomous channels, where every channel has high and low precedence queues. Every high priority queue has served one PU while the lowest precedence queue served several SUs on the first-come-first-serve (FCFS) basis.
4. A handoff decision protocol is divided into the transmission and sensing time slots. The interarrival of PU is noticed by the cognitive radio user (CRU) in its current channel. It ought to spend the initial part of every timeslot in monitoring the free channels while the communication is completed in the second part of the time slot.
5. When the multiple unused channels are assessed, the CRUs will choose any vacant channel for its next transmission and this random decision is according to the uniform distribution. Furthermore,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  
**Reactive Handoff** & \begin{tabular}{c} DFHC [18] \\ **Schemes** \\ \end{tabular} & No Backup & Maximum & Maximum & \begin{tabular}{c} **-** \\ **-** \\ **-** \\ \end{tabular} & \begin{tabular}{c} Improved waiting time** \\ **Improved handoff delay** \\ **-** \\ \end{tabular} \\  & \begin{tabular}{c} MG/1 \\ **Queuing Based** [50] \\ \end{tabular} & No Backup & Maximum & Maximum & \begin{tabular}{c} **-** \\ **-** \\ **-** \\ \end{tabular} & \begin{tabular}{c} Handoff processing time is \\ not considered \\ **Improved handoff delay** \\ **-** \\ \end{tabular} \\  & \begin{tabular}{c} Zigbee Scheme [52] \\ \end{tabular} & No Backup & Maximum & Maximum & \
\begin{tabular}{c} **-** \\ **-** \\ **-** \\ **-** \\ \end{tabular} &
\begin{tabular}{c} False alarm probability \\ **-** cannot work Under multi-path \\ **-** \\ **-** \\ **-** \\ \end{tabular} \\
**Hybrid Handoff** \\ **Schemes** & Hybrid Scheme [53] & No Backup & Average & \begin{tabular}{c} Can be \\ maximum \\ \end{tabular} & \
\begin{tabular}{c} **-** \\ **-** \\ **-** \\ \end{tabular} &
\begin{tabular}{c} Improved waiting time** \\ **Improved handoff delay** \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of some of the existing handoff strategies with regards to the channel backup, bandwidth utilization and the handoff delay.  
if no other channel is found to be free, the CRU will wait and stay on its current channel until the free channel is available.