3.2 Relocation Strategy
3.2.2 Relocation Timing Selection
In order to reduce the load of ASN-GWs and avoid ASN-GWs performing too many relocations, selecting a proper time to perform relocation is essential for ASN-GWs. We define a measurement for the ASN-GW’s load which is modified from [12]. This measurement is based on drop rate. We apply Random Early Detection (RED) [13] to mark and drop packets. If the queue of an ASN-GW is not full but the traffic of the ASN-GW is getting heavy, we mark some arrived packets randomly as same as RED. While the queue is full, we randomly drop those marked packets and new arrived packets. At every time interval i, we find a drop rate ri(0ri 1) as
where di is the number of marked and dropped packets during time interval i and fi is the number of received packets during time interval i. Thus, the load of an ASN-GW at time interval k can be defined as a weighted moving average [14] of
))
where wi is a weighting factor of ri and wk > wk−1 > . . . > wk−(h−1). According to Lk, the ASN-GWs are able to know the current load condition. Note that the load Lk will fall into interval [0, 1]. The older drop rate records may have effect on the load of ASN-GW. Thus, we define a constraint for Lk, h which is the maximum number of latest drop rate records should be taken to calculate Lk.
As Figure 8 showing, although the ASN-GW already records drop rates of each 6 interval, only 5 records are used during the calculating of the Lk of the 6th interval.
The record of first interval is not used because the h of the ASN-GW is given as 5. In this way, the Lk can represent the current load condition more actually.
h=5
Figure 8. An example of Lk calculating
This thesis defines three thresholds of load ℓL, ℓM and ℓH (0 < ℓL < ℓM < ℓH < 1), and the ASN-GW divides the load condition into four states. The table 3 shows four states for which MSs should be relocated and what kind of mobility will be used by MSs.
The operations ASN-GW will perform as follows.
Table 3. Four states relocation strategy
State Relocation Target Mobility Used
State 4
H k
M L
Relocate all of Anchored MS ordered by ability to cope with all of traffic generated by MSs. However, in order to avoid the load getting heavier, the ASN-GW selects the MS Mj with
th ASN-GW asks all MSs use CSN anchored mobility when they handoff to other ASN.
(4) State 4 (M LkH): The load of the ASN-GW is very heavy. The ASN-GW may not be able to handle the traffic from all of mobile stations.
In order to prevent the load of the ASN-GW keeping very heavy, the
ASN-GW has to perform relocation Mj ordered by the R from the Mj anchored MS set. The relocation will continually perform until the load of ASN-GW lower than ℓM. The number of MSs which will be relocated at the interval k starts at 1. The number will double at every interval until the number reaches the maximum MS number of once relocation. In addition, the ASN-GW asks all MSs use CSN anchored mobility when they handoff to other ASN.
Figure 9 demonstrates the detail flow chart of the ART-Based Algorithm.
Every Detection Interval Judge the of Loading ASN-GW
Lk< ℓH
No
Perform MS Relocation in Descending order According to
Lk< ℓM
The Number of Relocation MSs m=m*2
If m > MAX which is maximum number of MS for once relocation
m=MAX No
The Number of Relocation MSs m=1
Perform Relocation for anchor MS with RMj Rth
Perform Relocation for anchor MS with RMj Rth
Figure 9. Average Residence Time Based Relocation Algorithm
Consider an example of a WiMAX network with three ASN-GWs, G1, G2, G3 and six MSs M1, M2, . . . , M6 as shown in Figure 10. At the interval k, M4 will handoff from G2 to G3.The ARTs T(a, b) and R , a = 1, 2, 3, b = 1, 2, . . . , 8, of time interval Mb k are showing at Table 4.
Figure 10. An example for 4 states relocation strategy
Table 4. ARTs and R of time interval k for the example of Fig. 10 Mb
interval k M1 M2 M3 M4 M5 M6
T(a, b)
G1 70 60 15 30 5 35
G2 10 10 80 30 15 25
G3 15 60 20 30 10 20
Mb
R 7 6 -- -- 0.67 0.8
Although the network system has the same condition, if the load condition of the anchored ASN-GW is different, there will be different results at the next interval. In the following, we will show four outcomes of the network system after go through each four states of load condition with the same ARTs and movement at time interval
k.
Suppose that the load of G2 is at state 1 (i.e. 0LkL) at interval k. No relocations will occur and the M4 uses ASN anchored mobility to handoff from G2 to G3. Figure 11 shows the system condition at interval k+1 after the state 1 operation is performed at interval k.
M1 M2 M3 M4 M5 M6
Figure 11. System condition at interval k+1 after the state 1 operation
Suppose that the load of G2 is at state 2 (i.e. LLkM) at interval k. G2 selects anchored mobility to handoff from G2 to G3. Figure 12 shows the system condition at interval k+1 after the state 2 operation is performed at interval k.
M1 M2 M3 M4 M5 M6
Figure 12. System condition at interval k+1 after the state 2 operation from G2 to G3. Figure 13 shows the system condition at interval k+1 after the state 3 operation is performed at interval k.
M1 M2 M3 M4 M5 M6
Figure 13. System condition at interval k+1 after the state 3 operation
Suppose that the load of G2 is at state 4 (i.e. H Lk 1) at interval k. G2 selects the MS Mb according to their R from anchored MS set, and the relocation priority is Mb M1 > M2 > M6 > M5 at interval k. The number of MS should be relocation starts at 1.
Therefore, at interval k, only M1 will be relocated. At this state, M4 uses CSN anchored mobility to handoff from G2 to G3. Figure 14 shows the system condition at interval k+1 after the state 4 operation is performed at interval k. Suppose that the load of G2 is still at state 4 at interval k+1. The relocation priority at interval k+1 will change as M2 > M6 > M5 and the number of relocation will double as 2. Thus, two MSs (i.e. M2 and M6) will be relocated at interval k+1. Figure 15 shows the system
condition at interval k+2 after the state 4 operation is performed at interval k+1.
M1 M2 M3 M4 M5 M6
G1 70 60 15 30 5 35
G2 10 10 80 30 15 25 G3 15 60 20 30 10 20 -- 6 -- -- 0.67 0.8 interval k+1
T(a, b)
Mb
R
Figure 14. System condition at interval k+1 after the state 4 operation
M1 M2 M3 M4 M5 M6 G1 70 60 15 30 5 35 G2 10 10 80 30 15 25 G3 15 60 20 30 10 20 -- -- -- -- 0.67 --interval k+2
T(a, b)
Mb
R
Figure 15. System condition at interval k+2 after the state 4 operation