ISN -TCP-PLU S
4.5 Summary
0.8 0.85 0.9 0.95 1 1.05 1.1
10 20 30 40 50 60 70 80 90 100
Efficiency
Mean Handover Interval (sec) WP-TCP ISN-TCP ISN-TCP-PLUS
Figure 4.15: TCP Efficiency
fails for high UMTS delay because while estimating initial DeviationU M T S is taken as zero, whereas actual DeviationU M T S is very high. Therefore, the convergence requires considerable amount of rounds, before which the RTOs occur. However, this much of high one way delay for the UMTS network never occurs in practice.
4.4.4 TCP Efficiency
TCP efficiency is defined as the ratio of the number of packets transmitted successfully to the actual number of packets transmitted. In Figure 4.15, the efficiency for TCP flow is shown with respect to mean handover interval. Mean handover interval is defined as the average time between two consecutive handover. It can be seen from the figure that the efficiency of ISN-TCP-PLUS is significantly higher compared to the ISN-TCP and the WP-TCP, when mean handover interval is very high. The figure shows that the proposed ISN-TCP-PLUS is very adaptive with frequent handover.
4.5 Summary
This chapter provides an improved TCP variant for the vertical handover between the UMTS and the WLAN networks using the ISN based framework. The shortcomings of the WP-TCP is analyzed using the simulation results. It has been observed that the main problem during the handover from UMTS to WLAN network(slow to a fast network)
4.5 Summary
is the unnecessary retransmissions and that from WLAN to UMTS handover(fast to a slow network) is the spurious RTOs. The proposed TCP variants, namely the ISN-TCP and ISN-TCP-PLUS, use an estimation mechanism for measurement of the new RTT and the RTOs after the handover thereby avoids spurious RTOs during handover from WLAN to UMTS network. The unnecessary retransmissions during UMTS to WLAN handover is avoided through the filtering of unnecessary DACKs. The sending rate for the TCP is estimated after the handover is over, and the CWnd is updated accordingly so that TCP can rapidly adopt to the new link characteristics after the handover process gets completed. Simulation results confirm that ISN-TCP and ISN-TCP-PLUS perform considerably better than WP-TCP.
Ensuring QoS for the mobile users during vertical handover between IEEE 802.11 WLAN and data network provided by UMTS is another key requirements for seamless mobility and transfer of existing connections from one network to another. QoS fulfillment is a complex problem and requires participation of both the mobile users as well as the connection networks. The QoS assurance criteria for existing connections can be affected by fluctuations of data rates when a user moves from the high speed WLAN network to the low speed UMTS network, even in the presence of another WLAN network in its vicinity. This can happen if the alternate WLAN network is highly loaded. Therefore handover from a high speed network to a low speed network should be avoided, whenever possible. The next chapter proposes a QoS based handover procedure that prioritizes the existing connection over the new connections as a result of which the rate fluctuations due to handover can be avoided if there exist another WLAN network in the range of the mobile user.
Chapter 5
Vertical Handover over Intermediate Switching
Framework: Assuring Service Quality for Mobile Users
An efficient handover mechanism between hybrid networks is becoming a critical issue due to data rate differences among the underlying networks. Various handover methods for inter-networking of heterogeneous networks like UMTS and WLAN have been proposed in literature which aim at providing seamless handover from one network to another, as discussed in Chapter 2. However most of the existing handover schemes use Received Signal Strength Identifier (RSSI) as the metric for handover initialization decision. RSSI based handover is not a good solution because of the following reasons,
RSSI based handover decision may result in a premature handover between IEEE 802.11 and UMTS [65], even though the achievable data rate from the IEEE 802.11 network for the mobile node is much higher than the one it may get from UMTS network.
In case of heterogeneous networks, divergent networks may have dissimilar values of channel coding loss factor, noise and interference power which makes RSSI incomparable for different wireless technologies.
Proper load balancing between different network is required to design effective call admission control mechanism.
5 Vertical Handover over Intermediate Switching Framework
Then we have these schemes which are based on Signal to Interference Ratio(SINR) which is a better metric than RSSI [67]. Experimental results show that SINR-based handover methods results in better throughput than RSSI-based handover ones but it still doesnot consider any QoS parameters like bandwidth, delay, jitter etc.
Therefore, a better metric is required for making vertical handover decisions. This chapter proposes a QoS based handover mechanism over the ISN based framework as proposed in Chapter 3. UMTS network provides more coverage compared to any IEEE 802.11 WLAN network. An umbrella like topology is considered where IEEE 802.11 networks create coverage holes under the scope of an UMTS network. Whenever a mobile node enters under the coverage of an IEEE 802.11 WLAN network, it is preferable to switch the connection from the UMTS network to the IEEE 802.11 WLAN network.
The handover decision is taken cooperatively by the mobile node and its current point of attachment based on QoS requirement. The UMTS network is available all the time.
Therefore, if the QoS of the current WLAN network where the mobile node is attached goes down, it is always preferable to handover to another IEEE 802.11 WLAN, if available, than to the UMTS network. This is because sudden performance degradation can be observed if the mobile node is handed over to the UMTS network from an IEEE 802.11 WLAN network. Further, existing connections are preferred over new connections. This chapter considers a per-node QoS requirement where the QoS for every mobile user is based on the economic subscription with the service provider. The scheme proposed in this chapter has following contributions;
A per-node QoS differentiation strategy is proposed where mobile users can avail network service based on their economic subscription with the service provider.
The new connections are admitted to the network if sufficient bandwidth is available after allocating required bandwidth to all the existing connections. To maintain consistency in QoS, existing connections are preferred over new connections.
A pre-handover bandwidth reservation scheme is proposed to reserve bandwidth at alternate connection points to avoid QoS degradation during handover. Further, pre-handover bandwidth reservation helps to prioritize the handover from one IEEE 802.11 network to another IEEE 802.11 network over the handover from UMTS to IEEE 802.11 network.
The effectiveness of the proposed scheme is justified using simulation results.