An enhanced TCP for upward vertical handoff in integrated WLAN and cellular networks

Abstract

SUMMARYThe integration of wireless local area networks (WLANs) and third generation (3G) cellular networks has been recently a subject of great interest, mainly aimed at augmenting cellular networks with high-rate data services by WLANs in hotspots. The complementary characteristics of 3G cellular networks and 802.11 WLANs are expected to offer the best of both technologies. On the other hand, the drastically different characteristics of both networks could be a serious obstacle to providing seamless mobility in the integrated WLAN and cellular networks. In particular, mobile users suffer from a drastic decrease in data rate after a vertical handoff from a WLAN to a 3G cellular network. However, current TCP congestion control cannot adapt to the change in the data rate after the vertical handoff, resulting in significant throughput degradation. Thus, we propose a novel TCP scheme to enhance the throughput when a vertical handoff occurs from a WLAN to a cellular network. For the proposed scheme, the throughput performance is investigated via analytic modeling and simulation.

Improving TCP performance for vertical handover in heterogeneous wireless networks

“…• The TCP sender gets to know that the ACK for the first packet sent over the cellular network is lost by restarting the current RTO timer with an initial value of r RTT c where r (>1) can be selected according to the system dynamics and is typically three in [15] and RTT c denotes the average RTT, which is measured in the cellular network as in [9].…”

“…To prevent the premature expiration of RTO, the TCP sender resets the RTO so that the first packet delivered over the cellular network is acknowledged. That is, after a link change is detected through the handover notification from the TCP receiver as in [9], the TCP sender continues to wait for the ACK for the first packet sent over the cellular network although the RTO (used in the WLAN) expires. This prevents premature timeout for the first packet transmitted over the cellular network.…”

“…• The TCP sender gets to know that the ACK for the first packet sent over the cellular network is lost by restarting the current RTO timer with an initial value of r RTT c where r (>1) can be selected according to the system dynamics and is typically three in [15] and RTT c denotes the average RTT, which is measured in the cellular network as in [9]. • If the transmitted packets are lost regardless of VHO, the TCP sender retransmits the packets through the link currently attached and does not consider the RTT of the retransmitted packets in the calculation of a new RTT according to Karn's algorithm [15,16].…”

“…In an attempt to handle the reordering problem during VHO, our TCP avoids activating false fast retransmission due to spurious dupacks by making the TCP receiver send normal ACKs for the out-of-order packets received from the WLAN rather than sending dupacks, because the in-flight packets would be received over the previous link soon after the out-of-order packets are received. Our proposed TCP also prevents premature timeouts due to sudden increase of RTT after UVHO by resetting the RTO to the average RTT, which was measured in the cellular network [9] during UVHO. In addition, the proposed TCP can suppress premature timeouts, which occur since the RTO in the WLAN is updated before the in-flight packets are acknowledged, by maintaining the old timer for the previous link until all the in-flight packets are acknowledged even after moving into the new link.…”

Handover Vertical y convergencia hacia IP como factores clave del desarrollo de las redes 4G

“…MU experimenta por lo general una degradación del throughput debido a la diferencia de tasas de transmisión [22] mientras que en MD es menos crítico en el manejo del tiempo dado que, por lo general, el dispositivo tiene la posibilidad de permanecer conectado a la celda más grande durante todo el proceso. El proceso de handover se puede dividir en tres fases [19] [18].…”