A Handoff Technique to Improve TCP Performance in Next Generation Wireless Networks

INTRODUCTION

High capacity wireless access for mobile users is increasing rapidly. Rapid progress in the research and development of wireless networking and communication technologies has created several wireless communication systems such as GSM (Global System for Communication), GPRS (General Packet Radio Services), UMTS (Universal Mobile Telecommunication Services) and Wireless LAN (WLAN), Bluetooth for personal area and Satellite networks for Global networking. These networks complementary to each other and hence their integration can realize unified Next-Generation Wireless Systems (NGWS). In the integrated NGWS, users are always connected to the best available networks and switch between different networks based on their service needs (Akyyildiz et al., 2005). A challenging issue in NGWS is to support seamless mobility management. Mobility Management consists of two components as shown in Fig. 1 . Location Management enables the system to keep track of the location of mobile users between consecutive communications. Handoff management is to keep the user connection active when they move from one Base Station to other. There are several solutions for Location Management, but the solution for handoff management suffers from handoff delay. The Handoff types in NGWS are shown in Fig. 2 .

Horizontal handoff: Handoff between two Base Station of the same system. It is further classified into

Fig. 1:	Mobility management

Fig. 2:	Type of handoff

Vertical handoff: Handoff between two Base Station that belong to two different systems.

In the literature there are several algorithms to support seamless link-layer handoff (Perkins et al., 2002). The solution for Intra system and intersystem handoff is still a research issue. The efficient intra and inter system handoff depends on the following characteristics for seamless handoff and are:

Fig. 3:	Handoff management protocols in TCP/IP stack

There exist many solutions for Handoff management in different layers of Internet Protocol. The different layers and the techniques used are shown in Fig. 3 . The Application Layer handoff is initiated using the Session Initiation Protocol (SIP) and it doesn`t require any changes to the TCP/IP Protocol Stack (Wedlund and Schulzrinne, 1999). In the Transport Layer TCP-Migrate is used to support end-to-end Transport Layer Handoff Management (Snoeren and Balakrishnan, 2000). An architecture called MSOCKS is proposed (Matz and Bhagwat, 1998) for handoff using split-connection proxy architecture. In the Network Layer Mobile IP (Perkins et al., 2002) is proposed to support mobility management in IP based Networks. It uses Tunneling to forward IP packets when the MS moves away from the Home Network (HN).

Mobile IP is simple to implement but suffers from:

Triangular routing is eliminated by optimized Mobile IP, the handoff latency is not addressed by it. At present the use of link layer information to reduce the handoff requirement detection delay has gained attention in (Akyyildiz et al., 2004; Akyyildiz and Wang, 2004). The Link Layer information is used to anticipate the possibility of inter and intra system handoff in advance so that the handoff procedures can be carried successfully before the MT moves out of the coverage area of the serving Base Station (BS). The use of link layer information significantly reduces the handoff latency and handoff failure probability of handoff management protocols (Akyyildiz et al., 2004).

PROPOSED SYSTEM

Handoff architecture: In the proposed system we have used architecture as shown in Fig. 4 for Physical and data link layer. In the physical layer it has two units: Speed Estimation unit and RSS measurement unit. In the Data Link Layer it has Neighbor Discovery unit and the Handoff signaling delay estimation unit.

The functions of each unit and the information used by them are as follows. Information is collected from the Physical and Data Link Layer and is used to carry out the handoff procedures, handoff trigger unit and handoff execution unit. The functions of these units are listed below.

where, v is the speed of the mobile device, c is the speed of light in free space, fc is the carrier frequency of the received signal, fm is the Doppler frequency related to the speed of the mobile user.

Operation: The proposed architecture uses Data Link and Network Layer information to initiate and manage the handoff process. It depends on the mobile`s speed and handoff signaling delay as the major information. The operation of the proposed system is subdivided into five stages as shown in Fig. 5.

Fig. 4:	Modules in handoff management architecture

Fig. 5:	Stages in handoff operation

Each stage and the happening are explained below. The flow of the proposed mechanism is shown in Fig. 6.

Neighborhood discovery: In a Wireless system, Mobile Station (MS) is served by a Base Station (BS) and it learns about the neighboring BSs (BSs that are the immediate neighbors of the serving BS) using the Neighbor Discovery unit. Some of the neighboring BSs can present in the serving FA, whereas other may belong to different FA. When the MS moves into the coverage of a neighboring BS that belongs to its serving FA, the resulting handoff is Link Layer Handoff. In this case the existing handoff algorithms (Zhang and Holtzman, 1996) can be used without the proposed algorithm. When the neighboring BS belongs to a different FA under the serving system, it is known as the Intra system handoff. When the neighboring BS belongs to a different system other than the serving BS, the resulting handoff is an Intersystem handoff. The proposed algorithm is used in Inter and Intra system handoff.

Fig. 6:	Flow of handoff operation

Handoff signaling delay estimation: During change in location, it is difficult to predict to which particular BS the MS will move. The main objective of the proposed system is to estimate the handoff signaling delay in advance without knowing to which particular BS the MS will move. In the proposed algorithm, we estimate the handoff signaling delay of a possible handoff to a particular neighboring BS in advance we use the following steps. We have used Invalid authentication extensions to just learn the handoff signaling delay without changing the mobility binding at GFA or HA.

The handoff signaling delay is estimated based on the difference between the transmission time of the HMIP registration request message and the reception time of the Handoff registration reply message. Using the above steps the MS learns (i) handoff signaling delay in the event of movement to the BS, (ii) signaling delay of the associated handoffs to other neighboring BSs. From the above study it shows that for intrasystem handoff, as few messages are exchanged the handoff delay is lower compared to intersystem handoff.

Handoff anticipation: This stage requires information from the RSS measurement unit. If the RSS of the serving BS decreases continuously it shows a handoff is anticipated. The Handoff Trigger learns the signaling delay for that particular BS from the handoff signaling delay estimation unit.

Handoff initiation: Once the MS learns the BS that it is going to move, it estimates the right time to start the HMIP registration. The Handoff Trigger unit uses the speed and handoff signaling delay information to estimate the threshold T1. When the RSS of the serving BS drops below T1, the Handoff Trigger Unit sends a trigger to the Handoff execution unit to start the HMIP handoff procedure.

Handoff execution: The Handoff execution unit receives the handoff trigger from the handoff trigger unit it starts the handoff registration. Once the handoff registration is competed, the MT is switched to the new BS. The MS keeps its registration with the old BS for a specified time period to avoid the ping-pong effects during handoff using the binding method. The MS binds the CoA of the old FA and new FA at the GFA in intrasystem handoff and at HA in intersystem handoff. Thus GFA and HA forwards packets destined for the MS to both CoAs during this time interval (Zhang and Holtzman, 1996).

The operation of Handoff management is explained with the diagram shown in Fig. 6. The Mobile Station (MS) learns about the neighbor BSs using the neighbor discovery protocol and determines the type of handover in the movement to the new Base Station (BS). Once the neighboring BS is learnt, the Handoff Signaling Delay Estimation Unit estimates the signaling delay associated with the neighboring BS. The RSS monitoring unit starts to monitor the RSS of the serving BS and anticipates a handoff when the RSS decreases continuously and in the selection of the next BS. The different types of handover are as follows.