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In heterogeneous wireless networks, seamless vertical handover is important for migrating from one network to another network; it's still a challenging one. Handover performances are affected in terms of packet loss, delay and latency. PMIPv6 is a network based protocol that performs handover in network itself, thus reduces signaling overhead and it does not need the mobile node modifications as like host based protocols. This efficient handover is affected by the handover performances such as packet loss, delay, latency, throughput etc. In this paper we discuss about the techniques in PMIPv6 to enhance the performance and their simulation results are also discussed.
Keywords: Vertical handover, PMIPv6, HC, MIH, Seamless handover.
In 4G wireless networks, service continuity among diverse networks is eminent. Handover is a process that provides the service continuity without any disruptions. Efficient handover should provide the seamless connection that the IP address should not be changed and the continuity should not break anywhere while transferring the data and it is transparent to upper layers. Horizontal handover is one in which the data are migrated between the same network (i.e. intra domain). Whereas, in Vertical handover the data's are migrated between the different networks (i.e. inter domain). Achieving seamless handover in vertical handoff is still a challenging one. In data transmission during handover period, we focused on their performances by reducing the delay, packet loss, latency, signaling overhead. Improving the efficiency of vertical handover is important for traffic optimization. Several techniques are there in wireless networks to reduce the packet delay and loss; still it's a challenging one to achieve handover performances.
IPv6  in wireless network, has the capacity of auto configuring the neighboring networks and made possible to N no of connections. IPv6 is readily available to anyone, and easy to configure. IPv6 are of two types host based and network based. Host based protocols forces the MN (Mobile node) to perform the handover and needs modifications in it. Host based protocols are shortcoming of signaling overhead. Seamless handover is not achieved by changing of the IP address. The host based protocols such as hierarchical, transient binding and mobile protocols are involved at IP layer. Networks based protocols performs handoff in the network itself and does need the mobile node modifications. Same IP address is used as the Home address for transmitting within that domain. So the data flow will be efficient without any interruption. PMIPv6 (Proxy mobile IPv6)  is a network based protocol performs handover in network and does not force the MN modifications. Handover is done with the proxy care of address within that domain. In host based, signaling overhead is more because the mobile node performs handover. Several Host based protocols are there to perform vertical handover, but it needs the IP to be changed during mobility so it does not achieve efficient handover. As a result the latency will be more in that domain. In PMIPv6 signaling overhead is reduced and there are several techniques to reduce the latency, delay and packet loss, but still it's a challenging one in that Domain. Handover are performed within the PMIPv6 domain itself so there is no need of any architecture. But in wireless networks they performed either on tightly or loosely coupled architecture. They achieve proactive handoff in that architecture by making the mobile node to perform handoff by vertical handoff management, which consist of handover initiation, network selection, handoff decision and handovering data in either loosely or tightly coupled architecture. So many algorithms are used to perform that operation well, and their data are stored in the MAC layer of their networks. Architecture, algorithms and protocols are differ according to their considering networks. Real time applications are sensitive to delay, latency and throughput. Packet loss may occur while transmitting because it does not provide any buffer for data. Even though if we are implementing buffering mechanism there will be a chance of misrouting, overlapping of data and packet bottleneck problem can also arise. Several buffering mechanism are implemented in PMIPv6 but still it's not fulfilled.
PMIPv6 is a network based protocol, which performs handover in network itself. It consists of two major entities for performing handover that are MAG (Mobile Access Gateway) and LMA (Local Mobility Anchor). Where the MN is accessed through the MAG and the data are transmitted through the LMA and the LMA updates all the mobile node information's. Figure 1 shows the PMIPv6 access network, where the mobile node has a stable identifier. After the mobility management entities in a PMIPv6 network identify the mobile node and acquire the mobile node's identity, the mobile node can be authorized for the network-based mobility management service, i.e., permitted by the network to obtain an access address. Once the mobile node enters its PMIPv6 domain , the link layer Link Up trigger occurs when the link layer link between the MN and the MAG is established. The mobile node sends the Link Up trigger message to the MAG. Network assigns a unique home network prefix to each MN, i.e. Per-MN-Prefix, and conceptually this prefix follows the MN wherever it moves within the PMIPv6 domain.
Figure 1: PMIPv6 DOMAIN
The mobile nodes are differentiated by a Network Access Identifier (NAI), which has an associated set of information's stored on the network, including a profile containing the home prefix. Figure:2 shows that the MAG located in the access router, retrieves the MN profile information from AAA server and sends the customized Router Advertisements (RAs) to the MN, emulating the home network behavior. MN information is stored on the MAG, which tracks the mobility of MN. There are many mobile nodes connected to the MAG, which authenticate all the nodes and assure as a authorized one. It takes care of the mobility related signaling. And there are many MAGs that are connected to the LMA. The MN configures its Home Address (MN HoA) on the network interface, because the MN always receives the same home prefix, it believes that it is in the Home Domain. After allocating a prefix for the mobile node, the LMA sends a Proxy Binding Acknowledgement (PBA) message which includes the Home Network Prefix option containing the allocated prefix value. It creates a Binding Cache entry and establishes a bi-directional tunnel to the mobile access gateway. LMA stores all the mobile node information necessary for routing, which act as a home agent for PMIPv6 domain. LMA provides a home address where the IP address is not changed during mobility within this domain, which is responsible for providing exact path to the destination node, and it manages the mobile node reachability state. It also sets up a route to the mobile node's home network over the tunnel. Upon receiving PBA message, the MAG set up a bidirectional tunnel to the LMA and adds a default route over the tunnel to the LMA and all traffic from the mobile node gets routed to the mobile node's LMA over the tunnel. Now the MAG has all the information for it to emulate the mobile node's home network on the access link. The MAG also starts sending periodic Router Advertisements to the mobile node advertising its home network prefix. After receiving the Router Advertisement messages on the access link, the mobile node will configure its interface using stateful address configuration modes. At this point, the mobile node has a valid home address from its home network prefix at the current point of attachment. The serving MAG and LMA have proper routing states for handling the traffic sent to and from the mobile node. From the perspective of the mobile node, the entire PMIPv6 domain appears as its home link or a single link.
Figure2: Signaling flow of PMIPv6
1. MAG1 registers the mobile node information and authenticates it. After MN get authorized it sends to LMA through PBU (Proxy Binding Update).
2. LMA stores the information which contains of MN_ID and PCoA (Proxy Care of Address)
3. MN information is stored in the binding cache of LMA and it sends the PBA (Proxy binding Acknowledgement) to MAG1
4. Likewise MAG2 authenticate and registered in the LMA.
5. And then the data are transmitted from MAG1 to MAG2 through LMA.
PMIPv6 TECHNIQUES FOR HANDOVER PERFORMANCES:
PMIPv6 and PMIPv6 with MIH
In PMIPv6 Domain , network assigns a unique home network prefix to each MN, i.e. Per-MN-Prefix, and this prefix follows the MN wherever it moves within the domain. And it is not necessary to re-configure the care-of-address (CoA) at the MN for every change of PoA, hence handover delay is reduced. Figure 3 shows the PMIPv6 Domain with IEEE 802.21 MIH services, in that the MN, the PMIPv6 network entities, and the MAG in the access routers are informed about the values of the relevant parameters necessary in handover decision making prior to the actual handover process. Functions that are provided by the Media Independent Handover Function (MIHF)  which employs three functional components are Media Independent Information Service (MIIS), Media Independent Event Service (MIES), and Media Independent Command Service, where MIIS provides the static information about the characteristics and services of the neighboring networks. With the necessary information, MN may discover available neighboring networks and communicate with the elements within these networks to optimize handover. Figure 3shows that, MAG2 contains information about the neighboring MAG ,that includes authentication information also. When an MN is migrating from MAG1 to MAG2, MAG 2 would already be having the information about the MN ahead of time through the MIIS server MIES  provides services to upper layers by reporting about dynamic changes to the lower layer events. These events which trigger the services based on the lower layer reports that are based on delay, packet loss, and signal strength etc, MICS that are in the upper layer enable to control and manage the handover related functions of the lower layer. MIHF commands are used to make connectivity decisions about the lower layers and perform upper layer mobility.
Figure 3: Signaling flow of PMIPv6 enhanced with MIH
PMIPv6 WITH HC AND MIH:
HC (Handover Coordinator) is an internetworking multiple-interface base station network level entity that operates in the overlap area of the interworking heterogeneous wireless networks in a PMIPv6 domain . HC triggers relevant network entities both the authentication and pre authentication of the mobile node. HC is a network-based entity  that coordinates the facilitation of handover activities on behalf of the MN, thus the handover delay and packet loss were reduced without incurring extra signaling overhead in the air interface. Furthermore, data packets were delivered to the MN as real-time as possible even during the actual handover period without any need for buffering either at the source or target access. In figure4, the HC performs the handover activities for the MN by communicating with both the old MAG and new MAG. when the MN enters the overlap region, it performs the handover functions on behalf of the MN. Normally, the MN attaches to a MAG through connecting to an access point (AP) or base station (BS). The actual handover between the interworking heterogeneous wireless networks start later in the PMIPv6-HC scenario when compared to the PMIPv6 scenario. This is due to the fact that the HC continues to relay ongoing communication packets to MN in the overlap region. While with the PMIPv6 scenario the disconnection is abrupt and hence it happens earlier than that in PMIPv6-HC.
Figure 4: PMIPv6 with HC Architecture
The implementation of the PMIPv6-HC  handover scheme is focused mainly on the proof-of-concept in terms of demonstrating the capability of the scheme to reduce handover delay and packet loss without incurring extra signaling overhead in the air interface. In PMIPv6 Domain, figure5 shows that the MIH services provide a report mechanism that conveys useful network status information to entities where a decision is made to cause a command to be executed at some specific network elements to facilitate seamless handover. Hence, the handover process is facilitated by the information provided from the network to the MN, in addition to the information that the MN collects from the lower layers. This cooperative information exchange enhances the handover optimization. MIH services enable some operations to be performed prior to the handover process while the MN is still connected to the old MAG's link.
Figure 5: IEEE 802.21-enabled PMIPv6 domain and Mobile Node.
Figure 6: MIH enabled PMIPv6 domain
Figure 6 shows PMIPv6 domain enabled with MIH, considering two different networks and performing handover in that domain. Furthermore, the increase of simultaneous signaling messages from the many different MNs increases the delay in the connection links due to the saturation in the channel. However, even in this situation the CN continues to send ongoing communication packets to the MN. And the configuration says the corresponding average packet loss.
PMIPv6 with MIH
PMIPv6 as a mobility management protocol in the simulation of mobility across overlapping heterogeneous wireless access networks, WiMax and WLAN. Figure 7 shows the NS2 simulation setup with MAG1, MAG2, LMA and CN. MIH functionality was incorporated in the MN and the MAGs within the PMIPv6 domain.
Figure 7: NS2 Simulation setup
A flow of CBR traffic was simulated and transmitted from the CN to the MN using UDP. The CBR packet size was set to 1000 bytes while the interval between successive packets was fixed at 0.01 seconds. The simulation time was 20 seconds for each of 30 random simulated handovers among the heterogeneous networks .In one simulated handover, the CBR traffic started to flow between the CN and MN at 0.5 seconds through MAG 1. Then the PMIPv6, Proxy binding update (PBU) and proxy binding acknowledgement (PBA) messages were exchanged between MAG 1 and the LMA for registration purposes before the flow of the CBR traffic.
At 1 second the MN started moving towards MAG 2 at a speed of 30m/s. At around 14.334 seconds the MN received its last packet from MAG 1. It next received its first packet from MAG 2 at around 14.776 after performing proxy binding to LMA through MAG 2 which had detected the mobile node attachment.
Figure 8: Illustration of Handover delay and Packet loss
The simulation results of the performance comparison in terms of handover delay and packet loss obtained from the simulation of the two scenarios of PMIPv6 and PMIPv6 with MIH showed in Figure 8. Thus, the handover delay was about 0.4 seconds and the corresponding number of dropped packets during the handover period was about 38. And the handover delay in PMIPv6 with MIH was reduced about 0.12 seconds while the number of dropped packets was 13.And the handover performances was improved compared to PMIPv6 Domain when it is incorporated with MIH.
PMIPv6 WITH HC AND MIH
Consider the two different networks WiMax and WLAN in PMIPv6 Domain by incorporating HC and MIH services. The WiMax interface on the MN generates a 'link detect' event when it gets in the neighbor of the WiMax base station. This notifies the MIHF in the MN which then uses the MIH protocol to relay the message to the MAG's MIHF. A 'link up' event then triggers the PMIPv6 agent in the network to start performing the necessary proxy bindings. This happens immediately the MN senses the new point of attachment (WiMax base station) possibly before the link to the WLAN is disconnected. Thus, before the MN was disconnected from MAG 1, intensive communication was ongoing between MAG 2 and the MN via the MIH protocol. The MN knew before hand that it would lose the connection with MAG 1 and also knew that the next viable point of attachment was MAG 2.During simulation the packet size was set to 1000 bytes while the inter packets duration was fixed at 0.01s.
Figure 9. Packet loss
In Figure 9, the packets dropped in PMIPV6 are shown in red lines. Here in PMIPV6 the number of packets dropped is 13, during the handover time. The packets dropped in PMIPV6- MIH are shown in green lines. In PMIPV6-MIH the number of packets dropped is 3, during the handover time. During simulation the PMIPV6-MIH-HC produces the zero packet drops during the handover time. Here the packet drops are explained only during the handover time.
Figure 10: Packet delay
Here the above graph shows the handover delay in PMIPV6, PMIPV6-MIH, and PMIPV6-MIH-HC. The red line shows the handover delay in PMIPV6 and it is 1.2. The green line shows the handover delay in PMIPV6-MIH and the delay is 0.4. The blue line shows the handover delay in PMIPV6- MIH-HC and the delay is nearly zero. Here all the delays are calculated at the time of handover
Figure11: Impact of number of simultaneous handing over MNs on packet loss
Figure 12: Impact of number of simultaneous handing over MNs on handover delay
In a mobility management scenario, handover delay and packet loss increased with the increase in MN. Increasing the number of simultaneously handing over MNs, increases the mobility related signaling messages that must be handled at the same time by the relevant network elements. This scenario overloads the elements hence cause delays in the processing of these signaling messages and this messages cause delay in the connection link. Figure 11 and 12 shows the delay and loss occurred due to increase of MN.
This paper analyses about the techniques used in the PMIPv6 Domain. PMIPv6 domain mainly suffers on handover delay and packet loss. A Technique PMIPv6 with MIH performs better in PMIPv6 domain by reducing the delay and loss during handover period. Another Technique that incorporates PMIPv6 with MIH-HC, this improves the handover performances by reducing the delay and loss compared to previous technique. Thus PMIPv6 with MIH-HC performs efficient vertical handover with improved handover performances in PMIPv6 Domain.