This section presents the proposed E-CSPMIPv6 scheme highlighting its main advantages over the CSPMIPv6 and GB-FH schemes. The proposed E-CSPMIPv6 scheme introduces an efficient scheme for grouping the MNs’ binding messages with a special consideration for minimising handover latency and signalling cost. In the proposed scheme, two mechanisms, named Clustered neighbouring MNs (CN-MN) and the Clustered Remote MNs (CR-MN) are introduced to efficiently enhance the mobility management process in the IP-WSNs. In the CN-MN mechanism, a clustering technique is used to group the neighbouring MNs which move simultaneously as a group between two different networks within the CSPMIPv6 domain, as shown in Algorithm 1. In this mechanism, every MN has to continually calculate the RSS value () and compare it with the pre-configuration threshold (), as shown in Algorithm 1. If the MN’s RSS value exceeds the threshold, this MN becomes a Head Cluster (HMN) with an ability to group the neighbouring MNs using the Request Joining (Req-join) and Accept Joining (Acc-join) messages. This is performed to register/de-register them in advance. The HMN, after becoming a head cluster, sends a broadcast message to its neighbouring MNs to form a cluster. Finally, the HMN classifies the successfully joined MNs based on their serving MAG into lists to send their requests to the related MAG to process connections/de-connections a priori.
At the MAG side, when the RS is received by the related MAG, the MAG updates its Binding Update List (BUL) and sends a PBU message to the related HMAG, which in turn sends this request to the associated LMA after performing the on side processing, as shown in Algorithm 2. The LMA updates its Binding Cash Entry (BCE). It further creates new prefixes for the HMN and its neighbouring MNs upon successfully receiving the LPBU message sent by the HMAG. Finally, the LMA forwards the created prefixes by sending an LPBA message to the related HMAG. The HMAG now sends it to the related MAG to deliver these new addresses to the connected MNs when they announce their presence. This mechanism is meant to overcome the issues associated with the GB-FH scheme mentioned earlier.
With the CR-MN mechanism, the MNs that arrive simultaneously to the same MAG are processed together to reduce the signalling cost, as shown in Algorithm 3. In this mechanism, the MAG simultaneously processes the mobility-related signalling of the MNs that request a new link at the same time. This is done by using one PBU message for all the connected MNs and sending this message to the related HMAG. All subsequent messages and their processing between the HMAG-LMA and LMA-HMAG are similar to the CN-MN mechanism. After the MAG receives the PBA sent by the HMAG, it sends the created HNPs to the new connected MNs using broadcast message (e.g., Media Access Control Address (MAC)) or individual RA message for each connected MN. E-CSPMIPv6 considers the CSPMIPv6’s entities (e.g., LMA, HMAG, or MAG) without any modification to both mechanisms.
The Flow Diagram of the E-CSPMIPv6 Scheme
and Figure 3
show the message flow diagram for the proposed E-CSPMIPv6 scheme according to the functions of the CN-MN and CR-MN mechanisms, respectively.
In the proposed E-CSPMIPv6 scheme, the MNs have to exchange some messages before the handover takes place in order to reduce the handover latency. This pre-exchange of messages is initiated to handle the control signalling of a group of MNs. This consequently shortens the handover latency and signalling cost.
The introduction of MNs in this study is very pertinent, as it improves the overall system performance in terms of handover latency and signalling cost. It thus makes the system suitable and deployable for critical real-time applications. Several studies (e.g., [23
]) added extra functions (e.g., IEEE 802.21 Media Independent Handover (MIH)) to improve the system efficiency [26
In Figure 2
, each MN continuously monitor its RRS for any change. When the RSS value surpasses the
threshold, the MN sends Req-join to neighbouring MNs requesting them to form a cluster in which it acts as the cluster Head (HMN). The neighbouring MNs that join the cluster successfully become members of the cluster. To enhance the system performance in terms of handover latency and signalling cost, the proposed scheme reduces the cluster size (i.e., clustering the nearest neighbouring nodes) by minimising the Req-join coverage area. Moreover, the mechanism for grouping the MNs is performed at an appropriate time to reduce false prediction. This is achieved by setting
threshold to measure the MNs’ RSS from their old MAG and timing threshold value to measure their connected period time in order to prevent the recently connected MNs from being grouped, as depicted in Algorithm 1. This leads to minimal handover latency and signalling cost. After that, the MNs associated with same serving MAG, together with those associated with the target MAG respond by sending an Acc-join message to their HMN. Upon receiving the Acc-join messages, the HMN stores the senders’ addresses (e.g., MAC addresses) in its database for future use (pre-registration/de-registration). Since the HMN receives Acc-join messages from MNs belonging to different networks, it keeps them in different lists based on their network. All the aforementioned messages are exchanged before the handover takes place. Thus, there is no preparatory stage for the handover process (handover initiation and handover execution).
When the HMN reaches
threshold value, the HMN sends an RS message to the new MAG requesting to be registered. The RS message also carries information about the created list of the cluster members into its database, which is to be included in the registration process. The RS message is modified to carry the addresses stored in the HMN database lists. Moreover, the HMN sends a report message to the serving MAG and its new MAG, as in [27
], to inform them to pre-register/de-register their members. In addition, the HMN keeps receiving new Req-join messages from new neighbouring MNs as long as these messages are coming from MNs that are still connected to their old MAG. This is performed in order to increase the opportunity to pre-register/de-register MNs, especially when the MNs come in a sequential order (sparse network).
Thereafter, the new MAG registers the HMN and temporarily registers the neighbouring MNs that belong to the serving MAG. It also temporary de-registers the neighbouring MNs that are near to leave its coverage area. This is done by sending a modified Local Proxy Binding Update (LPBU) message that is compatible with the proposed E-CSPMIPv6 scheme to the related Head MAG (HMAG). Similarly, the serving MAG temporarily registers/de-registers the MNs based on their CoAs. This temporary pre-register/de-register process increases the prediction accuracy.
After the HMAG receives the LPBU messages from serving MAG, the new MAG looks up the information of the HMN together with its associated members (i.e., its neighbouring MNs) and updates this information accordingly inside its database based on the request of the HMN and its neighbouring MNs.The HMAG sends a PBU message to its related LMA requesting that its HMN be registered, and the neighbouring MNs be temporarily registered/de-registered.
Once the LMA receives the request message, it updates its BCE for each MN based on the type of their registration. A new flag, named S, is used for the mobility-related signalling messages to distinguish between the temporary registration and the actual registration. In addition, the LMA sends a PBA message carrying the HNPs to the corresponding HMAG. Fields such as number of HNP options and the HNP options in the PBU and PBA messages are utilised to determine the requested number of HNP. Then, the corresponding HMAG sends a Local Proxy Binding Acknowledgment (LPBA) message, which carries the HNPs of the MNs, to the requesting MAGs after updating its Binding Update table (BUL) table. When the MAGs successfully receive the LPBA message from the corresponding HMAG, the new MAG registers the HMN together with the neighbouring MNs that joined for temporary registrations/de-registrations. This registration is performed by the serving MAG and the new MAG updates its BUL tables according to the neighbouring CoAs. Subsequently, the new MAG sends a Router Advertisement (RA) message including the HMN’s HNP and the addresses of the MNs that wish to join its network in a broadcast manner. The aim of the broadcasted RA message is to deliver the HNP to the HMN and to inform the neighbouring MNs about their successful joining. To group the MNs’ signalling efficiently, the control messages are either extended such as RA and RS or the existing fields such as HNP are utilised. The original RS and RA control message that are used by the standard PMIPv6 protocol can be represented as Header, ICMP, MN-ID and Link-ID, where the Header contains the source and destination addresses, ICMP is a TCP/IP layer and MN-ID and Link-ID refer to MN-identifier and link-layer identifier, respectively. The Link-ID identifier contains a Header, ICMP and HNP, where HNP contains the MN’s home network prefix. These messages are extended to carry several MNs’ addresses and the new format become: Header, ICMP, MN-No, MN-ID1, Link-ID1, MN-ID2, Link-ID2, MN-IDn, Link-IDn, etc. The Reg-join and Acc-join messages are the same for RS and RA except that the link layer identifier represents the serving network address only. Similarly, there are multiple HNP options in the PBU message, and the number of HNP options indicates the amount of requested prefixes. The Prefix field of each HNP option is set to ZERO.
The HMN completes its registration processes by configuring its IP address when it receives the RA message from the new MAG successfully. Subsequently, any other neighbouring MN that has been temporarily pre-registered by the new MAG, sends an RS message to the new MAG to inform it of its presence. The new MAG sends an RA message to a specific neighbouring MN including the HNP. Similarly, it sends an LPBU message to the corresponding HMAG to activate the HNP of this MN. The HMAG activates the HNP of this neighbouring MN and then redirects the packets to the new CoA if the neighbouring MN belongs to this HMAG. Otherwise, the HMAG sends PBU to the LMA to activate the HNP of this neighbouring MN, as depicted in Algorithms 1 and 2. Therefore, the time required by the MN to configure its IP address is performed concurrently with the last two steps. Note that the time of the last two steps is negligible.
The proposed E-CSPMIPv6 scheme has several benefits with respect to its CN-MN mechanism, as illustrated below.
The CN-MN greatly reduces the false prediction of MNs movement by clearly preventing newer MNs connected to the serving MAG from joining the cluster. This advantage can be justified by carefully observing Figure 4
. As shown in Figure 4
, the issue of the diamond interchange in the overlapping area that is covered by multiple MAGs is taken into consideration in the proposed E-CSPMIPv6 scheme. This is done by applying a time threshold value that prevents an MN from sending an Acc-join message if this MN has been connected to its serving MAG for a period less than the threshold value.
The handover latency is reduced by eliminating the de-registration step from the handover process. Instead, the list created earlier by the HMN is sent to both MAGs (i.e., the serving and the new MAG) during the HMN handoff. The prior de-registration increases the system prediction accuracy by increasing the number of handoff MNs in the list prediction, which invariably reduces the handover latency and the signalling cost, and minimises bandwidth waste.
The HMN keeps receiving the request joining messages after completing its registration processes until a predefined threshold is reached. This is applied to increase the pre-registration of the MNs as much as possible, especially in the sparse networks.
In the second mechanism of the proposed protocol (i.e., CR-MN), the MNs that arrive at the same time at the same MAG are considered, as shown in Figure 3
. In the reviewed protocols in Section 2
, the mobility-related signalling is performed for each MN, even if the MNs arrive simultaneously to the covering area of the same MAG. This increases both signalling cost and bandwidth waste. To address this issue, the proposed CR-MN mechanism allows simultaneous registration of several MNs that arrive at the same time, as illustrated next.
When the movements of several MNs are detected by the MAG at the same time, the MAG has two options for registering these MNs. In the first option, after the MAG detects the MNs movements, it immediately sends an LBPU message together with the MNs information to pre-register the detected MNs. In the second option, the MAG must wait for a while to collect the RS messages sent by the MNs that are detected by the MAG. The waiting time should be very short to avoid degradation in system performance. In the proposed mechanism, a prediction technique is used to alleviate the delay that could occur in the second option (i.e., timer). The scenario of this mechanism is implemented as follows:
In this mechanism, the threshold is employed by the MAGs to measure the RSS. When MNs are detected by the new MAG and the RSS threshold is reached, the MAG expects that the MNs will soon change their link layer. Subsequently, the MAG adds the addresses of these MNs to a list that has been created to group the MNs that are expected to perform handoff simultaneously. The MAG has to wait until one of these expected MNs, which is already added to the list, requires an actual handover. At this time, the MAG sends an LPBU including all the MAC addresses of the MNs within the list to the related HMAG to register them.
The HMAG, LMA and MAG behave similarly to CSPMIPv6 protocol with regards to registering the MNs by exchanging LPBU, PBU, PBA, LPBA and RA messages, as shown in Figure 3
. Finally, the MNs receive the RA message, which is sent in a broadcast manner by the MAG to inform the MNs about their successful joining. When detected by the New MAG (NMAG), each MN informs the NMAG of its presence by the exchange of RS and RA messages. Accordingly, the NMAG sends the MN’s HNP to the present MN to complete its registration (i.e., configures the new CoA).
In the CR-MN mechanism, it is clearly observed that the signalling cost and the burden on the bandwidth are reduced as a result of processing the mobility signalling for several MNs simultaneously.