Mobility management in IEEE 802.11 WLAN using SDN/NFV technologies
© The Author(s). 2017
Received: 19 November 2016
Accepted: 1 April 2017
Published: 13 April 2017
IEEE 802.11 wireless LAN is proliferating since the increased trend in the wireless network utilization on mobile devices. Accuracy, fast content delivery, and reliable mobility support are essential features of any network to support the changing trend in a wireless network. However, traditional architectures in wireless LAN (WLANs or WiFi) always endured from challenges such as the provision of consistent mobility, real-time packet flow, and seamless handoff. Generally, most of the WLAN only relies on signal strength for handoff which is not sufficient enough for fair selection of an access point (AP) and therefore causes imperfect performance of the network. We present a novel mobility management scheme for WLANs to deal with the mobility management issues, and load balancing by software-defined network (SDN) and network function virtualization (NFV) technologies. The proposed scheme is based on logical AP (LAP) that keeps a connection with the user/mobile terminal (MT) during handoff triggered by either the user or the SDN controller for seamless mobility. It also involves the current state of each AP in addition to traditional parameters of WLAN. We implemented the proposed scheme on a real testbed in a WLAN environment. The evaluation results authenticate that our proposed scheme provides robust handover without throughput degradation and load imbalance among adjacent APs, and allocates the best AP in the neighboring region. Moreover, our proposed scheme is feasible to implement since it did not require any modification at the mobile terminal.
The rapid growth in mobile Internet applications and innovation of mobility services, location-based services, WiFi calling and cloud services, and the IEEE 802.11-based wireless LANs (WLANs/WiFi) has gained a lot of attention in recent years, because the WLAN provides economical solution to support the Internet access services with high throughput in contrast to cellular networks. According to the Juniper Research report , the volume of data traffic generated by smart mobile devices will hit nearly 197,000 petabytes (PB) by 2019 that mainly offloaded to WiFi. Meanwhile, some mobile operators also have begun the deployment of WiFi hotspots in high-density areas to overwhelm the threat of LTE spectrum that is near to Shannon’s capacity limit . The proliferation of WiFi consumers has raised some new requirements such as seamless handover, quality of service (QoS), high throughput, load balancing, and ubiquitous network access.
Unfortunately, the existing WiFi infrastructure is unstructured which could lead to the prevailing issues in terms of mobility management. There are some prominent issues: (1) The current WLAN structure is tightly coupled between the control and data plane that restricts the integration of innovative network applications, services, and policies . Thus, it makes the intelligent mobility management functions difficult to implement according to the real-time network state that is required for future network state perception. (2) The traditional IEEE 802.11 APs do not have any built-in feature for centralized network management . Thus, users need to pass through an authentication procedure during handover among APs, which is a time-limited task. In the case of timeout, the network connection will be lost. (3) The deployments of APs in a closed region for overwhelming the signal gaps among APs, the purpose is to provide a perfect and continuous connection to the user during mobility. However, the deployment of APs in an adjacent region is caused by signal interference . Using the IEEE 802.11 standard , MTs scanned multiple APs within the radio range and received an extensive list of SSIDs. To establish a connection, the user must select a SSID, most probably SSID selection is solely based on the strongest received signal strength indication (RSSI). In the case of association with already congested AP, the user could experience low throughput. The IEEE 802.11 standard  did not provide any load balancing mechanism that makes a comprehensive imbalanced network, and therefore, network confronted throughput degradation at the cost of packet loss ratio. The aforementioned issues restrict the perfect utilization of WiFi to fulfill the requirements of the future Internet which results in an inefficient mobility management.
The concept of software-defined network (SDN) is a promising solution in the wireless network personification that is able to provide programmable control plane and data plane into AP. These features boost the WLAN performance regarding the fine-grained packet control and provide a configuration interface using the OpenFlow . The OpenFlow-based switch can perform various jobs, defined by the controller based on predefined rules, e.g., it can act as a router, firewall, NAT , or other user-defined functions. The OpenFlow may incorporate handover parameters in existing applications that could lead to smooth performance for handover activity . The OpenWRT  assists in the reconfiguration of wireless protocols to enable the SDN programmable control plane in IEEE 802.11 AP to enhance the conventional handover process in a wireless network. Moreover, the emergence of the network function virtualization (NFV)  is highly encouraged to implement within the SDN platform to offer multiple benefits to the service operators and the user/mobile terminals (MT). The NFV has been considered as a useful abstraction to hardware functionalities to reduce the infrastructure cost and also helpful to minimize the power consumption. Therefore, it is evident that the SDN and NFV technologies have potential to build the prospective WLAN environment by introducing mobility management features that include seamless handover, wireless resource optimization, centralized management, and fine-grained controllability.
In the current research work, we propose a Logical AP-based Mobility Management (LAPM) scheme for WLAN in the SDN environment with the further functionality of the NFV, which better describe how to design an ideal mobility management environment for WLAN. We construct the logical AP (LAP) with an extended SDN/NFV abstraction, which release the IEEE 802.11 protocol stack complexity to forward the operations to the centralized controller. Each LAP corresponds to a MT that is associated with the AP. The LAP acts as the virtual AP (VAP), an abstraction of the physical AP (PAP), which provides auxiliary network functions. The LAP has the ability to maintain same VAP for each associated MT with adjacent PAPs concurrently to enable the seamless handover which can be achieved when MT associated with several PAPs in the signal overlapping range. We also examine the performance of both conventional handover and our proposed algorithm LAPM. Load balancing among APs that have common signal interference area is also a focus of this research. We build a testing environment, based on the SDN and the NFV technologies, to analyze the functionality of PAP. After implementing the LAPM scheme in a testbed, with the real traffic from users, it is evident from evaluation results that the proposed scheme could significantly minimize the handover latency in WLAN. In addition, LAPM scheme maintains the load balance with the concurrent provision of the seamless handover. This experimental study helps to evaluate the performance of installed PAPs for further enhancement of the QoE.
The rest of the paper is organized as follows: Section 2 presents the previous studies. Section 3 reports the functionality of mobility management scheme and implementation of LAPM scheme. In Section 4, we present the experimental results. Finally, in Section 5, we conclude the paper.
2 Related works
Mobility management is an obligation of the wireless networks. This investigation emphasizes on the WLAN mobility management schemes.
2.1 The traditional IEEE 802.11 handover schemes
The IEEE 802.11 AP handoff scheme is defined in three steps : (a) scan the new APs for the association, (b) send the authentication probe to target AP that has strong RSSI, and (c) begin the re-association procedure to connect with the new AP. Consequently, the lengthy traditional handover process is the primary cause of throughput degradation. Besides, there are also certain reasons  for packet loss such as electromagnetic interference, transmission impairment, and the hidden terminal problem. In the past decade, various handover approaches have been announced based on the surrounding channels  and the neighboring graph schemes . These schemes [13, 14] perform passive scanning to gather the results. However, channel synchronization during the scanning process declines the frequent handoff in a dense network environment that leads to the poor QoS. Compared with the traditional schemes mentioned above, our proposed scheme collects the neighboring PAP information through LAP to provide a gateway between the SDN controller and the MT. In our proposed scheme, we restrict the frequent handover of MTs in the closed region APs in order to reduce the latency and the ping-pong ratio.
2.2 The SDN-based WLAN infrastructure
Recently, the OpenFlow protocol has been considered in the wireless access network to provide the fine-grained packet control, and SDN makes the separation of underlying physical infrastructure and the network services [15–19]. The OpenRoads  was the first project in the SDN-based WLAN environment that enables wireless network slicing using the FlowVisor to assign different SSIDs to the MT. The OpenRoads introduced the OpenFlow-based testbed to control mobility between the WiFi and the WiMax base stations. Mortier et al.  proposed a prototype using the OpenFlow protocol to enable flow-based scheme with the fine-grained packet control in home networks. CloudMAC  is a distributed architecture that enables transferring the processing of MAC layer functions on the central servers to minimize the IEEE 802.11 AP pressure, and the AP is responsible only for the MAC frames forwarding among the virtual APs using the OpenFlow. Contrarily, CloudMAC did not declare the switching procedure of associated stations among APs simultaneously that is required for per-user handover. Besides, the CloudMAC drives all traffic towards the Cloud which increases the load of the control plane. In contrast, the LAPM scheme provides the seamless mobility without any change on the MTs to reduce the network complexities and the deployment cost. Suresh et al.  introduce an SDN framework Odin to empower the seamless mobility in WLAN. The Odin builds the logical virtual access point (LVAP), which is similar to the concept of the virtual APs in the CloudMAC architecture. The LVAP offers a dedicated logical connection to the client with a unique BSSID. During the handoff process, the client does not necessitate re-association with the target AP. The Odin offers seamless handover in a wireless network to reduce the handover delay in comparison with the IEEE 802.11 traditional handover. However, the handover process depends on a formal parameter RSSI which could lead to a load imbalance situation. The SDN-based handover approach  proposes for IEEE 802.11WLAN in which the handover procedure depends on neighbor AP response and permits the MT to connect with several APs simultaneously. It also enables fast switching among APs to improve the performance of the video streaming applications. However, the scope of the proposed approach is limited to video streaming applications without the inclusion of video conferencing and peer-to-peer-based applications. SDWLAN  presented an architecture that sustains client-unaware handoff on 802.11 AP MAC layer and provides a unified control platform for wireless APs and wired backbone. The wireless access switch (WAS) is a device that configured with the OpenFlow protocol for transferring several module functions of AP onto the centralized controller. However, it is challenging to incorporate WAS into the existing WLAN environment. The proposed LAPM scheme has advantages over the abovementioned SDN-based solutions with the main feature of the mobility management. The LAPM scheme is primarily distinctive from those techniques that support individual mobility of MTs with the load balancing approach among PAPs in the overlapping signal range without any amendments at the MTs. We performed several experiments on the real testbed to evaluate the performance of handoff and the load balancing in both traditional environment and the SDN-based environment. We will discuss in more detail about the evaluation and results in Section 4 of this research article.
3 The design and implementation of LAPM
3.1 Logical access point abstraction
3.2 Overall procedure
3.3 Implementation of LAPM scheme
4 Evaluation cases and experimental results
4.1 Case 1: handover between PAPs
4.2 Case 2: LAPM scheme for MT relocation and load balancing
The goal of the next experiment is to demonstrate how the LAPM scheme can enhance the network throughput of those MTs that were overlapping in region A. We installed two PAPs with the same overlapping region and selected six MTs for this experiment.
The emergence of the SDN/NFV paradigm provides us with a new chain of innovative prospects for solving the mobility issues in the traditional WLAN. To address the problems of seamless handover and MT association with overloaded APs in the signal-overlapping area, in this paper, we proposed an integrated mobility management scheme which considers both seamless handover and load balancing. Furthermore, the proposed scheme can be easily deployed without involving any modification at the client side. The proposed scheme makes it feasible for the users to connect with the best available AP. Our experiments demonstrate how to create LAP, assign LAP, and make the migration of LAP according to a real-time decision algorithm that was executed at the SDN controller. The evaluation results reveal that our LAPM-based WLAN scheme efficiently increases the throughput of the network and significantly decreases the packet loss ratio while simultaneously balances the network. For further study, the mobility management scheme will be investigated for a power saving module, and location awareness would be integrated into the current scheme, LAPM, to offer more true mobility in the future Internet architecture.
This work was supported by National Nature Science Foundation (61402065,61501075), Prospective Research Project on Future Networks (BY2013095-2-03) fund by Jiangsu Future Networks Innovation Institute and Project Foundation of Chongqing Municipal Education Committee (No. KJ1500429).
SMMG (PhD student) is the main author of current paper, he has raised the main idea, proposed the scheme, and wrote the manuscript. TH is the PhD advisor, she has suggested improvements in the main idea of the research and writing of manuscript. WJ has designed the experiments for seamless handover, and HMH has assisted to execute the experiments and gathered the results for evaluation. ZG has analyzed the proposed the approach and suggested improvements in the implementation and reviewed the content of the paper. CX has contributed with the expert’s advice for developing real testbed in the WLAN. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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