The end-to-end QoS guarantee framework for interworking WiMAX PMP and mesh networks with Internet

Computers and Electrical Engineering 39 (2013) 1905–1934

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Computers and Electrical Engineering journal homepage: www.elsevier.com/locate/compeleceng

The end-to-end QoS guarantee framework for interworking WiMAX PMP and mesh networks with Internet q Ing-Chau Chang a, Yi-Ting Mai b,⇑ a b

Department of Computer Science and Information Engineering, National Changhua University of Education, Changhua, Taiwan, ROC Department of Information and Networking Technology, Hsiuping University of Science and Technology, Taiwan, ROC

a r t i c l e

i n f o

Article history: Available online 14 June 2013

a b s t r a c t A network environment, in which 802.16 WiMAX PMP and mesh networks interwork with the Internet, is becoming prevalent and thus ensuring an end-to-end QoS of a media stream after the handoff of mobile host is critical. In this paper, an end-to-end QoS guarantee framework and its handoff flows are revised first. An end-to-end QoS adjustment algorithm is proposed to dynamically adjust the L2 bandwidth allocated in the WiMAX PMP and mesh networks and the L3 data rate in the Internet after the handoff. A CBQ + RED QoS adaptation algorithm is further proposed to adapt queue lengths among different service types and provide efficient queue and traffic management mechanisms as each packet arrives. Finally, simulation results indicate that this framework with these two algorithms outperform three QoS management schemes by supporting higher throughputs, lower packet loss ratios and smaller delays on the end-to-end path under different parameter combinations and two handoff scenarios. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction In the provision of a global network access service to modern wireless users, a heterogeneous network operator may operate different types of wireless access networks such as IEEE 802.11 (Wireless Fidelity, Wi-Fi) and 802.16 (Worldwide Interoperability for Microwave Access, WiMAX) [1], simultaneously. A variety of traffic flow demands have to pass through the common Internet backbone using the Differentiated Services (DiffServ) quality of service (QoS) scheme [2]. Therefore, utilizing the common backbone efficiently offers a better QoS to users and maximizes the operator’s revenues [3]. As Wi-Fi and WiMAX wireless networks, which adopt the Integrated Services (IntServ) QoS scheme [1], have become major access networks for mobile hosts (MH) in recent years, providing end-to-end (E2E) QoS services [4] over heterogeneous wireless access networks and the Internet backbone has become more and more challenging for MHs. The specific challenge arises when the MH changes its wireless network attachment point, i.e., when it hands over to a new wireless cell. When the handoff of the MH happens, the E2E path between the MH and its contacted node, i.e., correspondent node (CN), may change, which in turn introduces extra network operations and delays when attempting to guarantee the E2E QoS in this new path. Furthermore, if needed network resources such as network bandwidth, buffers, etc., in the new E2E path after the handoff of the MH are not available, ongoing real-time multimedia services executed by the MH will be interrupted, which may significantly degrade their QoS levels. To avoid the degradation that could occur in these situations, an E2E QoS guarantee framework [5], which covers the IntServ and DiffServ QoS networks, for the heterogeneous network operator is necessary to satisfy each MH’s resource

q

Reviews processed and recommended for publication to Editor-in-Chief by Associate Editor Dr. Paul Cotae.

⇑ Corresponding author. Tel.: +886 955900587.

E-mail addresses: [email protected], [email protected] (Y.-T. Mai). 0045-7906/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compeleceng.2013.05.007

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requirements and service QoS levels of all traffic classes when the handoff of the MH occurs. However, most previous research has only focused on some support issues of E2E QoS when heterogeneous wireless networks interwork with the Internet. First, [6] designed the integrated QoS signaling protocol for supporting E2E QoS of the MH when the MH crossed between different IntServ and DiffServ network domains. Second, because IntServ and DiffServ QoS domains adopt totally different appro1aches to process user packets, there must be a traffic mapping mechanism between these two domains to maintain the E2E QoS of transmitted packets [4,6]. Furthermore, for integrating QoS mechanisms of the layer 3 (L3) IP and layer 2 (L2) media access control (MAC) layers, the E2E framework also needs a cross-layer traffic mapping mechanism between these two layers [7–10]. Finally, to support uninterrupted service and reduce handoff latency, [5,11] proposed seamless handoff mechanisms, such as the context transfer one to accelerate the handoff process by transferring the MH’s current context as soon as the MH has received the L2 handoff trigger. Therefore, significant media playback interruption and QoS degradation can be avoided. As a standard-based technology enabling the delivery of last mile wireless broadband access, the IEEE 802.16 WiMAX network standard describes the physical and MAC layer specifications to support higher data rates, larger coverage, easier and more cost effective deployment than the Wi-Fi network [9]. Hence, the WiMAX network is more suitable to support four L2 QoS service types, i.e., unsolicited grant service (UGS), real-time polling service (rtPS), non-real-time polling service (nrtPS) and best-effort (BE). The WiMAX base station (BS) has the responsibility of managing and maintaining the QoS for all packet transmissions. Resource management in the WiMAX network is centrally controlled by the BS in two different ways for two IEEE 802.16 modes, i.e., point to multi-point (PMP) and mesh, respectively. When the handoff of the MH occurs, the most important thing for the BS to consider is whether the L2 bandwidth reserved for the MH is large enough in the new E2E path. If both of the old and new WiMAX networks utilize the PMP mode, the BS only needs to reassign the mini-slots used by the subscriber station (SS) in the old path to the SS in the new path without triggering the L2 bandwidth re-allocation process [6]. Nevertheless, if they both adopt the mesh mode, the BS must reserve enough L2 bandwidth in the new WiMAX network, according to the hop count between the BS and the current SS, which the MH is connecting to, after the handoff [8]. Further, [12] has shown that the handoff and temporary disconnection periods caused by user mobility significantly degrade the E2E QoS level, especially streaming video over the Internet. Hence, WiMAX QoS mechanisms should be integrated into this E2E framework to guarantee E2E QoS levels of all traffic classes [6,8]. In this paper, we consider how to support the E2E QoS of the MH when the MH moves between the 802.16 WiMAX PMP and mesh networks, which interwork with the Internet, as shown in Fig. 1, while maintaining high network utilization. To seamlessly integrate the Internet L3 and the WiMAX L2 QoS mechanisms, we assume that the L3 gateway router (GR) also supports functions of the WiMAX L2 BS to become the cross-layer GR/BS, which is located at the boundary of the Internet and the WiMAX networks. Important modules and two types of control flows in the cross-layer GR/BS are shown in Fig. 2. The first is the mobility management flow, which comprises the traditional context transfer, admission control and

Fig. 1. Integration of 802.16 wireless networks with the Internet.

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GR/BS

Context transfer (for InterWiMAX handoff)

QoS parameter extraction

CrossLayer (L2+L3)

Cross-layer and one-to-one QoS mapping

CBQ+RED QoS adaptation rtPS

UGS

BE

nrtPS

L1

Admission control

Cross-layer and end-to-end QoS adjustment

Physical Layer

QoS Traffic Control Flow for Each Arriving Packet Mobility Management Flow for the Handoff Fig. 2. Modules and two flows of the cross-layer GR/BS.

our proposed cross-layer and E2E QoS adjustment processes, for each user session when the handoff occurs. This cross-layer and E2E QoS adjustment process focuses on dynamically adjusting L2 and L3 resource allocations between the WiMAX networks and the Internet after the MH’s handoff. The second is the QoS traffic control flow which determines how one-to-one packet mapping and our proposed QoS adaptation processes are performed for each arriving packet by dynamically adjusting E2E service qualities of the MH to available network resources when the handoff occurs [11,12]. Traffic management and queue management are two key issues in the QoS traffic control flow. Rare research has addressed the interaction of these two inter-dependent issues in the provision of efficient E2E QoS support. For example, [13,14] have applied the class-based queuing (CBQ) scheme for traffic management only by sharing resources among different traffic classes in wired and wireless networks and [15,16] have adopted the random early detection (RED) scheme in network devices for the queue management of a traffic class. Contributions of this paper are summarized as follows. 1. To guarantee the E2E QoS of the MH that moves between WiMAX PMP and mesh networks, which interworks with the Internet, the E2E QoS guarantee framework and its E2E signaling flows will be completely revised for this specific network environment. 2. A cross-layer and E2E QoS adjustment algorithm in the mobility management flow will be proposed to dynamically adjust the Internet L3 and two types of WiMAX, i.e., PMP and mesh, L2 network resource allocations after the MH’s handoff. To our knowledge, this algorithm is the first one that supports L2 and L3 QoS adjustment in the E2E path that crosses the Internet and both modes of WiMAX networks. 3. An integrated CBQ + RED QoS adaptation algorithm for both traffic management and queue management of the QoS traffic control flow will be proposed to handle situations when the associated traffic queue of each arriving packet does not have enough bandwidth to meet the packet’s requirement. With the borrowing and retrieval processes in the CBQ + RED QoS adaptation algorithm, the integrated GR/BS will re-adjust resources among all traffic classes of MHs by allocating as many resources as possible to higher priority traffic classes of MHs to maintain their E2E QoS levels at the cost of sacrificing those of lower priority traffic classes. This paper is organized as follows. In Section 2, we will give a detailed survey of previous research devoted to E2E QoS guarantees and compare their advantages and disadvantages. In Section 3, complete handoff flows in the E2E QoS guarantee framework will be reviewed. Details of the proposed E2E QoS adjustment and CBQ + RED QoS adaptation algorithms, which are performed in the GR/BS, will be described there. Section 4 compares simulation results of our E2E framework with two proposed algorithms to those of three well-known QoS schemes for four traffic classes. Section 5 contains the conclusions drawn from this research.

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2. Related researches In terms of integrating the DiffServ Internet backbone and IntServ LANs to achieve E2E QoS support, the IntServ domain usually adopts the resource reservation protocol (RSVP) as its signaling protocol to reserve resources in advance. To reduce processing complexities in the Internet, RSVP control messages should not be processed when they pass through the Internet. However, this work has not considered how to manage user mobility on wide-spreading heterogeneous wireless networks that adopt the IntServ QoS approach. There has been a lot of research that has considered the QoS issue in heterogeneous wireless networks. In [9,17], the authors have surveyed MAC-based QoS implementations for the IEEE 802.16 WiMAX and IEEE 802.11 Wi-Fi networks respectively. For a WiFi-WiMAX heterogeneous network, an integrated protocol stack has been proposed to blend the generic virtual link layer (GVLL) and media independent handover (MIH) above the MAC layer [10]. Furthermore, the authors have analyzed the impact of GVLL on the guarantee of the QoS and the impact of MIH in achieving seamless handover. The authors in [18] have focused on QoS traffic engineering (TE) in a wireless physical layer environment and wireless mesh network. They have proposed an algorithm that optimizes the distribution of the outgoing traffic of each node among all feasible next-hops. By efficiently exploiting the multi-radio and multi-channel method, a novel cross-layer QoS-aware routing protocol on the optimized link state routing (CLQ-OLSR) has been proposed to support realtime multimedia communication [19]. The proposed CLQ-OLSR is based on a distributed bandwidth estimation scheme that estimates the available bandwidth on each associated channel used by the node. On the other hand, network traffic in heterogeneous networks exhibits increasing diversities so that large-scale IP traffic matrix estimation is a significant challenge. In [20], the authors have investigated traffic matrix estimation in large-scale backbone networks by building a time–frequency model and then proposing a multi-input and multi-output estimation scheme to solve this problem. However, this aforementioned research has not focused on supporting E2E QoS for MH’s handoff. The authors in [5] have developed a framework to categorize interworking solutions by defining a generic set of interworking levels and its related key interworking mechanisms for the WLAN and cellular integration and the WiMAX and 3GPP LTE/SAE interworking. They have also proposed network and link layer handover optimization mechanisms to achieve an inter-system seamless service continuity and satisfy service requirements during mobility. The seamless vertical handoff architecture, i.e., Vertical Handoff Translation Center (VHTC), has been proposed in [11] to guarantee MH’s QoS in IEEE 802.16 WiMAX and 802.11 Wi-Fi heterogeneous networks. An efficient bandwidth borrowing management (EBBM) module of VHTC has let the lower priority queues borrow spare bandwidth from the higher priority ones when they have owned spare bandwidth. Conversely, higher priority queues were forbidden to grab the bandwidth from lower priority queues in EBBM, which would degrade QoS levels of higher priority traffic flows if enough bandwidth has not been allocated to them in advance. Moreover, this VHTC with EBBM architecture has considered the queue management issue for QoS adaptation only, without supporting the cross-layer E2E QoS adjustment. Further, Karimi and his colleagues have proposed [12] to make the rate adaptation decisions in the application layer for multimedia applications over wireless networks. Based on the application-level wireless multi-level explicit congestion notification (ECN) marking, graceful degradation and the decision making

Fig. 3. The flow of the Intra-WiMAX handoff.

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(a)

(b) Fig. 4. The binding cache of GR/BS1 (a) before and (b) after case 1 handoff of the MH.

Fig. 5. The flow of the proposed cross-layer and E2E QoS adjustment algorithm.

algorithm, this work has presented its rate and QoS adaptation schemes. In summary, the aforementioned work has not considered how to adjust L2 and L3 resources for E2E QoS as our proposed cross-layer and E2E QoS adjustment algorithm does when the handoff occurs.

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Table 1 Notations used in the cross-layer and E2E QoS adjustment algorithm. Notation

Description

Riold Rinew RiIntav BW iold BW inew BW irem

The E2E L3 data rate of the MH’s flow, which belongs to L3 traffic type i, before handoff

Hold Hnew

The hop count from the GR/BS to the associated SS of the MH in the old WiMAX network before handoff The hop count from the GR/BS to the associated SS of the MH in the new WiMAX network after handoff

The E2E L3 data rate of the MH’s flow, which belongs to L3 traffic type i, after handoff The available L3 data rate of traffic type i in the Internet after handoff The total L2 bandwidth requirement of the MH’s flow, which belongs to L2 service type i, in the old WiMAX network before handoff The total L2 bandwidth requirement of the MH’s flow, which belongs to L2 service type i, in the new WiMAX network after handoff The remaining L2 bandwidth of service type i in the new WiMAX network after handoff

Table 2 Values of BW inew and Rinew for all handoff cases in Fig. 1. Handoff cases/values

BW inew

Rinew

Case 1 (Intra-WiMAX handoff, in the mesh mode)

BW iold

2 3

Case 2 (Intra-WiMAX handoff, in the mesh mode)

2 3

Case 3 (Intra-WiMAX handoff, in the PMP mode)

BW iold

Riold

Case 4 (Intra-WiMAX handoff, in the PMP mode)

BW iold

Riold

Case 5 (Inter-WiMAX handoff)

 BW iold 3  BW iold

Riold

Case 6 (Inter-WiMAX handoff)

1 3

 BW iold

 Riold

Riold

Riold

On the other hand, Packet scheduling for QoS support (PS_QS) [21] has introduced a packet scheduling algorithm in the BS and an admission control policy in each SS for supporting IEEE 802.16 QoS. To support all types of 802.16 service flows, the PS_QS packet scheduling has used fixed bandwidth allocation, the earliest deadline first (EDF) and the weight fair queue (WFQ) for UGS, rtPS and nrtPS, respectively. The remaining bandwidth is equally allocated to each BE connection. Further, the work in [6] considered the fast signaling mechanism for IEEE 802.16 PMP and mesh modes and has proposed the Integrated QoS Control (IQC) architecture. A two-layer packet scheduling has been deployed to manage the bandwidth of six queues, according to their transmission directions and service classes, in the BS. IQC has used the priority queue (PQ) as the first layer scheduling for UGS. It has further adopted the EDF for rtPS, the WFQ for nrtPS, and the round robin (RR) for BE as the second layer scheduling to match requirements of all service classes. Hence, both PS_QS and IQC have considered the packet scheduling approach for queue management in the QoS traffic control flow only. In order to achieve end-to-end multimedia services, 802.16 QoS must be well integrated with IP QoS. Our previous work has proposed a framework of cross-layer QoS support in the IEEE 802.16 PMP network only [7]. Performance problems in each standard scheme for QoS support in the IEEE 802.16 mesh network have been pointed out in [8]. That research also proposed efficient routing and scheduling mechanisms for the 802.16 mesh network. However, none of the aforementioned research has supported the cross-layer E2E QoS adjustment between IP L3 and 802.16 L2 in both mesh and PMP networks nor the traffic management for E2E QoS adaptation as our proposed CBQ + RED QoS adaptation algorithm in this paper. 3. The E2E QoS guarantee framework Characteristics of our E2E QoS guarantee framework used for the interworking environment, as shown in Fig. 1 above, are listed as follows: 1. The wired Internet backbone adopts the DiffServ QoS architecture, which is equipped with a Bandwidth Broker (BB), to reserve the requested bandwidth for the service request, manage internal resources and search for the optimal route in the Internet backbone. Functions of the BB are similar to those of the Network Resource Manager in [1]. 2. Local WiMAX networks use RSVP as the IntServ E2E signaling protocol to handle the change of resource requirements after the MH’s handoff. The E2E RSVP process for initializing the media stream will be described in Section 3.1. 3. As mentioned above, the integrated cross-layer GR/BS is located at the boundary of the Internet and the WiMAX networks. GR/BS adopts traditional Common Open Policy Service for Service Level Specification (COPS-SLS) and Context Transfer Protocol (CXTP) as its negotiation and context transfer protocols in the Internet to accelerate the handoff process as soon as it has received the L2 handoff trigger. Further, our QoS guarantee framework performs a cross-layer and one-to-one traffic mapping mechanism in the integrated GR/BS as done by the AQUILA organization for converting three Internet L3 traffic types into four WiMAX L2 QoS service types [1], and vice versa. Please refer to related work for these traditional protocols.

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Fig. 6. The flow of the Inter-WiMAX handoff.

Table 3 Notations used in the proposed CBQ + RED QoS adaptation algorithm. Notation

Description

Avg(Qi), Max(Qi) Max_th(Qi), Min_th(Qi) Array_L[k][i] Arr_pkt(Si) R(Qk), B(Qk) Insuff(Qi)

The The The The The The The

P Si

average and maximal queue lengths of class i maximal and minimal queue length thresholds of class i queue length lent from class i to class k packet length of the arriving packet S that belongs to class i retrieved and borrowed queue lengths from class k insufficient queue length of class i packet dropping probability of the arriving packet Si

4. The integrated GR/BS performs the proposed cross-layer and E2E QoS adjustment algorithm to support L2 and L3 QoS adjustment in the E2E path that crosses the Internet and the WiMAX networks. By integrating the queue management of RED and bandwidth sharing of CBQ, GR/BS also adopts the proposed CBQ + RED QoS adaptation algorithm to simultaneously handle dynamic QoS adaptation for both traffic and queue managements. Details of these two algorithms will be described later. In the following, the RSVP initiation process to reserve resources for E2E QoS guarantee before the MH’s handoff will be reviewed first. With the proposed E2E QoS adjustment and CBQ + RED QoS adaptation algorithm, revised E2E QoS guarantee flows for two handoff scenarios, i.e., the Intra-WiMAX handoff and Inter-WiMAX handoff, will then be considered. 3.1. RSVP initiation before the handoff For the MH, which is attached to 802.11 access point (AP), AP1 under SS3 in the WiMAX mesh network as shown in Fig. 1, to start a media streaming service with the CN, it must execute the following conventional procedures shown in Fig. 3 for RSVP initialization: (a) As soon as the CN receives a request from the MH to start the service, it issues an L3 RSVP PATH message, which contains requested QoS parameters like Traffic Specification (TSPEC), Previous Hop Router Address (PHOP) and Advertisement Specification (ADSPEC), to the MH at step 0.1. This PATH message is considered a normal traffic message when passing

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Fig. 7. The complete flow of the proposed CBQ + RED QoS adaptation algorithm.

through the Internet between two edge routers ER1 and ER2. As the L3 PATH message reaches the integrated GR/BS1 at the boundary of the 802.16 WiMAX mesh network, GR/BS1 handles this PATH in different ways according to which 802.16 mode, i.e., PMP or mesh, is utilized in the attached WiMAX network. If GR/BS1 operates in the PMP mode, it translates QoS parameters carried in the L3 RSVP PATH message into the QoS information in the type-length-value (TLV) encoded field of the 802.16 L2 Dynamic Service Change Request (DSC-REQ) message [1]. It then sends this DSC-REQ to the associated SS, i.e., SS3 in Fig. 1, of the MH. Conversely, GR/BS1 in the mesh mode issues an L2 Mesh Centralized Schedule (MSH-CSCH) message with the flow scale exponent field to SS3 for specifying desired 802.16 bandwidth requirements to SS3 [1]. Finally, SS3 re-issues an L3 RSVP PATH message to the MH. (b) After the MH receives the RSVP PATH message from the CN, it sends back an RSVP RESV message containing the negotiated QoS parameters along the reverse path of PATH, which is shown at step 0.2 in Fig. 3. As SS3 in the 802.16 network receives this RSVP RESV, it issues a corresponding Dynamic Service Change Response (DSC-RSP) message for the PMP mode or an MSH-CSCH for the mesh mode to GR/BS1 for carrying the negotiated QoS parameters of RSVP RESV. At GR/BS1, these L2 DSC-RSP or MSH-CSCH messages are converted back to the L3 RSVP RESV message and then forwarded to the entrance point, i.e., ER2, in the Internet. (c) When this RESV message arrives at ER2, ER2 retrieves the negotiated QoS parameters from this message and conveys them to the BB of the Internet by the COPS-SLS message at step 0.3. The BB records these QoS parameters and corresponding policy configurations at step 0.4. After that, ER2 forwards this RESV message back to the CN to complete the E2E QoS signaling process at step 0.5. After that, the CN begins its media stream to the MH at step 0.6.

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Fig. 8. The flow of the retrieval process in the CBQ + RED algorithm.

Fig. 9. The retrieval process of class i in the CBQ + RED algorithm: retrieving lent queue lengths from higher priority queues.

3.2. The proposed Intra-WiMAX handoff flow As shown in case 1 of Fig. 1, when the MH enters the overlapped service range between the original 802.11 AP1 under SS3 and the new 802.11 AP2 under SS4 in the same WiMAX mesh network, the handoff is called an Intra-WiMAX handoff, which consists of the following two processing stages to achieve E2E QoS guarantee as the handoff occurs. 3.2.1. The L2 handoff procedure (a) As soon as the received signal strength of the new 802.11 AP, i.e., AP2, is high enough to trigger the handoff, the MH begins exchanging six 802.11 L2 control messages, i.e., Probe-REQUEST (Probe-REQ), Probe-RESPONSE (Probe-RSP), Authentication-REQUEST (Authentication-REQ), Authentication-RESPONSE (Authentication-RSP), Reassociation-REQUEST (Reassociation-REQ) and Reassociation-RESPONSE (Reassociation-RSP), with AP2 to complete the L2 handoff process, as shown at steps 1.1–1.6.

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Fig. 10. The borrowing process flow in the CBQ + RED algorithm.

Fig. 11. The borrowing process of class i in the CBQ + RED algorithm: borrowing abundant queue lengths from lower priority queues.

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(b) In this handoff scenario, because the MH is still within the control of the same SS, it does not have to change its associated IP subnet and IP address. Thus, it does not issue any L3 handoff message. On the other hand, at step 1.7, the MH’s L2 has to send a DSC-REQ or MSH-CSCH message to notify the L2 of GR/BS1 of the MH’s current corresponding SS, i.e., SS4 instead of SS3, in the binding cache of GR/BS1 for the case 1 handoff, as shown in Fig. 4.

3.2.2. QoS adjustment and local RSVP signaling (a) As GR/BS1 receives the L2 DSC-RSP or MSH-CSCH message, it adopts the proposed cross-layer and E2E QoS adjustment algorithm, as shown in Fig. 5. This is to confirm whether it owns enough network resources to fulfill the QoS requirement of this MH’s flow after the handoff at step 2.1. Notations used in this algorithm are listed in Table 1. Instead of only considering the 802.16 PMP mode [7,22] or the 802.16 mesh mode [8,23], our cross-layer and E2E QoS adjustment algorithm operates over both WiMAX modes and the Internet backbone. As mentioned above, GR/BS adopts the one-to-one traffic mapping mechanism to convert Internet L3 traffic type i into WiMAX L2 QoS service type i. This algorithm performs the following steps for each traffic/service type such that its time complexity is O(n), where n is the number of total traffic/service types in the WiMAX network. According to three important input values, i.e., the hop count, i.e., Hold , from the GR/BS to the associated SS of the MH in the old WiMAX network before handoff, the hop count, i.e., Hnew , from the GR/BS to the associated SS of the MH in the new WiMAX network after handoff and the E2E L3 data rate, i.e., Riold , of the MH’s flow, which belongs to L3 traffic type i, before handoff, this algorithm will output the total L2 bandwidth requirement, i.e., BW inew , of service type i and the E2E L3 data rate, i.e., Rinew , of traffic type i for the MH’s flow in the new WiMAX network and the Internet respectively after the handoff. As this algorithm executes for each type i, it first calculates the total L2 bandwidth requirement, i.e., BW iold , of the MH’s flow in the old WiMAX network before the handoff and predicts the possible value of BW inew for the new WiMAX network after the handoff by multiplying Riold with Hold and Hnew [8], respectively, which is formulated as Eq. (1). If the GR/BS and its underlying 802.16 network operate in the PMP mode, then Hold is equal to Hnew . When the Intra-WiMAX handoff occurs in such a situation the GR/BS only has to allocate the old E2E L3 data rate, i.e., Riold , of the MH before its handoff to the MH as the new E2E L3 data rate, i.e., Rinew , after the handoff. This in turn allocates the old L2 downlink bandwidth, i.e., BW iold , before the handoff to the MH as the new L2 downlink bandwidth, i.e., BW inew , after its handoff by using the 802.16 DL_MAP. Both case 3 and 4 handoffs in Fig. 1 belong to this handoff scenario. Hence, steps 2.2 to 2.5 have to be skipped for the PMP mode. Conversely, WiMAX resources have to be shared among all underlying MHs in the 802.16 mesh network. If the mesh mode is adopted in the WiMAX network when the Intra-WiMAX handoff occurs, the MH’s handoff, though under the same GR/BS, may change the route and corresponding hop count between the GR/BS and the associated SS of the MH. This affects the corresponding E2E L3 data rate allocated to the MH after its handoff. As shown in Fig. 1, routes before and after the case 1 Intra-WiMAX handoff of the MH are GR/BS1 ? SS1 ? SS3 with the hop count of 2 and GR/BS1 ? SS1 ? SS2 ? SS4 with the hop count of 3 respectively. Without consuming more L2 bandwidth after the handoff, GR/BS1 allocates the old

Fig. 12. Simulation environment.

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802.16 L2 link bandwidth, i.e., BW iold , before the handoff to the MH as BW inew after the handoff. Hence the E2E L3 data rate, i.e., Rinew , after the handoff is equal to two thirds of the allocated L3 data rate in the old WiMAX network, i.e., Rinew ¼ BW iold =Hnew ¼ Riold  Hold =Hnew ¼ Riold  2=3, by Eq. (2). If Rinew is higher than the available L3 data rate, i.e., RiIntav , in the Internet after the handoff, Rinew should be assigned as RiIntav . This avoids allocating a higher data rate than the Internet can provide to the MH. Finally, GR/BS1 has to request the BB in the Internet to release one third of Riold at steps 2.2–2.5 for the case 1 handoff. However, if the hop count of the new route after the Intra-WiMAX handoff is smaller than or equal to that of the old route before the handoff, i.e., Hnew 6 Hold , the E2E L3 data rate, i.e., Rinew , after the handoff is set as the allocated L3 data rate, i.e., Riold , in the old WiMAX network. This in turn sets the total L2 downlink bandwidth, i.e., BW inew , after the handoff as Rinew  Hnew ¼ Riold  Hnew ¼ ðRiold  Hold Þ  ðHnew =Hold Þ ¼ BW iold  ðHnew =Hold Þ. Hence, BW inew is set as 2=3  BW iold for case 2 handoff in Fig. 1. On the other hand, when the Inter-WiMAX handoff occurs, e.g. cases 5 and 6 handoffs in Fig. 1, the GR/BS will check whether the predicted BW inew of type i is higher than the remaining L2 bandwidth, i.e., BW irem , in the new WiMAX network after the handoff. If it is, the GR/BS allocates BW irem as the total L2 bandwidth requirement in the new WiMAX network after the handoff, which in turn sets Rinew as BW irem / Hnew using Eq. (3). Otherwise, the GR/BS sets Rinew as Riold . As mentioned above, if Rinew is higher than RiIntav after the handoff, Rinew should be reset as RiIntav . Table 2 lists values of BW inew and Rinew , which are calculated by this proposed cross-layer and E2E QoS adjustment algorithm, of type i for all handoff cases in Fig. 1, where BW irem and RiIntav are assumed to be larger than BW inew and Rinew , respectively.

(

BW iold ¼ Riold  Hold

ð1Þ

BW inew ¼ Riold  Hnew Rinew ¼

BW iold Riold  Hold ¼ ; if ðHnew > Hold Þ Hnew Hnew

ð2Þ

Rinew ¼

 BW irem  ; if BW inew > BW irem Hnew

ð3Þ

(b) If GR/BS1 has to adjust the E2E L3 data rate in the Internet, its L3 adopts a COPS-SLS message to transmit the value of the newly allocated L3 data rate, i.e., Rinew , of type i to the BB via ER2 in the Internet, as shown at step 2.2 in Fig. 3. The BB then allocates this L3 data rate of type i in the Internet and creates a new QoS routing path between ER2 and the corresponding edge router, i.e., ER1, of the CN at step 2.3. Then at step 2.4, BB sends the COPS-SLS ACK, which contains the adjusted L3 data rate in the Internet, to GR/BS1. At step 2.5, GR/BS1 issues an RSVP REFRESH message with the adjusted L3 data rate back to the CN to modify the reserved L3 data rate along the path from the CN to GR/BS1. (c) After that, the L2 of GR/BS1 will perform the localized RSVP signaling process inside the WiMAX network by issuing a DSC-REQ/MSH-CSCH message with new QoS parameters, i.e., 802.16 L2 bandwidth BW inew and L3 data rate Rinew , to SS4 at step 2.6. SS4 then converts this DSC-REQ/MSH-CSCH to an RSVP PATH message with the new L3 data rate for the MH, which then sends back a corresponding RSVP RESV to SS4 at step 2.7. After GR/BS1 receives the DSC-RSP/MSHCSCH message from SS4, the new 802.16 L2 bandwidth and L3 data rate for the MH’s flow have been reserved. (d) Finally, the media stream can follow the new route from the CN to the MH via ER2, GR/BS1 and SS4 after the MH’s handoff with the guaranteed E2E QoS at step 2.8. As each packet of the media stream arrives at GR/BS1, the proposed CBQ + RED QoS adaptation algorithm will be executed at step 2.9 to check whether the associated traffic queue of this arriving packet has enough bandwidth to meet this packet’s requirements. Details of the proposed CBQ + RED QoS adaptation algorithm will be described in Section 3.4. 3.3. The proposed Inter-WiMAX handoff flow As shown in case 5 of Fig. 1, an Inter-WiMAX handoff occurs when the MH moves from the service range of AP2 under GR/ BS1 of the WiMAX mesh network to that of AP3 under GR/BS2 of the WiMAX PMP network. An Inter-WiMAX handoff consists Table 4 Simulation parameters used in the GR/BS. Parameters

Values

Total 802.16 bandwidth Flow data rate Number of total flows GS, CLSRT, CLSMM, CLSCM UGS, rtPS, nrtPS (Max_th, Min_th) of each queue

20 Mbps 600–950 Kbps 120 30%, 40%, 15%, 15% of the load 30%, 40%, 30% of the load (90%, 10%), (80%, 20%), (70%, 30%)

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Fig. 13. Average throughputs of five schemes for the UGS service type.

Fig. 14. Average throughputs of five schemes for the rtPS service type.

of the following three processing stages. As mentioned above, steps 0.1–0.5 in Fig. 6 for RSVP initialization in the E2E path must be executed first to start the media stream before the handoff. 3.3.1. L2 handoff and CXTP procedures (a) As soon as the received signal strength of the new 802.11 AP, i.e., AP3, is high enough to trigger the handoff, the MH exchanges six 802.11 L2 control messages, i.e., Probe-REQ, Probe-RSP, Authentication-REQ, Authentication-RSP, Reassociation-REQ and Reassociation-RSP, with AP3 to complete the L2 handoff process, as shown at steps 1.1–1.6 in Fig. 6. (b) Because the MH has changed its SS and associated IP subnet in this handoff scenario, it has to follow the L3 Dynamic Host Configuration Protocol (DHCP) procedure, as shown at steps 1.7–1.10, to acquire a new care-of address (CoA) from the new GR/BS, i.e., GR/BS2. In this paper, the GR/BS also acts as the foreign agent (FA) and the mobility anchor point (MAP) in the hierarchical mobile IPv6 (HMIPv6) protocol [24,25]. Then the MH sends an 802.16 L2 DSC-REQ/MSH-CSCH message to notify GR/BS2 of its current corresponding SS, i.e., SSa, and an L3 HMIPv6 Local Binding Update (LBU) message to GR/BS2 via SSa to update the local binding cache of GR/BS2 with the MH’s new CoA, as shown at steps 1.11 and 1.12. After these steps, the MH further executes the corresponding L3 CXTP procedures to exchange the MH’s current context and QoS parameters from the old GR/BS1 to the new GR/BS2 with three CXTP messages, i.e., Context Transfer

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Fig. 15. Average throughputs of five schemes for the nrtPS service type.

Fig. 16. Average throughputs of five schemes for the BE service type.

Activate Request (CTAR), CT Request and Context Transfer Data (CTD) at steps 1.13–1.15 in Fig. 6. This allows the MH to continue media streaming service after the handoff. Finally, GR/BS2 returns an HMIPv6 Local Binding ACK (LBA) message at step 1.16 to the MH to complete the L3 handoff process in the WiMAX network. 3.3.2. QoS adjustment and route re-configuration (a) As GR/BS2 has received the L2 DSC-RSP/MSH-CSCH and the L3 CXTP CTD messages, it confirms whether it can allocate enough WiMAX L2 bandwidth and the required E2E L3 data rate to fulfill the QoS requirement of this MH at step 2.1. Similar to the Intra-WiMAX handoff scenario, GR/BS2 also follows the cross-layer and E2E QoS adjustment algorithm in Fig. 5 to allocate the most appropriate L2 bandwidth, i.e., BW inew , and the L3 data rate, i.e., Rinew , for type i in the WiMAX PMP network to the MH after the handoff. (b) Then GR/BS2 adopts a COPS-SLS message to transmit new QoS parameters, i.e., Rinew , to the BB via ER3 on the Internet, as shown at step 2.2 in Fig. 6. As shown in Fig. 5, the BB compares the available L3 data rate, i.e., RiIntav , on the Internet with the allocated L3 data rate in the WiMAX PMP network to adjust the final E2E L3 data rate, i.e., Rinew , after the handoff. Afterward the BB creates a new QoS routing path between ER3 and the corresponding edge router, i.e., ER1, of the

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Fig. 17. Average packet loss ratios of five schemes for the UGS service type.

Fig. 18. Average packet loss ratios of five schemes for the rtPS service type.

CN. These operations are shown at steps 2.2 and 2.3. After GR/BS2 receives the COPS-SLS ACK, which contains the adjusted E2E L3 data rate, from the BB at step 2.4, it issues an HMIPv6 Global BU (GBU) message, including the IP address of GR/BS2 and these QoS parameters, to the CN to update its binding cache at step 2.5. This GBU message in turn triggers the CN to modify its resource usage and re-execute the E2E RSVP signaling process with the MH after the CN has sent the HMIPv6 Global Binding ACK (GBA) back to GR/BS2. 3.3.3. End-to-end RSVP signaling (a) At this stage, the CN first issues the RSVP PATH message with the E2E L3 data rate, i.e., Rinew , through the new QoS routing path on the Internet to ER3 and GR/BS2 at step 3.1. To reconstruct the local path and reserve local resources in the new WiMAX network, GR/BS2 converts the L3 RSVP PATH message and then issues the L2 MSH-CSCH or DSC_REQ message to SSa for the WiMAX mesh or PMP network respectively as GR/BS2 receives the RSVP PATH message from ER3. GR/BS2 in turn lets SSa convey a new L3 RSVP PATH message with the E2E L3 data rate to the MH. (b) After the MH accepts the RSVP PATH message, it replies to SSa with the RSVP RESV message. Consequently SSa issues the MSH-CSCH or DSC_RSP message to GR/BS2, which then sends the RSVP RESV message back to the CN to complete the E2E RSVP signaling process at step 3.2.

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(c) Finally, the media stream can follow the new E2E path from the CN to the MH via ER1, ER3, GR/BS2 and SSa to continue the service for the MH, which is shown at step 3.3. As each packet of the media stream arrives at GR/BS2, the proposed CBQ + RED QoS adaptation algorithm will perform the necessary QoS adaptation operations for both traffic and queue managements at step 3.4. 3.4. The CBQ + RED QoS adaptation algorithm In the traditional CBQ, a single queue is facilitated for each traffic class, which may contain multiple user connections of similar characteristics. Each class can lend its available network bandwidth to other classes if allowed in its initial configuration. On the other hand, the original RED provides the queue management mechanism for handling forthcoming congestion. It first marks or drops packets when they arrive at the queue and may notify the traffic source to reduce the emission rate to mitigate the congestion. By integrating the multi-queue management of RED and bandwidth sharing of CBQ, we propose a QoS adaptation algorithm, which is denoted as CBQ + RED in this paper. CBQ + RED is executed in GR/BS to solve the problems that occur when the associated traffic queue of the arriving packet does not have enough queue length to meet the MH’s QoS requirement as each packet arrives. Notations used in the proposed CBQ + RED QoS adaptation algorithm are listed in Table 3 and the complete flow of this algorithm is illustrated in Fig. 7. In this algorithm, the highest priority queue is allocated to traffic class 1. Whenever the GR/BS receives an arriving packet Si which belongs to class i, it will add the length of this packet, i.e., Arr_pkt(Si), to the average queue length, i.e., Avg(Qi), of class i. According to the RED algorithm, if the modified Avg(Qi) is larger than the maximal queue length threshold, i.e., Max_th(Qi), of class i, this arriving packet should be marked with the dropping probability PMax . Conversely, if Avg(Qi) is less than the minimal queue length threshold, i.e., Min_th(Qi), of class i, this arriving packet will be queued. Additionally, RED adopts Eq. (4) to assign a dropping probability, i.e., P Si , to an incoming packet. The value of P Si increases linearly from 0 at Avg(Qi) = Min_th(Qi) to the maximal value PMax at Avg(Qi) = Max_th(Qi). To reduce the dropping probability of each arriving packet, which in turn guarantees its E2E QoS, the CBQ + RED algorithm can dynamically adjust queue lengths among different traffic classes with the following two queue length adaptation approaches; one is the retrieval first and the other is the borrowing first. CBQ + RED uses an array, i.e., Array_L[k][i], to record the queue length lent from class i to class k. With the retrieval first approach, the GR/BS first executes the retrieval process. The flow of the retrieval first approach for class i is shown in Fig. 8. The GR/BS retrieves the lent queue length of class i from the queue of the next higher priority class k (k = i  1) if the average queue length, i.e., Avg(Qk), is smaller than the minimal queue length threshold, i.e., Min_th(Qk), of class k. Hence, the retrieved queue length from class k, i.e., R(Qk), is equal to the smaller value of Array_L[k][i] and the abundant queue length of class k. Here the abundant queue length of class k is defined as the maximal queue length, i.e., Min_th(Qk)  Avg(Qk), which can be lend to class i without increasing dropping probabilities of packets already in its queue. Queue parameters like the minimal queue length threshold, i.e., Min_th(Qk), the maximal queue length threshold, i.e., Max_th(Qk), and the maximal queue length, i.e., Max(Qk), of class k are modified as Min_th(Qk)  R(Qk), Max_th(Qk)  R(Qk) and Max(Qk)  R(Qk), respectively. Further, Min_th(Qi), Max_th(Qi) and Max(Qi) of class i are modified as Min_th(Qi) + R(Qk), Max_th(Qi) + R(Qk) and Max(Qi) + R(Qk), respectively. The GR/BS continues this retrieval process by advancing to class k  1 until Avg(Qi) of class i is not larger than Max_th(Qi) or the highest priority class, i.e., k = 1, has already returned the queue length borrowed from class i, as shown in Fig. 9. However, if retrieved queue lengths from all higher priority classes are not enough to let Avg(Qi) become smaller than Max_th(Qi), the GR/BS then executes the borrowing process, which flow is shown in Fig. 10, to borrow queue length from the lowest priority class j (j = M, where M is the number of traffic classes in the GR/BS). The borrowed queue length, i.e., B(Qj), from class j is equal to the smaller value of the insufficient queue length, i.e., Insuff(Qi), of class i and the abundant queue length, which is equal to Max_th(Qj)  Avg(Qj), of class j. Here the insufficient queue length, i.e., Insuff(Qi), of class i is defined as Max_th(Qi)  Avg(Qi). Thus, this borrowing process reduces dropping probabilities of packets already in the queue of class i at the cost of raising those of class j to the maximal value of 1 when class j has lent all abundant queue length to class i. At this point, Insuff(Qi), Array_L[k][i], Min_th(Qi), Max_th(Qi) and Max(Qi) of class i and Min_th(Qj), Max_th(Qj) and Max(Qj) of class j have to be modified accordingly. The GR/BS continues this borrowing process by advancing to class j  1 until Avg(Qi) of class i is not larger than its Max_th(Qi), i.e., Insuff(Qi) is equal to 0, or the next lower priority class, i.e., j = i + 1, has already lent its abundant queue length to class i, as shown in Fig. 11. Conversely, the borrowing first approach initially executes the borrowing process from lower priority classes and then the retrieval one from higher priority classes instead. Once the aforementioned two processes have been executed, the GR/BS afterward adopts Eq. (4) where P Max = 1 as the RED algorithm to re-calculate and re-mark the dropping probability, i.e., PSi , of this arriving packet Si. However, if Avg(Qi) of class i is still larger than its Max(Qi), each arriving packet at queue i will be dropped immediately.

PSi ¼ P Max 

Av gðQ i Þ  Min thðQ i Þ Max thðQ i Þ  Min thðQ i Þ

ð4Þ

As discussed above, GR/BS has to perform this CBQ + RED algorithm for each media packet issued from the CN to the MH in the WiMAX network. In the worst case scenario, all (n  1) priority queues except those to which the arriving packet belongs have to return or lend bandwidth to this packet’s queue in the CBQ + RED retrieval and borrowing processes, where n is the number of total traffic/service types in the WiMAX network. Hence, the time complexity of CBQ + RED is proportional to

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Fig. 19. Average packet loss ratios of five schemes for the nrtPS service type.

Fig. 20. Average packet loss ratios of five schemes for the BE service type.

n and the total packet arrival rate, which is equal to the product of the average data rate, i.e., r, of each stream destined to the MH, the average number of streams, i.e., s, per MH and the number of active MHs, i.e., m, in the WiMAX network controlled by a GR/BS. Because the aggregate traffic load, i.e., q, of all media streams in GR/BS is proportional to the total packet arrival rate, the time complexity of CBQ + RED can be further expressed as O(nq) = O(nrsm). 4. Performance evaluation 4.1. Simulation environment In this paper, we adopt the Internet and WiMAX interworking environment as shown in Fig. 12 to perform simulations that are extensions of our previous work [7,8]. In this environment, there are six 802.16 networks and each is controlled by a GR/BS, which is further connected to the Internet through a corresponding ER. To evaluate performances within a general interworking environment, four 802.16 base stations, i.e., GR/BS1, 2, 5 and 6, and two base stations, i.e., GR/BS3 and 4, execute 802.16 PMP and mesh modes respectively. As shown in Fig. 12, each GR/BS in the PMP mode simultaneously connects to

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Fig. 21. Average E2E delays of five schemes for the UGS service type.

Fig. 22. Average E2E delays of five schemes for the rtPS service type.

four underlying 802.16 SSs to form an 802.16 1  4 PMP network and each SS further connects to an 802.11 AP located at the center of one 802.11 wireless cell. In contrast, each GR/BS in the mesh mode only connects to an 802.16 SS, which is located at the upper left corner of the 802.16 4  4 mesh network. This 4  4 mesh network consists of total 16 SSs, which further connect to their corresponding 802.11 APs. As the simulation starts, 120 MHs are uniformly distributed over total 4  12 802.11 wireless cells in this environment and each MH associates with the nearest 802.11 AP. We assume the traditional one-to-one traffic mapping mechanism is used to convert five traffic classes, i.e., guaranteed service (GS), control load service real-time (CLSRT), control load service multimedia (CLSMM)/control load service control message (CLSCM) and best effort (BE), on the Internet to four 802.16 service types, i.e., UGS, rtPS, nrtPS and BE, in the WiMAX network, and vice versa. In the following section, we compare three performance metrics, i.e., average/aggregate throughputs, average packet loss ratios and average delays on E2E paths, of UGS, rtPS, nrtPS and BE service classes for five different QoS schemes adopted by the GR/BS under two handoff scenarios. The first handoff scenario is the Inter-WiMAX one, which allows the MH to move across the boundary of two adjacent 802.16 WiMAX networks which use different modes. For example, the MH can perform its handoff from an 802.11 AP under the 802.16 1  4 PMP network controlled by GR/BS2 to a neighboring 802.11 AP under the 802.16 4  4 mesh network controlled by GR/BS3, as shown in Fig. 12. Conversely, the second handoff scenario is the Intra-WiMAX one, which only allows the MH to move within the same 802.16 WiMAX network. According to the random

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Fig. 23. Average E2E delays of five schemes for the nrtPS service type.

Fig. 24. Average E2E delays of five schemes for the BE service type.

waypoint mode, each MH can move from its current wireless cell to one of its neighboring 802.11 wireless cells with equal probability at each time period during the simulation. The first two QoS schemes are combinations of the CBQ + RED QoS adaptation algorithm with or without the cross-layer and E2E QoS adjustment one, which are denoted as CBQ + RED with adjusted E2E QoS and CBQ + RED (without adjusted E2E QoS) respectively. Both CBR + RED schemes adopt the retrieval first approach by default, which allows the GR/BS to retrieve the lent queue lengths from higher priority types first and then borrow the abundant queue lengths from lower priority ones if needed, for the incoming packet when the packet reaches the GR/BS. The third scheme is VHTC with EBBM [11], which adapts QoS by allowing the lower priority queues borrow spare bandwidth from the higher priority ones only when the lower ones are full. Conversely, the higher priority queues are forbidden to borrow the bandwidth from lower ones in VHTC with EBBM. Additionally, VHTC with EBBM does not support the cross-layer E2E QoS adjustment. IQC [6] with two-layer scheduling, as mentioned in Section 2, is the fourth scheme. However, IQC does not provide both the QoS adaptation algorithm and cross-layer QoS adjustment one between the Internet and 802.16 WiMAX network as the CBQ + RED does. Lastly, PS_QS [21], which has introduced a packet scheduling algorithm in the GR/BS and an admission control policy in each SS for supporting IEEE 802.16 QoS, is chosen as the fifth scheme. However, PS_QS supports neither the cross-layer E2E QoS adjustment algorithm nor the QoS adaptation one. This weakness means that PS_QS exhibits the worst performance among these five schemes.

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Fig. 25. Aggregate throughputs of five schemes over all service types.

Fig. 26. Average packet loss ratios of five schemes over all service types.

Simulation parameters used in the GR/BS are listed in Table 4. As the traffic load of the GR/BS is q, queue lengths allocated to all three high priority types are equal to m  q, where m denotes total queue lengths of the GR/BS. All five QoS schemes initially allocate fixed 30%, 40% and 30% of total allocated queue lengths m  q to UGS, rtPS and nrtPS service types respectively. At most, the BE type consumes the remaining queue length, i.e., m  (1  q), if needed. Three (Max_th, Min_th) combinations of each queue are used to compare their effects on three performance metrics, where the (90%, 10%) of the queue length is the default value. In this simulation, each MH generates a traffic flow, which belongs to one of four traffic types, to exchange Constant Bit Rate (CBR) data with a CN on the Internet. All flows follow the possion arrival process with average data rates ranging from 600 Kbps to 950 Kbps. 4.2. Simulation results In the following section, simulation results of the Inter-WiMAX handoff scenario are shown first. Then, those of the Intra-WiMAX handoff are explained.

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Fig. 27. Average E2E delays of five schemes over all service types.

Fig. 28. Average throughputs for the UGS service type with three (max_th, min_th) combinations.

4.2.1. Scenario 1: the Inter-WiMAX handoff Average throughputs of the UGS, rtPS, nrtPS and BE service types versus the traffic load q of the GR/BS for the Inter-WiMAX handoff scenario are shown in Figs. 13–16, respectively. Specific performance behavior can be delineated from this data. First, as the traffic load increases, average throughputs of the UGS, rtPS and nrtPS types under these five QoS schemes increase accordingly. As mentioned above, VHTC with EBBM only allows the borrowing of spare bandwidth from the higher priority queues and both IQC and PS_QS do not have the QoS adaptation mechanism. However, the two CBQ + RED schemes can dynamically adapt queue lengths among the four service types using the retrieval first approach. Hence, average throughputs of the UGS, rtPS and nrtPS types achieved by these two CBQ + RED schemes are higher than those of VHTC with EBBM, IQC and PS_QS, as shown in Figs. 13–15 respectively. Conversely, as the traffic load q is not higher than 0.8, average throughputs of the BE type achieved by IQC and PS_QS, which both allocate fixed queue lengths for all classes, are higher than those of two CBQ + RED schemes and VHTC with EBBM, as shown in Fig. 16. This is because the remaining queue length, i.e., m  (1  q), is enough for the lowest priority BE type under IQC and PS_QS. However, as the traffic load q rises higher than 0.8, the remaining queue length decreases accordingly such that the BE types of IQC and PS_QS suffer from even lower average throughputs than those of two CBQ + RED schemes and VHTC with EBBM. In summary, the proposed CBQ + RED scheme achieves larger average throughputs for higher priority types at the cost of smaller throughputs for lower priority

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Fig. 29. Average throughputs for the rtPS service type with three (max_th, min_th) combinations.

Fig. 30. Average throughputs for the nrtPS service type with three (max_th, min_th) combinations.

ones. As mentioned above, CBQ + RED, VHTC with EBBM, IQC and PS_QS are schemes that do not use the adjusted E2E QoS mechanism. None of these consider the influence of the varied hop count, which is between the GR/BS and the associated SS of the MH in the 802.16 mesh network, on allocation of the total 802.16 L2 bandwidth and the Internet L3 data rate after the handoff. Therefore, CBQ + RED with adjusted E2E QoS achieves the largest improvement on average throughputs for the highest priority type UGS among all five QoS schemes, as shown in Fig. 13. In contrast, CBQ + RED with adjusted E2E QoS has smaller improvements on average throughputs of lower priority types over the other four schemes, as shown in Figs. 14– 16. In terms of the average throughput, CBQ + RED with adjusted E2E QoS has the strongest capability among these five QoS schemes of allocating resources to higher priority types in preference to lower priority ones. Average packet loss ratios of all service types versus the traffic load q of the GR/BS under these five QoS schemes are shown in Figs. 17–20 respectively. As the traffic load increases, average packet loss ratios of three higher priority types achieved by two CBQ + RED schemes and VHTC with EBBM, which supports QoS adaptation among different traffic classes, rise accordingly. In contrast, those of IQC and PS_QS remain relatively stable because neither of these supports the QoS adaptation mechanism. Hence, average packet loss ratios of two CBQ + RED schemes and VHTC with EBBM are lower than those of IQC and PS_QS, even when the traffic load approaches 0.95. It should be noted that VHTC with EBBM suffers higher packet loss ratios than the two CBQ + RED schemes, because VHTC with EBBM only allows the lower priority types to borrow spare

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Fig. 31. Average throughputs for the BE service type with three (max_th, min_th) combinations.

Fig. 32. Aggregate throughputs over all service types with three (max_th, min_th) combinations.

bandwidth from the higher priority ones, whereas both the CBQ + RED schemes can retrieve the lent queue length from higher priority types and borrow abundant queue length from lower priority ones. Further, with the adjusted E2E QoS mechanism to consider the influence of varied hop counts, in the 802.16 networks before and after the handoff, on allocation of the total 802.16 L2 bandwidth and the Internet L3 data rate, CBQ + RED with adjusted E2E QoS achieves lower average packet loss ratios than those of the other four schemes without adjusted E2E QoS. Higher priority types achieve lower average packet loss ratios within CBQ + RED with adjusted E2E QoS. These results are shown in Figs. 17–20. Figs. 21–24 show average E2E delays of the UGS, rtPS, nrtPS and BE service types versus the traffic load q of the GR/BS respectively. As the traffic load increases, average E2E delays of all four service types achieved by five QoS schemes increase accordingly. Because two CBQ + RED schemes perform the proposed QoS adaptation algorithm with the retrieval and borrowing processes to adjust queue lengths among four service types, they both achieve considerably lower average E2E delays for higher priority types. By considering the old and new hop counts in the 802.16 networks where the MH is located before and after the handoff, the CBQ + RED scheme with adjusted E2E QoS supports the cross-layer and E2E QoS adjustment algorithm to allocate the most appropriate 802.16 L2 bandwidth and the most appropriate Internet L3 data rate to the MH. This E2E QoS adjustment algorithm is especially effective in reducing the total traffic load of the Internet backbone when the MH

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Fig. 33. Average throughputs for the UGS service type with the retrieval first and the borrowing first approaches.

Fig. 34. Average throughputs for the rtPS service type with the retrieval first and the borrowing first approaches.

executes the Inter-WiMAX handoff between two neighboring WiMAX networks of different modes, i.e., from the PMP network to the mesh one or from the mesh network to the PMP one. Hence, the CBQ + RED scheme with adjusted E2E QoS has the lowest average E2E delays for all service types among these five QoS schemes. On the other hand, because VHTC with EBBM only allows the lower priority types to borrow spare bandwidth from the higher priority ones, it has higher average E2E delays for all service types than two CBQ + RED schemes. For both IQC and PS_QS schemes, which support neither the adjusted E2E QoS mechanism nor the QoS adaptation one, IQC outperforms PS_QS in terms of average E2E delays for all service types due to the two-layer packet scheduling adopted by IQC. Finally, PS_QS suffers the highest average E2E delays for all service types among these five QoS schemes. Here we compare aggregate throughputs, average packet loss ratios and average delays on E2E paths of all four service types under the five QoS schemes. Please note that the aggregate throughput of each QoS scheme is defined as the sum of the average throughput of each service type but the average packet loss ratio and average delay of each scheme are averaged over all service types when this QoS scheme is adopted. As shown in Figs. 25–27 respectively, aggregate throughputs, average packet loss ratios and average delays of these five schemes rise as the traffic load grows to 0.8. However, as the traffic load q rises higher than 0.8, the BE types of IQC and PS_QS suffer from even lower average throughputs, higher average packet loss ratios and larger average delays than those of the other three QoS schemes, as shown in Figs. 16, 20 and 24

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Fig. 35. Average throughputs for the nrtPS service type with the retrieval first and the borrowing first approaches.

Fig. 36. Average throughputs for the BE service type with the retrieval first and the borrowing first approaches.

respectively. Hence, IQC and PS_QS experience lower aggregate throughputs, higher average packet loss ratios and higher average delays on E2E paths when q P 0.8. Conversely, because of the queue length retrieval and borrowing processes in two CBQ + RED schemes, aggregate throughputs achieved by both of them are higher than those of IQC and PS_QS. But average packet loss ratios and delays achieved by both are lower than those of IQC and PS_QS, especially when the traffic load is larger than 0.8. Additionally, aggregate throughputs, average packet loss ratios and average delays of VHTC with EBBM are middle values among those of all these schemes. Finally, aggregate throughputs, average packet loss ratios and average delays achieved by CBQ + RED with adjusted E2E QoS are better than those of the other four schemes without adjusted E2E QoS. In terms of results discussed above, both of our proposed CBQ + RED QoS adaptation and adjustment algorithms significantly improve average/aggregate throughputs. They also reduce average packet loss ratios and average delays, especially for higher priority service types. We further observe influences of two CBQ + RED parameters on one of three performance metrics, i.e., the average/aggregate throughput. The first parameter consists of values for the maximal and minimal queue length thresholds, i.e., Max_th and Min_th, of each traffic queue. The second parameter is the execution sequence of the retrieval and borrowing processes. The other two metrics, i.e., average packet loss ratio and delay, exhibit similar results. First, as shown in Figs. 28–31, we use three combinations of Min_th and Max_th, i.e., (10%, 90%), (20%, 80%) and (30%, 70%) of the maximal queue length, and the

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Fig. 37. Aggregate throughputs over all service types with the retrieval first and the borrowing first approaches.

Fig. 38. Aggregate throughputs of five schemes over all service types for the Intra-PMP handoff.

default retrieval first approach in the proposed CBQ + RED scheme with adjusted E2E QoS to execute simulations. As the traffic load increases, average throughputs of all four service types under these three (Min_th, Max_th) combinations rise accordingly. With the larger Max_th value, the lower priority service type j has more abundant queue lengths, which are equal to Max_th(Qj)  Avg(Qj) as shown in Fig. 10. These abundant queue lengths can be borrowed by higher priority queues, which results in the average throughputs of UGS and rtPS, achieved by the (10%, 90%) combination, surpassing those of (20%, 80%) and (30%, 70%) combinations, as shown in Figs. 28 and 29 respectively. However, lower priority service types like nrtPS and BE have the largest average throughputs with the (30%, 70%) combination, as shown in Figs. 30 and 31 respectively. On the other hand, with the lower Min_th value, the lower priority type i can theoretically retrieve more lent queue lengths, which are equal to min(Min_th(Qk)  Avg(Qk), Array_L[k][i]) in Fig. 8, from the higher priority type k. However, this retrieval process of the lower priority type rarely occurs because the average queue length of the higher priority type k, i.e., Avg(Qk), has a high probability to be larger than its Min_th(Qk) under the middle or heavy load (q = 0.6) of this simulation. Finally, aggregate throughputs of all four service types with the (10%, 90%) combination are the largest ones among the three combinations, as shown in Fig. 32.

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Fig. 39. Average packet loss ratios of five schemes over all service types for the Intra-PMP handoff.

Fig. 40. Average E2E delays of five schemes over all service types for the Intra-PMP handoff.

Finally, we compare the simulation results of the retrieval first and the borrowing first approaches with the default (10%, 90%) combination in our proposed CBQ + RED scheme with adjusted E2E QoS. As shown in Figs. 33–37, average throughputs of these four service types and the aggregate throughput achieved by these two approaches rise accordingly as the traffic load grows. With the borrowing first approach as mentioned above, the CBQ + RED scheme first borrows the available queue lengths from lower priority types and then retrieves lent queue lengths from higher priority types if needed. Hence, we can observe that average throughputs of higher priority service types, i.e., UGS and rtPS, with the borrowing first approach are larger than those with the retrieval first one as shown in Figs. 33 and 34 respectively. Conversely, average throughputs of lower priority service types, i.e., nrtPS and BE, using the borrowing first approach are smaller, as shown in Figs. 35 and 36 respectively. Thus, the borrowing first approach prefers high priority types but lacks resilience for high traffic bursts. In contrast, the retrieval first approach compromises queue length distributions among all types such that it achieves slightly higher aggregate throughputs than those of the borrowing first approach, as shown in Fig. 37, especially when the traffic load, i.e., from 0.6 to 0.95, in this simulation are heavy. 4.2.1.1. Scenario 2: the Intra-WiMAX handoff. In the Intra-WiMAX handoff scenario, 20 MHs are randomly distributed over a single PMP or mesh WiMAX network and are confined to move within the same 802.16 network as shown in Fig. 12. Thus,

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Fig. 41. Aggregate throughputs of five schemes over all service types for the Intra-Mesh handoff.

Fig. 42. Average packet loss ratios of five schemes over all service types for the Intra-Mesh handoff.

the Intra-WiMAX handoff can be further classified into two subtypes, i.e., intra-PMP and intra-mesh handoffs. In the following section, values of three performance metrics over all four service types are presented to evaluate the effectiveness of the five QoS schemes for intra-PMP and intra-mesh handoffs. Due to space limitation of this paper, average values of three performance metrics for each service type are omitted. In the intra-PMP handoff, e.g., cases 3 and 4 in Fig. 1, it does not matter whether the E2E QoS adjustment mechanism is adopted, both CBQ + RED schemes achieve the same aggregate throughputs, average packet loss ratios and average delays over all service types on E2E paths as shown in Figs. 38–40 respectively. This is because the GR/BS allocates the same E2E L3 data rate, i.e., Riold = Rinew , to the MH, which in turn allocates the same L2 downlink bandwidth, i.e., BW iold = BW inew , to the MH before and after the intra-PMP handoff, as discussed in Section 3.2 and shown in Table 2. Conversely, for the intra-mesh handoff, e.g., cases 1 and 2 in Fig. 1, the routes and corresponding hop counts between the GR/BS and the associated SS of the MH before and after the intra-mesh handoff may change, which therefore affects the corresponding E2E L3 data rate and 802.16 L2 bandwidth allocated to the MH after the intra-mesh handoff. Hence, the CBQ + RED scheme with adjusted E2E QoS achieves better aggregate throughputs, lower average packet loss ratios and lower average delays over all service types on E2E paths than those of the other four schemes without adjusted E2E QoS, as shown in Figs. 41–43 respectively. Because

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Fig. 43. Average E2E delays of five schemes over all service types for the Intra-Mesh handoff.

there is only one physical wireless link in the mesh network, a longer transmission path implies that a packet goes through the link many times and results in an increasing consumption of link capacity [8]. If the average hop count is 4, a path in the mesh network needs nearly four times the link capacity consumed in the PMP network. Hence, traffic loads of these five schemes in the mesh network have almost become saturated when q = 0.6. Consequently, aggregate throughputs of five schemes over all service types, which are shown in Fig. 41, increase slowly for the intra-mesh handoff. Moreover, performance improvements of the CBQ + RED scheme with adjusted E2E QoS over the CBQ + RED scheme for the intra-mesh handoff of the Intra-WiMAX handoff scenario are less significant than those for the Inter-WiMAX handoff scenario, as shown in Figs. 42 and 26 for the average packet loss ratios and Figs. 43 and 27 for the average E2E delays respectively. This is because no handoff occurred between the neighboring PMP and mesh networks in the Intra-WiMAX handoff scenario. Without this handoff, the ability of the E2E QoS adjustment algorithm to reduce the total traffic load of the Internet backbone is reduced. 5. Conclusion In this paper, we have first revised the cross-layer E2E QoS guarantee framework over the integrated Internet, 802.16 WiMAX PMP and mesh networks for supporting MHs handoff. This designed framework employs the CXTP for a fast and seamless handoff, the COPS-SLS as the coordination protocol between the Internet and the WiMAX networks, the cross-layer and one-to-one traffic mapping mechanism to reduce overheads for converting diverse QoS types between L2 and L3, and the receiver-oriented RSVP as the E2E signaling protocol to continue the ongoing media streaming service after the handoff. Furthermore, analysis of this cross-layer framework has produced several new findings that can contribute the body of literature. We have proposed a cross-layer and E2E QoS adjustment algorithm to dynamically adjust allocated L2 bandwidth and L3 data rates between the Internet and the WiMAX networks. The proposed CBQ + RED scheme provides efficient queue and traffic management by dynamically adapting queue lengths among different service types when the new GR/BS after the MH’s handoff does not have enough resources to meet the MH’s QoS requirement. We have also proposed the E2E QoS flows to handle Intra-WiMAX and Inter-WiMAX handoffs for both WiMAX PMP and mesh networks. Simulations results obtained from five QoS schemes, three combinations of queue length thresholds, the retrieval or borrowing first approaches, and two handoff scenarios suggest that this framework with the E2E QoS adjustment and CBQ + RED QoS adaptation algorithms can support higher average throughputs, lower packet loss ratios and smaller delays for higher priority service types. These in turn improve aggregate throughputs, reduce packet loss ratios and reduce delays over all service types. Consequently, this cross-layer framework with proposed QoS adaptation and adjustment algorithms can achieve improved E2E QoS guarantees for Intra-WiMAX and Inter-WiMAX handoffs when both WiMAX PMP and mesh networks interwork with the Internet. References [1] IEEE Std 802.16-2009. IEEE standard for local and metropolitan area networks. Part 16: Air interface for broadband wireless access systems; 2009. [2] Podlesny M, Gorinsky S. Leveraging the rate-delay trade-off for service differentiation in multi-provider networks. IEEE J Sel Area Commun 2011;29(5):997–1008. [3] Leu J-S, Lin C-K. On utilization efficiency of backbone bandwidth for a heterogeneous wireless network operator. Wirel Net (WINET) 2011;17(7):1595–604 [ACM/Springer].

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