source optimizing and high-performance solution for business environments and private ... application (i.e. Android App) running on mobile phones that manages ..... UMTS interfaces (i.e., ISDN cards) involve higher costs than. VoIP interfaces.
23rd IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC 2012)
Cloud Voice Service for Seamless Roaming in Heterogeneous Networks Thang Tran, Maike Kuhnert and Christian Wietfeld Communication Networks Institute (CNI) Faculty of Electrical Engineering and Information Technology TU Dortmund University, Germany Email: {Thang.Tran, Maike.Kuhnert, Christian.Wietfeld}@tu-dortmund.de
Abstract—Today’s heterogeneous networks are still in growing progress with the current introduction of Long Term Evolution. Despite the given infrastructure, hardly any service provider offers seamless voice call services due to the high integration cost. For addressing this issue, we introduce a feasible Virtual Room based Vertical Handoff (VOOH) cloud solution that only applies latest existing wireless access technologies (UMTS, LTE, WiFi) for connectivity purposes. Therefore, VOOH does not require any changes in the underlying network layers that result in low integration effort. Moreover, this solution uses the circuit-switched instead of packet-switched channel of UMTS to guarantee QoS for calls while handing over seamlessly from VoIP (via LTE or WiFi) to UMTS. Besides introducing the VOOH concept and its communication procedure, we present an analytical three-dimensional Markov model for performance evaluation that is validated by our developed event-discrete simulation model. Based on this model, a scalable and costeffective design of VOOH as cloud service for specific use case examples and the obtained findings are discussed. The results presented in this paper illustrate that an enhanced VOOH (eVOOH) solution improves the VOOH performance up to 56% and thus allows to further minimize economic costs.
I. I NTRODUCTION A. Motivation The IT landscape consisting of heterogeneous networks is in constant growth that is clearly visible by the ongoing introduction of the 4G mobile communication technology Long Term Evolution (LTE). Despite the given infrastructure, hardly any service provider offers seamless voice services across these networks (UMTS/HSPA, LTE, WiFi) because of the high technical and economic effort [1]. Although latest smartphones enable circuit-switched and Voice over IP (VoIP) calls, seamless handoff between these both technologies for uninterruptible voice call is not deployed. At the present time, circuit-switched based mobile telephony plays a dominating role, but it is still relatively expensive for private consumers in comparison to the rising popularity of VoIP [2]. Unlike circuit-switched calls, most commercial network operators forbid VoIP via the packet-switched channel of 3G UMTS, whereby the current 3G UMTS/HSPA does not satisfy the Quality of Service (QoS) for VoIP calls [3]. For improving QoS of VoIP, the upcoming LTE standard presents a promising option. However, it remains uncertain if network operators will support mobile telephony via VoIP over LTE in
the future. Apart from that, it is assumed that circuit-switched and VoIP calls will coexist in the foreseeable future. Due to this given trend and to benefit at an earlier stage from seamless voice calls, service providers require a costeffective and scalable Over-the-Top (OTT) [4] cloud solution that only uses today’s existing wireless access technologies (UMTS/HSPA, LTE, WiFi) for connectivity purposes without necessary changes in the underlying network layers. In particular, the availability and accessibility of cloud voice services can be increased by the today’s heterogeneous networks that will have more attracted attention for cloud services as a resource optimizing and high-performance solution for business environments and private customers [5]. B. State-of-the-art Analysis There exist network operator dependent solutions based on, e.g., the Unlicensed Mobile Access (UMA) [6] standard, IP Multimedia Subsystem (IMS) [7] [8] [9], and SIP Session Border Controller (SBC) [10]. For enabling seamless voice call continuity between UMTS and WiFi, these approaches require changes in the network infrastructure as well as on client side. In terms of handoff between VoIP via LTE and circuitswitched calls, approaches like Voice over LTE via Generic Access (VoLGA) [11], Circuit Switched Fall Back (CSFB) [1] and Single Radio Voice Call Continuity (SRVCC) [12] have been proposed. However, using these solutions is related to high technical and economic cost. For instance, SRVCC needs significant changes of operator’s legacy core that prohibit the immediate use of seamless voice calls in the today’s heterogeneous network environment. In [13], the authors suggested a network operator independent approach based on PBX (Private Branch Exchange) gateways to support seamless handoff calls between WiFi and UMTS. For this purpose, the Call Forwarding Function of ISDN standard has to be modified. Consequently, the compliance of the standard is not guaranteed any more. To benefit earlier from seamless cloud voice services, we presented in [14] and [15] a cost-effective and feasible Virtual Room based Vertical Handoff (VOOH) solution implemented as prototype for uninterruptible voice call continuity. One of the major strengths of this solution is that VOOH only applies today’s existing heterogeneous networks (UMTS, LTE,
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WiFi) for connectivity purposes. This means that the network operator independent VOOH service only rides on top of the current networks (i.e., UMTS/HSPA, LTE, 802.11g) without any required modifications in the underlying network layers. Therefore, this results in low integration costs unlike the previously presented solutions (e.g., SRVCC). Another additional advantage is that VOOH uses the circuit-switched instead of packet-switched channel of UMTS for calls while handing over seamlessly from VoIP (via LTE or WiFi) to UMTS. This has the nice property that QoS for calls can be guaranteed. Since VOOH is deployed as a cloud voice service, high availability among other things plays an important role to reach high user acceptance. Therefore, the main contribution of this paper is the development of an analytical three-dimensional Markov chain and an event-discrete simulation model used for analyzing the scalable design of VOOH. With the help of these models, the cost-effective resource allocation of VOOH for dynamic traffic loads can be determined to fulfill specific requirements (i.e., call blocking probability as QoS). The rest of the paper is structured as follows: In Section II, the basic concept including the communication procedure of VOOH is introduced that is needed to understand the later on presented analytical three-dimensional Markov and simulation model. Following this, we describe the analytical Markov model from VOOH server’s point of view and an event-discrete simulation model where the latter is used to validate the analytical model (Section III). Based on the computationally more efficient simulation model, the results for cost-effective VOOH design as a cloud service are discussed for three different use case examples, followed by a conclusion in Section IV. II. M OBILE VOOH C LOUD S ERVICE FOR S EAMLESS VOICE C ALL C ONTINUITY This section briefly describes the essential concept of VOOH for ensuring seamless handoff calls. These basics are required for understanding the analytical three-dimensional Markov and simulation models which are presented in Section III. For detailed information about the components of VOOH and its services, we refer to [14]. A. VOOH System Design as a Cloud Service Figure 1 illustrates the basic components and communication channels of networks used by VOOH. The software based VOOH solution consists of a VOOH application and a VOOH cloud server. On client side, communication partners have a VOOH application (i.e. Android App) running on mobile phones that manages circuit-switched (CS) calls via UMTS and VoIP calls via LTE/WiFi. All signaling procedures between the VOOH client and server are handled by our developed Overlay Hybrid Signaling Protocol (OHSP) [14]. In case of handoffs, the client always initiates the handoff processes using the protocol OHSP. The VOOH server includes two different kinds of interfaces (ISDN, Ethernet) to establish circuit-switched calls via ISDN and VoIP calls over an IP based connection of a corresponding
UMTS Bob with VOOH Application
CS: Circuit-Switched PS: Packet-Switched
Only for calls CS-UMTS
PSTN
VOOH Cloud Server
PS-UMTS Alice
LTE Not used for calls, only VOOH signaling IP based Network
WiFi
VOOH signaling and calls via LTE/WiFi
Fig. 1. System Design of VOOH with Overlay Signaling and Voice Channels
Internet Service Provider (ISP), whereas the VOOH client receives VoIP calls via LTE or WiFi (see Fig. 1). In our concept, the server plays a proxy role between two communication partners where all calls and signaling processes have to pass through. This does not apply if both communication partners are directly reachable via UMTS. For enabling direct calls, VOOH clients periodically notify their available wireless connections (UMTS, LTE, WiFi) to the server via OHSP. The server will then forward this information to authorized users needed to create direct UMTS calls. This feature with direct calls is referred to as enhanced VOOH (eVOOH). As shown in Figure 1, all signaling processes are transmitted either via LTE/WiFi or the packet-switched (PS) channel of UMTS, whereas calls are carried out over LTE/WiFi or the circuit-switched channel of UMTS. While handing over from VoIP (via LTE or WiFi) to UMTS, using CS channel of 3G UMTS has the advantage that QoS for calls can be guaranteed in contrast to VoIP via the PS of 3G UMTS. In terms of realizing cloud services, the network operator independent VOOH solution is deployable as Software as a Service (SaaS) providing an application for seamless voice calls. This requires a local VOOH client running on smartphones. In addition, the VOOH server can also be realized as Platform as a Service (PaaS) that offers a various number of interfaces for ensuring seamless voice call continuity. Due to the assuming scalable structure of PaaS, this allows dynamic handling of interfaces to fulfill specific requirements (i.e., call blocking probability as QoS), which is cost-effective for service providers as well as for end users. B. Communication Procedure of VOOH using OHSP Figure 2 depicts the essential communication procedures for a call establishment and handoff initiation (i.e., UMTS to VoIP via LTE). For further detailed information about the VOOH processes, we refer to [14]. It is assumed that Bob has no LTE/WiFi access, whereas Alice is only available via WiFi. In order to contact Alice, Bob’s VOOH App sends a CALL message included with Alice’s alias via the packet-switched channel of UMTS to the VOOH cloud server (see message 1 in Fig. 2). Following this, the VOOH server creates a virtual call room with a call number (CN) and answers with the VCR CREATED message
VOOH Cloud Server
Bob
z
Alice
VoIP via WiFi available
Only UMTS available
S(0,0,2)
mλ
Virtual Call Room created (2)
2μ
S(0,0,1)
2HOV
VoIP via LTE available *HANDOFF_INIT (7)
Virtual Call Room
HOU uλ * Message types of VOOH
S(1,0,0)
t
t
t
Fig. 2. VOOH Communication Process Sequence Example for Vertical Handoff using UMTS/HSPA, LTE, and WiFi
x
vλ
mλ
μ
S(0,0,0)
HOV
2HOU
vλ
μ
μ
μ
μ S(1,1,0)
S(0,1,0)
uλ
y
Fig. 3. Three-Dimensional Markov Chain Example from VOOH Server’s Point of View for two VoIP and two UMTS Interfaces
containing the call number (see messages 2 & 3 in Fig. 2). For enabling uninterruptible voice call continuity, the client needs the call number to make timely circuit-switched calls via UMTS or VoIP calls via LTE/WiFi to the virtual call room. In parallel to the previous process, the VOOH server reads out Alice’s contact data from a secured local database using the alias that has been sent by Bob. After identifying Alice’s data, the server calls Alice (i.e., VoIP call via WiFi) (see message 4 in Fig. 2). A successful call between Bob and Alice is created if both parties are entered into the virtual call room (see messages 5 & 6 in Fig. 2). In case of a required handoff for ensuring seamless call, it is assumed that the VOOH client of Bob initiates a handoff from UMTS to LTE. This process is locally announced by the HANDOFF INIT message. In that case, the client applies the specific call number (CN) to establish a second call to the VOOH server. This means we have two parallel connections (circuit-switched and VoIP) to the virtual call room for a short time to guarantee seamless handoff. The old connection (i.e., UMTS) is kept alive until the new connection (i.e., VoIP via LTE) is successfully created to the virtual call room (see messages 7 & 8 in Fig. 2). III. P ERFORMANCE E VALUATION This section presents an analytical three-dimensional Markov chain and an event-discrete simulation model used for a scalable system design of the VOOH server. Afterwards, the performance of VOOH and its enhancement with direct UMTS calls (eVOOH) are evaluated and discussed for three different use case examples (i.e., Low Cost, Standard, Business). For performance evaluation, we choose the call blocking probability as key performance indicator (KPI). In the following, ISDN interfaces of the VOOH server are referred to as UMTS interfaces and Ethernet is a synonym for a specific number of VoIP interfaces.
A. Analytical Three-Dimensional Markov Model For determining the long-term distribution of occupancy states, a transition state diagram from VOOH server’s viewpoint is modeled. For this analysis, we assume that the call arrival rate 𝜆 is Poisson distributed [16]. In our model, 𝜆 consists of percentages of UMTS (u), VoIP (v), and mixed (m) calls: 𝜆=𝑢⋅𝜆+𝑣⋅𝜆+𝑚⋅𝜆
(1)
where 𝑢 + 𝑣 + 𝑚 = 1. Furthermore, 𝜇 describes the exponential mean service time representing the call duration. It is assumed that the call duration is regardless of call types. Figure 3 shows a transition state diagram example of VOOH with two UMTS and two VoIP interfaces (𝐼𝑈 = 2, 𝐼𝑉 = 2). From VOOH server’s point of view, VOOH handles three types of incoming calls: ∙ UMTS calls: pure circuit-switched calls via UMTS. ∙ VoIP calls: pure VoIP calls via LTE and/or WiFi. ∙ Mixed calls: calling party only reachable via circuitswitched of UMTS and called party only available via VoIP, and vice versa. For instance, VoIP calls describe pure VoIP calls between two communication partners that are connected via LTE and/or WiFi. A state 𝑆(𝑈, 𝑉, 𝑀 ) of the three-dimensional VOOH Markov chain includes three different numbers of calls: ∙ U := Number of circuit-switched calls via UMTS ∙ V := Number of VoIP calls ∙ M := Number of mixed calls (circuit-switched ↔ VoIP) In terms of resource reservation, a UMTS and a VoIP call always occupies two interfaces of its corresponding type (i.e., UMTS, VoIP), whereas a mixed call requires one UMTS and one VoIP interface. State transitions from pure calls to mixed calls and vice versa represent vertical handoffs. This means
z S(0,0,M) S(0,0,M-1) S(0,0,M-2) S(1,0,M-2) S(1,0,M-3) S(0,0,M-4) S(2,0,M-4) S(2,0,M-5)
S(0,V-U,M) S(0,V-U,M-1) S(0,V-U+1,M-2) S(0,V-U+1,M-3) S(1,V-U+1,M-2) S(1,V-U+1,M-3) S(0,V-U+2,M-4) S(0,V-U+2,M-5) S(2,V-U+2,M-4) S(2,V-U+2,M-5) S(0,V-1,2)
S(0,0,2)
S(0,V-1,1) S(U-1,0,2)
S(U-1,V-1,2)
S(U-1,0,1)
S(U-1,V-1,1) S(0,0,0)
y
S(0,V,0)
S(U,0,0)
S(U,V,0)
x Fig. 4. Staircase Shape of the three-dimensional Markov Chain of VOOH resulting from V>U
that handoffs are characterized either by closure of mixed calls or establishment of new mixed calls. As shown in Figure 3, 𝐻𝑂𝑈 describes the exponential transition rate for handoffs to UMTS calls and 𝐻𝑂𝑉 for handoffs to VoIP calls. Considering Figure 3, the transitions to UMTS calls are plotted on x-axis, whereas the transitions to VoIP calls are located on y-axis and the transitions to mixed calls on z-axis. Based on the Markov chain example depicted in this figure, a maximum of one VoIP and one UMTS call can be handled simultaneously. The maximum number of possible mixed calls is calculated as the minimum of VoIP and UMTS interfaces: 𝑀 𝑖𝑛(𝐼𝑈 , 𝐼𝑉 ). In this example, two mixed calls are possible. As illustrated in Figure 3, S(0,0,0) presents the starting state without any active calls. With a given transition rate of 𝑢 ⋅ 𝜆, state S(0,0,0) changes to S(1,0,0) indicating a new UMTS call that occupies two UMTS interfaces. Transitions back to state S(0,0,0) occur with a probability of 𝜇 if the VOOH system is located in one of the states like S(1,0,0), S(0,1,0) or S(0,0,1), where corresponding calls are finished. State transitions, e.g., from S(0,0,1) to S(1,0,0) happen if the calling party loses its VoIP connection via LTE/WiFi with a transition rate of 𝐻𝑂𝑈 . This causes that both parties create a new UMTS call to the VOOH server to continue their call. Unlike the switch from state S(0,0,1) to S(0,1,0) with a transition rate of 𝐻𝑂𝑉 , the reverse transition from S(0,1,0) to S(0,0,1) is described by the transition rate of 2𝐻𝑂𝑈 since one of two interfaces in state S(0,1,0) will be released, which results in a double transition rate. This reason is also given for the state change from S(1,0,0) to S(0,0,1) with a transition rate of 2𝐻𝑂𝑉 . Using VOOH Markov model allows to compute the call
blocking probabilities for three different call types (UMTS, VoIP, Mix). To determine the call blocking probability for a specific call type, all states have to be considered in which a call is rejected due to the insufficient available resources of the VOOH server. For instance, the blocking probability for UMTS calls is expressed by the sum of all state probabilities 𝑃 𝑟 that prohibit a successful call. These states are S(1,0,0), S(1,1,0), S(0,0,2) and S(0,0,1). In case of S(0,0,1), a pure UMTS call cannot be created since only one UMTS interface is available. Figure 4 shows a general Markov model of VOOH with its three dimensions (U,V,M) for the case of V>U. This case is considered since the number of VoIP calls is only affected by the provided bandwidth of the Internet connection and the server performance. Therefore, the number of VoIP calls is higher compared to the number of circuit-switched calls. Furthermore, it is expected that the physical installations of UMTS interfaces (i.e., ISDN cards) involve higher costs than VoIP interfaces. As evident from this figure, the staircase shape is the characteristic appearance of this VOOH system. This is caused by the use of mixed calls that apply two types of resources (i.e., UMTS, VoIP) simultaneously. Moreover, an increasing number of mixed conversations decreases the number of possible pure UMTS and VoIP calls, which is also the reason for this special shape. Derived from the Markov model illustrated in Figure 4, the blocking probabilities for VoIP (𝑃𝑏,𝑣𝑜𝑖𝑝 ), UMTS (𝑃𝑏,𝑢𝑚𝑡𝑠 ), and mixed calls (𝑃𝑏,𝑚𝑖𝑥𝑒𝑑 ) are defined as follows: 𝑃𝑏,𝑣𝑜𝑖𝑝 = +
𝐺 𝑈 −𝑖 ∑ ∑
𝑃 𝑟{𝑆(𝑥, 𝑉 − 𝑖, 2𝑖)}
(2)
𝑖=0 𝑥=0 𝐺 𝑈 −𝑖 ∑ ∑
𝑃 𝑟{𝑆(𝑥, 𝑉 − 𝑖, 2𝑖 − 1)}
𝑖=1 𝑥=0
𝑃𝑏,𝑢𝑚𝑡𝑠 =
𝐺 𝑉 −𝑖 ∑ ∑
𝑃 𝑟{𝑆(𝑈 − 𝑖, 𝑦, 2𝑖)}
(3)
𝑖=0 𝑦=0
+
𝐺 𝑉 −𝑖 ∑ ∑
𝑃 𝑟{𝑆(𝑈 − 𝑖, 𝑦, 2𝑖 − 1)}
𝑖=1 𝑦=0
𝐺 𝑈 −𝑖 ∑ ∑
𝑃𝑏,𝑚𝑖𝑥𝑒𝑑 = +
𝑖=0 𝑥=0 𝐺 𝑉 −𝑖 ∑ ∑
𝑃 𝑟{𝑆(𝑥, 𝑉 − 𝑖, 2𝑖)} 𝑃 𝑟{𝑆(𝑈 − 𝑖, 𝑦, 2𝑖)}
𝑖=0 𝑦=0
−
𝐺 ∑
𝑃 𝑟{𝑆(𝑈 − 𝑖, 𝑉 − 𝑖, 2𝑖},
𝑖=0
𝐺 = 𝑚𝑖𝑛{𝑈, 𝑉 }.
(4)
Therefore, the total blocking probability 𝑃𝑏,𝑡𝑜𝑡𝑎𝑙 is expressed by:
User 1
VOOH Server
1
𝑃𝑏,𝑡𝑜𝑡𝑎𝑙 = 𝑃𝑏,𝑢𝑚𝑡𝑠 ∪ 𝑃𝑏,𝑣𝑜𝑖𝑝 ∪ 𝑃𝑏,𝑚𝑖𝑥𝑒𝑑 −𝑃𝑏,𝑢𝑚𝑡𝑠 ∩ 𝑃𝑏,𝑣𝑜𝑖𝑝 −𝑃𝑏,𝑢𝑚𝑡𝑠 ∩ 𝑃𝑏,𝑚𝑖𝑥𝑒𝑑 −𝑃𝑏,𝑣𝑜𝑖𝑝 ∩ 𝑃𝑏,𝑚𝑖𝑥𝑒𝑑 +𝑃𝑏,𝑢𝑚𝑡𝑠 ∩ 𝑃𝑏,𝑣𝑜𝑖𝑝 ∩ 𝑃𝑏,𝑚𝑖𝑥𝑒𝑑 𝑃𝑏,𝑡𝑜𝑡𝑎𝑙 = +
−𝑖 𝐺 𝑈 ∑ ∑
(5)
𝐺 𝑉 −𝑖 ∑ ∑
𝑃 𝑟{𝑆(𝑥, 𝑉 − 𝑖, 2𝑖)}
𝐺 ∑
Service Rate
Incoming new calls
4
Incoming handoff calls 3
2
Handoff needed
Call processing
(b)
(6)
𝑃 𝑟{𝑆(𝑈 − 𝑖, 𝑦, 2𝑖)}
𝑃 𝑟{𝑆(𝑈 − 𝑖, 𝑦, 2𝑖 − 1)}
𝑖=1 𝑦=0
−
Call Arrival Rate Ȝ
1
μ
Fig. 5. Simulation Model (a) and Simulated Communication Procedures (b)
𝑖=0 𝑦=0
+
User 2
2
(a)
𝑃 𝑟{𝑆(𝑥, 𝑉 − 𝑖, 2𝑖 − 1)}
+
4
User n
𝑖=0 𝑥=0 𝐺 𝑈 −𝑖 ∑ ∑
𝑖=1 𝑥=0 𝐺 𝑉 −𝑖 ∑ ∑
Simulated Communication Procedures
3
𝑃 𝑟{𝑆(𝑈 − 𝑖, 𝑉 − 𝑖, 2𝑖)}
𝑖=0
𝐺 = 𝑚𝑖𝑛{𝑈, 𝑉 }. B. Simulation Model of VOOH In order to validate the analytical model, we develop a simulation model using the discrete event-based simulation tool OMNeT++4.2.1 [17]. Figure 5a depicts the simulation model of VOOH with incoming calls of users that are represented by the call arrival rate 𝜆. Accepted calls are processed with a given service rate 𝜇 describing the average call duration between two communication partners. In this simulation model, the communication procedure starts with incoming calls consisting of percentages of different call types (UMTS, VoIP, mixed). A successful call is only established if the VOOH server has enough non-occupied resources (i.e., free interfaces). In terms of occupied interfaces, a pure call always requires two of its corresponding interfaces, whereas a mixed call needs one UMTS and one VoIP interface. During calls, handoffs are initiated from client side with a predefined probability similar used for the analytical model. In this handoff process, a pure call (e.g., VoIP ↔ VoIP) can switch to a mixed call (e.g., UMTS ↔ VoIP) and vice versa (see Fig. 5b). The simulation time is limited to 10000 seconds since this time is sufficient to obtain a reliable statement about the call blocking probabilities of specific calls. The random generator is based on an exponential distribution method provided by OMNeT++. C. Comparison of Analytical and Simulation Model To validate the analytical Markov model with the simulation model, same configuration settings are achieved for both models. Based on the result of our survey about the call
duration via UMTS with 225 persons at the social networking service Facebook, the call duration is set to 𝑇 = 6 minutes which represents a service rate of 𝜇 = 𝑇1 . The VOOH server is configured with 40 VoIP and 40 UMTS interfaces where the latter is applied to create circuit-switched calls to the UMTS network. VoIP calls are running over an IP based connection of a corresponding Internet Service Provider (ISP) from VOOH server’s point of view, whereas the VOOH client receives VoIP calls via LTE or WiFi. According to [18], one in four of the EU citizens have used VoIP in 2011. Therefore, the percentages of UMTS, VoIP and mixed calls in the call arrival rate 𝜆 are defined as follows: 𝑢=0.65, 𝑣=0.25, and 𝑚=0.1. The percentage of UMTS calls (u) is the highest value since it is assumed that the current network coverage for UMTS calls is larger than for VoIP calls. Furthermore, we presume that the handoff probability from UMTS to VoIP is higher than to the opposite direction. This is explained by the fact that users will stay longer, e.g., in WiFi zones like at the company or at home. Due to low user mobility in those zones, it is assumed that users will close successfully a call without performing a handoff. Thus, the handoff probabilities are defined as follows: 𝐻𝑂𝑉 =0.05 (handoff from UMTS to VoIP) and 𝐻𝑂𝑈 =0.02 (handoff from VoIP to UMTS). The upper part of Figure 6 compares the blocking probabilities of UMTS, VoIP and mixed calls obtained by the analytical and simulation models. As illustrated in this figure, the analytical results are closely similar to the corresponding curves of the simulation model. By viewing the differences, the analytical results of the blocking probability for UMTS calls are 3% different on average to the simulation model, whereas the difference at VoIP and mixed calls varies only 1% on average. In practice, systems with very high blocking probabilities are not acceptable for certain applications (e.g., emergency use cases). Therefore, the lower part of Figure 6 underlines the area with low blocking probability, which is major of interest. For instance, if we have a closer look under the threshold of 20%, the difference between the results derived from the models are approximately negligible indicating that the analytical model fits very well to the simulation model (up to 0.01% on average). Furthermore, the blocking probabilities of the three call types obtained by the models show the same tendencies as
Simulation:
0.5
Analytical:
UMTS Mix VoIP
0.4
Number of Interfaces
Blocking Probability
0.6 UMTS Mix VoIP
0.3 0.2 0.1
500 400
VoIP
300
UMTS
200 100 0 Low Cost
0 2
3
4
5
6
7
8
9
Blocking Probability
Call Arrival Rate λ [call/min] 0.2
0.15
0.05
0.02%
Negligible difference between analytical and simulation model
0.01%
0 1
2
3
4
5
Call Arrival Rate λ [call/min]
Fig. 6. Comparison of Blocking Probability as QoS obtained from Analytical and Simulation Model
demonstrated in Figure 6. In both cases, UMTS calls have the highest blocking probability due to the high percentage of UMTS calls (65%) in the call arrival rate 𝜆. Considering the parameter settings, the blocking probability 𝑃𝑏,𝑚𝑖𝑥𝑒𝑑 of mixed calls is always higher than 𝑃𝑏,𝑣𝑜𝑖𝑝 of VoIP calls. This behavior is influenced by the high percentage of UMTS calls in 𝜆 and the higher handoff probability 𝐻𝑂𝑉 compared to 𝐻𝑂𝑈 . Consequently, the curve of 𝑃𝑏,𝑚𝑖𝑥𝑒𝑑 is not only higher, but also closer to the curve of VoIP than to UMTS calls. Based on this finding, the use of these models enables a cost-effective and scalable design of the VOOH server for specific use cases. This section has shown that two independent methods (i.e., three-dimensional Markov model and simulation model) achieve closely similar results. D. Influence of VOOH Use Case Examples on Blocking Probability as QoS To evaluate the results of our developed models, we define three different use case examples that allow determining a cost-effective and scalable design of the VOOH server by considering the maximum permissible call blocking probability. The following use cases are chosen: ∙
∙
∙
Business
Fig. 7. Results of Use Case Examples with different QoS Requirements described by Blocking Probabilities 𝑃𝑏,𝑡𝑜𝑡𝑎𝑙 500
1%
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Use Cases with λ=40 [call/min]
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Low Cost: Maximum permissible total blocking probability is 5%. A relatively high blocking probability is taken into account to minimize cost. Standard: Maximum permissible total blocking probability is 0.1%. In this use case, a balance between low cost and high availability is aspired. Customers must really count on a rejection of calls in very few cases. Business: Maximum permissible total blocking probability is 0.0001%. Business customers need an almost non-blocking system and accept the associated increased costs.
Number of UMTS Interfaces
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Fig. 8. Comparison of required UMTS interfaces of VOOH and enhanced VOOH (eVOOH) considering the maximum permissible total Blocking Probability 𝑃𝑏,𝑡𝑜𝑡𝑎𝑙 for different Use Case Examples
To obtain the total blocking probability 𝑃𝑏,𝑡𝑜𝑡𝑎𝑙 , we apply the simulation model due to the more efficient calculation time in contrast to the Markov model. The parameter settings are the same like presented in Section III-C. Figure 7 illustrates the cost-effective design of the VOOH server for three use cases (Low Cost, Standard, Business) to achieve the optimal capacity usage of all interfaces with 𝑐𝑎𝑙𝑙 an assuming call arrival rate 𝜆 = 40 𝑚𝑖𝑛 , while respecting the predefined blocking probabilities. It can be seen that the number of the corresponding interfaces increases with rising QoS (i.e., low 𝑃𝑏,𝑡𝑜𝑡𝑎𝑙 ). For instance, the use case Business requires 17% more VoIP and 36% more UMTS interfaces compared to the use case Low Cost to fulfill the call blocking probability of 0.0001%. In order to reduce the load of the VOOH server, the VOOH solution also allows direct UMTS calls without passing through the server. This direct call is possible if both communication partners are directly reachable via UMTS. For this purpose, it is necessary that all participants have to notify their UMTS availability to the server that forwards this information to authorized users via the overlay signaling protocol OHSP. For further detailed information about the enhanced VOOH (eVOOH) concept, we refer to [14]. Applying eVOOH with direct calls reduces the call arrival rate 𝜆 and the handoff probability 𝐻𝑂𝑈 from VoIP to UMTS. It is assumed that eVOOH decreases the total blocking probability in comparison to VOOH without direct calls. As a result, this might minimize economic costs (i.e., reducing number of interfaces). In order to validate this statement, Figure 8 demonstrates the added value of eVOOH with the assumption that 75% UMTS calls are direct calls. This has the effect that the percentage of UMTS calls in 𝜆 decreases from server’s point of view,
whereas the percentages of VoIP and mixed calls in 𝜆 increase slightly. Since it is expected that the physical installations of UMTS interfaces (i.e., ISDN cards) involve higher costs than VoIP interfaces, this figure only shows the number of UMTS interfaces. Confirmed by the results, eVOOH lowers the load of the VOOH server and permits cost saving by reducing the number of UMTS interfaces up to 56% (i.e., use case Business). To sum up, the use of the validated models allows costeffective and scalable design of the VOOH solution as a cloud service to fulfill a predefined maximum permissible call blocking probability. This ensures an improved resource usage of VOOH (i.e., number of interfaces for calls) and minimizes economic costs. Moreover, the previously presented results have shown the gain of the eVOOH feature with direct UMTS calls that further improves the overall VOOH performance. As proof of concept and for experimental performance analysis, we presented in [19] a prototype implementation that has been tested over a public network infrastructure. IV. C ONCLUSION AND F UTURE W ORK In this paper, we proposed a Virtual Room based Vertical Handoff (VOOH) service for seamless voice calls across the today’s existing heterogeneous networks (UMTS, LTE, WiFi). Unlike related solutions, VOOH is a readily deployable voice cloud service that only applies wireless access technologies for connectivity purposes and does not require any changes in the underlying network layers. Therefore, this results in low integration costs. Moreover, the VOOH service does not violate contract plans of most commercial network providers since the circuit-switched instead of packet-switched channel of UMTS is used for calls while handing over seamlessly from VoIP (via LTE or WiFi) to UMTS. In order to enable a cost-effective and scalable design of VOOH as cloud service, we modeled an analytical threedimensional Markov chain that considers the specific property of VOOH’s communication procedure. For validating the Markov model, we also developed an event-discrete simulation model that differs up to 0.01% on average from the results of the analytical model. This paper has shown that two independent methods (i.e., Markov model and simulation model) achieve closely similar results. To evaluate the results of the validated model, we define three use case examples with different maximum permissible call blocking probabilities as QoS. As evident from the obtained results, an increasing QoS requires more UMTS and VoIP interfaces at the VOOH server to guarantee seamless voice calls. For improving the VOOH performance, VOOH also enables direct UMTS calls without passing through a third party. This feature with direct calls is referred to as enhanced VOOH (eVOOH). Confirmed by the results, eVOOH outperforms VOOH up to 56%. Consequently, this allows to further minimize economic costs (i.e., reducing number of UMTS interfaces), whilst also taking into account the required QoS. In future work, we plan to develop a distributed and more reliable system architecture for enhancing the scalability.
ACKNOWLEDGMENT The authors would like to thank Nils Dorsch for his technical assistance. Our work has been conducted within the 𝑆𝑒𝑐2 project (Secure Ad-hoc On Demand Virtual Private Storage), which is funded by the German Federal Ministry of Education and Research (BMBF) (01BY1031). R EFERENCES [1] Paisal, V.: Seamless Voice over LTE, IEEE 4th International Conference on Internet Multimedia Services Architecture and Application (IMSAA), IEEE, pp. 1-5, 2010. [2] Federal Statistical Office: Statistical Yearbook 2011 For the Federal Republic of Germany including International Tables, 2011. [3] Prokkola, J., Perala, P.H.J., Hanski, M., Piri, E.: 3G/HSPA Performance in Live Networks from the End User Perspective, IEEE International Conference on Communications (ICC), IEEE, pp. 1-6, 2009. [4] Bertin, E., Crespi, N., L’Hostis, M.: A few myths about Telco and OTT models, 15th International Conference on Intelligence in Next Generation Networks (ICIN), IEEE, pp. 6-10, 2011. [5] Zhang, S., Zhang, S., Chen, X., Huo, X.:Cloud Computing Research and Development Trend, Second International Conference on Future Networks (ICFN), pp. 93-97, 2010. [6] Yarali, A., Saleeba, K.: Unlicensed Mobile Access: Leading Technological Alternative in the Fixed Mobile Convergence Stable, Third International Conference on Communication Theory, Reliability and Quality of Service, IEEE, pp. 1-8, 2010. [7] Kou, G., Tang, Y., Jiang, Y.: A Novel Scheme of Vertical Handover for IP Multimedia Subsystem, 2nd International Conference on Advanced Computer Control, IEEE, pp. 30-33, March 2010. [8] Bellavista, P., Corradi, A., Foschini, L.: IMS-Compliant Management of Vertical Handoffs for Mobile Multimedia Session Continuity, IEEE Communications Magazine, vol. 48, no. 4, pp. 114-121, 2010. [9] Dutta, A., Lin, F.J., Das, S., Chee, D., Lyles, B., Chiba, T., Yokota, H., Schulzrinne, H.: A-IMS Architecture Analysis and Experimental IPv6 Testbed, International Conference on IP Multimedia Subsystem Architecture and Applications, pp. 1-5, 2007. [10] Salsano, S., Veltri, L., Martiniello, G., Polidoro, A.: Seamless Vertical Handover of VoIP Calls based on SIP Session Border Controllers, IEEE International Conference on Communications (ICC), IEEE, pp. 20402047, 2006. [11] Stepaniuk, O.: Voice over LTE via Generic Access (VoLGA) as a possible solution of mobile networks transformation, International Conference on Modern Problems of Radio Engineering, Telecommunications and Computer Science (TCSET), IEEE, pp. 205, 2010. [12] Namakoye, J., Van Olst, R.: Performance Evaluation of a Voice Call Handover Scheme between LTE and UMTS, IEEE AFRICON, IEEE, pp. 1-5, 2011. [13] Mehta, A., Forte, A. G., Schulzrinne, H.: Using Conference Servers for SIP-based Vertical Handoff between IP and Cellular Networks, Proceedings of the 6th ACM International Symposium on Mobility management and Wireless Access (MOBIWAC), pp. 28-34, 2008. [14] Tran, T., Wietfeld, C.: Cost-Effective and Feasible VOOH Solution for Seamless Service Roaming, IEEE International Conference on Wireless and Mobile Computing, Networking and Communications (WIMOB), IEEE, pp. 280-287, 2011. [15] Daniel, K., Tran, T., Wietfeld, C.: IP-Based Overlay Signaling for Seamless Service Roaming in Heterogeneous Networks, IEEE Wireless Communications and Networking Conference (WCNC), IEEE, pp. 1-6, 2009. [16] Kleinrock, L., Gail, R.: Queueing Systems: Problems and Solutions, John Wiley & Sons, 1996. [17] Varga, A.: The OMNeT++ Discrete Event Simulation System, Proceedings of the European Simulation Multiconference, pp. 319-324, Prague, Czech Republic, 2001. [18] Federal Association for Information Technology, Telecommunications and New Media (BITKOM), http://www.bitkom.org, 2012. [19] Tran, T., Kuhnert, M., Wietfeld, C.: Seamless Context-aware Voice Service in the Cloud for Heterogeneous Network Environment, 21st International Conference on Computer Communication Networks (ICCCN), 2nd International Workshop on Context-aware QoS Provisioning and Management for Emerging Networks, Applications and Services, IEEE, Munich, Germany, pp. 1-6, 2012.
