A Comprehensive Simulation Study of Low Latency Handoffs in Mobile IPv4 for VoIP in IEEE 802.11b WLAN∗ Asimakis Lykourgiotis
Stavros Kotsopoulos
Tasos Dagiuklas
Dept. of Electrical and Computer Engineering University of Patras Rio, Hellas
Dept. of Electrical and Computer Engineering University of Patras Rio, Hellas
Dept. of Telecommunications Systems and Networks TEI of Mesolonghi Nafpaktos, Hellas
[email protected]
[email protected]
[email protected]
ABSTRACT
1.
The inexpensive deployment combined with the high bandwidth availability of IEEE 802.11 wireless local area networks led to their proliferation. Although WLANs provide the means of portable computing and networking, the transition to full mobility support, despite the significant progress, has not yet achieved. As an effect, minimization of Handoff Delay is the one of the crucial factors and challenges to deliver multimedia services of high quality (i.e. VoIP over WLANs). The aim of this paper is to access and evaluate the impact of mobility protocols in terms of Handoff Delay on VoIP quality. Low Latency Handoffs in Mobile IPv4 has been used since it is the most promising extension of the standard network-layered Mobility Management protocol of Mobile IPv4. According to the performed analysis, this paper presents VoIP Quality (in terms of R-factor) during handoff scenarios for various VoIP codecs using the aforementioned mobility protocol through OPNET simulation. Through simulation, it is shown that the achieved reduction of Handoff Delay can lead to VoIP Quality improvements in terms of service continuity in all-IP Wireless Access Networking environments.
Due to low cost infrastructure deployment and high data rate 802.11 WLANs became a common wireless technology not only for private areas but for public places as well, such as public hotspots. Mobility Management (MM) mainly comprises two tasks: Location Management and Handoff Management. While Location Management is responsible for keeping track of the location of the Mobile Node (MN) between successive calls, Handoff Management is in control of service continuity when the MN changes its point of attachment to the network. The importance of an efficient MM protocol in 802.11 WLAN increases as the coverage of a WLAN Access Point (AP) is much smaller than that of a cellular base station leading to more frequent handoffs. Upon the accomplishment of transparent mobility support, WLANs will complement Third Generation (3G) cellular networks and substantiate the vision of next generation networks (NGN). Internet was initially designed to facilitate fixed hosts for data exchange. Nonetheless, due to the explosive growth of the Internet, wireless IP networks have gained considerable popularity. The Mobile IPv4 (MIPv4) [11] is a mature protocol for handling Network Layer MM in an all-IP environment like NGN. However, MIPv4 has not been deployed widely in real networks as it mainly suffers from high Handoff Delay. To improve protocol performance many extensions have been proposed. The most well promising solution is Low Latency Handoffs for Mobile IPv4 (LLH) [5] which utilizes Link Layer triggers to optimize handoff procedure. The idea of expediting handoff process using Link Layer information is very pervasive in the research community resulting to numerous cross layer proposals. LLH is an experimental protocol proposed by the Internet Engineering Task Force (IETF) and comprises two schemes named Preregistration and Post-registration which both eliminate the Network Layer component of the Handoff Delay. Doing so, IP Mobility will guarantee QoS for time-sensitive multimedia applications. One key application which will take advantage of NGN is Voice over IP (VoIP). VoIP is considered to be a competitive alternative to conventional connection-oriented Public Switched Telephone Networks. The challenge to deliver voice services over the unreliable and connectionless Internet is to support the required QoS in order to satisfy the end user demands. A variety of speech codecs with diverse characteristics have been proposed to support VoIP. One
Categories and Subject Descriptors C.2.1 [Computer-Communication Networks]: Network Architecture and Design—Wireless communication; C.2.2 [Computer-Communication Networks]: Network Protocols—Protocol verification; I.6.4 [Simulation and Modeling]: Model Validation and Analysis
Keywords Mobile IPv4, Low Latency Handoffs, VoIP, IEEE 802.11b, R-factor ∗Work presented at ACM MSWiM’12 Poster Session, October 21–25, 2012, Paphos, Cyprus.
Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. PM2 HW2 N’12, October 21–22, 2012, Paphos, Cyprus. Copyright 2012 ACM 978-1-4503-1626-2/12/10 ...$15.00.
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INTRODUCTION
way to evaluate the quality of speech is through subjective testing where a group of listeners independently rate the service. One the other hand, subjective methodology considers coding (distortion) and network parameters (delay, jitter, packet loss) to evaluate the perceived VoIP quality. A standardized method to estimate the user’s satisfaction is the E-Model, which was defined by the International Telecommunication Union (ITU) to assist the transmission planning [12]. Consequently, the evaluation of LLH with E-Model for supporting VoIP is of great importance for the NGN concept. Most of the previous work on the topic of MM mainly focused on either Network Layer metrics (e.g. Handoff Delay, Packet End-to-End Delay) or a specific VoIP codec configuration. Moreover, the outcome of VoIP QoS for mobile users depends on the specific implementation of the overall protocol architecture. Hence, a thorough understanding and comprehensive evaluation of varying conditions are necessary to effectively assess the level of the achieved QoS. Thus, in this paper the performance of LLH is estimated for a large range of possible network and application parameterizations using E-Model. The reminder of this paper is organized as follows: Section 2 describes related work , Section 3 gives a brief outline of the LLH, Section 4 presents the basic stages of VoIP process, Section 5 introduces the E-Model, Section 6 describes the 802.11b processes that affect LLH performance, Section 7 defines the modeling of LLH in OPNET Modeler, Section 8 presents and interprets the results and finally Section 9 concludes the paper.
oFA MN oFA Subnet Internet
HA Home Network
MN nFA nFA Subnet
CN CN Network
Figure 1: Architectural Model network prefix changes too, destroying the socket and losing its IP connection. The MIPv4 was introduced to alter this problem by allowing the MN to utilize two IP Addresses, one for identification called Home Address (HoA), the other for routing called Care-of-Address (CoA). In the following, a brief presentation of MIPv4 and LLH will be given.
3.1
Mobile IPv4
According to MIPv4, two new functionalities must be added to the wireless routers of every domain for supporting IP Mobility. The first is called Home Agent (HA) and allows roaming of its users to other subnets and the other is called Foreign Agent (FA) and is capable to accept visitors (Fig. 1). Every time a MN enters a Foreign Network, it registers with the FA and obtains a CoA , which most commonly is the FA’s IP address. Then the FA registers the MN’s current CoA to the HA using a control message called Registration Request. Upon successful registration, the HA replies to the FA with a Registration Reply message. Every time a Correspondent Node (CN) attempts to establish a connection with the MN, it sends the packets to the MN’s HoA. The HA encapsulates them using the CoA as Destination Address of the outer IP header field and forwards them through a tunnel to the FA. On receiving the packets, the FA decapsulates them and sends the original packets to the MN.
2. RELATED WORK Fathi et al. [6] evaluated the disruption time of LLH for various schemes using an analytical model. Their results showed the upper bound for the delay between involved entities, under which LLH can support real-time services. However, they consider these delays as constant parameters. Blondia et al. [2] investigated the performance of LLH through analysis and simulation. The results obtained by their analysis estimated the expected number of dropped packets and Packet End-to-End Delay during the handoff. Their simulation captured the peak buffer usage of the APs as well as the TCP goodput during handoff. Tseng et al. [14] conducted experiments of a Pre-registration handoff scheme to measure the Handoff Delay and Packet Loss for Constant Bit Rate (CBR) traffic. The results showed that LLH could meet the delay requirement of VoIP application. Although, all previous work was motivated by the vision of IP-based real-time services, little research has been done on the impact of LLH on VoIP performance. Moreover, no results have yet been presented in terms of user perceived quality of voice. To the best of our knowledge, this is the first paper that presents results for various codecs and utilizes E-Model for perceptual QoS estimation.
3.2
Low Latency Handoffs for MIPv4
In MIPv4, a MN initiates the handoff procedure when it detects that it has moved to a new subnet. Movement detection is based upon the ICMP Router Advertisement messages which are sent periodically (no less than 3 seconds) and can be received only after Link Layer (L2) handoff. This can introduce significant disruption which is unacceptable by real-time applications such as VoIP. To address this problem, the IETF Network Working Group has proposed a cross layer solution where L2 triggers are used to optimize the Network Layer (L3) handoff procedure [5]. An L2 trigger is an event related to the L2 condition or the L2 handoff process. An initial event is an upcoming change in L2 point of attachment due to signal deterioration. The protocol defines three triggers for this event based on the entity that the trigger occurs. This event can occur either at the MN (L2-MT), the old FA (L2-ST) or the new FA (L2-TT). The next event is the trigger that occurs at the
3. MOBILITY MANAGEMENT In order to set up and establish an Internet connection, applications make use of the Sockets Application Programming Interface (API). Sockets uniquely identify the communications endpoints by an IP Address and a TCP/UDP port. Moreover, the IP Address comprises a network prefix and a host identifier. When a MN changes subnets, the
30
is a consequence of an L2-MT trigger. The oFA will reply with a Proxy Router Advertisement (Message 2b).
HA
iii. MN sends a Registration Request (Message 3) to the nFA routed through the oFA since the MN is not directly connected to the nFA yet.
5. RegRply
4. RegReq
iv. At the end, Messages 4 and 5 complete the standard MIPv4 Registration. Moreover, if the MN is not already connected to the nFA, the Registration Reply is buffered and unicast to the MN as soon as it connects. If the registration is successful, packets are tunneled from the HA to the nFA and then to the MN.
1a. PrRtSol oFA
2a. PrRtSol
1b. PrRtAdv
nFA
2b. PrRtAdv 3. RegReq MN
3.2.2
In the second scheme, illustrated in Fig. 3, the MN postpones the registration to the nFA while it has moved to a new subnet but it still receives and sends packets. To do so, a tunnel is established between the oFA and the nFA without the involvement of the MN. The HA still sends packets to the oFA and then the oFA (or also called anchored FA, aFA) forwards the packets to the nFA. If the MN moves to a third FA before registering to serving FA, the latter signals the aFA to move the other end of the tunnel to it. The PostRegistration procedure without involving a third FA will be discussed next.
Movement
Figure 2: Pre-registration Handoff Protocol L2 - ST
1a. HRqst 1b. HRply oFA
Post-Registration
nFA BET
i. Firstly, the L2-ST trigger informs the oFA that a specific MN is about to move to a nFA.
MN Movement
ii. oFA then sends a Handoff Request (Message 1a) to nFA.
Figure 3: Post-registration Handoff Protocol
iii. nFA responds with a Handoff Reply (Message 1b) in which it requests a reverse tunnel with a specific lifetime. During MN’s L2 handoff, packets are forwarded from the oFA and buffered at the nFA.
old FA (oFA), informing the oFA that MN has moved to another subnet (L2-LD). Finally, the last event is the consequent establishment of the new link (L2-LU) which can occur at the MN or the new FA (nFA). The L2 triggers that are made available to the MIPv4 are assumed to be generic and technology independent. As these events are very common in cross layer design, IEEE has developed a standard to support Handoff Management and interoperability between heterogeneous network types. The result is the 802.21 Media Independent Handover [8], which is a standard that can perform such signaling and assists LLH functionalities. LLH introduces the following two methods for transparent L3 handoff.
iv. When the L2 handoff is completed, the nFA sends the packets to the MN. Finally, the standard MIPv4 Registration is initiated by the MN in short time.
4.
VOICE OVER INTERNET PROTOCOL
VoIP is a well promising application for voice delivery service over packet-switched networks like the Internet. The basic stages of voice transmission are as follows. Initially the analog voice signal is modulated and compressed into a low bit rate packet stream by the codec at the speaker. Then, the generated payload is inserted into an IP packet for delivery to the destination. At the listener’s side same functions are performed in reverse order. Various codecs have been standardized for voice compression, differing in data rate and perceived voice quality. ITU H.323 is a standard for multimedia communications over packet-based networks. Audio codecs used in this standard are, among others, G.711 and G.729. The G.711 codec is the most widely used codec in VoIP applications and often used as a reference. G.711 uses Pulse Code Modulation (PCM) sampling at 20 ms which results in a 64 kbps stream. The codec has very low algorithmic complexity and consequently high bandwidth usage, while achieving very good voice quality. G.729A is a compatible extension of the G.729 codec, but requires less computational power. G.729A uses
3.2.1 Pre-Registration In this scheme as shown in Fig. 2, the MN becomes aware of an anticipating L3 handoff and performs it before completing the L2 handoff. L3 handoff can be either networkinitiated or mobile-initiated. Below, the successive steps of the latter will be discussed. i. To perform a Pre-registration, necessary information must be exchanged between the oFA and the nFA prior to the handoff initiation. Therefore, the oFA solicits (Message 1a) and caches (Message 1b) advertisements from neighboring nFAs. ii. MN sends a Proxy Router Advertisement (Message 2a) to solicit an advertisement from a router (nFA) different from the one receiving the message. This message
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Table 1: R-factor 90 - 93.2 80 - 90 70 - 80 60 - 70 50 - 60 0 - 50
sidetone and quantizing distortion. None of these parameters is function of the network plane. The delay impairment factor Id inserts the effect of talker and listener echoes to the voice quality and is the only that depends on network delay components. The effective equipment impairment factor Ief represents impairments caused by low bit-rate codecs. It also includes impairment due to randomly distributed packet losses. The values of Ief depend on subjective mean opinion score test results. Finally, as mobile users tend to tolerate lower service quality in exchange for mobility, the advantage factor A is used. This factor counterbalances the effects of impairment factors when there are other advantages of access to the user and its value ranges from 0, for conventional wired systems, to 20, for hard-to-reach locations. The aforementioned transmissions impairments are additive and only Id and Is are influenced by underlying packet transport. As a result, Cole et al. [3] used measurements of these factors and fit them to the following simple mathematical expressions.
Relationship of R-factor and MOS MOS User satisfaction 4.34 - 4.50 very satisfied 4.03 - 4.34 satisfied 3.60 - 4.03 some users dissatisfied 3.10 - 3.60 many users dissatisfied 2.58 - 3.10 nearly all users dissatisfied 1.00 - 2.58 not recommended
Code Excited Linear Prediction method with a fixed Algebraic codebook structure (ACELP). The codec operates on speech frames of 10 ms corresponding to an 8 kbps stream. Global System for Mobile communications (GSM) is the European standard for digital cellular communications. GSM uses codecs to compress the speech signal, while maintaining an adequate quality of the decoded output. Among the three proposed speech codecs of the standard is the Full Rate (FR) codec, which belongs to the class of Regular Pulse Excitation - Long Term Prediction (RPE-LTP) codecs. In this case, a frame of 20 ms is encoded as a block of 260 bits, leading to a bit rate of 13 kbps. Additionally at the encoding stage, silence suppression is possible through the Voice Activity Detection (VAD) procedure. As voice communication is mainly half duplex taking into account that only one side speaks at a time, the VAD procedure can significantly reduce the required resources. At the other end of the communication, the codec will generate comfort noise for the silence periods. At the end, a playout buffer is implemented at the receiver to compensate for variance in the Packet End-to-End Delay. This de-jitter buffers incoming packets before playing them out, letting slower packets arrive on time and play out at their right order. The ITU considers one-way delay between 0 and 150 ms as acceptable for voice applications in Recommendation G.114 [10]. As a result, this range can be used for a fixed playout buffer. There are also adaptive playout buffer schemes but they are generally vendor-specific.
Id = 0.024 · d + 0.11 · (d − 177.3) · u(d − 177.3) { Ief =
30 · ln(1 + 15e),
for G.711
(3)
11 + 40 · ln(1 + 10e),
for G.729
(4)
where, d is the average, absolute one-way mouth-to-ear delay, u(·) is the step function and e is the total packet loss probability.
6.
IEEE 802.11 WLAN
The IEEE 802.11 is a set of standards that operates at the license free ISM band (for Industrial, Scientific and Medical use) which makes easy and cost effective the deployment of such a network. As a result the proliferation of 802.11 WLANs makes them a possible candidate for delivering multimedia services to mobile users. IEEE 802.11 introduces two operational modes, ad hoc and infrastructure. In ad hoc mode, all wireless nodes within range communicate directly with each other through a distributed medium access control. In infrastructure mode the stations communicate with others through an AP, which also connects them to the Internet. Afterwards, some key aspects for accurate simulation of 802.11 networks will be reviewed. The mandatory medium access control (MAC) protocol for 802.11 WLANs is the Distributed Coordination Function (DCF). DCF is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) with exponential backoff and the exchange of control messages (RTS/CTS) in order to reserve the medium and transmit large packets. Additionally, after the successful reception of packet the station must transmit an acknowledgment packet (ACK). If an ACK is not received within a specific time the sender retransmits the packet. The 802.11 standard defines two types of retry limit, long and short. A packet that reaches the retry limit is discarded by the MAC. The short retry limit is used for frames whose size are less than or equal to RTS Threshold. Typical values for short and long retry limits are 7 and 4 respectively. The 802.11b standard operates in the 2.4 GHz ISM band and use 11 of the 14 available channels. The adjacent channels are separated by 5 MHz. As the protocol requires 25 MHz channel separation between adjacent APs so as not to interfere with each other, a 5 channels gap is needed. The
5. E-MODEL The complexity of modern networks involves many transmission parameters with combined effects. E-Model, defined in Recommendation ITU-T G.107 [12], is a computational model that attempts to interpret all these parameters. The output from the E-model is a scalar quality rating value called R-factor, which varies directly with the overall VoIP QoS. The transmission rating factor R lies within the range of 0 to 100, representing extremely bad to very high quality respectively. Moreover, R-factor provides a statistical estimation of quality measures and can be transformed into other ones. Such a measure is the Mean Opinion Score (MOS) which is in the scale 1 to 5 and has been used extensively in transmission planning. Table 1 summarizes the relationship between R-factor, MOS and user satisfaction. This rating factor R is composed of: R = Ro − Id − Is − Ief − A
(2)
(1)
The basic signal-to-noise ratio Ro represents noise sources such as circuit noise and room noise at both ends of the communication. The simultaneous impairment factor Is is a combination of all impairments which occur more or less simultaneously with the voice signal such as non-optimum
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most commonly used combination of non overlapping channels is the set 1, 6 and 11. When the MN is moving out of the coverage area of the serving AP, it must scan for another available AP. This is done by the MAC layer scanning functionality by changing its operating channel and dwells to a new one for a certain period of time. Scanning can be either passive or active. In passive mode, the MN listens to the medium and waits to receive a special message broadcast by the AP, called Beacon. In active mode the MN transmits a Probe Request frame expecting a Probe Response as reply from an AP. Afterwards, two-way handshake processes, called Authentication and Association complete the handoff. Finally, another crucial factor for the 802.11 WLAN simulation and evaluation is the buffer size at the AP. This factor is critical for packet loss and delay. In a loaded WLAN the larger the buffer size the higher the queuing delay will be. On the other hand, the smaller the buffer size the faster the buffer overflows. As the channel utilization increases the expected response time increases too, from some milliseconds to some tens of milliseconds. This is a significant delay for the transmission of PrRtAdv message and can lead to intermittent connectivity or poor performance. However, 50 packets is a typical value for the buffer size. All the aforementioned factors are important for 802.11b simulations and are investigated at [13]. Therefore, in our simulations the above typical values for 802.11 WLAN configuration were used for more reliable results.
Table 2: 802.11b Simulation Settings Parameter Value Parameter Value Basic Rate 1 Mbps CWmin 31 Data Rate 11 Mbps CWmax 1023 PLCP Preamble Short Short Retry 7 SIFS 10 µs Long Retry 4 DIFS 50 µs AP buffer 50 packets Slot time 20 µs Load 3.3 Mbps
Codec
Table 3: VoIP Simulation Settings G.711 G.729A GSM-FR
Modulation
PCM
CS - ACELP
RPE - LTP
Sample Period
20 ms
10 ms
20 ms
VAD
No
Yes
Yes
Playout Buffer
100 ms
100 ms
100 ms
To override this problem, the MN followed a predefined trajectory and the oFA was a priori informed for the nFA.
7.2
Signaling and Tunneling
In the Pre-registration scenario, new fields were added to the original control messages of the MIPv4. In addition, as described in Section 3, oFA must have access to the IPv4 address of the nFA. To this end, a new mechanism was added to oFA to allow manual configuration of a list of nFA addresses upon which a MN could possibly perform L3 handoff. Furthermore, a new process enabled the oFA to solicit its neighboring nFAs and cache the most recent advertisement from each one. Inter-FA solicitation occurs, as the protocol defines, at a predefined time interval with minimum value of 1 sec. Moreover, oFA is also able to receive and process PrRtSol messages from MNs and reply with the correct PrRtAdv. The rest steps of the procedure are the same with the standard MIPv4 Registration. In the Post-registration scenario, the oFA is capable of sending HRqst and establishing the one end of the tunnel for forwarding MN’s traffic to the nFA, and subsequently the nFA can send HRply and establishes the other end of the tunnel. Finally, the standard MIPv4 Registration is initiated by the MN immediately after L2-LU event.
7. SYSTEM MODELING An LLH implementation for both Pre-registration and Post-registration scenarios was developed in OPNET Modeler 15.0 [7] to evaluate the impact of the protocol on the performance of VoIP quality. OPNET Modeler is a powerful communication system simulator which provides detailed parameterization modeling, supports a variety of VoIP codecs and can obtain R-factor measurements. The LLH was developed by extending the existing MIPv4 implementation. Below, the most important extensions that were applied to MIPv4 module are briefly presented.
7.1 Triggering Mechanism In the Pre-registration scenario, the mobile initiated handoff was implemented and in the Post-registration the source trigger scheme. However, the results can be used without loss of generality. In the network initiated handoff the difference is only the transmission of the PrRtSol message from the MN to the network while in the target trigger scenario there is no difference at the signaling delay and it is only a matter of implementation. Both cases need a triggering mechanism that at first can predict that a certain MN is about to move from the oFA and then that it completed an L2 handoff with the nFA. For that purpose, a cross layer module was developed as an intermediate layer between WLAN MAC and IP layers. Through this interface the L2-MT, L2-ST, L2-LD and L2LU triggers described at Section 3 became available to the L3 protocol suite. Once the L2 trigger was received, the handoff processes described in the aforementioned section were initiated. Finally, although the detection of an imminent handoff was feasible by monitoring the uplink RSSI, the identification of the target agent demands an accurate movement detection algorithm which is out of the scope of this work.
8.
RESULTS
8.1 8.1.1
Simulation Setup Background traffic
Saturation throughput is defined as the limit reached by the system throughput as the offered load increases, and represents the maximum load that the system can carry in stable conditions. The 802.11 DCF function was investigated and modeled in [1]. Through this analysis, it was shown that the normalized saturation throughput can be computed as a function of the number of stations and the packet length for a specific WLAN configuration. For our simulation set up, the normalized saturation throughput was computed equal to 0.60 for a data rate of 11 Mbps and constant-sized (1500 bytes) UDP packets. For background traffic, the packets were generated following a Poisson distribution and destined to and from one wireless station at each cell, equally loading
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0.1
handoff. For this purpose the movement detection mechanism must supply an adequate amount of time to the handoff module. On the other hand, to accurately predict movement the mechanism must generate the outcome as close to the event as possible so that the probability of change of the MN’s velocity and direction is minimized. Below, these two counter effects are analyzed by means of Queueing Theory [9]. Assuming exponential service time, both MN and FA are modeled as M/M/1/k queues because the buffer size of the AP significantly influences the network’s performance as presented in [13]. Therefore, k is equal to 50 packets which is a typical value for the buffer size of most wireless cards. The service rate of the DCF is, according to the saturation throughput analysis in [1]:
E[P] = 1500 Bytes E[P] = 1024 Bytes E[P] = 512 Bytes
0.09
Signaling Delay (sec)
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Utilization
S= Figure 4: Signaling Delay of Various Packet Lengths
(6)
where, Ps is the probability that there is at least one transmission in the considered slot time, Ptr is the probability that a transmission occurring on the channel is successful, E[P ] is the average packet payload size, Ts is the average time the channel is sensed busy, Tc is the average time the channel is sensed busy during a collision and σ is the duration of a slot time. All the aforementioned parameters can be retrieved by the WLAN configuration. The expected response time for an M/M/1/k queue is:
both the uplink and the downlink up to 50 percent of the saturation throughput.
8.1.2 WLAN configuration As it was analyzed in Section 6, the configuration of the WLAN MAC affects the VoIP QoS significantly and has a crucial impact on the LLH performance as well. For more accurate results these parameters were set to the default values that are used by the most wireless cards as it was investigated in [13] . Table 2 shows that set of parameters and their default values. The same set of values was used for the calculation of the saturation throughput described above. In addition, overlapping coverage areas between adjacent APs is assumed, meaning that once the MN is triggered to initiate the handoff process it has already enter the coverage area of the nFA.
E[R] =
1 − (k + 1)ρk + k · ρ(k+1) 1 · µ−λ 1 − ρk
(7)
where, µ is the service rate, λ is the arrival rate, k is the queue size and ρ is the utilization factor. The signaling that takes place prior of the L2 handoff in the Pre-registration is the exchange of PrRtSol, PrRtAdv and RegReq messages between the MN and the oFA, as described in Section 3. If the time interval between the completion of the movement detection and the link loss is less than this signaling delay the handoff will be unsuccessful. Fig. 4 shows this signaling delay with respect to the channel utilization for various values of the E[P ]. It can be observed that as the utilization or the average packet payload size increases the aforementioned signaling delay increases too. For low values of ρ the signaling delay varies around 10 ms and as the ρ increases the delay reaches 100 ms.
8.1.3 VoIP and E-Model The VoIP process, as it was described in Section 4, involves a diverse set of modulation techniques, sampling periods and silence suppression. The default values are also used for setting the VoIP application and are shown in Table 3. For more accurate simulation model, the experimental results presented in [4] were used for modeling the VAD procedure. In this analysis it was shown that the On and Off periods in a packetized model of voice sources can be modeled accurately by the generalized Pareto distribution instead of the classical source model which uses 650 ms mean value for silence duration and 352 ms for talkspurt duration, respectively. Finally, for the calculation of the R-factor the model introduced in [3] and described at Section 5 was used. The following expression is obtained by replacing the constants Ro , Ief , A as in [13]: R = 93.2 − Id − Ie
Ps · Ptr · E[P ] (1 − Ptr ) · σ + Ptr · Ps · Ts + Ptr · (1 − Ps ) · Tc
8.3
Simulation Results
To achieve statistical significance, each simulation was repeated 20 times with different random seeds. During each run the MN conducted 10 handoffs resulting to a total set of 200 handoffs for each simulation scenario. The link delays between HA and FA and between oFA and nFA have exponential distributions with mean values of 10 ms and 5 ms respectively. The following figures depict the average value and the standard deviation of the R-factor of the G.711 codec for the investigated parameters. The R-factor was calculated for each packet separately and the results include 5 packets before the L2-LD trigger and 15 packets after the L2-LU trigger. As described in Table 1 a value of 70 is the lower limit of region “some users dissatisfied” and 60 is the lower limit of region “many users dissatisfied”. Initially, Pre-registration and Post-registration schemes are compared. As described in Section 8.2, for a successful execution of a Pre-registration handoff, the L2-MT trigger
(5)
8.2 Analysis As mentioned above, the MN used predefined trajectory to eliminate the need of a movement detection mechanism. Nevertheless, the accuracy of this mechanism is essential for the successful execution of the handoff process. Especially, in the case of Pre-registration were signaling messages must be exchanged between the MN and the network prior the L2
34
80
80
R Factor
85
R Factor
85
75
75
ToFA-nFA = 5 ms ToFA-nFA = 15 ms
Pre-Registration Post-Registration 70
0
2
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6
8
10
12
14
16
18
ToFA-nFA = 25 ms 70
20
0
2
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6
Packet Sequence #
8
10
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14
16
18
20
Packet Sequence #
Figure 5: R-factor for both schemes
Figure 7: Impact of ToF A−nF A
85
85
80
80
R Factor
R Factor
75
75
70
65
60
Prereg G.729A(VAD) Prereg G.711 Prereg GSMFR (VAD) Postreg G729A(VAD) Postreg G.711 Postreg GSMFR (VAD)
70
TMT-LD = 50 ms
55
TMT-LD = 75 ms TMT-LD = 100 ms 65
0
2
4
6
8
10
12
14
16
18
50
20
Packet Sequence #
0
2
4
6
8
10
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Packet Sequence #
Figure 6: Impact of TM T −LD
Figure 8: Comparison of Various Codecs
must occur some tens of seconds before the L2-LD event. This delay between the L2-MT and L2-LD is denoted by TM T −LD and it was set to 75 ms. Moreover, in the Postregistration the crucial factor is the link delay between oFA and nFA, denoted by ToF A−nF A . This delay was set to 5 ms. Fig. 5 shows that at a Pre-registration handoff the first packets experienced higher delays than that of a Postregistration. This is a result of the time interval between the L2-MT and L2-LD triggers. The longer the time interval the earlier the HA registration and the more time the MN will not receive packets from the oFA although it is still connected to it. Instead packets will be forwarded to and buffered at the nFA, experiencing higher End-to-End Delay. On the other hand, on the Post-registration scheme packets are delivered through the oFA until the L2-LD, and only after this event are forwarded to the nFA. However, R-factor reaches the initial level in the Pre-registration case whereas in the Post-registration it is stabilized at a lower level. This is a result of the impact of the additional delay imposed by tunneling packets from the oFA to the nFA and it is not eliminated even after the standard MIPv4 Registration. Below, the impact of TM T −LD and ToF A−nF A on the LLH performance is investigated. As explained above, in the Pre-registration scheme the nFA registers the MN to the HA after the L2-MT event.
From that moment on, packets will be sent to the nFA and will be buffered until the L2-LU event. Consequently, the smaller the TM T −LD delay less packets will be buffered. Fig. 6 shows that the R-factor degradation is highly affected by the increase of the TM T −LD . Combining the results of Fig. 4 and Fig. 6 we conclude that TM T −LD must range between 50 and 100 ms to successfully meet the requirements of LLH protocol and VoIP application. As described above, the value of ToF A−nF A is the main drawback of Post-registration. Fig. 7 shows the R-factor degradation for different values of the ToF A−nF A . The Rfactor decreases more as the ToF A−nF A increases. Additionally, an increase is observed after the standard MIPv4 Registration but the R-factor level after the registration is lower the higher the ToF A−nF A delay. Furthermore, it is worth noting that the number of the packets that will be tunneled between oFA and nFA depends on the transmission delay between nFA and HA and can be some tens to hundreds of seconds. As a result, we can conclude that Preregistration depends on the accuracy of the movement detection mechanism and in this case less packets experience a higher degradation of the R-factor. On the other hand, Post-registration depends on the MN’s location and more packets experience a lower degradation. Finally, Fig. 8 compares G.711, G.729A and GSMFR.
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11.
Among these codecs, G.729A and GSMFR, which use VAD, experienced lower degradation of the R-factor as the silence suppression reduces the packet loss probability. Moreover, it can be inferred that the time needed for the value of the Rfactor to reach the initial level is equal for the various codecs. The G.729A with double the sampling rate compared with G.711 and the GSMFR needs double the number of packets to reach the initial level.
REFERENCES
[1] G. Bianchi. Performance Analysis of the IEEE 802.11 Distributed Coordination Function. IEEE Journal on Selected Areas in Communications, 18(3):535–547, March 2000. [2] C. Blondia, O. Casals, L. Cerda, N. V. D. Wijngaert, and G. Willems. Performance Evaluation of Layer 3 Low Latency Handoff Mechanisms. Mobile Networks and Applications, 9(6):633–645, April 2004. [3] R. Cole and J. Rosenbluth. Voice over IP Performance Monitoring. Computer Communication Review, 31(2):9–24, April 2001. [4] T. D. Dang, B. Sonkoly, and S. Molnar. Fractal Analysis and Modeling of VoIP traffic. Telecommunications Network Strategy and Planning Symposium, pages 123 – 130, June 2004. [5] K. El-Malki. Low-Latency Handoffs in Mobile IPv4. IETF, RFC 4881, June 2007. [6] H. Fathi, R. Prasad, and S. Chakraborty. Mobility Management for VoIP in 3G systems: Evaluation of Low-Latency Handoff Schemes. IEEE Wireless Communications, 12(2):96 – 104, April 2005. [7] OPNET Modeler 15.0, http://www.opnet.com. [8] IEEE Std 802.21-2008, IEEE Standard for Local and Metropolitan Area Networks: Media Independent Handover Services. [9] L. Kleinrock. Queueing Systems, Volume 1: Theory. John Wiley & Sons, Inc., 1975. [10] ITU-T Recommendation G.114 : One-way transmission time. [11] C. Perkins. IP Mobility Support for IPv4. IETF, RFC 5944, November 2010. [12] ITU-T Recommendation G.107 : The E-model: a computational model for use in transmission planning. [13] S. Shin and H. Schulzrinne. Measurement and Analysis of the VoIP Capacity in IEEE 802.11 WLAN. IEEE Transactions on Mobile Computing, 8(9):1265 –1279, September 2009. [14] C.-C. Tseng, L.-H. Yen, H.-H. Chang, and K.-C. Hsu. Topology-aided Cross-Layer Fast Handoff Designs for IEEE 802.11/Mobile IP Environments. IEEE Communications Magazine, 43(12):156–163, December 2005.
9. CONCLUSION In this paper, the LLH performance was evaluated for supporting MM in 802.11b WLANs where mobile clients use real-time VoIP application. Our evaluation was based on the E-Model which is a computational model to estimate user’s satisfaction. The results showed that the protocol’s performance is dynamic in relation to the varying network conditions (i.e. time delay from the Home Network) and implementation (i.e. codec, movement detection). Moreover it was investigated the range that these parameters may vary while maintaining a satisfactory level of VoIP QoS. Finally, the results showed that the handoff process can be significantly improved by LLH which exploits L2 triggers to execute a rapid or even transparent L3 handoff. The overall procedure can achieve an over 70 score of the R-factor even for the most stretched scenarios.
10. ACKNOWLEDGMENTS This work was supported by the ROMEO project (grant number: 287896), which was funded by the EC FP7 ICT collaborative research programme.
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