C ONCLUSIONS - IPSEC-BASED VOIP PERFORMANCE IN WLAN ENVIRONMENTS

CHAPTER 3 IPSEC-BASED VOIP PERFORMANCE IN WLAN ENVIRONMENTS

3.5 C ONCLUSIONS

This chapter investigated the performance of IPsec-based VoIP service measured in the WLAN environment. The overheads caused by IPsec encapsulation were discussed in several aspects.

After IPsec encapsulation, the performance of IEEE 802.11b without packet loss degrades by 4% (in terms of the number N of RTP streams supported) for G.729 and 5% for G.711, respectively.

The IPsec encapsulation causes more packet processing, packet retransmisstions, and packet loss, and therefore results in extra latency. When N < 15, the IPsec overhead is less than 9.26%. When 15 ≤ N ≤ 20, the latency overhead for IPsec significantly increases (can be up to 570.97%). For N > 20, the latency overhead for IPsec drops to less than 4.38%.

Similarly, IPsec encapsulation results in extra jitter overhead. When N < 15, the IPsec overhead is about 10%. When 15 ≤ N ≤ 25, the jitter overhead for IPsec significantly increases (can be up to 26.3%). For N > 25, the jitter overhead for IPsec drops to less than 13.47% for G.711 and 8.24% for G.729.

Our study provides guidelines to select appropriate system parameters setups for the VoIP service over WLAN environment. Specifically, for an IEEE 802.11b AP, the system saturates if we pump more than 20 original RTP streams or 19 IPsec RTP streams for G.729 (or 18 original RTP streams or 17 IPsec RTP streams for G.711). Also, for transmission in IEEE 802.11b, at least one G.711 or G.729 RTP packet (i.e. 20 ms) should be queued in the jitter

buffer to achieve acceptable VoIP performance specified by [33]. We also observed that the jitter buffer size is not affected by IPsec encapsulation in the IEEE 802.11b configuration in our study.

Chapter 4 M-Taiwan Experience in VoIP-WiMAX Trial

Considering voice as a dominant telecommunication service, the performance of Voice over IP (VoIP) plays a critical role in deployment of WiMAX technology providing All-IP network services. To that effect, in this chapter we investigate the performance of a WiMAX-based VoIP established under the Mobile Taiwan (M-Taiwan) field-trial funded program. To achieve the objectives of the trial the measurement results expressed in the form of Mean Opinion Score (MOS), packet loss, packet delay and jitters. For the worst-case-scenario, the tests were conducted under a stringent condition of both communicating devices, wirelessly connected to the same WiMAX base station under a heavy background traffic and interference, were experiencing simultaneous handovers during the communication.

Upon our analysis the field measurements confirm an excellent performance when both communicating devices kept stationary and show an acceptable quality for the service when both communicating devices are on the move at a speed of 50 Km/h.

4.1 Introduction

Taiwan's Wi-Fi industry accounts for more than 90% of the global market share. In its quest to identify the next generation products, the Taiwan government has chosen Worldwide Interoperability for Microwave Access (WiMAX) [5] as one of the major directions for Taiwan’s wireless industry, and has established the Mobile Taiwan (M-Taiwan) Program as the blueprint for an island-wide WiMAX environment. M-Taiwan aims at developing chip

sets and base stations (BS). For example, WiMAX chip sets have been developed by Mediatek, and the BSs have been developed by T-Com and ZyXEL. Furthermore, by creating its own WiMAX ecosystem, Taiwan offers not only manufacturing capabilities, but also an entire service and application test-bed for mobile services, mobile learning and mobile life.

Since 2006, 18 large-scale WiMAX service trials have been deployed in Taiwan [34, 35].

In M-Taiwan, the Voice over IP (VoIP) service is considered as an enabling technology integrating broadband data applications with the voice. In particular, IP Multimedia Core Network Subsystem (IMS) [15] is utilized for voice and data integration. Under the support of the M-Taiwan Program, this chapter investigates the VoIP performance for a WiMAX deployment in Taipei City. In this chapter, we elaborate on the VoIP experimental environment, describe the output measures, and demonstrate the VoIP performance with and without mobility. The remainder of this chapter is organized as follows. Sections 4.2 and 4.3 provide a brief overview of VoIP service and WiMAX system. The general configuration set-up for the experimental field tests explained in Section 4.4 followed by the service performance measurement system in Section 4.5 and detailed results in Section 4.6.

4.2 VoIP Overview

With the explosive growth of the Internet subscriber population, VoIP has become the most promising low-cost option for voice communication over the IP network. In the M-Taiwan program, VoIP is implemented by using the Session Initiation Protocol (SIP) [22] and Real-Time Transport Protocol (RTP) [23].

4.2.1 Session Initiation Protocol and RealTime Transport Protocol

IETF RFC 3261 defines SIP for Internet telephony [22]. As an application-layer signaling protocol over the IP network, SIP is designed for creating, modifying, and terminating multimedia sessions or calls. A SIP customer premise equipment (CPE) is installed with a user agent. The user agent contains both a User Agent Client (UAC) and a User Agent Server (UAS). The UAC (or calling user agent) is responsible for issuing SIP requests, and the UAS (or called user agent) receives the SIP requests and responds.

The SIP message specifies the Real-Time Transport Protocol (RTP) [23], which delivers the data in the multimedia sessions. Implemented on top of UDP, RTP detects packet loss and ensures an ordered delivery. The RTP packet also indicates the packet sampling time from the source media stream. The destination application can use this time stamp to calculate delay and jitter to provide the QoS feedback.

SIP conjuncts with protocols such as Session Description Protocol (SDP) [36] to describe the multimedia information. It conveys sufficient information to enable applications to join a session. During the session initiation, SDP describes the media type, media protocol, and codec number supported by the session endpoints to announce the endpoints capabilities. SDP provides the RTP information such as the network address and the transport port number of the RTP connection. Details of SIP and RTP can be found in [15].

4.2.2 EModel

The quality of a communication service is traditionally based on subjective perception and typically measured by the Mean Opinion Score (MOS), which considers the effects of

equipment and impairment factors to subjectively quantify the perceived quality of a transmission such as voice based on typical users’ perceptions ]. The MOS values range are quantized to 5 levels, from 1 to 5, where 1 is unacceptably bad, 2 is poor, 3 is fair, 4 is good, and 5 is excellent. The ITU-T G.107, however, defines an E-Model which provides a computational model for rating the end-to-end transmission performance for the VoIP service [37]. The E-Model considers different kinds of transmission impairments add on linearly to the scale of the rating factor R. The model then converts the value of R into a MOS scale that quantifies an overall conversational quality.

The rating factor R is then expressed as follows,

R = Ro – Is – Id – Ie-eff + A. (4.1)

In the right-hand side of (4.1), these factors are described as follows:

R^o: The basic signal-to-noise ratio includes the noise sources such as circuit noise and room background noise.

I^s: The simultaneous impairment factor combines the impairments that occur simultaneously with the voice signal. These impairments include the quality degradation caused by the overall loudness, non-optimum sidetone and quantizing distortion.

I^d: The delay impairment factor represents the impairments due to delay in arrival of the voice signal.

I^e-eff: The effective equipment impairment factor represents impairments caused by low bit-rate CODEC and the impairments due to random packet loss.

A: The advantage factor allows for compensation of impairment factors when there are other advantages of access to the user. ITU-T G.107 suggests the default value 0 for A.

The rating factor R is then converted into an estimated MOS value as follows,

For R < 0: MOS = 1

For 0 < R < 100: MOS = 1 + 0.035R + R x (R – 60) x (100 – R) x 7 x 10^{– 6} (4.2)

For R > 100: MOS = 4.5

Therefore, the estimated MOS values range from 1 to 4.5

The relation (4.2) between the estimated MOS value and the rating factor R is illustrated in Figure 1.

1 1.5 2 2.5 3 3.5 4 4.5 5

0 10 20 30 40 50 60 70 80 90 100

Rating Factor R

Estimated MOS

Figure 4.1 Estimated MOS Value as a Function of Rating Factor R

4.3 WiMAX Overview

Following the success of the Internet technology, broadband data communication services have been provisioned to the expert communities for decades, which for the wired and fiber connections have been achieved with the turn of the century. For wireless it is due any time within the next decade where superior mass production of quality wireless components to extend the frequency range and overcome shadowing and multipath fading issues using super sensitive receivers [38]. Now, with the industry capable of providing the WiMAX technology for superiority of virtually nil infra-structure costs, we are able to offer a data-enabled very low cost wireless metropolitan area network (WMAN) style wireless broadband access (WBA) solutions which in long run may overshadow competitive solutions [39] due to the fact that WiMAX is able to provide broadband wireless access with wide service coverage, high data throughput, high mobility and greater service flexibility [40, 41]. Figure 4.2 shows a simplified WiMAX network architecture, which consists of the access service networks (ASNs; see Figure 4.2 (a)) and the connectivity service networks (CSNs; see Figure 4.2 (b)).

An ASN provides radio access (such as radio resource management, paging and location management) to the WiMAX mobile station (MS; Figure 4.2 (e)). The ASN comprises ASN gateways (ASN-GWs; see Figure 4.2 (c)) and WiMAX BSs (see Figure 4.2 (d)). Every ASN-GW connects to several BSs. The ASN-GWs are also connected to each other to coordinate MS mobility. A CSN consists of network nodes such as the mobile IP (MIP) home agent (HA; see Figure 4.2 (f)) [3], the authentication authorization, and accounting (AAA) server (see Figure 4.2 (g)) and the dynamic host configuration protocol (DHCP) server (see Figure 4.2 (h)). The CSN provides IP connectivity (such as Internet access and IP address allocation) to a WiMAX MS and interworks with the ASNs to support capabilities such as AAA and mobility management. Before an MS is allowed to access WiMAX services, it must

be authenticated by the ASN-GW (which serves as the authenticator) and the AAA server in

Figure 4.2 Simplifiied WiMAX Network Architecture

The WiMAX Physical (PHY) and Media Access Control (MAC) layers are defined in IEEE 802.16 standard to support multiple services with point-to-multipoint and mesh broadband wireless access [35]. The point-to-multipoint mode defines one-hop communication between a BS and an MS, while the mesh mode allows traffic to be directly exchanged and forwarded among neighboring BSs. IEEE 802.16 is initially designed as an access technology for WMAN. The first specification IEEE 802.16-2004 targets on fixed and nomadic accesses. In IEEE 802.16e-2005 amendment, the IEEE 802.16e system (Mobile WiMAX) further provides functions to facilitate mobile accesses. We introduce the functions of MAC and PHY layers in the following subsections. Details of WiMAX technology can be found in [38]. Figure 4.3 illustrates the IEEE 802.16 protocol stack. The functions of the WiMAX PHY and MAC layers are described in the following subsections.

4.3.1 The Media Access Control Layer

There are three sublayers in IEEE 802.16 MAC layer: service-specific convergence sublayer (CS; see Figure 4.3 (a)), the MAC common part sublayer (see Figure 4.3 (b)), and the security sublayer (see Figure 4.3 (c)).

Figure 4.3 IEEE 802.16 Protocol Stack

The service-specific CS performs packet classification, header suppression, and converts packets between the upper layer and the MAC layer. The IEEE 802.16 currently supports packet CS and ATM CS to interface with IP and ATM protocol layers, respectively. In IEEE 802.16, the connections between the MSs and the BSs can be identified with unique connection identifications (CIDs). The packet CS may check the IP or TCP/UDP header of a packet to determine its CID. Besides the CID mapping, the CS may perform the optional payload header suppression to eliminate the redundant parts of the packets during the transmission over the air interface.

The MAC common part sublayer provides the medium access, connection management, and QoS functions that are independent of specific CSs. After the packets are processed by the CS, the MAC common part may perform automatic repeat request (ARQ) for retransmitting lost packets. ARQ is optional in IEEE 802.16 but is mandatory for IEEE 802.16e.

In IEEE 802.16, QoS functions are implemented in the MAC common part sublayer. Several service classes are defined to satisfy various QoS requirements. For example, a VoIP connection is often associated with unsolicited grant service (UGS) to support constant bit-rate (CBR) or CBR-like flows with constant bandwidth allocation. According to the QoS associated, the BS schedules radio resources with various scheduling disciplines, such as round-robin and first-in-first-out.

The security sublayer provides privacy and protections through encryption, decryption, and authentication. In IEEE 802.16, an MS is requested to perform the authentication and authorization before attaching to a WiMAX network. During the authorization procedure, the MS negotiates with the BS to generate the session key. To perform packet encryption and decryption, each connection is linked with a security association (SA), which contains the security information and settings such as encryption keys. Packet encryption and decryption are exercised based on the information in the SA.

Before accessing the WiMAX network, an MS should perform a complete spectrum search and synchronize the time and frequency with a BS through the ranging procedure. Then the MS starts the network entry procedure to negotiate the capabilities with the BS and performs authorization process to generate the keys used between the MS and the BS. Finally, the MS obtains an IP address from the BS, and establishes data connections with the BS.

4.3.2 The Physical Layer

In the PHY layer (see Figure 4.3 (d)) IEEE 802.16 defines several specifications for different frequency ranges and applications. For example, orthogonal frequency division modulation (OFDM) is used for non-line-of-sight operations in the frequency bands below 11 GHz. By

extending the OFDM technology, orthogonal frequency division multiple access (OFDMA) allows one channel to be shared by multiple users. The IEEE 802.16 standard defines a set of adaptive modulation and coding rate configurations that can be used to trade off data rate against system robustness under various wireless propagation and interference conditions. The allowed modulation types are binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (16QAM), and 64QAM [35].

Several duplexing technologies are provided in IEEE 802.16. In time division duplex (TDD), a WiMAX frame consists of a downlink subframe and an uplink subframe and a short transition gap is placed between the downlink and uplink subframes for receive and transmission transitions in the radio. The gap between the downlink burst and the subsequent uplink burst is called transmit/receive transition gap (TTG). The gap between the uplink and the subsequent downlink is called receive/transmit transition gap (RTG).

The duration of an OFDM symbol includes the useful symbol time and a prefix. In OFDM, all users within the same cell or sector use orthogonal subcarriers to carry the OFDM symbols.

The OFDM symbol uses a fixed-length cyclic prefix (CP) to counteract the intersymbol interference. The ratio of the CP length to the useful symbol time is defined as the guard interval, which is used by the receiver to collect signals from multiple paths and improve system performance.

4.4 VoIP Experimental Environment

The main bulk of this trial service performance measurement has been conducted during 2007-08 in the Taipai area under various communication conditions. Based on the abstract network in Figure 4.2, Figure 4.4 illustrates the network architecture for one of the WiMAX

deployments in the M-Taiwan Program. Based on mobile WiMAX (IEEE 802.16e-2005) technology [5], more than 52 WiMAX BSs have been deployed. The WiMAX ASN-GW (a Foundry’s Netlron XMR400 plus Motorola’s CAP Controller) is located in Taipei County.

The distances between the BSs to be tested in our study and the ASN-GW range from 18.5 Km to 21 Km. Every BS is connected to the ASN-GW through a 50 Mbps optical fiber link.

The ASN-GW connects to the Foreign Agent (FA; which is a Redback’s SmartEdge 400) through Gigabit Ethernet (GE). The FA connects to a core router (Juniper’s M120) through another GE. The core router connects to an L2 switch (Cisco’s Catalyst 3560E) through GE.

The L2 switch connects to the HA (a Starent’s ST-16 Intelligent Mobile GW) through GE, and connects to an FTP server through a 10/100M Fast Ethernet (FE). In the above configuration, backup for ASN-GW controller, FA, and core router are also deployed to support reliability and availability.

Figure 4.4 M-Taiwan VoIP Experimental Environment

In this experimental environment, the WiMAX MSs are installed SIP call agents, and serve as SIP CPEs. The VoIP calls are generated and measured between WiMAX CPE1 (Figure 4.4 (1)) and WiMAX CPE2 (Figure 4.4 (2)). Our experiments also include the background data traffic, which is generated from WiMAX CPE3 (Figure 4.4 (3)) to the FTP server Figure 4.4

(7)). These three CPEs are notebooks connected to Quanta/Beceem’s WiMAX (wave 2) USB dongles, and are all located in a minivan (see Figure 4.4 (4) and Figure 4.6 (left)). As illustrated in Figure 4.5 (a), it is clear that a one-way VoIP link between CPE1 and CPE2 consists of 12 hops (CPE1 ÅÆ BS ÅÆ ASN-GW ÅÆ FA ÅÆ Core Router ÅÆ L2 Switch ÅÆ HA ÅÆ L2 Switch ÅÆ Core Router ÅÆ FA ÅÆ ASN-GW ÅÆ BS ÅÆ CPE2). In Figure 4.5 (b), the data path between CPE3 and the FTP server includes 6 hops (CPE3 ÅÆ BS ÅÆ ASN-GW ÅÆ FA ÅÆ Core Router ÅÆ L2 Switch ÅÆ FTP Server).

(a) VoIP Path between CPE1 and CPE2 (b) FTP Path between CPE3 and FTP Server

Figure 4.5 Data Paths in the Experiments

Figure 4.6 WiMAX CPEs in the Minivan (left) and the WiMAX Antenna (right)

愛買百貨-60km-TCP-UL Rate(Beceem)

(a) TCP uplink transmission rate (Kbps) vs CPE speed (Km/h)

婦幼院區路線-45km-TCP-DL Rate(Beceem)

(b) TCP downlink transmission rate (Kbps) vs CPE speed (Km/h)

Figure 4.7 Real-Time Measures of TCP Transmission Rate at Various CPE Speeds

In our study, a 3-sector WiMAX BS (Figure 4.6 (right)) is typically installed at the roof of a building with the coverage of 1.5 Km in diameter. To fully utilize existing cellular infrastructure, the WiMAX antenna may be collocated with the WCDMA antenna. The

WiMAX antenna is an adaptive system with beamforming. In this WiMAX network deployment, the TDD ratio for downlink and uplink can be 3 to 1 or 3 to 2. In our experiments, 3-to-1 ratio is considered. The modulation schemes are 16QAM 3/4,16QAM 1/2, QPSK 3/4, QPSK 1/2 for uplink, and 64QAM 5/6, 64QAM 3/4, 64QAM 2/3, 64QAM 1/2, 16QAM 3/4,16QAM 1/2, QPSK 3/4, QPSK 1/2 for downlink. We observed that the bandwidth performance is significantly improved by up to 100% in our measurements when the modulation scheme is enhanced from 64QAM 1/2 to 64QAM 5/6 for downlink, and from 16QAM 1/2 to 16QAM 3/4 for uplink. Through measurements of 14 experiments, the average TCP uplink transmission rate is 3.668 Mbps. Figure 4.7 (a) plots a typical experiment of TCP uplink transmission rate as the CPE speed changes. The sample points are measured for every 2-3 seconds. The figure indicates that the transmission rate drops significantly as the CPE suddenly accelerates (e.g., when the speed increases from 6 Km/h to 59 Km/h).

The average downlink TCP transmission rate of the BSs is 10.01 Mbps (per sector). Figure 4.7 (b) plots a typical experiment of TCP downlink transmission rate as the CPE speed changes.

For a stationary CPE, the maximum and minimum uplink TCP bandwidths are 2.879 Mbps and 2.306 Mbps, respectively. The average uplink bandwidth is 2.492222 Mbps. The maximum and minimum downlink TCP bandwidths are 7.781 Mbps and 4.741 Mbps, respectively. The average downlink bandwidth is 6.881778 Mbps.

We also measure the handover delays. The average handover delays of 5 measurements are 67.78 ms for inter-BS handover at 30 Km/h, 68.125 ms for inter-BS handover at 50 Km/h, 63.5 ms intra-BS handover at 30 Km/h, and 65 ms for intra-BS handover at 50 Km/h.

Therefore, as the CPE speed increases form 30 Km/h to 50 Km/h, the handover delay increases by 0.51%-2.3%. The inter-BS handover time is 4.8%-6.7% longer than the intra-BS

handover.

Figure 4.8 Moving Path for Mobility Tests (the solid path is covered by one base station, and the dashed path is covered by another base station)

In the stationary tests, the distance between the CPEs and the BS is about 210 meters, and the output data are measured at a base station located at the 7th floor of a building in Nei-Hu area of Taipei City. In the mobility tests, two WiMAX BSs are involved. These BSs are located near the Taipei City Hall. To produce the handover effect under the controlled condition, the minivan carrying the CPEs repeatedly drove on the roads around a square area adjacent to the City Hall (see Figure 4.8). This path is covered by two WiMAX BSs and the distances between the BSs and the CPEs range from 150 meters to more than 400 meters.

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