Voice over Wi-Fi

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Mobile computers and wireless networking increasingly entice a mobile workforce to carry more computing power and maintain a real-time Internet connection to their company resources and customers. The next logical move is to provide voice over Wi-Fi.

Voice over Wi-Fi is already making its way into the vertical markets. According to the high-tech research firm, In-Stat/MDR, there has been additional demand from verticals such as education, healthcare, retail, and logistics for VoIP using Wi-Fi networks. While an estimated 80,000 wireless IP handsets shipped in 2002, shipments of voice over Wi-Fi handsets are expected to surpass half a million units by 2006.

Even though the Wi-Fi WLAN installed base is increasingly adding voice to existing wireless networks, and the market is projected to grow significantly, there is little demand for wireless voice beyond the previously mentioned vertical markets. Nonetheless, VoWLAN vendors bank on the fact that if the solution is easy and cheap enough to implement, it will eventually find its way into areas outside those markets. As a result, two vendors, Symbol and Spectralink, have partnered with PBX, wireless LAN, and LAN Telephony vendors to sell their Voice over Wi-Fi products. Industry experts predict that as the demand for voice over WLAN increases, more vendors will enter the market, bringing handset prices down, and pushing VoWLAN handsets out to the more mainstream business environments.

Although Spectralink has its own proprietary QoS method, even that company understands that a standard QoS specification would allow for more competitors to enter the market, driving prices down, which will result in faster market growth. So while existing VoIP technology can provide voice services over a wireless network, to make Voice over Wi-Fi a reality, engineers must tackle QoS, security, and roaming issues.

While we touched on some the issues engineers must address when provisioning QoS in a wireless environment earlier, let's now look at the same issues as they apply to Voice over Wi-Fi. Then we will consider some solutions being prepped by the industry and standards bodies to attack those issues.

Note 

I want to thank CMP Media LLP and Ravi Kodavarti, the author of "Overcoming QoS, Security Issues in VoWLAN Designs," published on the CommDesign.com website on April 3, 2003, as the following relies on that article.

Stacking on UDP

The 802.11 specification supports two modes of operation: infrastructure and ad hoc. In infrastructure mode, all end stations communicate to the wired network and to each other through an access point (AP). The AP must provide bridging functions to facilitate traffic either between the end stations or between end stations and the wired networks. An ad hoc network allows connectivity between two end stations, without the need for an AP, using a peer-to-peer type protocol.

The 802.11 standard controls the interface mechanisms at the OSI's Data Link MAC sublayer and the Physical (PHY) Layer, with higher layer protocol support left to the user. In the case of voice communications, an implementation using RTP/UDP/IP would reside on top of the 802.11 MAC and PHY. Fig. 16.7 shows the different layers of the UDP protocol stack that could reside on top of the MAC and PHY.

Figure 16.7: In VoWLAN systems, the UDP protocol sits on both the MAC and PHY layers.

In theory, the architecture described above would provide an effective way to deliver voice capabilities over 802.11 links. But, in reality, designers still will face some QoS issues when working with an 802.11 link. Let's look at this issue in more detail.

Achieving QoS

There are significant differences between wireless and wireline networks with respect to QoS issues. QoS in wired networks range between guaranteed service and best-effort service. Guaranteed service works when bandwidth of the network is typically larger than the bandwidth of the service that is guaranteed. In best-effort service, the individual bandwidth allocated changes over time, and the user adjusts the bandwidth requested based on the congestion of the network. In effect, each of these types of network implementations enables QoS by decreasing packet loss, latency, and jitter.

The UDP protocol can be used in networks that can provide guaranteed service. UDP dumps packets on the network and hopes that it goes through to the other side. It relies on higher layers to deal with the issues of a packet that does not make it through. The TCP protocol can also be implemented in best-effort networks (e.g. IP networks). As part of TCP, there is an acknowledgement sent from the destination. If an acknowledgement is not received, the transmission will be re-sent at a slower rate—the assumption here is that the network is congested.

Typically, VoIP implementations use UDP even for best-effort networks, and they account for the lost packets using various higher layer techniques. These implementations assume that the underlying network will be designed to account for the latency and jitter requirements of the higher-layer application.

In a wired network, accounting for latency and jitter are fairly straightforward—that's not the case in a wireless network. Unlike the wired network, WLANs must deal with tough propagation issues in order to determine channel performance. Thus, during the design of a WLAN system, engineers must combat issues such as multipath and Rayleigh fading,

Note 

Rayleigh fading occurs when an end-user device moves about while transmitting and receiving data. Rayleigh fading is caused by multipath reception. When a mobile antenna in a Wi-Fi network receives a large number of reflected and scattered waves, wave cancellation effects cause the instantaneous received power, as seen by that moving antenna, to become a random variable, dependent on the antenna's location.

To account for the uncertainty in the wireless medium, the 802.11 MAC includes an acknowledgement (ACK) protocol. When a packet is transmitted, the sender firsts listens for any activity on the air, and if there is none, waits a random amount of time before doing a transmission. This methodology is called carrier sense multiple access/collision avoidance (CSMA/CA), which can be viewed as a "listen first, talk later" methodology. If an ACK is not received, either due to interference or collision, then the entire process is repeated. The MAC layer ACK protocol is independent of the higher layer protocol, whether it is UDP or TCP.

The ACK protocol builds a layer of reliability on the WLAN transmission, making it very useful in data transmissions. However, in voice applications, this protocol adds jitter and latency to voice traffic. In order to account for jitter, buffers need to be used, which in turn add more latency.

Figure 16.8: This diagram illustrates Wi-Fi's RTS and CTS mechanisms.

The ACK function is not the only QoS headache for designers looking to deliver voice services over WLAN systems. The WLAN MAC also includes a request to send/clear to send (RTS/CTS) mechanism. When used together, RTS and CTS decrease the chance of collision on a system by making sure that end stations in the vicinity of the source and destination hear the RTS and CTS respectively. RTS and CTS add robustness to the system, at the cost of adding latency to the packets that are transmitted using this protocol. Fig 16.8 shows the cone of influence of an RTS and CTS frame exchange.

Avoiding the QoS Problem

To avoid the problems caused by the ACK protocol, designers can implement other techniques to reduce retransmissions. One way to accomplish this task is by fragmenting a packet into smaller packets.

While an ACK function is still required during transmission of fragmented packets, it is expected that overall latency and jitter will decrease as the likelihood of a smaller transmission getting corrupted is reduced. This would benefit wireless VoIP implementations, especially if low-bit-rate vocoders were used to compress the voice traffic. For example, designers can use a G.729 or G.723 codec to decrease overall latency and jitter on a wireless system, even though these vocoders add some fixed latency to the voice path. Since digital signal processors (DSPs) work very well in vocoding algorithms, it would vastly improve the voice quality of a wireless VoIP implementation if a DSP were present as part of the mobile computing device.

The 802.11e draft specification provides another alternative for dealing with QoS problems. This draft specification defines an enhanced distributed control function (EDFC) that allows a WLAN access point to provide up to eight virtual channels to every computing device. In order to ensure the highest priority channel is transmitted first, each of these eight channels has associated QoS parameters.

Additionally, under 802.11e, an AP could also support a hybrid control function (HCF). Through this function, the AP can take control of the channel before any of the stations do, thus reducing collision overhead and the number of retransmissions. Since the 802.11e draft standard is supplementary to the 802.11 MAC sublayer, however, it would reduce overall latencies for wireless VoIP if implemented as part of a MAC hardware implementation.

The 802.11g specification also helps to solve some of the QoS problems caused by interference on wireless channels. This spec defines the use of either orthogonal frequency division multiplexing (OFDM), or packet binary convolutional coding (PBCC) coding schemes. To provide enhanced error protection, these modulation schemes employ convolutional coding, thus allowing them to deliver better packet error rate, latency, and jitter, due to the superior coding nature. (Note: The OFDM-based 802.11a spec also supports convolutional coding.)

Security Issues

QoS is not the only issue designers must tackle when pitching VoIP services over 802.11 links. Unlike VoIP over wired systems, VoIP over a WLAN system entails comprehensive security for all aspects of a call. The reason network administrators implementing VoIP over a wireline network are not be overly concerned about an attack on their secure network is that typically all Ethernet drops are well protected, and it is virtually impossible for a hacker to get access to the network without breaking into the facility. Whereas, the main aspects of security in a WLAN environment are the privacy of a voice call and protection from denial-of-service attacks. Thus it is imperative that authentication and packet traffic are secure in order to ensure security in these cases.

The 802.11i standard is a MAC sublayer enhancement allowing support of both packet security and authentication security. The authentication security stems from the 802.1X protocol. 802.1X does not provide any cipher support. Instead, it provides a framework for authentication and key management functions using the extensible authentication protocol. The 802.1X protocol allows for a mechanism where a server on a network can provide dynamic keys to each WLAN client. The draft 802.11i proposal also supports 802.11X enhancements with respect to the pre-authentication of clients. This work is primarily driven to support roaming on WLAN networks.

Current mechanisms of 802.11 cipher-based security methods revolve around using the Wired Equivalent Privacy (WEP) protocol. However, WEP is not considered adequate for enterprise applications, since hackers can easily decode the underlying key that is used for data traffic. Additionally, since WEP is a static implementation, it is a chore for network administrators to change the key on an AP, because this would entail changing it on every station as well. Some implementations use access control lists that authenticate based on the MAC addresses of computing devices. However, MAC addresses can be easily duplicated to spoof the AP.

To address this security issue, the Wi-Fi Alliance has adopted a subset of 802.11i for immediate certification. This program is referred to as Wi-Fi protected access (WPA).

While the security features of cipher support and authentication support in the 802.1 li standard afford a layer of protection for WLAN networks, they also add complexity for voice traffic. Authentication for server-based methods adds latency to the setup of a call, and ciphering using WEP, WPA, or AES adds latency to each packet (if these were to be implemented in software). The 802.11 standard treats 802.11i as a MAC layer enhancement. Therefore, in order to minimize delay, it is imperative that silicon vendors add support for the authentication and cipher security as part of their hardware.

Note 

Chapter 17 has more detailed information on all of the security methods mentioned in this section.

Roaming and Interoperability

Roaming and interoperability also play a critical role in the development of WLAN systems that support voice. On the roaming front, WLAN system calls must support fast handoff and authentication between access points when handling voice calls. If fast handoff is not supported, designers will encounter delay during the probe, authentication, and re-association stages.

The Inter Access Point Protocol (IAAP) and other proprietary methods support roaming between different APs. Studies show that handoff delays between different APs can be as high as 400 ms. The 802.1X additions to pre-authentication, as part of the 802.11i draft, and the AP roaming protocols, as part of the 802.11f draft, address methods to decrease handoff delays. Silicon vendors must provide support for these features as part of their MAC implementations in order to support VoIP mobility.

In addition to dealing with roaming between WLAN devices, 802.11 designers must be concerned with roaming voice calls between cellular and WLAN systems. Right now, standards on this front are pretty crude. The cellular sector has defined some packet-based specs, but most of the efforts to date have focused on data services. Thus, to provide true VoIP roaming across cellular and WLAN networks, standards will need to be established to promote wide scale deployment and adoption. Currently, vendors are working on proprietary ways of solving this issue.

As engineers have observed, a lot of pitfalls lay ahead in the delivery of voice services over wireless links. Fortunately, the 802.11 committee, through its e, i, and f drafts, is addressing this issue head on. In the interim, however, designers looking to add voice capabilities to their WLAN designs must deal with packet loss and jitter issues carefully.

Available VoWLAN Systems

Some vendors haven't waited for the 802.11e standard to be ratified. Many have struck out on their own in order to fulfill the growing market for voice over Wi-Fi. The beneficial features Wi-Fi phones bring to the end-user include the ability to converge the telephony function directly into an already existing data network infrastructure, and the ability to operate with any access point. Customers also turn to IP telephony because it simplifies their network infrastructure and can lower an organization's overall communications costs since a mobile IP phone allows them to add mobility without paying for cell phone airtime.

Calypso Wireless recently launched its new C1250i videophone. The phone uses Wi-Fi networks for both videoconferencing and data access. This mobile videophone also is capable of seamlessly switching back and forth between the cellular networks and Wi-Fi networks.

Cisco Systems announced a "new" VoIP 802.11 handset in April 2003, which it dubbed the "7920 IP Phone." Cisco plans to ship this Wi-Fi phone to U.S. channel partners sometime in June 2003, with availability in other countries soon after.

Note, however, that the 7920 IP Phone communicates only with 802.11b technology and "is designed for use within enterprises rather than totally replacing a cell phone," at least according to Charlie Giancarlo, Cisco's senior vice-president of switching, voice, and carrier systems.

Of course, Cisco isn't new to the voice over Wi-Fi marketplace. Its 7960 IP phone, which sits on a desk and plugs into a wired Ethernet network, has an installed user-base. However, the 7920 IP phone offers the added benefit of mobility around a building or campus that has a wireless LAN.

According to Giancarlo, the 7920 could be ideal for environments such as retail stores, where employees need to move around a site during the workday. "The phone should deliver two hours of talk time and 24 hours of standby time before the need to recharge," he adds.

Motorola is also in the process of producing Wi-Fi equipped phones. Those phones supposedly will be able to switch automatically and seamlessly from one Wi-Fi location to another.

Nokia has demonstrated VoIP over WLAN. The Nokia system uses Mobile IPv6, fast handovers, and context transfer for security, QoS, buffers, and header compression state. The company's measurements indicate that the Mobile IP parts add very little to the delay overhead. What the company found problematic, though, is the device driver for the 802.11 NIC. According to the company, if it can find a solution for that little dilemma, the WLAN phone's handover time would be well under 5 milliseconds.

Intermec Technologies added VoIP capabilities to its wireless-enabled 700 series of mobile computers in a bid to keep retail clerks accessible to answer questions, make decisions, and take customer requests—whether on the floor or in the back room-improving customer service and employee productivity.

The 700's VoIP technology enables unit-to-unit voice communications over existing 802.11b wireless LAN infrastructures. Unlike current in-store voice communications technology, the 700 with VoIP provides enterprise-level communications, allowing workers to talk to anyone else on a corporate network using a 700 device—whether they are in the same store, a store across town, or the distribution center across the country.

"Now workers in every store and warehouse can have instant voice communication to locate merchandise and solve problems for customers," said Scott Medford, director of retail business development for Intermec. "And it's done with the existing wireless and wired LAN infrastructure simply by adding software to a retailer's in-store data collection device."

The 700 mobile computer, which weighs less than 16 ounces with battery, is designed to fit comfortably in the hand. It uses Microsoft's Pocket PC 2002 and runs virtually all 32-bit Windows CE applications. To support the voice application, the device comes standard with full duplex audio capability and a standard 2.5mm headset jack designed to accept the widest possible variety of headset options available. The user-replaceable, rechargeable lithium ion battery delivers 8-to-10 hours of continuous use on a single charge; more than adequate for a full shift's work. The 700's rugged case can withstand multiple four-foot drops onto concrete and its high-contrast VGA touch screen delivers excellent readability indoors and out.

The 700 with a VoIP solution package includes application software, installation guide, installation CD, and a standard cell phone-style hands-free earphone with a microphone. The package delivers consistently high, conversational quality voice throughout a WLAN's coverage area. All that's required to get VoIP up and running over an existing WLAN are: some 700 handheld devices with integrated 802.11b, the VoIP application, and, of course, the 802.11b network. No servers or gateways are needed. Furthermore, both the voice application and unit directory services can be centrally managed with all other aspects of the 700 via the Intermec 6920 enterprise device manager.

SpectraLink's NetLink IP Wireless Telephone is being tested at North Carolina State University. This phone is compatible with 802.11b wireless LAN, and uses the Skinny Client Control Protocol that is supported on Cisco's Call Manager (which provides caller ID, call forward, conference calling, multiple line capabilities, and many other common functions). When used in conjunction with the SpectraLink Voice Priority (SVP) server, the system allows QoS for voice packets. The SVP server helps to recognize voice packets being transmitted over the network, and gives these packets a higher priority to lessen the number of packets lost on the network. However, once 802.11e is ratified, the SVP server will be obsolete; but, since 802.11e will be a new standard, it could be a year or two before the standard migrates into usable service.

To incorporate the Wi-Fi phones onto the network involves three simple steps: (1) the phones need to be setup on the Cisco Call Manager in the same way that any other VoIP compatible client would be configured; (2) the MAC address of the phone and the phone's DNS information need to be configured; (3) the final step is to activate each phone when it is within the WLAN's coverage area, so it can receive its network information from the server.

Once a phone is activated, an individual could, for example, roam throughout the WLAN's coverage area with the phone, switching from access point to access point without loss of any voice packets. Since the access points are monitoring activity, they can allow the user of the VoIP wireless phones to use the phone anyplace on the campus as long as there is an access point available.

Symbol Technologies continues its wireless product and technology leadership. In 2001, Symbol offered the first wireless VoIP handset that supports worldwide both IEEE 802.11b 11 Mbps Direct Sequence (DS) wireless local area networks and International Telecommunications Union (ITU) H.323 standard-based telephony systems. The Spectrum24 NetVision Phone is offered directly to Symbol's core customer base and indirectly to its community of wireless VoIP telephony and channel partners through OEM and reseller agreements. The NetVision handset allows enterprise customers to add wireless voice connections to their in-building wireless LANs, and to achieve mobility with the same level of functionality as their existing Ethernet desktop phone systems. Symbol expects the demand for in-building VoIP wireless handsets, like NetVision, to grow proportionally with the adoption of 802.11b wireless LANs, as enterprise customers extend their wired networks to support wireless PDAs, laptop PCs, and yet-to-be developed converged voice and data devices.

The Symbol wireless VoIP product set features the NetVision family products resold with the Ericsson WebSwitch 100 G4, a four port desk top gateway for the small business. The wireless VoIP handset is also designed to integrate directly into the Nortel Networks ITG Product line, Mitel IPERA 2000, and Cisco AVVID/Call Manager, while firmware has been developed to support a host of other telephony systems from Alcatel, Innova-phone, Motorola, Vegastream, and more.

When integrated with gateway products, the NetVision wireless VoIP handset supports PBX (Private Branch Exchange) supplementary services, such as call waiting, call transfer, conferencing, call park, paging, as well as Symbol's added mobility features, including intercom mode (walkie-talkie), text messaging, and pre-emptive roaming. Furthermore, the NetVision phone supports Symbol's Quality-of-Service (QoS) voice prioritization to guarantee high voice quality.

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