A wireless mobile computer that is also connected to the enterprise private branch exchange (PBX) and to the larger public switched telephone network (PSTN) supports calls to associates, customers and vendors, making it a powerful tool for improving associate productivity and enhancing customer service.  The converged solution of voice and data places all the value and benefit of a traditional desktop telephone into the hands of mobile associates while leveraging the investment of the data capture capabilities.  The result is a single device on a single network that is used for multiple purposes, which enables organizations to do more for less.

This white paper explores the benefits and challenges of consolidating the voice communication features of multiple devices onto a single wireless local area network (WLAN) and into a single mobile handheld device.

Overview

Adoption of the Institute for Electrical and Electronics Engineers (IEEE^®) 802.11 standards for wireless LANs and the maturing of voice-over-IP (VoIP) technology have set the stage for a new productivity tool – converged voice and data on mobile computers.  Utilizing the installed 802.11 wireless infrastructure for both voice and data is the ideal approach to solving most employee communication requirements.  This converged solution allows for leveraging wireless LAN investments to eliminate limitations found in other communication alternatives.    

Today's Opportunity

There is a strong requirement for a durable, reliable, converged voice/data mobile computer that also provides key telephony functions.  However, most consumer-grade PDAs are unable to meet the durability requirements for high-volume usage and at the same time provide high-quality voice capability.  A durable mobile computer for the execution of business data applications combined with PBX and PSTN connectivity and toll quality voice fills an important market need.

Strategic Approach

The successful convergence of voice and data onto a mobile computer requires attention to three specific technology areas:  

• Hardware: A fast CPU to handle the voice compression/ decompression and a mechanical design that provides for microphone, speaker or speakerphone with the appropriate acoustical characteristics.
• Software: Addressing all the unique wireless VoIP challenges -- wireless quality of service (QOS), wireless latency and secure fast roaming.  
• Telephony integration: Adopting wireless VoIP often requires integration into host PBX systems and emerging PBX and iPBX solutions. 

Making the right hardware product decisions is only part of the solution for a robust converged voice/data implementation.  Addressing both the software and integration challenges requires domain knowledge.  This includes an in-depth understanding of the challenges in implementing VoIP on wireless in addition to knowing how to implement a best-in-class 802.11 wireless VoIP solution.  The following sections detail some of the available technical challenges, solution options and implementation choices.

Challenges

A wireless mobile computer uses the same transport services that a desktop computer uses to support business applications.  These applications are based on transmission control protocol/internet protocol (TCP/IP), which is configured with appropriate wireless security services.  Support of converged voice and data is similar, because it is typically a TCP/IP-based application, but the challenges faced by a wireless voice application are unique.  A robust wireless voice application also incorporates solutions to each of the following challenges.

Security

For chief information officers (CIOs), security is the biggest concern when considering the deployment of a wireless LAN.  Fear of a hacker or spy stealing or corrupting valuable company data forces information technology (IT) managers and CIOs to take a conservative position.  The security technologies applied to address these data security concerns often impact the ability to deliver good quality voice over a wireless LAN.  The primary negative factor to voice quality is the additional burden of latency resulting from the implementation of the security policies.  However, some security options have minimal impact on wireless voice quality.

There are a number of choices for wireless security schemes, mechanisms and standards.  The basic 802.11 standard defines wireless equivalent privacy (WEP) which is supported by all 802.11 product manufacturers.  However, WEP has inherent security flaws.  To address that, 802.11i provides two fixes. The first is designed for the generations of equipment already in the field and changes the way WEP works, wrapping it in additional protection protocols. The second, for new generations of devices, replaces WEP with security based on the Advanced Encryption Standard (AES), the latest high-security global standard for encryption.

The Wi-Fi Alliance, a nonprofit international association formed in 1999 to certify interoperability of wireless LAN products based on the IEEE 802.11 specification, is helping to drive support of two modes:  Wi-Fi Protected Access (WPA), versions 1 and 2.  Neither WPA1 nor WPA2 enhances the mobility support of 802.11 networks to provide good voice quality when the devices are in motion. And, unlike laptop computers used on a desk, voice-enabled mobile devices are meant to be held, moved and carried around.  

While WPA1 and WPA2 dramatically improve the security of 802.11 data networks, they are security standards first and foremost. These new security mechanisms provide stronger encryption services and add important authentication capabilities.  However, they are also less than optimal in supporting real-time audio applications like VoIP because security mechanisms take time to operate and impose a latency penalty that degrades voice quality.  In addition, all of the extra processing that is required is directly at odds with the kind of low-power operation needed in battery-operated devices. This leads to a requirement for hardware (ASIC) assist circuitry in order to get optimum voice quality when using mobile devices.

An alternative to 802.11i-based security is virtual private networks (VPNs). The use of VPN is viewed as a straightforward solution to wireless security problems, as it provides security control at the highest level with end-to-end encryption across a connection.  VPNs meet authentication and data security requirements but impose a penalty on real-time applications like VoIP.  In many cases, VPNs operate in a way that is contradictory to what is needed for good mobility. The most common and interoperable VPNs based on the IPSEC protocol assume a fixed connection link and are unsuited to mobile operations.  By comparison, VPNs based on WTLS were developed as part of an advanced mobile cellular architecture and are optimized for secure operation across multiple bearers and unreliable networks.

Unfortunately, VPNs impose a significant computational burden on the networked device.  Without an assist from a high-end processor or coprocessor, the resultant voice quality of a wireless PDA-class device is degraded.  The tunneling of the packet flow through a VPN also adds to the overall latency within the system and further reduces the voice quality.  It almost seems that VPNs and wireless VoIP are mutually exclusive.  This is not quite true, but it means that care must be taken in implementing and deploying a wireless VPN solution.  Consideration for the computing power of the mobile device, its battery capacity and the VPN design within the network fabric must all be assessed in an attempt to guarantee a secure, high-quality VoIP solution.

Another standard is Kerberos, which is a security architecture developed at MIT and implemented in most Unix products and in Microsoft^® Windows Server 2003.  Used as a standard security scheme for over 20 years, Kerberos provides a method of unit authentication and key management.  It is implemented to support mutual authentication when every device within the network fabric, including access points, is authenticated.  Once authenticated within a secured network, roaming from access point to access point is secure through the passing of pre-secured credentials, verified between parties.  This architectural approach supports the concept of a fast, secured roam without requiring a complete re-authentication upon each roam.  In addition, Kerberos goes beyond a simple authentication scheme to provide for dynamic key management for compliant devices.

Standards-Based Solutions

In the absence of a mobility-ready wireless security standard, manufacturers are moving forward with additional solutions that are good for secure mobility and build upon the 802.11i standard to make it mobility ready. 

Both in standards bodies and collaborative associations, companies are working to enhance the wireless security options available and to ensure cross-vendor interoperability of products and mobility.  The goal is to add value and mobility – enabling extensions that enable mobility.  Good examples of these in-process works include:

• Using the Wi-Fi WPA interoperability specifications to require some of the 802.11i options like pre-authentication
• Extending 802.11i within Wi-Fi to add PMK (Pair-wise Master Key) caching
• Opportunistic pre-authentication for improved roaming

Quality of Service (QoS)

Good voice quality demands the timely delivery of audio packets to the receiving device.  Typically, a delay of more than 60 msec in delivering audio packets results in a deterioration of the overall voice quality.  Management of this delivery problem requires enforcement of quality of service (QoS) that guarantees delivery of selected packet types within certain latency limits.  For 802.3 networks, QoS is mandated by the IEEE 802.1p standard, a mechanism that tags each packet or frame with a priority label.   As that frame traverses a network fabric, each switch and router enforces the indicated QoS in preference of lower QoS tagged frames. 

The wireless segment of a network, by its nature, is the more fragile component with regard to reliability frame transmission/reception – where QoS is important.  The IEEE committee has made a lot of headway in defining the 802.11e standard for QoS.  However, this technology is in its first stages and hasn't met mass acceptance and customer adoption.  Until then, today's product offerings meet this challenge using a proprietary QoS architecture.    

Wireless Congestion

VoIP traffic places a unique demand on the wireless infrastructure, specifically the high packet/second rate.  Most data applications are "bursty" and transmit large data frames.  This kind of traffic is addressed through the data throughput capacities of the wireless infrastructure such as a wireless switch (in kilobits per second).  Voice traffic is isochronous with small packets and places a different demand on the wireless infrastructure.  Because much of a voice frame is header, there are a fixed number of frames per second that can be processed.  In this case, it is possible that a large number of VoIP applications might reach the maximum capacity of a wireless LAN.  This creates points of congestion within a wireless infrastructure, degrades voice quality (even with QoS) and causes data application failures resulting from the voice traffic congestion.

Addressing this potential congestion problem are general approaches that either:

• Maximize the throughput (number of calls) through any single access point, or
• Distribute the traffic demand more evenly across multiple network resources.

Maximizing throughput is best achieved through aggregation of the audio stream packets to minimize the packet per second through any one access point or access port.  Implementing this technique has a demonstrable impact on being able to control congestion problems.  Traditionally, distribution of traffic to minimize wireless infrastructure congestion has been approached in one of two ways:

• Creating intelligent mobile client RF modules, or
• Using a centralized server within the network.

Managing potential congestion via intelligence within the mobile device requires collaboration with the infrastructure to provide additional access point/access port loading information that permits making such intelligent roam decisions.  This typically requires a modification of certain 802.11 signaling elements to carry this information.  Fortunately, the 802.11 standard is written in such a way that this is possible. 

Others attempt to manage the wireless infrastructure congestion issue with a server-based architecture where each mobile unit registers and derives roam information and roam permission from the server.  However, this is an application-level architecture, a vendor/device specific solution that requires additional devices be added to the network.

Voice/Data Mobile Computer Design Considerations

Once the network and RF challenges are addressed, the feature set of the mobile device itself is considered.  In order to provide the best user experience with wireless VoIP, the following items need to be addressed at the mobile unit level:

• A fast CPU is needed to supply the necessary compression CODEC services.  Support of CODECs like G.726 and G.729 requires that mobile computers have a very fast CPU clock (400Mhz or greater) for VoIP applications.
• Acoustic design is important because the mechanical and electrical design of a VoIP-enabled mobile unit reflects clear design intent and support:  microphone or speakerphone and/or headset.  Also, assessing the basic ergonomics of the device and how a user holds the device while in a conversation is a critical consideration, because this is not usually a factor for PDA-class devices.

PBX Integration Considerations

Finally, it is vital to provide cost-effective, feature-rich solutions for integration into the customers' telephony systems (PBX or iPBX).  Through these systems, a simple phone call to the vendor or customer is made.  Two solutions are available to provide such integration:

• Gateway/adjunct solutions
• Direct integration solutions

Gateway solutions address the legacy market opportunities.  Organizations with traditional PBXs connecting them to the PSTN are able to install a wireless VoIP solution by adding a gateway product, which also supports an analog or digital telephony interface.   The gateway also provides a network interface and acts as an application bridge in support of VoIP-to-analog/digital translation of the signaling.  This approach allows a business to retain their PBX investment while extending the services to include wireless VoIP.

Direct integration solutions are designed to complement the VoIP services being offered by the host PBX or iPBX.  As VoIP technologies are deployed, more and more telephony vendors offer proprietary native VoIP support.  Typically implemented to support an IP-desktop phone, these solutions also support wireless VoIP solutions that conform to the vendor's VoIP architecture and protocol.  These solutions offer the tightest and most feature-rich solutions, but require the customer to already have made a VoIP decision with the internal telephony services.

The Solution

A robust wireless VoIP solution requires attention to supported components and functions across multiple technology disciplines.  Attention to RF infrastructure design, Ethernet network configuration, security policies, acoustics, telephony and mechanical design converge across a multi-vendor landscape to result in a strong wireless VoIP solution.

• Support with wireless infrastructure features (power save mode, QoS, security)
• Collaboration with telephony customer premises equipment (CPE) providers
• Acoustical and mechanical design requirements
• Adherence to standards

Return on Investment – Demonstrating the Value

Having telephony support in a terminal requires a demonstration of real value-add to drive market demand.  There are hard and soft ROI realizations with voice-enabled mobile computers.  The hard ROI examples identify specific and calculable savings as a result of deploying such solutions, and the soft ROI considerations are valuable and real but often subjective in any attempts to quantify them.  The detailed ROI analysis is different for each market segment based on cost structure and business dynamics in considering communication technologies.

All ROI analyses start from a customer's direct communication requirements.  Whether this is between employees, employees and customers or employees and vendors, most organizations have a real business need for reliable, quick communications to keep their companies competitive and successful. 

Hard ROI values are derived through the mitigation of current expenses.  Whether it is the result of lowered material costs or elimination of ongoing operational expenses, these economic considerations are easily quantified.  ROI metrics are derived from:

• Elimination of duplicate infrastructures:  Some wireless VoIP solutions require deployment of a separate wireless network just for voice.  Converged solutions (utilizing the 802.11 wireless LAN) provide a built-in reuse of the existing wireless LAN infrastructure, which also mitigates the cost and total cost of ownership (TCO) of the entire system.  In addition, when consideration of a converged wireless LAN solution is made with new construction, significant savings are realized without the need to deploy a hardwired telephony infrastructure and rely solely on the wireless services for voice and data.
• Elimination of duplicate devices:  Deploying mobile computers that provide both telephony services and in-building communications such as walkie-talkie features eliminates the need for additional devices.  In addition to voice, support of text messaging or paging on the same wireless device eliminates the need for pagers.  Again, the application of converged voice and data to use one device for many functions lowers overall expenditures. 
• Elimination of ongoing service charges:  Some companies deploy cell phones to provide for in-building communication.  However, this option includes annual service contracts with the service providers.  Also, some walkie-talkie products require a license to use the spectrum, which is eliminated when using a wireless LAN solution.  Converged mobile computers are purchased as capital expenditures.  Ongoing service charges and license fees for devices such as pagers and walkie-talkies are mitigated.
• Elimination of unnecessary associate functions:  When deploying wireless communication devices, many companies restructure their personnel organization.  This results in a streamlined operation that achieves business goals with fewer associates.  The fact that associates are mobile and still reachable is an important factor in how a business allocates its people resources.