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Can Voice be the Killer App for WiMAX?
By Elvis Tucker, Director of Solutions and Alliances, Aperto Networks
For every new technology, the debate always arises: what is the “killer app” for this technology. In other words, what is the application that will be highly desired by the users of the technology and able to drive a healthy revenue stream for the service provider, thus ensuring the long-term survivability of the technology?
Not surprisingly, this debate is now raging for WiMAX (Worldwide Interoperability for Microwave Access). Considering the growing popularity of Voice over IP (VoIP) in developed and emerging markets, where the cost of laying new fiber may be prohibitive, many believe that voice services may emerge as the “killer app” for WiMAX for many service providers.
The early data already supports this belief. Voice is a dominant service being offered in the early WiMAX deployments of Aperto Networks and other WiMAX system vendors. In fact, Aperto reports that a staggering 70% of its WiMAX operator customers either offer voice services on their network or transport voice as an overlay service from a partner or third party service provider. Thus, WIMAX is already proving itself to be an effective delivery system for voice services. With WiMAX being an all IP-based technology, analog voice is translated into VoIP packets before being delivered over the network. Thus this discussion is about the suitability of WiMAX for VoIP and can VoIP be this killer application.
Examining the Requirements
Is it enough for WiMAX to be good at delivering voice? Does this attribute alone ensure its survivability? However important voice may be as a revenue source for operators, WiMAX system designers understand that voice is never the only application on an IP network. If it were, then WiMAX would only need to be a wireless manifestation of the Public Switched Telephone Network (PSTN)—delivering a constant stream of bits from one end-point to another—without the overhead involved with IP packetization, routing, fragmentation, etc.
The PSTN was designed from its inception to carry voice, and only voice, and it still does so today. However, unlike the PSTN, IP network applications cover a wide range of voice, video, data and multi-media services like real-time voice calls, streaming audio and video, instant messaging, e-mail, file downloading, internet gaming and, of course, web browsing. On the IP network, all of these traffic types are considered “data” and are carried within the payload of the IP packet. However, the requirements for effectively dealing with each traffic type can be quite different. While voice is showing revenue growth today, operators will also want to exploit the full potential of other multimedia and data services.
Voice-based services are characterized by the need for low end-to-end latency and predictable performance (low variation in the performance or “jitter”). This gives callers the real-time experience that they have come to enjoy on pure analog telephone networks. However, voice applications tend to have low bandwidth requirements and can overcome the occasional loss or delay in signal since the human ear can “fill in the gaps” based on the context of the discussion.
Various data applications, on the other hand, have significantly different requirements on the network from those of voice. Some data applications are even more tolerant of unpredictable performance since the user may not be expecting real time delivery anyway. Email is such an application. Still other data and multimedia applications that require high bandwidth may tolerate fluctuations in the availability of the bandwidth. Downloading large files falls into this category.
Streaming multimedia applications can work over a relatively slow link since they typically buffer data before playing, and only require a moderate level of predictability to prevent exhausting the buffer before a file is transmitted. Hence, network requirements for bandwidth, latency, and error correction can vary greatly between different data applications.
The real issue, however, is the delivery of services over a single network. Since voice is typically not the only application on an IP network, service providers need to deploy technology that enables them to adapt to the individual requirements of each application in real time. Even more daunting, the network needs to manage these applications simultaneously while maximizing performance for all—even on the same wireless channel.
WiMAX architects paid close attention to the usage trends on other all-IP networks like cable and DSL. Voice continues to be a growing and dominant application. Worldwide, VoIP users have grown at the staggering rate of more than 1 million new users each quarter since the middle of 2004. During the same time period, North America has observed a doubling of the number of VoIP users each quarter. Industry watchers are predicting global subscriber levels for voice over broadband (VOBB) subscribers in excess of 140 million by 2010. With the proliferation of Skype, Vonage, and other low-cost, highly- featured Internet-based calling services; these increases are likely to continue.
Wired and wireless services offered today are addressing the VoIP growth phenomena. VoIP usage is moving to WiFi in the form of WiFi enabled phones and access points with integrated VoIP user agents. Coupling a telephone adapter or integrated access device with the DSL or cable modem can retrofit these services for VoIP. However, these services cannot deal with the explosion in multimedia and multi-services including voice, video and other data. Bandwidth limitations, lack of service availability, high operations and maintenance costs and the inability to identify and prioritize delivery of voice against other data prevent these services from fully addressing the worldwide demand for VoIP and other services.
The implication of these challenges is clear: The IP Network should be designed from the ground up to manage multi-services including voice as a key revenue generating application.
To handle voice as a key application in a multi-service environment, IP networks require both technical and non-technical attributes not found in most other access networks. These attributes include an effective and flexible quality of service (QoS) regime, an ability to economically offer the service in underdeveloped areas and emerging economies, and a range of bandwidth management strategies for delivering differentiated services to consumers, small business, and enterprises over a common platform.
But Can WiMAX Really Do All This?
WiMAX can be deployed to serve virtually every user segment over a wide economic and geographic spectrum. Even today, WiMAX is addressing the needs of corporate users in New York City as well residential users in the emerging economies of India, Sri Lanka, and Pakistan. It is serving users in the open stretches of rural Montana as well as the disparate islands of Indonesia.
In each of these cases, typical service offerings include voice as a key revenue-generating application, in addition to various forms of data.
As a result, WiMAX network operators are beginning to reap the benefits of a technology that does not require investing in expensive new wired infrastructure—infrastructure that often does not exist in developing countries or in rural areas of developed nations. With WiMAX, service providers can inexpensively deploy a robust, multi-service network (and even a very flexible management system) that translates into more affordable and competitive costs for subscribers.
Moreover, the time for provisioning multi-service networks is substantially reduced with WiMAX. Outdoor WiMAX systems can be deployed in minutes versus the days or weeks often required to properly qualify and provision a DSL or cable line. Indoor WIMAX systems can be self-installed by the customer, providing plug-and-play access to broadband wireless service.
Traditional wire line operators are beginning to offer WiMAX as a “fill-in” service providing access to customers outside the central office range of DSL. This enables rural villages that previously had no phone service or who shared a single phone because of the lack of wired infrastructure (and the cost of building it out) to enjoy affordable voice and data services. The benefits also extend to rural small and medium businesses that, with newly affordable T1/E1 access, can now more effectively compete with larger businesses in developed, urban areas.
Technology That Supports Differentiable Service Levels and More
WiMAX Customer Premise Equipment (CPE) and network-side infrastructure (primarily base stations) are evolving quickly to accommodate voice users. Voice and multi-services are predominantly provided today through a discrete WiMAX CPE interconnected with voice elements-hosting POTS phones and computers. In the near future, WiMAX CPEs will be widely available with IAD functions built in—eliminating the discrete voice device. Consumers and businesses can plug POTS phones, IP Phones, and computers directly into the WiMAX CPE with SIP, H.323 capabilities, and router functions. Other advanced options include dual mode WiFi / WiMAX CPEs that allow extended wireless access to WiFi clients.
WiMAX base stations are responsible for smartly delivering the voice traffic throughout the network and ultimately to the called and calling party. Some base station vendors are architecting features into their systems allowing operators to tune the system for handling voice and multi-services..
WIMAX systems vendors are implementing algorithms to discretely classify and prioritize traffic delivered to and from the subscriber and associate it with the appropriate service level and charging policy. Advanced WiMAX systems will allow operators to tune the system for the unique characteristics of voice (low latency and jitter), and therefore simultaneously deliver toll quality voice and satisfy bandwidth-hungry data and streaming media applications. After the data is classified, WiMAX systems assign a delivery priority based on the classification result. Specific priorities can be set for real-time services like voice that require guaranteed data rates of near real-time access to the channel, versus non real-time services like email that require only “best effort” service delivery. Priorities are provisioned and observed at both ends of the WiMAX link. WiMAX systems implemented with leading edge QoS features will offer a distinctive advantage to operators intending to offer voice, data, and multi-media services.
Latency and jitter requirements for voice traffic can be very different from that of most other data. WiMAX infrastructure tightly controls this latency and jitter on links used for voice, while appropriately adjusting for various other types of data.
Automatic Repeat Request (ARQ) is a WiMAX MAC function boosting performance in harsh RF conditions where the network resends missing or corrupt frames before the upper layers are aware of the error condition, making the network more efficient. WiMAX base stations should have this feature configurable where it can be disabled for voice traffic since voice can be very forgiving of a small number of dropped frames, but enabled for data services demanding error-free transport.
WiMAX uses clearly defined service classes for prioritizing traffic over the wireless link. Unsolicited Grant Service (also commonly referred to as constant bit rate), real-time polling service, non real-time polling service and best effort delivery prioritization can be assigned to each traffic type within the sector.
Service classification and prioritization is not necessarily observed over the wired network, but effective quality of service needs to be an end-to-end function. Therefore, base stations should support traditional IP-styled quality of service (DSCP) and the mapping of WiMAX service classes into DSCP at the edge of the wired network.
Voice, Multimedia and Data over WiMAX Infrastructure
WiMAX Secret Sauce: Intelligent Bandwidth Management
Delivering high quality voice and multimedia services is ultimately dependent on the quality of the transport channel—in this case the wireless link. In order to achieve this, WiMAX incorporates certain technology enhancements over and above most other wired and wireless technologies.
WiMAX uses orthogonal frequency division multiplexing (OFDM) (256 carriers for to 802.16d-2004 and up to 1024 for 802.16-2005 based systems) to transmit data. The OFDM waveform travels in parallel, versus serially, mitigating the effects of selective fading and increasing the effective data rate of the system. The WiMAX OFDM waveform offers network operators the advantage of being able to operate with the larger delay spread of the NLOS environment. By virtue of the OFDM symbol time and use of a cyclic prefix, the OFDM waveform eliminates the inter-symbol interference (ISI) problems and the complexities of adaptive equalization, ensuring highest quality voice and services on par with that of wired networks.
Radio spectrum is a precious and scarce resource for small and large operators. Therefore is it critical that spectrum is utilized economically, carrying the maximum payload both upstream from the subscriber and downstream to the subscriber. Voice traffic is typically symmetric with both callers having an equal opportunity to talk. Conversely, file downloading and streaming multimedia require more bandwidth in the downstream direction allowing the subscriber to receive the data as quickly as possible. Therefore, a flexible bandwidth management scheme for dealing with bursty, asymmetric data traffic simultaneously with voice is another key technology driving WiMAX success.
WiMAX employs a Time Domain Duplex (TDD) scheduling scheme that allows the link to be specifically tuned for the application in use. If a user's primary requirement is for asymmetrical data traffic—in which the aggregate downlink traffic is exceeding the aggregate upstream traffic—TDD systems can assign more bandwidth in the downlink direction. Advanced TDD systems will dynamically adjust and synchronize the bandwidth allocation for hundreds of subscribers according to application traffic requirements.
WiMAX For Everyone
Many believe that voice could be the killer app for WiMAX and there is increasing data to support this belief. Subscribers in urban and rural settings, and in developed and underdeveloped areas, are beginning to turn to VoIP as a lower cost alternative to traditional circuit-switched voice.
However, voice is not the only service on IP networks. A system that is good at carrying voice must also be good at carrying the same services delivered on today's wired networks: voice, multimedia, and other data. WiMAX allows voice to fulfill its role as a proven revenue-generating service while enabling the operator to simultaneously offer profitable, high quality multimedia and data services.
WiMAX has been architected from the ground up for multi-service delivery with key built-in technologies. Leveraging the inherent strengths of wireless connectivity for rapid, flexible, and low cost provisioning and management, WiMAX has an opportunity to dramatically increase the availability of low cost, high quality voice, data, and multimedia services.
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