Posted by: Paul DeBeasi
Introduction
Sprint recently launched their WiMAX service, dubbed Xohm, in Baltimore with plans to expand the service to Chicago, Washington, and other cities. Should Sprint be optimistic about their new WiMAX service? Well, Intel thinks so. Sriram Viswanathan, general manager of Intel’s Worldwide Interoperability for Microwave Access (WiMAX) program office, once stated that WiMAX “could potentially be the biggest thing since the Internet itself.” That is quite a bold statement and underscores the sentiment of many WiMAX proponents. But as the WiMAX industry has grown, so has WiMAX hype and confusion.
This blog post is the first in a five-part WiMAX tutorial series based upon articles I wrote for Search Mobile Computing. The series begins by introducing WiMAX technology, applications, and terminology. Follow-on blog posts will discuss WiMAX services, performance, security, and devices.
Fixed WiMAX
The IEEE originally formed the IEEE 802.16 working group in 1998 to provide a standard for wireless metropolitan area networks. The primary application was for high-speed fiber access solutions using high frequency line-of-sight (LOS) fixed wireless connections. The original standard was referred to as 802.16 and evolved to support fixed broadband wireless access over lower frequency non-line-of-sight (NLOS) wireless connections. The evolved standard, 802.16-2004, is often referred to as fixed WiMAX.
Mobile WiMAX
Once the 802.16-2004 standard was complete, the IEEE committee began work to further evolve the standard to support mobile applications. Mobile communication is more complex than fixed communication. The technology must be able to hand off a wireless connection from one base station to another while the user is moving, without dropping the connection. The new 802.16e-2005 standard was completed in December 2005, and not only supports mobile applications but also nomadic and fixed applications. 802.16e-2005 is often referred to as mobile WiMAX.
Physical Layer
The fixed/mobile WiMAX PHY layer uses a technology called Orthogonal Frequency Division Multiplexing (OFDM). OFDM techniques have been around for decades. The technology is now commonplace in wireless systems such as WiFi. OFDM evolved from earlier single-carrier modulation systems and frequency division multiplexing systems. OFDM is a type of frequency division multiplexing system that provides better channel throughput because each of the underlying sub carriers are orthogonal to each other.
Media Access Control Layer
The primary job of the MAC layer is to provide an interface between the PHY layer and upper layer protocols. Each instance of the MAC layer in a fixed/mobile WiMAX station has a 48-bit address as defined by the IEEE Std 802. Unlike the distributed and connectionless 802.11 MAC, the WiMAX MAC is centralized and connection-oriented. A 16-bit connection identifier (CID) identifies each WiMAX connection. Each WiMAX MAC layer protocol data unit uses the CID, instead of the MAC address, to identify source and destination. WiMAX has a rich quality of service mechanism that is based upon the Data Over Cable Service Interface Specifications (DOCSIS).
WiMAX applications
At the most basic level, WiMAX supports mobile, fixed, and nomadic wireless applications. A mobile application provides communication while the user is in transit. A good example is a business traveler that communicates while on a train. The Sprint Xohm service provides Internet access while moving at high speeds using WiMAX technology. It will compete with 3G technologies like EV-DO (from Verizon Wireless) and HSDPA (from AT&T).
Fixed wireless applications often provide last mile connections in rural or underdeveloped areas that do not have Digital Subscriber Line (DSL), Hybrid Fiber-Coax (HFC), or other last mile wired infrastructure. Enterprises can either purchase fixed WiMAX technology for use in their own private network (e.g., a point-to-point wireless connection between two buildings) or can purchase fixed WiMAX service from a wireless Internet service provider (WISP). Towerstream is an example of a WISP that provides fixed, high-speed WiMAX access for business users in many urban areas.
Finally, a nomadic application is one where a user moves from location to location but communicates only while stationary. A good example is a repair-person that needs high speed network access while parked at a customer location, but not while they are driving. The WiMAX service from Clearwire can be used for nomadic (and fixed) applications.
Summary
Table 1 summarizes the how WiMAX technology relates to WiMAX applications. Over time, 802.16e-2005 will likely become the dominant standard for fixed, nomadic, and mobile applications thus limiting the use of 802.16-2004.
Table 1: WiMAX applications and technologies
As we can see from this blog post, terminology is important. Therefore, throughout the rest of this WiMAX tutorial series will use the following terminology to ensure clarity.
• Fixed WiMAX will refer to technology that adheres to the 802.16-2004 standard.
• Mobile WiMAX will refer to technology that adheres to the 802.16e-2005 standard.
• Fixed WiMAX application will refer to a last mile wireless application irrespective of the underlying technology standard.
Looking ahead to part 2
In the next blog post we look at WiMAX services. We will discuss what services will be offered, who will offer the services, and when they will be generally available.
More information
For a much deeper treatment of this subject, see “Fundamentals of WiMAX” by Jeffrey G. Andrews, et. Al.
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