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This is the first in a multi-part series of posts I am working on. I am attempting to familiarize myself with the 802.16 protocol standards and the WiMAX forum efforts. I find that the best way to learn stuff is to write it down. I hope this helps others out there, and by all means, if you see and error, please comment and I will correct it.

Introduction

The IEEE 802.16 working group is the standards body developing the specifications for the next generation of broadband wireless networks – both fixed and mobile. The WiMAX (Worldwide Interoperability for Microwave Access) forum is an interoperability initiative that will certify that broadband wireless radios manufactured by various vendors comply with the IEEE 802.16 specification and – more importantly – are interoperable from vendor to vendor through testing.

WiMAX Range and Throughput

The throughput and range of a WiMAX link is very dependent upon a number of factors including; transmit power, antenna gain, directionality, modulation scheme, forward error correction codes, the terrain, density, height of tree cover, presence of hills and valleys, bodies of water, etc. (nothing new here, these are all the typical issues that need to be dealt with in wireless communications). Despite this, it is not uncommon to see statements in the media describing WiMAX multipoint coverage as being capable of extending coverage up to 30 miles from a base station or tower location. In certain (very specific) cases this is true. However, typical operating ranges fall into the 8-10 mile range for line of site (LOS) installations and 4 – 5 miles for non or near-line of sight (NLOS) installations.

Realized throughput is dependent upon many of the same factors that address the achievable link distance. WiMAX supports a number of different modulation schemes and coding rates. Also, it is important to remember though that WiMAX is a shared network service, meaning that this bandwidth will be shared amongst all of the users within a given WiMAX cell – much like cable modem users share their Internet bandwidth with other subscribers in the neighborhood.

802.16 Reference Model

802.16 is essentially a wireless implementation of Layers 1 and 2 of the OSI model (Physical Layer and Link Layer). The Physical layer is referred to as the PHY. The link Layer is referred to as the MAC (Media Access Control) layer. The MAC is comprised of 3 sub-layers; the Service Specific Convergence Sub-layer (CS), the Common Part Sub-layer (CPS), and the Security Sub-layer (SS).

The Service Specific Convergence Sub-layer provides the interface between the WiMAX network and the higher level protocols. Data received from the higher layer protocols is mapped into the appropriate WiMAX identifiers for transmission over the network (for example, Ethernet Addresses are converted to WiMAX station identifiers). Currently there are only two CS interfaces defined; one for ATM traffic, and one for packet (IP) traffic.

The MAC CPS performs the bulk of the MAC layer processing. It is responsible for managing system access, bandwidth allocation, and connection management.

The MAC Security Sub-layer provides authentication, secure key exchange, and payload encryption.

The WiMAX Physical Layer (PHY) actually consists of multiple sub-specifications, each one dependent upon the application and frequency spectrum to be utilized. There are three basic frequency bands to be concerned with:

• 10 GHz to 66 GHz Licensed Frequencies
• Licensed Frequencies below 11 GHz
• Unlicensed frequencies below 11 GHz (primarily 5 to 6 GHz)

Coming next….a look at the Convergence Sublayer of the MAC.