CWAP Certified Wireless Analysis Professional Official Study Guide

Chapter 1

802.11 Overview

In this chapter, you will learn about the following:

• OSI model
• Packets, frames, and bits
• Data-Link layer
  • MSDU
  • MPDU
• Physical layer
  • PSDU
  • PPDU
• 802.11 architecture
• 802.11 services
  • Station service
  • Distribution system service
• 802.11 frames
  • Management frames
  • Control frames
  • Data frames
• IEEE 802.11-2007 standard and amendments
  • 802.11b
  • 802.11a
  • 802.11g
  • 802.11n-2009
• Wi-Fi Alliance

Before this book starts to get deep into the technical stuff, we want to take some time to discuss a little of the “what and why” of wireless LAN analysis. Since you are reading this book, it is likely that you have already read the CWNA and CWSP study guides. If you have not done so, do not worry. However, be prepared because after this overview chapter, this book is going to get very technical.

If you read the CWNA: Certified Wireless Network Administrator Official Study Guide (Exam PW0-104), by David D. Coleman and David A. Westcott (Sybex, 2009), you learned about the fundamentals of wireless LANs, the history of them, and the basic technologies relating to them. You should have a good understanding of how they work, in the same way that the typical person has an understanding of driving an automobile. Most drivers have a reasonable understanding of how a car accelerates, turns, and maneuvers. For the average person using the roads, a basic knowledge of how a car handles and maneuvers is adequate for driving on the roads. While traveling along various roads, congestion due to traffic and accidents will occasionally occur. When these problems occur occasionally, the additional time it takes for people to travel to their destinations is usually tolerated. However, when unusually heavy traffic occurs on the roads or excessive accidents happen, specialists are needed to analyze the situation to determine how to improve the traffic flow and to decrease the occurrences of accidents.

The people who analyze the roads and traffic need to have a much more in-depth knowledge of them than the individual who is driving on them. They need to understand the inertial forces that vehicles exert on the road so that they understand how well the road can handle the typical speeds at which the vehicles are traveling. They need to understand the type of traffic that is using the roads (cars, trucks, motorcycles, bicycles, and even pedestrians) so that they can optimize the flow of the traffic. They also need to understand outside factors that affect traffic such as weather, fog, and the location of the sun. These people need to monitor the roadway so that they can analyze and identify where and why bottlenecks and accidents are occurring and ideally come up with solutions to improve the flow of traffic.

Just as civil engineers need advanced training and experience to properly analyze and maintain roads, people who manage wireless LANs need specialized training and experience to properly design and maintain them. In this book, we hope to provide you with the advanced knowledge and skills needed to analyze and troubleshoot wireless LANs. To gain this knowledge and skill, you will need to learn and understand the underlying architecture and principles of wireless LANs and RF communications. This will include an understanding of the IEEE 802.11 standard along with RF signals.

In Chapter 1, we will provide some overview and fundamentals. You will learn about the OSI model and the two key layers that involve wireless network: Physical and Data-Link. You will learn about the different 802.11 services and the three types of frames that are used for 802.11 communications: management, control, and data. A review of many of the core 802.11 standard and amendments will make sure that you understand the key components of wireless 802.11 wireless networking. After that, the rest of the book is hard-core analysis and the principles and technologies needed to perform this level of analysis.

OSI Model

The International Organization for Standardization (ISO) is a global, nongovernmental organization that identifies business, government, and society needs and develops standards in partnership with the sectors that will put them to use. The ISO is responsible for creating the Open Systems Interconnection (OSI) model, which has been a standard reference for data communications between computers since the late 1970s.

Why Is It ISO and Not IOS?

ISO is not a mistyped acronym. It is a word derived from the Greek word isos, meaning equal. Because acronyms can vary among languages, the ISO decided to use a word instead of an acronym for its name. With this in mind, it is easy to see why a standards organization would give itself a name that means equal.

The OSI model is the cornerstone of data communications. The layers of the OSI model are as follows:

• Layer 7: Application
• Layer 6: Presentation
• Layer 5: Session
• Layer 4: Transport
• Layer 3: Network
• Layer 2: Data-Link
  • LLC sublayer
  • MAC sublayer
• Layer 1: Physical

The IEEE 802.11-2007 standard defines communication mechanisms only at the Physical layer and MAC sublayer of the Data-Link layer of the OSI model. By design, the 802.11 standard does not address the upper layers of the OSI model, although there are interactions between the 802.11 MAC sublayer and the upper layers for parameters such as quality of service (QoS).

It is important to have an understanding and knowledge of all layers of the OSI model when working with packet analyzers. When captured data is displayed in an analyzer, it is typically separated and grouped by the OSI layers that the data represents. When capturing wireless frames, if wireless encryption is implemented, all of the layer 3 through layer 7 information will be grouped and displayed as the encrypted payload.

You should have a working knowledge of the OSI model for both this book and the CWAP exam. Make sure you understand the seven layers of the OSI model and how communication takes place at the different layers. If you are not comfortable with the concepts of the OSI model, spend some time reviewing it on the Internet or from a good networking fundamentals book prior to taking the CWAP exam. You can find more information about the ISO at www.iso.org.

Packets, Frames, and Bits

When learning about any technology, it is important at times to step back and focus on the basics. If you have ever flown an airplane, it is important, when things get difficult, to refocus on the number-one priority— to fly the airplane. Navigation and communications are secondary to flying the airplane. When dealing with any complex technology, it is easy to forget the main objective; this is as true with 802.11 communications as it is with flying. With 802.11 communications, the main objective is to transfer user data from one computing device to another.

As data is processed in a computer and prepared to be transferred from one computer to another, it starts at the upper layers of the OSI model and moves down until it reaches the Physical layer, where it is ultimately transferred to the other devices. Initially, a user may want to transfer a word processing document from their computer to a shared network disk on another computer. This document will start at the Application layer and work its way down to the Physical layer, get transmitted to the other computer, and then work its way back up the layers of the OSI model to the Application layer on the other computer.

As data travels down the OSI model, for the purpose of being transmitted, each layer adds header information to that data. This enables the data to be reassembled when it is received by the other computer. At the Network layer, an IP header is added to the data that came from layers 4–7. A layer 3 IP packet or datagram encapsulates the data from the higher layers. At the Data-Link layer, a MAC header is added, and the IP packet is encapsulated inside a frame. Ultimately, when the frame reaches the Physical layer, a PHY header with more information is added to the frame.

Data is eventually transmitted as individual bits at the Physical layer. A bit is a binary digit, taking a value of either 0 or 1. Binary digits are a basic unit of communication in digital computing. A byte of information comprises 8 bits.

Data-Link Layer

The 802.11 Data-Link layer is divided into two sublayers:

• The upper portion is the IEEE 802.2 Logical Link Control (LLC) sublayer, which is identical for all 802-based networks, although not used by all IEEE 802 networks.
• The bottom portion of the Data-Link layer is the Media Access Control (MAC) sublayer, which is identical for all 802.11-based networks. The 802.11 standard defines operations at the MAC sublayer. The MAC sublayer acts as an interface between the lower layer PHY and the upper LLC sublayer.

Chapter 3 is dedicated to the MAC sublayer. In Chapter 3, the MAC sublayer frame format will be examined, along with an explanation of 802.11 MAC layer addressing.

MSDU

When the Network layer (layer 3) sends data to the Data-Link layer (layer 2), the data is handed off to the LLC and becomes known as the MAC Service Data Unit (MSDU). The MSDU contains data from the LLC and layers 3–7. A simple definition of the MSDU is that it is...