MAC and PHY Layer Fundamentals in IEEE 802.11 Wireless Networks



In the complex world of wireless networking, understanding the interactions between the MAC (Medium Access Control) and PHY (Physical) layers is crucial for comprehensive network analysis. The IEEE 802.11 standard implements these layers within the OSI model, providing a structured approach to wireless data transmissions.


Before we begin our deep-dive into MAC and PHY layers, it is important to understand the OSI Layered Model and function of each layer. The OSI model comprises seven layers, each playing a critical role in data communication:
Figure -1: OSI Layers and functions


As seen in the image above, the IEEE 802.11 Standard operates in these Layer 1 (PHY) and Layer2 (MAC) Layers of the OSI Model. 

Figure-2 MAC and PHY Layer Functions


MAC Layer:

The MAC layer serves as a critical component in WiFi networks, managing several essential functions:

Key MAC Layer Responsibilities:

1. Frame Construction: Transforming raw data into standardized 802.11 frames

2. Medium Access Control: Implementing CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) medium access to share the Wireless Medium among the stations.

3. Error Handling: Ensuring reliable data transmission through acknowledgments and retransmissions.


Concept of MSDU’s and MPDUs
  • MSDU (MAC Service Data Unit): The payload received from higher layers at the MAC sublayer.
  • MPDU (MAC Protocol Data Unit): MSDU encapsulated with MAC header and trailer, including:
    • Source and destination MAC addresses
    • Sequence control information.
    • Frame Check Sequence (FCS) for error detection.
PHY Layer: Signal Transformation

The PHY layer converts the frames received from the MAC layer into wireless signals, operating through two critical sublayers:
 
PHY Layer Sublayers:
  • PLCP (Physical Layer Convergence Procedure)
  • PMD (Physical Medium Dependent)

Key PHY Layer Responsibilities::
  • Prepares frames for transmission
  • Adds synchronization and header information
  • Modulates bits into radio frequency (RF) signals
  • Manages signal transmission
PHY Data Units
  • PSDU (PHY Service Data Unit): Raw frame data from MAC layer
  • PPDU (PHY Protocol Data Unit): PSDU is PHY layer view of MSDU with additional PHY layer header

Transmission Workflow: From Application to Bits

Lets review the transmission process from the Layer 7 to the Layer 1. We would focus on what is important and in the modern networking and align to learning about the fundamentals of Wireless.  Layer 1 through Layer 4 is more important when we talk about the data flowing through and Access Point to the Receiving Station.


At the Transmitter:

1. Application Layer: Generates original message

2. Transport Layer: Adds TCP/UDP headers

3. Network Layer: Appends IP addressing information

4. MAC Layer: Encapsulates payload into MSDU and Converts MSDU to MPDU

5. PHY Layer: Transforms MPDU to PSDU and Converts PSDU to PPDU for transmission. The PMD transforms PPDU into bits and using Modulation Techniques, it sends those bits using RF signals to the intended receiver. 
 
Reception Process

1. At the receiver, PMD sublayer demodulates RF signals to bits

2. PHY layer extracts PSDU from PPDU

3. MAC layer reconstructs MSDU from MPDU

4. Higher layers process and deliver data to the intended application. 

Significance of Data Units

Each data unit represents a crucial transformation stage:
  • MSDU: Original payload
  • MPDU: MAC-level addressing and integrity
  • PSDU: Raw frame at PHY layer
  • PPDU: Wireless transmission preparation

Conclusion

Understanding the interactions between MAC and PHY layers is fundamental to wireless network analysis. The systematic encapsulation and transformation of data ensure efficient, reliable wireless communication across the network environments and between the Station and the Access Points.

Ref: https://www.cwnp.com/certifications/cwap

Comments

Post a Comment

Popular posts from this blog

Understanding RSSI and LQI Metrics of IOT

Image
 In the rapidly increasing adaption of Internet of Things (IoT), understanding  the basics network performance metrics is crucial for building reliable and efficient systems. Two fundamental metrics that play a n important role in the IOT Device communication are the Received Signal Strength Indicator (RSSI) and Link Quality Indication (LQI). Let's dive deep into what these metrics mean and why they matter for IoT implementations. Received Signal Strength Indicator (RSSI) RSSI serves as a fundamental measurement of RF power received by a wireless device. What makes RSSI particularly interesting is that it measures all RF power in a channel, regardless of the source. This means it captures: Signals from IEEE802.15.4 transmitters Interference from Bluetooth devices WiFi signals Background radiations This comprehensive measurement makes RSSI an essential tool for Clear Channel Assessment (CCA), helping devices determine if a channel is free befo...
Read more

Understanding "Invalid FTE" Error with 802.11r Roaming

Image
 When investigating Fast Transition (FT) roaming issues in enterprise wireless networks, packet captures often hold the key to diagnosis. Let's analyze this specific roaming failure case where a client fails to roam from one access point to another. Problem Statement: Random Devices are facing FT roaming issue and requires reauthentication.  While the problem statement sounds easy, capturing roaming failures could be challenging when the problem is random/intermittent. It is also important to know where to capture; Remember CWAP Guidelines? For roaming we need to capture near multiple AP's.  In this case we used RUCKUS AP Remote PCAP feature to stream captures from multiple AP's to the Wireshark and Voila! We were lucky to capture the failure  use case.  Now lets see what does it look like: The packet captures shows us an "Invalid FTE (0x0037)" error. FTE stands for Fast Transition Element, a critical component of the 802.11r protocol that facilitates seaml...
Read more

Association Failures with Legacy Printers due to Management Frame Protection- A Technical Analysis

Image
 Let's understand first, what is Management Frame Protection? Based on the IEEE 802.11w amendment, Protected Management Frames (PMF), also known as Management Frame Protection (MFP), is a security feature that provides integrity protection for both unicast and broadcast management frames, while also encrypting unicast management frames in the same way as data to provide confidentiality. Without the Protected Management Frames feature, all management frames are sent unprotected in the open. Transmitting open frames makes connections vulnerable to attack. To leverage Protected Management Frames, both the AP and the STA need to be capable of using it, and it must be activated for each encrypted Wi-Fi network of the AP. If those conditions are met, Protected Management Frames are automatically invoked during client association. Understanding Management Frame Protection Failures MFP is one of the common challenge when working with legacy devices in modern wireless networks. My recent t...
Read more