WiMAX Protocol Stack: Layers and Functions

This article describes the WiMAX protocol stack and the functions of each layer in the system, including the WiMAX physical layer, WiMAX MAC layer, and upper layers.

The protocol stack is depicted below in the figure, along with its sub-layers.

Physical Layer

WiMAX physical layers are of five types: SC, SCa, HUMAN, OFDM, and OFDMA, as described in the IEEE 802.16-2004/802.16e standards. Any one of these will be used in the system. For example, fixed WiMAX uses the OFDM type of physical layer, and mobile WiMAX uses the OFDMA type.

The physical layer takes the MAC PDU (consisting of MAC GMH, MAC payload, and CRC) and performs the following functions:

1. Scrambling
2. Forward error correction
3. Interleaving
4. Modulation
5. IFFT (Inverse Fast Fourier Transform)
6. Cyclic prefix insertion
7. Pass the IQ data to the RF module for radio frequency modulation and transmission into the air. Hence, this layer is often referred to as the transmission layer.

WiMAX supports 2.5 GHz/3.5 GHz and 5.8 GHz frequency bands.

MAC Layer

The MAC layer consists of three sub-layers: the MAC privacy sub-layer, the MAC common part sub-layer, and the MAC convergence sub-layer, as mentioned in the figure.

• MAC Privacy Sub-layer: Performs authentication, encryption, and key management functions.

• MAC Common Part Sub-layer: Performs ranging, scheduling, connection setup, bandwidth allocation, hybrid ARQ, and QoS functions. Various QoS schemes and applications of each supported in WiMAX are as follows:

  It is a connection-oriented protocol that assigns a connection ID (a 16-bit value that identifies a connection to equivalent peers in the MAC) to each service flow on both uplink and downlink pairs between the BS and SS.

  Each service flow (uniquely identified by an SFID, a 32-bit value) has its own QoS parameter settings (latency, jitter, & throughput).

  • UGS (Unsolicited Grant Service): The SS periodically receives service from the network. It is used for VOIP without silence suppression.

  • rtPS (Real-time Packet Services): The BS asks the SSs to send bandwidth requests periodically (no contention involved here). It is used for streaming audio/video.

  • ertPS (Extended Real-time Packet Services): Piggybacking is taken care of. It is used for VOIP with silence suppression or activity detection.

  • nrtPS (Non-real-time Packet Services): Whenever the BS finds some interval, it will ask the SSs to send a bandwidth request message (contention-based). It is used for FTP.

  • BE (Best Effort): The SS can request bandwidth at any time; whether the SS gets it depends on the WiMAX system. It is used for data transfer and web browsing.

• MAC Convergence Sub-layer: Functions performed by the MAC convergence specific sub-layer are mentioned below:

  • Makes upper-layer frames compatible for use by WiMAX MAC/PHY layers.
  • Maps upper-layer addresses into WiMAX protocol addresses.
  • Translates upper-layer QoS fields into WiMAX MAC format.
  • Classifies external network data and associates them with the proper MAC service flow identifier (SFID) and connection ID (CID).
  • Handles TCP/IP-based traffic.

Upper Layers

Above the MAC layer, there are IP, TCP, UDP, and Application layers.

One can download and read RFCs from IETF (https://www.ietf.org/rfc.html) and WiMAX forum (https://www.wimaxforum.org/) published documents to gain more knowledge on the same.