What is Protocol ?
A Protocol is a set of Rules and Frame formats that govern the way that two entities Communicate. Since all networking devices are dumb in the sense that they only understand fixed commands, a Protocol provides commands to send and receive information.
Block diagram - Wireless LAN:
IEEE 802.11 MAC – Block Diagram:
Description of Blocks:
Device Driver:
• The device driver configures various 802.11 parameters through host interface. Values of these parameters are written into SRAM at predefined locations.
• Device driver reads the status registers of MAC Controller.
• Device driver invokes 802.11 MAC operations by writing into command register.
• The SDU (Service Data Unit) is transferred by the device driver to SRAM.
• The SDU is read by the device driver from SRAM.
Host Interface Unit:
• Host Interface Unit contains configuration, command and status register banks which can be accessed by the device driver.
• The device driver communicates with embedded microcontroller using register banks.
• Host Interface Unit supports Configuration Registers defined by bus standard.
Embedded Microcontroller:
• 16 bits RISC microcontroller.
• Implements all the functionality of 802.11 MAC as software.
• The 802.11 protocol program lies in the Flash RAM, external to the controller.
• Device driver sets command registers of Host Interface to request the 802.11 service.
• This command register invokes microcontroller operation through interrupt.
• Transceiver Attachment Unit can be mapped to IO or memory space of the microcontroller.
• Embedded Microcontroller sets various command register of TAI to invoke the operation and reads the status register of TAI to track the state of operation.
• It configures the Baseband processor and hence controls its functionality.
• It also generates MAC/Physical layer frame header and data.
Transceiver Attachment Interface Unit:
• Implements the time critical functionality of the protocol.
• Set of FSM implementation will be in VHDL as hardware.
• Interfaces the Baseband controller to receive and transmit the frame.
• Contains various configuration, command and status register banks.
• Implements transmit and receive FIFOs to temporarily store the network data.
• Provides channel access indication.
External Memory:
• Flash RAM contains program of 802.11 protocols, which is executed by the microcontroller.
• Flash RAM also contains various system routine to control the operation of Baseband controller.
• External memory provides higher flexibility in protocol implementation at the cost of higher fetch/access time.
• SRAM contains network data and temporary variables.
• The SDU (Service Data Unit) to be transmitted is stored in SRAM by device driver.
• The SDU received from the peer MAC is stored into SRAM by TAI unit.
• All the interface to the SRAM is controlled by the Bus Interface Unit.
Standards of MAC Protocol:
The world has some international organizations which decide about standards. Then locally, in each region or country, other bodies are empowered to regulate. Historically, Europe, USA and Japan (plus now the emerging China) are the regions. Each may decide completely different systems for the same applications, or make harmonized decisions. There is some complexity to overview all the actors. For example:
ISO: International Standardization Organization, reporting to United Nations (UN). Local represent of ISO are: ANSI in USA, AFNOR in France, DIN in Germany, BSI in UK, and HSC in Japan.
Wireless has been really impacted by this world organization:
Ø 5 to 6 GHz frequency plan and usage regulation: each region has decided something unique.
Ø HiperLAN2 and IEEE 802.11a are incompatible
Ø Nightmare with IEEE 802.16/a and HiperMAN. These differences are also in the high level vision.
Ø USA: non-centralized system, IP protocol stack non compatible with ISO.
Ø Europe: centralized systems, ISO protocol stack.
Introduction To IEEE 802.11 Family
