N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 7 - LAN, WAN, VPN, WLAN, MAN, FDDI and Optical MANS                                                      Slide 24 of 45 _________________                                                                                  __________

                                                How does a WLAN work?

                                            802.11 Standard

                                            Physical Layer - Operational Frequencies

                                            MAC Layer - CSMA / CA

                                            Information Transmission

_________________                                                                                  __________

Recently, the market for wireless  communications has enjoyed tremendous growth. This growth momentum is benefiting personal and business computing. Wireless LANs makes it easier to set up networks or extend the existing Wire Line LANs. IEEE has developed 802.11 standard, defining Physical and Media Access Control (MAC) layers, for the Wireless LANs.

802.11 Standard - Original standard made provisions of data rates of either 1 M bps or 2 M bps. Now 802.11 standard has defined a, b and g versions for higher data rates.

Physical Layer - defines operational frequencies. The original version specifies operations in the 2.4 - 2.4835 H Hz frequency band for spread spectrum transmission and 300 - 428,000 G Hz for IR transmission.

MAC Layer - defines a set of protocols, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), responsible for maintaining order in the use of a shared medium. This protocol is very similar to the protocol used in the Ethernet LANs. In this protocol, when a node receives a packet to be transmitted, it listens to the carrier to ensure that no other node is transmitting. If the channel is clear, it than transmits the packet. Otherwise, it will wait for a time chosen by algorithm and than transmit the packet. The time chosen by two nodes being the same is very small.

Information Transmission - When a node wants to transmit a packet, it sends ready-to-send (RTS) including the length of the packet. The receiving node responds with clear-to-send (CTS) message packet. The transmitting node after receiving CTS sends the information packet. The receiving node does the Cyclic Redundancy Check (CRC) on the information packet. When the packet is received successfully, the receiving node sends an acknowledgement (ACK) packet to the transmitting node.