ess to the shared medium therefore needs to be actively managed to ensure that the available bandwidth is not wasted through repeated collisions of this type. This is the main task of the MAC layer. Wireless Network Logical Architecture Length (bytes) 0 to 2312 Frame Address Address Address Address checksum Protocol Frame Frame version sub-type Length (bits) Management, Association Request/Response Control, Data Beacon, RTS, CTS, ACK, Figure 2-5: MAC Frame Structure Carrier Sense Multiple Access/Collision Detection (CSMA/CD) The most commonly used MAC method to control device transmission, and the one specified for Ethernet based networks, is Carrier Sense Multiple Access/Collision Detection (CSMA/CD) (Figure 2-6). When a device has a data frame to transmit onto a network that uses this method, it first checks the physical medium (carrier sensing) to see if any other device is already Table 2-6: Elements of the 802.11 MPDU Frame Structure MPDU element Description Frame control A sequence of flags to indicate the protocol version (802.11 a/b/g), frame type (management, control, data), sub-frame type (e.g. probe request, authentication, association request, etc.), fragmentation, retries, encryption, etc. Duration Expected duration of this transmission. Used by waiting stations to estimate when the medium will again be idle. Address 1 to Destination and source, plus optional to and from addresses Address 4 within the distribution system. Sequence Sequence number to identify frame fragments or duplicates. The data payload passed down as the MSDU. Frame check sequence A CRC-32 checksum to enable transmission errors to be detected. Chapter Two Device A Data packet Medium busy Medium free Collision Device B attempts to send Carrier Random sensing backoff Data packet Medium busy Medium free Collision Medium busy Medium free Device C attempts to send Carrier Random Carrier sensing backoff sensing Data packet Slot time Figure 2-6: Ethernet CSMA/CD Timing transmitting. If the device senses another transmitting device it wai