Enhancements of the IEEE 802.11 Medium Access Control ... - PATS

Chapter 3. IEEE 802.11 standardconfigurations. The CS (Carrier Sense) is performed through a physical and a virtualmechanism. Both sensing mechanisms are used to determine the state of themedium. The physical carrier sensing (PHY CS) is performed by the air interface(PHY - Physical Layer). A virtual carrier sensing (VCS) mechanism is provided bythe MAC. Before transmitting, a node senses the medium to determine if anothernode is transmitting. If the medium is not determined to be busy, the transmissionmay proceed. The CSMA/CA distributed algorithm mandates that a gap of aminimum specified duration exists between contiguous frame sequences, duringwhich a transmitting node ensures that the medium is idle before attempting totransmit. If the medium is determined to be busy, the node defers until the end ofthe current transmission. After deferral, or prior to attempting to transmit againimmediately after a successful transmission, the node selects a random backoffinterval and decrements the backoff interval counter while the medium is idle. Atransmission is successful when an ACK frame is received from the receiver.Taking into account that nodes in ad hoc networks have asymmetric information(they are not all within radio range of one another), they cannot be based onlyon PHY CS, but also on virtual CS (VCS). A VCS mechanism is executed by overhearingRequest-To-Send/Clear-To-Send (RTS/CTS) control packets, that containa Duration/ID field which defines the duration of time required to transmit the currentdata frame and corresponding ACK packet. All nodes within the receptionrange of either the originating node (which transmits the RTS) or the destinationnode (which transmits the CTS) learn of the medium reservation 1 .Fig. 3.3 depicts the RTS/CTS channel reservation. Node A transmits an RTSFigure 3.3: CSMA/CApacket to node B. Node 1 hears the RTS packet and it defers (3.3.a). Node Banswers to the node A with a CTS packets. Nodes 2 and 3 hearing this transmissionalso defer (3.3.b). Node A transmits Data packet without a collision from1 The neighbors of the receiver (sender) which are not neighbors of the sender (receiver) are calledhidden nodes to the sender (receiver), see details in Section 5.1. RTS/CTS control packet exchange isproposed in order to avoid collision from hidden nodes.36