5s74 LEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS,VOL 14.NO.10.OCTOBER 2015 RTS is only initiated by a client.so the AP does not g nerate A DATA to AP any RTS frame but respondstoaclient's RTS with a CTS frame an RTS cas Fig. CAP find AP others_ A and then transmits the packet to client B immediately.Note that in each frame the calle whic Fig.4.An example showing no dual link frame,it finds out that the duration in the CTS frame is longer The delay for Client A serves two purposes.First,for a legacy the pa time.The delay time can be computed by Eg.2).There are the packet to Client B is sent first two scenarios in this case.If the transmission time of the packet protected without interference rom th n the above procedur Client A in Fig.3 vaits a tim b busytone signal to ensure two transmissions finish at the same Frame Spacing (DIFS)time.Thus,the ACK may collide with me Otherwis as denoted by the "AP-longer h3(b).Client the packet from the other client.In our design.the CTS frame problem.I dura中 n in CI covers the enti and the AP finish their missions,Client B first returns an CTS fran update the NAV value ordins to the duration in ACK frame to the AP and then the AP returns an ACK frame the CTS frame.As a result.the ACK from the AP can also be to Client A upon re protected the aph L rly.Sin in the beeinnin the Client A does not know whether or not the AP will establish dual links.it computes the duration as if the A hepCteoraohercietomypiesaCTS es not set up From the cts frame Client A finds out that the duration in CT up a li he half me w A knows th duplex case.If the AP does set up dual links.it just uses th time and then starts a packet transmissio .When the packet is duration in the RTS frame to compute the duration of the CTS expla ed be transmission cove In the th and the AP As be set up i ot support full duplex communications.so symmetrical dual inks (1) selve the queue status as well as the length of the first packet in 3()the nC as follows.In the the AP knows exactly which client is about to send a pa et and the DurationcTs DurationgTS-TCTs-2TsIFS+Tp TACK. es how to establish dual links with another client.In [6] an业 time.In the case of Fig.3(b),the the packet is transmitted by OFDM symbols such as in 802.11a network,the AP cannot decode the packet header to get the Durationcrs =T2+TSIFs +2TACK ransm itting ent ad dress until it receives the wl e packet where t is the transmission time of the packet from the ap to AP Client B.When A receive the CTS frame,it decides the delay the packet is not interfered by the packet from Client A to the time according to can decod e the packet more easily throug Delay=DurationcTs-T-TsIFs-2TACK 215874 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 14, NO. 10, OCTOBER 2015 RTS is only initiated by a client, so the AP does not generate any RTS frame but responds to a client’s RTS with a CTS frame. The first case is shown in Fig. 3. Client A first transmits an RTS frame to the AP. When the AP finds that it has a suitable packet for Client B, it replies a CTS frame to Client A and then transmits the packet to client B immediately. Note that in each frame there is a field in the MAC header called duration, which is used for other nodes to update the network allocation vector (NAV) value. When Client A receives the CTS frame, it finds out that the duration in the CTS frame is longer than its duration. Thus, Client A knows that the AP intends to establish dual links. It then transmits the packet to the AP after a certain delay, which is at least longer than a preamble time. The delay time can be computed by Eq. (2). There are two scenarios in this case. If the transmission time of the packet from the AP to Client B is shorter than the packet transmission time of the other link plus a preamble time, as denoted by the “AP-shorter” scenario in Fig. 3(a), then the AP transmits a busytone signal to ensure two transmissions finish at the same time. Otherwise, as denoted by the “AP-longer” scenario in Fig. 3(b), Client A delays its transmission for enough time such that two transmissions finish simultaneously. When Client A and the AP finish their transmissions, Client B first returns an ACK frame to the AP and then the AP returns an ACK frame to Client A upon receiving the ACK frame from Client B. To ensure correct operation of A-Duplex, the duration in RTS and CTS needs to be computed properly. Since in the beginning Client A does not know whether or not the AP will establish dual links, it computes the duration as if the AP does not set up dual links. In case that the AP does not set up dual links, the duration field in the RTS frame works exactly as that in the half duplex case. If the AP does set up dual links, it just uses the duration in the RTS frame to compute the duration of the CTS frame as explained below. Since the AP’s transmission covers all clients, the duration field in the CTS frame can gracefully protect the transmissions between Client A and the AP. As a result, the duration in RTS frame is computed as DurationRTS = 3TSIFS + TCTS + T1 + TACK, (1) where TSIFS is the Short Inter-Frame Spacing (SIFS) time, TCTS is the time of CTS frame, TACK is the time of ACK frame, and T1 is the time of data packet to the AP. With the duration from RTS, the AP computes the duration in CTS as follows. In the case of Fig. 3(a), the duration in CTS is computed as DurationCTS = DurationRTS − TCTS − 2TSIFS + Tp + TACK, where Tp is the preamble time. In the case of Fig. 3(b), the duration in CTS is computed as DurationCTS = T2 + TSIFS + 2TACK, where T2 is the transmission time of the packet from the AP to Client B. When A receive the CTS frame, it decides the delay time according to Delay = DurationCTS − T1 − TSIFS − 2TACK. (2) Fig. 4. An example showing no dual links. The delay for Client A serves two purposes. First, for a legacy node, when it finishes a data packet transmission, it waits a fix time for ACK. If no ACK is received after the preset timeout, it starts the retransmission procedure. Second, the delay ensures the packet to Client B is sent first and its preamble can be protected without interference. In the above procedure, Client A in Fig. 3 waits a time period of TSIFS + TACK before it receives its ACK. This time period is longer than Distributed Coordination Function Inter Frame Spacing (DIFS) time. Thus, the ACK may collide with the packet from the other client. In our design, the CTS frame can solve this problem. The duration in CTS covers the entire period of the above procedure, so all other clients receiving the CTS frame update the NAV value according to the duration in the CTS frame. As a result, the ACK from the AP can also be protected. In the second case when the AP has no packet for a client, the operation procedure is shown in Fig. 4. When AP finds no suitable packet for all other clients, it only replies a CTS frame. From the CTS frame, Client A finds out that the duration in CTS frame is the same with its duration. Thus, Client A knows that the AP will not establish dual links, so it only delays an SIFS time and then starts a packet transmission. When the packet is received correctly, the AP returns an ACK to Client A. In the third case, the AP may get the channel first by sending a data packet without RTS/CTS exchange. No dual link can be set up in this case for two reasons. First, a client does not support full duplex communications, so symmetrical dual links cannot be formed. Second, clients cannot decide among themselves about which one can start asymmetrical links to explore capture effect. If the decision is done by the AP, then the queue status as well as the length of the first packet in queue of each client must be reported to the AP. To avoid such complexity, dual link setup is ignored in the third case. In A-Duplex, the RTS/CTS frame exchange procedures ful- fill the following functions. Firstly, from the RTS frame, the AP knows exactly which client is about to send a packet and then decides how to establish dual links with another client. In [6], the AP transmits packet to another client to establish dual links when it decodes the header of the received packet. However, if the packet is transmitted by OFDM symbols such as in 802.11a network, the AP cannot decode the packet header to get the transmitting client address until it receives the whole packet. Secondly, the RTS/CTS mechanism guarantees that the packet from the AP to Client B is transmitted first and the preamble of the packet is not interfered by the packet from Client A to the AP. Thus, Client B can decode the packet more easily through capture effect. Thirdly, the RTS frame contains signal strength information that can be used by the AP to select a proper client