| 5 | | This application note presents results from a simple experiment using the [wiki:802.11 Mango 802.11 Reference Design] and its [wiki:802.11/wlan_exp experiments framework]. We use three WARP v3 nodes to build a basic 802.11 network, with one AP and two stations (STA). |
| | 5 | The [https://en.wikipedia.org/wiki/Hidden_node_problem Hidden Node Problem] is a classic topology studied in wireless networking where traditional carrier sensing breaks down since one or more nodes are "hidden" from the carrier sensing capability of the rest of the network. |
| | 6 | |
| | 7 | || [[Image(wiki:802.11/wlan_exp/app_notes/dcf_with_hidden_nodes/figs:hidden_node.png, width=800)]] || |
| | 8 | || '''Classic Hidden Node Topology''' || |
| | 9 | |
| | 10 | As we have seen in the [wiki:802.11/wlan_exp/app_notes/dcf_with_multiple_flows multiflow app note], carrier sensing plays a crucial role in the DCF's ability deal with contention in a shared wireless medium. In the classic hidden node topology shown above, STA 1 can carrier sense the AP and vice versa. STA 2 can carrier sense the AP and vice versa. However, STA 1 and STA 2 are out of range of one another and cannot detect each other's transmissions. The traditional solution to this problem is the [https://en.wikipedia.org/wiki/IEEE_802.11_RTS/CTS RTS/CTS handshake]. The basic timeline of this handshake is as follow: |
| | 11 | |
| | 12 | 1. STA 1 wishes to send a data MPDU to the AP. Since STA 2 cannot directly hear this transmission, the likelihood of a collision might be large. Instead, STA 1 "requests to send" the MPDU by sending a very short RTS frame to the AP. This transmission may still collide, but it is very short, so the penalty to lost medium time is significantly mitigated. |
| | 13 | 1. When the AP receives this RTS frame, it checks the medium by looking at its NAV. If the NAV says the medium is clear, it sends the "clear to send" CTS frame in response. Critically, this CTS frame contains a duration field that species how long the medium will be occupied for the upcoming MPDU and ACK transmissions. |
| | 14 | 1. Since STA 2 is within the transmission range of the AP, it overhears this CTS transmission and updates it NAV for the entire duration specified in the CTS frame. This will prevent STA 2 from colliding with the upcoming MPDU transmission from STA 1. |
| | 15 | 1. Finally, STA 1 receives this CTS message and begins the transmission of the actual MPDU. This MPDU is now protected from collisions from anyone who overheard the AP's CTS transmission. |
| | 16 | |
| | 17 | RTS/CTS support was added in [wiki:../../../Changelog#a1.3Release 802.11 Reference Design (version v1.3)]. In this application note, we'll use the reference design and its [wiki:802.11/wlan_exp experiments framework] to investigate the hidden node problem. |
