Outdoor 802.11 Mesh MAC Protocol Issues & Considerations

1. Outdoor 802.11 Mesh MAC Protocol Issues & Considerations Portland, Oregon 12-15 July 2004 Tricci So <tso@nortelnetworks.com> Tricci So, Nortel Networks

2. Objectives Discuss the MAC Protocol issues and considerations when deploying 802.11 wireless mesh configuration. Tricci So, Nortel Networks

3. 802.11 MAC Protocol Issues & Considerations To Support Outdoor Long Distance Mesh Configuration • Contention-based 802.11 MAC Protocol Considerations for Mesh Networking • The common 802.11 MAC ACK timer, aSlot time, NAV, CWsize, MUST be re-examined to adapt the longer round trip delay because of the increased distance between Mesh Points. • Hidden Node is a REAL problem in the Mesh configuration. The use of RTS/CTS Virtual Carrier-sense, although, can prevent some hidden node problems, but NOT ALL. In addition, it worsens the Exposed Node problem, which is especially harsh for the mesh deployment in term of optimal wireless link throughput. The problem can forbid the independent parallel communication among other mesh points over the same channel. In addition, extra Round Trip Delay introduced by RTS/CTS exchanges over the longer distance is waste of precious wireless link capacity. • The enabling of indiscriminate broadcast and multicast traffic within the mesh can INTENSIFY the hidden node interference problems among the mesh APs especially with omni-directional antenna. The result will degrade the overall system throughput. More hidden node problems are described later. . . . Tricci So, Nortel Networks

4. PG MANET PG MANET PMD_SAP 802.11 MAC ACK Timer Considerations for Outdoor Mesh MAC_SAP IEEE 802.11 , 1999 Edition: . . .After transmitting an MPDU that requires an ACK frame as a response, the STA shall wait for an ACKTimeout interval, starting at the PHY-TXEND.confirm. If a PHY-RXSTART.indication does not occur during the ACKTimeout interval, the STA concludes that the transmission of the MPDU has failed, and this STA shall invoke its backoff procedure upon expiration of the ACKTimeout interval. …. MAC Sublayer 4 3 1 2 PHY-RXSTART. indication PHY-RXSTART. end PHY-TXEND. request PHY-TXEND. confirm PHY_SAP PLCP Sublayer PMD_SAP PMD Sublayer ACK timer MUST be long enough to accommodate the long distance for the outdoor 802.11 mesh environment. TACK response < ACKTimeout Tricci So, Nortel Networks

5. Hidden Nodes Issue - Example • Two nodes are hidden from one another when both attempt to send information to the same receiving node, resulting in a collision of data at the receiver. Hidden Node Hidden Node B C B C A A Hidden Node Hidden Node 1. A transmits to B (C does not hear) 2. C transmits to B (Collision!!!) Tricci So, Nortel Networks

6. Reviewing RTS/CTS Virtual Carrier Sensing • Each node has a NAV table which contains the remaining time of packet transmission of its neighbors • Nodes conduct Virtual Carrier-sense: when the channel is physically sensed to be idle and NAV empty, source node determines to sent RTS packet • All other idle nodes, upon hearing RTS, update their NAV table and defer their transmission • Destination node sends CTS packet to respond to the RTS polling • Neighboring of the destination node overhear the CTS and update their NAV table • After receiving CTS, source transmits Data and receives ACK Tricci So, Nortel Networks

7. Hidden Node Virtual Carrier Sense Solution • To avoid collision in the multi-access environment, all of the receiver’s neighboring nodes need to be informed that channel will be occupied. • RTS (Request to send) indicates that node wants to transmit data • CTS (Clear to send) is used to grant the transmission. • However, Virtual Carrier Sensing does NOT always solve the problem of hidden node! Tricci So, Nortel Networks

8. A B C D More Hidden Node Problem for Mesh • Overhearing a CTS from neighboring nodes over the same channel can inhibit remote node from transmitting to its neighboring nodes. Hidden Node Hidden Node RTS RTS Time CTS RTS Data Transmission CTS Tricci So, Nortel Networks

9. A B C D Another Hidden Node Problem for Mesh • Overhearing a data transmission from neighboring nodes over the same channel can inhibit one node from transmitting to other nodes. This type of interference is especially more frequent when using the omni-directional antenna. Hidden Node RTS RTS CTS Time CTS Data Transmission Data Transmission Interference Tricci So, Nortel Networks

10. A B C D Exposed Node Problem for Mesh • Overhearing a data transmission from neighboring nodes over the same channel can inhibit neighboring node from transmitting to remote node. • Due to the un-intentional backoff, in the mesh configuration, this behavior has even more impact to the overall system performance. Exposed Node Remote Node CTS Time RTS Tricci So, Nortel Networks

11. Conclusions • Common 802.11 MAC ACK timer is geared towards shorter distance indoor deployment. The duration of such timer should be examined carefully for the longer distance outdoor 802.11 mesh deployment such as for the community network. • Hidden Node and Exposed Node are real but are conflicting issues. These issues are especially real for the auto-configured mesh deployment regardless that it is indoor or outdoor. RTS/CTS Virtual Carrier-sense is not sufficient to resolve those problems for 802.11 mesh architecture. Intelligent frequency management and power control among the mesh points should be considered. • The enabling of broadcast and multicast traffic within the mesh can INTENSIFY the Hidden Node and Exposed Node interference problems. The result will degrade the overall system throughput. Tricci So, Nortel Networks