1. Outdoor 802.11 Mesh MAC Protocol Issues & Considerations
Month 2002
doc.: IEEE /xxxr0
July 2004
Outdoor Mesh MAC Protocol Issues & Considerations
Portland, Oregon
12-15 July 2004
Tricci So
Tricci So, Nortel Networks
John Doe, His Company

2. Month 2002
doc.: IEEE /xxxr0
July 2004
Objectives
Discuss the MAC Protocol issues and considerations when deploying wireless mesh configuration.
Tricci So, Nortel Networks
John Doe, His Company

3. Month 2002
doc.: IEEE /xxxr0
July 2004
MAC Protocol Issues & Considerations To Support Outdoor Long Distance Mesh Configuration
Contention-based MAC Protocol Considerations for Mesh Networking
The common MAC ACK timer, aSlot time, NAV, CWsize, parameters 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 the overall 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
John Doe, His Company

4. 802.11 MAC ACK Timer Considerations for Outdoor Mesh
PG MANET
July 2004
PMD_SAP
MAC ACK Timer Considerations for Outdoor Mesh
MAC_SAP
IEEE , 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 3 4 1 2
PHY-RXSTART.
indication
PHY-RXSTART.
end
PHY_SAP
PHY-TXEND.
request
PHY-TXEND.
confirm
PLCP Sublayer
PMD_SAP
PMD Sublayer
ACK timer MUST be long enough to accommodate the long distance for the outdoor mesh environment.
TACK response < ACKTimeout
Tricci So, Nortel Networks

5. Hidden Nodes Issue - Example
July 2004
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
Month 2002
doc.: IEEE /xxxr0
July 2004
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
John Doe, His Company

7. Hidden Node Virtual Carrier Sense Solution
July 2004
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. More Hidden Node Problem for Mesh
July 2004
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.
Source
Node
Destination
Node
Hidden/Destination
Node A B C D
RTS
RTS
Time
CTS
RTS
Data Transmission
CTS
Tricci So, Nortel Networks

9. Another Hidden Node Problem for Mesh
July 2004
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.
Source
Node
Destination
Node
Source/Hidden
Node A B C D
RTS
CTS
RTS
Time
CTS
Data Transmission
Data Transmission
Interference
Tricci So, Nortel Networks

10. Exposed Node Problem for Mesh
July 2004
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 A B C D
CTS
Time
RTS
Tricci So, Nortel Networks

11. July 2004
Conclusions
Common MAC ACK timer. aSLOT, NAV and CWsize is geared towards shorter distance indoor deployment. The duration of such parameters should be examined carefully for the longer distance outdoor 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 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