1. MAC Considerations for Mesh
July 2004
doc.: IEEE /760r1
July 2004
MAC Considerations for Mesh
L. Lily Yang (Intel Corp.)
Akira Yamada (NTT DoCoMo)
Lily Yang, Akira Yamada
Lily Yang, Akira Yamada

2. Overview Existing 802.11 MAC Architecture 11s MAC Considerations:
July 2004
doc.: IEEE /760r1
July 2004
Overview
Existing MAC Architecture
Preliminary Analysis on 11e for 11s
11s MAC Considerations:
Level of QoS Support
Examples of Mesh Specific MAC issues
Practical Considerations
Possible MAC Architecture Alternatives for 11s
Summary and Call to actions
Lily Yang, Akira Yamada
Lily Yang, Akira Yamada

3. Context: Major Functional Components for 802.11s Mesh Points *
July 2004
Context: Major Functional Components for s Mesh Points *
Configuration/ Management
Internetworking
Interfaces
IEEE802.11s Amendment
.11s Mesh Network Measurement
Layer 2 Mesh Routing and Forwarding
.11s Mesh Security
MAC/MLME enhancement for .11s Mesh
IEEE MAC
IEEE802.11　PHY
IEEE a/b/g/j/n
*[1] slide#3
Lily Yang, Akira Yamada

4. Existing 802.11 MAC Architecture Overview
July 2004
doc.: IEEE /760r1
July 2004
Existing MAC Architecture Overview
Point
Coordination
Function
(PCF: optional)
HCF
Controlled
Access
(HCCA)
HCF
Contention
Access
(EDCA)
11n
MAC
Enhancement
(based on 11e?)
MAC
Distributed Coordination Function
(DCF)
In a non-QoS STA, HCF is not present.
In a QSTA implementation, both DCF and HCF are present. PCF is optional in all STAs.
PCF is required for contention-free services for non-QoS STA.
EDCA (enhanced distributed channel access) is required for prioritized QoS services.
HCCA (HCF controlled channel access) is required for parameterized QoS services.
11a/11b/11g/11j PHY
11n PHY
* Based on e D8.0 Clause 9.1, Figure 47
Lily Yang, Akira Yamada
Lily Yang, Akira Yamada

5. 11e Components and Their Relevance to 11s
July 2004
11e Components and Their Relevance to 11s
EDCA
Prioritized QoS support and performance enhancement
Important to 11s
HCCA
Parameterized QoS support, need HC
Not directly applicable in mesh
Block Ack
Performance optimization, optional for QSTAs
Useful but not essential to mesh
DLP and APSD
Optimization for QSTAs
Orthogonal to mesh
APSD
Block Ack
EDCA
HCCA
DLP
*courtesy of
Duncan Kitchin
Most Relevant Component to Mesh Points: EDCA
Lily Yang, Akira Yamada

6. EDCA: Important Enhancements over DCF
July 2004
EDCA: Important Enhancements over DCF
Prioritized QoS support by separate Channel Access Functions for four ACs
Separate QoS parameters: CWmin, CWmax, TXOP duration, AIFSN
Provide differentiated services
Optional: admission control (by QAP)
Performance enhancement over basic 11 MAC (e.g., TXOP)
Reduce channel access overhead
Fix “slow client” problem
Lily Yang, Akira Yamada

7. EDCA: Not Sufficient for 11s
July 2004
EDCA: Not Sufficient for 11s
Designed for QBSS:
Assume one hop traffics between QAPQSTA
Assume QAP responsible for QSTA settings: BSS QoS parameters are set by QAP
how to apply that to mesh points?
Admission control administered by QAP
NOT designed for WLAN Mesh (infrastructure network of Mesh Points)!
None of the Mesh Point has complete visibility into the whole WLAN Mesh => Really need distributed coordination between mesh points
Not designed for multi-hop: issues like hidden/exposed terminals worsen the performance
No end-to-end considerations at all: cause network inefficiency.
Lily Yang, Akira Yamada

8. MAC Considerations (1): QoS Support
July 2004
MAC Considerations (1): QoS Support
From Usage Models:
Multi-media applications requires QoS support
But we need to carefully determine the type of QoS support for 11s:
Is prioritized QoS sufficient?
EDCA can help.
Do we need parameterized QoS support like what HCCA provides?
HCCA not directly applicable due to distributed nature of mesh
Much harder problem
The level of QoS support required will have great impact on the 11s standard completion time
Need to determine the level of QoS support in 11s Functional Requirements
Lily Yang, Akira Yamada

9. MAC Considerations (2): Mesh Specific Issues
July 2004
MAC Considerations (2): Mesh Specific Issues
Fundamentally different network assumptions from 11/e/n:
Distributed data plane: peer-to-peer (no Coordinator)
Multi-hop & end-to-end aspects
Much more complicated traffic pattern in the network than the “star” pattern traffic in the BSS
Mesh points are used to build infrastructure network:
Mesh Point can be co-located with AP or application end points
A mixture of traffic: from own BSS (or own applications), and from WDS
Example of mesh specific issues that are NOT addressed in 11, 11e or 11n:
Hidden/exposed nodes
Lack of flow control in congested network
No prioritization consideration between source traffic and relay traffic
Lily Yang, Akira Yamada

10. Hidden Node and Exposed Node Problem *
July 2004
Hidden Node and Exposed Node Problem *
Expected to be more common and profound in mesh and hence have more severe negative impact on performance
Virtual carrier sensing by RTS/CTS: is supposed to solve this problem, but not really effective, esp. for mesh
Main consequence: NAV is not set correctly
Collision becomes severe (hidden nodes)
Waste wireless bandwidth (exposed nodes)
* See [2] and other literature for more details.
Lily Yang, Akira Yamada

11. Hidden Nodes: Example AP1 AP2 AP3 AP4 AP1 AP2 AP3 AP4 AP1 AP2 AP3 AP4
July 2004
Hidden Nodes: Example
AP1
AP2
AP3
AP4
Case (a)
Case (c)
AP1
AP2
AP3
AP4
Case (b)
Case (d)
Case (b)
AP1
AP2
AP3
AP4
Case (a)
Case (c)
AP1
AP2
AP3
AP4
Case (d)
Lily Yang, Akira Yamada

12. Lack of Flow Control: Example
July 2004
Lack of Flow Control: Example
Individual flow throughput (b/s) N0 N4 N1 N2 N3
Flow 1
Flow 2
Load (mb/s)
Lily Yang, Akira Yamada

13. Lack of Flow Control: Example (cont.)
July 2004
Lack of Flow Control: Example (cont.)
Actual Scheduling Result when load=700kb/s
e2e throughput
674k
449k
355k
235k
Congested nodes
238k
347k
450k
697k
Rx: k k k k 235k
Tx: k k k k k
Wasted TX
Ideal Scheduling, when the network is overloaded
430k
430k
430k
430k
430k
430k
430k
430k
e2e
throughput
Rx: k k k 860k 860k
Tx: k k k 860k 430k
Lily Yang, Akira Yamada

14. Traffic Delivery Prioritization
July 2004
Traffic Delivery Prioritization
Relay traffic
Mesh AP
STA N4 N0 N1 N2 N3
BSS traffic
When Mesh Point co-located with AP on one RF interface:
How to coordinate the relay traffic for mesh point and the BSS traffic for AP?
Does it make sense to prioritize the traffic streams depends on type of traffic (relay vs source), how far they’ve traveled,
Lily Yang, Akira Yamada

15. MAC Considerations (3): Practical Matters
July 2004
MAC Considerations (3): Practical Matters
Best if compatible with existing MAC architecture (DCF, EDCA etc.)
Hard to adopt if completely different MAC
Simplicity & low cost are always desirable
Encourage implementation flexibility
Mesh AP
Two radio interfaces (one for AP, one for Mesh Point)
Same radio interface for both AP and Mesh Point
Mesh Point being application end points (clients)
Mesh Point only as relay
We want to get the standard out in a reasonable time frame
Address the most important requirements first: Good enough
TGs need to specify such a prioritized list in its functional requirements for MAC
Lily Yang, Akira Yamada

16. MAC Considerations (4): Architectural Alternatives
July 2004
MAC Considerations (4): Architectural Alternatives
Point
Coordination
Function
(PCF: optional)
HCF
Controlled
Access
(HCCA)
HCF
Contention
Access
(EDCA)
Mesh
Coordination
Function
11n
MAC
MAC
Distributed Coordination Function
(DCF)
11a/11b/11g/11j PHY
11n
PHY
Option#1: Mesh Coordination Function builds on top of DCF, independent of 11e
Lily Yang, Akira Yamada

17. MAC Considerations (4): Architectural Alternatives (cont.)
July 2004
MAC Considerations (4): Architectural Alternatives (cont.)
Point
Coordination
Function
(PCF: optional)
HCF
Controlled
Access
(HCCA)
Mesh Coordination
Function
11n
MAC
MAC
EDCA
Distributed Coordination Function
(DCF)
11a/11b/11g/11j PHY
11n
PHY
Option#2: Mesh Coordination Function builds on top of EDCA (could still enhance DCF as well)
Lily Yang, Akira Yamada

18. MAC Considerations (4): Architectural Alternatives
July 2004
MAC Considerations (4): Architectural Alternatives
Point
Coordination
Function
(PCF: optional)
HCF
Controlled
Access
(HCCA)
HCF
Contention
Access
(EDCA)
Mesh
Coordination
Function
11n
MAC
MAC
Distributed Coordination Function
(DCF)
11a/11b/11g/11j PHY
11n
PHY
Option#3: Mesh Coordination Function independent of DCF (very different MAC)
Lily Yang, Akira Yamada

19. July 2004
Summary
Only a very preliminary analysis on 11e and its implications to 11s
Clearly there are MAC performance issues beyond multi-hop routing that 11s must address
Lets start discussing MAC issues and agree on functional requirements
Ad Hoc Group?
Focus on problems and requirements, not solutions (yet)
Lily Yang, Akira Yamada

20. July 2004
doc.: IEEE /760r1
July 2004
References
[1] s-suggested-major-functional-components s.ppt
[2] s-outdoor mesh-mac-protocol-issues-and-considerations.ppt
Lily Yang, Akira Yamada
Lily Yang, Akira Yamada

21. July 2004
Thank you
Send me if you are interested in ad hoc group discussion on MAC issues:
Lily Yang, Akira Yamada