1. Class-based Contention Periods (CCP) for the 802.11n MAC
August 2004
doc.: IEEE /xxxr0
August 2004
Class-based Contention Periods (CCP) for the n MAC
A. Dasylva, Z. Yao, D.Y. Montuno, W. Chen, M. Ouellette, J. Aweya, and K. Felske
Nortel Networks
Abel Dasylva, Nortel Networks
Abel Dasylva, Nortel Networks

2. Acronyms HC: Hybrid Coordinator CP: Contention Period
August 2004
doc.: IEEE /xxxr0
August 2004
Acronyms
HC: Hybrid Coordinator
CP: Contention Period
CFP: Contention Free Period
CCP: Class-based Contention Periods
ECP: Explicit Contention Period. A type of CP explicitly allocated by the HC and where a subset of the ACs may contend. It starts and ends with specific control frames
LCP: Legacy Contention Period. A traditional CP.
STA: Station. Unless mentioned other wise an STA indicates a legacy STA (with or without QoS support)
HT STA: High Throughput STA, an STA supporting the new features described in this document and other n features.
HT AP: High Throughput AP, an AP supporting the features described in this document and other n features.
Abel Dasylva, Nortel Networks
Abel Dasylva, Nortel Networks

3. General description of CCP
August 2004
doc.: IEEE /xxxr0
August 2004
General description of CCP
Two types of contention periods
Explicit CPs (ECPs) allocated by the HT AP
Legacy CPs (LCPs)
In each ECP a subset of ACs contend according to EDCA rules
ECPs are delimited by
ECP-Start
ECP-End or
ECP-Start+ECP-end frames
Two access modes for ECPs
Default mode: a channel access function can access the channel within an ECP if its AC is allowed in the ECP
QoS negotiation mode: the HT AP grants access to the channel access function after a QoS negotiation phase
Abel Dasylva, Nortel Networks
Abel Dasylva, Nortel Networks

4. August 2004
doc.: IEEE /xxxr0
August 2004
Motivation for CCP
The need for a simple and effective QoS provisioning mechanism
Improve the throughput efficiency of EDCA by
Allowing non-QoS (TCP) traffic to contend more aggressively for the available bandwidth
Maintain and improve the performance of QoS traffic (with EDCA) by better isolation from non-QoS flows
The complexity and polling overhead (especially the associated preamble+PLCP overhead) of HCCA
The difficulty of accurate QoS provisioning with EDCA
Solution CCP: blend features of HCCA and EDCA
Centralized allocation and scheduling of ECPs
Distributed channel access within ECPs
QoS provided by the proper selection of ECP lengths and scheduling
Abel Dasylva, Nortel Networks
Abel Dasylva, Nortel Networks

5. ECP Scheduling (Informative)
August 2004
doc.: IEEE /xxxr0
August 2004
ECP Scheduling (Informative)
Abel Dasylva, Nortel Networks
Abel Dasylva, Nortel Networks

6. Control frames/ ECP-Start frame
August 2004
Control frames/ ECP-Start frame
A frame sent by the HT AP to initiate a new ECP
Fields:
RA: set to the broadcast group address
ECP type: 1 byte field giving the ECP type. Each ECP type maps to a subset of ACs.
Duration: this field is set to the length of the ECP
Abel Dasylva, Nortel Networks

7. Control frames/ ECP-End frame
August 2004
Control frames/ ECP-End frame
A frame sent by the HT AP to end an ECP
Fields:
RA: set to the broadcast group address
Duration: frame duration
Abel Dasylva, Nortel Networks

8. Control frames/ ECP-End+ECP-Start frame
August 2004
Control frames/ ECP-End+ECP-Start frame
A frame sent by the HT AP to end the current ECP and start the next ECP
Fields:
RA: set to the broadcast group address
Duration: duration of the ECP
ECP type: type of the next ECP
Abel Dasylva, Nortel Networks

9. Control frames/ ECP-Access Req. frame
August 2004
Control frames/ ECP-Access Req. frame
A frame sent by a HT STA to the AP to request access to ECPs of one or more types
The ECP access request is on a per-flow basis. It is not doe for each data frame
Fields:
Duration: frame duration
TA: address of the requesting HT STA
ECP type n: n-th ECP type for which access is requested
TSPEC n: TSPEC of the traffic to be transmitted in ECPs of type n
Abel Dasylva, Nortel Networks

10. Control frames/ ECP-Access Req. ACK. frame
August 2004
Control frames/ ECP-Access Req. ACK. frame
A frame sent by an HT AP to an HT STA to acknowledge the receipt of an ECP-access request frame
Fields:
Duration: frame duration
RA: address of the requesting HT STA
Request number: request number assigned by the HT AP
Abel Dasylva, Nortel Networks

11. Control frames/ ECP-Access Resp. frame
August 2004
Control frames/ ECP-Access Resp. frame
A frame sent by the HT AP to a requesting HT STA in response to an ECP access request access
Fields:
Duration: frame duration
RA: address of the requesting HT STA
ECP type n: n-th ECP type for which access is requested
Resp n: admission decision for ECP n
Abel Dasylva, Nortel Networks

12. Mgmt frames/ ECP capability element
August 2004
Mgmt frames/ ECP capability element
Information element advertising ECP capability by the HT AP or HT STAs
Fields:
ECP capability: bit indicating whether the HT AP is able to allocate ECPs, or HT STAs are able to interpret ECP frames
ECP length: maximum ECP length
Num. ECP types: the number of ECP types that are supported by the HT AP
Abel Dasylva, Nortel Networks

13. Mgmt frames/ ECP parameter element
August 2004
Mgmt frames/ ECP parameter element
An information element giving the parameters of an ECP type
Fields:
ECP type: type of the ECP between 0 and 255
Mode: access mode for the ECP type, i.e. default (0) or through QoS negotiation (1)
AC mask: ACs that are allowed to contend for channel access
Abel Dasylva, Nortel Networks

14. MAC sublayer functional description 1/5
August 2004
MAC sublayer functional description 1/5
ECP allocation and scheduling
The HT AP allocates ECPs by sending ECP-Start or ECP-Start+ECP-End frames
The duration of a new ECP is set in the duration field of the corresponding ECP-Start, or ECP-Start + ECP-End frame
The length of an ECP cannot exceed the value of MAX_ECP_LENGTH (larger than the CFP length) set in the ECP length field of the ECP capability element
An ECP-Start or ECP-End+ECP-Start frame may be allocated
A PIFS interval after the completion of a CFP or ECP
A PIFS interval after the transmission of a frame in LCPs
Channel access during ECPs
Essentially EDCA rules with minor modifications:
A frame exchange sequence initiated within an ECP must complete within that ECP
A TXOP obtained within an ECP must complete within the ECP
The HCCA function cannot obtained polled TXOPs within an ECP
Abel Dasylva, Nortel Networks

15. MAC sublayer functional description 2/5
August 2004
MAC sublayer functional description 2/5
Channel access during LCPs
All HT STAs may contend according to EDCA rules
The HCCA function may allocate polled TXOPs
Interaction with the power save feature
Consider a HT STA emerging from power-save mode
This HT STA may not have knowledge of the current CFP/ECP/LCP
The HT STA resets an ECP-length timer with the value MAX_ECP_LENGTH and waits for of the following events to occur:
The timer expires: then the HT STA concludes that it is within an LCP, and the states of the channel access functions are set accordingly
A CFP-End, ECP-Start, ECP-End or ECP-Start+ECP-End frame is received and the states of the channel access functions may be properly set
Abel Dasylva, Nortel Networks

16. MAC sublayer functional description 3/5
August 2004
MAC sublayer functional description 3/5
Support of frame aggregation (class-based frame aggregation): it is possible to transmit aggregate frames within ECPs subject to the following limitations:
For an ECP with no required QoS negotiation: only frames of ACs allowed within the ECP may form an aggregate
For an ECP with required QoS negotiation: only frames from channel access functions of the allowed ACs that have been granted access to the ECP type by the HC
Abel Dasylva, Nortel Networks

17. MAC sublayer functional description 4/5
August 2004
MAC sublayer functional description 4/5
ECP allocation examples
Abel Dasylva, Nortel Networks

18. MAC sublayer functional description 5/5
August 2004
MAC sublayer functional description 5/5
ECP allocation examples
Abel Dasylva, Nortel Networks

19. MAC sublayer Mgmt/ ECP capability
August 2004
MAC sublayer Mgmt/ ECP capability
ECP capability information: included by the HT AP in association or re-association messages with the following info
Whether ECPs are supported
The maximum ECP length (MAX_ECP_LENGTH)
The number of supported ECP types
ECP parameters: included by the HT AP in association or re-association messages with the following info for each ECP type
The mode: default or through QoS negotiation
AC mask: allowed ACs
Abel Dasylva, Nortel Networks

20. MAC sublayer Mgmt/ IBSS operation
August 2004
MAC sublayer Mgmt/ IBSS operation
Currently not supported with CCP
Abel Dasylva, Nortel Networks

21. MAC sublayer mgmt/ Coexistence with legacy STAs
August 2004
MAC sublayer mgmt/ Coexistence with legacy STAs
Legacy STAs: STAs not able to interpret ECP control frames
Channel access within ECPs: the setting of the duration field in ECP-Start, and Ecp-Start+ECP-End frames ensure that legacy STAs do not interfere with ECP traffic
Channel access within LCPs: all STAs including legacy ones may contend according to EDCA
No conflict with HCCA
Abel Dasylva, Nortel Networks

22. Simulations 1/10 Settings August 2004 Parameter Value PHY Tx Antennae
Rx Antennae
Nominal Channel Width
20 MHz
PHY Options enabled
None
Type
802.11a
Rate
54Mbps
TCP
Model
Reno
Duplicate ACK threshold 3
ACK policy
Delayed ACK
Maximum ACK delay
200ms
Settings
Abel Dasylva, Nortel Networks

23. Simulations 2/10 Settings August 2004 Parameter Value MAC
Channel access methods
EDCA and CCP
Note: the channel access method used is shown in the CCP results table using color coding.
HCCA
Not used
TXOP
Block ACK
AIFS VO
2 (slots) VI 2 BK 7 BE 3
Cwmin 15
CWmax
1023
Settings
Abel Dasylva, Nortel Networks

24. Simulations 3/10 AC mapping August 2004 Abel Dasylva, Nortel Networks
Application
AC_BE
Content download
Internet File transfer
AC_BK
VoD control channel
Local File transfer, printing
Remote user interface
AC_VI
DV Audio/video
SDTV
HDTV (Video/Audio)
DVD
Video Conf
Internet Streaming video/audio
Internet Streaming audio
MP3 Audio
Interactive Gaming
Netmeeting application/desktop sharing
Video phone
AC_VO
VoIP
AC mapping
Abel Dasylva, Nortel Networks

25. Simulations 4/10 ECP scheduling Two ECP types are defined
August 2004
Simulations 4/10
ECP scheduling
Two ECP types are defined
An ECP type for AC_VO+AC_VI
An ECP type for AC_BK+AC_BE
Within an ECP type
EDCA rules
Strict priority among the ACs allowed (differet AIFS, and internal contention resolution)
A round-robin schedule
The ECPs alternate and are delimited by ECP-Start+ECP-End control frames
Abel Dasylva, Nortel Networks

26. Simulations 5/10 Other The PHY rate is small at 54Mbps
August 2004
Simulations 5/10
Other
The PHY rate is small at 54Mbps
CCP does not support ad-hoc traffic
The traffic demand of the SSs have been slightly modified as follows
To avoid excessive overload by QoS traffic: some QoS sources have been turned off
To allow a fair comparison with EDCA: ad-hoc traffic has been removed
The same traffic demand is used for EDCA and EDCA+CCP
Abel Dasylva, Nortel Networks

27. Simulations 6/10 Comparison criteria
August 2004
Simulations 6/10
Comparison criteria
CC3: the overall system throughput (QoS+non-QoS) is increased by CCP. The increase is substantial in SS4
CC18: in all SSs the non-QoS throughput is significantly increased by CCP
CC19: the QoS throughput is about the same with EDCA and with CCP. The best possible case is the EDCA throughput where QoS traffic has strict priority over non-QoS traffic
CC#
Name
Result
EDCA
CCP
CC3
List of goodput results for usage models 1, 4 and 6.
SS1 (Mbps), see Table 13
23.7
24.8
SS4, see Table 17
16.3
29.3
SS6, see Table 21
15.3
19.4
CC18
HT Usage Models Supported (non QoS)
SS1(Mbps/ratio)
0.0/1.0
1.0/1.0
SS4
7.2/450
20.2/450
SS6
0.0/20.0
4.4/20.0
CC19
HT Usage Models Supported (QoS)
SS1
23.7/27.0
23.8/27.0
9.1/9.1
9.0/9.1
15.3/21.88
15.02/21.8
Abel Dasylva, Nortel Networks

28. Simulations 7/10 Comparison criteria
August 2004
Simulations 7/10
Comparison criteria
CC20: CCP does not affect the performance of QoS traffic. However, in SS1 and SS6 there is an overload of RT traffic (at a PHY rate of 54Mbps) and some large HDTV and SDTV flows do not meet their QoS requirements.
CC24: the throughput efficiency is significantly increased with CCP
CC#
Name
Result
EDCA
CCP
CC20
BSS Aggregate Goodput at the MAC data SAP
SS1(Mbps)
23.7
24.8
1.65
1.75
SS4
16.3
29.3
SS6
15.3
19.4
2.83
7.25
CC24
MAC Efficiency(%)
SS1
43.89
45.93
30.19
54.26
28.33
35.9
Abel Dasylva, Nortel Networks

29. August 2004
Simulations 8/10
SS1
CCP increases the non-QoS throughput and the overall throughput
The effect is minimal on the QoS of real-time flows
Offered load
Throughput
Flows
Compliant flows
EDCA
CCP
QoS flows
27.04
23.74
23.86 13 9
Non-QoS flows
1.0
0.0 1
N/A
Abel Dasylva, Nortel Networks

30. August 2004
Simulations 9/10
SS4
CCP significantly increases the non-QoS throughput and the overall throughput
The effect is minimal on the QoS of real-time flows
Offered load
Throughput
Flows
Compliant flows
EDCA
CCP
QoS flows
9.15
9.13
9.09 18
Non-QoS flows
450
7.24
20.22 26
N/A
Abel Dasylva, Nortel Networks

31. August 2004
Simulations 10/10
SS6
CCP significantly increases the non-QoS throughput and the overall throughput
The effect is minimal on the QoS of real-time flows
Offered load
Throughput
Flows
Compliant flows
EDCA
CCP
QoS flows
21.88
15.34
15.02 34 30
Non-QoS flows
20.0
0.0019
4.41 10
N/A
Abel Dasylva, Nortel Networks

32. Conclusion A simple framework for effective QoS provisioning
August 2004
Conclusion
A simple framework for effective QoS provisioning
A wide variety of bandwidth allocation and QoS policies supported
Full backward compatibility with /802.11e
The requirement for admission control to ensure QoS within real time ECPs (beyond the scope of this work)
Abel Dasylva, Nortel Networks