C
HAPTER
8
| Wireless Settings
Quality of Service (QoS)
– 96 –
802.1D priorities is specifically intended to facilitate inter operability with
other wired network QoS policies. While the four ACs are specified for
specific types of traffic, WMM allows the priority levels to be configured to
match any network-wide QoS policy. WMM also specifies a protocol that
access points can use to communicate the configured traffic priority levels
to QoS-enabled wireless clients.
WMM Operation — WMM uses traffic priority based on the four ACs; Voice,
Video, Best Effort, and Background. The higher the AC priority, the higher
the probability that data is transmitted.
When the access point forwards traffic, WMM adds data packets to four
independent transmit queues, one for each AC, depending on the 802.1D
priority tag of the packet. Data packets without a priority tag are always
added to the Best Effort AC queue. From the four queues, an internal
“virtual” collision resolution mechanism first selects data with the highest
priority to be granted a transmit opportunity. Then the same collision
resolution mechanism is used externally to determine which device has
access to the wireless medium.
For each AC queue, the collision resolution mechanism is dependent on two
timing parameters:
◆
AIFSN (Arbitration Inter-Frame Space Number), a number used to
calculate the minimum time between data frames
◆
CW (Contention Window), a number used to calculate a random backoff
time
After a collision detection, a backoff wait time is calculated. The total wait
time is the sum of a minimum wait time (Arbitration Inter-Frame Space, or
AIFS) determined from the AIFSN, and a random backoff time calculated
from a value selected from zero to the CW. The CW value varies within a
configurable range. It starts at CWMin and doubles after every collision up
to a maximum value, CWMax. After a successful transmission, the CW
value is reset to its CWMin value.
Table 4: WMM Access Categories
Access
Category
WMM
Designation
Description
802.1D
Tags
AC_VO (AC3)
Voice
Highest priority, minimum delay. Time-sensitive
data such as VoIP (Voice over IP) calls.
7, 6
AC_VI (AC2)
Video
High priority, minimum delay. Time-sensitive
data such as streaming video.
5, 4
AC_BE (AC0)
Best Effort
Normal priority, medium delay and throughput.
Data only affected by long delays. Data from
applications or devices that lack QoS
capabilities.
0, 3
AC_BK (AC1)
Background
Lowest priority. Data with no delay or
throughput requirements, such as bulk data
transfers.
2, 1
Summary of Contents for E21011
Page 1: ...EliteConnectTM 802 11 a b g n Access Point SMCE21011 USER GUIDE...
Page 21: ...TABLES 21 Table 32 1000BASE T MDI and MDI X Port Pinouts 255 Table 33 Console Port Pinouts 256...
Page 22: ...TABLES 22...
Page 25: ...INDEX OF CLI COMMANDS 25...
Page 109: ...CHAPTER 10 Status Information Event Logs 109...
Page 111: ...SECTION Command Line Interface 111 VLAN Commands on page 228 WMM Commands on page 231...
Page 147: ...CHAPTER 16 DHCP Relay Commands 147 RELATED COMMANDS show interface wireless...
Page 235: ...CHAPTER 30 WMM Commands 235...
Page 263: ...INDEX 263...
Page 264: ...SMCE21011 149100000016A R01...