provide privacy and security of user data.
Figure shows the relationship between the OSI reference
model and the IEEE 802.11 Standard. Figures and list amendments
to the original IEEE 802.11 Standard affecting wireless LAN.
This lesson examines 802.11e in detail.
Content
6.1 Implementing WLAN QoS 6.1.2
WLAN QoS Description The IEEE standardized 802.11e in
2005 as a set of technologies for prioritizing traffic and
preventing packet collisions and delays to improve voice and
video performance over WLANs. The 802.11e specification uses
the same eight priority level structure that 802.11p uses. They
are grouped into four access categories or traffic classes as
follows: - Voice: Priority 7 or 6 for
toll-quality VoIP calls requiring low latency
-
Video: Priority 5 or 4 for SDTV or HDTV video
streams
- Best Effort: Priority 3 or 0 for
latency-insensitive, interactive applications
-
Background: Priority 2 or 1 for batch data transfer
applications and indicates less than best effort
treatment
Each access level provides a separate
queue. To identify the class of each packet, the standard uses
markers that are similar to markers used in wired Ethernet.
Seeing those markers, an access point handles packets according
to the packet’s assigned priority. To speed the adoption of QoS
in the 802.11 marketplace, the Wi-Fi Multimedia (WMM) was
implemented before the 802.11e standard was approved. WMM is a
subset of the 802.11e standard using four access categories
rather than the eight priority values of 802.11e. By default,
these are voice, video, best effort, and background, but
definitions of the four classes can be changed from the default
definitions. WMM provides a group of features for wireless
networks to improve audio, video, and voice application
performances. The resulting QoS allows translation from 802.1p
or differentiated services code point (DSCP) to the appropriate
RF techniques giving high-priority traffic an increased
probability of RF transmission over lower-priority traffic. WMM
uses traffic-prioritization capabilities based on the four
defined access categories. RF prioritization gives a higher
access category the increased probability of being transmitted
first. This allows the platinum level to obtain RF access for
transmission before the gold, silver, or bronze levels. The
access categories correspond to 802.1p or DSCP priorities to
facilitate interoperability with QoS policy-management
mechanisms. WMM priorities coexist with legacy devices that are
not WMM-enabled. Packets that are not assigned to a specific
access category are categorized by default as the best-effort
access category. Figure depicts the WMM traffic prioritization
scheme that the Wi-Fi Alliance has approved for assigning
application data that is being sent to the client. WMM is an
enhancement to the MAC sublayer that adds QoS functionality to
Wi-Fi networks. The MAC sublayer handles access mechanisms,
fragmentation, and encryption. WMM prioritization maps four
independent transmit queues to eight 802.1e priority levels, as
shown in Figure .
Content 6.1
Implementing WLAN QoS 6.1.3 WLAN QoS RF
Backoff Timing The fundamental access method of 802.11 is
CSMA/CA. CSMA/CA works by a "listen before talk
scheme." WMM uses the CSMA/CA-based distributed
coordination function (DCF) mechanism that gives all devices
the same priority and is based on a best-effort,
“listen-before-talk” algorithm. If the medium is not busy, the
transmission may proceed. CSMA/CA uses a random backoff time to
avoid collisions among stations sharing the medium, if the
sending station’s physical or logical sensing mechanism
indicates a busy medium. Each client waits a random backoff
time and then transmits only if no other device is transmitting
at that time. This collision-avoidance method gives all the
devices the opportunity to transmit, but when traffic demand is
high and networks can become overloaded, the performance of all
devices will be affected. Using Enhanced DCF (EDCF), WMM
stipulates different fixed and random wait times for the four
access categories to provide more favorable network access for
applications that are less tolerant of packet delays. Devices
that have less time to wait have a better chance of being able
to transmit than those that have a longer wait. The access
categories—voice, video, best effort and background—map to
Ethernet 802.1d prioritization tags to allow consistent QoS
across wireless and wired network segments. WMM provides
priority access to the RF medium in two ways: - First,
the wireless access point must prioritize the data into four
access categories (from highest to lowest: platinum, gold,
silver, and bronze).
- Second, the lower-priority
traffic must use longer interframe wait timers to allow
higher-priority traffic access to the wireless network first.
The timers result from a summarization of the fixed short
interframe space (SIFS), slot times (fixed-length time
intervals based on priority), and a random slot timer (based on
priority) shown in Figure for various QoS levels. The random
backoff slot timers prevent media contention among traffic from
within the same access category.
Content
6.1 Implementing WLAN QoS 6.1.4
Lightweight Access Point—Split MAC Architecture Two
philosophically opposite approaches to providing wireless
access exist. The first approach, which is becoming less
popular, uses autonomous access points (AP) with a lot of
intelligence. Intelligent access points can communicate with
existing routers and thus support robust applications. This
style of WLAN management, referred to as a distributed
architecture, works well but is expensive and requires access
points that can work within a specific vendor's particular
infrastructure. The second approach takes the intelligence out
of APs and puts the intelligence into switches or routers to
allow networks to scale and to reduce the overall cost of WLAN
deployment. This approach led to the development of the
Lightweight Access Point Protocol (LWAPP), resulting in an
entirely new paradigm for managing WLAN deployments. LWAPP uses
the concept of a “split MAC”, which is the ability to separate
the real-time aspects of the 802.11 protocol from most of the
protocol’s management aspects. Cisco has designed a
centralized, lightweight access point wireless architecture to
address the unique RF management needs of enterprises. A core
component is the split MAC architecture, which distributes the
processing of 802.11 data and management protocols between a
lightweight access point and a centralized WLAN controller.
More specifically, the access point handles time-sensitive
activities, such as beacon handling, handshakes with clients,
MAC layer encryption, and RF monitoring. The WLAN controller
handles all other functions and provides system-wide
visibility. These functions include IEEE 802.11 management
protocol, frame translation, and bridging functions, as well as
system-wide policies for user mobility, security, QoS, and,
perhaps most importantly, real-time RF management. Figure
distinguishes real-time, or time-critical, MAC functions that
the access point performs, and non-real-time, or network
critical, MAC functions that the controller performs. The
figure also introduces the concept of an LWAPP tunnel.
Forthcoming topics explain that all traffic coming from
wireless clients goes into LWAPP packets that are tunneled
through the underlying network to the wireless LAN controller.
Content 6.1 Implementing WLAN QoS
6.1.5 WLAN QoS Challenges Providing QoS in a
WLAN is a challenge due to the following : - End-to-end
QoS uses DSCP for packet marking.
- Wireless RF uses
Layer 2 marking.
- In the Cisco deployment model,
traffic destined to access points does not contain 802.1p QoS
tag information because the access points connect to a Cisco
Catalyst switch port, which is not a trunk.
- As a