result, packets transmitted over a WLAN will also
lose Layer 2 QoS information. Since it is vital to
use the Layer 3 DSCP information to provide QoS in the absence
of Layer 2 QoS information, a solution is needed. Cisco WLAN
controllers using software version 3.2 or later ensure that
packets receive proper QoS handling end-to-end during
transmission. WLAN controllers ensure that the packet maintains
the QoS information while the packet traverses the network
using RF wireless IEEE 802.1e markings or WMM mappings, as
appropriate. Figure depicts how to overcome this challenge
using LWAPP to encapsulate and tunnel packets from the access
point to the controller. In this way, end-to-end QoS
implementation requires mapping between Layer 2 802.1p or Layer
3 DSCP and wireless RF using 802.11e or WMM. The lightweight
access point WLAN solution enhances the way access points use
Layer 3 information to ensure that packets receive the correct
over-the-air prioritization when the packets are transmitted
from the access point to the wireless client.
Content
6.1 Implementing WLAN QoS 6.1.6
WLAN QoS Implementation When voice or video uses a
WLAN, the service must integrate with the wired network and
Voice over IP (VoIP) systems to deliver consistent, high
quality end-to-end service. QoS protocols used on the WLAN and
wired network media must be mapped to one another while traffic
transits the boundary between the two media. In particular,
WLAN 802.11e packets must be mapped to LAN 802.1p packets and
vice versa. Figure [1] provides a brief overview of how to
implement QoS on a WLAN. When a WLAN client sends 802.11e
traffic, it will have a User Priority (UP) classification in
its frame (802.1p or 802.11e). The access point needs to map
this 802.11e classification into a DSCP value for the LWAPP
packet carrying the frame to ensure that the packet is given
the appropriate priority on its way to the wireless LAN
controller. A similar process needs to occur on the wireless
LAN controller for LWAPP packets going to the access point.
Also needed is a mechanism to classify traffic on both the
access point and the wireless LAN controller for non-802.11e
clients, so that their LWAPP packets can also be given the
appropriate priority. Figure shows how traffic that originated
in the wired network passes through a LAN switch to a
controller, through LWAPP tunnels to an access point, and then
to a wireless client. The bottom of the diagram shows the
process from the wireless client to the wired LAN. There are
four steps in the process: Step 1 The traffic travels
from the Ethernet switch to the controller. Step 2 The
traffic travels from the access point to the wireless client.
Step 3 The traffic travels from the client to the
access point. Step 4 The traffic travels from the
controller to the Ethernet switch. This series of subtopics
gives details on the steps of QoS implementation: WLAN QoS
Implementation Step 1: From Ethernet Switch to Controller and
LWAPP Tunnel
When a LAN controller forwards an LWAPP
packet to an access point, it must contain the QoS information
from the original Ethernet packet coming from the Ethernet
switch. The WLAN controller puts this information into the
outer header in an LWAPP packet. Figure shows how the
controller encapsulates the incoming Ethernet packet retaining
the original DSCP value and payload. The controller also
translates the DSCP value of the incoming packet to the
appropriate 802.1p priority value. These values are placed in
the outer header of the LWAPP frame. LWAPP control packets are
always tagged with an 802.1p value of 7, while the encapsulated
LWAPP data packets derive the DSCP and 802.1p value from the
original packet. QoS Packet Marking Translations
Default mapping between DSCP, 802.1p, and 802.11e exist as
shown in the table in Figure . The following are notes on Layer
3 QoS packet marking enhancements: - Layer 3 QoS is not
supported when you are using Layer 2 LWAPP encapsulation. You
must use 802.1p tagging for QoS marking to ensure that packets
receive the proper level of QoS.
- The Layer 3 QoS
packet marking translations cannot be configured.
There are benefits of Layer 3 QoS packet marking enhancements:
- The enhancements ensure that packets receive the
proper QoS handling from end-to-end.
- Policing of
802.11e priority (or WMM), 802.1p, and IP DSCP values ensures
that wireless endpoints conform to network QoS policies.
The following are notes on network control (IP DSCP 56,
Cisco Unified Communications 802.1p 7, and IEEE 802.1e 7):
- The 802.1p priority level 7 requires special handling
because this level is reserved for LWAPP control.
-
Data packets with a priority of 7 are always degraded to
priority 6 or DSCP 46.
- An LWAPP control priority of 7
also translates to DSCP 46 because there are no other logical
options.
WLAN QoS Implementation Step 2: Out of
the Tunnel and Through the Access Point to the Wireless
Client
When a packet goes from an access point to the
client, the DSCP value from the incoming LWAPP packet is mapped
to the 802.11e priority value as shown in Figure . For a WMM
client, the access point translates the DSCP value of the
incoming LWAPP packet to the 802.11e priority value. The access
point polices the value to ensure that the value does not
exceed the maximum value allowed, based on the WLAN QoS policy
assigned to that client. Then the access point places the
packet in the 802.11e transmit queue that is appropriate for
that WMM access category or 802.11e priority level. For a
regular (non-WMM) client, the access point places the packet in
the default 802.11e or WMM transmit queue based on the WLAN QoS
policy that is assigned to that client. WLAN QoS
Implementation Step 3: From Client to Access Point and Into the
Tunnel
When a packet is sent from the client to the
access point, the 802.11e priority value is mapped to the DSCP
value on the access point. Figure shows this process. When the
access point receives an 802.11 frame from a WMM client, the
access point polices the 802.11e priority value to ensure that
the value does not exceed the maximum value that is allowed for
the QoS policy assigned to that client and then maps the
802.11e priority value to the DSCP value.
When the
access point receives a frame from a regular (non-WMM) client,
the access point uses the default 802.11e priority or WMM value
for the QoS policy that is assigned to that client or WLAN ID
and translates the value to the DSCP value. Mapping Traffic
from Access Point to Controller
When packets from an
access point go to the controller, the access point translates
a DSCP value based on the incoming 802.11e value as shown in
Figure . The access point does not send tagged packets because
doing so causes a problem with Cisco switches that are not
configured for trunking from the access point. Because the
access point is not trunked, the access point does not copy the
802.11e incoming value to an 802.1p (outer) packet. WLAN QoS
Implementation Step 4: Out of the Tunnel Through the Controller
to Ethernet Switch
The controller uses the incoming
outer DSCP value from an LWAPP packet to generate an 802.1p
priority value in the packets, which are sent to the Ethernet
switch by the controllers as shown in Figure . When the
controller receives an LWAPP packet, the controller generates
the IEEE 802.1p priority value for the wired side using the
incoming DSCP (outer) value.
Content 6.1
Implementing WLAN QoS 6.1.7 Packet Tagging
There are two situations to consider: tagged and untagged
packets. - Tagged packets: The 802.1p- or
DSCP-tagged packets are received from the LAN:
- The tag
is propagated to the LWAPP frame.
- The WLAN
ID-configured QoS takes priority for the assigned access
category; if the tag is lower than the configured QoS, the
access point queues the packet at a lower access
category.
- An authentication, authorization, and