Cisco aironet 1522 Design And Deployment Manual page 108

Connecting the Cisco 1520 Series Mesh Access Point to Your Network
The collision avoidance mechanism uses two values, called CWmin and CWmax. CW stands for
contention window. The CW determines what additional amount of time an endpoint should wait, after
the interframe space (IFS), to attend to transmit a packet. Enhanced distributed coordination function
(EDCF) is a model that allows end devices that have delay-sensitive multi-media traffic to modify their
CWmin and CWmax values to allow for statically greater (and more frequent) access to the medium.
Cisco access points support EDCF-like QoS. This provides up to eight queues for QoS.
These queues can be allocated in several different ways:
Based on TOS / DiffServ settings of packets
•
Based on Layer 2 or Layer 3 access lists
•
Based on VLAN
•
Based on dynamic registration of devices (IP phones)
•
AP1520s, in conjunction with Cisco controllers, provides a minimal integrated services capability at the
controller, in which client streams have maximum bandwidth caps, and a more robust differentiated
services (diffServ) capability based on the IP DSCP values and QoS WLAN overrides.
When the queue capacity has been reached, additional frames are dropped (tail drop).
Encapsulations
There are several encapsulations used by the mesh system. These include CAPWAP control and data
between the controller and RAP, over the mesh backhaul, and between the mesh access point and its
client(s). The encapsulation of bridging traffic (non-controller traffic from a LAN) over the backhaul is
the same as the encapsulation of CAPWAP data.
There are two encapsulations between the controller and the RAP. The first is for CAPWAP control, and
the second for CAPWAP data. In the control instance, CAPWAP is used as a container for control
information and directives. In the instance of CAPWAP data, the entire packet, including the Ethernet
and IP headers, is sent in the CAPWAP container. (See
Figure 69
Controller
802.1/802.1P
802.1/802.1P
For the backhaul, there is only one type of encapsulation, encapsulating MESH traffic. However, two
types of traffic are encapsulated: bridging traffic and CAPWAP control and data traffic. Both types of
traffic are encapsulated in a proprietary mesh header.
In the case of bridging traffic, the entire packet Ethernet frame is encapsulated in the mesh header (See
Figure
Cisco Aironet 1520, 1130, 1240 Series Wireless Mesh Access Points, Design and Deployment Guide, Release 6.0
108
Encapsulations
CAPWAP
CAPWAP
IP/IP DSCP
Encapsulation
CAPWAP
CAPWAP
IP/IP DSCP
Encapsulation
70).
Figure
RAP
Control
CAPWAP Control Information
Data
Client Packet
69.)
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