Chapter 10 Policy and Static Routes
NXC Series User’s Guide
185
Assured Forwarding (AF) PHB for DiffServ
Assured Forwarding (AF) behavior is defined in RFC 2597. The AF behavior group defines four AF classes.
Inside each class, packets are given a high, medium or low drop precedence. The drop precedence
determines the probability that routers in the network will drop packets when congestion occurs. If
congestion occurs between classes, the traffic in the higher class (smaller numbered class) is generally
given priority. Combining the classes and drop precedence produces the following twelve DSCP
encodings from AF11 through AF43. The decimal equivalent is listed in brackets.
WMM
WiFi Multimedia (WMM) provides basic Quality of Service (QoS) features to wireless networks. The four
categories of QoS described by WMM are: voice (VO), video (VI), best effort (BE), and background (BK).
These categories, known as a “access categories” (AC), are mapped to 802.1D priority values which
can then be mapped to their corresponding DSCP hex values.
The WMM ACs as implemented on the NXC have the following functions:
VOICE: All wireless traffic to the SSID is tagged as voice data. This is recommended if an SSID is used for
activities like placing and receiving VoIP phone calls.
VIDEO: All wireless traffic to the SSID is tagged as video data. This is recommended for activities like video
conferencing.
BEST EFFORT: All wireless traffic to the SSID is tagged as “best effort,” meaning the data travels the best
route it can without displacing higher priority traffic. This is good for activities that do not require the best
bandwidth throughput, such as surfing the Internet.
BACKGROUND: All wireless traffic to the SSID is tagged as low priority or “background traffic”, meaning
all other access categories take precedence over this one. If traffic from an SSID does not have strict
throughput requirements, then this access category is recommended. For example, an SSID that only
has network printers connected to it.
Table 79 Assured Forwarding (AF) Behavior Group
Class 1 Class 2 Class 3 Class 4
Low Drop Precedence AF11 (10) AF21 (18) AF31 (26) AF41 (34)
Medium Drop Precedence AF12 (12) AF22 (20) AF32 (28) AF42 (36)
High Drop Precedence AF13 (14) AF23 (22) AF33 (30) AF43 (38)
Table 80 WMM to DiffServ Conversion on the NXC
Priority WMM AC 802.1D Priority DSCP Hex Value
Lowest
Highest
BK 1 0x08
BK 2 0x10
BE 0 0x00
BE 3 0x18
VI 4 0x20
VI 5 0x28
VO 6 0x30
VO 7 0x38
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