Allied Telesis Network Router AT WR4600 User Manual

How To | Configure the WLAN with QoS  
Introduction  
This How to Note explains how to configure Quality of Service (QoS) in a wireless  
Local Area Network to prioritize traffic under busy conditions.  
There is an example scenario provided at the end of this document, which  
describes the configuration for prioritizing voice traffic. This example could be  
extended to cover a wider installation case.  
For a complete description of the commands used in this document, please refer  
to the following guide:  
“AT-WR4600 Series – Command Reference Guide”.  
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Table of contents  
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Overview  
What is Wi-Fi Multimedia (WMM)  
The Wi-Fi Alliance defined WMM as a subset of the IEEE 802.11e standard to  
support QoS on wireless networks. WMM offers a mechanism to prioritize traffic  
demands from different applications and grants an adequate support to  
multimedia applications, thereby reducing the harmful effects caused by high  
latency and throughput reductions.  
Why use Wi-Fi Multimedia?  
Multimedia and data-centric applications typically share the same network  
resources, despite their conflicting requirements for data delivery. Without QoS,  
the network offers a best-effort service, which means that applications have equal  
opportunities to transfer data. WMM enables network managers to prioritize traffic  
based on bandwidth requirements, especially delay-sensitive applications such as  
Voice over Wireless LAN and streaming multimedia.  
QoS policies play a key role when traffic load exceeds the capability of network  
resources, or when some applications, like file transfers, consume the available  
bandwidth. Based on services classification, WMM individually manages the  
traffic classes to provide low latency, reliability, and a more predictable delay to  
critical services.  
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How Wi-Fi Multimedia works  
In the IEEE 802.11 standard, the wireless nodes access the medium using the  
“listen-before-talk” algorithm CSMA/CA-based 1 Distributed Coordination Function  
(DCF). The wireless node transmits a frame once its checks indicate that the  
medium is free for a period of distributed inter-frame space (DIFS) and a random  
back-off time has elapsed. Otherwise, it defers the frame delivery. Note that  
contention reduces the available bandwidth, because nodes must back off before  
retrying to communicate.  
DCF offers an efficient mechanism for transferring bursty traffic, but all nodes use  
the same access method, consequently a real time data source will face  
continuous contention with a large data transfer happening at the same time.  
WMM provides two features that resolve contention and enable QoS:  
.
.
Enhanced Distributed Channel Access (EDCA)  
Hybrid Controlled Channel Access (HCCA)2  
EDCA gives precedence to higher-priority traffic by means of a shorter arbitration  
interframe space (AIFS) and contention windows (CW). The back off time is the  
sum of predefined AIFS and a random value from 0 to CW, which is doubled at  
each collision till the limit (CWmax) is reached. Figure 1depicts the timing. In  
addition, transmit opportunity (TXOP) bounds the time allocated to the node for  
data delivery.  
HCCAis a centralized access control facility coordinated by the access point,  
which acts as point coordinator (PC). The PC collects the status of registered  
stations (via polling) and provides them the permission for channel occupancy  
and assigns, for example, the TXOP, the transmit start time and duration.  
1 Carrier Sense Multiple Access with Collision Avoidance.  
2 HCCAfacility is not delivered by current software release.  
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minimum wait (AIFS)  
random backoff (CW)  
AIFS = DIFS + 7 slots  
CW = 0~15 slots  
background  
best-effort  
video  
AIFS = DIFS + 3 slots  
AIFS = DIFS + 2 slots  
AIFS = DIFS + 2 slots  
CW = 0~15 slots  
CW = 0~7 slots  
CW = 0~3 slots  
voice  
slot time  
EIFS (optional)  
DIFS  
PIFS  
SIFS  
contention window  
backoff slots  
busy medium  
Interframe spaces:  
SIFS:  
PIFS:  
DIFS:  
AISF:  
EIFS:  
short interframe space  
PCF interframe space  
DCF interframe space  
arbitration interframe space  
extended interframe space  
- used by HCCA facility  
- used by QoS facility  
- used to extend the DIFS in case no ACK is received in time  
Note, the node delivering data waits for DIFS before sending next frame and, within that interval, the receiver replies  
the ACKnowledgement frame to confirm the reception succeeded.  
It may happen the receiver delays the replay, due to the signal propagation on long distance link or additional time is  
required by receiver for decoding and error recovery; in that case, EIFS is included giving the opportunity to return the  
ACK.  
Figure 1 WMM access timing  
WMM requires that the clients running multimedia applications assign the  
appropriate priority level to the generated data traffic and that network  
infrastructure consistently applies QoS policies.  
WMM derives four Access Categories from IEEE 802.1D Class of Service (CoS)  
as depicted by Table 2.  
Within the wireless node, the WMM manages four independent transmit queues,  
one per access category, and dispatches to them incoming packets depending on  
the assigned Layer 3 differentiated service field (TOS/DSCP). Packet forwarding  
will happen basing on transmit opportunities.  
Table 2 - WMM Access Categories and IEEE 802.1d tag mapping  
Access  
Category  
IEEE 802.1D  
Traffic Types  
Priority  
Description  
Voice  
Highest  
Low-latency and toll voice quality  
SDTV and HDTV streaming  
Traffic less sensitive of latency, but  
affected by delays  
6, 7  
4, 5  
0, 3  
Video  
Highest -1  
Lower + 1  
Best-effort  
Background  
Lower  
Low priority traffic without throughput  
constraint (i.e. file transfer)  
1, 2  
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How to configure Wi-Fi Multimedia  
Use the command: traffic-class to set the EDCA back off criteria for the required  
WMM access category:  
traffic-class [background|best-effort|video|voice]  
cw-min <0-10> cw-max <0-10> fixed-slot <2-20>  
no traffic-class [background|best-effort|video|voice]  
On collisions, WMM sets the back off time within the CWmin range and uses a  
persistence factor (PF) as the coefficient that multiplies the contention window  
size in successive retransmission attempts following a collision.  
WMM setup may only be entered on a wireless node with an access point role.  
Note:  
It will distribute the QoS policies to “slave” nodes, for example, non-root bridge,  
workgroup-bridge, etc.  
Parameter descriptions  
cw-min (Contention Window minimum)  
That is the random wait time before retrying the transmission; the initial size of the  
contention window is within 0~CWmin back off slots.  
The value specifies the number of back off slots as a power of 2n -1; in example,  
cw-min 3  
cw-min 0  
means 23-1=7 back off slots.  
means 20-1=0 back off slots.  
cw-max (Contention Window maximum)  
That is the maximum size of the contention window.  
The value specifies the number of back off slots as a power of 2n -1.  
fixed-slot (Arbitrary Inter-Frame Space)  
That is the predefined idle time that the station waits before starting the contention  
window (CW) .  
The value specifies the number of back off slots.  
Refer to “AT-WR4600 – Command Reference Guide”, for default settings and  
recommended configuration for the different WLAN topologies (access point,  
Note:  
bridging, etc.).  
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Connecting AT-WR4600 products  
There are two options for accessing the Command Line Interface of the devices:  
.
.
SSH remote shell, using any text-mode SSH client software.  
RS-232 Line Driver/Receiver that provides a standard DTE console interface;  
note, this is optional and is not always available.  
Accessing the device via SSH  
The Secure Shell (SSH) is an encrypted networking protocol, allowing the user to  
configure the device through an Ethernet network. Microsoft does not supply SSH  
tool. You can use an alternative program such as Tera Term Pro, which is  
licensed under GPL2. The device is factory configured with the static IP address  
192.168.1.1. If the device is receiving an address from a DHCP server, please  
check the assigned IP address.  
Accessing the device via RS-232  
To access the device, you can use either the build-in Microsoft HyperTerminal  
(note it was removed from Windows Vista and later versions) or an alternative  
program such as Tera Term Pro.  
The settings of the serial terminal are as follows.  
Transmission rate  
Data length  
Stop bit  
115200bps  
8 bit  
1bit  
Parity  
None  
Flow control  
None  
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Accessing the CLI  
Enter the login and password via the terminal console (SSH or RS-232). Note, by  
default, the login name is manager and the password is friend.  
AT-WR4600 login: manager  
Password:  
Feb 9 10:35:07 AT-WR4600 auth.info login[6898]: root login on 'ttyS0'  
AA TTTTTTTTTTTTTTTTTT ooooo  
AAAAA TTTTTTTTTTTTTTT oooooooo  
AAAAAAAA  
TTTTTTTT  
I oooooo  
IIIIIIIIII  
IIIIIIIIII  
AAAAAAAAAAA TTTTTTT  
AAAAAAA AAAAA TTTT  
AAAAAAA  
AAAAA T  
IIIIIIIIII  
Copyright (C) 2012 Allied Telesis International  
All rights reserved.  
AT-WR4600> enable  
Password:  
AT-WR4600#  
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Configuration example  
This configuration example starts with the Command Line Interface already set to  
Global configuration mode:  
AT-WR4600#configure terminal  
AT-WR4600(config)#  
On setup completion, run the command show running-config to verify the  
applied configuration.  
All of the examples in this document provide the minimum set of commands  
Note:  
useful for configuring the WMM, regardless of other services like bridging, and  
security etc.  
Using QoS to prioritize voice traffic  
This example applies the QoS policy to support voice traffic.  
Wireless nodes belong to a Wireless Distribution System working in 5GHz bands,  
while the access to a wireless LAN is offered in 2GHz bands.  
STA AP  
Bridge1  
AP  
AP1  
Bridge0  
STA  
Bridge2  
Figure 2 QoS giving preference to voice traffic  
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Common setup  
To restore the factory configuration on any device, enter the following commands:  
AT-WR4600#copy nvram:/factory-config-bridged startup-config  
AT-WR4600#reload  
Configure the common SSID for open authentication:  
AT-WR4600(config)#dot11 ssid ATIopen  
AT-WR4600(config-ssid)#authentication open  
AT-WR4600(config-ssid)#guest-mode  
AT-WR4600(config-ssid)#exit  
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[Bridge0] station role: root bridge  
Assign the static IP address:  
AT-WR4600(config)#interface vlan1.1  
AT-WR4600(config-if)#no ip address dhcp  
AT-WR4600(config-if)#ip address 10.0.0.100/24  
AT-WR4600(config-if)#exit  
Configure the station role, the SSID and WMM to the radio interface 0 for bridging:  
AT-WR4600(config)#interface dot11radio 0  
AT-WR4600(config-if)#protocol-family an  
AT-WR4600(config-if)#station-role root bridge  
AT-WR4600(config-if)#ssid ATIopen  
AT-WR4600(config-if)#bridge-group 1  
AT-WR4600(config-if)#traffic-class voice cw-min 2 cw-max 4  
fixed-slot 2  
AT-WR4600(config-if)#traffic-class best-effort cw-min 4 cw-max 10  
fixed-slot 3  
AT-WR4600(config-if)#no shutdown  
AT-WR4600(config-if)#end  
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[Bridge1] station role: non-root bridge  
[AP1] station role: root access-point  
Assign the static IP address:  
AT-WR4600(config)#interface vlan1.1  
AT-WR4600(config-if)#no ip address dhcp  
AT-WR4600(config-if)#ip address 10.0.0.1/24  
AT-WR4600(config-if)#exit  
Configure the station role and the SSID to the radio interface 0 for bridging:  
AT-WR4600(config)#interface dot11radio 0  
AT-WR4600(config-if)#protocol-family an  
AT-WR4600(config-if)#station-role non-root bridge  
AT-WR4600(config-if)#ssid ATIopen  
AT-WR4600(config-if)#bridge-group 1  
AT-WR4600(config-if)#no shutdown  
AT-WR4600(config-if)#exit  
Configure the station role, the SSID and WMM to the radio interface 1 for access:  
AT-WR4600(config)#interface dot11radio 1  
AT-WR4600(config-if)#protocol-family bg  
AT-WR4600(config-if)#station-role root access-point  
AT-WR4600(config-if)#ssid ATIopen  
AT-WR4600(config-if)#bridge-group 1  
AT-WR4600(config-if)#traffic-class voice cw-min 2 cw-max 4  
fixed-slot 2  
AT-WR4600(config-if)#traffic-class best-effort cw-min 4 cw-max 10  
fixed-slot 3  
AT-WR4600(config-if)#no shutdown  
AT-WR4600(config-if)#end  
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[Bridge2] station role: non-root bridge  
Assign the static IP address:  
AT-WR4600(config)#interface vlan1.1  
AT-WR4600(config-if)#no ip address dhcp  
AT-WR4600(config-if)#ip address 10.0.0.2/24  
AT-WR4600(config-if)#exit  
Configure the station role and the SSID to the radio interface for bridging:  
AT-WR4600(config)#interface dot11radio 0  
AT-WR4600(config-if)#protocol-family an  
AT-WR4600(config-if)#station-role non-root bridge  
AT-WR4600(config-if)#ssid ATIopen  
AT-WR4600(config-if)#bridge-group 1  
AT-WR4600(config-if)#no shutdown  
AT-WR4600(config-if)#end  
©
2012 Allied Telesis, Inc. All rights reserved. Information in this document is subject to change without notice. Allied Telesis is a trademark or registered trademark of Allied Telesis, Inc. in the United States and other countries.  
All company names, logos, and product designs that are trademarks or registered trademarks are the property of their respective owners.  
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