The WiSE Article Series: Wi-Fi Subtleties Explained (Quality of Service [QoS] Controls)By CWNP On 04/16/2013 - 22 Comments
In this fourth installment of the WiSE article series from AirTight Networks, you will learn about some controls that matter in QoS implementations along the Wi-Fi data path.
Guest Blogger: AirTight Networks Author: Hemant Chaskar Series Editor: Tom Carpenter
About the WiSE article series: Wireless is inherently complex; its study spans at least two engineering disciplines: Electrical Engineering and Computer Science. Add to this the nuances of various standards, vendor implementations, RF environments, and protocol interactions, and it is not uncommon to feel a little lost in understanding the various aspects of Wi-Fi network operation. In this series of short articles, we explain various Wi-Fi subtleties, to work toward a better understanding of Wi-Fi network deployments.
WiSE Article No.4 Wi-Fi Network Performance – Controls that Matter for QoS in the Wi-Fi Data Path
One aspect of Wi-Fi network performance is wireless throughput, which was the topic in earlier WiSE articles. Quality of Service (QoS) is another aspect of the network performance that is relevant for applications such as VoIP over Wi-Fi. In this context, QoS is provided by prioritizing the packets belonging to specific applications such as VoIP over others so that they encounter minimal latency in transit. It takes three different sections of the data path to use three different techniques for the end-to-end handling of wireless QoS-sensitive packets, as discussed below. The idea of this article is not to provide overview of standard Wi-Fi QoS mechanisms such as WMM, but to point out some subtleties in using them in the network.
[caption id="" align="aligncenter" width="554"] Typical Wi-Fi Data Path[/caption]
1. Client to AP Upstream Wireless QoS (Notable: client support is essential): On this section of the data path, the key requirement is to prioritize in the wireless medium the transmission of packets from the client that carry QoS-sensitive payload. In order to get access to the channel, these QoS-sensitive packets have to compete with: a) packets waiting to be transmitted at other clients connected to the same AP, b) packets waiting to be transmitted at the same AP, and c) packets waiting to be transmitted at APs and clients in the BSS’s in the radio neighborhood of the client operating on the same (and adjacent in case of 2.4 GHz) channel.
To quickly win access to the channel among all these competing packets, the QoS-sensitive packets at the wireless client will have to use the aggressive channel access parameters (backoff window and inter-frame space) associated with the high priority access category of the 802.11WMM. The subtlety here is that independent of any other QoS mechanisms that the WLAN infrastructure may offer (such as admission control), support in the client is essential to make this happen.
This support for prioritizing VoIP traffic on the client to AP wireless upstream is typically natively available in clients that are custom made for QoS application such as VoIP phone. However, if the client runs a mix of QoS and non-QoS applications (e.g., laptop running softphone and data application), QoS may have to be explicitly configured in the OS and/or application to ensure that correct 802.11 WMM access categories are indeed used by wireless NICs for the QoS packets.
Shown below are packet headers showing DSCP and WMM classifications for packets transmitted by VoIP phone, PC running softphone without any QoS configuration, and PC running softphone with Windows policy-based QoS configured to mark softphone application’s packets with DSCP code of 46 (which is the EF class of DiffServ corresponding to VoIP). As can be seen, the VoIP phone by default uses DSCP of 46 which gets mapped to the WMM access category of 6 by its Wi-Fi NIC; WMM classes 6 and 7 are for voice and hence phone will get QoS on the client to AP wireless link. The softphone without any specific QoS configuration in the PC uses both the DSCP and the WMM of 0, which is for background traffic and hence softphone will NOT get any QoS on the client to AP wireless link. Finally, in the QoS configured PC the softphone packets are marked with the DSCP of 46 and are correctly translated by the Wi-Fi NIC to WMM of 6.
VoIP phone (QoS parameters natively chosen):
[caption id="" align="aligncenter" width="540"] VoIP Phone QoS Natively Chosen[/caption]
[caption id="" align="aligncenter" width="778"] VoIP Phone QoS Natively Chosen[/caption]
Softphone on laptop (no QoS parameters by default):
[caption id="" align="aligncenter" width="345"] Softphone no QoS Parameters[/caption]
[caption id="" align="aligncenter" width="694"] Softphone no QoS Parameters[/caption]
Softphone on laptop with policy based QoS (QoS paramerters properly chosen):
[caption id="" align="aligncenter" width="393"] Softphone with QoS Parameters[/caption]
[caption id="" align="aligncenter" width="769"] Softphone with QoS Parameters[/caption]
2. AP to AP Midstream Wired QoS (Notable: packet marking/re-marking may be required): On this section of the data path, QoS handling is facilitated by wired network elements – switches and routers – by different types of queuing, prioritization, and VLAN segregation techniques. There are typically no queuing delays inside the AP as packets are transferred from wireless to wired link, since packets coming in on a slower wireless link are immediately and serially transferred to the higher speed wired link. As such, the AP does not have major role to play in offering QoS on this section of the path.
However, a non-obvious point is that the wired network may depend on the packet markings (DSCP and 802.1p) done at the ingress to provide QoS differentiation among packets after the packets have left the AP. If the client application did not do the packet classification and marking in the client to AP upstream wireless section (and hence no QoS was afforded on the wireless upstream), the AP or the ingress switch port can perform the packet marking for the wired section of the path. Different techniques can be used to do the marking such as mark packets coming on a dedicated SSID as VoIP packets, snoop session control messaging to identify VoIP packets, identify RTP packets to mark VoIP packets, use pre-configured protocol templates to match with packet fields, etc.
Another subtle point is that even if the client did the marking (and hence QoS was afforded on the wireless upstream), the AP or the ingress switch port may still be required to re-mark the packets to address factors such as i) client marking scheme is different from the marking scheme followed in the wired network (e.g., client uses DSCP of 46 for WMM voice access category, but rest of the network is configured to use DSCP of 40 for VoIP, or client uses DSCP of 48 for WMM voice access category, but rest of the network is configured to use DSCP of 46 for VoIP), ii) the AP was configured for connection admission control, but the softphone did not ask permission before using high priority access category of WMM and hence the AP marked the softphone’s packets down to best effort before forwarding to the wired network, etc.
3. AP to Client Wireless Downstream QoS (Notable: entirely the AP’s responsibility): On this section of the path, the AP has a major role to play in providing QoS. The subtlety here is that there will be periods of time when the AP will receive incoming traffic from the wired side at a higher rate than it can transmit over the wireless link and hence has to queue up the packets for transmission. Since queues can build up, prioritized queuing is necessary so that the QoS sensitive packets get scheduled for wireless transmission from these queues ahead of the other traffic. Once chosen from transmission they will use high priority access category of the 802.11 WMM to swiftly get access to the channel.
In order to implement prioritized queuing and prioritized channel access, the egress AP can identify QoS-sensitive packets by varied mechanisms such as: i) the markings available in the packets received at the wired interface, ii) VLAN segregation at the wired interface, iii) SSID where packets are destined if that SSID is dedicated to VoIP, iv) other identification and classification techniques similar to those used at ingress AP.
In summary, we always hear terms such as packet marking, priority queuing, WMM, connection admission control etc. in the context of VoIP over Wi-Fi. These techniques have different roles to play in different sections of the end to end data path. The network designer has to configure these different segments of the path correctly to achieve end to end QoS for latency sensitive applications; QoS may be affected even if one of the three parts does not provide the required support.
Authors Bio: Hemant Chaskar is VP for Technology and Innovation at AirTight Networks. In this role, he looks after AirTight’s technology R&D and also performs roles in product design, business development, and various customer facing activities. At AirTight, Hemant has been working on Wi-Fi networking and security for the past 8 years; and has held positions at Nokia Research and Lucent Technologies prior to that. He holds Ph.D. in Electrical Engineering from the University of Illinois at Urbana-Champaign.