Aruba Advanced QoS Part 1: Optimizing Application Delivery
Back to the future with this Airheads Online article from 2007
The term Quality of Service (QoS) refers to the ability to sustain a given service level at or above the minimum performance by the applications running on the network. Certain network services need to be delivered within specific performance levels for reliable operation. For example, voice and video streams will break up if the bandwidth is inadequate or the packets take too long to traverse the network from source to destination.
QoS is used to prioritize and assure defined performance levels to real-time traffic streams by ensuring adequate bandwidth, low packet loss, and controlled jitter and latency, while also ensuring that low
priority traffic flows are also successfully sent and received.
To ensure end-to-end QoS, each device in the network must be capable of two fundamental functions:
- Packet tagging and marking to reflect the relative priority of the packet and ensure QoS between network components (e.g., switch, router, etc.).
- Queuing, prioritization and traffic shaping QoS within each network component.
Why is QoS Important?
Different applications have differing QoS requirements. Voice and video are delay-sensitive but can tolerate low levels of packet loss, whereas data applications are delay-tolerant but cannot handle packet loss. (Note: For information on the effects of jitter, delay and packet loss on voice traffic, refer to Appendix B.)
As a packet traverses the network from source to destination, congestion in the network can result in the packet getting dropped, delayed and/or corrupted, resulting in retries and lost packets. The packets may also arrive at the destination with varying delays (jitter) or out of sequence. All of these problems adversely affect
the quality of real-time traffic like voice and video.
The 802.11 radio frequency (RF) bands, or Wi-Fi, face additional resource constraints compared to traditional wired switched networks. 802.11 networks operate in a half-duplex mode on a shared media similar to older wired hubs. Delay and interference in the air can increase retries, further increasing the chances of congestion, jitter and dropped packet while also reducing the net throughput of the transmission cell.
Configuring QoS settings correctly on network components ensures that the various streams are given the correct, corresponding service level, resulting in a better end-user experience. Proper QoS mechanisms in the air ensure that real-time traffic gets access to the media without delay and interference. QoS mechanisms like WMM and 802.11e ensure proper QoS from the devices to the infrastructure and vice versa.
What Applications Require QoS?
Traffic from applications and services that are delay-sensitive require QoS to ensure smooth operation.
Examples of such applications are:
- IP Telephony or Voice over WLAN (VoWLAN) which requires strict limits on jitter and delay.
- Streaming multimedia which requires guaranteed throughput.
- Video Teleconferencing (VTC) which requires low jitter.
Mission-critical applications like alarms and medical monitors which don’t require a lot of bandwidth, but are still highly latency-sensitive.
These types of services and applications are described as inelastic meaning that that they require a certain level of bandwidth to function and are latency-sensitive; low bandwidth and / or high delay will render these applications non-functional.
The actual end-to-end QoS capabilities of the network are referred to as service levels. The service levels differ in their degree of QoS strictness which directly translates into queue prioritization, bandwidth allocation, latency, jitter and packet loss characteristics.
From the Aruba Networks system’s perspective, the network can be divided into three major segments: wireless segment, LAN edge, LAN core. Different QoS mechanisms are used to ensure that right service levels are maintained on each network segment.
The wireless segment falls outside the realm of the traditional mechanisms used to enforce QoS on the wire. The WLAN system must ensure that real-time traffic receives priority queuing and processing. In addition, the system needs to ensure adequate RF bandwidth for high priority traffic, and intelligently implement the system’s ongoing RF management operations such that the QoS requirements of the traffic are not compromised.
On an Aruba network, traffic on the wireless segment is queued for transmission according to priority.
Downstream (from AP to client) wireless QoS is controlled by the Aruba system whereas upstream traffic prioritization is controlled by the client. In a wireless multimedia (WMM)-supported environment, QoS is enforced by the infrastructure and client as per the 802.11e standards.802.11e / WMM is the only WiFi QoS standard available today that defines the upstream QoS mechanisms.
However, even in a non-WMM environment, the Aruba system can enforce QoS by ensuring priority queuing on a per-session basis. Other wireless QoS enforcement features on the Aruba system include:
- Per SSID-based bandwidth allocation
- QoS-aware RF scanning
- Fast roaming to minimize packet loss during roaming and provide better the end-to-end QoS
- Call Admission Control – Voice aware bandwidth and call management
The LAN edge is the segment between the mobility controller and the AP. Traffic on the LAN edge is GRE encapsulated for tunneling purposes. Therefore, to ensure that devices in this LAN segment prioritize the traffic properly, the tags carried by the actual data packets have to be reflected in the GRE tunnel headers.
Traffic from the controller to the AP is tagged by the controller. Exclusively on Aruba WLAN systems, the AP is intelligent enough to remember tags on downstream traffic and match the tag on upstream traffic on a per client basis.
The LAN core is the area of the network upstream from the mobility controller. The controller serves as the entry and exit point for all the traffic to and from wireless users. Traffic entering the wireless LAN system may carry native tags (802.1p or DSCP) set by the application or by other network elements that reside in the core. The Aruba system can be configured to respect the existing tags or apply new tags based on various policies and prioritize the traffic accordingly.
The Aruba System uses various QoS mechanisms to preserve the service levels of the traffic in each of the network segments. The Aruba mobility controller interfaces with both the wired and wireless medium and implements QoS mechanisms for both. However, most traffic through the system is either originated from or destined to a wireless client. This document highlights many of the Wi-Fi QoS features available on the Aruba system.
It is important to remember that QoS is a network characteristic and for the network to be QoS-capable, it is essential that every device in the network honors the tagging and traffic prioritization settings.