IEEE 802.11e: Quality of Service (QOS)

This article mainly focuses on Quality of Service (QOS) for Video conferencing that helps for end users, customers and network admins resulting in better video quality and its performances. This article also covers the recommended standard set of DSCP markings and their usage in the IPv4/v6 headers.

QOS impacts:

IEEE 802.11e standard introduced Quality of Service and its enhancements in 2005 Wi-Fi amendment.

Quality of Service (QOS) is a key topic that has wide roles in Wi-Fi network technologies. It is mainly used to prioritise the network traffic to minimize few factors like time-delay, Jitter and packet loss.

Nowadays, Modern networks are typically converged networks in which Voice traffic, Video traffic and data traffic, etc. are sharing the IP network which has greater advantages in terms of cost savings for Voice and Video traffic.  For example: Google Meet, Microsoft Teams, and etc.

Since different kind of traffic is sharing the same network, QOS provides prioritisation to the network traffic depending on the following characteristics namely Bandwidth, Latency, Jitter and Packet Loss.

Quick elaboration on these key factors:

Bandwidth:

Overall capacity of the network link, measured in bits per second.

Latency:

Amount of time it takes the traffic from source to destination, measured in milli seconds.

Jitter:

Difference in how long it takes for each paket from source to destination, measured in milli seconds.

Packet Loss:

Packets that not be able to reach their destination networks, measured in %.

Standard Recommendations for Audio and Video Traffic network requirements are listed below:

Latency: 150 ms or less

Jitter: 30 ms or less

Packet loss: 0.5 – 1 %

Now, we know that there are different kind of network traffic that is present in the network. Many of you may wonder, how to classify and mark the network traffic and give prioritise to only those network traffic at a highest consideration.

Classification of network traffic can be done in many categories such as Access Control List (ACL), Network Based Application Recognition (N-BAR), Priority Code Point (PCP), Differentiated Service Code Point (DSCP).

In this article, I will speak about how classication of network traffic type used in DSCP and its marking values for each packet as because DSCP is most preferred QOS tool.

DSCP markings and its significance:

RFC 2474 defines DSCP field in 1998. This DSCP is present in Layer 3 IP version header, and it is one of the QOS tools to determine the high/low priority traffic.

From the above figure, you can infer that IPV4 header has a Type of Service (TOS) byte which has 8 bits in total.

Old TOS byte consists of IP Precedence (3 bits) and unused (5 bits). Later, it was decided to give 3 more bits in the TOS byte (8 bits) for DSCP (Total 6 bits) and Explicit Congestion Notification (2 bits) in DiffServ field.

Now, DSCP uses 6 bits which has 64 values (0-63) and ECN (unused) uses 2 bits in the total TOS 8 bits.

Normally, every QOS tool is designed to identify the network traffic type and each QOS tool has its own standard markings in their fields.

Let us look into the standard markings of DSCP in our case study and it has few types that will be used depending on the end user’s network traffic types (Voice Traffic, Video Traffic, Data Traffic) namely,

·         Default Forwarding (DF)

·         Expedit Forwarding (EF)

·         Assured Forwarding (AF)

·         Class Selector (CS)

Elaborating one by one below and their corresponding DSCP marking values:

Default Forwarding (DF):

It refers to best effort traffic (default) which does not require any QOS priorities.

DSCP markings for DF is 0.

Expedited Forwarding (EF):

It is used when traffic requires low latency, low jitter and low & of loss than the standardized one which I mentioned above. It is highly suitable for Voice traffic over the network communication.

DSCP markings for EF is 46, in binary format – 101110.

Assured Forwarding (AF):

It defines 4 different traffic class namely AF1x, AF2x, AF3x and AF4x where x = Drop Precedence.

·         If DSCP markings has highest drop precedence value, then there is a chance that packet is dropped during congestion and it is considered as low-priority network traffic.

·         If DSCP markings has low drop precedence value, then it is considered as high-priority network traffic.

How this DSCP AF values are calculated?

Below figure shows the TOS byte (8 bit) and first 3 bits are referred as class, and next 2 bits are for drop precedences and 5th bit is always 0 and last 2 bits are unused bits.

Above Figure 2, depicts that first 5 bits can be either 0 or 1. Now we’ll see how to calculate DSCP AF values from the above figure. DSCP AF values can be calculated based on the decimal number of classes and decimal number of drop precedence values.

DSCP AF values are represented as AF<XY> where X = decimal number of classes and Y = decimal number of drop precedence values.

Illustration 1:

It is the first example to calculate DSCP AF values, if suppose 001010 are the first 6 bits which is participating in calculation of AF values.

According to Illustration 1, X = decimal no. of class which is 2^0 = 1 and Y = decimal no. of drop precedence which is 2^0 = 1. Hence, DSCP AF values are AF 11 for this case and corresponding binary value for 001010 are DSCP 10.

Illustration 2:

According to Illustration 2, X = decimal no. of class which is 2^0 = 1 and Y = decimal no. of drop precedence which is 2^1 = 2. Hence, DSCP AF values are AF 12 for this case and corresponding binary value for 001100 are DSCP 12.

From the above 2 illustrations, we can infer that Highest drop precedence value will have lowest priority network traffic, thus AF 11 has highest priority than AF 12.

Similarly, we can calculate any AF values in these AF1x, AF2x, AF3x and AF4x different classes. For example, AF 41 will be given highest QOS priority than AF 42, AF 43 and AF 44.

Class Selector (CS):

It has 8 different DSCP CS values for backward compatibility with IP Precedence. Its values are listed from CS 0, CS 1, CS 2, CS 3, CS 4, CS 5, CS 6 and CS 7.

Now, we got all these values (DF, EF, AF, CS). But we need to know how these values are contributing for different network traffic types.

RFC 4594 Standard:

RFC 4594 defines the list of techniques and guidenlines for configuring traffic classes and their corresponding DSCP values. It also elaborates the study in different network traffic type with DSCP marking values for all types in DSCP. Standards recommendations are shown below.

From the Figure 6 we have arrived to conclusion that, for different network traffic type (Voice, Video, Best effort) uses specific DSCP markings. As this DSCP performs traffic classification and prioritization to provide the appropriate QOS treatment.

And every DSCP marking configurations which is showed in this article is applicable to all the interfaces defined in the network.  article focuses on Video conferencing (Ex: Google Meet, Microsoft Teams etc.), DSCP markings are considered as AF4x. And this should be the DSCP markings must be configured in the end interfaces defined to the network devices, so that it helps for all end users and customers for seamless video call communication traffic with less complaints that enables this WI-Fi communication better and better.

Upon reading this article, If you want to understand more on QOS and its practical problems in Video Conferencing and how to tackle this problem in this wireless communication era. Read this brilliant article by Saulo Meneghini de Britto who brought all the real time problems and solutions.

Link: https://www.linkedin.com/posts/saulobritto_how-to-improve-video-conferences-on-wi-fi-activity-7283879696176418816-1qky?utm_source=share&utm_medium=member_desktop

If anyone has more insights about QOS for Video conferencing, or if there are any points I might have missed, please let me know. Additionally, I encourage you to check my other articles and share your thoughts as well. Your input is highly valued in expanding our understanding of these exciting new wireless technologies in this wireless world.

Thanks.

Sudharsan E