Quality of Service (QoS) is a method to prioritize/penalize predefined types of traffic. QoS is a complex topic so this text will discuss only enough basic principles to help you configure an initial setup.

QoS is not the same as FreshTomato's Bandwidth Limiter. QoS analyzes all traffic and follows rules for prioritizing one type of traffic over another, based on all clients on the network. Bandwidth Limiter limits the bandwidth used by one or more client devices on the network. Bandwidth Limiter does not prioritize one type of traffic over another type. It prioritizes one client device or group of client devices over others.

If you need a simple, crude tool for one or more known client devices, Bandwidth Limiter may be what you need.

QoS is more sophisticated, and in many ways, more refined tool.

However, Bandwidth Limiter shares a few settings with QoS. When Bandwidth Limiter is enabled, QoS should be disabled, and vice versa. If both are enabled, they will conflict with each other and neither will work.

Basically, QoS performs the following:

• Identifies traffic.
• Classifies this traffic.
• Marks packets.
• Defines classes and subclasses.
• Sets class parameters (such as reserved bandwidth).

You might have heard the above points when describing “diffserv” (differential services). Diffserv is the only type of QoS that FreshTomato can perform.

It's often said that QoS is a technique to mitigate lack of bandwidth when an upgrade is too costly or even impossible. More precisely, QoS also allows you to prioritize latency-sensitive traffic (such as voice) even when there's no network congestion. Thus, QoS introduces controls to allows you to protect certain traffic.

Here are some commonly used QoS terms:

• Traffic: This is the stream of data between two nodes. Streams could be unidirectional (such as over UDP)
  or bidirectional (such as over TCP).
  
  
• Best effort: This is also known as “no QoS” or “first come, first served” (FIFO).
  This is usually the default queuing mechanism applied to network interfaces.
  When QoS is operating, best effort usually means “that class of traffic that handles everything else”.
  
  
• Classification: This is the process of internally identifying packets based on predefined parameters.
  For example: “from IP”, “to port”, “protocol=tcp”.
  
  
• Marking: This is the process of changing the Type of Service/DSCP field in the IP header of a packet
  with a locally significant meaning. This will help (a router?) to match packets, as it can simply look at
  their ToS/DSCP headers to know what type of data they contain. Marking it not always performed.
  
  
• Queuing: This (Outbound only process) is the buffering of data which the network can't process in real time.
  There are many forms of Queuing in QoS. This is divided into classes.
  
  
• Policing: This is the (Inbound and Outbound) process of dropping packets in a controlled way.
  
  
• Class Discipline: These are the actual rules of priority applied to all the classes of data.
  Packets In the Priority class always come first. If Priority is empty, WFQ (Weighted Fair Queuing)
  is applied among all the other classes.
  There are dozens of queuing disciplines, FreshTomato only supports the following:
  • asdfgjkl
  • asdfghjkl
    

The diagram below should help you to understand these principles:

Have you ever experienced a poor quality VoIP call from your LAN onto the Internet?

Do your media, such as videos, buffer too much?

Is someone on your LAN generating too much traffic?

These are all problems that QoS can help you to solve. Note that QoS is NOT auto-regulating. You need to manually define most of the QoS parameters, and tweak them, as needed.

As shown in the above screenshot, QoS doesn't work when CTF is enabled. This is because CTF causes network packets to be routed in hardware, outside of firmware control or monitoring.

This Network Scheduler Wikipedia entry may help you further understand the above principles:
https://en.wikipedia.org/wiki/Network_scheduler