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The evolution of the data networks entails an increase in the volume of the transmitted traffic, which requires the usage of the a quality of service policy. The implementation of the policy will allow the classification of the network traffic and the distribution of the network resources between different traffic classes.
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In packet networks, the traffic is transmitted from the sending node to the receiving node through communication channels and intermediate devices. Generally a data packet is processed by each intermediate device independently. Let's look at an example of data packet processing performed by an intermediate network device (Figure 1):
- Node-2 forms generates a data packet frame and transmits it to Medium-2. The data is encapsulated in a frame based on the L2-protocol that is used in Medium-2.
- The data frame is distributed in Medium-2. The : the frame is converted into a modulated signal according to the physical properties of the environment. The signals used in wired and wireless environments are different , so and this affects their propagation properties and their usage scenarios.
- The signal arrives at the incoming network interface of the intermediate network device; after demodulation, the received data frame is checked for integrity: the damaged frames are discarded.
- Next, the frame must be processed by the switching module in order to determine its path. If the frame is addressed to this intermediate network device, it will be passed for processing to the internal services. If the frame is addressed to another node, two scenarios are possible: the frame is passed to further processing until it reaches the output interface, or it is discarded (if Medium-2 is a common environment, where all signals will be received by all devices connected to the medium, according to the L2 protocol's operational principles, if the destination address in the frame's header does not belong to the device, then the device should discard it).
- In case that the frame should be processed and transferred to another node, before exiting the device it will be placed into a packet queue. A packet queue is a set of buffers that contain the data received by the incoming interfaces. The number and size of the memory buffers used for the packet queue storage are not standardized and depend on the equipment's manufacturer. For example, the InfiLINK 2x2 family of devices has 32 queues, 17 of which are available for configuration to the user.
- The data frame passes through the packet queue to which it was assigned and arrives at the outgoing interface.
- Since packet queues are a link between incoming and outgoing interfaces, a device should have a controller that fills the queues with the incoming data and picks data from the queues for transmission to the outgoing interfaces. Usually, these functions are performed by the central processing unit (CPU). As it will be shown below, the filling and the emptying of data into and from the queues can be performed unevenly and depends on the classification of the data streams.
- The outgoing interface generates a modulated signal and transmits it to Medium-5 which is connected to Node-5, the destination of the original data frame.
- Node-5 receives the signal, demodulates it , and processes the received data frame.
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A network device can be intermediate for several pairs of nodes , and each node can transmit the data of several services (Figure 2a). Let's look at the scheme where the "Network device" is intermediate for the traffic coming from the following pairs of nodes: Node-1 - Node-4, Node-2 - Node-5 and Node-3 - Node-6. The first pair transmits data for three services, the second for two and the third for one service. If there are no QoS settings, the data of all services get through the general queue in the order they are received at the "Network device" and in the same order they will be transferred from the queue to the outgoing interfaces.
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