User Guide > Switch
A Switch is a network hardware that can be connect to other Switches and Host Ethernet Ports to form a computer Ethernet network. Switches use packet switching to receive Ethernet packets that encapsulate data and forward these to the destination Host(s) were the original data is extracted and reassembled. More precisely, a Switch is a multiport Ethernet network bridge that uses MAC addresses to forward data at the data link layer (layer 2) of the OSI model.
To add a new Switch to your system model, click the Switch button in the Toolbar.
This will open the Switch dialog:
In the upper part, this dialog allows you to specify a name for the new Switch as well as the number of available Ethernet ports. Parameters part of the switch memory model are also available.
The “PSFP” tab allows configuration of Per stream filtering and policing - for a detailed explanation see PSFP.
In the center pane, Ethernet port configuration settings are available - for a detailed explanation see Ethernet port configuration.
Note that the memory model is the same irrespective of the Scheduling Policy selected (Strict Priority Scheduling, Audio Video Briding or Time-Aware Shaper).
Each Switch port has a separate memory that stores outbound Ethernet frames that must be queued temporarily due to contention at the egress interface of a port. A Switch port memory is divided into blocks composed of a number of bytes. To store a new outbound Ethernet frame, that is to be transmitted by an egress port, the memory manager of the port reserves a number of memory blocks to hold the new frame. For example, if an Ethernet frame of size 1500 bytes is to be queued up by an egress port and the block size is set to the default size of 128 bytes per block, the number of memory blocks reserved are ceiling(1500/128) = 12.
The default block size of 128 bytes was deduced experimentally by performing stress test measurements on a three port automotive grade switch. It should be a good compromise block size; if one typically has many minimum sized Ethernet frames in a network, a small block size should be preferable. If one instead has many video-streams with maximum sized Ethernet frames in the network, a larger block size is probably more efficient.
The memory of a Switch port is further divided into two separate buffers, one for high traffic class frames, the High Buffer, and the other one for low traffic class frames, the Low Buffer. The High buffer block size and the Low buffer block size thus effectively specifies the size of these two buffers, respectively, and the byte size of a frame buffer is computed as (Buffer block size) * (Block byte size).
This parameter is set individually for each Switch port and specifies which frames are considered as high traffic class frames. If it is set to 7, then only the Ethernet frames mapped to the highest traffic class queue will be stored in the High Buffer of a port. Ethernet frames mapped to traffic classes 0-6 on the other hand will be stored in the Low Buffer. If instead this parameter is set to 6, then Ethernet frames mapped to traffic class 6 and 7 will be stored in the High Buffer and frames mapped to traffic classes 0-5 will be placed in the Low Buffer etc. This means that if this parameter is set to 0, all frames will be stored in the High Buffer and the Low Buffer will not be used at all.
Now, assume we have a situation where a switch is overwhelmed by a large amount of low priority frames coming in on several ports and all of these should exit through one and the same egress port. The port buffer memory quickly fills up and packet drop sets in. If an Ethernet frame arrives, wants to exit through the same port and is mapped to the highest traffic class, there is a high probability that it is dropped, if there is a single frame buffer per port. This is especially true if it is a large frame. However, with a separate High Buffer for high traffic class frames, there is a much higher probability that there is memory available for the high traffic class frame and it should get through the switch with much higher probability as compared with a single frame buffer per port storing all types of frames.
The following actions are available for existing Switches - right-click on a Switch in the topology or the tree view:
Edit | Opens the Switch dialog in edit mode. | |
Delete | Removes the Switch from the
topology. Note! this will affect for example flows where this Switch is used. |
The following actions are available for existing Switch ethernet ports - right-click on a port in the topology or the tree view:
Edit | Opens the Switch port dialog in edit mode. |