User`s guide
Signamax Connectivity System 065-7708, 065-7728, 065-7764
www.signamax-us.com www.signamax-eu.com
2.4.1.1 Theory
Understanding IEEE 802.1p Priority
Priority tagging is a function defined by the IEEE 802.1p standard designed to provide
a means of managing traffic on a network where many different types of data may be
transmitted simultaneously. It is intended to alleviate problems associated with the
delivery of time critical data over congested networks. The quality of applications that
are dependent on such time critical data, such as video conferencing, can be severely
and adversely affected by even very small delays in transmission.
Network devices that are in compliance with the IEEE 802.1p standard have the ability
to recognize the priority level of data packets. These devices can also assign a priority
label or tag to packets. Compliant devices can also strip priority tags from packets.
This priority tag determines the packet's degree of expeditiousness and determines
the queue to which it will be assigned.
Priority tags are given values from 0 to 7 with 0 being assigned to the lowest priority
data and 7 assigned to the highest. The highest priority tag 7 is generally only used for
data associated with video or audio applications, which are sensitive to even slight
delays, or for data from specified end users whose data transmissions warrant special
consideration.
The Switch allows you to further tailor how priority tagged data packets are handled on
your network. Using queues to manage priority tagged data allows you to specify its
relative priority to suit the needs of your network. There may be circumstances where
it would be advantageous to group two or more differently tagged packets into the
same queue. Generally, however, it is recommended that the highest priority queue,
Queue 1, be reserved for data packets with a priority value of 7. Packets that have not
been given any priority value are placed in Queue 0 and thus given the lowest priority
for delivery.
A weighted round robin system is employed on the Switch to determine the rate at
which the queues are emptied of packets. The ratio used for clearing the queues is
4:1. This means that the highest priority queue, Queue 1, will clear 4 packets for every
1 packet cleared from Queue 0.
Remember, the priority queue settings on the Switch are for all ports, and all devices
connected to the Switch will be affected. This priority queuing system will be
especially beneficial if your network employs switches with the capability of assigning
priority tags.
VLANs Description
A Virtual Local Area Network (VLAN) is a network topology configured according to a
logical scheme rather than the physical layout. VLAN can be used to combine any
collection of LAN segments into an autonomous user group that appears as a single
LAN. VLAN also logically segment the network into different broadcast domains so
that packets are forwarded only between ports within the VLAN. Typically, a VLAN
corresponds to a particular subnet, although not necessarily.