Allied Telesis AT-RPS8000 Installation Guide download pdf (Page 24)

Product Description

STP addresses this problem by ensuring that there is only one path

between the end nodes. Where multiple paths exist, STP places the extra

paths in a redundant or standby mode, leaving only one main active

path. The redundant paths can be activated by STP if the main path goes

down. So not only does STP guard against multiple links between end

nodes, but it also activates backup redundant paths in case a main link

fails.

Quality of Service

(QOS)

The AT-9006 Series Gigabit Ethernet Switches support the IEEE 802.1p

standard and Quality of Service. QoS can be important in network

environments where there are time-critical applications, such as voice

transmission or video conferencing, that can be adversely affected by

packet transfer delays.

Prior to QoS, network traffic was handled in a best-effort manner. File

transfer delays did occur, but were mostly transparent to network users.

But with the introduction of time-critical applications, packet transfer

delays have proven problematic. For example, delays in packets carrying

voice transmission can result in poor audio quality.

The QoS feature was designed to address this problem. The IEEE 802.1p

standard outlines eight levels of priority, 0 to 7, with 0 the lowest priority

and 7 the highest priority. The Ethernet switches have two priority

queues, normal and high. Packets with priority levels 0 to 3 are placed in

the normal queue and packets with priority levels 4 to 7 are placed in the

high queue.

When a tagged packet enters a switch port, the switch responds to the

priority in the tag and forwards the packet accordingly. If desired, you

can configure the individual ports on the switch so that the priority level

in a tagged frame is ignored and that the tagged packets received on a

port are automatically assigned to either the normal or high priority

queue, regardless of the priority level in the packet. Consequently, the

switch will forward a tagged frame according to the port priority level

and not to the priority level in the tagged packet itself. However, the

switch does not alter the priority level in the packet, so that when the

packet egresses the switch, its original packet priority level is

unchanged.

Packets in the two queues are handled in a round robin manner. The

default algorithm specifies that packets in the high priority queue

receive six times more importance than packets in the normal priority.

That is, the switch can handle six high priority packets before checking

for the presence of a packet in the normal priority queue.

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Allied Telesis AT-RPS8000 Installation Guide Page 24