Allied Telesis AT-9724TS specifications 18 QoS, Advantages of QoS, Understanding QoS

6-18 QoS

The AT-9724TS supports 802.1p priority queuing Quality of Service.The following section discusses the implementation of QoS (Quality of Service) and benefits of using 802.1p priority queuing.

The Advantages of QoS

QoS is an implementation of the IEEE 802.1p standard that allows network administrators a method of reserving bandwidth for important functions that require a large bandwidth or have a high priority, such as VoIP (voice-over Internet Protocol), web browsing applications, file server applications or video conferencing. Not only can a larger bandwidth be created, but other less critical traffic can be limited, so excessive bandwidth can be saved.The Switch has separate hardware queues on every physical port to which packets from various applications can be mapped to, and, in turn prioritized.View the following map to see how the AT- 9724TS implements 802.1P priority queuing.

How 802.1p works (Switch default settings)

7 priority queues

Queues

Priority

Port

Port default priority

Class 0				Class 1					Class 2		Class 3
	0				1			2		3		4
1	2	3	4		5	6	7
0	0	0	1		4	2	6		.........................................

Class 4

5

Class 5

6

Class 6

7

Figure 6- 47. Mapping QoS on the Switch
The picture above shows the default priority setting for the Switch. Class-6 has the highest priority of the seven priority classes of service on the Switch. In order
to implement QoS, the user is required to instruct the Switch to examine the header of a packet to see if it has the proper identifying tag tagged.Then the user
may forward these tagged packets to designated classes of service on the Switch where they will be emptied, based on priority.
For example, lets say a user wishes to have a video conference between two remotely set computers.The administrator can add priority tags to the video
packets being sent out, utilizing the Access Profile commands.Then, on the receiving end, the administrator instructs the Switch to examine packets for this tag,
acquires the tagged packets and maps them to a class queue on the Switch.Then in turn, the administrator will set a priority for this queue so that will be
emptied before any other packet is forwarded.This results in the end user receiving all packets sent as quickly as possible, thus prioritizing the queue and allowing
for an uninterrupted stream of packets, which optimizes the use of bandwidth available for the video conference.
Understanding QoS
The Switch has eight priority classes of service, one of which is internal and unconfigurable.These priority classes of service are labelled as 6, the high class to 0,
the lowest class.The eight priority tags, specified in IEEE 802.1p are mapped to the Switch's priority classes of service as follows:
Priority 0 is assigned to the Switch's Q2 class.
Priority 1 is assigned to the Switch's Q0 class.
Priority 2 is assigned to the Switch's Q1 class.
Priority 3 is assigned to the Switch's Q3 class.
Priority 4 is assigned to the Switch's Q4 class.
Priority 5 is assigned to the Switch's Q5 class.
Priority 6 is assigned to the Switch's Q6 class.
Priority 7 is assigned to the Switch's Q6 class.
For strict priority-based scheduling, any packets residing in the higher priority classes of service are transmitted first. Multiple strict priority classes of service are
emptied based on their priority tags. Only when these classes are empty, are packets of lower priority transmitted.
For weighted round-robin queuing, the number of packets sent from each priority queue depends upon the assigned weight. For a configuration of 8 CoS queues,	66
Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch