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AN-30 System User Manual 

 

 
802.1p QoS  
 
The AN-30 uses IEEE 802.1p to ensure QoS to TDM based applications such as mobility 
backhaul and enterprise PBX traffic. IEEE 802.1p allows traffic to be assigned into one 
of two (high or low priority) queues.  Ethernet packets are queried to determine if they 
are priority tagged.  Packets tagged with a priority value greater than or equal to four are 
automatically placed in the high priority queue. All other packets go into a default queue. 
The AN-30 always services the traffic in its high priority queue before the default, lower 
priority queue.  A low priority packet will only be transmitted once the high priority 
queue is empty.  
 
Flow Control 
 
The AN-30 uses standard IEEE 802.3x flow control, for improved traffic handling.  
 
IEEE 802.3x defines a control protocol between interfacing Ethernet port for the purpose 
of pausing transmission from an upstream device.   Upon reception of a pause control 
frame, an Ethernet port will not transmit it’s next normal data frame until the timer 
specified in the pause control frame expires, or another pause control frame is received 
with it’s timer set to zero.  During the flow controlled period, only flow control packets 
can be transmitted between across the Ethernet link. 
 
The AN-30 uses this mechanism internally transmitting pause control frames as required 
based on the availability of its own internal resources, including buffers, transmit queues 
and receive queues. 
 
Since Ethernet traffic typically runs at 100 Mbps it can easily exceed the maximum 
capacity of the wireless link. Left unchecked this situation will trigger indiscriminate 
packet discarding in the wireless transmit buffer.  To avoid this condition the AN-30’s 
wireless interface issues pause control frames to flow control upstream traffic. This 
effectively provides an automatic traffic shaping capability that automatically triggers in 
advance of a buffer overflow condition occurring. The system will therefore adjust itself 
to the available wireless throughput. If wireless throughput is decreased the system will 
automatically throttle-back (traffic shape) the lower priority traffic streams by first 
discarding the packets in its default queue.  
 
Upon receiving additional packets, the system will issue pause control frames to halt 
upstream transmission until it can service its wireless transmit buffers.  
 
Once wireless resources are again freed up, a new pause control frame is issued to resume 
transmission. 
 

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