Appendix 2 – An-30 Support For Tdm Explained - Redline AN-30 System User Manual

AN-30 System User Manual
11.2. Appendix 2 – AN-30 Support For TDM explained
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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