User's Manual

ALFOplus 24GHz (North America) - Release 01.05.0x - MN.00395.E - 004 47
Fig.15 -
Means that the Serving Priority of the frames present in the queues will be:
• the packets present in the Queue 7 will be transmitted with a Race 8/35
• the packets present in the Queue 6 will be transmitted with a race 8/35
• ......
• the packets present in the Queue 0 will be transmitted with a rate 1/35.
7.4 ETHERNET FRAME FRAGMENTATION
QoS preserve High priority traffic, by giving it precedence during traffic congestions. However, in case of
real time traffic also latency and jitter are important factors. Latency is strictly related to the line speed
and usually can be managed by designing the network topology in a proper way (e.g. by limiting the max-
imum number of hops in link chains). Jitter is instead a more sensitive parameter because it depends on
the traffic conditions.
In fact, when a High priority packet has to be sent over the radio link it is scheduled on a High Priority
queue. However, before to be sent over the radio link it has to wait that the packet currently in transmis-
sion (even a Best Effort packet) will be entirely sent. This waiting time can considerably change depending
on the best effort packet size (from 64bytes to 1518 bytes of even more in case of jumbo frames). One
technique used to mitigate this phenomenon is packet fragmentation, i.e. longer frames are subdivided in
smaller fragments at Tx side. A label is added to the packet in order to number these subframes. At Rx
side the original frame is rebuilt after all the fragments are received. In this way, the maximum waiting
time for a High Priority packet is reduced to the sub-frame size (some hundreds of bytes), providing sen-
sitive benefits to the packet jitter.
The SIAE switch allows to fragment Ethernet frames with two options: 256 or 512 Bytes.
For example: in case the radio is serving a 1024Byte frame in the lowest (queue 0) and there is an incoming
frame (256 bytes) in the highest priority queue (queue 7). The packet in the highest priority should be
served first, but since the servant is busy processing the packet in the lower queues, the 256Byte frame
has to wait until the radio has processed the 1024 Byte frame, see Fig.16
.