EtherHaul Operation, Administration and Maintenance Manual
Page 87
9.8.4
PFC - Priority-based flow control
Priority-based flow control (IEEE standard 802.1Qbb) prevents frame loss from
congestion by pausing traffic based on the congested priority without affe cting the
traffic of uncongested priorities.
Instead of pausing all traffic on a link as in IEEE 802.3x Ethernet PAUSE (flow control),
PFC allows you selectively pause traffic according to its class.
Standard Ethernet does not guarantee that a packet injected into the network will arrive
at its intended destination. Reliability is provided by upper-layer protocols. A problem
arises when transmitters send packets faster than receivers can accept them. When
receivers run out of available buffer space to hold incoming flows, they silently drop
additional incoming packets.
Applications that require reliability in Layer 2 must have flow control that includes
feedback from a receiver to a sender regarding buffer availability. Using IEEE 802.3x
Ethernet PAUSE control frames, a receiver can generate a MAC control frame and send a
PAUSE request to a sender when a specified threshold of receiver buffer has been filled
to prevent buffer overflow. Upon receiving a PAUSE request, the sender stops
transmission of any new packets until the receiver notifies the sender that it has
sufficient buffer space to accept them again. The disadvantage of using Ethernet PAUSE
is that it operates on the entire link, which might be carrying multiple traffic flows.
Some traffic flows do not need flow control in Layer 2, because they are carrying
applications that rely on upper-layer protocols for reliability. PFC enables you to
configure Layer 2 flow control selectively for the traffic that requires it.
Implementation concept:
This feature should work only when the ODU is set as “pipe” i.e. as transparent
bridge.
FPC can be set towards one UNI port only (one of the 4 Eth ports), for traffic set
between one of ETH ports and the radio port (Eth0/RF).
Priorities should be set for the PFC for the RF port only (Eth0). It can be done for all
8 queues but the mandatory requirement is for the first 4 queues.
Once the BW in the radio drops down, as a result of modulation change or manual
change, if the out queues of ETH0/RF cross a maximum threshold a pause frame
should be send to the UNI port.
Min and max thresholds defined in Mb.
Note:
PFC configuration supported in the CLI only.
