Frame And Packet-Based Bandwidth Allocation - Alcatel-Lucent 7950 Quality Of Service Manual

Egress Port-Based Schedulers

Frame and Packet-Based Bandwidth Allocation

A port-based bandwidth allocation mechanism must consider the effect that line encapsulation
overhead plays relative to the bandwidth allocated per service
definition (at the queue level) operates on a packet accounting basis. For Ethernet, this includes the
DLC header, the payload and the trailing CRC. This does not include the IFG or the preamble.
This means that an Ethernet packet will consume 20 bytes more bandwidth on the wire than what
the queue accounted for.
The port-based scheduler hierarchy must translate the frame based accounting (on-the-wire
bandwidth allocation) it performs to the packet based accounting in the queues. When the port
scheduler considers the maximum amount of bandwidth a queue should get, it must first determine
how much bandwidth the queue can use. This is based on the offered load the queue is currently
experiencing (how many octets are being offered the queue). The offered load is compared to the
queues configured CIR and PIR. The CIR value determines how much of the offered load should
be considered in the "within-cir" bandwidth allocation pass. The PIR value determines how much
of the remaining offered load (after "within-cir") should be considered for the "above-cir"
bandwidth allocation pass.
For Ethernet queues (queues associated with an egress Ethernet port), the packet to frame
conversion is relatively easy. The system multiplies the number of offered packets by 20 bytes and
adds the result to the offered octets (offeredPackets x 20 + offeredOctets = frameOfferedLoad).
This frame-offered-load value represents the amount of line rate bandwidth the queue is
requesting. The system computes the ratio of increase between the offered-load and frame-offered-
load and calculates the current frame based CIR and PIR. The frame-CIR and frame-PIR values
are used as the limiting values in the "within-cir" and "above-cir" port bandwidth distribution
passes.
From a provisioning perspective, queues and service level scheduler policies are always
provisioned with packet-based parameters. The system will convert these values to frame-based
on-the-wire values for the purpose of port bandwidth allocation. However, port-based scheduler
policy scheduler maximum rates and CIR values are always interpreted as on-the-wire values and
must be provisioned accordingly.
distribution from the port to the queue level and shows the packet or frame-based provisioning at
each step.
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Figure 19 and Figure 20
7950 XRS Quality of Service Guide
. The service level bandwidth
or or
provide a logical view of bandwidth