Alcatel-Lucent 7450 Quality Of Service Manual page 514

Overview
means that an Ethernet packet will consume 20 bytes more bandwidth on the wire than what
the queue accounted for. When considering HDLC encoded PoS or SDH ports on the
7750 SR, the overhead is variable based on '7e' insertions (and other TDM framing issues).
The HDLC and SONET/SDH frame overhead is not included for queues forwarding on PoS
and SDH links.
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.
For PoS or SDH queues on the 7750 SR, the packet to frame conversion is more difficult to
dynamically calculate due to the variable nature of HDLC encoding. Wherever a '7e' bit or
byte pattern appears in the data stream, the framer performing the HDLC encoding must place
another '7e' within the payload. Since this added HDLC encoding is unknown to the
forwarding plane, the system allows for an encapsulation overhead parameter that can be
provisioned on a per queue basis. This is provided on a per queue basis to allow for
differences in the encapsulation behavior between service flows in different queues. The
system multiplies the offered load of the queue by the encapsulation-overhead parameter and
adds the result to the offered load of the queue (offeredOctets *
configuredEncapsulationOverhead + offeredOctets = frameOfferedLoad). The frame-
offered-load value is used by the egress PoS/SDH port scheduler in the same manner as the
egress Ethernet port scheduler above.
From a provisioning perspective, queues and service level (and subscriber 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.
Figure 25
and shows the packet or frame-based provisioning at each step.
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provide a logical view of bandwidth distribution from the port to the queue level
Figure 24 and
Quality of Service Guide