1
1
ECE 284
ECE 284
The Link Layer
Additional Material
This material will not be part of the exam.
It is included here for completeness.
2
ECE 284
ECE 284
Leaky Bucket Mechanism
Leaky Bucket Mechanism
Leaky bucket: traffic policing
●Bucket holds up to
B
tokens
●
b
is the number of tokens currently in the bucket
●Tokens fill up the bucket at rate
λ
●Transferring a packet from the input queue consumes a token from the bucket
The number of packets admitted in a time int erval is bounded:
Variants
●Packet-level leaky bucket
●Bit-level leaky bucket
λ
Bb
(
)
BttttD +−⋅≤ 122,1 ][
λ
3
ECE 284
ECE 284
Wireless Fair Queuing
Wireless Fair Queuing
Goal: provide fairness despite temporary localized channel errors
●Channel errors should be almost transparent to the user
If a scheduled flow is experiences a bad channel, one with a good channel will
be allowed to transmit instead to avoid wasting capacity
Flows most be compensated for capacity lost due to channel errors
Issues:
●Monitoring the channel state
●Separation between flows: flows that always percei ve a good channel should
not be impacted
4
ECE 284
ECE 284
Building Blocks of Wireless Fair Queuing
Building Blocks of Wireless Fair Queuing
Error free service model: used as a reference
Lead: ahead of error free service, received more band width so far
●Number of slots that have to be relinquished in the future
Lag:
●Definition 1: difference between error-free and actual service (independent
whether another flow could transmit or not)
●Definition 2: number of slots during which another flow transmitted instead
Compensation model: when are slots of a leading flow relinquished and how
are they distributed amongst lagging flows
●Giving up lead: all at once or more graceful degradation
●Distributing lag: preferential or fairly (proportional to its lag)
Decouple slot queues from packet qu eues
●Slot are scheduled, based on packet arrivals
●A separate mechanism selects which packet is transmitted
5
ECE 284
ECE 284
Wireless Fair Service (WFS)
Wireless Fair Service (WFS)
A variant of WFQ is used as the reference model
●Separate rate weight
wi
and delay weight
φi
●Decouple rate and delay constraints
Lead:
l
i
number of extra slots received
Lag:
b
i
number of slots used by other flows
In sync: allocation same as error free service, neit her leading nor lagging
⎥
⎦
⎤
⎢
⎣
⎡−
+−=
i
i
ii w
kL
kSRkS )1(
)1(,max)(
i
i
ii
kL
kSkF
φ
)(
)()( +=
6
ECE 284
ECE 284
WFS Behavior
WFS Behavior
Compensation model:
●Leading flow relinquishes of its slots
●Lagging flow receives a fraction of the relinquished slots
Properties
●Short-term and long-term fair
●Achieves delay and throughput bounds
●Delay and bandwidth decoupling
●Graceful relinquishing of slots
●Fair compensation of lagging flows
max
i
il
l
∑
kk
ib
b
