2. Improving QoS…one algo at a time!
Scheduling algorithms are important components in the
provision of guaranteed quality of service parameters
such as delay, delay jitter, packet loss rate, or throughput.
The design of scheduling algorithms for mobile
communication networks is especially challenging given
highly variable link error rates and capacities, and the
changing mobile station connectivity.
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5. Wireless Network Scheduler Challenges...
Characteristics of wireless links
Subject to time- and location-dependent signal
attenuation, fading, interference, and noise that result
in bursy errors and time-varying channel capacities.
Wireless channel model
Discrete-time Markov chain with two states:
error-free (“good”) or error-prone (“bad”)
A packet is successfully received if and only if the link
stays in the good state throughout the packet
transmission time.
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6. Wireless Network Scheduler Challenges cont....
Information needed to make scheduling decisions
Number of sessions
Session reserved rates
Link states
Statuses of session queues
Information availability
For the down-link:
The scheduler is located at the base station (BS)
This information is easily obtianed
For the up-link:
Some means must be provided to collect queue status
information and to inform mobile stations (MSs) of their
transmission times.
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7. Wireless Network Scheduler Challenges cont....
To maximize MS battery life
To transmit/receive in contiguous time slots and then
go into a sleep mode rather than to rapidly switch
among transmit, receive and sleep modes.
Handoffs
Following a handoff, any packets for S that are queued
at previous cell C1’s BS will be forward to current cell
C2’s BS
For timestamp-based scheduling
Timestamp update
Fairness gap: low timestamp extra service
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8. Wireless Network Scheduler Challenges cont....
In CDMA network,
the total interference at an MS must be small enough
to ensure an adequate signal-to-interference ration
(SIR) for each session, thereby enabling its target bit
error rate (BER) to be met.
The scheduler must ensure that the number of
simultaneous transmissions in the network is not so
high as to result in excessive interference.
In multihop networks
No BSs
Rapidly changing topology
Routing
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9. Our Paradigm Shifting Approach…
Components
(1) An error-free service model
(2) A lead/lag counter
Whether the session is leading, in sync with, or lagging its
error-free model and by how much
(3) A compensation model for each session
A lagging session is compensated at the expense of leading
sessions
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10. Our Paradigm Shifting Approach cont….
(4) Separate slot queues and session queues for each
session
When a packet arrives, it is timestamped and placed in the
packet queue;
A slot with the same timestamp value is added to the slot
queue.
If the HOL (Head of line) packet for a session is dropped
due either to excessive delay (delay-sensitive) or an
excessive number of retransmissions (error-sensitive), the
precedence of the session for accessing the channel is
maintained by the slot queue.
(5) A means for monitoring and predicting the channel
state for every backlogged session.
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12. Our Scheduler Algorithms…
We Build Work-conserving…
The scheduler is never idle if there is a packet awaiting
transmission.
Generalized Processor Sharing (GPS)
Weighed Fair Queueing (WFQ)
Virtual Clock (VC)
Weighted Round Robin (WRR)
Self-Clocked Fair Queueing (SCFQ)
Deficit Round Robin (DRR)
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13. Our Scheduler Algorithms cont…
VS. Non-work-conserving
The scheduler may be idle even if there is a
backlogged packet in the system because it may be
expecting another higher-priority packet to arrive.
Hierarchical Round-Robin (HRR)
Stop-and-Go Queuing (SGQ)
Jitter-Earliest-Due-Date (Jitter-EDD)
Higher average packet delays than work-conserving
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14. Our Scheduler Algorithms cont.…
We use Timestamps
Incoming packets are timestamped before being placed
in their respective session queues.
The HOL packets are then sorted in increasing order of
their timestamps, and the packet with the lowest
timestamp value is selected for transmission.
Better QoS guarantees
VS. Round-robin
No timestamps
Easily implemented but lower QoS
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15. Our Approach To Scheduling in CDMA
Networks…
Advantages of CDMA over TDMA and FDMA
higher (soft) system capacity
soft handoff
simple frequency planning
inherent frequency diversity against multipath fading
Voice activity factor and antenna sectorization are
readily exploited using CDMA.
Drawback
an accurate power control mechanism is required.
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16. We leverage…
Advantages of CDMA
The soft capacity feature of CDMA allows a new
session to be established provided that the for all
transmitting sessions can be maintained above their
target levels a certain percentage of the time.
The packets sent from a number of MSs can be
successfully received simultaneously at the BS,
provided an adequate power control scheme is used.
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17. Our Unique Scheduling Algorithms have…
Topology transparency:
The scheduler works efficiently regardless of how frequently
and unpredictably the topology changes.
Our topology independent algorithm reduces the burden of
having to recompute and reassign time slots.
Low connectivity information requirement:
Some of our algorithms need global network connectivity
information while others require only local (e.g., one- or
two-hop) connectivity information.
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