Sistem Antrian dengan Prioritas

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S1 Teknik Telekomunikasi Fakultas Teknik Elektro

REKAYASA TRAFIK | TTH3J3 | Kur. 2016 | 2017/2018

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Target Pembelajaran

• _{Priority Queuing}

• _{Custom Queuing}

• _{Weighted Fair Queuing (}_WFQ₎

• _{Weighted Random}

Early Detection (WRED)

• _{Token Bucket}

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Usage Parameter Control non-conforming

contract tagging

UPC

passing discarding

An example of UPC algorithm

Conforming TAT

Non-conforming

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Shaping increases network efficiency by reducing output bandwidth

without shaping Re-space the input cells

With shaping

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CAC examines whether network resources are sufficient or not for a new

Time B

it r at e

CAC

Necessary bandwidth

<Space of bandwidth ?

accept (or reject)

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QoS Design

Contoh : Kualitas Voice dalam Jaringan

VoIP dipengaruhi langsung oleh dua faktor utama :

 Packet loss  Delay Packet

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Packet Delay

Packet delay can cause either voice-quality degradation, due to the

Packet Loss

There are several reasons for packet loss, including:

1. Congestion caused by queues overflowing their packet limits 2. Network nodes running out of buffer space

3. Memory limitations in network nodes

4. Policing, or controls that watch traffic flows, ensuring that

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Echo

Echo bukan masalah khusus terhadap packet switching

tetapi berpengaruh pada teknologi Circuit switching yang disebabkan oleh dua hal :

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Komponen Fixed Delay

• _{Propagation—six microseconds per kilometer}

• _{Serialization}

• _Processing

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Komponen Variable Delay

• _{Queuing delay}

• _{Dejitter buffers}

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Contoh dalam VoIP QoS :

Classification Queuing

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Queuing’s Premise

• _{Getting better service is a matter}

of managing congested queues

• _{Over-all latency and bandwidth are}

constant

– _{Make some trafc absorb latency, and}

therefore give up bandwidth

– _{Shield other trafc from latency, and}

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FIFO Queuing

• _Premise

– _{“Packets leave in order of arrival”}

• _{Fixed queue lengths}

– _{Results in}_{dropping from tai}_{l of}

queue under load

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Simplest QoS Algorithm: Priority

Queuing

• _{Stated requirement:}

– _{“If <application> has trafc waiting,}

send it next”

• _{Commonly implemented}

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Priority Queuing Implementation Approach

• _{Identify interesting trafc}

– _{Access lists}

• _{Place trafc in various queues}

• _{De queue in order of queue}

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Priority Queuing

(PQ)

Traffic Destined for Interface

Interface Buffer

Transmit

Queue Output Line Interface Hardware

• _Ethernet • _{Frame Relay} • _ATM

• _{Serial Link} • _Etc.

High

Medium

Normal

Low

Q Length Defined by Q Limit

Classify

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Priority Queuing

Priority (Access)

List Forwarder

Low Priority

Medium Priority

High Priority

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Pitfalls of Priority Queuing

• _{Can cause trafc lockout}

if not well engineered

• _{FIFO by priority:}

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Class-Based Queuing

• _{One standard : custom queuing}

– _{“Trafc with <characteristics>}

needs a guaranteed rate or latency”

• _{“Characteristics” may be:}

– _Application

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Custom Queuing Implementation Approach

• _{Identify the trafc}

• _{Distribute into FIFO queue by class}

• _{Dequeue in some rotation}

– _{Remove some number of bytes at most}

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Custom Queuing (CQ)

Q Length Deferred by Queue Limit

Up to 16 3/10

1/10

Weighted Round Robin Scheduling (byte count)

Classify

Interface Hardware

• _Ethernet • _{Frame Relay} • _ATM

• _{Serial Link} • _Etc.

2/10

3/10 2/10

Link

Utilization Ratio

Transmit

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Custom Queuing

Queue #0

Queue #1 Queue #2

Queue #9 Queue #10

Queue #11 Queue #12

Queue #13 Queue #14

Queue #15 Queue #7

Queue #16 Queue #8

Control Traffic

Priority (Access)

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Pitfalls of Custom Queuing

• _{FIFO by class:}

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Weighted Fair Queuing

(WFQ)

– _{“Trafc latency must be} predictable”

– _{“Reserved fows must achieve}

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Fair Queuing Approach

• _{Enqueue trafc in the sequence}

the TDM would deliver it

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Efects of Fair Queuing

• _{Low-bandwidth fows get}

– _{As much bandwidth as they can use}

– _{Timely service}

• _{High-bandwidth fows}

– _{Interleave trafc}

– _{Cooperatively share bandwidth}

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What Weighting Does

• _{In TDM}

– _{Channel speed determines message}

“duration”

• _{In WFQ}

– _{Multiplier on message length changes}

simulated message “duration”

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Weighted Fair Queuing (WFQ)

Interface Configurable

Number of Queues

Flow-Based Classification by: Weight Determined by:

Weighted Fair Scheduling

Classify

Transmit

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Interactive Traffic Timing

Milliseconds

500 1000 1500 2000 2500 3000

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Interactive Traffic Timing

Milliseconds

Fair Queuing

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Pitfalls of Weighted Fair Queuing

• _{Requires more sorting}

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RED Algorithm

(Random Early Detection)

• _{Maintain mean queue depth}

• _{Drop trafc at a rate proportional to}

– _{Mean queue depth}

– _{Time since last discard}

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Behaviour of a TCP Sender

• _{Sends as much as credit allows}

• _{Starts credit small}

– _{Avoid overloading network queues}

• _{Increases credit exponentially}

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Behaviour of a TCP Receiver

• _{When in receipt of “next message,”}

schedules an ACK

• _{When in receipt of something else,}

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Sender Response to ACK

• _{If ACK acknowledges something}

– _{Update credit and send}

• _{If not, presume it indicates a lost}

packet

– _{Send frst unacknowledged message}

right away

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Implications for Routers

• _{Dropping a message is a way of}

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Weighted Random Early Detection

• _{Stated requirement}

– _“_{Avoid congestion} _{in the frst place”}

– _{“Statistically give some trafc better}

service than others”

• _{Congestion avoidance, rather than}

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Weighted Random

Early Detection (WRED)

FIFO Scheduling Pass

Fail

Discard Test

Transmit

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Weighted Random Early Detection

Minimum Threshold Maximum Threshold

Drop Rate

Weighted RED Behavior with TCP

50 Maximum Threshold

Minimum Threshold

Drop Rate

Mean Queue Depth

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Pitfalls of RED

• _{This is again FIFO queuing}

– _{Not predictable}

• _{Depends on host behaviour}

for efectiveness

– _{Not deterministic}

• _{Specifcally dependent on bulk}

of trafc being TCP

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Trafc Shaping

• _{Use token bucket model which}

output packet stream are governed

by token stream

• _{Trafc Shaping Functions :}

– _{Smoothing trafc burst, all the trafc is}

bounded by a confgured fow rate

– _{Enforce a long-term average trafc rate}

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Token Bucket

Output Line

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Traffic Shaping Queue Structure

 _{One queue per}

– Sub-interface – Access List

– DLC

 _{Feeds into queues at hardware layer}

Forwarder

Within Token Bucket?

Interface Congested?

Sub-Interface Fancy Queues

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Generic Trafc Shaping (GTS)

Transmit

Queue Output Line Traffic

Destined for Interface

Configured Queuing (e.g. WFQ, CQ, etc.) Match

No Match

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