Quality of Service (QoS)

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Quality of Service (QoS)

in networking and telecommunications

Baw Ch’ng


Presented to:


Biographical Background


•  Consulting practice in the Wireless Networking & Telecommunications field

•  Clients from small start-up (vendor) and investment firm to Tier-1 service provider (operator)

•  Services offered include:

•  Strategic Consulting – e.g., technology positioning, product positioning, roadmap development

•  Standards Consulting – e.g., standards development, standards & trade group representation

•  Technology Consulting – e.g., technology development, architecture development, trial orchestration •  Please visit www.bawman.com for more information

Baw Ch’ng

•  Founder & President of BAWMAN LLC, proud member of CONET

•  Serve start-ups, investors, and large corporations; vendors & operators, domestic & international

•  Developed wireless mobile broadband technologies, products, and standards

•  3G, 4G, Wi-Fi, 3GPP & 3GPP2, WiMAX Forum, Small Cell Forum, Broadband Forum, Wi-Fi Alliance

•  Specialize in wireless networking, particularly in small cell technologies (e.g., femtocell, picocell)

•  Patents awarded in the areas of mobility management, network management, network security &

authentication, device provisioning, network selection, service activation, network planning, etc.


Presentation Outline

Biographical Background

What is Quality of Service (QoS)?

QoS Considerations

QoS Principles and Techniques

Real-World Impact of QoS


Quality of Service (QoS)

What is QoS in the networking and

telecommunications context?

–  Intuitively

•  Overall performance of telephony or computer network,

particularly as seen by end users [Wikipedia]

–  Technically

•  For classical telephony (ITU-T E.800)

–  Service response time, loss, signal-to-noise ratio, crosstalk, echo,

frequency response, loudness levels, etc.

–  Basically things that impact end-user perception of network


•  For computer network (including “packet voice”)

–  How quickly, how steadily, and how reliably data gets from


QoS Considerations

Why would we need QoS? When would we need QoS?


QoS Considerations

Classical telephony

–  Circuit switched

–  Mechanical (circuit) switch board:

–  Some one mechanically connects ports by wires

Picture  credit:  Chris  Limek  

Ports   Ports   Wires  


Classical Telephony

A classical telephone connection

Caller   A  

Caller   B  


QoS Considerations

•  Classical telephony

–  Establishment of physical circuit creates a dedicated channel

for end users

•  No contention once a circuit is established

–  QoS considerations •  Voice call quality

–  Engineering efforts to ensure call quality focused on getting signals appropriately amplified and filtered along the circuit.

•  Service availability

–  How many circuits are needed to support a given population of telephone users? –  Use statistical modeling to size switching network capacities to (reasonably)

meet demand


Considerations of QoS

Computer networking (and “packet voice”)

– Packet switched

•  User data chopped up into “packets” for transportation

–  Issues: lost packets, duplicated packets, out-of-order

delivery of packets

– No physically dedicated channel

•  Links are “shared resources”

–  Issue: resource contention ç most QoS studies deal with this

How to deal with each issue?


QoS Considerations

Revisit QoS definition for computer network:

– How quickly, how steadily, and how reliably data gets from computer A to computer B

How quickly, how steadily, and how reliably

do we


the data to get from computer A to

computer B?

– It depends ...


Different Applications, Different Needs

•  … different expectations from end users •  Applications that are not time-sensitive

–  No real hurry for data to get from computer A to computer B

–  E.g. general web surfing, text messages, e-mail, file transfer

•  Applications that are time-sensitive

–  Data must arrive within strict (sub-second) time limits to be


•  Streaming

–  Time-sensitive data that travels one way

–  E.g., music, movie •  Interactive

–  Time-sensitive data travels bi-directionally


Different Applications, Different Needs

•  File-Transfer and E-mail

–  OK if data do not arrive quickly or steadily …

•  High latency is tolerable, large jitter is tolerable

–  … as long as all the packets eventually arrive

•  Packet loss is bad, must be able to recover lost packets

•  Music and Video streaming

–  OK if some data get lost

–  First bits of data need not be delivered to the user fast, subsequent bits of data

need to be delivered to the user steadily in close succession

•  High latency is tolerable, but large jitter is not

•  Telephony (voice-only or video chat)

–  OK if some data get lost

–  Packets must be delivered to the user quickly and steadily in close succession

•  Neither high latency nor large jitter is tolerable

•  “Mission-Critical,” High-Frequency Trading, “First-Person Shooter” Gaming

–  Data must be transmitted and delivered quickly and reliably





Techniques to Deal with Packet Switch Issues

Issue: Packet Loss

–  Technique to deal with packet loss

•  Have the sender retransmit the missing packets

Issue: Packet Duplication

–  Technique to deal with duplicates

•  Receiver to discard the duplicates

Issue: Packet Arrives Out-of-Order

–  Technique to deal with out-of-order arrival

•  Receiver to cache and rearrange packets into correct order





Common Question

How to know that a packet is lost, duplicated, or

has arrived out-of-order to be begin with?

–  Answer: Use sequence numbers

–  Further issue:

•  Sequence number space cannot be infinitely large

•  Have to deal with sequence number wrap-around

–  For efficiency, generally design protocols with sequence number

space to be large enough to uniquely identify the maximum

number of “in-flight” packets that can be accommodated in a link or connection …

–  … which is in turn dependent on the link or connection’s

maximum speed and minimum packet size


When is QoS needed?

When would we need QoS?

– Under what circumstances would we need QoS?

– Under what circumstances would QoS be helpful?

100%   0  

QoS  not  needed   helpful  QoS  is   QoS  not  helpful  


QoS Basic Principles

Resource (capacity) reservation

– May be constant or statistical

Limiting input

– Rate-limiting, traffic shaping

Prioritize, prioritize, prioritize

– First come not necessary first served

•  Packet Queuing, Packet Scheduling


Resource Reservation

Signal the switches/routers to reserve capacity

from end to end before sending user data

– Example: RSVP (RFC 2205)



Rate-Limiting, Traffic-Shaping

Limit the input to the network

–  Limit the load put onto the network


–  Leaky Bucket

•  Need only two parameters to specify traffic profile

–  Leak rate

–  Bucket size

–  Random Early Detection/Discard (RED)

•  Used with self-limiting sources (e.g., TCP)

–  Artificially trigger the sources’ self-limiting features

In pu t     O utp ut   Overflow   (Discard)  



Classify, then Prioritize

Packet classification

–  By type

•  Usually by protocol or by explicit indication of class or type in

packet header

–  By source/destination

–  By flow

•  Combination of type, source, destination

! May be collapsed into a single identifier inside the network





Example: DiffServ

DiffServ classes of service

– Expedited Forwarding (EF)

•  Low loss, low latency, meant to be prioritized above all else

•  Stricter admission control and policing

– Assured Forwarding (AF)

•  Four priority classes

•  Three different drop rates (high, med, low) for each class

– Default





Example: 802.1Q-2005

IEEE 802.1Q-2005 classes of service

– NC – network control (highest priority)

– IC – internetwork control

– VO – voice, < 10 ms latency & jitter

– VI – voice, < 100 ms latency & jitter

– CA – critical application

– EE – excellent effort

– BE – best effort





Packet Classi


cation by Source / Destination

Commercial Motivation

– Provide differentiated (premium) services to customers who are willing to pay for it

– Support “Service Level Agreement” (SLA)

•  Guaranteed capacity / performance for paying customers

Policy Impact

– Net Neutrality

•  “Differentiated service” is also “discriminated service”

•  Society may want to have a say as to what forms of “discrimination” should be discouraged





Packet Classi


cation and Label Mapping

•  For efficiency or by necessity, classify at the “edge”

–  Map into a label (single identifier)

•  Switch or route by label in the “core”



Packet Queuing, Packet Scheduling

Multiple packets waiting to be transmitted

– Which one gets to go first?


– First In First Out (FIFO)

– Strict priority

– Maximize throughput

•  Relevant in blocking networks and channels with time-varying capacity (e.g., radio frequency channels)

– (Weighted) Fair Queuing

Does  maximizing   throughput  not  

always  lead  to   beYer  QoS?  


Maximizing Throughput

Blocking network consideration

– Relevant to a switch or router’s internal interconnection network


Maximizing Throughput

Channels with time-varying capacity


(Weighted) Fair Queuing

Fair queuing seeks to evenly distribute network

capacity to all users

– Fairness may we “weighted”


– Packet-by-Packet Round Robin

•  Fair only if all packets are the same size

– Bit-by-Bit Round Robin (Theoretical / Idealized)

•  Not practical, but provides important theoretical framework that guides subsequent works

–  Initially proposed by Demers, Keshav, Shenker in 1989


(Weighted) Fair Queuing

Practical (implementable) fair queuing schemes

–  Fair Queuing by Demers, Keshav, Shenker

•  Virtual Clock FQ by Zhang

–  Stochastic Fair Queuing

–  Random Early Detection / Discard (RED)

–  Deficit Round Robin (DRR) by Shreedhar & Varghese

•  Distributed DRR (dDRR) by Baw Ch’ng

–  Proportionally Fair (for wireless links)

–  etc.

Generalized, “weighted” versions of the above are





Proportionally Fair Scheduling

•  A compromise between two competing considerations

–  Maximizing network throughput

–  Guaranteeing some minimal level of service to link-wise disadvantaged users

•  Scheduling priority is inversely proportional to

anticipated per-unit throughput resource consumption


•  Used in wireless air


•  A modified version





Fair Queuing & DiffServ

Recall DiffServ classes:

– Expedited Forwarding

•  Strict priority

– Assured Forwarding (4 classes, 3 drop rates)

•  Weighted fair queuing among classes

•  Weighted RED within each class

– Default (Best-Effort)


QoS In Every Day Services

•  You benefit from QoS every time you …

–  Talk on the phone

–  Use cellular data (3G or 4G/LTE)

•  Modern telephony are packet switched

–  Common infrastructure used to carry “voice” and “data”

•  Contention between “voice” and “data” traffic

–  QoS is essential to meet user expectation for “acceptable call quality”

•  3G and 4G cellular data all implement Proportional Fair Queuing

–  Wireless spectrum is a scarce and valuable resource

–  Proportional Fair Queuing keeps network throughput high while


QoS in Networking Products

All enterprise class and carrier class switch and

router platforms support some subset of QoS


Consumer class routers marketed to gamers

Look for the following keywords in product

packaging or data sheets


Summary and Thought on QoS

•  Network capacity is always limited

–  Communications channel’s capacity is always limited

•  Demand for data transfer is potentially unlimited

–  Consumers can always dream up more ways to use more data

•  Unlimited demand have to contend for limited capacity •  Always a need to mediate such contention

–  QoS is essentially the field of study to mediate such contention

–  New technologies, new applications, new socio-economic

developments require constant re-evaluation of QoS goals and refinement of QoS techniques


Thank You




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