Quality of Service (QoS)
in networking and telecommunications
Baw Ch’ng
info@bawman.com
Presented to:
Biographical Background
BAWMAN LLC
• 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
performance
• 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
need
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
useful
• 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
Side
bar
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
Side
bar
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)
A
Rate-Limiting, Traffic-Shaping
•
Limit the input to the network
– Limit the load put onto the network
•
Examples:
– 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)
Prioritization
•
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
Side
bar
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
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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
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Packet Classi
fi
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
Side
bar
Packet Classi
fi
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”
A
Packet Queuing, Packet Scheduling
•
Multiple packets waiting to be transmitted
– Which one gets to go first?
•
Options
– 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”
•
Examples
– 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
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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
TransmiYer
• Used in wireless air
interfaces
• A modified version
Side
bar
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
standards
•
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
