Quality of Service Anjali Agarwal
Quality of Service
» the ability of a network to provide better service to selected network traffic over various technologies including Ethernet, Wireless networks, IP-routed networks, ATM, and FR that may use any or all of these underlying technologies
» method to provide preferential treatment to some arbitrary amount of network traffic, as opposed to all traffic being treated as “best effort”.
What is QoS
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Factors affecting QoS
Delay – end-to-end
Jitter (Delay Variation) – causes signal to be
distorted
Packet Loss and Out of Order Packets
Bandwidth Available
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Factors affecting QoS
Delay
• Accumulation (speech samples to frame and frames to form packets)
• Processing (gateways and routers time to process the packets)
• Queuing (packets wait for their turn)
• Emission (minimal for small packets and/or high bandwidth links
• Propagation
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Factors affecting QoS
Delay (cont.)
5 ms 30 ms
Propagation Coding Packetization
20 ms Transmission Queuing (Jitter)
30 ms Decoding Depacketization Playout Buffering
5 ms Propagation
90 ms One-Way
Gateway
IP Network
PSTN
Telephone
Telephone
Delay results in echo and talker overlap
» echo if round trip delay > 50ms
echo cancellers (ITU recommendation G.168 )
»Talker overlap if one way delay > 250ms Concordia University Anjali Agarwal
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Gateway
Factors affecting QoS
Network Problems
• Voice packets are continuous – no gap between packets
• Due to variations in inter-packet arrival time, gaps known as jitter occur between packets
Router
– must be removed by receiving gateway by collecting packets in buffers
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Factors affecting QoS
Network Problems (cont.)
Packet Loss: Congestion may drop some packets
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•Missing packets are detected
•the last received packet re-played at a decreased
volume
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Factors affecting QoS
Network Problems (cont.) Out of Order Packets: due to different routes
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•Missing packet replaced by its last received packet as if it is lost
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Factors affecting QoS
Bandwidth
Maximal data transfer rate that can be sustained between two endpoints – limited by
• physical infrastructure of traffic path
• number of other flows which share this path
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Voice Quality
– fidelity of the reproduced speech
– intelligibility (ability to extract the information)
Factors influencing delivery of high voice quality
clarity
packet loss
speech codecs
silence suppression
comfort noise generation end-to-end delay
echo
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Factors influencing delivery of high voice quality 1. Clarity
– telephone devices through the quality of loudspeaker and microphone – echo generated between speaker and microphone
– Gateways that attach to PSTN
– type of transcoding, speech codecs – possible silence suppression
– comfort noise generator
– IP network
– excessive jitter and packet loss
– Generic Media Gateways – speech codecs
– voice activity detection mechanism
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Factors influencing delivery of high voice quality 2. Packet Loss
– routers discard packets when experiencing congestion – late arriving packets equivalent to lost packet
– information must arrive in certain time window
– retransmissions add an extensive delay
– Flow control and traffic prioritization algorithms necessary to avoid packet loss – Modern routers must be designed to implement such schemes
– RSVP
– MPLS
– traffic classification with Differentiated Services
– PSTN does not suffer from this – bandwidth reserved for duration of the call Concordia University Anjali Agarwal 14
Factors influencing delivery of high voice quality 3. Speech Codecs
– transforms between analog voice and digital bit-streams
– may use compression techniques (removing redundant information)
– compromise between – voice quality
– computation power – delay
– network bandwidth
– lost or severely damaged information has a more noticeable effect on lower bit-rate speech codecs than with G.711
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Factors influencing delivery of high voice quality 4. Silence Suppression
(Voice Activity detection)
– no packets send when caller silent
– VAD can realize reduction in bandwidth requirement
5. Comfort Noise Generation
(complimentary of VAD)
– muting the channel gives impression that channel has gone dead – dead-air syndrome
– low-level noise signal generated at receiver
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Factors influencing delivery of high voice quality 6. End-to-End Delay
Packet delay primarily determined by the following – Packet capture delay
– Switching or routing delay
– Queuing time
Gateways and Terminals
– Voice Signal processing at sending and receiving sides – Delay due to packet variation at receiver side
– Packetization delay at the transmit side
7. Echo
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Service Levels
refers to the actual QoS capabilities of a specific network application from end-to-end, with some level of control over bandwidth, jitter, delay, and loss, provided by the network
Different Service Levels
» Best-Effort Service
» Differentiated Services » Guaranteed Services
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Different Service Levels
Best-Effort: lack of QoS
» no priorities or guarantees
» basic queuing with FIFO packet delivery » e-mails and general file transfers
Differentiated Service: qualitative QoS / Soft QoS
» treats some traffic better than the rest
faster handling, more bandwidth on average, lower loss rate on average no hard and fast guarantee
» course level of packet classification each class receiving a particular QoS
» lower delay for mission-critical interactive applications, packet voice applications
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Different Service Levels
Guaranteed Service: quantitative QoS / Hard QoS
» absolute reservation of network resources, typically bandwidth
implies reservation of buffer space appropriate queuing disciplines
» for applications requiring a fixed delay and for delay- sensitive traffic, such as voice and video
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QoS mechanisms Divided into basic groups that align with OSI model
Physical Layer
» can provide for alternate physical path for redundancy
can provide differentiated services if paths have different characteristics (best-effort on lower, QoS traffic on higher speed)
Data Link Layer
» MAC to provide service differentiation
»ATM
CBR and VBR best suited for telephony and voice applications, and
multimedia applications such as video
ABR and UBR for best-effort delay-insensitive traffic such as file transfers and e-mail
inherent complexity of ATM and its QoS mechanisms
provides only part of the end-to-end path for mostly TCP/IP networks
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Data Link Layer (cont.) » Frame Relay
QoS mechanisms (cont.)
Committed Information Rate (CIR) confirms network delivery
» IEEE 802.1p (bright future for Ethernet technology)
allows preferential queuing on the basis of “priority” value
provides consistent method for Ethernet, Token Ring, etc.
3-bit priority field ( 0 assigned as lowest priority and 7 as the highest priority)
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QoS mechanisms IEEE 802.1p
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QoS mechanisms IEEE 802.1p
Proposed IEEE 802.1p Priority Values and Associated Traffic Types
Priority
Binary
Traffic Types
7
111
Network Control
6
110
Interactive Voice
5
101
Interactive Multimedia
4
100
Controlled Load Applications (Streaming Multimeia)
3
011
Excellent Effort
0
000
Best Effort (Default)
2
010
Spare
1
001
Background
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Network Layer
QoS mechanisms (cont.)
» IP precedence: the three precedence bits in the IPv4 header’s Type of Service (ToS) field utilized to specify class of service for each packet.
» Packet marking: The ingress router must mark the packets as they enter the network with appropriate values so that interior routers can handle packets differentially.
» Packet classification: Routers must check all received packets to determine if the packets should receive differential treatment.
» Packet queuing: The routers may employ multiple queues along with some scheduling disciplines such that delay-sensitive traffic will be serviced sooner.
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QoS mechanisms (cont.)
Packet queues
Marked packets Packet
arrived
classification Scheduling
Packet classification and queuing
Transmit queue
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QoS mechanisms (cont.)
Scheduling Algorithms:
– FIFO queuing – a traditional IP router (basic store and forward)
– fair algorithm, same delay is imposed on all queued packets – Priority queuing
– queue for each distinct priority levels
– serviced in order of priority with highest priority traffic receiving minimal delay
– lower priority may be prevented from being serviced
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Scheduling Algorithms (cont.):
– Weighted Round-Robin: when network is shared with minimal bandwidth or latency requirements
– bandwidth is guaranteed at a potential congestion point, however, no application achieves more than a predetermined proportion of overall capacity
– Queues serviced round-robin in proportion to a weight assigned for each queue
QoS mechanisms (cont.)
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Scheduling Algorithms (cont.): – Weighted Fair Queuing:
QoS mechanisms (cont.)
– interactive traffic is scheduled in front of the queue to reduce response time
– the remaining bandwidth is fairly shared among high- bandwidth flows
– ensures queues do not starve for bandwidth
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QoS mechanisms (cont.)
Transport and Application Layer
» packets may be marked and classified at these layers
» routers can use port numbers, however will have to locate transport-level header that might be behind optional IP header
» routers need to know many application-level protocols
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End-to-End Implementation
Every element in the network path should deliver its part of QoS
Access device
QoS in the access network
Edge Router Edge Router
QoS in the backbone network
Access device
QoS in the access network
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End-to-End Implementation (cont.)
Edge Router
ATM Core
IP Network
Edge Router
Delay-sensitive traffic channeled to ATM core
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Service Level Agreements
A service contract between a customer and a service provider
specifies the service classes supported and the amount of traffic allowed in each class.
Static SLAs – negotiated on a regular, e.g. monthly and yearly, basis. Dynamic SLAs must use a signaling protocol, to request for services
on demand.
The classification, policing and shaping rules used at the ingress routers are derived from the SLAs.
The amount of buffering space needed for these operations is also derived from the SLAs.
DS field may be re-marked, as determined by the SLA between the two domains.
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