Transcript Teenus_77 - Raadio- ja sidetehnika instituut

Kommunikatsiooniteenuste
arendus
IRT0080
Loeng 7
Avo Ots
telekommunikatsiooni õppetool,
TTÜ raadio- ja sidetehnika inst.
[email protected]
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Packet switching versus circuit switching
packet switching
• Great for bursty data
– resource sharing
– simpler, no call setup
• Bad for applications with hard resource requirements
– Excessive congestion: packet delay and loss
– Need protocols for reliable data transfer, congestion control
– Applications must be written to handle congestion
• How to provide circuit-like behavior?
– bandwidth guarantees needed for audio/video apps
– Common practice: over-provision
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How do loss and delay occur?
packets queue in router buffers
• packet arrival rate to link exceeds output link capacity
• packets queue, wait for turn
• when packet arrives to full queue, packet is dropped (aka lost)
– lost packet may be retransmitted by previous node, by source end system,
or not retransmitted at all
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
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Four sources of packet delay
• 1. nodal processing:
• 2. queueing
– check bit errors
– determine output link
– time waiting at output
link for transmission
– depends on congestion
level of router
transmission
A
propagation
B
nodal
processing
queueing
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Delay in packet-switched networks
3. Transmission delay:
• R=link bandwidth (bps)
• L=packet length (bits)
• time to send bits into link
= L/R
transmission
A
B
nodal
processing
4. Propagation delay:
• d = length of physical
link
• s = propagation speed in
medium (~2x108 m/sec)
• propagation delay = d/s
s and R are very
different quantities!
propagation
queueing
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Nodal delay
d nodal  d proc  d queue  d trans  d prop
• dproc = processing delay
– typically a few microsecs or less
• dqueue = queuing delay
– depends on congestion
• dtrans = transmission delay
– = L/R, significant for low-speed links
• dprop = propagation delay
– a few microsecs to hundreds of msecs
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Queueing delay
• R=link bandwidth (bps)
• L=packet length (bits)
• a=average packet arrival
rate
traffic intensity = La/R
• La/R ~ 0: average queueing delay small
• La/R -> 1: delays become large
• La/R > 1: more “work” arriving than can be
serviced, average delay infinite!
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Transport service requirements of common apps
Data loss
Bandwidth
Time Sensitive
file transfer
e-mail
Web documents
real-time audio/video
no loss
no loss
no loss
loss-tolerant
no
no
no
yes, 100’s msec
stored audio/video
interactive games
instant messaging
loss-tolerant
loss-tolerant
no loss
elastic
elastic
elastic
audio: 5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
elastic
Application
yes, few secs
yes, 100’s msec
yes and no
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QoS architectures for the Internet
1. Best effort: no guarantees
2. Integrated Services (IntServ): Per-flow guarantees
3. Differentiated Services (DiffServ): Per aggregate guarantees
2
1
Best Effort
2
3
Differentiate
d
Services
Integrated
Services
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Edge Router/Host Functions
• Classification: marks packets
according to classification
rules to be specified.
• Metering: checks whether the
traffic falls within the
negotiated profile.
• Marking: marks traffic that
falls within profile.
• Conditioning: delays and then
forwards, discards, or remarks
other traffic.
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Motivation
• Traffic sensitive flow can be redirected to
long path
• Loss sensitive flow can be redirected to
lossy network path
• Jitter sensitive flow may be sent along two
paths of high jitter/ or on a single path w/
queuing delay and have high jitter
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Traffic Engineering Objectives
• Traffic Engineering (TE) concerned with
performance optimization
• The key performance objectives
– traffic oriented e.g. minimization of packet loss
– resource oriented - optimization of resource
utilization e.g. efficient management of
bandwidth
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Objectives (cont’d)
• Minimizing congestion is a major traffic and
resource oriented performance objective
• Congestion manifest under two scenarios
– Network resources insufficient or inadequate
• Solved by capacity expansion or classical congestion control
techniques
– Inefficient mapping of traffic streams onto available
resources
• Reduced by adopting load balancing policies
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Example
• All links have capacity of 1000
300
1
600
1
1
2
2
300
What if these are the
delay sensitive flows?
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Voice over IP Issues
• modify capacity management and routing
methods in IP to support IP telephony
– delay less than 300ms
– loss rate <1%
• First Model: RSVP
• Second Model: voice service uses Virtual
Private Network
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RSVP Routing
• Shortest Path First
• Shortest Available Path First
• Widest Available Path First
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Virtual Private Networks
•
•
•
•
•
interconnects telephony switches
Direct Path Only
Success to the Top
State-Dependent Routing
Approximate State-Dependent Routing
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Lingid
http://en.wikipedia.org/wiki/Client_server
http://www.tarrani.net/mike/docs/TrafficEngineeri
ng.pdf
http://en.wikipedia.org/wiki/Traffic_engineering_
%28telecommunications%29
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