introduzione - sti.uniurb.it didattico/introduzione_1.pdf · sistemi di comunicazioni multimediali...

Legal Informatics – Computer law01 (Information Society

Domenico Condello isti information science and technology institute 1/number

Università degli studi di Urbino “Carlo Bo”Informatica ApplicataSistemi di Comunicazioni Multimediali

Massimo Parrucci Facoltà di Scienze e Tecnologie 1

Introduzione

Parte 1





QoS Specification

USER

Application level

System level (OS and network)

Multimediadevice

Networksubsystem





Jitter Control

(a) High jitter. (b) Low jitter.





Quality of Service

• Requirements• Techniques for Achieving Good Quality

of Service• Integrated Services• Differentiated Services• Label Switching and MPLS





Network OS Parameters

• Bandwidth• Jitter• Delay• Reliability





RequisitiHow stringent the quality-of-service

requirements are.5-30





QoS Framework

• Static functions– Traffic and QoS specifications (traffic

types/parameters)– QoS negotiation and signalling– Admission control– Resource reservation

• Dynamic Function– Traffic shaping and policing– Queuing and scheduling– Congestion control





Traffic source types

• CBR Constant Bit Rate: Transmit traffic at a fixed rate, such as 64Kbps voice

• VBR Variable Bit Rate: traffic rate is notfixed





Traffic parameters

• Different flows have different trafficpatterns

• A give traffic pattern can be describedusing several traffic parameters– Peak rate– Average rate– Burst size





Traffic parameters

Burst size

Peak rate

bps

time

Average rate





Traffic patternsAll patterns have the same average rate (10kbps), but different peak rateand burst size

time

10Kbps

time

time

20Kbps

40Kbps





QoS Parameters• Required QoS can be defined by several

parameters• Delay: how long it takes for a packet to

traverse the network?• Jitter: what is the variance nel delay?• Loss: how often packets get lost in the

network and never show up at the destination”?





Signalling• Signalling is a mechanism used by the users to

communicate QoS related infomation to the network before the communication start

• Static: relevant table entries are createdmanually

• Dynamic Negotiation: the users passes its trafficand QoS parameters and their associatedvalues to the network and the network providesdetails of the QoS guareentees to the user.

• Automatic negotation is called signaling





Admission Control

• First line of defence against attacks on QoS

• Network should not commit any guaranteeif availableresources are not enough tomaintain requested QoS

• Two Ways– During QoS signalling





Resource reservation

• To guarantee any QoS, network resourcesmust be reserved in advance

• Type of network resources– Bandwidth– Buffer space– CPU time

• Reservation could be dynamic, usingsignalling, or static (manual)





How much to reserve

• Easy for constant bit rate sources (CBR)– Reserve at the peak rate

• Difficult for VBR sources– Peak rate reservation wastes bandwidth (no

statical gain)– Average rate reservation may cause

excessive packet delay





Admission Control 1/2

• First line of defence against attacks on QoS

• Network should not commit any guaranteeif availableresources are not enough tomaintain requested QoS

• Implementation– Dynamic: using signalling protocol/sw– Static: manual process (no signalling

required)





Admission Control 2/2

• The admission control function isperfomed by means of two techniques, called traffic policing and traffic shaping

• Traffic policing is perfomed through twoalgorithms: leaky bucket and token bucket





Traffic shaping

• Questa tecnica fluidifica il traffico sul lato server anziché sul lato client

• Il traffic shaping regola la velocità media della trasmissione dei dati





Buffering

Smoothing the output stream by buffering packets.





The Leaky Bucket Algorithm

(a) A leaky bucket with water. (b) a leaky bucket with packets.





The Token Bucket Algorithm

(a) Before. (b) After.

5-34





Token bucket 1/2

• S la lunghezza della raffica (secondi)• C capacità del token bucket• ρ Byte/sec la velocità di arrivo dei token• M byte/sec la velocità di output massima• Una raffica di output contiene al max

– C + ρS byte





Token bucket 2/2

• Numero di byte in una raffica lunga S secondi è di MS

• Quindi– C + ρS = MS

• Risolvendo– S=C/(M-ρ)





Esempio

• C=250KB• M=25MB/sec• ρ= 2mb/sec





The Leaky Bucket Algorithm

(a) Input to a leaky bucket. (b) Output from a leaky bucket. Output from a token bucket with capacities of (c)250 KB, (d) 500 KB, (e)750 KB, (f) Output from a 500KB token bucket feeding a 10-MB/sec leaky bucket.





Le slide riprodotte in questa presentazione sono esclusivamente di ausilio alla didattica. Alcune illustrazioni sono estratte da “A. S. Tanenbaum Competer network” 4th editionPrentice Hall

introduzione - sti.uniurb.it didattico/introduzione_1.pdf · sistemi di comunicazioni multimediali...

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