1 ATM Logical Connect ions Vir t ual channel connect ion (VCC) - - PDF document

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20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 1

ATM – Asynchronous Tr ansf er Mode

• General
• P

r ot ocol ar chit ect ur e

• Logical connect ions
• ATM cells
• Service cat egories
• AAL – ATM Adapt ion Layer

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 2

ATM - General

• Asynchr onous Transf er Mode
• Connect ion or ient ed packet swit ched net wor k
• Spr ings f r om t elephony, “high-speed swit ching”
• Cont r ol signaling

– Allocat e resources (QoS) – Traf f ic paramet er negot iat ion and usage monit oring

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 3

ATM – Prot ocol Archit ect ure

• Two layer s

– ATM Adapt ion Layer (AAL)

• Maps higher -layer inf or mat ion int o ATM cells

– ATM Layer

• Def ines t he t r ansmission of dat a in f ixed-size cells
• Def ines t he use of logical connect ions

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20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 4

ATM – Logical Connect ions

• Vir t ual channel connect ion (VCC)

– A var iable-rat e, f ull-duplex f low f or f ixed-size cells – Used f or user-user exchange, user-net work exchange and net work-net work exchange

• Vir t ual pat h connect ion (VP

C)

– A bundle of VCCs t hat have t he same end point s

• To set up a virt ual channel, t her e must f irst be a

VP C t o t he requier ed dest inat ion node wit h suf f icient available capacit y t o suppor t t he VCC, wit h t he appropr iat e qualit y of service

• Fig 4.3

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 5

ATM - Cell f ormat

• UNI - User -Net wor k I nt er f ace

– GFC: Gener ic Flow Cont rol - (lokal kont roll) – VPI : Vir t ual Pat h I dent if ier – VCI : Virt ual Cir cuit I dent if ier – Type: Management , congest ion cont rol, AAL5 – CLP: Cell Loss Priorit y – HEC: Header Er ror Check (CRC-8)

• NNI - Net wor k-Net wor k I nt er f ace

– GFC är del av VPI

GFC HEC (CRC-8) 4 16 3 1 8 VPI VCI CLP Type Payload 384 (48 bytes) 8 20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 6

ATM – Service Cat egories

• Real-t ime ser vice

– Const ant bit rat e (CBR)

• Fixed dat a r at e
• Relat ively t hight upper bound on t r ansf er delay

– Real-t ime variable bit r at e (rt -VBR)

• Tight ly const r ained delay and delay var iat ion (t ime-sensit ive

applicat ions)

• The t r af f ic is somewhat bur st y
• The r t -VBR ser vice allows t he net wor k mor e f lexibilit y t han

CBR

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20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 7

ATM – Service Cat egories (2)

• Non-r eal-t ime ser vice

– Non-real-t ime variabel bit rat e (nrt -VBR)

• The end syst em specif ies a peak cell rat e, a sust ainable or

avar age cell rat e, and a measur e of how bur st y or clumped t he cells may be

• The net wor k allocat es r esour ces t o pr ovide r elat ively low

delay and minimal cell loss

• Can be used f or dat a t r ansf er s t hat have cr it ical r esponse-

t ime r equir ement s

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 8

ATM – Service Cat egories (3)

• Non-r eal-t ime ser vice (cont )

– Unspecif ied bit rat e (UBR)

• Make use of unused capacit y (not used by any ot her

cat egor y)

• Cells ar e f orwar ded on a f ir st -in-f ir st -out (FI FO) basis
• Delays and var iable losses ar e possible
• A best -ef f or t ser vice

– Available bit r at e (ABR)

• The end syst em specif ies a peak cell rat e (PCR) and a

minimum cell r at e (MCR)

• The net wor k allocat es r esour ces f or MCR capacit y
• Unused capacit y is used up t o PCR
• Congest ion f eed-back
• Fig 4.8

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 9

ATM – ATM Adapt ion Layer (AAL)

• Two logical sublayers

– The convergence sublayer (CS)

• Pr ovides t he f unct ions needed t o suppor t specif ic

applicat ions using AAL

• Each AAL user at t aches t o AAL at a ser vice access point

(SAP)

• Ser vice dependent

– The segment at ion and reassembly sublayer (SAR)

• Responsible f or packaging inf or mat ion r eceived f r om CS

int o cells f or t r ansmission and unpacking t he inf or mat ion at t he ot her end

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20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 10

ATM – AAL

• AAL - ATM Adapt ion Layer

– AAL 1 and 2 f or applicat ions wit h r eal-t ime const raint s – AAL 3/ 4 f or packet dat a – AAL 5 alt ernat ive t o AAL 3/ 4

AAL ATM AAL ATM

… …

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 11

ATM - AAL 3/ 4, AAL 5

• AAL 3/ 4

– Fig 4.10

• AAL 5

– Fig 4.12

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 12

ATM – Summary

• General
• P

r ot ocol ar chit ect ur e

• Logical connect ions
• ATM cells
• Service cat egories
• AAL – ATM Adapt ion Layer