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

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 1 Jonny Pettersson, UmU 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 2 Jonny Pettersson, UmU 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 20 March 2001 Computer Networks, advanced course 3 Jonny Pettersson, UmU 1

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 4 Jonny Pettersson, UmU ATM - Cell f ormat 4 8 16 3 1 8 384 (48 bytes) GFC VPI VCI Type CLP HEC (CRC-8) Payload • 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 20 March 2001 Computer Networks, advanced course 5 Jonny Pettersson, UmU 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 20 March 2001 Computer Networks, advanced course 6 Jonny Pettersson, UmU 2

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 7 Jonny Pettersson, UmU 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 8 Jonny Pettersson, UmU 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 20 March 2001 Computer Networks, advanced course 9 Jonny Pettersson, UmU 3

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 AAL … … ATM ATM 20 March 2001 Computer Networks, advanced course 10 Jonny Pettersson, UmU ATM - AAL 3/ 4, AAL 5 • AAL 3/ 4 – Fig 4.10 • AAL 5 – Fig 4.12 20 March 2001 Computer Networks, advanced course 11 Jonny Pettersson, UmU 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 20 March 2001 Computer Networks, advanced course 12 Jonny Pettersson, UmU 4