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ATMATM Asynchronous Transfer Mode An Introduction to John A. Clark.

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Presentation on theme: "ATMATM Asynchronous Transfer Mode An Introduction to John A. Clark."— Presentation transcript:

1 ATMATM Asynchronous Transfer Mode An Introduction to John A. Clark

2 ATM VOICE VIDEO DATA Unlike other standards, ATM has been designed from the outset to support VOICE, VIDEO and DATA. ATM SWITCH

3 ATM Unlike contemporary shared bandwidth LANs, ATM uses parallel bandwidth allocation for each user 10 Mb/s User bandwidth = 10 Mb/s 1 Ethernet/Token Ring User 5 Mb/s User bandwidth = 5 Mb/s 2 Ethernet/Token Ring Users

4 ATM Unlike contemporary shared bandwidth LANs, ATM uses parallel bandwidth allocation for each user 155 Mb/s User bandwidth = 155 Mb/s 1 ATM User User bandwidth = 155 Mb/s 2 ATM Users 155 Mb/s Provided the switches have sufficient aggregate bandwidth, NO degradation will occur under any load - NON BLOCKING SWITCH Bandwidth is allocated initially and is then reserved: thus it can never be exhausted

5 ATM Unlike traditional variable packet size LAN standards, ATM uses fixed length CELLS Ethernet or Token Ring ATMI.D. PAYLOAD 5 bytes 48 bytes 53 BYTES TOTAL The payload contains protocol independent data of ANY bit pattern - these 48 bytes are completely un-interrogated by the network switches The lack of a trailing payload CRC field allows very rapid CUT-THROUGH switching, based on the VCI (Virtual Channel Identifier) in the ID

6 ATM Although the fixed cell size makes ATM similar to Time Division Multiplexing (TDM), empty time slots waste bandwidth on TDM where no data is ready ATM TDM 1 2 3 4 1 2 3 = Data frame = Empty slot = Data cell = Video cell = Voice cell ATM channels are comprised of a continuous stream of voice, video and data cells - The small cell size is a result of the delays concerned with packetising digitised voice at 64 Kb/s that give unacceptable between the North American proposal of 69 bytes and the European proposal of 36 bytes!

7 ATM Unlike the connectionless approach used in Bus or Ring Layer 2 standards, ATM is connection orientated Physical Data Link Data Link Network Transport Application Session Presentation ATM ADAPTATION ATM The ATM layer is concerned with the cell structures and handling of virtual connections - The ATM Adaptation Layer (AAL) is where a higher layer service (voice, video or data) is adapted for transport over the ATM network

8 ATM Unlike the connectionless approach used in Bus or Ring Layer 2 standards, ATM is connection orientated The ATM switch simply maps input and output port numbers to Virtual Channel Identifiers to produce a true end to end switching capability ATM SWITCH 11 22 41 42 3 5VCIPORTPORTVCI In Port VCI Out Port VCI3 5 11 42 5 41 322 Thus, ATM is essentially fixed size fast packet switching, superimposed on the circuit switching scheme

9 ATM Standards now approximately 2/3 complete ATM FORUM Ad-hoc committee CCITT Consultative committee for International Telegraph & Telephony - now the ITU - International Telecommunications Union Collective agreements DE FACTO STANDARDS FORMAL STANDARDS In practice, the ATM Forum produces all standards concerned with LAN services (e.g. Ethernet, Token Ring etc.). The CCITT is not involved with this at all and concentrates heavily on telephony and PTT usage of ATM - The IETF (Internet Engineering Task Force) also defines the encapsulation of various protocols over ATM in RFCs (Request For Comment), such as RFC1483 (multiprotocol), RFC894 (Ethernet), RFC1042 (Token Ring) etc.

10 ATM ATM LANs ran first in 1992 ATM HISTORY * 1983First ATM papers published * 1987CCITT selected ATM for B-ISDN * 1990CCITT issued 13 ATM recommendations * 1992First ATM LANs installed * 1993First ATM WANs installed Several manufacturers have ATM switch products shipping already (e.g. Bay Networks !) and many will role out switches during 1995 (e.g. 3Com, Chipcom, Dec etc.). Public WAN services could be several years away - The European ATM WAN pilot scheme was launched in Brussels and Dublin during November 94 and involved 15 cross connects running over 34 Mb/s E3 bearers.

11 ATM Private UNI, the NNI & public UNI PUBLIC ATM PRIVATE ATM UNINNIUNI The private UNI (User/Network Interface) and NNI (Network/Network Interface) define the physical, electrical, optical, management and data structures between the edge station and switch and between two switches respectively - as the whole private network is considered a user edge to the PTOs, the interface to the public ATM switch is also denoted as a UNI The PNNI (Private NNI) is yet to be ratified by the ATM Forum (Q3 95?), when the functionality will be extended to re-route failing switch links

12 ATM PVCs & SVCs PVC PVCs (Permanent Virtual Circuits) must be statically configured using network management before the circuit is used SVCSignalling SVCs (Switched Virtual Circuits) are dynamically configured using signalling - Two ATM Forum signalling standards exist (V3.0 & V3.1) as well as the ITU Q2931 standard - It will be the extension of Q2931 that will re-route down NNI links Signalling between UNI NNI etc. requires global addressing agreement!

13 ATM ATM uses network address & telephone numbers VOICE E.164 ISDN Addresses DATA 48 bit MAC addresses VIDEO No Addresses The addressing scheme must account for any virtual circuit, regardless of the type of traffic - Three forms currently exist : DCC (Data Country Code), ICD (International Code Designator) and E.164 Private Address: 45 E.164 ISDN BCD Number MAC Address E.164 addressing is generally used with Q.2931 signalling today over the a version of the ATM Adaptation Layer called AAL5

14 ATM ITU have defined 5 Classes of Service The ITU B-ISDN 5 classes of service have been designed to account for the fact that voice, video and data have differing requirements on the bit rate VOICE CONSTANT BIT RATE DATA VARIABLE BIT RATE VIDEO CONSTANT BIT RATE A B C D A B C D Yes Yes No No Cnst Var Var Var Yes Yes Yes No End to end timing Bit Rate Connection orientated ITU SERVICE CLASS AAL5 - SEAL AAL5, sometimes known as SEAL (Simple & Efficient Adaptation Layer) was the ATM Forums response to the ITUs complex AAL3/4 - AAL5 is being increasingly used for data and variable data rate video and audio

15 ATMATM LUNI the LAN Emulation UNI LAN Emulation Server Broadcast Unknown Server LAN Emulation Client Client The LAN Emulation part of the UNI is needed to translate Ethernet or Token Ring MAC addresses to ATM addresses and to mimic broadcasts in the ATM connection orientated environment The standard defines an LEC (Lan Emulation Client), an LES (Lan Emulation Server) which does the address mapping and a BUS (Broadcast Unknown Server) which provides broadcasts for ARP etc.

16 ATM 0 10 100 1000 1985 1990 1995 386 486 MIPS Token Ring 16 Mb/s Ethernet 10 Mb/s FDDI 100 Mb/s ATM 155 Mb/s ATM 622Mb/s ATM 2 Gb/s ALPHA,R6000SPARC,Pentium AMDAHLs LAW : 1 MIP requires 1 Mb/s I/O Fibre optic interfaces running 155 Mb/s SONET (Synchronous Optical NETwork) / SDH (Synchronous Digital Heirachy) standards are available now - these will be scaled to 622 Mb/s. 155 Mb/s Interfaces for Category 5 UTP are also available. ATM is fast enough for the foreseeable future !

17 ATM Data Rate SDH Designation SONET Designation Framing Type 51.84 Mb/s -------------OC-1STS-1 155.52 Mb/s STM-1 OC-3STS-3 622.08 Mb/s STM-4 OC-12STS-12 1.244 Gb/s STM-8OC-24 STS-24 2.488 Gb/s STM-16OC-48 STS-48 STM-n : Synchronous Transfer Mode - n x 155.52 Mb/s OC : Optical Carrier STS : Synchronous Transport Signal ATM is fast enough for the foreseeable future !

18 ATM Floor Hubs Desk Stations BasementSwitch Bridge/Router 100 Mb/s FDDI 10 Mb/s 10BaseT 64Kb/s - 34 Mb/s E1/E3/ISDN Frame Relay/ SMDS 10 Mb/s 10BaseT Todays typical collapsed backbone LANs, with switched or routed basement hubs and varied WAN bearer services are anything but homogeneous Fibre Campus backbone Category 5 UTP

19 ATM Initially, the availability of ATM switch modules for basement and top range floor hubs will make 155 Mb/s fibre risers and campus backbones a viable upgrade - Compute intensive workstations will also take advantage of 12/16 port fibre ATM switches to gain 155 Mb/s dedicated pipes at competitive price per port - 100 Mb/s switching hubs are likely to offer 155 Mb/s fibre interfaces, or Cat 5 copper interfaces Floor Hubs Desk Stations BasementSwitch 100 Mb/s 100BaseT Category 5 UTP 155 Mb/s ATM Fibre Campus backbone 155 Mb/s ATM Risc/PentiumWorkstation ATM Fibre 64Kb/s - 34 Mb/s E1/E3/ISDN Frame Relay/ SMDS

20 ATM ATM switching Floor Hubs Desk Stations ATM Switch ATM Switch 155 Mb/s ATM Public ATM Bearers Eventually, when the PTOs make full wide area ATM bearer services available, it will be possible to implement a completely ATM homogeneous LAN/WAN supporting voice, video and data for bandwidth hungry, multimedia applications PABX ATM Edge Switch 2 Gb/s ATM Fibre Campus backbone VideoConference 155 Mb/s ATM 622 Mb/s ATM ATM Cat 5 UTP


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