ATM Asynchronous Transfer Mode these slides are based on USP ATM slides from Tereza Carvalho 1 ATM Networks Outline ATM technology designed as a support for ISDN Definitions: STM and ATM Standardization bodies: ITU-T and ATM Forum ATM Basics: UNI and NNI interfaces transmission paths, virtual paths and virtual channels ATM cells Reference Model 2 1
ATM Basics ITU-T approved standard for Broadband ISDN packet switching technique with fixed length packets (53 bytes) cells and cell switching support for multimedia applications (data, voice and video) transmission media high-speed data links ATM service model includes support to constant bit rate variable bit rate available bit rate CBR VBR ABR 3 support for burst traffic ATM Basics support for applications with requirements regarding: packet loss packet delay support for dynamic allocation of bandwidth according to the current traffic demand no retransmission on a link to link basis erroneous cells that cannot be fixed are discarded 4 2
Characteristics: Cell Switching evolution of Frame Relay based on the very low BER of the current physical media fixed length cells processing overhead is reduced dynamic bandwidth allocation 5 Characteristics: Cell Switching several logic connections multiplexed over a single PHY interface end-to-end error and flow control are NOT implemented constant or variable bit rate services data, voice and video over a single network the number of networks needed is reduced* 6 3
Computer 1 B C Computer 3 A D ATM Switch Computer 2 E F Computer 4 cell1 Cell Switching cell 2 cell 3 cell 4 cell 5 cell 1 cell 2 cell 3 cell 4 cell 1 cell 2 cell 3 A E F 4 7 STM Synchronous Transfer Mode a data unit is identified according to its position in the transmission frame bandwidth allocation is defined during the subscription phase transmission frames have a rigid structure that is periodically repeated throughput is fixed time slot trailer Chan 1 Chan 2... Chan n Chan 1 Chan 2... Chan n Chan 1 Chan 2... Synchronously repeated 8 4
ATM Asynchronous Transfer Mode transfer refers to transmission and switching aspects transfer mode is a specific way to transmit and switch data in a network asynchronous, in the context of a multiplexed transmission, refers to asynchronous dynamic bandwidth allocation according to the demand a data unit (cell) associated to any channel can appear at anytime i.e. any time slot an ATM network is based on cell transmission and switching a single interface for several kinds of services data, audio & video 9 ATM Asynchronous Transfer Mode dynamic bandwidth allocation asynchronously repetition of channels variable bit rate cell header Chan 1 Chan 3 Chan 7 FREE Chan 1 Chan 5 Chan 1 Asynchronously repeated 10 5
UNI and NNI Interfaces 2 interface types are defined in ATM UNI User Network Interface ATM switch and ATM user host private ATM switch and public ATM switch NNI Network Network Interface between public ATM switches between private ATM switches 11 UNI and NNI Interfaces ATM User Public Network ATM User ATM User Private UNI Private ATM Switch Private NNI Private ATM Switch Public UNI Public Network Public ATM Switch Public NNI Public NNI B-ICI* Public ATM Switch Public NNI Public ATM Switch 12 *BISDN Inter Carrier Interface 6
Virtual Paths and Virtual Channels starting with a good analogy the road system cells vehicles transmission paths (TP) virtual paths (VP) virtual channels (VC) highways directions lanes 13 Virtual Paths and Virtual Channels Transmission Path (TP) Göteborg VP4 Malmö VCC5 VP2 VP3 Transmission Path (TP) VCC5 VCC6 VCC1 VCC2 VCC3 VCC4 Karlstad VP1 = Highway VP2, VP3, VP4 = Secondary Road VP1 Stockholm VCC1 = 100 Km/h VCC2 = 80 Km/h VCC3 = 60 Km/h VCC4 = Service Lane VCC6 VCC1 VCC2 VCC3 VCC4 14 7
Virtual Paths and Virtual Channels a transmission path (TP) has: one or more virtual paths (VP) a virtual path (VP) has: one or more virtual channels (VC) VP VCs Transmission Path VP VP VCs VCs 15 Virtual Paths and Virtual Channels Switch VP4 Switch VCC5 VCC5 VCC6 VCC6 VP2 VP3 VCC1 VCC1 VCC2 VCC2 Switch VP1 Switch VCC3 VCC3 VCC4 VCC4 a VP and VC example 16 8
Virtual Paths and Virtual Channels Örebro VP10 Västerås VC11 Arvika A VC10 VC41 B Kil VP10 VP20 VC40 VP40 Karlstad VC42 VC42 Stockholm Communication between: A/C and A/D B/C and B/D VP50 VC44 VC14 VP60 VP30 Uppsala VC15 Kista 17 C D Virtual Paths and Virtual Channels Virtual Channel - VC unidirectional transport of ATM cells associated to a single VCI Virtual Channel Link - VC link unidirectional transport of ATM cells between a point where a VCI value is assigned and a point where that value is terminated Virtual Channel Connection - VCC a concatenation of VC links that extends between 2 points where ATM service users access the ATM layer 18 9
Virtual Paths and Virtual Channels Örebro VP10 Västerås VC11 Arvika A VC10 VC41 B Kil VP10 VP20 VC40 l1 VP40 Karlstad VC42 l2 VC42 Virtual Channel Link Stockholm VP50 VC44 Virtual Channel Connection (VCC) = l1 + l2 + l3 VC14 l3 VP60 VP30 Uppsala VC15 Kista 19 C D Virtual Paths and Virtual Channels Virtual Path - VP unidirectional transport of ATM cells of VC links that have the same destination Virtual Path Link - VP link a group of VC links, identified by a common VPI, between a point where a VPI value is assigned and a point where it is terminated Virtual Path Connection - VPC a concatenation of VP links that extends between 2 points where ATM service users access the ATM layer Transmission Path - TP between the network elements - the transmission medium 20 10
Virtual Paths and Virtual Channels Örebro VP10 Västerås VC11 Arvika A VC10 VC41 B Kil VP10 VP20 VC40 VP40 Karlstad VC42 VC42 Virtual Path Link Virtual Channel Link Stockholm VP50 VC44 VC14 VP60 VP30 Uppsala VC46 VC15 Kista 21 C D Virtual Paths and Virtual Channels VPI and VCI are used for cell switching switching can occur in different levels transmission path virtual path virtual channel VP switching VPI is changed VP switches terminate VP links VC switching VCI and VPI are changed VC switches terminate VC and VP links VPI and VCI are unique values for a single transmission path (TP) VPI and VCI have local meaning only 22 11
Virtual Paths and Virtual Channels VCI 1 VCI 2 VCI 3 VCI 4 VCI 5 VCI 6 VPI 1 VPI 2 VPI 3 VPI 4 VPI 5 VPI 6 VCI 3 VCI 4 VCI 5 VCI 6 VCI 1 VCI 2 VC Switch VCI 3 VCI 1 VCI 4 VCI 2 VP Switch Virtual Path Switching VCI 1 VCI 2 VCI 1 VCI 2 VPI 1 VPI 2 VPI 3 VPI 4 VPI 5 VCI 4 VCI 3 VCI 1 VCI 2 VP Switch Virtual Path and Virtual Channel Switching 23 Virtual Paths and Virtual Channels VCC - Virtual Channel Connection a concatenation of one or more VC links, where the VCI changes on every VC switch VPC - Virtual Path Connection a concatenation of one or more VP links, where the VPI changes on every VP switch 24 12
Virtual Paths and Virtual Channels VP switches terminate VP links change an incoming VPI to an outgoing VPI VCI remains unchanged VC switches terminate VC links and, therefore VP links too! both VC and VP are switched both VCI and VPI are changed 25 VP and VC Switching VC VCI = a1 VC VCI = a2 VC VP VP VP VP VP VP VP VPI = x1 VPI = x2 VPI = x3 VPI = y1 VPI =y2 VPI = y3 VPC x VPC y VCC VC = Switch VC VP = Switch VP 26 13
VC11 Arvika A VC10 VC41 B Kil VP10 VP20 VC40 Örebro VP40 VP and VC Switching Karlstad VP 20 VC 40 VC42 VP 40 VC 43 VP10 VP 10 VC 43 VC42 Stockholm Västerås VP50 VC14 VP 50 VC 43 VC44 VP60 VP30 Uppsala VP 60 VC 43 VC15 Kista 27 C D VP and VC Switching virtual path technique reduce the control cost by grouping connections sharing common paths some advantages of virtual paths: simplified network architecture network transport functions can be separated in those related to VC and VP increased network performance and reliability network deals with fewer, aggregated units reduced processing and short connection setup time much of the work is done when the VP is set up VC establishment can be executed with simple control functions 28 14
ATM works with cells ATM cells packets with fixed length - 53 bytes 5 bytes for the header + 48 bytes for payload some advantages of using cells include queuing delay for high-priority cells is reduced support for CBR and VBR traffic at the same time switching is more efficient it is easier to implement switching hardware 29 ATM cells 2 ATM header formats are defined for UNI interfaces 4 bits for flow control (generic flow control field) for NNI interfaces extra 4 bits for VPI (12 bits in total) 5 bytes 48 bytes Header Payload an ATM cell 30 15
UNI ATM Cell Header 1 2 3 4 5 6 7 8 Bytes GFC VPI VPI VCI VCI VCI PT CLP HEC 1 2 3 4 5 GFC - Generic Flow Control VPI - Virtual Path Identifier VCI - Virtual Channel Identifier PT - Payload Type CLP - Cell Loss Priority HEC - Header Error Control 31 NNI ATM Cell Header 1 2 3 4 5 6 7 8 Bytes VPI VPI VCI VCI VCI PT CLP HEC 1 2 3 4 5 VPI - Virtual Path Identifier VCI - Virtual Channel Identifier PT - Payload Type CLP - Cell Loss Priority HEC - Header Error Control 32 16
Header Fields of ATM cells GNF - Generic Flow Control only for UNI cells alleviate short-term overload conditions in the network VPI - Virtual Path Identifier routing field for the ATM network 8-bit long in UNI / 12-bit long in NNI more paths in the network VCI - Virtual Channel Identifier routing field to and from the end user 33 Header Fields of ATM cells PT - Payload Type type of information in the payload field 0 as first bit user data 1 as first bit network management or maintenance info. CLP - Cell Loss Priority guidance to the network in case of congestion 0 is a high-priority cell and 1 is used for low-priority cell HEC - Header Error Control calculated over the other 32 bits of the header 34 17
ATM Protocol Architecture Management Plane Control Plane User Plane Higher Layer ATM Adaptation Layer ATM Layer Physical Layer Plane Management Layer Management ATM protocol reference model consists of 3 planes User plane user information - includes error and flow control Control plane call and connection control Management plane management of resources of the ATM sub layers management and coordination between all plans 35 ATM Protocol Architecture ATM protocol reference model consists of 3 layers Physical Layer transmission in the physical medium between 2 ATM devices ATM Layer common to all services provide cell switching ATM Adaptation Layer (AAL) 2 layers (VP and VC) service dependent maps high layer information in cells to be transported over the ATM network 36 collect information from cells and deliver it to the higher layers 18
ATM Services ATM Forum has defined 5 types of services CBR Constant Bit Rate e.g. uncompressed audio & video RT-VBR Real Time Variable Bit Rate e.g. real time video conferencing NRT-VBR Non-Real Time Variable Bit Rate e.g. banking transactions, airline reservation ABR Available Bit Rate e.g. burst applications (browsing) UBR Unspecified Bit Rate e.g. data transfer 37 Physical Layer Physical layer is divided in 2 sub layers Physical Medium Dependent (PMD) Transmission Convergence (TC) Physical Medium Dependent (PMD) physical medium transmission fiber optic, coaxial cable, twisted pair bit alignment and bit timing optical - electrical conversion 38 19
Physical Layer Transmission Convergence (TC) places ATM cells (from the ATM layer) in the transmission frame structure of the PMD layer extracts ATM cells from transmission frame structure of the PMD layer and deliver it to the ATM layer header error control (HEC) 39 ATM Layer Functions Generic Flow Control Inserting and Removing Cell Headers adds a header to outgoing cells delivered by the AAL remove header of incoming cells and deliver the data to the AAL VPI / VCI switching VPI is changed in VP switches VPI and VCI are changed in VC switches Cell multiplexing and demultiplexing cells from different VP and VC are multiplexed when they are sent demultiplexed during reception 40 20
ATM Layer ATM Switch 10 22 11 Port 1 Port 2 Port 3 04 Port VPI/VCI Port VPI/VCI 1 10 3 04 1 22 2 11 VPI/VCI Switching 41 Virtual Paths and Virtual Channels Örebro VP10 Västerås VC11 Arvika A VC10 VC41 B Kil VP10 VP20 VC40 VP40 Karlstad VC42 VC42 Stockholm Communication between: A/C and A/D B/C and B/D VP50 VC44 VC14 VP60 VP30 Uppsala VC15 Kista 42 C D 21
ATM Adaptation Layer AAL adapt services provided by the ATM layer and the requirements of the higher layers PDU from higher layers are mapped in the fields of the ATM cell 2 sub layers in the AAL Convergence Sublayer - CS provides the functions needed to support specific appl using AAL Segmentation and Reassembly Sublayer SAR responsible for packaging information received from CS into cells for transmission and unpacking the information at the other end 43 Service Classification for AAL Timing relation between source and destination Bit Rate Class A Class B Class C Class D Required Not Required CBR VBR Connection mode Connection oriented Conn.less AAL protocol Type 1 Type 2 Type 5 Type 3 / 4 44 22
AAL 5 ATM Adaptation Layer Type 5 low-overhead AAL to transport IP datagrams over ATM networks AAL header and trailer are empty padding is used to guarantee that the data is multiple of 48 bytes 45 ATM applications ATM networks are being used as wide area networks (WAN) network backbone to interconnect LAN in different sites (MAN) applications that need QoS guarantees integrated data, voice & video 46 23