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A T M ( A SYNCHRONNOUS T RANSFER M ODE ). 1. (ATM) has been recommended and been accepted by industry as the transfer mode for Broadband Integrated Services Digital Networks (B-ISDN).
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A T M (ASYNCHRONNOUS TRANSFER MODE) Prof. Ing. Anton Čižmár, CSc.
1. (ATM) has been recommended and been accepted by industry as thetransfer mode for Broadband Integrated Services Digital Networks (B-ISDN). • 2. ATM has beendesigned to be able to handle different types of services and applications such as voice, data, image,text and video and mixture of all these. • ATM provides a good bandwidth flexibility and can be used efficiently from local area networks(LANs) and wide area networks (WANs). • ATM is a connection-oriented packet switching techniquein which all packets are of fixed length of 53 octets. • No processing like error control is done on the information field of ATMcells inside the network and it is carried transparently in the network. Prof. Ing. Anton Čižmár, CSc.
ATM is a connection-oriented technology ! • In contrast to the popular connection-less IP-protocol where each IP-datagrammust find its way through the internet (helped by static or dynamic routing tables),ATM is inherently a connection-oriented technology, where each point-to-point orpoint-to-multipoint connection must be set up first before it can be used. • This connection setup can be done either statically by manual configuration orautomatically using the signalling protocols described later in this chapter.Once a connection between two ATM end points has been found and set up,each ATM cell in this connection takes the same route through the ATM network.ATM cells also keep their relative order in which they were sent by the transmittingnode. Prof. Ing. Anton Čižmár, CSc.
From a technical point of view, the fundamental underpinning of ATM is: • to support all existing services as well as emerging services in the future • fixed-size cells with VPI and VCI to minimises the switching complexity • statistical multiplexing to utilises network resources very efficiently • to minimise the processing time at the intermediate nodes and supports very hightransmission speeds as well as very low speed by negotiate service contract for aconnection with required quality of services • to minimise the number of buffers required at the intermediate nodes to bound the delayand the complexity of buffer management • guarantees performance requirements of existing and emerging applications, • layered architecture, and • capable of handling bursty traffic. • ATM is a connection oriented mode: • The header values (i.e. VCI and VPI etc.) are assigned duringthe connection set up phase and translated when switched from one section to other. • Signallinginformation is carried on a separate virtual channel than the user information. • In routing, there aretwo types of connections, i.e., Virtual channel connection(VCC) and Virtual path connection(VPC).A VPC is an aggregate of VCCs. Switching on cells is first done on the VPC and then on the VCC. Prof. Ing. Anton Čižmár, CSc.
ATM is connection-oriented and the establishment of the connections includes the allocation of avirtual channel identifier (VCI) and/or virtual path identifier (VPI). It also includes the allocation ofthe required resources on the user access and inside the network. These resources, expressed interms of throughput and quality of service, can be negotiated between user and network eitherbefore the call-set up or during the call. ATM sieť a typy rozhraní Prof. Ing. Anton Čižmár, CSc.
Asynchronous Inputs • Several asynchronous data streams (voice, multimedia, data, etc.)arrive from oneor several CPEs (customer premises equipment) at the UNI (user-network interface).The data rate of each stream can be time-varying. • The arriving data octets of the individual streams are packaged into constant sizeATM cells, consisting of a 48 octet payload part and a 5 octet header part. Two fieldsin the header (VPI/VCI) are used to give each individual data stream a unique address. • All ATM cells formed from the various asynchronous data streams arrive at randomintervals at the inputs of an ATM multiplexer where they are temporarily stored in alarge buffer. Synchronous Output • The ATM multiplexer choses among the incoming ATM cells on a first-come /first-served basis or according to some negotiated priorities and multiplexes theselected cells onto an outgoing synchronous ATM link. Such a link can be carried over2 Mbit/s or 34 Mbit/s PDH connections or 155 Mbit/s , 622 Mbit/s or 2.5 Gbit/s STM-Nconnections. • If the input buffer runs empty then in order to keep up synchronous operation„empty cells“ are mapped onto the outgoing ATM stream. • On the other hand if the input buffer is about to overflow, ATM input cells arediscarded according to the parameters settled in the traffic contract for theindividual data streams. This operation is called Usage Parameter Control (UPC)or Policing. Prof. Ing. Anton Čižmár, CSc.
%Overhead Delay (ms) Overhead Delay 0 20 60 80 Payload (bytes) Why Small Cells ? Prof. Ing. Anton Čižmár, CSc.
Cell Size: 32 bytes or 64 bytes? • Cell size of 32 and 64 bytes: • 64 bytes cells have better transmission efficiency • 32 bytes cells have small delay • both sizes are integer power of 2 • Europe wanted 32 bytes size, US and Japan wanted 64 • Compromise: 48 bytes Prof. Ing. Anton Čižmár, CSc.
ATM cell structure Prof. Ing. Anton Čižmár, CSc.
Virtual Channel Connection (VCC) • A virtual channel connection is a single point-to-point connection through an ATM network. A physical link between two ATM nodes can be shared by a nearly unlimited numbers of VCCs, as long as the maximum transmission capacity of the link is not exceeded. • A virtual channel between two ATM nodes is uniquely characterized by its combined VPI / VCI identifiers. Virtual Path Connection (VPC) • A virtual path connection is a bundle of several virtual channel connections that have the same endpoints. • A virtual path between two ATM nodes is uniquely characterized by its VPI identifier. • Treating several VCCs as a single VPC increases switching efficiency, because an ATM switches must look only at the VPI field of each ATM cell. • As a service for its customers, an ATM carrier can assign and set up a virtual path between two customer end points. The customer is then free to choose the number and the capacity of the individual virtual connections within this predefined VPC. Prof. Ing. Anton Čižmár, CSc.
VPI, VCI connection Prof. Ing. Anton Čižmár, CSc.
VP-Switch • A VP-switch, often also called an ATM cross-connect, switches ATM cells on a virtual path level by looking at the VPI identifiers only. The VCI field is usually left unchanged. Thus VP switches can be implemented very efficiently. VC-Switch • A VC-switch handles ATM cells on a virtual channel level by looking at both the VPI and VCI identifiers. Both VPI and VCI values usually change on the outgoing connection. Combined Switches • In a typical ATM node, path level switching and channel level switching are often combined as it is shown in the above figure. Acting as a first stage, a VP-switch handles the large ATM transit volume, whereas a smaller percentage of cells that must be switched on the individual channel connection level are fed into a separate VC-Switch. Prof. Ing. Anton Čižmár, CSc.
I/O Structure an ATM Switch • An ATM switch can have a variable number of interface cards (e.g. Ethernet, PDH, SDH cards) and each card has a certain number of bidirectional physical ports (usually 1- 8 ports per card). • An ATM switch should be able to switch any virtual channel arriving at any of the input ports to any virtual channel leaving through any output port. • In order to configure the input / output mapping, a switching table with an entry for each virtual channel or path connection is needed. Bidirectional Unicasts • For bidirectional connections usually only one direction has to be configured explicitly. The required inverse direction entry in the switching table (using the same VPI/VCI values) is automatically generated. Unidirectional Multicasts • ATM easily supports unidirectional multicasts. ATM cells can be duplicated and copied onto as many output ports as desired. • A multicast from an ATM root or server node to several ATM leaf or client nodes will share a single virtual channel per physical line as long as possible and can thereby save valuable bandwidth. Actual duplication of ATM cells will only occur in those intermediate nodes where virtual channel connections to different endpoints must fork. Prof. Ing. Anton Čižmár, CSc.
Connectionless: Packet Routing • Path 1 = S1, S2, S6, S8 • Path 2 = S1, S4, S7, S8 • Data can take different path and can arrive out of order Connection Oriented: Cell Switching • VC = S1, S4, S7, S8 • Data takes the same path and arrives in sequence Prof. Ing. Anton Čižmár, CSc.
VPI/VCI tables in network equipmentupdated byadministrator Prof. Ing. Anton Čižmár, CSc.
Dynamically set up connectionsvia signaling Prof. Ing. Anton Čižmár, CSc.
Transfer data over newly established link Prof. Ing. Anton Čižmár, CSc.
Dynamically tear down connectionsvia signaling Prof. Ing. Anton Čižmár, CSc.
PVC established manually across UNI anddynamically across NNI Prof. Ing. Anton Čižmár, CSc.
ATM – protocol reference model Customers To transmission channel Prof. Ing. Anton Čižmár, CSc.
AAL AAL 1 : Circuit Emulation 2 : Audio / Video 3/4 : Data Transfer 5 : Lower Overhead AAL for Data Audio 48 bytes Video To ATM Layer Data Data AAL - ATM Adaptation Layer Prof. Ing. Anton Čižmár, CSc.
AAL type 1 for Class A AAL1 for Class A, illustrating the use of the 48-byte payload. This illustratesthat one of the bytes of the payload must be used for this protocol. Prof. Ing. Anton Čižmár, CSc.
AAL2 for Class B AAL2 is being defined for Class B, but it’s still under development. This will be important though,because it will allow ability of ATM to support the bursty nature of traffic to be exploited for packetvoice, packet video, etc. Prof. Ing. Anton Čižmár, CSc.
ATM over SDH Prof. Ing. Anton Čižmár, CSc.
ATM TU ’99 EF TU, Letná 1, Košice miesto konania ATMTU ’99 Infotel ST, Horná 79, Banská Bystrica konferenčná miestnosť videokonferenčný prenos FEI STU, Ilkovičova 3, Bratislava výstavný stánok ST kostrový ATM prepínač videokonferencia JPEG Sieť ATM Slovenských telekomunikácií ATM prepínač videokonferencia PC LAN rýchly prístup na Internet Školiace stredisko dátových sietí ST, Sliačska 17, Bratislava Internet Backbone Prof. Ing. Anton Čižmár, CSc.
THANK YOU! Prof. Ing. Anton Čižmár, CSc.
