prof. ing. anton Čižmár, csc. a t m (a synchronnous t ransfer m ode ) a t m (a synchronnous t...

Prof. Ing. Anton Čižmár, CSc.

A T M

(ASYNCHRONNOUS TRANSFER MODE)

A T M

(ASYNCHRONNOUS TRANSFER MODE)


1. (ATM) has been recommended and been accepted by industry as the transfer mode for Broadband Integrated Services Digital Networks (B-ISDN).

2. ATM has been designed to be able to handle different types of services and applications such as voice, data, image, text and video and mixture of all these.

3. ATM provides a good bandwidth flexibility and can be used efficiently from local area networks (LANs) and wide area networks (WANs).

4. ATM is a connection-oriented packet switching technique in which all packets are of fixed length of 53 octets.

5. No processing like error control is done on the information field of ATM cells inside the network and it is carried transparently in the network.


ATM is a connection-oriented technology !

• In contrast to the popular connection-less IP-protocol where each IP-datagram must 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 or point-to-multipoint connection must be set up first before it can be used.

• This connection setup can be done either statically by manual configuration or automatically 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 transmitting node.


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 high

transmission speeds as well as very low speed by negotiate service contract for a connection with required quality of services

• to minimise the number of buffers required at the intermediate nodes to bound the delay and 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:

1. The header values (i.e. VCI and VPI etc.) are assigned during the connection set up phase and translated when switched from one section to other.

2. Signalling information is carried on a separate virtual channel than the user information.

3. In routing, there are two 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.


ATM is connection-oriented and the establishment of the connections includes the allocation of a virtual channel identifier (VCI) and/or virtual path identifier (VPI). It also includes the allocation of the required resources on the user access and inside the network. These resources, expressed in terms of throughput and quality of service, can be negotiated between user and network either before the call-set up or during the call.

ATM sieť a typy rozhraníATM sieť a typy rozhraní


Asynchronous Inputs

• Several asynchronous data streams (voice, multimedia, data, etc.)arrive from one or 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 size ATM cells,

consisting of a 48 octet payload part and a 5 octet header part. Two fields in 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 random intervals at the

inputs of an ATM multiplexer where they are temporarily stored in a large 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 the selected cells onto an outgoing

synchronous ATM link. Such a link can be carried over 2 Mbit/s or 34 Mbit/s PDH connections or 155

Mbit/s , 622 Mbit/s or 2.5 Gbit/s STM-N connections.

• 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 are discarded according to

the parameters settled in the traffic contract for the individual data streams. This operation is called

Usage Parameter Control (UPC) or Policing.


Why Small Cells ?

Overhead Delay

% O

verh

ead

Del

ay (

ms)

Payload (bytes)0 20 60 80


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


ATM cell structure


Virtual Channel Connection (VCC)

• A virtual channel connection is a single point-to-point connection through an ATMnetwork. A physical link between two ATM nodes can be shared by a nearly unlimitednumbers of VCCs, as long as the maximum transmission capacity of the link is notexceeded.

• A virtual channel between two ATM nodes is uniquely characterized by its combinedVPI / VCI identifiers.

Virtual Path Connection (VPC)

• A virtual path connection is a bundle of several virtual channel connections thathave 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, becausean 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 pathbetween two customer end points. The customer is then free to choose the numberand the capacity of the individual virtual connections within this predefined VPC.


VPI, VCI connection


VP-Switch

• A VP-switch, often also called an ATM cross-connect, switches ATM cells on avirtual path level by looking at the VPI identifiers only. The VCI field is usually leftunchanged. 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 theVPI and VCI identifiers. Both VPI and VCI values usually change on the outgoingconnection.

Combined Switches

• In a typical ATM node, path level switching and channel level switching are oftencombined as it is shown in the above figure. Acting as a first stage, a VP-switchhandles the large ATM transit volume, whereas a smaller percentage of cells thatmust be switched on the individual channel connection level are fed into aseparate VC-Switch.


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 theinput ports to any virtual channel leaving through any output port.• In order to configure the input / output mapping, a switching table with an entry foreach virtual channel or path connection is needed.

Bidirectional Unicasts• For bidirectional connections usually only one direction has to be configuredexplicitly. The required inverse direction entry in the switching table (using thesame VPI/VCI values) is automatically generated.

Unidirectional Multicasts• ATM easily supports unidirectional multicasts. ATM cells can be duplicatedand copied onto as many output ports as desired.• A multicast from an ATM root or server node to several ATM leaf or client nodeswill share a single virtual channel per physical line as long as possible and canthereby save valuable bandwidth. Actual duplication of ATM cells will only occur inthose intermediate nodes where virtual channel connections to different endpointsmust fork.


Connectionless: Packet Routing

• Path 1 = S1, S2, S6, S8• Path 2 = S1, S4, S7, S8• Data can take different pathand can arrive out of order

Connection Oriented: Cell Switching

• VC = S1, S4, S7, S8• Data takes the same path andarrives in sequence


VPI/VCI tables in network equipment updated by administrator


Dynamically set up connections via signaling


Transfer data over newly established link


Dynamically tear down connections via signaling


PVC established manually across UNI and dynamically across NNI


ATM – protocol reference model

To transmission channel

Customers


AAL1 : Circuit Emulation

2 : Audio / Video

3/4 : Data Transfer

5 : Lower Overhead AAL for Data

Video

Data

Audio

ToATM Layer

48 bytes

AAL

Data

AAL - ATM Adaptation Layer


AAL type 1 for Class A

AAL1 for Class A, illustrating the use of the 48-byte payload. This illustrates that one of the bytes of the payload must be used for this protocol.


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 packet voice, packet video, etc.


ATM over SDH


Sieť ATM Slovenských telekomunikácií

EF TU, Letná 1, EF TU, Letná 1, KošiceKošice

miesto konania ATMTU ’99

FEI STU, Ilkovičova 3, FEI STU, Ilkovičova 3, BratislavaBratislava

APEX-MAC1

Školiace stredisko dátových sietí Školiace stredisko dátových sietí ST,ST,

Sliačska 17, BratislavaSliačska 17, Bratislava

Infotel ST, Horná 79, Banská Infotel ST, Horná 79, Banská BystricaBystrica

konferenčná miestnosť

výstavný stánok ST

Internet Backbone

LAN

APEX-M AC 1

ATM TU ’99

rýchly prístup na Internet

videokonferencia PC

videokonferencia JPEG

videokonferenčnýprenos

ATM prepínač

kostrový ATM prepínač


THANK YOU!THANK YOU!

prof. ing. anton Čižmár, csc. a t m (a synchronnous t ransfer m ode ) a t m (a synchronnous t...

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