ATM

MONIKALAGWALCSE MITS

Contents• Definition• Features• ATM Devices• Architecture• Service Categories• Quality of Service (QoS)• Applications• Benefits• References

ATM Definition

• Asynchronous Transfer Mode (ATM) is an International Telecommunication Union-Telecommunications Standards Section (ITU-T) standard for cell relay wherein information for multiple service types, such as voice, video, or data, is conveyed in small, fixed-size cells.

• ATM networks are connection-oriented.

ATM Features• Service is connection oriented, with data transferred over a Virtual

Circuit (VC).

• A cell-switched network.

• Fixed-size cell (53-Bytes)

• Uses Asynchronous time-division multiplexing (Asynchronous TDM)

• ATM is independent of the transmission medium. ATM cells can be sent on a wire or fiber, and can also be packaged inside the payload of other carrier system.

ATM Multiplexing

MUX

`

Wasted bandwidth

ATM

TDM

4 3 2 1 4 3 2 1 4 3 2 1

4 3 1 3 2 2 1

Voice

Data packets

Images

ATM Devices and the Network Environment

• An ATM network is made up of an ATM switch and ATM endpoints. An ATM switch is responsible for cell transit through an ATM network. The job of an ATM switch is well defined:

• It accepts the incoming cell from an ATM endpoint or another ATM switch. It then reads and updates the cell-header information and quickly switches the cell to an output interface toward its destination.

• An ATM endpoint (or end system) contains an ATM network interface adapter.

• Examples of ATM endpoints are workstations, routers, digital service units (DSUs), LAN switches, and video coder-decoders (CODECs).

ATM Architecture

ATM Architecture

ATM ArchitectureThe ATM reference model is composed of the following planes, which span all layers:• Control - This plane is responsible for generating and managing

signaling requests.

• User - This plane is responsible for managing the transfer of data.

• Management - This plane contains two components:

Layer management manages layer-specific functions, such as the detection of failures and protocol problems.Plane management manages and coordinates functions related to the complete system.

The ATM reference model is composed of the following ATM layers:

• Physical layer - Analogous to the physical layer of the OSI reference model, the ATM physical layer manages the medium-dependent transmission.

• ATM layer - The ATM layer is responsible for the simultaneous sharing of virtual circuits over a physical link (cell multiplexing) and passing cells through the ATM network (cell relay). To do this, it uses the VPI and VCI information in the header of each ATM cell.

• ATM adaptation layer (AAL) - The AAL is responsible for isolating higher-layer protocols from the details of the ATM processes. The adaptation layer prepares user data for conversion into cells and segments the data into 48-byte cell payloads.

• Finally, the higher layers residing above the AAL accept user data, arrange it into packets, and hand it to the AAL.

A/D

Voice

s1 , s2 …

Digital voice samples

A/D

Video

… Compression

compressed frames

picture frames

DataBursty variable-length

packets

cells

cells

cells

AAL

AAL

AAL

ATM Adaptation Layers

ATM Cell Header Format

ATM Services

There are two general ATM services:

• Permanent Virtual Connection (PVC):

Leased Line

• Switched Virtual Connection (SVC):

Telephone call

Permanent Virtual Connection (PVC):• A PVC is a circuit that a network operator over an

ATM-switched network between a specific source and a specific destination manually provisions.

• A permanent virtual circuit (PVC) is a connection that is permanently established between two or more nodes in frame relay and asynchronous transfer mode (ATM) based networks.

• It enables the creation of a logical connection on top of a physical connection between nodes that communicate frequently or continuously.

This diagram shows how a permanent virtual circuit (PVC) is used to allow the transfer of data through a communications network through a pre-established logical (virtual) path.

Switched Virtual Circuit (SVC)• A switched virtual circuit (SVC) is a type of virtual

circuit in telecommunication that is used to establish a temporary connection between two different network nodes until completion of a data transfer session, after which the connection is terminated.

• SVCs are implemented in data, voice or video communication connection-based packet and circuit switching networks that have small or limited time usability.

• A remote user requests a connection with the host server/device, and a virtual circuit/connection is created between both nodes.

• When the circuit connection's purpose is complete, or it becomes idle, the SVC is suspended.

• For example, a SVC created between a remote user and server for a file download is closed when the download process is completed.

This diagram shows how a switched virtual circuit (SVC) uses an address provided by the user to establish a logical (virtual) path through a communications network.

• In this example, a SVC is created by using the destination address to determine the required programming of routing tables in switches within the data network.

• These routing tables assign data transfer connections between input and output channel on each switch.

• For example, as data from the sending computer (portable computer) is sent into input channel 3 of the first switch, it is transferred to the output channel 4.

• This process will repeat for any data that is sent from the sending computer to the destination computer during the switched connection.

ATM Services:

Virtual Circuits and Paths

Quality of Service (QoS)

• A networking term that specifies a guaranteed throughput level.

• One of the biggest advantages of ATM over competing technologies such as frame relay and fast Ethernet.

• It allows ATM providers to guarantee to their customers that end-to-end latency will not exceed a specified level.

ATM Provides the following QoS:

• Constant Bit Rate (CBR)• Variable Bit Rate (VBR)• Unspecified Bit Rate (UBR)• Available Bit Rate (ABR)

Constant Bit Rate (CBR)• This is a service class where cells are

transmitted in a continuous bit-stream in order to meet voice and video QoS needs.

• Specially suitable for voice-over-IP (Transfer fixed rate uncompressed video), and other traffic that requires steady, guaranteed throughput.

• The configuration on CBR may vary with different platforms.

Variable Bit Rate (VBR) Non-Real Time (VBR-nrt)

• This service class is used in order to transmit non-real-time applications that are bursty in nature.

• The traffic characteristics are defined in terms of the Peak Cell Rate (PCR), Sustained Cell Rate (SCR), and Minimum Burst Size (MBS).

Variable Bit Rate (VBR) Real Time (VBR-rt)• This service class is used in order to transmit

real-time data that is sensitive to time delays, like compressed voice over IP and video conferencing. VBR-rt, as well as VBR-nrt.

Available Bit Rate (ABR)

• This is a service class where ATM switches make no guarantee of cell delivery, but do guarantee a minimum bit rate and that cell loss is kept as low as possible with the use of a feedback mechanism.

• Aimed at data traffic such as file transfer and Email.• Adjust the amount of bandwidth based on the amount

of traffic in the network

Unspecified Bit Rate (UBR)• This is a service class where the network

management makes no Quality of Service (QoS) commitment.

• It models the best-effort service that the Internet normally provides and is suitable for applications tolerant to delay and does not require real-time responses.

• Examples include email, fax transmission, file transfers, Telnet, LAN and remote office interconnections.

Service categories (ATM Forum)ATM Layer Service Category

CBR RT-VBR NRT-VBR

more less

more stringent less stringent

UBRABR

user has to pay ...

Benefits of ATMThe benefits of ATM are the following:

• high performance via hardware switching. • dynamic bandwidth for bursty traffic. • class-of-service support for multimedia. • scalability in speed and network size. • common LAN/WAN architecture. • opportunities for simplification via VC architecture. • international standards compliance.

ApplicationsApplications suitable for ATM Service include:

• Voice and video • Packetized voice and video • Systems Network Architecture (SNA) • WAN/VPN connectivity • Web hosting • E-commerce • Client-server (terminal-host) data • LAN interconnection • LAN emulation • Remote access • File transfer • Internet/intranet/extranet access

– E-mail messaging – Text imaging – Forms processing

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