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An ATM Primer

ATM can be described as having several key

attributes that makes it effective in supporting diverse,

high speed applications:

Cell Relay Technology,

High Speed Interfaces,

Highly Flexible Connectivity,

Multiple Service Classes,

Logical Functional Distinctions,

Quality of Service Standards, and

Comprehensive Management Structure.

Cell Relay

ATM is a cell relay technology, which is based onthe segmentation of an incoming data stream into a

series of small, fixed-length (53 octets) data units.

These units are called cells. As illustrated below, data,

voice and video traffic is all broken into cells and trans-

ported across the network by interleaving the combined

stream of cells onto the transmission media.ATM is

officially defined by the International

Telecommunication Union-Telecom Sector. ATM isalso standardized by the American National Standards

Institute (ANSI).

TrafficSources Interface Device

Interleaved Cells

10 AT M - T he Universal Infrastructure

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User and Network Interfaces

Presently, as illustrated below, there are three inter-

face standards for use with public ATM networks.

User-Network Interface (UNI)

Network- Network Node Interface (NNI)

Broadband-Inter Carrier Interface (B-ICI) (ATM

Forum only)

The UNI is the standard interface between

customer premises equipment (CPE) and public ATM

network service. By complying with this standard, users

are assured that premises equipments such as PBX,

routers, hubs, ATM Data Service Units (DSUs), andATM switches, are compatible with public ATM

network service. The basic interface between two ATM

switches is specified by the NNI standard. The NNI

standard provides for additional flows of management

data beyond what is available with the UNI standard.

The interface between two public ATM networks is

defined by the Broadband Inter Carrier Interface (B-

ICl) specification. Based on the NNI standard, the B-

ICI specification provides the exchange of

management, configuration, and performance data

that is needed when connecting public networks. The

B-ICI will allow end-to-end management of the inter-

connected networks.

UN I NN I B-ICI

Device ATM Switch Public ATM Public ATM


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Virtual Paths and Connections

Virtual Connections allow multiple connections to

be defined simultaneously across a single network

facility, with each connection having flexible

bandwidth. In addition to virtual connections (VCs),

ATM defines a higher level of system-to-system

connectivity. These are called Virtual Paths (VP). A VC

is used to connect individual end systems through the

network. A VP is simply a logical collection, or

bundle of multiple VCs, as shown above. The prede-

fined VP assigned between end systems provides a

tunnel over which VCs destined for the remote loca-

tion can be routed. IN a VP tunnel configuration, noneof the Virtual Channels are switched by the network,

only Virtual Paths.

Virtual Paths allow users to provision their own

end-to-end VCs among locations that are connected

with predetermined VPs. This can be accomplished

without interacting with the ATM Service provider,allowing a quick response to end user needs. In addi-

tion, both permanent and switched VCs may be

included in existing permanent VPs, so that users can

efficiently load balance traffic flow between the two

locations. A single port may support both VPs and VCs

of differing service classes. VCs and VPs support a

Virtual Path (VP)

Virtual Connections

- Permanent - PVC- Switched - SVC


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Center, for example, maintains full responsibility for

the operation and maintenance of the ATM services.

This includes all access facilities and their customer

premises equipment (CPE).

With these services, they are able to provide early

identification and resolution of network issues as theyoccur and deliver high quality customer service.

The Broadband Operations Center

The Broadband Operations Center includes

responsibility for:

ATM Service monitoring for seven days a week

for 24 hours a day,

Coordination of all physical and logical aspects of

the ATM service,

Monitoring network circuits and switches,

Real time performance and fault monitoring of all

access and interoffice trunk facilities,

Proactive monitoring of selected switch parame-

ters and tables to allow for early identification of

potential problems,

Assigning all VP and VC addressing for ATM

switch and CPE,

Performing Modifications to switch routing tables;

providing routing and configuration control,

Optimizing throughput by modifying switch para-

meters and settings, and

Monitoring and analyzing network performance.


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called a VCC. A VCC is a specific instance of a SVC

or PVC. A VCC may traverse one end-to-end VPC or

several sequential VPCs.

Virtu al Ch an nel Iden tifier (VCI)The 16-bit

number in an ATM cell header identifying the specific

virtual channel on which the cell is traversing on the

current physical circuit.

Virt ua l Circuit (VC)(1) A portion of a virtual path

or a virtual channel used to establish a virtual connec-

tion between two end nodes. (2) Logical channels estab-

lished as a result of the call initiation procedure to a

network address that exists for a period of time.

Virtu al Path Conn ection (VPC)Virtual paths in

two or more sequential physical circuits can be concate-

nated to create a logical connection, called a VPC.

VPCs must be pre-configured. All cells traversing VCs

in a VPC are routed the same way.

Virtua l Path Identifier/ Virtua l Chan nel

Iden tifier (VPI/ VCI)Combined, these fields iden-

tify a connection in the ATM network.

Virtual Path Link (VPL)Between switches, may

carry many connections (ATM).

Virtu al Tribu tar y (VT)A payload structure that

specifies where and how a sub-STS-1 signal will fit into

an STS-1 SPE.


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NOTES


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Produced and Published by

One Apple Hill, Suite 216, Natick, MA 01760

Tel: (508) 651-1155 Fax: (508) 651-1171 E-mail: [email protected]

http://www.techguide.com

This Technology Guide is one

of a series of guides, publishedby ATG, designed to put complex

communications and netw orking

technology concepts into practica l

and understandable terms.

Each guide provides objective,

non-biased information to assist in

the internal education, evaluation

and decision making process.

This Technology Guide, as w ell

as the other communications and

netw orking technology guides

in the series, are available

on ATGs Web Site.

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atm by rahul rk(9986510206)