Tech Guide ATM in Premises

This guide has been sponsored by

ATG’s Communications &Networking Technology

Guide Series

SimplifiedATM Access

at the CustomerPremises

Table of ContentsIntroduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2ATM Network Access Requirements . . . . . . . . . . . . 4Clear Division of Responsibility Between . . . . . . . . .

Carriers and Users. . . . . . . . . . . . . . . . . . . . . . . . 4Quality of Service (QoS) . . . . . . . . . . . . . . . . . . . . . 7End-to-end Traffic Management . . . . . . . . . . . . . . 10Network and Service Interworking . . . . . . . . . . . . . 12

Methods for Accessing ATM-based WANs . . . . . 12Three Alternatives for Building an ATM-NTU . . . 16Fault Localization: CPE vs. Central Office . . . . . . . 18Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Media and Rate Conversion . . . . . . . . . . . . . . . . . 18Network and Service Interworking . . . . . . . . . . . . . 19Simplifying the ATM Access Network . . . . . . . . . . 19

Carrier Perspective . . . . . . . . . . . . . . . . . . . . . . . . 23Benefits of the Simplified ATM-NTU and. . . . . . . . .

Interworking-NTU . . . . . . . . . . . . . . . . . . . . . . 23Summary and Conclusions . . . . . . . . . . . . . . . . . 26Case Study 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Case Study 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Glossary of Terms . . . . . . . . . . . . . . . . . . . . . . . . . 31

About the Editor…Gerald P. Ryan is the founder of Connections Telecommunications Inc., a Massa-

chusetts-based company specializing in consulting, education and software tools which

address Wide Area Network issues. Mr. Ryan has developed and taught numerous

courses in network analysis and design for carriers, government agencies and private

industry. Connections has provided consulting support in the areas of WAN network

design, negotiation with carriers for contract pricing and services, technology acquisi-

tion, customized software development for network administration, billing and audit-

ing of telecommunications expenses, project management, and RFP generation. Mr.

Ryan is a member of the Networld+Interop program committee.

This book is the property of The Applied Technologies Group and is made avail-

able upon these terms and conditions. The Applied Technologies Group reserves

all rights herein. Reproduction in whole or in part of this book is only permitted

with the written consent of The Applied Technologies Group. This report shall be

treated at all times as a proprietary document for internal use only. This book may

not be duplicated in any way, except in the form of brief excerpts or quotations

for the purpose of review. In addition, the information contained herein may not

be duplicated in other books, databases or any other medium. Making copies of

this book, or any portion for any purpose other than your own, is a violation of

United States Copyright Laws. The information contained in this report is

believed to be reliable but cannot be guaranteed to be complete or correct.

Copyright © 1997 by The Applied Technologies Group, One Apple Hill,

Suite 216, Natick, MA 01760, Tel: (508) 651-1155, Fax: (508) 651-1171

E-mail: [email protected] Web Site: http://www.techguide.com

Simplified ATM WAN AccessThe ACE-101 extends the service providers ATMmanagement capability to the customer premises,clearly demarcating QoS parameters and respons-ibility between the carrier and the user.

ACE-101 offers Quality of Service control, end-to-endtraffic management, fault localization, and rate andmedia conversion.

Benefits include improved service quality, reducedmaintenance costs and better diagnostics, greater service flexibility, and reduced operating costs.

For more information about RAD’s full range of ATMaccess solutions, contact: [email protected]

www.rad.com

U.S. Headquarters International HeadquartersTel: (201) 529-1100 Tel: 972-3-6458181Fax: (201) 529-5777 Fax: 972-3-6498250

email: [email protected]

E3/T3, STM-1/STS-3C

End-to-End OAMFault ManagementContinuity Check

Performance ManagementLoopback

Customer Premises Customer Premises

ATMUNI

ATMUNI

ATMUNI

SwitchATMUNI

SwitchACE-101

• Monitor• Police

• Monitor• Police

ACE-101ATM

Backbone

As ATM takes its place as the dominant switching fabric

within both carrier networks and very large corporate enterprise

networks, there is a growing need for inexpensive, QoS-enabled

solutions for customer access into an ATM-based wide area

network. Currently, fully configured routers or ATM edge

switches, originally designed for use in a central office, are being

used for this purpose. These devices can perform traffic concen-

tration for large numbers of users, but are most often too big to

support a single site cost-effectively. What is necessary is a

simplified ATM access device to serve as the formal access and

management boundary between the user equipment and the

carrier’s network. There has been a need for an access system

that has a specific application focus, supports QoS to the end

system, promotes simplicity in deployment and management, and

has a superior cost-value ratio. This Guide analyzes this new

requirement and describes the range of functions that such devices

will have to provide. This Guide can help the network developer

to plan the separation between network domains and to specify

the functionality needed at the customer interface point.

IntroductionAsynchronous Transfer Mode (ATM) networks are

capable of simultaneously transporting voice, data,

graphic and video content at very high speeds, with

well-defined qualities of service (QoS), using advanced

cell-based switching techniques. Support for real time

multimedia applications and the promise of better

scalability, flexibility and integration make ATM the

pervasive switching technology for next generation

wide area networks. Network service providers are

moving quickly both to make ATM the dominant

infrastructure for the core of their wide area networks

and also to implement access to the ATM networks

directly from customer premises.

2 • Simplified ATM Access at the Customer Premises

As a result, ATM is now widely deployed through-

out the core of carrier and corporate enterprise net-

works, but as it has extended to the outer perimeter, or

edge, of the network, there has been a lack of properly

sized and fully featured access devices that provide

seamless and comprehensive ATM connectivity.

Carriers as well as corporate implementers have

found themselves deploying high priced, fully featured

ATM edge switches at the remote location in order

to provide adequate service. Alternatively, they have

depended on the user to provide connectivity through

a variety of CPE devices. This has left a serious gap in

the ability of carriers to provide good end-to-end ATM

service because there has been, until recently, a serious

lack of properly sized edge devices that incorporate all

of the needed features for effective ATM access.

The small number of user ports and the straight-

forward nature of the traffic mix to and from some

remote locations, do not warrant the use of high-priced

edge switches or routers at the customer site, and yet

the need remains for a seamless, managed interface

between the customer and the network provider.

To avoid unnecessary costs, ATM access from these

locations needs to be as simple as possible without

sacrificing the functionality and manageability that

is essential to successful ATM-based networking.

It is clear that what is needed is a suite of customer

network termination devices that are suitable for low to

medium ATM access requirements. The following

Network Edge

Figure 1 — Core vs. Edge

CarrierCore

Technology Guide • 3

sections examine the issues involved in providing such

devices and identifies the key benefits of formalizing

the customer interface into a simple, low cost, fully

functioned and manageable network termination unit

(NTU).

ATM Network Access RequirementsIn addition to the requirement that access devices

be properly sized and priced to accommodate the

needs of different size locations, customer premises

ATM access devices must also allow:

• Clear division of responsibility between carriers

and users

• QoS control

• End-to-end traffic management

• Fault localization (CPE vs. Central Office)

• Service flexibility—media and rate conversion

Clear Division of Responsibility BetweenCarriers and Users

Any network can be divided into three distinct

parts: the customer premises equipment (CPE), the

access connection that links the customer to the carrier

central office, and the carrier core network infrastruc-

ture used for interoffice transport. Each of these three

parts could be used to provide the access interface into

the ATM core since each serves as a natural boundary

that can be used for ATM adaptation, the conversion

of network media and traffic rates, as well as opera-

tional management and administration.

The diagram on the next page (Figure 2) illustrates

how the network components relate to each other and

indicates the spans of control for a typical network.

4 • Simplified ATM Access at the Customer Premises Technology Guide • 5

Each part of the network contributes to the

achievement of user-to-user connectivity, but each

one is typically developed, operated and managed

independently. The carrier is most often responsible for

network operation but not for the customer premises

equipment or for the applications that need to commu-

nicate over the system.

In order to meet performance standards, carriers

need to monitor and control every aspect of the ATM

connection. This is most important at the point where

user traffic enters the public network at which service

types and QoS are assigned. In the classical WAN

access approach, shown in the above illustration, all of

the access conversion takes place at the carrier central

Figure 3 — Classical WAN Approach

Customer Premises

Carrier Premises

Router

T1 - E1 - A

TM

T1 - E1 - ATM T1 - E1

T1 - E1

n x 64K

FUNI/FR

PBX

TDMMUX

ATMEdgeMUX

CarrierATMCore

Router

FRAD

ATMWide AreaNetwork

Figure 2 — Essential Network Elements

EdgeSwitch

CustomerPremises

NetworkInterface ATM

Access Network

LocalNetworks

Edge-to-Edge

End-to-End

User-to-User

office where native traffic is converted to ATM. This,

unfortunately, does not allow end-to-end control but

only edge-to-edge (from central office to central office).

Also, the CPE oftentimes fails to provide appropriate

traffic management capabilities since it was designed

for use in a LAN environment and does not have the

monitoring and control capabilities to support WAN

access. As a result, carriers need to identify, and have

available for widespread deployment, a cost efficient,

highly functional NTU for use at the customer

premises.

ATM CPE-Based Access

This approach, illustrated below, provides a clear

definition of the boundary between the user’s CPE and

the provider’s parts of the network.

It allows the carrier to extend its control end-to-end

to protect the network from abuse; to provide assurances

of conformity to user-network contract terms; to

improve network reliability and management; and to

support different types of user applications and traffic

types. In essence, the carriers need to provide an on-site

“agent” that looks out for their interests and provides

them with better management and control.

The ATM access device provides the point of

attachment to the network for the customer’s equipment.

This can include remote connectivity for voice (E1/T1

Figure 4 — ATM CPE Based Approach

CustomerPremises

AccessNetwork

Carrier Premises

Router

PBX

InterworkingNTU

InterworkingNTU

Inter-workingNTUSwitch

ATMEdgeMUX

CarrierATMCore

Router

10Base T

T1 - E1155 Mbps

T1 - E1/T3 -E3

155 Mbps

100Base T

155Mbps


interfaces), Ethernet, Frame Relay, IP, native ATM and

other traffic types. In most cases, the requirement is for

a single attachment on the network side and a defined

set of customer interfaces (thereby minimizing the

need for multiplexing and switching), supporting a

variety of media types and rates (from E1/T1 to 155

Mbps over copper, fiber or coax).

Benefit to the Customer

The customer also benefits from a customer

premises access device which acts as a common refer-

ence point that can be specified in his service contract.

Traditional networks have always been offered on a

“best effort” basis, with little in the way of penalties for

non-performance, but this is changing as a result of the

more stringent requirements of some types of applica-

tion (voice and video, for example). ATM changes this

approach by defining standard mechanisms to meet

specific Quality of Service and support the service level

agreements. Where the network interface is, and how

performance is measured at that point, certainly

becomes a key differentiator between carrier solutions.

Quality of Service (QoS)ATM networks are classed as “multi-service net-

works” because they permit different types of data to

be transported concurrently. Each traffic flow is differ-

ent, requiring an individual Quality of Service (QoS)

contract for each user. These QoS specifics must be

both negotiated at the user-network interface (UNI)

and enforced through traffic shaping and traffic policing

policies. The carrier must check during the set-up

phase to ensure that the requested QoS is achievable

and must monitor performance on an end-to-end basis

throughout the life of the connection. The access

interface, of course, must be part of the QoS-enabled

network in order to guarantee performance.


These QoS classes are implemented on the Virtual

Channels (VCs) that are established as the traffic path

through the network between end points. The QoS

standards essentially result in rules governing these key

performance criteria:

Localized Parameters—Measured at the CPE

interface (the point where user traffic enters

the public service)

• Peak Cell Rate (PCR)—The maximum cell

transfer rate allowed at the interface

• Sustainable Cell Rate (SCR)—The average cell


• Cell Delay Variation Tolerance (CDVT)—The

“jitter”, or mean variation in the inter-cell timing

• Minimum Cell Rate (MCR)—The lowest cell


• Burst Size (BS)—The maximum number of cells

that can be transmitted at the peak cell rate

Network Parameters—Measured between two

points in the network

• Cell Delay—Maximum allowable end-to-end cell

delay

• Cell Delay Variation—Maximum allowable

variation in cell delay

• Cell Loss—Maximum allowable percentage of

traffic that can be lost or discarded.

The ATM access device enables traffic conformity

checks to be performed at the point where user traffic

enters the public network by letting carriers monitor

these localized parameters. It also provides a user-

oriented point of observation where end-to-end

measurements of cell delay, cell delay variation and


QoS Management

Each data stream from a user device must declare

its requirements for QoS every time a virtual channel

or virtual path connection is established. This is accom-

plished via ATM signaling at the UNI.

Four categories of QoS have been defined for

ATM services:

• Constant Bit Rate (CBR) offers strict bandwidth

guarantees and minimal delay and delay varia-

tion. It is suitable for real time applications such

as telephony and video. It is intended to be the

equivalent of digital leased line service. This class

of service can not tolerate delay or loss.

• Variable Bit Rate (VBR) which is suited to LAN

interconnection and similar applications that do

not make stringent demands on bandwidth, delay

or delay variation. Within the category, the ATM

forum has defined real time (RT) and non-real

time (NRT) traffic. VBR-RT is close to CBR in

terms of its specifications and is intended for time

sensitive applications that can tolerate some mini-

mal variation in delay or cell loss. The VBR-NRT

is used for applications such as on line transaction

processing and static video.

• Available Bit Rate (ABR) makes use of the band-

width that is available from moment to moment

for applications such as file transfer that can

tolerate low priority handling, but which still need

minimal cell loss. These applications are not as

time sensitive and therefore can be superseded by

more time sensitive applications.

• Unspecified Bit Rate (UBR) is an economical class of

service, which is similar to ABR in that it uses

available bandwidth on a moment to moment

basis, but without service level guarantees.

cell loss can be taken. Without the access device the

carrier is restricted to edge-to-edge measurements (cen-

tral office to central office). The ability to efficiently

monitor and report upon what is happening is funda-

mental to network management as well as QoS control

and traffic assurance.

End-to-end Traffic ManagementThe current state of service provisioning reveals

that even with the standards in place and user supplied

edge devices in the last mile, ATM service providers

are not in a position to assure that their networks are

provisioning the guaranteed levels of service. To provide

end-to-end control of the ATM network, whether it

conveys native ATM or existing LAN, voice or Frame

Relay interconnect services, carriers need to extend

management to the very edge of the network, i.e., to

the customer premises. But if the carriers rely on the

customer CPE, they forfeit vital traffic management

and control capabilities. On the other hand, edge

muxes offer a solution, enabling QoS control and end-

to-end traffic assurance but at too high a price.

The efficient transmission of user traffic is the only

purpose of networks. This traffic input to the network

is also the cause of network congestion. The QoS

agreements define, preferably in a service contract,

what type of traffic is to be carried, what quantities

and volumes are expected, and what level of security

and accuracy is needed. This contract needs to be

enforced both to protect the network and to manage

the user’s costs. The ATM access device can monitor

these localized parameters for user data entering the

network on a real time basis by checking the Peak Cell

Rate (PCR), Sustainable Cell Rate (SCR), Minimum

Cell Rate (MCR), Cell Delay Variation Tolerance

(CDVT), and Maximum Burst Size (BS). Any failure

on the user’s part to honor the service agreement (by

sending too much data, for example) can impact the


ability of the provider to meet other user’s needs, and

so must be prevented. In addition to monitoring, the

ATM access device may be involved in policing access

(avoiding misuse) and traffic shaping.

Traffic Management Functions

The ITU, in its I.371 and I.610 recommendations,

describes a set of five mechanisms for controlling traffic

in an ATM network so that it conforms to the localized

and network performance parameters. There are three

specific mechanisms used to control local access, which

need to be implemented in the ATM NTU device:

• Traffic Monitoring: Checks to CPE conformance to

the negotiated QoS standards,

• Traffic Policing: Identifies and tags non-conforming

cells by changing the cell loss priority (CLP) bit,

causing them to be discarded when congestion

occurs in the network,

• Traffic Shaping: Buffers the traffic entering the

network and does rate adaptation to assure

compliance with the local parameters.

The enforcement of traffic contracts protects

the carrier’s network from failures on the user’s part.

Traffic that is submitted in violation of the contracted

parameters could seriously affect other customers shar-

ing the WAN. The carrier needs to examine the traffic

presented for acceptability and reject (or flag) any out-

of-bounds requests before accessing the public network.

A network interface device at the customer premises

minimizes network vulnerability while providing addi-

tional flexibility.

The carrier can seldom dictate the technologies

and products to be used by the customer, and must

therefore be able to interconnect to, and work together

with, a variety of possible configurations and standards.

The CPE can range from simple ATM-enabled work-



industry hype and yet has seldom been realized. The

integrated network implies that all parts make use of

ATM protocols in a “seamless” fashion. The ability to

harmonize all levels of network, including the CPE, is

easiest in a private or outsourced network and most

difficult in shared public networks.

This is the only solution that allows all QoS and

management controls to be directly user-driven.

Nonetheless, a division of responsibility between the

customer and the provider still exists, even if it is in the

same company. Although an ATM Network Interface

Card (NIC) could provide the boundary functions,

these are usually deemed to be part of the user’s

responsibility. Even in a private network (i.e., a network

with a single administrative domain) it would be bene-

ficial to provide a control point that is separate from

the user’s workstation.

Edge-to-Edge Network Integration

The scenario of network integration only between

the edge switches would arise if different technologies

were being used in the different parts of the network

(an Ethernet LAN accessing an ATM service via

Frame Relay is one example). Integration is restricted

to the core network that links carrier wire centers and,

even if the access network is ATM-based, it is not

treated as being QoS-enabled.

ATMWide AreaNetwork

Figure 5— Integrated End User Solution

Network End

CustomerPremises

ATMAccess Network

Network Edge

EdgeSwitch

ATMNIC

stations to complex legacy networks (a host-oriented

SNA network, for example). Speed and media conver-

sions will often be required. For example, copper wiring

within a site may need to interconnect to a fiber optic

access network. Protocol conversions might also be

needed—converting the internal switched fast Ethernet

frames to ATM cells for transmission over a T1 data

link, for example.

Network and Service InterworkingNetwork and service interworking define the

mechanisms to support ATM service overlay, i.e., to

transport other protocols and traffic types such as

Frame Relay, IP or voice over an ATM network. Most

carriers today have deployed ATM switches in their

backbone and use the ATM backbone to transport

other services such as LAN and voice. An ATM access

concentrator at the customer premises performs the

service interworking, carrying all the user traffic into

the carrier’s network over a single ATM link.

Methods for Accessing ATM-based WANs

The access network, which connects the CPE to

the ATM-based WAN, may use ATM or may use other

protocols, may be constructed out of different media,

and may operate at various speeds. The four categories

of network configuration that are considered to be

basic are described below.

User-to-User Network Integration

The fully integrated, ATM-based user-to-user

network—defined as having the “edge” of the network

at the workstation—has been the subject of much

The edge of the network would be the ATM

switch, not the customer’s premises. The access network

could be treated as the user’s responsibility or it may

not support QoS controls at all. The service provider

provides edge-to-edge service level guarantees but

cannot ensure end-to-end performance.

End-to-End Network Integration

The third category of access method places

the “edge” of the carrier’s network in the customer

premises, at the end of the WAN but not all the way

to the workstation. The access network can either be a

separate, interconnected sub-network under the control

of the carrier, such as a Frame Relay, or can be treated

as an extension of the core WAN switch. These two

alternatives are illustrated below and on the next page.

ATMWide AreaNetwork

Figure 7 —Carrier Supplied Customer Premises Access Device

Network End Network Edge

CustomerPremises

ATM or non-ATMAccess Network

EdgeSwitch

Router orLAN Switch

ATMWide AreaNetwork

Figure 6 — Edge Switch-Based Implementation

Network EndNetwork Edge

CustomerPremises

ATM Network

Network Switch or

Edge Switch

EdgeSwitch


Placing a large edge switch in the customer

premises would be analogous to using a PBX in

telephony. It extends the WAN and dedicates a fully

functional network node to the customer. Since this is

a relatively expensive solution, it would be targeted to

the needs of larger offices or buildings. It might also

be used in very complex networks requiring a variety

of voice, data and multimedia interfaces to be concen-

trated over the same access network.

Establishing the customer’s site as an extension

of the central office has some advantages (such as

allowing multiple input) but there are also constraints.

For example maintaining a complex, carrier owned

switch that is remote from the central office is more

time consuming and may lack diagnostic and fault

management capabilities normally found in the customer

premises equipment. In summary, the premises-based

edge switch, while suitable for larger organizations

that warrant a full-featured, high-cost device, is not

cost-effective for smaller offices.

A variation on this theme is illustrated below.

In this case some functions of the edge switch are

located at the customer premises while other remain at

the central office. This effectively creates a distributed

edge switch—much the way an ISDN connection

distributes some functionality to the terminal adapter.

ATMWide AreaNetwork

Figure 8 —Carrier Supplied ATM NTU/Internetworking NTU

Network EndNetwork Edge

CustomerPremises

ATM Access Network

EdgeSwitch

ATM NTU orInternetworking

NTU



access to or control over the customer’s equipment,

and may not even be familiar with its operation.

Responsibilities for service restoration in a failure

situation are not clear. The carrier would certainly be

unable to perform moves, adds and changes without

considerable coordination with the customer. The need

for testing for conformance to standards would also

need to be ongoing, since changes in either end can

affect the other. This approach is seldom satisfactory

except in a centrally controlled private network (where

all components are under the control of a single

manager). Even in this case the switches will not be

able to support and control QoS and will not be able

to prevent users from affecting one another.

Provide a low-cost network termination device

at the customer site.

An ATM-NTU is a fixed function device whose

role is to create the formal boundary between the

network-provided components and the customer’s

equipment. It would not replace the carrier edge

switch but will simplify its operation. The ATM-NTU

supports the provider’s needs for remote monitoring

and control of the access network without duplicating

expensive switching functions. It acts as a guard—

checking and possibly correcting the user traffic that

is entering the public network.

An Interworking-NTU is similar to an ATM-NTU,

performing all the functionality described above: traffic

monitoring and control. In addition it performs concen-

tration of multiple services (voice, LAN, Frame Relay,

ATM, etc.) over a single ATM link. Most offices have

routers, Frame Relay multiplexers, PBXs and other

legacy devices. The Interworking NTU concentrates

the heterogeneous traffic types from all these devices

over a single ATM link.

A carrier owned, special-purpose network termina-

tion unit (NTU) located at the customer premises is

a solution that satisfies many of the requirements

discussed earlier. ATM functions, as well as network

interworking, can be provided as close as possible to the

user source, the size and complexity of the ATM edge

switch will be reduced since many functions will occur

at the NTU and the overall cost of operation will be

lower.

Three Alternatives for Building an ATM-NTU

Use a provider-supplied edge router or switch

in the customer premises.

The carrier could install a full edge switch or a

router to support the local networking environment (the

end-to-end integration approach). The switch needs to

be modular and scaleable enough to handle physical

moves, adds and changes. It would need to be config-

urable to support a wide range of protocols (whether

the customer has the need or not) and manageable

both remotely and locally. The owner of the switch

must manage and control faults and performance from

end-to-end, identify and report user errors and allow

integration with customer management systems.

Current edge switching solutions are intended for heavy

traffic, multiple applications, traffic monitoring, band-

width grooming and shaping, and VC management.

Multiple modular platforms and the heavy processing

power required to perform the QoS tests on a per cell

per customer basis make this an expensive solution.

Use an existing user LAN or workgroup switch

interface.

The carrier could rely on the ATM capabilities

of the CPE to provide a network interface point. This,

however, does not establish a stable reference point for

service access. The carrier is unlikely to have remote

Fault Localization: CPE vs. Central OfficeOne of the key issues in network control and diag-

nostics is the ability to isolate performance difficulties

between the customer CPE and the network. By having

the correct, fully functional NTU device located on the

customer premises, carriers can quickly identify the

source of network problems. The network management

equipment that is implemented both in the carrier and

the private network environment is usually built around

the SNMP protocol and its associated operational suite

of tools. By incorporating these functions within the

ATM NTU architecture, carriers are able to identify

the source of problems in the general sense and can

localize problems according to their nature and appli-

cation.

ManagementNTU devices that support the SNMP management

protocol and standard SNMP MIBs together with a

mature and intuitive GUI interface should support

both UNIX and PC based platforms. This helps to

insure the ability to integrate management into an

existing backbone management platform, enabling

centralized management and maintenance functional-

ity such as remote firmware download and diagnostics.

Media and Rate ConversionRate Conversion is accomplished between dissimilar

interfaces by extracting ATM cells from one interface

at one speed and sending them over a second interface

at another speed. For example, rate conversion could

enable the connection of an OC-3 155 Mbps fiber

interface to a T-3 or E-3 carrier interface. Effective

NTU devices use high capacity buffering with multiple

levels of priorities to convert between rates and signal

types, while preserving the Virtual Channel and Virtual

Path traffic characteristics. CBR traffic is assigned to


the high priority queue while VBR –RT to a lower

priority, then VBR-NRT lower still, and finally UBR

traffic to the lowest priority. Media conversion, which

essentially allows the interoperability between diverse

media type such as fiber, or twisted pair copper wire,

requires that interconnectivity happen at the lowest,

electrical, layer of the protocol stack. This addresses

the nature of the physical port configuration as well as

the method used to propagate a signal. For example,

fiber typically uses the SONET signaling architecture

which is different from what is used for T1 or E1. A

properly equipped NTU has the capacity to intercon-

nect a wide range of media and rate characteristics

and assure effective end-to-end signaling.

Network and Service InterworkingAlmost every office has multiple networking

devices: PBX for voice connectivity, routers for LAN

traffic, possibly Frame Relay access devices to concen-

trate voice and data traffic over a single link or even

ATM LAN switches. The ATM access device should

be able to simultaneously support several of these

applications and concentrate the different traffic sources

over a single ATM link. This simplifies the user-carrier

relationship: service billing, installation and control, by

limiting them to a single link and a single service.

Simplifying the ATM Access NetworkThe key to simplifying any network is to provide

a comprehensive management system that covers all

parts of the network, and which provides a clear division

of authority among the network’s managers and crisp

specifications for the services to be provided. The most

critical division, between the customer and the carrier,

is where an ATM-NTU or Interworking-NTU can be

utilized.


Using such an NTU as a network interface, as

described earlier, meets the following key requirements:

• It is the most cost-effective platform for the

medium or small office, for both public and

private networks;

• It can be restricted to specific services or single

link ATM access concentration, thereby reducing

complexity and a potential source of congestion;

• It allows dedicated or flexible functionality in a

compact package focused on specific

applications; and

• It reduces life cycle costs by increasing

manageability and reducing component

inter-dependencies.

Since an ATM access facility is required for every

attached customer network, it can represent a significant

component of the per-user costs. Simplifying and

improving the access network devices can reduce

deployment costs, improve management effectiveness

and offer a competitive advantage for the carrier.

Some functions of ATM networks must be located

at the ends of the network while others are distributed

throughout the network itself. These functions can

generally be categorized according to whether they act

on a single traffic flow or on all traffic in a link. The

ATM-NTU or Interworking-NTU provides a simplified

version of an edge multiplexer with all its benefits:

QoS control, traffic assurance, access concentration,

service interworking, etc. In addition, all the benefits

previously listed, such as end-to-end control, protecting

users from each other, etc., are also available. Imple-

menting these in a single user device is much easier

than would be possible in a highly shared environment.


Simplifying the Carrier Edge Switch

The edge switches at the carrier end of the access

network can also be simplified as a result of implement-

ing the NTU concept. Since all the media conversions,

performance management and traffic concentration

functions can be done at the customer premises, the

edge switches can be optimized for ATM-to-ATM

switching.

The ATM-NTU or Interworking-NTU, being an

application-specific device, offers other design and

implementation simplifications—it does not have to be

large modular solution, but a simple dedicated device

or small hub. They are built to address specific applica-

tions (voice and LAN, voice and Frame Relay, etc.), and

not to adapt to every possible situation. They are also of

limited size—1-3 ports on the user side and one network

port (with optional redundancy) would be a typical

requirement for a smaller office, or slightly more with

possible modularity for larger sites.

Additional ATM-NTU or Interworking-NTU

Functions

The fundamental purpose of the ATM customer

premises access device is to insert a “border” between

the network facilities and the CPE, thereby providing a

stronger measure of provider control over their resources.

This is similar to the specifications for such devices as

CSU/DSU or ISDN terminal adapters.

The NTU is a remote access device that meets

ATM Forum standards. The diagram on the next page

illustrates functions that this type of device could

usefully incorporate.

Additional ATM- NTU Functions

The ATM-NTU acts on behalf of the carrier in

performing the following functions:

• Device Management: The functions performed by

the ATM-NTU must be managed by the carrier

remotely, requiring support for SNMP and

TCP/IP. Remote control over the ATM-NTU

is needed by the carrier both to minimize opera-

tions costs and to allow sophisticated OAM

support. Device management functions include

fault management, security management, and

configuration management. The ATM-NTU

and Interworking-NTU should be able to perform

loopback testing in order to localize faults. It is

also an ideal point of attachment for software

and hardware monitoring systems.

• Access Concentration: Access concentration performs

multiplexing of data, but not full switching, at

the ATM customer premises access device level.

• Access Circuit Diversity: It may be desirable to provide

multi-line capabilities in the NTU. Having two

connections to the ATM WAN would permit a

backup capability and would allow incremental

increases in speed. This is not always a require-

ment in smaller offices.

Figure 9 — ATM NTU / Interworking NTU

USER

• Device Management

• Network & Service Interworking

• Rate & Media Conversion

• QoS Control

• Fault Management

• OAM Support

Traffic:

PCRSCR

CDVTMCRBS

PUBLICNETWOR


Carrier Perspective

As ATM is deployed to larger numbers of end

users, the value of the NTU concept grows stronger. It

enables cost effective, high quality service that is readily

deployed and offers the following benefits:

Improved Service Quality

• End-to-end control

• Protects users from each other

• Reduced maintenance costs

• Fewer service calls—user access to fault

information

• Fault prevention

• Better fault localization and diagnostics

• Service flexibility—media and rate conversion

• Reduced cost—simplifies edge devices

• Service interworking

Benefits of the Simplified ATM-NTU andInterworking-NTU

Using an ATM-NTU or Interworking-NTU

provides a number of important benefits that can make

ATM-based networks more successful. The advantages

can be technical, organizational, operational and

economical.

Cost-effectiveness

The ATM-NTU or Interworking-NTU is likely to

be constructed in compact, standalone or rack-mounted

units that are geared for volume purchases. It is designed

to allow either local or remote setup and monitoring,

thereby improving life cycle costs. As a result of its sim-

plicity, the NTU is much more cost-effective than a full


edge switch for its target market.

Also contributing to the cost-effectiveness is the

fact that by performing all the functionality at the

customer premises simplifies the operation of edge

switches—they no longer need to support multiple

technologies to perform the access concentration. They

only have to “know” ATM both on the user side and

on the network side. This reduces the cost of the edge

multiplexes and reduces the overall cost of the ATM

network for the carrier.

Network Management

Perhaps the most significant advantage of the

ATM-NTU or Interworking-NTU is its potential as a

tool for managing the network. Having a well-defined

boundary between the user and provider reduces

management finger-pointing and provides test and

monitoring access at a point which represents a common

view for both user and provider.

The ATM-NTU can also help to separate archi-

tecture and planning decisions. Things on one side of

the device are less dependent on what’s on the other

side. Without an ATM-NTU the service provider would

need to be involved in upgrades and changes to the

user’s interface equipment which, in a large network,

can become a costly exercise. It is therefore a more

effective growth path for service deployment.

Supporting rate and media conversions, including

the ability to handle the various flavors of ATM,

provides greater flexibility. This can make network

transitions and upgrades easier to plan and to justify.

User-Provider Relationship

The advantages of an ATM-NTU or Interworking-

NTU can also be understood by examining the benefits

from the service provider’s and the user’s perspective.

The NTU provides the necessary separation

between the user portion of the network and the


provider’s portion, thereby allowing end-to-end control

and management by the provider. The NTU is a point

where traffic is formally handed over to the network—

but only if it conforms to contract characteristics.

Protecting the network from abuse has to be a primary

consideration of the network provider. The use of an

NTU approach meets both these needs at an acceptable

cost.

Achieving control over the traffic coming onto the

network at the customer premises reduces the load on

the access network, avoids overhead in the ATM access

network switch, and ensures lower overall costs. Faults

can be localized more easily, and additional monitoring

points can be provided.

Assuring QoS conformance at the customer site

leads to better service, ease of deployment and lower

cost.

The network termination unit, which serves as the

provider’s “representative” at the customer site, should

also be able to identify user problems and report them

to the manager, thereby providing traffic assurance up

to the customer premises.

The network user, of course, wants a problem-free

environment that satisfies the organization’s require-

ments for speed, QoS, and accessibility. The availability

of the ATM-NTU or Interworking-NTU allows better

service and problem resolution, isolation from other

users, flexibility of service media, lower cost, speedy

deployment, elegant and simple solution, and a custom

fit.


Summary and Conclusions

Customer premises access devices such as ATM

NTUs or Interworking-NTUs extend carrier control to

the customer premises bringing important benefits such

as increased user satisfaction, lower maintenance costs,

flexibility of service offerings and greater service assur-

ances. The ATM NTU functionality can be accommo-

dated in a suite of interface access devices, enabling

the carrier to have standard control of both native

ATM and multi-service contracts.

It has been claimed that ATM cell switching is a

major step forward in the evolution of networks. ATM-

based WANs allow network service providers to offer

multi-service, multi-protocol, QoS-enabled transport

capabilities, thereby providing a platform for a new

generation of distributed applications. Since both large

and small offices will require access to ATM networks,

the need for scalability in an ATM network access

device is evident.

Today’s solutions for user access to ATM can be

too costly and over-sized for many of today’s needs. An

elegant and cost-effective solution is the ATM-NTU or

Interworking-NTU. It has been shown that the ATM-

NTU provides an answer that meets the need. It is easy

to deploy, sized right for its task, provides QoS support,

and is easily managed remotely.

The ATM-NTU supports the traditional network

provider’s requirement to maintain control over their

resources and to protect the network against misuse.

But it is also beneficial to end users who want end-to-

end support.

26 • Simplified ATM Access at the Customer Premises Case Study • 27

CASE STUDY 1:Efficient and Simple Control of ATM Quality of Service

Following large-scale ATM service trials, and prior

to rolling out nationwide and international native

ATM services, a major European carrier determined

the need for a carrier owned and controlled ATM

Network Termination Unit to be provisioned at the

interface between the customer premises and the public

ATM network. This would serve as the demarcation

point between the ATM environment at the customer

premises and the carrier’s network. The device had to

be QoS enabled, offer a superior cost-performance

ratio, and support a minimum number of VP/VCs (256).

The carrier also required that flexible LAN/WAN con-

nectivity options be built into the device, for maximum

on-the-spot compatibility between diverse end user

equipment and carrier transmission interfaces.

The ACE-101 answers all these specifications. In

particular, ACE-101 satisfies the carrier’s requirements

for implementing Operation, Administration and

Maintenance (OAM) support for end-to-end ATM

service monitoring, including Continuity Check (CC),

Alarm Indication Signal (AIS), and Remote Defect

Indication (RDI).

ACE-101 also provides Performance Management

(PM) features, checking peak cell rate, cell delay varia-

tion tolerance, sustainable cell rate, and maximum burst

size of traffic entering the public network. Loopback

cells are used to measure cell loss and cell delay para-

meters end to end.

Of special interest to the carrier is the ACE-101’s

ability to carry out “Reverse PM,” collecting statistics

on what is transmitted by the NTU into the network

as well as information on what is received from the

customer premises into the NTU device.

Policing of ATM traffic is applied to all connections,

preventing users from violating their service contract

by transmitting unauthorized ATM cells into the public

network. In parallel, the carrier implements Policing at

the edge switch to stop illegal user attempts to circum-

vent the ATM NTU.

As ACE-101 demonstrates, the carrier is able to

exercise maximum control over ATM parameters,

ensure traffic conformity with a low-cost dedicated

device managed from a central location. For end users,

ACE-101 offers modularity—adapting to diverse inter-

faces—redundancy, and essential traffic shaping (buffer-

ing of multiservice traffic according to predetermined

priority settings).

The benefits of the ACE-101 family may also be

applied to other ATM service applications, such as at

the boundary between carrier jurisdictions in interna-

tional networks.

Figure 10 — Efficient and Simple Controlof ATM Quality of Service

Switch

NMS

Public Network

• QoS control• Segment OAM between edges, including:

Fault Management - AIS, RDI, CC, LBPerformance Management - Forward and BackwardContinuity checks

SwitchSwitch

Customer Premises

ACE-101

Customer Premises

ACE-101

Customer PremisesATM datain-band managent

ACE-101User

Network

UserNetwork

Check ingresstraffic conformance

• Policing• Monitoring

UserNetwork


CASE STUDY 2:Corporate Network Migrationto ATM Services

How organizations with large multi-branch networks

can migrate from Frame Relay services to ATM, and

achieve lower communication costs.

Current Network Configuration:

• Headquarters—the corporation purchased a new

PBX for it headquarters with an ATM network

interface, which can support thousands of users.

The corporation plans to support 240 simultane-

ous calls to its 30 branch offices. Also, an ATM

LAN switch is used.

•Regional offices—have both PBX and Routers with

a Frame Relay interface. Their E1/T1 network

link is expected to suffice for the next 2 years.

After that time, however, they will require higher

data rates.

• Branch offices—use Frame Relay access concentra-

tors to concentrate voice, LAN and legacy SNA

traffic over a single Frame Relay Link at

fractional E1/T1 rates.

ATM migration objectives

• Minimal investment in new equipment—use

existing Frame Relay devices as much a possible.

• Migrate regional offices and headquarters to

ATM access concentrators to future-ready their

network to broadband (above E1) ATM services.

The ACE family solved the corporate needs as

described in the following diagram.

Case Study • 29

ACE-2005, a large modular ATM access concen-

trator (AAC), was used to concentrate eight E1 ports

from the PBX with the E3 ATM port from the LAN

switch over 155 Mbps ATM link.

Each Regional office installed an ACE-20, low-end

low-cost access concentrator, which provided an E1

PBX interface and E1 or V.35 Frame Relay interface

to the router. As bandwidth requirements increase, the

ACE-20 can later scale up to the ACE-200, which

supports multiple E1 PBX interfaces and multiple

Ethernet or Frame Relay ports as well as higher ATM

rates (up to 155Mbps).

ACE-2 preserved the existing investment in Frame

Relay equipment. This simple low-cost Frame Relay

to ATM converter supports both network and service

interworking, converting the Frame Relay traffic to

ATM cells.

As this study shows the ACE family can offer a

simple low-cost migration to ATM giving organizations

the flexibility to use existing equipment or migrate to

ATM-based equipment as bandwidth requirements

grow.

Figure 11 — Corporate WAN Migration to ATM

ATM

ACE-20

ACE-2

n x 64 / E1

PBX Router

Regional Branch Office

Frame RelayE1

PBX

ACE-2005

LAN Switch

Headquarters

E3 8 x E1

155 Mbps

FRAD

PBX Router

Branch Office

Frame RelayE1

E1

30 • Simplified ATM Access at the Customer Premises Glossary • 31

Glossary

Bridge—A device that connects and passes packets

between two network segments. Bridges operate at

Layer 2 of the OSI reference model (the data-link

layer) and are insensitive to upper-layer protocols. A

bridge will examine all frames arriving on its ports and

will filter, forward, or flood a frame depending on the

frame’s Layer 2 destination address.

Cell—For ATM, most vendors have agreed that this

information “package” will be developed consisting of

53 bytes or “octets”. Of these, the first 5 constitute the

header; 48 carry the payload

Cell Delay Variation (CDV)—ATM performance

parameter which specifies the potential variation (+/-)

from the expected average transit delay through the

network over a given virtual circuit.

Cell Error Ratio (CER)—ATM performance para-

meter which specifies the ratio of errored cells to the

total cells transmitted over a given virtual circuit.

Cell Loss Priority (CLP)—A 1-bit field in an ATM

cell header that provides a two level priority indicator.

Used to bias the discarding of cells toward lower prior-

ity cells in the event of congestion. Similar to the DE

bit in frame relay.

Cell Loss Ratio (CLR)—ATM performance para-

meter which specifies the ratio of lost (non-delivered)

cells to the total cells transmitted over a given virtual

circuit.

Cell Transfer Delay (CTD)—ATM performance

parameter which specifies the average transit delay of

cells between a source and destination over a given

virtual circuit.

Central Office (CO)—(1) A local telephone company

office which connects to all local loops in a given area

and where circuit switching of customer lines occurs.

(2) A local Telephone Company switching system,

where Telephone Exchange Service customer station

loops are terminated for purposes of interconnection

to each other and to trunks. In the case of a Remote

Switching Module (RSM), the term Central Office

designates the combination of the Remote Switching

Unit and its Host.

Channel Service Unit/Data Service Unit

(CSU/DSU)—A digital interface unit that connects

end user equipment to the local digital telephone loop.

Circuit—A two-way communications path. (2) A

communication path or network; usually a pair of

channels providing bidirectional communication.

Circuit Emulation—A connection over a virtual

circuit-based network providing service to the end users

that is indistinguishable from a real, point-to-point,

fixed-bandwidth circuit.

Committed Burst (Bc)—Amount of data allowed

in time T=Bc/CIR without being marked DE.

Concentrator—Device that serves as a wiring hub

in star-topology network. Sometimes refers to a device

containing multiple modules of network equipment.

Congestion—Excessive network traffic.

Congestion control—Network management issue

for the controlling of traffic flow so switches and end-

stations are not overwhelmed with information and

cells subsequently dropped.

Constant Bit Rate (CBR)—Delay intensive applica-

tions, such as video and voice, that must be digitized

and represented by a continuous bit stream. CBR

traffic requires guaranteed levels of service and

throughput.


Data Exchange Interface (DXI)—(1) ATM: A

variable-length frame-based ATM interface between

a DTE and a special ATM DSU/CSU. The ATM

DSU/CSU converts between the variable-length DXI

frames and the fixed-length ATM cells. (2) Defines the

format for transmitting information that has gone

through the ATM convergence sublayer.

Data Service Unit—Device on the customer end

of a digital circuit that provides framing of sub-rate

(under 64 Kbps) customer access channel(s) onto

higher rate data circuits. May be combined with a

CSU in a single device.

Digital PBX—A private branch exchange (PBX)

that operates internally on digital signals. Thus, voice

signals must be digitized for use in the PBX.

DSU/CSU—Equipment used to terminate a Switched

56 line and convert a PC’s digital data signal into a

digital transmission signal.

E1—The term for a digital facility used for transmitting

data over a telephone network at 2.048 Mbps. The

European equivalent of T1.

E3—The highest transmission rate generally available

in the European digital infrastructure (34 Mbps).

Edge Switch/Access Router—Device used to take

frames from LANs and send them over an ATM network

as cells. Normally provides LAN emulation and, if

used for PBX connections, circuit emulation.

Excess Burst (Be)—Transient capacity above CIR

in FR net.

Frame—A logical grouping of information sent as a

link-layer unit over a transmission medium. The terms

packet, datagram, segment, and message are also used

to describe logical information groupings at various

layers of the OSI reference model and in various tech-

nology circles.

Glossary • 33

Frame Relay—High-performance interface for

packet-switching networks. Considered more efficient

than X.25 which it is expected to replace. Frame relay

technology can handle “bursty” communications that

have rapidly changing bandwidth requirements.

LAN Emulation—A technique for legacy LAN

MAC-layer protocols like Ethernet and token ring, to

work transparently across an ATM network.

LAN Emulation Client (LEC)—ATM Forum-

defined specifications in support of LAN-to-LAN

connectivity, called LAN Emulation. LEC defines that

set of functions implemented in an ATM network that

provide LAN DTEs with information regarding the

location of the other LAN Emulation services.

Local Area Network (LAN)—(1) A network cover-

ing a relatively small geographic area (usually not

larger than a floor or small building). Compared to

WANs, LANs are usually characterized by relatively

high data rates. (2) Network permitting transmission

and communication between hardware devices, usually

in one building or complex.

Local Loop—The line from a telephone customer’s

premises to the telephone company Central Office.

Multiprotocol encapsulation over ATM—The

process for enabling an ATM device or application to

add a standard protocol identifier to the LAN data

which allows higher-layer protocols, such as IP, to be

routed over ATM.

Network Terminating Equipment (NTE)—A

grouping of ISDN functions at the boundary between

the ISDN and the subscriber.

Postal, Telegraph and Telephone Company

(PTT)—Generic term for a provider of these services.

A governmental agency in many countries.

34 • Simplified ATM Access at the Customer Premises Glossary • 35

Quality Of Service (QOS)—Term for the set of

parameters and their values which determine the

performance of a given virtual circuit.

Router—(1) An OSI Layer 3 device that can decide

which of several paths network traffic will follow based

on some optimality metric. Also called a gateway

(although this definition of gateway is becoming

increasingly outdated), routers forward packets from

one network to another based on network-layer infor-

mation. (2) A dedicated computer hardware and/or

software package which manages the connection

between two or more networks.

Routing—The process of finding a path to the desti-

nation host. Routing is very complex in large networks

because of the many potential intermediate destinations

a packet might traverse before reaching its destination

host.

Switch—In the context of Frame or LAN switching,

this refers to a device which filters, forwards and floods

frames based on the frames destination address. The

switch learns the addresses associated with each switch

port and builds tables based on this information to be

used for the switching decision. Some switches are high

speed implementations of bridges where switching

decisions are made in silicon, usually an Application

Specific Integrated Circuit (ASIC).

Switched Virtual Circuit (SVC)—A virtual link,

with variable end-points, established through an ATM

network. With an SVC, the user defines the end-points

when the call is initiated that are subsequently termi-

nated at the end of the call. With a PVC, the end-

points are predefined by the network manager. A single

virtual path may support multiple SVCs.

T1—(1) Digital transmission facility operating with a

nominal bandwidth of 1.544 Mbps. Also known as

Digital Signal Level 1 (D1). Composed of 24 DS-0


Virtual Path Connection (VPC)—Virtual paths

in two or more sequential physical circuits can be

concatenated 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.

Virtual Path Identifier/Virtual Channel

Identifier (VPI/VCI)—Combined, these fields

identify a connection in the ATM network.

Glossary • 37

channels in many cases. The T1 digital transmission

system is the primary digital communication system in

North America. (2) A high-speed 1.5 mbits/sec leased

line often used by companies for access to the Internet.

Traffic Shaping—Allows the sending station to

specify the priority and throughput of information

going into the ATM network and subsequently monitor

information progress to meet required service levels.

Undefined Bit Rate (UBR)—Traffic class defined

by the ATM Forum.

Unshielded Twisted Pair (UTP)—Four-pair wire

medium used in the transmission of many different

protocols such as Ethernet, 10BaseT, and CDDI.

User Parameter Control (UPC)—Traffic policing

to ensure that the defined peak traffic rate is not

exceeded in the ATM switch.

Variable Bit Rate (VBR)—Applications which

produce traffic of varying bit rates, like common LAN

applications, which produce varying throughput rates.

Virtual Channel—A defined route between two end

nodes that may access multiple virtual paths.

Virtual Channel Connection (VCC)—Virtual

channels in two or more sequential physical circuits can

be concatenated to create an end-to-end connection,

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.

Virtual Channel Identifier (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.

Virtual Path—A group of virtual channels, which

can support multiple virtual circuits.

38 • Simplified ATM Access at the Customer Premises Notes • 39

NOTES NOTES

Visit ATG’s Web Site

to read, download, and print

all the Technology Guides

in this series.

www.techguide.com

“The significant problems we face cannot be solved

by the same level of thinking that created them.”

Albert Einstein

www.rad.com

U.S. Headquarters International HeadquartersTel: (201) 529-1100 Tel: 972-3-6458181Fax: (201) 529-5777 Fax: 972-3-6498250

email: [email protected]

PABX

ACE-2005

FR/ATM Device

PABX

ACE-20

E1/T1 E1/T1

E1/T1, E3/T3, STM-1, OC-3C

E3/T3, STM-1, STS-3C

E1/T1, E3/T3, STM-1, STS-3C

FR/ATM Device

PABX

ACE-200 ACE-2 ACE-101

ATMTransport

Frame RelayATM/DXI

ATMBackbone

ACE Family of ATM Access ProductsThe ACE family of ATM Network Termination Units(NTUs) and Interworking Access Devices offers real benefits to carriers and their ATM customers:

• Improved service quality—protecting users fromeach other

• Reduced maintenance costs through fewer service calls

• Better diagnostics

• Greater service flexibility

• Faster deployment of native ATM services

The ACE family includes:

• ACE-101 ATM Network Termination Unit

• ACE-2 ATM Single Service Interworking NTU

• ACE-200 ATM LAN and Voice Interworking NTU

• ACE-20 ATM Triple Service Interworking NTU

• ACE-2005 ATM Multiple Service Inteworking AccessConcentrator

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, published

by ATG, designed to put complex

communications and networking

technology concepts into practical

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 well

as the other Communications and

Networking Technology Guides

in the series, are available

on ATG‘s Web Site.

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