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SONET vs. ATM: an either-or choice?

If you want a single network that can transport video and high-speed data and integrate low-end facility communications, you may not have to choose between SONET and ATM.

Life for the network manager was, if not exactly a bed of roses, at east easier when the typical end-user enterprise used one network for data and a separate one for voice.

Organizations today expect a single, integrated network to handle voice, data, video and multimedia traffic, from personal computers sharing data files on a local area network (LAN), to off-site employees videoconferencing with headquarters, to customers accessing information from the corporate World Wide Web site.

The industry offers two solutions for achieving a large integrated network: SONET/SDH (synchronous optical network and synchronous digital hierarchy) and ATM (asynchronous transfer mode). ATM, in particular, has been positioned as the ideal networking solution.

In reality, ATM handles some applications perfectly and others less well. Sometimes presented as competing solutions, SONET and ATM really are complementary technologies. Although the two technologies compete in certain markets, choosing SONET or ATM is not necessarily an either-or decision.

In this article, we will look at the differences between SONET and ATM, discuss the applications each technology handles best, and provide guidelines for the communications manager on selecting the right solution for his or her organization.


The more SONET and ATM are bandied about as be-all-and-end-all solutions, the muddier the distinctions between the two become. SONET is a globally accepted, non-proprietary standard for broadband transmission through fiber-optic cables. It handles transmissions from 51 Mbps to 10 Gbps.

SDH is SONET's international equivalent. SONET/SDH is a physical transport medium that occupies the two bottom layers (physical and data link layers) of the seven-layer Open Systems Interconnect (OSI) model.

ATM is a high-speed packet switching technique suitable for LAN, wide-area network and broadband ISDN (integrated services digital network) transmissions. It occupies the network and transport layers, the third and fourth layers of the OSI model.

SONET is a physical structure, while ATM is a transmission protocol. If the ATM protocol is used, a transport medium (such as SONET) is still needed to carry traffic over the network.


ATM handles data traffic smoothly, but runs into delay problems with voice transmissions. These delays introduce echoes similar to those heard on international telephone calls.

A voice network can be retrofitted with echo cancellation circuits, but these require expensive and difficult infrastructure modifications, which increase the cost of implementing an ATM network.

A second limitation to ATM stems from the large installed base of communications equipment without ATM capabilities. This older equipment was designed for low-speed data and voice applications, and uses slower, low-cost-per-channel protocols such as RS-232.

For example, most facilities today have separate, low-end security, voice, fire alarm and energy management systems. The security application usually includes surveillance cameras, badge readers, and low-end video systems. The voice network handles the facility's telephone communications.

Facility systems of this sort are not ATM-compatible, because their manufacturers have not adopted ATM as a standard communications protocol. If an end-user organization wanted ATM to support facility systems, it would have to install units to convert these low-tech functions to ATM.


Communications equipment has a 15-year life cycle, with network operators replacing 10% of their installed equipment annually. At that rate, it will be another decade before ATM is widely implemented.

ATM handles high-speed data and multimedia applications well, and is ideal for high-end videoconferencing applications such as distance learning and telemedicine.

The dilemma for the communications manager is how--and whether--to integrate slower, low-tech applications such as the facility systems just described with newer applications that are well-suited to ATM.

Instead of installing a single, integrated network to handle data, voice and multimedia, the information systems department might install an ATM backbone network for high-speed data and video applications, and not worry about integrating voice communications or lower-tech applications.

SONET fills the needs of the organization that cannot justify creating a separate ATM network which does not also support existing networks. A SONET ring is designed for larger facilities with building-to-building applications and multiple protocols, each handling a different task.

SONET is the ideal transport medium if the enterprise accesses the public network to tie together separate facilities as well as private networks within facilities. All major public U.S. networks carry ATM on top of SONET rings. While the public network still uses some DS3 (44.736 Mpbs) channels, SONET rings usually replace DS3s in newer applications.


SONET is a carrier adaptable to any kind of communications, and its flexible, open architecture has built-in redundancy in its transmission paths. The organization that needs to protect mission-critical applications can achieve redundancy with a SONET network.

As a transmission protocol, ATM lacks SONET's built-in redundancy. While an ATM network can be equipped with redundancy, this approach increases costs and moves away from standardization.

With a SONET network with ATM capabilities, the organization has a single network that can transport sophisticated video and high-speed data applications and integrate low-end facility communications systems.

The communications manager trying to choose the right network for his or her organization often focuses on the percentage of high-speed vs. low-speed applications. Here is a more comprehensive checklist for the manager who is evaluating network technologies:

* What size is the facility? Large facilities need SONET with an ATM overlay, while a smaller facility can install an ATM network and hard-wire other systems.

* Is access to the public network necessary? Will the network be spread over long distances? Is a large private network planned? If the answer to any of these questions is yes, the network requires SONET.

* What applications will the network support? A dedicated high-speed data or high-quality video network that does not need to integrate voice or low-speed facility systems is ideal for ATM.

* Does the network need to support voice as well as low-end security, fire alarm and energy management systems? If yes, a SONET ring with ATM capabilities is the best solution.

* Is redundancy required? If so, a combination SONET/ATM network is best.


Applications that need SONET

Larger facilities (Solution: install SONET ring with ATM overlay)

Access to the public network needed to tie together separate facilities and private networks within facilities

Network spread over long distances

Large private network

Need to integrate low-speed data, voice and existing low-end facility systems (fire alarm, security, voice, energy management)

Need for single, integrated network to handle enterprise's multimedia, voice, data and video traffic

Building-to-building applications and multiple protocols, each handling different tasks

Need for built-in redundancy to protect mission-critical applications

Applications that need ATM

Smaller facilities (Solution: install ATM network; hard-wire other systems)

Access to the public network not needed

Short-distance network

Smaller private network

No need to integrate low-speed data, voice of facility systems

Dedicated high-speed data application

High-quality video or videoconferencing applications -- i.e., telemedicine and distance learning

Built-in redundancy is not as vital
COPYRIGHT 1997 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1997 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Technology Information; includes related article on Cell Transfer Mode
Author:Shaunfield, John
Publication:Communications News
Date:Jul 1, 1997
Previous Article:Retooling education.
Next Article:Keep tabs on your ISP.

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