Computer Networks: The Physical Layer

ATM and SONET

This page will provide a compilation of outside sources for understanding ATM and SONET, and their relationship to each other and the communications world.

Tips and Hints on ATM

Kumar's Advantages of ATM

  • flexibility--ATM can be easily evolved for future services
  • efficiency in the sue of its available resources
  • it is one simple universal network
  • reduction of operation, administrative, and maintenance costs
  • reduction of transport costs (by statistical multiplexing)
  • provision of dynamic bandwidth allocation
(from Broadband Communications, page 142)

ATM vs. Traditional TDM

ATM has an advantage over the normal TDM multiplexing in that it will pack information from its various sources into cells, discard the empty cells, and then place the valid cells into the multiplexed line. This way no bandwidth is wasted on empty cells. (Kumar 143)

The ATM Cell

The ATM cell is a 53-byte entity, with 48 bytes being payload (i.e., data) and the other 5 being a header that contains routing information. It was designed to be small and fixed-sized.

Is the information on ATM confusing?

Part of the reason might lie in the fact that ATM is a *very* new technology. Standards are being defined as you read this, but it is not that easy. ATM is so popular that many people are researching ways to improve it, and standards in the making will likely go with the best methodology, so if you read one thing about ATM today and a different thing next week, don't worry: ATM technology is just changing that fast.

Source: Roberts, James, Ugo Mocci, and Jorma Virtamo, eds. Broadband Network Traffic: Performance Evaluation and Design of Broadband Multiservice Networks. Berlin: Springer, 1996.

Tips and Hints on SONET

Kumar's Advantages of SONET

  • The direct synchronous multiplexing used in SONET is the key to flexible, cost-effective worldwide telecommunications networking.
  • SONET has built-in maintenance capabilities which aids in its own transmission, as well as aiding the upper levels of the network.
  • SONET is backward-compatible with all of the currently existing networks, being able to transmit appropriate signals for any network.
  • SONET is suitable for use in all of the major networking areas: long-distance networks, LANs, local loops, and cable TV.

SONET Data Transmission Rates

SONET data transmission rates are stated in two ways: OCn and STS-n. They mean the same thing, and the numbers correspond. All OCn and STS-n are multiples of OC1/STS-1. Unlike the T series lines (e.g., T1, T2, T3), SONET standard values do not increase by adding 1. The possible values of n (so far) are 1, 3, 12, 24, 48, 192. Also unlike the T series lines, SONET's numbers correspond to the multiple of the OC1/STS-1 value--so OC3 = 3 * OC1. The basic rate (OC1 or STS-1) is a speed of 51.84 Mbps.

The SONET Frame

Section Overhead
[3 bytes wide]
[3 rows tall]		 Payload Envelope Area
[87 bytes wide]
[9 rows tall]
Payload Pointer
.
Line Overhead
[3 rows tall]
.

Statistics:

  • 1 Row x 1 Column = 1 byte
  • Total Frame Size = 810 Bytes (9 rows x 90 Columns)
  • Transport overhead = 27 Bytes (9 rows x 3 columns)
  • Payload Capacity = 783 bytes (9 rows x 87 Columns)
  • Payload Overhead = 27 bytes
  • TOTAL USEABLE PAYLOAD = 756 Bytes
The overhead is used for system maintenance tasks, such as frame alignment.

(from Kumar Chapter 9)

Understanding SONET and ATM together

First things first. ATM and SONET are somewhat confusing. They sound alike in that they are fast ways of sending information across a line. But their names are contradictory (asynchronous vs. synchronous). Yet they are used together on B-ISDN service. What is going on?
  • ATM is the switching technology.
  • SONET is the transmission technology.
ATM is a logical way to organize the bits on a wire, but ATM does not care what kind of wire it is on. SONET, however, is concerned with getting the bits from one end of the line to the other, and it is very concerned with the wire type: it must run on fiber optics (its name stands for Synchronous Optical Network after all).

ATM and SONET can work together despite their opposing nomenclatures. ATM is asynchronous in that it will take information on a variation of first-come, first-served. SONET is synchronous in that it will multiplex according to a rigid clock schedule, sending a frame for each input line, whether actual information is on it or not. BUT this is precisely why ATM and SONET work so well together. SONET does not have to worry about wasted bandwidth too much if it works with ATM because ATM will do its best to optimize before SONET gets a chance to send an empty frame.

The Future of ATM and SONET

The networking world is very excited about the possibilities offered by ATM and SONET for B-ISDN. It is hoped and believed that these technologies will eventually replace out current worldwide network (phones, cable, and more). But it will not happen all at once, and it will not happen soon. The costs are high, and the technology is new. There is a good side and a bad side to waiting: the good side is that by waiting, we don't find out that a new network is obsolete right after we put it in, but the bad news is that if we always wait for the next good thing to come along, we will never get around to installing a new network.

Suggested Readings

Kumar, Balaji. Broadband Communications: A Professional's Guide to ATM, Frame Relay, SMDS, SONET, and B-ISDN. New York: McGraw-Hill, 1995.
This book does a good job of explaining the inner workings of broadband communication techniques, specifically the technologies that B-ISDN uses to provide its services. ATM (Chapter 8) and SONET (Chapter 9) are explained thoroughly, and in more simplistic terminology without leaving out technical details. A good place to start gaining a better understanding of ATM and SONET and how they work together to make B-ISDN work.

Killen, Harold B. Digital Communications with Fiber Optics and Satellite Applications. Englewood Cliffs: Prentice Hall, 1988.
For a more technical explanation of how sending data over fiber optics really works, try Chapter 10 of this book. It explains all of the physical principles that are involved in the propagation of light through the fiber strands.

Chaffee, C. David. The Rewiring of America: The Fiber Optics Revolution. Boston: Academic Press, Inc., 1988.
Refer to this book for an analysis of how the fiber optics technology has affected communications in America, as well as a discussion of the evolution of fiber optics technology. This book covers a broad sampling of applications of fiber optics.

Tantawy, Ahmed N., ed. High Performance Networks: Frontiers and Experience. Boston: Kluwer Academic Publishers, 1994.
This book is a compilation of essays on the hot topics of the mid/late 1990's. Articles discuss topics such as Aurora (Gigabit Technology), B-ISDN for multimedia, parallelism in communication, and ATM Networks These essays are not as easy to read as Kumar's book because they are more scholarly in nature, leading them to be technical and make assumptions about the reader.

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