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PDH, Broadband ISDN, ATM, and All That:

A G u i d e t o M o d e r n WA N N e t w o r k i n g , a n d H o w i t E v o l v e d .

by Paul Reilly MSD Marketing Silicon Graphics, Inc.

April 10, 1994

SUMMARY: This white paper looks into the wonderful world of ATM andBroadband ISDN. The intent of this paper is to explain these emerging technologies, how they evolved, and explore the impact they mighthave on the computer industry. Various related technologies such as X.25, ISDN, and PDH are also covered. While it is assumed that youhave some knowledge of LAN computer networking, we have tried to keep this paper focused on the non-expert.

Acknowledgments: We wish to thank the following people who have contributed to this white paper(in alphabetic order):

Nelson Bolyard, Scott Bovenizer, Greg Chesson, John Talbott, Jon Thompson,and Rob Warnock.

(C) Copyright 1994, Silicon Graphics, Inc. All Rights Reserved PDH, Broadband ISDN,ATM, and All That: A Guide to Modern WAN Networking, and How it Evolved.

Silicon Graphics, Inc. Mountain View, California

UNIX is a trademark of UNIX System Laboratories

Section 1 Introduction.................................................................................. 1

1.1 ISDN vs. B-ISDN ......................................................................................2 1.2 B-ISDN vs. ATM........................................................................................2 1.3 SONET and SDH......................................................................................2 1.4 LAN vs. WAN ............................................................................................ 3 1.5 Connectionless vs. Connection-Oriented Communications......................4 1.5.1 Connectionless .........................................................................................5 1.5.2 Connection-Oriented.................................................................................5 1.5.3 The Gray Area in Between. ......................................................................6 1.5.4 Summary...................................................................................................6

Section 2 The History of B-ISDN: How it all began.................................... 7

2.1 How It All Began. ......................................................................................7 2.2 The Development of Signaling..................................................................7 2.3 Transoceanic Telephony. ..........................................................................8

Section 3 Modern Telephony. ...................................................................... 9

3.1 The History of Analog Telephony. .............................................................9 3.2 Digital Telephony--The Basics. .............................................................. 10 3.3 Digital Telephony--The Sampling Theory...............................................13 3.4 Why Bother? ........................................................................................... 14 3.4.1 Noise Reduction in Digital Transmissions...............................................14 3.4.2 Digital Switches are Cheaper.................................................................. 16 3.4.3 Digital Transmission is Cheaper. ............................................................ 18 3.4.4 Signaling is More Secure........................................................................ 18 3.5 The Downside of Digital Telephony......................................................... 19

Section 4 B-ISDN -- The Raison d'Etre. ................................................... 21

4.1 The T1 Digital Transmission Lines.......................................................... 22 4.1.1 E-1--the European Equivalent of T1. ..................................................... 23 4.1.2 The Digital Services, or What the Wire Carries. .....................................23 4.2 The Issues. ............................................................................................. 24 4.2.1 The Grand Plan That Isn't....................................................................... 25 4.2.2 Why Ever Did They Do That? ................................................................. 26

Section 5 X.25 ............................................................................................. 28