AT&T 555-025-201
DEFINITY® Communications System
Network and Data Connectivity
CONTENTS
_______________________________________________________________________________________
_______________________________________________________________________________________
ABOUT THIS DOCUMENT |
xvii |
PREREQUISITE SKILLS AND KNOWLEDGE
DOCUMENT ORGANIZATION
HOW TO MAKE COMMENTS ABOUT THIS DOCUMENT
xvii
xvii
xix
1. INTRODUCTION TO CONNECTIVITY |
1-1 |
TRANSMISSION TYPES
ANALOG TRANSMISSION
DIGITAL TRANSMISSION
ANALOG VS. DIGITAL
TRANSMISSION STATES
VOICE TRANSMISSION
Entering the Switch
Exiting the Switch
DATA TRANSMISSION
Protocol Layers
Protocols Used
Protocol States
Connectivity Rules
MULTIPLEXED COMMUNICATION
TYPES OF MULTIPLEXING
Frequency-Division Multiplexing
Time-Division Multiplexing
Statistical Multiplexing
MULTIPLEXING OVER DS1 FACILITIES
Line Coding
Signal Inversion
Framing
Signaling
1-3
1-3
1-3
1-5
1-7
1-7
1-7
1-9
1-11
1-11
1-13
1-14
1-15
1-17
1-17
1-17
1-17
1-18
1-18
1-19
1-21
1-24
1-25 iii
iv CONTENTS
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MULTIPLEXING OUTSIDE THE SWITCH
Multiplexing onto T1 Trunks
Compressing the Signal
Altering Channel Assignments on T1 Trunks
Getting the Signal Ready for the Central Office
Changing the Transmission Medium from Metallic to Fiber-Optic
Multiplexing with Microwave
Demultiplexing
STATISTICAL MULTIPLEXING
TRUNKING
APPLICATION
Local Exchange Trunks
Tie Trunks
Special-Access Trunks
Auxiliary Trunks
Miscellaneous Trunks
CONNECTIVITY
ADMINISTRATION OPTIONS
System 85/G2 Administration
System 75/G1 Administration
DS1 Options
INTERFACE CIRCUIT
TRUNK TABLES
1-29
1-29
1-31
1-31
1-31
1-32
1-32
1-32
1-34
1-35
1-35
1-35
1-36
1-37
1-37
1-37
1-38
1-39
1-39
1-41
1-41
1-42
1-42
2. COMMUNICATION SYSTEM NETWORKING — AN OVERVIEW |
2-1 |
TYPES OF NETWORKS
NETWORK EVOLUTION
SOFTWARE CAPABILITIES
Basic Networking Software
Main-Satellite/Tributary Software — The UDP and Multipremises Packages
Electronic Tandem Network Software — The PNA and ETN (Standard Network) Packages
Distributed Communications System
NETWORK CALL PROCESSING
2-2
2-3
2-4
2-4
2-5
2-6
2-7
2-7
CONTENTS v
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Internal Dial Plan
Auxiliary Call Information
Automatic Alternate Routing
Automatic Route Selection
NETWORK ADMINISTRATION
PRIVATE NETWORK TRUNKS
NETWORKING FEATURE PARAMETERS
2-8
2-10
2-11
2-14
2-17
2-17
2-17
3. TANDEM TIE TRUNK NETWORKS |
3-1 |
INTERACTIONS WITH OTHER NETWORKS |
3-1 |
4. MAIN-SATELLITE/TRIBUTARY (MS/T) NETWORKS THROUGH THE UDP OR |
|
MULTIPREMISES PACKAGES |
4-1 |
MS/T CONFIGURATIONS
Main-Satellite Configuration
Main-Tributary Configuration
MS/T FEATURES
Routing Incoming Calls
Routing Outgoing Calls
Other Routing Capabilities
ENGINEERING CONSIDERATIONS
INTERACTIONS
4-2
4-2
4-3
4-4
4-7
4-7
4-8
4-8
4-9
5. ELECTRONIC TANDEM NETWORK (ETN) THROUGH THE ETN AND PNA |
|
PACKAGES |
5-1 |
ETN CONFIGURATIONS
ETN TRUNKS
Access and Bypass Access Tie Trunks
Off-Net Trunks
FEATURES
Uniform Numbering Plan
Automatic Alternate Routing
5-1
5-2
5-4
5-5
5-5
5-5
5-7
vi CONTENTS
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Automatic Route Selection
ETN INTERACTIONS WITH OTHER NETWORKS
Extension Number Portability Clusters
Software-Defined Network
Release Link Trunk Networks — CAS and ACD
Distributed Communications System Clusters
Main-Satellite/Tributary Networks
ENGINEERING CONSIDERATIONS
5-7
5-7
5-8
5-9
5-12
5-14
5-14
5-14
6. DISTRIBUTED COMMUNICATIONS SYSTEM (DCS) |
6-1 |
DCS CLUSTERS |
6-2 |
DCS LINKS |
6-2 |
Tie Trunks |
6-3 |
Signaling Links |
6-3 |
DCS SIGNALING LINK CONNECTIONS |
6-8 |
System 75 or Generic 1 (G1) to System 75 or G1 |
6-8 |
System 85 or Generic 2 (G2) to System 85 or G2 |
6-10 |
System 75 or Generic 1 (G1) to System 85 or Generic 2 (G2) |
6-12 |
DIMENSION Signaling Links |
6-14 |
TRANSPARENT FEATURES |
6-14 |
Transparent Attendant Features |
6-14 |
Transparent Voice Terminal Features |
6-17 |
Audio Information Exchange (AUDIX) Features Transparency |
6-19 |
ENGINEERING CONSIDERATIONS |
6-21 |
ADMINISTRATION CONSIDERATIONS |
6-23 |
7. DATA CONNECTIVITY — AN OVERVIEW |
7-1 |
DATA COMMUNICATIONS VARIABLES
Communications Protocols
Analog vs. Digital
Synchronous vs. Asynchronous
Simplex vs. Half-Duplex vs. Full-Duplex
7-2
7-2
7-4
7-4
7-5
CONTENTS vii
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Parallel vs. Serial
Transmission Speed
Type of Communications Channel
SPECIAL DATA FEATURES
Terminal Dialing
Data Call Setup From a Voice Terminal
Computer Dialing
Data Hot Line
Data Protection
Data Privacy
Data Restriction
Data Only Off-Premises Extension
7-6
7-6
7-7
7-9
7-9
7-10
7-10
7-10
7-11
7-11
7-11
7-11
8. DATA COMMUNICATIONS CAPABILITIES |
8-1 |
DTE CONNECTIONS
DTE COMPATIBILITY
Protocol Converters
Terminal Emulators
PC/PBX or PC/ISDN
DCE POSSIBILITIES
Data Modules
ADUs
Modems
DATA MODULE CHARACTERISTICS
MODEM POOLING CONNECTIONS
LOCAL AREA NETWORK CONNECTIONS
LAN TOPOLOGY
LAN PROTOCOL
LAN TRANSMISSION MEDIA
LAN CONNECTORS
ADVANTAGES OF A COMMUNICATIONS SYSTEM IN A LAN ENVIRONMENT
Communication Between the LAN and Outside DTEs
Communication Between the LAN and Other Networks
8-3
8-3
8-3
8-4
8-4
8-5
8-5
8-6
8-6
8-6
8-10
8-13
8-13
8-13
8-14
8-14
8-14
8-15
8-18
viii CONTENTS
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The Communication System as a LAN Backup
PUBLIC AND PRIVATE DATA NETWORK CONNECTIONS
PUBLIC DATA NETWORKS
Packet-Switched Public Data Networks
Circuit-Switched Public Data Networks
PRIVATE DATA NETWORKS
Terrestrial Private Data Network Links
Satellite Private Data Network Links
TELEMARKETING HOST CONNECTIONS
THE ISDN GATEWAY
THE INTEGRATED TELEMARKETING GATEWAY
DATA NETWORK ADMINISTRATION
8-20
8-21
8-21
8-21
8-22
8-26
8-26
8-30
8-31
8-31
8-31
8-35
9. DATA COMMUNICATIONS CONFIGURATIONS |
9-1 |
DSC AND PSC CONNECTIONS
SNA NODE CONNECTIONS
CONNECTIONS THROUGH PUBLIC OR PRIVATE DATA NETWORKS
File Transfers
Video Teleconferencing
Image Processing
FAX Transmittal
9-2
9-5
9-6
9-6
9-6
9-7
9-7
A. RELATED DOCUMENTS |
A-1 |
B. SYNCHRONIZATION OF DIGITAL FACILITIES |
B-1 |
THE NEED FOR SYNCHRONIZATION
SYNCHRONIZATION HIERARCHY
CHANGES TO THE SCS SOFTWARE MADE AVAILABLE VIA SOFTWARE PATCHES
NETWORK SYNCHRONIZATION AND ENGINEERING
AVAILABILITY OF SYNCHRONIZATION SOURCES
CONCLUSIONS ON SYNCHRONIZATION
B-1
B-3
B-11
B-18
B-28
B-29
CONTENTS ix
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USE OF GENERIC 2 AS A SYSTEM CLOCK REFERENCE
USE OF GENERIC 1 AS A SYSTEM CLOCK REFERENCE
B-29
B-30
C. TRUNKING TERMS AND CAPABILITIES |
C-1 |
D. COMMUNICATIONS PROTOCOLS |
D-1 |
OSI MODEL
STANDARD PROTOCOLS
AT&T PROTOCOLS
D-2
D-5
D-10
E. LEAD DEFINITIONS |
E-1 |
F. NETWORKING FEATURES——AVAILABILITY MATRIX |
F-1 |
ABBREVIATIONS |
AB-1 |
GLOSSARY |
GL-1 |
INDEX |
IN-1 |
x CONTENTS
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|
LIST OF FIGURES |
|
Figure 1-1. |
Analog-to-Digital Conversion |
1-4 |
Figure 1-2. |
Transmission States for Voice Communication |
1-8 |
Figure 1-3. |
Transmission States for Data Transmission |
1-12 |
Figure 1-4. |
Time Slot |
1-17 |
Figure 1-5. |
Alternating Mark Inversion |
1-19 |
Figure 1-6. |
Example of B8ZS Line Coding |
1-20 |
Figure 1-7. |
D4 Framing |
1-24 |
Figure 1-8. |
The Extended Super Frame |
1-26 |
Figure 1-9. |
T1 Trunk Multiplexing |
1-30 |
Figure 1-10. |
Possible Multiplexed Connections |
1-33 |
Figure 1-11. |
Statistical Multiplexing |
1-34 |
Figure 2-1. |
Internal Dial Plan Interactions |
2-9 |
Figure 2-2. |
AAR Processing |
2-12 |
Figure 2-3. |
ARS Processing |
2-15 |
Figure 4-1. |
Standalone MS/T Network |
4-1 |
Figure 4-2. |
MS/T Coordinated Numbering |
4-3 |
Figure 4-3. |
Dialing Patterns From Tributary or Satellite to Main |
4-5 |
Figure 4-4. |
Dialing Patterns From Main to Satellite or Tributary |
4-5 |
Figure 4-5. |
Dialing Patterns From Main to Tandem |
4-6 |
Figure 4-6. |
Dialing Patterns From Satellite or Tributary to Tandem |
4-6 |
Figure 5-1. |
Typical 2-Level Hierarchical Electronic Tandem Network (ETN) |
|
|
Configuration |
5-3 |
Figure 5-2. |
An Example of Intermachine Tie Trunk Usage (Overflow Routing) |
5-4 |
Figure 5-3. |
Private Network Uniform Numbering Plan |
5-7 |
Figure 5-4. |
Extension Number Portability Cluster Using Common RNXs |
5-8 |
Figure 5-5. |
Feasible ETN/SDN Configuration |
5-10 |
Figure 5-6. |
Call Flow SDN - to - ETN |
5-11 |
Figure 5-7. |
Call Flow ETN - to - SDN |
5-12 |
Figure 5-8. |
Centralized Attendant Service |
5-13 |
Figure 6-1. |
Distributed Communications System (DCS) Cluster |
6-1 |
Figure 6-2. |
Internally Connected DCS Signaling Channel Over a DS1 |
6-4 |
CONTENTS xi
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Figure 6-3. Direct Link DCS Connections
Figure 6-4. Minimized Link Connections
Figure 6-5. DCIU/PI Linkage in a DCS Cluster
Figure 6-6. Short Haul Analog Signaling: System 75/G1 to System 75/G1 Via DSU Figure 6-7. Long Haul Analog Signaling: System 75/G1 to System 75/G1 Via Modem Figure 6-8. Long Haul Digital Signaling: System 75/G1 to System 75/G1 Via AVD DS1 Figure 6-9. Long Haul Digital Signaling: System 75/G1 to System 75/G1 Via ISDN PRI
Figure 6-10. Short Haul Digital Signaling: System 75/G1 to System 75/G1 — Direct Via MPDM or MTDM
Figure 6-11. Short Haul Analog Signaling: System 85/G2 to System 85/G2 Via DSU Figure 6-12. Long Haul Analog Signaling: System 85/G2 to System 85/G2 Via Modem Figure 6-13. Long Haul Digital Signaling: System 85/G2 to System 85/G2 Via ISDN PRI
Figure 6-14. Short Haul Analog Signaling: System 85/G2 to System 85/G2 — Direct Via IDI (Isolating Data Interface)
Figure 6-15. Short Haul Analog Signaling: System 75/G1 to System 85/G2 — Direct Via DSU
Figure 6-16. Long Haul Analog Signaling: System 75/G1 to System 85/G2 — Direct Via Modem
Figure 6-17. Long Haul Digital Signaling: System 75/G1 to System 85/G2 Via AVD DS1 Figure 6-18. Long Haul Digital Signaling: System 75/G1 to System 85/G2 Via ISDN PRI
Figure 6-19. Short Haul Digital Signaling: System 75/G1 to System 85/G2 — Direct Via MPDM
Figure 6-20. AUDIX in a DCS Network
Figure 7-1. Possible Data Connections
Figure 7-2. Synchronous vs. Asynchronous Communication
Figure 7-3. Modes of Link Operation
Figure 7-4. Serial vs. Parallel
Figure 7-5. Off-Premises Data-Only Extension — Configurations
Figure 8-1. PC/PBX and PC/ISDN 3270 Emulation Connections
Figure 8-3. External Modem Pool Using Intelligent Modems (Hayes-compatible) Figure 8-4. Integrated Modem Pool
Figure 8-5. Modem Pooling over a DS1 Channel
Figure 8-6. Outside DTE Access to LAN
Figure 8-7. Outside PC Access to a LAN Operating with StarGROUP Software
Figure 8-8. Asynchronous Communications from LANs Operating with StarGROUP Software
xii CONTENTS
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Figure 8-9. |
Common ISN Configuration |
8-18 |
Figure 8-10. |
LAN-to-LAN Connections Through the Switch |
8-19 |
Figure 8-11. |
X.25/StarGROUP Connectivity |
8-20 |
Figure 8-12. |
LAN/SNA Connectivity |
8-20 |
Figure 8-13. |
ACCUNET Packet Service |
8-22 |
Figure 8-14. |
Public Switched Data Network with Robbed Bit Facilities (Mode 1) |
8-23 |
Figure 8-15. |
Domestic Public Switched Data Network with ISDN-PRI Facilities (Modes 0, 1, 2, |
|
|
3) |
8-25 |
Figure 8-16. |
International Public Switched Data Network with ISDN-PRI Facilities (Modes 0, |
|
|
1) |
8-25 |
Figure 8-17. |
Private Data Network with Robbed Bit Trunks (Mode 1) |
8-26 |
Figure 8-18. |
Private Data Network with Robbed Bit Facilities and D4 Connection at Far End |
|
|
(Mode 1) |
8-28 |
Figure 8-19. |
Private Data Network with 24th Channel or ISDN-PRI Signaling (Modes 0, 1, 2, |
|
|
3) |
8-28 |
Figure 8-20. |
Private Data Network with 24th Channel or ISDN-PRI Signaling (Modes 0, 1, 2, |
|
|
3) |
8-28 |
Figure 8-21. |
Private Data Network with Robbed Bit, Fractional T1 Facilities (Mode 1) |
8-30 |
Figure 8-22. |
Telemarketing Gateway Configuration |
8-32 |
Figure 9-1. |
Dedicated/Permanent Switched Connections — Configurations (Sheet 1-or- |
|
|
2) |
9-3 |
Figure 9-2. |
SNA-Node to SNA-Node Connection |
9-5 |
Figure 9-3. |
A Possible File-Transfer Configuration |
9-6 |
Figure 9-4. |
A Possible Teleconferencing Configuration |
9-7 |
Figure 9-5. |
A Possible Imaging Configuration |
9-8 |
Figure 9-6. |
A Possible FAX Configuration |
9-8 |
Figure B-1. |
Options for Synchronization |
B-2 |
Figure B-2. |
Synchronization Hierarchy |
B-4 |
Figure B-3. |
Stratum Levels for the Synchronization Hierarchy |
B-7 |
Figure B-4. |
SCS (Generic 2) |
B-8 |
Figure B-5. |
Duplicated Synchronization Architecture and Cross Coupling |
B-9 |
Figure B-6. |
Tone-Clock Synchronizer (Nonduplicated, Generic 1) |
B-12 |
Figure B-7. |
Public-Network External Clock |
B-15 |
Figure B-8. |
External Clock |
B-16 |
Figure B-9. |
External-Clock Interface |
B-17 |
Figure B-10. |
External-Clock Duplicated Synchronization |
B-17 |
CONTENTS xiii
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Figure B-11. |
External and Internal Reference Levels |
B-19 |
Figure B-12. |
Nonpublic Network without Digital Switches |
B-20 |
Figure B-13. |
Proper Use of Backup Facilities |
B-21 |
Figure B-14. |
Improper Use of Backup Facilities |
B-22 |
Figure B-15. |
Optimal Diverse Routing |
B-23 |
Figure B-16. |
Less Than Optimal Diverse Routing |
B-24 |
Figure B-17. |
Excessive Cascading |
B-25 |
Figure B-18. |
Minimized Cascading |
B-25 |
Figure B-19. |
Excessive Synchronization from One Node |
B-26 |
Figure B-20. |
Minimized Synchronization from One Node |
B-27 |
Figure D-1. |
OSI Reference Model |
D-4 |
Figure D-2. |
ISDN Protocols |
D-9 |
Figure D-3. |
Data mode 1 With DCP and DS1/DMI/BOS Frame Multiplexing |
|
|
Arrangements |
D-13 |
Figure D-4. |
Data mode 2 Frame Structure |
D-15 |
Figure D-5. DCP Frame Format |
D-17 |
xiv CONTENTS
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|
LIST OF TABLES |
|
Table 1-1. |
Terminal/Port Compatibility |
1-9 |
Table 1-2. |
Trunk Port Types |
1-10 |
Table 1-3. |
Protocol States for Data Communication |
1-14 |
Table 1-4. |
Physical-Layer Protocol vs. Character Code |
1-15 |
Table 1-5. |
DMI Mode vs. Character Code |
1-15 |
Table 1-6. |
Signal-Inversion Requirements |
1-21 |
Table 1-7. |
Data-Module Capabilities |
1-22 |
Table 1-8. |
24th-Channel Signaling Arrangement — DMI BOS |
1-27 |
Table 1-9. |
24th-Channel Signaling Types vs Trunk Types and Destinations |
1-28 |
Table 1-10. |
Generic 1 and Generic 2 Analog Trunks — Voice/Voice-Grade Data BCC |
1-43 |
Table 1-11. |
Generic 1 and Generic 2 Digital Trunks — Voice BCC |
1-45 |
Table 1-12. |
Generic 1 and Generic 2 Digital Trunks — 64 kbps User Rate — Modes 0,2,3 |
|
|
BCCs |
1-48 |
Table 1-13. |
Generic 1 and Generic 2 Digital Trunks — 56 kbps User Rate — Mode 1 |
|
|
BCC |
1-49 |
Table 2-1. |
Networking Software |
2-4 |
Table 2-2. |
Network Administration Terminals and Systems |
2-18 |
Table 2-3. |
Networking Feature Parameters |
2-19 |
Table 6-1. |
DCS Node Capacity |
6-2 |
Table 6-2. |
Link Capacity |
6-3 |
Table 6-3. |
Availability of Attendant Features |
6-15 |
Table 6-4. |
Availability of Transparent Voice Terminal Features |
6-17 |
Table 6-5. |
DCS Cluster Limits |
6-21 |
Table 7-1. |
Data Feature vs Communications Systems |
7-10 |
Table 8-1. |
DCP-Port Data Modules |
8-8 |
Table 8-2. |
BRI-Port Data Modules |
8-9 |
Table 8-3. |
LAN Transmission Media Characteristics |
8-14 |
Table 8-4. |
Gateway PC Software |
8-15 |
Table 8-5. |
LAN-to-LAN Communications Packages |
8-19 |
Table 8-6. |
Requirements for Connections from the PBX to SDS/SDDN |
8-24 |
Table 8-7. |
Requirements for PBX-to-PBX Private Network T1.5 and T45 Connections |
8-27 |
CONTENTS xv
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Table 8-8. |
Requirements for PBX-to-PBX Private Network Spectrum Connections |
8-29 |
Table B-1. |
SCS References Switches |
B-10 |
Table D-1. |
Some CCITT Digital Standards |
D-6 |
Table D-2. |
Some CCITT Analog Standards |
D-7 |
Table D-3. |
Some CCITT ISDN Standards |
D-8 |
Table D-4. |
Some Well-Known Protocol Standards |
D-10 |
Table D-5. |
DCP versus ISDN-BRI protocol parameters comparison |
D-17 |
Table E-1. |
EIA RS-232C (V.28) LEADS/DEFINITIONS |
E-2 |
Table E-2. |
EIA RS-232D (V.28) LEADS/DEFINITIONS |
E-3 |
Table E-3. |
EIA RS-449 (V.24) LEADS/DEFINITIONS |
E-4 |
Table E-4. |
EIA RS-366 LEADS/DEFINITIONS |
E-5 |
Table E-5. |
CCITT V.35 LEADS/DEFINITIONS |
E-6 |
Table E-6. |
EIA RS-232C (V.28) LEADS SUPPORTED FOR AT&T TERMINALS |
E-7 |
xvi CONTENTS
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This document explains how to arrange two or more communications systems in a network. It also tells how to use your communications system to connect one data communications device or network to another device or network.
This document is for AT&T sales teams and customers who are responsible for:
∙Designing telecommunications networks
∙Interfacing data processing devices or networks through a communications system
To use this document, you should be familiar with the communications systems you intend to include in the network. The following documents describe AT&T communications systems:
∙An Introduction to AT&T DEFINITY® 75/85 Communication System Generic 1 (555-200-024)
∙An Introduction to DEFINITY® 75/85 Communications System Generic 2 (555-104-020)
∙AT&T DEFINITY® 75/85 Communication System Generic 1 System Description (555-204-200)
∙DEFINITY® Communications System Generic 2 and System 85 System Description (555-104-201)
∙DIMENSION® System Description and Planning (554-111-100)
For network implementation particulars, you may also find the following documents helpful:
∙AT&T DEFINITY® 75/85 Communications System Generic 1.1 Implementation (555-204-654)
∙DEFINITY® Communications System Generic 2 and System 85 Feature Descriptions (555-104-301)
∙AT&T DEFINITY® 75/85 Communications System Generic 1 and System 75 and System 75 XE Feature Descriptions (555-200-201)
Appendix A, Related Documents, lists the names of other documents you may need to refer to for more detailed information.
This document has two major sections: Part I covers communications system networking, and Part II describes data connectivity. The document contains the following chapters.
∙Chapter 1 — Introduction to Connectivity — Explains concepts and devices that are common to both the communications system networking and the data connectivity sections of the manual. These areas of shared interest include: transmission types, transmission states, multiplexing, and trunking.
xvii
xviii ABOUT THIS DOCUMENT
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PART I
∙Chapter 2 — Communications System Networking — An Overview— Introduces the networking software packages that are used to implement a PBX network. Included is a matrix that maps node administration to the tools used given the type of switch and switch version.
∙Chapter 3 — Tandem Tie Trunk Networks — Defines tandem tie trunk networks and describes how they operate.
∙Chapter 4 — Main-Satellite/Tributary (MS/T) — Describes the MS/T configurations, features, and engineering and administration considerations.
∙Chapter 5 — Electronic Tandem Network (ETN) — Describes the ETN configurations, features, interactions with other types of networks (including Centralized Attendant Service networks and the Software-Defined Network), and ETN engineering and administration considerations.
∙Chapter 6 — Distributed Communications System (DCS) — Describes the DCS configurations, transmission facilities, features, feature transparency, and engineering and administration considerations.
PART II
∙Chapter 7 — Data Connectivity — An Overview — Describes general data communication concepts and capabilities, as well as defines the basic data communications features provided that your communication system provides.
∙Chapter 8 — Data Communications Capabilities — Describes how to use a communications system to connect data terminal equipment, local area networks, public and private data networks, and telecommunication hosts.
∙Chapter 9 — Data Communications Configurations — Provides template configurations for various data networking applications, e.g., medical imaging, FAX communication, file transfers, and so forth.
∙Appendix A— Related Documents — Lists the names of documents related to DIMENSION, System 75/85, and the DEFINITY Generic 1 and Generic 2 communications systems.
∙Appendix B — Timing and Synchronization — Provides detailed information on synchronization in the private switching systems within a network. This appendix also provides many examples of how and where synchronization is used.
∙Appendix C — Trunking Terms and Trunk Compatibilities — Provides a list of trunking terms and several tables that identify trunking compatibility between particular types of switches.
∙Appendix D — Protocols — Explains the open system interconnect (OSI) protocol model, describes protocol naming systems, defines some of the most widely implemented protocols, and describes DCP, DMI, and BX.25 proprietary protocols.
∙Appendix E — Lead Designations — Lists the pin-outs for RS-232C, RS-232D, RS-449, RS-366, and V.35 connectors.
∙Appendix F — Networking Features — Lists the networking features available on each communications system.
∙Appendix G — Robbed-Bit AVD (rbavd) — Explains how to implement rbavd trunks on the Generic 1 communications system.
∙Glossary
∙Index
ABOUT THIS DOCUMENT |
xix |
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xx ABOUT THIS DOCUMENT
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This book explains two aspects of communicating through a business communications system:
∙Part I, Communications System Networking, explains how to arrange two or more communications systems in a network so that they communicate with each other efficiently and cost effectively.
∙Part II, Data Connectivity, describes the flow of data both within and between communications systems.
This chapter explains some fundamentals that are common to both networking and data connectivity. These fundamentals are:
∙Transmission types
∙Transmission states
∙Multiplexed communication
∙Trunking
1-1
1-2 INTRODUCTION TO CONNECTIVITY
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Both analog and digital transmission can be used to carry information across telephone lines and data links.
Analog transmission carries information as continuously varying electrical waves. Variables like the wave amplitude, phase, and frequency impart the information contained in the signal.
Analog transmission can carry both voice and data.
∙Voice: Since voice originates as analog sound waves, analog voice terminals can transmit it over analog facilities without any conversion.
VOICE |
ANALOG |
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VOICE TERMINAL |
SIGNAL |
∙Data: The discrete binary pulses that are generated by digital terminal equipment cannot be carried across analog lines. However, they can be used to vary (modulate) a carrier signal, which analog facilities can carry. At the destination, the variation in the signal is mapped to the original binary pulses (demodulated), and the original pulses are regenerated. Modems perform this modulation/demodulation function.
The analog facilities over which voice and modulated data are carried are sometimes called voice-grade lines. Hence, the name voice-grade data has been used for data carried over analog facilities.
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Digital transmission carries information as discrete pulses transmitted at different voltage levels (for example, +3 volts/−3 volts). Each pulse represents a binary bit (for example, a +3-volt pulse might
represent a binary 1, while a −3-volt pulse might represent a binary 0). The bits are grouped into 8-bit units called bytes that convey the information. Like analog transmission, digital transmission can carry both voice and data.
1-3
1-4 TRANSMISSION TYPES
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1. Original
signal
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011001110001011110100
Adapted from: Data and Computer Communications, by William Stallings, Macmillan Publishing Co., N.Y., 1988
Figure 1-1. Analog-to-Digital Conversion
TRANSMISSION TYPES |
1-5 |
_______________________________________________________________________________________
_______________________________________________________________________________________
∙Voice: To carry voice over digital transmission facilities, analog voice signals must be converted into digital information. To do this, the voice signal is sampled 8000 times a second. The sample is encoded according to a process called pulse-code modulation or PCM. Pulse-code modulation is a four-step process (see figure 1-1).
1.The analog signal is sampled.
2.Through a process called pulse-amplitude modulation (PAM), discrete pulses are generated, of the same amplitude as the analog signal at the sampled point.
3.The amplitude of these discrete pulses is rounded to the nearest integer.
4.The amplitude is encoded as a binary number, which is then transmitted over the digital facilities.
The device that performs pulse-code modulation is called a codec. Each digital voice terminal attached to a System 75, System 85, and DEFINITY® communications system contains a codec, called a digitizer, where PCM occurs for voice communication. For this reason, voice that can be carried over digital facilities is sometimes called digitized voice. Digitization of communication originating at analog voice terminals or trunks occurs at the switch ports to which these facilities are attached.
ANALOG |
DIGITAL |
SIGNAL |
CODEC |
SIGNAL |
Note that voice-grade data may also be carried over digital facilities if it is put through the PCM process. In fact, when voice-grade data enters the switch at an analog line or trunk port, the codec treats the data identically to voice (see steps 1 through 4, above).
∙Data: For binary data to be carried over digital facilities, the bits need not be converted. However, headers and trailers that make the transmission conform to a particular protocol must be created and inserted. This can occur at a stand-alone data module, a voice terminal equipped with a data module, or a switch port.
PROTOCOL |
DIGITAL |
DIGITAL DATA |
SIGNAL |
CONVERTER |
Alexander Graham Bell invented the first practical telephone in 1876. For nearly 80 years thereafter, transmission occurred over analog facilities. Since primarily voice was being transmitted during these years, analog facilities more closely fit the need.
With the advent of computers, facilities that could transmit binary data were needed. The installation of public digital facilities remained uneconomical, however, until the facilities could be used to transmit both voice and data. It was not until the late 1950s that the digitization of voice and the transmission of digital
1-6 TRANSMISSION TYPES
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signals became feasible through the economic, operational, and reliability features of solid-state electronics. In 1962, the Bell System established the first commercial use of digital transmission when the first T1 carrier system went into operation in a Chicago area exchange. By 1985, 50% of the country’s exchangearea trunks were T-carrier digital facilities.
Currently, the national network is employing more and more digital facilities. This trend has occurred because of some of the advantages that digital transmission offers. These advantages include:
∙Integration of transmission and switching: The introduction of the No. 1 ESS™ in 1965 was a major milestone in the telecommunications industry since it was the first switch that was computer controlled.
Thus, inside the No. 1 ESS, both data and voice were digital. In the public network, the No. 1 ESS was followed in 1976 by the No. 4 ESS™ and by the No. 5ESS® in the early 1980s. AT&T customer-
premises switches became digital with the System 75 and System 85 offerings in the early 1980s. The advantages of servicing these digital transmission facilities with digital switches arose from the fact that the internal signal need not be converted back to analog for internal switch processing. In a totally integrated network, voice signals are digitized at or near the source and remain digitized until delivered to their destination.
∙Signal regeneration: With all transmissions, the signal weakens (attenuates) the farther it is transmitted. With analog transmission, the signal is amplified periodically in an attempt to recreate its original quality. This means, however, that any noise on the line, due to line disturbances and random variations in voltage or current, is also amplified.
With digital transmission, the signal is regenerated by repeaters placed strategically along the line. Since pulses are recreated during the regeneration process, any noise that had entered the line in the distance the signal traversed since the last repeater is eliminated.
∙Low signal/noise ratios: Noise and interference in an analog voice network become most apparent during speech pauses when the signal amplitude is low. Relatively small amounts of noise occuring during a speech pause can be quite annoying to the listener. In a digital link, the noise level during speech pauses is held to an acceptable level because it is controlled by encoding rather than random line conditions.
AT&T is rapidly replacing analog facilities with digital and expects to have this conversion completed in the early 1990s. However, local facilities terminating at customer premises are often analog. Therefore, today’s national network is essentially an analog/digital hybrid. This makes it imperative for network planners to be familiar with conversion techniques and devices so they can design a costeffective network that utilizes uncomplicated routing patterns and existing equipment, no matter whether analog or digital. The rest of this chapter explains the devices and techniques that are available for building a network for voice or data transmission or both.
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When you plan a communications network, there are certain rules that you cannot violate:
∙Analog signals cannot be carried over digital facilities without first being converted to digital.
∙Digital signals cannot be carried over analog facilities without first being converted to analog.
∙Digital communication must conform to a particular protocol. Unless a protocol converter is inserted in the transmission stream, both the originating and destination ends of the communication link must speak the same protocol. In addition, every protocol in the transmission link must be represented twice in the line — once as an initiator and, again, as a terminator.
To plan a network that adheres to these rules, you must be familiar with how voice or data is represented at every stage of switching and transmission. Its mode of representation at each stage is its transmission state at that point.
When you design a network, you must know at which points in the network the transmission state changes. At the switch, transition points for voice signals occur before the signal enters the switch and, again, as it exits the switch. (See figure 1-2 for the transmission states that occur with voice transmission.)
When voice is transmitted to the switch, the following processes occur depending upon the transmission device.
∙Analog telephones generate an analog signal that travels over twisted pair to an analog port on the switch.
—If the switch is digital — like the System 75, System 85, and DEFINITY communications systems — the port contains a codec, which converts the analog signal to a digital signal using PCM.
—On analog time-multiplexed switches, like the DIMENSION®, the analog port samples the analog signal and regenerates samples using pulse-amplitude modulation (PAM); or, on older switches, a path through a switching matrix is found and the analog signal is applied to it directly.
∙Digital voice terminals digitize the voice signal using PCM before transmitting it to a digital port on a digital switch. No code conversion occurs at the digital port. (Note that digital voice terminals are never attached to analog switches.)
∙Incoming analog-trunk calls to a digital switch are converted to PCM by a codec built into the analogtrunk circuit pack through which the signal enters the switch. On a time-multiplexed analog switch, the analog-trunk signal is also converted to PAM at the trunk port.
1-7
1-8 TRANSMISSION STATES
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DIGITAL SWITCH
123 456 789 *0#
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456
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Figure 1-2. Transmission States for Voice Communication
TRANSMISSION STATES |
1-9 |
_______________________________________________________________________________________
_______________________________________________________________________________________
∙Incoming digital-trunk calls to a digital switch need no conversion since the call is already PCM encoded. Incoming digital-trunk calls to analog switches are converted outside the switch at a D4 channel bank. In these cases, the digital trunk terminates at the D4 channel bank. From the D4 channel bank, incoming calls are forwarded to the switch via analog trunks. (See Multiplexed Communication later in this chapter.)
Inside a digital switch, therefore, voice is always coded as PCM. Inside an analog switch, voice is coded as PAM for time-multiplexed switches or left as a pure analog signal for matrix switches.
Below is a table that summarizes the types of signals that particular terminals generate and the types of circuit packs that can handle the signals.
Table 1-1. Terminal/Port Compatibility
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7400 series |
Digital |
Digital-line |
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ç TN754 |
ç SN270 |
TN754 |
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7500 series |
Digital |
BRI |
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N/A |
ç N/A |
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* Universal modules are available only on the G2 switch.
Note that the table names the circuit-pack type by the type of terminal that can use it. Multifunction electronic telephones use MFET circuit boards; multifunction analog telephones use MFAT circuit boards; DCP terminals use digital ports (called digital-line ports on the S75/G1, and general-purpose ports [GPP] on the S85/G2) and basic rate interface terminals use BRI ports.
To complete the picture of analog and digital switch ports, you also need to know the trunk port types, names, and numbers given in figure 1-2. (See the Trunking section of this chapter for an explanation of trunk types and protocols.)
For outgoing or tandem calls that are routed over analog trunks, and for intercom or incoming calls to analog telephone sets, another conversion must occur.
1-10 TRANSMISSION STATES
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Table 1-2. Trunk Port Types
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Signal |
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çAnalog |
ç CO |
LC08 |
TN747B |
SN230B |
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ç LC13 |
ç TN763C |
ç SN231 |
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LC11 |
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DS1 |
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ç TN767 |
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ç TN767 |
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TN555 |
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*Universal modules are available only on the G2 switch.
∙From a digital switch, the voice signal, which is represented in the switch as digital PCM code, is converted to analog via a codec built into the analog line or trunk circuit pack.
∙From an analog switch, conversion of the internal PAM code to continuous analog signals also occurs at the trunk or line port.
For outgoing or tandem calls that are routed over digital trunks, and for intercom or incoming calls to digital terminals, the following processes occur:
∙From a digital switch, PCM code treatment depends upon the port through which the signal exits.
—DS1 and primary rate interface (PRI) ports package the signal according to how the port is administered. (The line coding, framing, and signaling are administrative choices. Refer to the Multiplexed Communication section of this chapter for further details.) In addition, PRI ports package their supervisory messages according to ISDN-PRI protocol.
—Digital-line ports (System 75 and Generic 1) and general-purpose ports (System 85 and Generic 2) are both digital ports that package the signal according to DCP protocol. (See page 1-13 for an explanation of this protocol.)
—Basic rate interface (BRI) ports are digital ports that package the signal according to ISDN BRI protocol. (See page 1-13 for an explanation of this protocol.)
∙From an analog switch, no provision is made for connecting with digital voice terminals. Outside the switch, however, outgoing and tandem calls can be packaged for transmission over digital trunks. The voice signal exits the switch via analog trunks, then enters the digital trunk at a D4 channel bank. (See the Multiplexed Communication section of this chapter for further details.)