ATT DEFINITY Communications System Generic 3 Instructions Manual

							Data Management
Issue  3   March 1996
2-15
The EIA RS-232D protocol provides two interface connectors. The female side 
connector is known as data communications equipment (DCE). The male side 
connector is known as data terminal equipment (DTE). Data equipment 
manufacturers design either the DCE or DTE interface into their products. 
Products such as modems, data service units (DSUs), Digital Terminal Data 
Modules (DTDMs), and Processor Data Modules (PDMs) have a built-in DCE 
interface. Products such as some typ es of multiplexers, data terminals, printers, 
computer ports, and Trunk Data Modules (TDMs) have a built-in DTE interface. 
Modular Data Modules (MDMs) can be configured as either DCE or DTE.
The maximum c a ble length recommended by EIA for the EIA RS-232D protocol is 
25 feet (15 meters). However, practical applications have shown that the cable 
length can be much greater. Factors limiting cable length include transmission 
speed, cable capacitance, and nearness of noise sources such as fluorescent 
lights or electric generators. Each a p plication should be considered separately.
RS-449
This protocol allows longer c a bles than the EIA RS-232D. Maximum cable 
lengths for various data speeds are as follows:
n19.2 kb ps — 200 feet (61 meters)
n9.6 kbps — 400 feet (122 meters)
n4.8 kbps — 800 feet (244 meters)
n2.4 kbps — 1,600 feet (488 meters)
The RS-449 protocol is provided as a communications  link interface on the 
Applications Processor (AP). This standard uses a 37-conductor c a ble. The AP 
RS-449 interface contains unbalanced driver/receivers that also permit 
interconnection to the EIA RS-232D interface when used with a 37- to 25-pin 
cable adapter. Since the AP RS-449 interface is compatible with the EIA 
RS-232D protocol, it also is limited to the same maximum 19.2 k b ps data rate.
RS-366
The RS-366 communications protocol specifies the standards for interfacing 
computers to ACUs. This permits a computer to originate data calls over a 
switched telephone network. The AP provides one RS-366 interface for each six 
EIA RS-232D  interface ports.
Digital Communications Protocol (DCP) Interface
The DCP is used by the system’s d i gital switch, d i gital voice terminals, data 
modules, the 510D terminal, and the 515 BCT. This protocol permits 
simultaneous voice and data over the same communications link to the switch.
The DCP consists of a 160 k bps, four-wire serial data link that operates 
full-duplex over standard  twisted-pair building cable. For data-only transmission,  
						

							Functional Description
2-16Issue  3   March 1996 
the maximum cable length is 5,000 feet (1,524 meters). When voice and data 
transmission is carried over the same data link, as when a 510D terminal, 515 
BCT, or a DTDM is used , the cable length is limited by the voice transmission 
distance.
The D CP sends digitized voice and digital d ata in frames. Each frame consists of 
four fields or channels (see Figure 2-3). The first field is a unique three-bit framing 
pattern that defines the frame boundary. The second field is a one-bit control or 
signaling channel between the digital switch and di gital data endpoint. The third 
and fourth fields are two independent information (I) channels. The information 
channels are eight bits each and are used to send digitized voice or digital data.
Figure 2-3. Digital Communications Protocol Frame Structure
There are 8,000 frames per second. Therefore, the bit rate available is eight for 
the signaling  channel and 64 kbps for the information channel. The digital switch 
routes each information channel independently so that simultaneous voice and 
data can be completed to different destinations.
The full capacity of the information channels (64 kbps) is available for digitized 
voice. Data terminals typically operate at speeds from below 300 bps up to 19.2 
kbps, asynchronous or synchronous. The DCP uses data modules to map the 
data terminal data into a 64 kb ps information channel.
The framing rate of 8,000 per second and eight bits per information channel is 
consistent with other telecommunication systems such as the DS1/E1 carrier. 
This minimizes potential conversion problems when interfacing to d ifferent digital 
facilities.
BX.25 Packet Switching Protocol
The BX.25 protocol implements the international standard for packet switching. It 
is a multilayered protocol. [Layering is a structuring of specific p rotocol functions 
(for example, error detection and correction) that are grouped together as a 
unique layer or level.]
FRAMING
FIELDSIGNALINFORMATION
#1INFORMATION
#2FRAMING
FIELD
DCP FRAME 
						

							Data Management
Issue  3   March 1996
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The BX.25 protocol is similar to the ITU-T X .2 5   protocol and, from a user 
perspective, is compatible with the standard . The BX.25 protocol has three 
layers that are not specified for the X.25 protocol. These layers are Application, 
Presentation, and Session. The A pp lication and Presentation layers are defined 
in the Transa ction-Oriente d Protocol (TOP) of the BX.25.
The TOP is a high-level protocol, intended to standardize communications 
between transaction-oriente d systems. Transaction-oriented communications 
involve communication of small messages or requests describing a single unit of 
work that may result in a reply being sent b ack to the originating system. The 
Session layer is intended to establish, manage, and terminate sessions for use 
by higher-level protocols or, in some cases, by user applications directly. Other 
differences between X.25 and BX.25 are as follows:
nThe X.25 protocol specifies network standards only; the BX.25 protocol 
places requirements on the user interface as well.
nThe X.25 protocol p rovides for d atagram services while the BX.25 p rotocol 
does not. Datagram service has not been implemented within the 
continental United States.
nThe X.25 protocol leaves the users in a point-to-point environment to 
develop their own solutions to the following areas of potential conflict, 
while the BX.25 protocol provides solutions:
— Link layer a d dressing
— Logical channel selection
— Call collision
Basic elements of the Application and Presentation layers must be user defined 
under both protocols. The following figures shows the relationship and similarity 
between the BX.25 and X.25 protocols.
The BX.25 protocol is used in the system to provide communications between 
the switch  and the switch-related features. The BX.25 p rotocol is also used in the 
system to provide communications between the switch and the AUDIX and to 
provide communication between DCS switches 
						

							Functional Description
2-18Issue  3   March 1996 
Figure 2-4. BX.25 Packet-Switching Protocol
USER
APPLICATION
LAYER
PRESENTATION
LAYER
SESSION
LAYER
NETWORK
LAYER
DATA L IN K
LAYER
PHYSICAL
LAYER
TRANSPORT
LAYER
------ ------
---
BX.25 Protocol LayersUSER
PACKET
LAYER
LINK
LAYER
PHYSICAL
LAYER
SESSION
LAYER
T
O
P* 
						

							Data Management
Issue  3   March 1996
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Figure 2-5. X.25 Packet-Switching Protocol
USER
PACKET
LAYER
LINK
LAYER
PHYSICAL
LAYERNot
Specified
by
ProtocolUSER
APPLICATION
LAYER
PRESENTATION
LAYER
SESSION
LAYER
NETWORK
LAYER
DATA LI N K
LAYER
PHYSICAL
LAYER
TRANSPORT
LAYER
------ ------
---
X.25 Protocol Layers 
						

							Functional Description
2-20Issue  3   March 1996 
International Telecommunications
Union - Telecommunications,
Specifications Sector (ITU-T) Interface
X.25 Packet-Switching Protocol
The ITU-T (formerly CCITT) i s one  of  three divisions of the International 
Telecommunications Union, an agency of the United Nations. The standards set 
by the ITU-T generally deal with public networks. Two series of standards or 
recommendations specifically deal with data transmission:
nThe V-series p rovides recommendations for d ata transmission over analog 
or voice telephone networks.
nThe X-series  provides recommendations for data transmission over digital 
networks.
The V-series includes the V.10, V.11, V.24, V.28, and V.35. Also, V.26, V.27, and 
V.28 are mo dem recommendations for 2400, 4800, and 9600 bps, respectively.
V.10 and V.11 are the equivalent to the EIA RS-423 and RS-422.
V.24 provides definitions for all interchange circuits crossing the DTE/DCE 
interface.
V.28 defines a set of electrical characteristics comp atible with EIA RS-232D.
V.35 provides the constant current interface for 48 kbps operation.
The X.25 protocol is the ITU-T recommendation for imp lementing International 
Standards Organizations Reference Model of Open Systems Interc onnection. 
This is the international model for packet-switching networks and is a 
bit-oriente d, layered-type p rotocol. The transport, network, data link, and 
physical layers (levels) are defined functionally by the ITU-T.
The X.25 protocol specifies network requirements and procedures to provide the 
user interface for a packet-switching network. Typically, users generate 
low-speed asynchronous data. The X.25 software segments this data into 
packets, adds framing and routing information, and queues the packets into a 
buffer memory. User data p ackets, along with the added framing bits, are then 
transmitted over high-sp eed carriers. This permits efficient and dynamic sharing 
of these high-speed data links.
The X.25 protocol provides the communications links between multiple APs. 
						

							International Telecommunications Union - 
Telecommunications, Specifications Sector (ITU-T) 
Interface
Issue  3   March 1996
2-21
Wideband Switching
Wideband Switching  provides a range of data transmission sp eeds (from 128 to 
1984 k b ps for E1 services, 128 to 1536 kbps for T1 services). The higher 
transmission rates are needed for applications such as video conferencing, data 
backup, scheduled batch processing, and primary data connectivity. 
						

							Functional Description
2-22Issue  3   March 1996 
Data Management Features
The following features are associated with Data Management:
nAdministered Connections 
nAlphanumeric Dialing 
nData Call Setup [including Default Dialing  and Alphanumeric Dialing]
nData Hot Line
nData-Only Off-Premises Extensions
nData Privacy
nData Restriction
nDefault Dialing 
nDigital Multiplexed Interface
nDS1/E1 Tie Trunk Servic e
nEIA Interface
nInformation System Network (ISN) Interface
nModem Poolin g
nPC Interface
nPC/PBX Connection
nUniform Call Distribution (See Direct Department Dialing [a VOICE 
Management Feature])
nWideband Switching
nWorld Class Core BRI 
						

							Network Services
Issue  3   March 1996
2-23
Network Services
Network Services allows a group of switches (consisting of DEFINITY  Generic 1, 
Generic 2, Generic 3, System 75 and System 85, and/or other systems) to be 
configured to meet the communications needs of a medium- to larg e-size 
corporation. Possible arrangements include an Electronic Tandem Network 
(ETN), Distributed Communications System (DCS), and Main/Satellite/Tributary. 
Each is briefly described in this chapter.
Do not assume that the system has any c a pabilities other than those explicitly 
stated herein.
 Refer to the System 75/85 AT&T Network and Data Services 
Reference 
manual, 555-025-201, for differences between this system and other 
AT&T systems.
Network Services Features
The following features are associated with Network Services:
nAAR/ARS Partitioning
nARS to AAR Conversion
nAutomatic Alternate Routing
nAutomatic Circuit Assurance
nAutomatic Route Selection
nDCS Alphanumeric Display for Terminals
nDCS Attendant Control of Trunk Group Ac cess
nDCS Attendant Direct Trunk Group Selection
nDCS Attendant Display
nDCS Automatic Callback
nDCS Automatic Circuit Assuranc e
nDCS Busy Verification of Terminals and Trunks
nDCS Call Coverage
nDCS Call Forward ing All Calls
nDCS Call Waiting
nDCS Distinctive Ringing
nDCS Leave Word Calling
nDCS Multi-Appearance Conference/Transfer
nDCS Trunk Group Busy/Warning Indication
nNon-Facility Associated Signaling and D-channel Backup (G3i)
nFacility Restriction Levels and Traveling Class Marks 
						

							Functional Description
2-24Issue  3   March 1996 
nForced Entry of CDR Ac count Codes
nGeneralized Route Selection
nIntegrated Services  Digital  Network — Primary Rate Interface
nNetwork Access — Private
nNetwork Access — Public
nOff-Premises Station
nQSIG Global Networking
nRestriction — Toll
nSubnet Trunkin g
nTime of Day Routing
nUniform Dial Plan
Private Network Configurations
A p rivate network is a configuration of trunk and switching facilities d e dicated to 
the use of a b usiness or organization. It may have as few as two switches or it 
may have hundreds of switches located throughout the world. Although they 
normally serve moderate to heavy calling between locations, the following 
configurations make it p ossible for organizations of all sizes to realize the 
benefits of a private network.
nElectronic Tandem Network (ETN) — Serves the needs of customers with 
many locations in a large g eographic area. This configuration p rovides for 
calling between locations without accessing toll facilities.
nDistributed Communications System (DCS) — Serves the needs of 
customers with several locations in a small or large geographic area. A 
Distributed Communications System (DCS) a p pears as a single switch 
with respect to certain features.
nMain/Satellite/Tributary — Serves the needs of customers with a few 
locations in a small geographic area.
The system also can b e used within a Tandem Tie Trunk Network (TTTN). A TTTN 
is a nonhierarchical network of tie trunks interconnecting three or more switches. 
User dialing into each switch in the call’s p ath is required. That is, the user at one 
switch dials the trunk access code for a tie trunk group to another switch, 
receives dial tone from that switch, and then dials another trunk access code to 
reach another switch. When dial tone is received from the final (d esired) switch, 
the user dials the desired extension number.