ATT System 25 Reference Manual

							DS1 Facility Interface
When digital signals are transmitted over a network of digital communications links, switching
nodes, multiplexers, and transmission interfaces, all entities must be synchronized. This is
referred to as network synchronization.
For synchronous transmission,information is transmitted to the transmission facility at a
fixed rate. Each bit occupies a fixed unit time interval. All significant transitions must
correspond to multiples of the fixed unit time interval.Message and frame synchronization
are achieved by using special characters at the beginning and end of the message and by
knowing the number of bits contained in each frame.
If the average transmit rate is faster than the average receive rate, the buffer of the receiving
unit will overrun.If the average receive rate is faster than the average transmit rate, the
buffer of the receiving unit will underrun.It is necessary to control the overruns, (frame
deletions) and underruns (frame repetitions) and, when necessary, only allow an overrun or
underrun in 1-frame increments. The deletion or repetition of a single frame is termed a slip
or a controlled slip.
Controlling the slip rate is accomplished by synchronizing the clocks associated with the
switching equipment so that all transmissions have the same average line rate. The AT&T
digital network synchronization is based on a hierarchy of clocks. Four strata are used with
stratum 1 clock being the most accurate (+/- 0.00001 ppm) and stratum 4 clock the least
accurate (+/- 32 ppm). The stratum 1 clock is the AT&T Reference Frequency clock. AT&T
4ESS™ toll switches maintain stratum 2 clocks while AT&T 5ESS® central office digital
switches and Digital Access and Cross Connects maintain stratum 3 clocks. The System 25
meets accuracy requirements for a stratum 4 clock. If possible, the System 25 should be
synchronized to a higher stratum clock.
T1 Facility Network Connections
The System 25 can be connected to T1 transmission facilities with endpoints on private
networks and public networks.
Private Network Connections
Private network endpoints include connections to other PBXs, or computers which use
internal modems, and off-premises stations.
AT&T Digital PBX Endpoints
The T1 facility can provide digital tie trunk service between System 25, System 75, System
85, and DEFINITY™ Communications System, Generic 1 and Generic 2. The PBXs can be
co-located or geographically disbursed.When co-located, any combination of framing,
signaling, and line coding can be used. However, each PBX must be administered the same.
If the PBXs are separated, the carrier facility providing the T1 facility can place limitations on
the permitted options. In either case, when two or more PBXs are connected together, one
must be selected as the timing master and the others are administered as timing slaves.
2-119 
						

							FEATURES AND SERVICES
Non-AT&T Digital PBX Endpoints
To determine if the System 25 can be connected to another vendor’s PBX, several items
must be verified:
l The other vendor’s PBX must provide a DS1 interface as specified by AT&T.
l The other vendor’s PBX must provide at least one of the line
signaling options provided by the System 25.
l The other vendor’s PBX must implement the digital loss plancoding, framing, and
specified in EIA RS-
464/PN1378.
l The other vendor’s PBX must either maintain a stratum 4
synchronizing to System 25.
This list is not exhaustive. Many vendors have certification testing
products.
AT&T Analog PBX Endpoints
The System 25 DS1 interface can be connected to an analog PBXclock or be capable of
programs for their DS1
(such as a Dimension®
PBX or other vendor analog PBX) provided a D4 channel bank is used. The channel bank
demultiplexes the 24 DS0 digital channels and converts each to analog form. The analog
form supports a variety of telephony signaling arrangements. If a D4 channel bank is used,
D4 framing, robbed-bit signaling, and ZCS line coding must be used.
Analog Off-Premises Station Endpoints
The System 25 DS1 interface can be connected to 24 analog off-premises stations provided
a D4 channel bank is used.
Public Network Connection
Public network endpoint connections include central offices, Digital Access and Cross-
Connect (DACS) system frames, and toll offices.
Note:Most public network connections require a Customer Service Unit (CSU) to
be installed at the PBX location. The CSU provides proper termination of the
circuit (includesnetwork protection
and FCC compliance), signal
regeneration, and loopback testing. A CSU is required at each PBX location
when connecting to ACCUNET™ T1.5 service.
4ESS Toll Switch Endpoint
This connection is called Special Access connection.The physical connection is made either
directly from the System 25 location to the 4ESS toll switch or from the System 25 location
to a central office (a“nailed-up” connection) and then to the toll switch. In either case, toll
calls go directly from the System 25 to the AT&T Communications toll network.
2-120 
						

							DS1 Facility Interface
The trunks terminating on a 4ESS switch must be administered as either tie or direct inward
dialing trunk types. The tie trunks can be either one-way or two-way; either immediate start,
wink start or delay dial. The DID trunks are incoming only and wink start or immediate start.
Some 4ESS switches do not provide secondary dial tone. It is recommended that System 25
provide the secondary dial tone.Secondary dial tone can be provided through the Automatic
Route Selection (ARS) feature on all DS1 trunks that terminate on the 4ESS switch.
All framing and signaling modes may not be available on some 4ESS switches. However, D4
framing, ZCS line coding, and robbed-bit signaling is supported. Also B8ZS and Common
Channel signaling is supported.
Touch-tone (DTMF) dialing is not supported on some 4ESS switches. Therefore, the System
25 must support both touch-tone and pulse dialing.
The System 25 synchronization should be administered such that the 4ESS switch provides
the master clock source.
5ESS Digital Switch Endpoint
The 5ESS digital switch provides digital central office service and supports “digital” ground
start, reverse battery, and E&M trunk type.
E&M and direct inward dialing trunks are preferred terminations on a 5ESS switch,
The System 25 synchronization should be administered such that the 5ESS switch provides
the master clock source.
AT&T DACS Endpoint
The System 25 DS1 interface can be connected to a DACS endpoint. The DACS can be
thought of as an “electronic patch panel” for DS1 signals. Cross connections may be made
at either the DS1 or DS0 level. The DACS supports both D4 and ESF framing, both ZCS and
B8ZS line coding, and depending on the DACS software version, both robbed-bit signaling
and common channel signaling.
The DACS contains a stratum 3 clock. In all cases, the DACS is synchronized to the AT&T
Reference frequency. Therefore, a suitable synchronization reference may be optionally
obtained from the DACS.
Analog Central Office Endpoint
The System 25 DS1 facility interface may be connected to any analog central office provided
a D4 channel bank is used.
The D4 channel bank is located at the central office and it is the responsibility of the central
office to set the channel unit attenuators to the appropriate values.
If the D4 is a stand-alone unit, it should use the DS1 signal received from the System 25 as
its timing source. In this case the System 25 must maintain a stratum 4 clock. If the D4 is
2-121 
						

							FEATURES AND SERVICES
synchronized to the AT&T Reference Frequency, then it can be used as the synchronization
reference for the System 25.
DS1 Interface Circuit Pack
The TN767 circuit pack is the principal hardware element connecting System 25 to the T1
facility interface.Two DS1 circuit packs may be mounted in the System 25 cabinet(s). Each
DS1 facility interface CP supports up to 24 trunks (23 if common channel signaling is used).
The TN767 CP emulates the TN747 CO trunk CP, the TN753 Direct Inward Dialing CP, the
TN760B Tie Trunk CP, and the TN742 Analog Line CP. This means the TN767 can be
administered to replace existing CPs that support automatic tie trunk, delay dial tie,
immediate dial tie, wink start tie, ground start central office trunk type, loop start central
office trunk type, direct inward dial trunk, and off-premises station line.
The trunk type can be selected on a port-by-port basis for each of the 24 ports of the TN767
CP.
The TN767 monitors the T1 facility for errors such as loss of signal, framing errors, and bit
errors.
DS1 Interface Network Synchronization Circuit Pack
To provide T1 facility interface in System 25, in addition to the TN767 CP, one other CP is
required. The ZTN131 CP synchronizes the System 25 to a master DS1 data stream,
monitoring the reference clock,and maintaining a stratum 4 clock for synchronizing
transmission on all DS1 ports.
Wiring
The DS1 Interface uses shielded cable to connect the TN767 CP to the T1 facility or the CSU
if connecting to a public network.
Emergency Transfer
A T1 line cannot be switched over to an analog phone for emergency transfer. Therefore, the
System 25 should have at least one analog loop or ground-start line if emergency transfer is
needed.
2-122 
						

							DS1 Facility Interface
Administration Requirements
The administration of DS1 interface CP is completed by accessing the Port, High Density
Circuit (HI D CKT), and System entries of the main administration menu.
The items to be administered under Port include (default):
l Assign 24 DS0 channel slots
l Assign port type, the TN767 CP supports the following port types:
— 201
— Single line tip/ring station without a message waiting light
— 7xx —Ground start trunks
— 8xx —Loop start trunks
— 9xx —
Direct Inward Dialing trunks
— 10xx —Tie trunks
l Assign special signaling to each DS1 port. They are as follows:
— Foreign Exchange (default)
— Special Access Unit
l Set Central Office disconnect timing
l Set end-end signaling timing
l Set end-end pause timing
l Set answer supervision delay timing
The items to be administered under HI D include (default):
l Assign board type, 767
l Enable the T1 CP (disabled)
l Set the line compensation (1)
l Set line code suppression (ZCS)
l Set framing mode (D4)
l Set signaling mode (robbed-bit)
l Set Red Alarm activation time (3 seconds)
2-123 
						

							FEATURES AND SERVICES
The items to administered under System include (default):
l Assign Primary Synchronization Source (No slot)
l Enable Automatic Synchronization Source Switching (enabled)
Hardware Requirements
Requires a TN767 CP plus ZTN131. The TN767 CP provides 24 ports.
DS1 Interface Connection Information
Detailed connection information is provided in the following figures:
l Figure 2-14—
Direct DS1 Connection Between Adjacent System 25 Cabinets
l Figure 2-15—
Direct DS1 Connection Between System 25 Cabinets (Located 1310
Feet Apart, Maximum)
l Figure 2-16—System 25 Connection to DS1 Facility Located 4310 Feet (Maximum)
Away
l Figure 2-17
—System 25 Connection to
l Figure 2-18—System 25 Connection to
l Figure 2-19
—System 25 Connection
Interface)
l Figure 2-20—System 25 Connection toDS1 Facility Located 4311 Feet or More Away
DS1 Facility (Off-Premises Cabling)
to DS1 Facility (Non-Metallic Transmission
DS1 551 CSU
SYSTEM 25
TN767OCTOPUS
. CABLEOCTOPUS
CABLE

LEG 8LEG 8
SYSTEM 25
TN767
Figure 2-14.Direct DS1 Connection Between Adjacent System 25 Cabinets
2-124 
						

							DS1 Facility Interface
131O FEET MAXIMUM
CABLING DISTANCE
SYSTEM 25
SYSTEM 25
TN 767C6D CONNECTORCABLE (NOTE)TN767
< >
NOTE:
Figure 2-15. Direct
Apart,
FOR DISTANCES OVER 50 FEET (15.2 M)
USED C6E CONNECTOR CABLE(S) BETWEEN
C6C CONNECTOR CABLE AND DS1 TIETRUNK CIRCUIT PACK.
DS1 Connection Between System 25 Cabinets (Located 1310 Feet
Maximum)
S25
TN767CSU
CSU
DSX-1
DSX-1
OFFICE
OFFICEDS1
REPEATER
REPEATER
655 FEET
MAXIMUM
Figure 2-16. System 25
3000 FEET OR LESS
655 FEET
MAXIMUM
Connection to DS1 Facility Located 4310 Feet (Maximum) Away
2-125 
						

							FEATURES AND SERVICES
S25
TN767CSU
DSX-1
PROTECTION(NOTE)CIRCUIT
655 FEET
REPEATERED
MAXIMUM
DS1 LINE
CSU
DSX-1PROTECTIONDS1(NOTE)CIRCUIT
655 FEET
MAXIMUM
NOTE:USE OF THE OFFICE REPEATER IS OPTIONAL DEPENDING
UPON THE DISTANCE TO THE FIRST T1 REPEATER
Figure 2-17. System 25 Connection to DS1 Facility Located 4311 Feet or More Away
S25
TN767DSX-1T1 LINE
REPEATER
6000 FEET
3000 FEETOR LESS
655 FEET
MAXIMUMOR LESSCSU
CSU
DSX-1T1 LINEDS1REPEATER
655 FEET3000 FEET
MAXIMUMOR LESS
Figure 2-18.System 25 Connection to DS1 Facility (Off-Premises Cabling)
2-126 
						

							DS1 Facility Interface
MICROWAVE
INTERFACEMICROWAVE
INTERFACE
S25
TN767
DSX-1DSX-1
DS1
655 FEET
MAXIMUMLIGHT-GUIDE
LIGHT-GUIDE
INTERFACEINTERFACE
INFRAREDINTERFACEINFRAREDINTERFACE
ANY DSX-1
INTERFACE
ANY DSX-1INTERFACE655 FEET
MAXIMUM
Figure 2-19.
System 25 Connection to DS1 Facility (Non-Metallic Transmission Interface)
SYSTEM 25
TN767C6C
CONNECTOR
CABLE (NOTE)
< >
NOTE: FOR DISTANCES OVER 50 FEET (15.2 M)USED C6E CONNECTOR CABLE(S) BETWEENC6C CONNECTOR CABLE AND DS1 TIETRUNK CIRCUIT PACK.551 CSU
TB1
TO
CO
TO LOCALLY
PROVIDED -48V
POWER SUPPLY
Figure 2-20.
System 25 Connection to DS1 551 CSU
2-127 
						

							FEATURES AND SERVICES
Data Call Setup
This feature allows a user to originate calls from
25 provides three methods of data calling:
l Dialing from a data terminal, which is
feature description.a data terminal or a voice terminal. System
described in the“Data Terminal Dialing”
l Setting up data calls from a voice terminal,which is described in the “Transfer to
Data” feature description.
l Setting up data calls (or voice calls) for another terminal from a data terminal, which
is described in the “Third-Party Call Setup” feature description.
2-128