Line and System Interfaces of Digital Telephone Switching System


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This section contains the specifications for line and systems interfaces associated with the CO digital switching system. All trunk interfaces shall be digital.

Line and System Interfaces of Digital Telephone Switching System

5.31.1 Operating Limits

The following operating limits apply to the switching equipment and all associated miscellaneous circuits.
a) Operating Voltage
All digital exchange equipment requiring direct current, shall be designed to operate at a nominal voltage of -48 Vdc. The operating voltage may vary from -44 to -52 Vdc with positive polarity to ground, and the CO digital equipment shall be designed to operate normally within this voltage range.
b) Subscriber Lines Supervision Limits
1. Loop resistance
The loop resistance shall be 2000 ohms (maximum), including the telephone set (either rotary or Dual Tone Multi Frequency (DTMF) type). The switch shall support subscriber line impedances from 2000 to 3000 ohms by using external range extenders.
The digital switches shall provide a ground-referenced battery and loop current feed. Since the loop current will vary for each loop resistance, the maximum loop resistance of 2000 ohms shall maintain a minimum loop current of 18 to 24 mA, while the maximum current limit of line circuits is 48 mA.
2. Telephone set(s)
A maximum of four (4) telephone sets in parallel operation shall be allowed on the line.
3. Insulation resistance
The insulation resistance shall be 20000 ohms (minimum) measured between the a or b wire to earth and between the a to b wire.

4. A short circuit anywhere in a subscriber line shall not cause any damage to the exchange equipment.
5. The subscriber line circuit shall be protected against high voltage greater than 110 volts ac. Reference shall be made to SAES-T-903 “Protection Equipment in Communication Buildings.”
c) Digit Receivers
The following sub-sections state the requirements for the subscriber digit (dial pulse) receivers:
1. Dial Pulse Receivers
The equipment shall accept dial pulses within the following limits:
 Dial speed (decadic): 8-16 pulses per second.
 Make break ratio: 50-50 and 33-67.
 Inter-digit interval: 200 milliseconds minimum. (between dial pulse digits)
2. DTMF Receivers
The DTMF receivers shall accept two voice frequencies selected from separate groups of frequencies. The exchange must have the potential ability to cater the signals A, B, C, and D, corresponding to the fourth row of the key-board, when these signals are provided. The corresponding digital symbol, or letter, for the combination of a low-group and a high-group frequency is as follows:

The receiver’s performance shall comply with the following limits:
Receiving level (high, low group): -3 to 24 dBm.

Maximum level difference between two frequencies: 5 dB.
Frequency deviation: within plus/minus 2 percent (± 2%).
Signal length: not less than 40 milliseconds.
3. Incoming Calls
Receivers shall accept and register multifrequency pulsing from incoming trunks.
4. Digit Registration
 For LOC calls, receive a maximum of 7 digits.
 For LDD calls. Receive a maximum of 11 digits including prefix “0” for nationwide dialing.
 For IDD calls, receive a maximum of 16 digits including prefix “0” (zero).
d) Interconnection Control
The CO digital switch shall comply with the following sub-sections that state the requirements for the routing, interception, and trunk continuity test:
1. Routing
The outgoing traffic routing shall be flexible, so that it can be changed with minimum internal modification. It shall be possible to arrange each trunk subgroup for one-way outgoing and/or two-way (both ways) operation. The outgoing routes (one-way outgoing and two-way trunk subgroup) shall have available at least 512 routes in addition to the alternate routing capabilities.
2. Interception
The interception service shall be served by either a voice mail or a voice announcement machine.
3. Trunk Continuity Test
During the process of establishing a call through a switch and interconnecting with trunk, a continuity check of the trunk shall be made by the switch, prior to outpulsing the dialed digit.

e) Line Lockout
Line lockout shall be provided for all equipped lines to release the occupied links for the following cases:
1. When the subscriber fails to start dialing within an adjustable time interval of 10 – 20 seconds after receiving dial tone.
2. When the subscriber fails to end dialing within an adjustable time interval of 20 – 30 seconds after start dialing.
3. When a line is in an incorrect off-hook state.
Note: In each of the above cases, a busy tone and a “receiver off-hook” (howler) signal shall be applied to the subscriber line for one minute without the use of permanent trunks. It also shall be possible to route that line to an announcement machine, if required. Time periods indicated above shall be adjustable by software.
f) Subscriber Supervision
Subscriber supervision shall be effective over the operating range of -44 to -52 Vdc up to the line limit specified previously under section 5.31.1. To conserve energy, a constant current source shall be used as the feeding bridge.
g) Operating Features for Trunking Application
The CO digital switches shall perform the following functions depending on the signaling type used:

1. Signal Recognition
The exchange shall repeat supervisory signals from the called station toward the calling station whenever it is necessary.
2. Trunk Assignments
a. It shall be possible for each trunk route to operate as one-way incoming trunks, one-way outgoing trunks, and two-ways R2 trunks.
b. For trunk switch application, at least 512 outgoing (one-way and two-ways outgoing trunks) routes shall be available in the CO digital exchange in addition to the other types of trunks.
c. At least 500 trunks per route (incoming, outgoing, and/or two-ways trunks) shall be assignable.
d. The following trunk subgroup assignments shall be possible:
 Incoming (ICT), outgoing (OGT), and two-ways (2WT) trunk subgroups to one exchange unit.
 Either ICT or 2WT from or to one exchange unit.
 ICT, OGT, and 2WT from or to several exchange units.
 Either ICT or OGT or 2WT from or to several exchange units.
e. It shall be possible to assign all trunks in one trunk subgroup to any switching network group or, separately, to several switching network groups.

3. Route Advance
A method of advance route with a minimum of four trials in additional to the direct route for multi-alternate routing shall be provided. The call then shall be routed to overflow (“all trunks busy” tone) when all paths are busy.
4. Trunk Classes
The exchange shall be equipped with class of service indications for trunk group identification. The exchange shall also be equipped to increase the number of classes of services.
5. Service Restriction
The exchange shall be equipped to prevent connections that are restricted. A selective basis for prevention and a means to route such calls to busy-tone-trunks shall be provided.
6. Intercepting Facilities
a. Facilities shall be provided to intercept calls that cannot be completed due to the following conditions:

 Partially dialed or incorrectly dialed.
 Vacant codes dialed.
 Changed codes dialed.
 Switching machines congestion or network congestion.
b. Voice announcement machines for intercepting messages for the calls specified above shall be provided with the following capabilities.
 The machines shall have an announcement interval that may be varied from 3 to 24 seconds.
 Where required, an amplifier or several amplifiers of sufficient power to handle 200 intercepted calls shall be provided.
c. Intercept trunks for each announcement and reorder tone shall be provided. All of the intercept trunks shall be optionally assignable for any announcement or reorder tones. The exchange shall be equipped to send a reorder tone to the calling subscriber in case the calling subscriber still remains off-hook after the machine stops sending the recorded announcement.

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