DDS Line Coding
As mentioned elsewhere, DDS uses a form of Bipolar, return-to-zero, Alternate Mark Inversion for transmission over a 4-Wire metallic pair circuit.
At a transmission rate of 56K, the Nyquist frequency is 28 KHz. A binary zero is represented as zero volts on the line, while a binary one is transmitted in the form of a positive or negative pulse. Each binary one pulse is of opposite polarity from the previous pulse (Alternate Mark Inversion). The Alternate Mark Inversion prevents a build-up of the DC level on the line. Operation is similar to that of T1 AMI line encoding. HOWEVER, DDS/SW56K CIRCUITS EMPLOY UNIQUE BIPOLAR VIOLATION CODE SEQUENCES! THE STRUCTURE AND USE OF THESE SPECIAL BIPOLAR VIOLATION SEQUENCES IS DISCUSSED LATER ON IN THIS DOCUMENT.
With DDS, a single control station can communicate to multiple tributary stations (Multi-point DDS). This is accomplished through the use of a Multi-point Junction Unit (MJU). The MJU can open the “reverse” channel path upon detection of channel data activity (space-bit detection), or upon sensing a change in channel line activity (from “Control Mode Idle” to “Data Mode”). In the case of the latter; like standard data modems, the Inactive to Active states are controlled by the state of the end-device’s RTS lead. When RTS is low/off, the CSU/DSU reflects this state by transmitting a Control Mode Idle sequence to the network. When RTS is high/on, the CSU is actively transmitting data from the DTE device and is in “Data Mode”.
Bipolar Violation Codes
DDS and SW56K codes are expressed in the following notations:
0 = Zero volts transmitted B = Positive or Negative Pulse (Binary one) V = Positive or Negative Pulse in violation of the AMI rule X = Either a 0 or B, depending upon the required polarity of a violation N = Either a 0 or B is acceptable (Don't care)
The X and V bits are always separated by a 0, since consecutive pulses of the same polarity could degrade performance.
- This code is often referred to as Control Mode Idle (CMI).
- In DDS circuits, this code is usually transmitted when the RTS signal is low/off, depending upon CSU/DSU operation.
- Upon receiving this code from the network, the CD signal from the CSU/DSU is set low/off, depending again upon CSU/DSU operation/optioning.
- In SW56K circuits, this code is transmitted by network when the network is prepared to accept a call from a station.
- This code is also transmitted by the the Terminal Equipment when idle. This is analogous to an “ON-HOOK” state in the PSTN.BBBBX0V
- Can be transmitted by the network when network troubles exist.N00BX0V
- Can be transmitted by the network when network troubles exist.N0BBX0V
DTE Zero Suppression Sequence
- Transmitted toward the network from the DTE to provide adequate facility transition density. Any sequence of SEVEN (7) consecutive zeroes must be encoded as:0000X0VThe example below depicts the use of the Zero Suppression Sequence.
Data <–111100000000000000001 Line <–BBBB0000X0V0000X0V00B
Network Zero Suppression Sequence
- Transmitted by the network towards the DTE equipment to provide adequate facility transition density. Any sequence of SIX (6) consecutive zeroes must be encoded as:000X0V
More on Simplex Current
Simplex DC current (Sealing current) is provided between the Transmit and Receive pairs. This current helps to prevent oxide corrosion in cable junctions and splices.
However, this simplex current also provides a loopback function. When NORMAL, the Transmit Pair (T1, R1) is kept positive with respect to the Receive Pair (T,R). When a CSU LOOPBACK is desired, this polarity is REVERSED. The CSU detects the current reversal and enters a loopback state back towards the connected CO.
The DC simplex current has the following characteristics:
- For a MINIMUM voltage of 7 volts, the current should be at least 4 mA.
- For a MAXIMUM voltage of 28 volts, the current should not exceed 20 mA.
- For equal voltage on both wires of a pair, the current difference between the two wires should not exceed 1 mA.
Absence of simplex current should not affect data transmission operations of the station apparatus (CSU/DSU).
The phone network usually relies on centralized diagnostic centers for DDS testing. This section details the diagnostic capabilities available to the test centers.
Proper operation is tested by the service provider through the use of loopbacks. Additionally, testing is usually consolidated in centralized “hub” locations. Through the use of special DS0 codes, loopbacks can be effected at Remote Office Channel Units (OCU). Upon receipt of a CSU loopback code, the Remote OCU can reverse the simplex power, resulting in a loopback in the customer’s CSU equipment.
There are TWO types of loopbacks: Latching and Non-Latching. Standard DDS service utilizes Non-Latching loopbacks. DDS w/ Secondary Channel can use both Non-Latching and Latching loopbacks. A “new” service, 64K DDS, ONLY utilizes Latching loopbacks.
When a Latching loopback is invoked, a specific loopback activate and loopback deactivate sequence is sent.
When a Non-Latching loopback is invoked, the loopback is maintained as long as EVERY OTHER BYTE RECEIVED CONTAINS THE LOOPBACK CODE! Effective Bit Rate throughput for BERT tests during these loopbacks is actually 28 KBPS!