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An Overview of the DDS System

Introduction

The term DDS is an acronym for either the transport method, Digital Data System, or the name of the AT&T service itself, Dataphone Digital Service. In either case, it describes a North American digital transmission method that was initially deployed in the mid-1970s.

Up until 1984, when T1 facilities were tariffed, 56 KBPS DDS facilities were about the fastest digital systems commercially available. DDS facilities typically include rates of 2.4, 4.8, 9.6 and 56 KBPS. You were a big player when you had 56K DDS circuits!

Later, DDS services included optional "secondary channels" and included the rates of 19.2 KBPS and 64 KBPS.

DDS is available in point-to-point and multi-point synchronous configurations.

A 56 KBPS DDS circuit is normally referred to as a "56 KBPS" service, while 9.6, 4.8, and 2.4 KBPS lines are referred to as "Subrate" services.

Secondary Channel Operation

Optionally, in DDS-2, a "secondary" channel may be provided to the customer. Since this "secondary" channel operates at a relatively low bit- rate, this channel is typically used by customers to measure the end-to-end error-rate performance of the "primary" channel. Vendors of DDS equipment often use the secondary channel to provide remote unit configuration and monitoring capabilities. The following chart outlines the "primary", "secondary", and the local loop bit rates:

         Primary Rate         Secondary Rate        Local Loop Rate
         ____________         ______________        _______________

           2.4 KBPS             133.33 BPS              3.2 KBPS
           4.8 KBPS             266.66 BPS              6.4 KBPS
           9.6 KBPS             533.33 BPS             12.8 KBPS
          56.0 KBPS             2.666 KBPS             72.0 KBPS

Note that if "secondary" channel service is not provided, the actual local loop rate is equal to the "primary" rate (e.g. 56 KBPS service has a local loop bit-rate of 56 KBPS).

Customer Premise Equipment

At the customer's premise the following (CPE) equipment is required:

-- Data Service Unit (DSU)
-- Channel Service Unit (CSU)

Usually, these two functions are incorporated by vendors into a single CSU/DSU unit.

The Data Service Unit

The DSU converts unipolar customer data (RS-232, V.35, etc.) to a Bipolar Return-To-Zero (BPRZ) format for transmission over the local loop. As in T1 AMI encoding, this type of encoding is useful for transmitting timing information and ensures no DC offset.

Just like T1 AMI systems, large strings of zeroes may cause the complete absence of pulses on the line, causing timing recovery problems at the receiver. So, Bipolar Violation sequences are used to workaround this problem. Additionally, specific Bipolar Violation codes are used for diagnostic tests by the phone company.

For Subrate channels (2.4, 4.8, 9.6 KBPS) the Bipolar Violation codes used are based upon B6ZS (Bipolar 6 Zero Substitution). For 56 KBPS channels, the Bipolar Violation codes are based upon B7ZS (Bipolar 7 Zero Substitution).

Bipolar Violation codes typically have the following format:

          V 0 X C3 C2 C1   ----->
          ______________

          V = Violation bit
          X = Pulse/no pulse (ensures 0 DC bias - set according to the
                              desired + or - polarity of the violation)
          C1, C2, C3 = Control Code

The DSU provides the following functions:

Clock recovery and "loop timing" functions
Received signal amplification and equalization
Recognition and generation of network control codes
Optionally respond to the "Alternating" DSU loop code by putting a loop up towards the telephone company

An "Alternating" loop is one in which a loopback control byte is sent, followed by a test data byte. The loopback remains in effect as long as the loopback control bytes are received.

The Channel Service Unit (CSU)

The CSU provides the actual physical line interconnection and associated circuitry. The CSU provides the following functions:

Signal amplification and equalization
Termination for the simplex current ("span power")
Mandatory response to a polarity reversal of the local loop by initiating a loopback towards TELCO.

This CSU loop is sometimes referred to as a "Channel Loopback".
Optionally respond to the "Latching" loopback codes by initiating a loopback towards TELCO

A "Latching" loop is one in which one specific loopback code(s) is used to secure the loop, and another different loopback code(s) is used to remove the loop.

The Physical Line

DDS is delivered from the Central Office to the Customer Premise over unloaded, 4-Wire metallic-pair circuits. The termination impedance of DDS equipment is 135 ohms. DC Simplex ("sealing" or "Span Poser") current is provided between the Transmit and Receive pairs. This current helps to prevent oxide corrosion in cable junctions and splices.

There are no simple rules for loop distance. The requirement for loop length is that the insertion loss at the Nyquist Frequency (1/2 the bit rate) is less than 34 dB. Any bridged taps should be less than 2000 feet (at the 56K rate). As a general guideline only the following table can be used:

   ______________________________________________________________________
   |           |                                                        |
   |           |                        BIT RATE                        |
   |           |________________________________________________________|
   |           |       |      |      |      |      |      |      |      |
   |           |  2.4  | 3.2  | 4.8  | 6.4  | 9.6  | 12.8 | 56   | 72   |
   |  AWG #    |  KBPS | KBPS | KBPS | KBPS | KBPS | KBPS | KBPS | KBPS |
   |           |_______|______|______|______|______|______|______|______|
   |___________|_______|______|______|______|______|______|______|______|
   |           |       |      |      |      |      |      |      |      |
   |    19     |  112  | 100  |  86  | 78.5 |  67  | 65.5 | 36.5 |  35  |
   |    26     |   37  |  34  | 30.5 | 26.5 |  25  | 21.5 | 11.6 | 9.5  |
   |___________|_______|______|______|______|______|______|______|______|


DISTANCE IS EXPRESSED IN KILOFEET (1000 ft.)

Typically, the phone company will install repeaters at lengths much shorter than those expressed in this table.

Go here for more information on DDS line coding.

Central Office Equipment

At the C.O., the line will terminate into either a Sub-Rate Data Multiplexer (SRDM) or an Office Channel Unit (OCU).

The SRDM is employed for "Sub-Rate" services of 2.4, 4.8, 9.6, and 19.2 KBPS. Its function is to digitally multiplex (using Time Division techniques) many "subrate" channels, or "pulse-stuff" a single "subrate" channel, into a single 64K data signal for incorporation into a standard DS0 channel.

The OCU is used for direct handling of 56K and 64K DDS services. The OCU is usually a special card incorporated into T1 multiplexers at the Central Office.

More information about CO equipment and DS0 codes is here.