Return to the History page.
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.
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).
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 CodeThe DSU provides the following functions:
The CSU provides the actual physical line interconnection and associated circuitry. The CSU provides the following functions:
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.
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.
| Home | History | Multiplex | Xmission | Networking | Switching | Modulation |
ddsops.htm, ©1998 All rights reserved
Tampa Bay Interactive, Inc.
Last Revised on: Monday, 25-Oct-2004 19:45:30 EDT