Dismiss
InnovationQ will be updated on Sunday, Oct. 22, from 10am ET - noon. You may experience brief service interruptions during that time.
Browse Prior Art Database

Bit Rate Conversion Mechanism for Switched 56

IP.com Disclosure Number: IPCOM000114752D
Original Publication Date: 1995-Jan-01
Included in the Prior Art Database: 2005-Mar-29
Document File: 6 page(s) / 199K

Publishing Venue

IBM

Related People

Mandalia, BD: AUTHOR [+3]

Abstract

This invention describes a software method for preparing and recovering a 56 kbps byte stream transported over a 64kbps link.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 32% of the total text.

Bit Rate Conversion Mechanism for Switched 56

      This invention describes a software method for preparing and
recovering a 56 kbps byte stream transported over a 64kbps link.

      Switched 56 protocol provides a method of transporting a 56kbps
data stream in a 64kbps data stream.  For transmission, the 56kbps
data stream is padded by inserting a 1 bit after every 7 bits to
effectively increase the raw data rate to 64kbps.  On reception, the
64kbps stream is stripped of the extra 1 bits and the 56kbps stream
is recovered.

This protocol is described in the CCITT Recommendation V.110, Table
7b, page 415
               56 Kbps ISDN data stream
    ...  wwwwwwww  xxxxxxxx  yyyyyyyy  zzzzzzzz ...
  Last bit out     ///        //       /         First bit out
       ...  wwwwxxx1 xxxxxyy1 yyyyyyz1 zzzzzzz1 ...
               64Kbps ISDN data stream to network
  Fig. 1.   Transmit Stream Structure
      64Kbps ISDN data stream from network
          ...  wwwwwww1 xxxxxxx1 yyyyyyy1 zzzzzzz1 ...
  Last bit in       \\\       \\        \            First bit in
       ...  ..wwww   wwwxxxxx  xxyyyyyy  yzzzzzzz...
          56 KBps ISDN derived data stream
  Fig. 2.   Receive Stream Structure

      Byte Manipulation Mechanism (Switched 56 Rate Conversion)
Features - The innovative features of the bit rate conversion are:
  o  The protocol is available for enable/disable on any ISDN B
      channel (timeslot)
  o  The protocol is implemented in microcode, so it is flexible and
      easily removed/installed, enabled/disabled
  o  The transmit algorithm converts a group of 7 bytes to 8 bytes.

          By processing 7 bytes at each pass of the algorithm, the
processor registers are used efficiently and the minimum amount
of transmit state change information must be preserved between
passes.
  o  The receive algorithm converts a group of 8 bytes to 7 bytes.

          By processing 8 bytes at each pass of the algorithm, the
processor registers are used efficiently and the minimum amount
of receive state change information must be preserved between passes.
  o  The algorithm uses the barrel multiplier as a barrel shifter

          The processor instruction set only provides a single bit
shift (right or left) instruction, multiple shifts require
multiple instructions, which is not acceptable due to performance
constraints.  One feature that the processor provides is a barrel
multiplier, which allows a multiply operation to be completed in
a single cycle.  Since a multiply by 2n is equivalent to a shift
left of n, the multiplier is used to provide multiple shifts, and
the multiply value can be stepped for each sample with a single
left shift.

The transmit algorithm is:
  o  Get the first sample from the 56kbps circular buffer using the
      Microc...