Browse Prior Art Database

FF_7E Idle Flag Generation in HDLC Transmission

IP.com Disclosure Number: IPCOM000116543D
Original Publication Date: 1995-Sep-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 80K

Publishing Venue

IBM

Related People

Gerardin, P: AUTHOR [+4]

Abstract

Disclosed is a system to generate an idle flag in a HDLC transmission, involving hardware, picocode and microcode.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

FF_7E Idle Flag Generation in HDLC Transmission

      Disclosed is a system to generate an idle flag in a HDLC
transmission, involving hardware, picocode and microcode.

      Re-use of existing hardware leads to this implementation, but
it could be foreseen to apply this method to a very simple hardware
which eliminates the FF and 7E flags systematically on reception.

      The operator is not required to select any flag or pattern, the
decision is taken by this system.  Assume the following network and
HDLC transmission for a better understanding:
  Server            Network Node         Network Node         Server
    or                                                          or
  Communic.  >>>>>  Hard.Scanner  >>>>>  Hard.Scanner  >>>>>
Communic.
  Controler          rcv   xmit           rcv   xmit
Controler
  Figure 1.  NETWORK
  FFFFFF7E-(P)Fr.3-7EFFF..FFF7E--Fr.2--7EFFF..FFF7E--Fr.1--7EFFF...
  Transmitter side        -------->                     Receptor side
  Fr.  means Frame
  Figure 2.  HDLC TRANSMISSION

      Assumptions are that Transport Network Node (TNN) and HDLC are
known, and this implementation is limited to HDLC lowest level.  In
reception, a very simple hardware eliminates FF and 7E flags, only
data frames being provided to software application layer.  In
transmission, a Flag has to be regenerated.

      If the flag FF is always used, and if the Data Terminal
Equipment (DTE) follows the meaning of the Idle-State in HDLC, a
performance problem will be encountered, because the receiving DTE
will require the transmitting DTE to repeat the frames 2 and 3 on
reception of the FF flag after the frame 1; Then the receiving DTE
acknowledges the frame 2, and requires the transmitting DTE to repeat
the frame 3, (and so on if more than 3 frames).  The rule being that
if the flag FF is sent, it means that the sender has nothing to send
or has finished sending of I-Frames.  This Idle flag has a higher
priority than the Poll-bit.

      If the flag 7E is always used, and if the DTE follows the HDLC
specification a problem will be encountered in case of en errored
frame containing the Poll Bit, becaus...