Conversion of ISDN LAPB and ISDN LAPD frames
AssigneeInternational Business Machines Corporation
InventorsSteven Price, Raymond Jepson, Barry J. Worley, Steven P. Wood, Andrew J. Keogh
Life of this patent
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Abstract
An apparatus and method for transmitting X.25 communications over the D-channel of an ISDN link. This is achieved by terminal adapters connected between an X.25 terminal and an ISDN connection which contain converters to convert X.25 LAPB frames into LAPD frames and vice versa. The conversion process involves directly copying fields from the incoming LAPB frame into a LAPD frame with the minimum amount of verification. Essentially the only checking performed is to examine the number of octets in each field of the incoming frame to ensure that the conversion process itself does not fall down.
Claims11
A method of converting an ISDN LAPB frame to an ISDN LAPD frame comprising the steps of:
- determining a frame length of the ISDN LAPB frame;
- discarding any ISDN LAPB frames having a frame length less than two octets;
- examining an address field of the ISDN LAPB frame to determine whether it is a, COMMAND or RESPONSE;
- building an LAPD address field for the ISDN LAPD frame, and setting a COMMAND/RESPONSE bit in accordance with the address field of the ISDN LAPB frame;
- examining a control field of the ISDN LAPB frame to determine the length of the control field, and discarding the ISDN LAPB frame if the determined length of the control field is two octets and the total length of the ISDN LAPB frame is two octets, else copying the control field directly from the ISDN LAPB frame to the ISDN LAPD frame;
- and
- copying a fourth and any further octets directly into an information field of the ISDN LAPD frame.
A method of converting an ISDN LAPD frame to an ISDN LAPB frame comprising the steps of:
- determining a frame length of the ISDN LAPD frame:
- discarding any ISDN LAPD frames having a frame length less than three octets;
- examining an address field of the ISDN LAPD frame to determine whether it is a COMMAND or RESPONSE;
- building an LAPB address field for the ISDN LAPB frame, and setting an LAPB address field for the LAPB frame to the COMMAND or RESPONSE value in accordance with the address field of the ISDN LAPD frame;
- examining a control field of the LAPD frame to determine the length of the control field, and discarding the ISDN LAPD frame if the determined length of the control field is two octets and the total length of the ISDN LAPD frame is three octets, else copying the control field directly from the ISDN LAPD frame to the ISDN LAPB frame;
- and
- copying a fifth and any further octets directly into an information field of the ISDN LAPB frame.
A system for converting an ISDN LAPB frame to an ISDN LAPD frame comprising:
- means for determining a frame length of the ISDN LAPB frame;
- means for discarding any ISDN LAPB frames having a frame length less than two octets;
- means for examining an address field of the ISDN LAPB frame to determine whether it is a COMMAND or RESPONSE;
- means for building an LAPD address field for the LAPD frame, and means for setting a COMMAND/RESPONSE bit in accordance with the address field of the ISDN LAPB frame;
- means for examining a control field of the ISDN LAPB frame to determine the length of the control field, and discarding the ISDN LAPB frame if the determined length of the control field is two octets and the total length of the ISDN LAPB frame is two octets;
- means for copying the control field directly from the ISDN LAPB frame to the ISDN LAPD frame;
- and
- means for copying a fourth and any further octets directly into an information field of the ISDN LAPD frame.
A system for converting an ISDN LAPD frame to an ISDN LAPB frame comprising:
- means for determining a frame length Of the ISDN LAPD frame;
- means for discarding any ISDN LAPD frames having a frame length less than three octets;
- means for examining an address field of the ISDN LAPD frame to determine whether it is a COMMAND or RESPONSE;
- means for setting the address field for the ISDN LAPB frame to a COMMAND or RESPONSE value in accordance with the address field of the ISDN LAPD frame;
- means for examining a control field of the ISDN LAPD frame to determine the length of the control field, and discarding the ISDN LAPD frame if the determined length of the control field is two octets and the total length of the ISDN LAPD frame is three octets;
- means for copying the control field directly from the ISDN LAPD frame to the ISDN LAPB frame;
- and
- means for copying a fifth and any further octets directly into a information field of the ISDN LAPB frame.
Dependencies are inferred from the claim text.