Frame Format of BISYNC

Binary Synchronous Communication Protocol (BISYNC) is a byte oriented communication protocol and a data link layer protocol, developed by IBM. In a byte oriented approach, every frame is viewed as a set of bytes/characters.

Some of the favored byte oriented protocols are –

1. BISYNC (Binary synchronous communications protocol)
2. Point to point protocol (PPP)
3. DDCMP (Digital data communications message protocol)

BISYNC is also referred as BSC. BISYNC follows a sentinel approach. A sentinel approach is a guarding kind of behavior, exhibited by certain fields in frame format.

Frame Format of BISYNC :

• BISYNC has two (BI) synchronous fields (SYNC) in the starting of the frame.
• The frame format includes two synchronous fields of 8 bits each. These two fields are essential to spot the beginning of the frame.
• The third field is SOH (start of header) which of 8 bits, followed by fourth field – header.
• The fifth field is STX (start of text) which of 8 bits, followed by a body, of variable length.
• The body, is followed by ETX (end of text) which of 8 bits.
• CRC (cyclic redundancy check) is the last field of 16 bits, which is required for error detection.
• Frames are transmitted, beginning with leftmost field.
• The start of the frame is denoted by sending a special SYN (synchronize) character.
• Data portion of the frame, is contained between special sentinel character STX and ETX i.e., the body is guarded by STX & ETX.

STX and ETX :

• If the receiver encounters STX or ETX inside the body, it misunderstands them to be the actual STX or ETX.
• This problem, can be handled, using byte/character stuffing approach.

Note –
We use byte/character stuffing approach here, since it is a byte oriented protocol which employs character oriented approach. We use bit stuffing approach, in a bit oriented protocol.

• Byte/character stuffing approach can be defined as a process of adding one extra byte whenever there’s a flag or escape character within the text.
• A flag/escape character, say SYN/STX/ETX if found repeated twice in the body, an additional one byte will be added.
• This process of byte/character stuffing is handled by DLE (Data link escape) in BISYNC protocol. A DLE is a character used to carry non-printable characters in frame.