- A protocol in data communication is a group of Specifications used to implement one or more layers of the OSI model.
- Data link protocols can be classified as synchronous or asynchronous.
- Asynchronous protocols such as XMODEM, YMODEM, ZMODEM, BLAST, and Kermit are used in file transfer.
- Synchronous protocols can be classified into two groups:
1.Character-Oriented protocols.
2.Bit-Oriented protocols.
- In Character-oriented protocols, the frame is interpreted as a series of characters.
- In bit-oriented protocols, each bit or group of bits can have meaning.
- Binary synchronous communication (BSC) is the most well-Known character-oriented protocol.
- BSC operates in half-Duplex mode using stop-and-wait ARQ in a point-to-point
- There are two types of BSC frames:
1.Control frames.
2.Data frames.
- Control frames perform these functions:
a.Make a connection
b.Control flow and error
c. Sever a connection
- A bit pattern that resembles a BSC control character in the data field must not be recognized as a control character, it must be made transparent.
- Data transparency in BSC is achieved by a process called byte stuffing.
- Byte stuffing involves
a.Demarcation of the transparent region.
b.Addition of DLE(in the transparent region) before every DLE character.
- All bit-oriented protocols are related to high-level data link control (HDLC).
- HDLC operates in half- or full-duplex mode in a point-to-point or multi point link configuration.
- HDLC stations are categorized as follows:
a.Primary station - Sends commands.
b.Secondary station - Sends responses.
c.Combined station-Sends commands and responses.
- HDLC stations are configured as follows:
a.Unbalanced - One primary, one or more secondaries.
b.Symertrical - Two physical stations, each capable of switching from primary to secondary.
c.Balanced - Two combined stations, each of equal status.
- HDLC stations communicate in one of three modes :
a.Normal response mode (NRM) - the secondary station needs permission to transmit.
b.Asynchronous response mode (ARM) - the secondary station does not need permission to transmit.
c.Asynchronous balanced mode (ABM) - Either combined station may initiate transmission.
- HDLC protocol defines three types of frames :
a.Information frame (I-frame) - for data transmission and control.
b.Supervisory frame (S-frame) - for control.
c.Unnumbered frame (U-frame) - for control and management.
- HDLC handles data transparency by adding a 0 whenever there are five consecutive 1s following a 0. This is called bit stuffing.
End of Summary of Data Link Protocol
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