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CS601 - Data Communication - Lecture Handout 40

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YMODEM is similar to X-MODEM with only the following major differences:

  • 1024-Byte data unit
  • Two CANsto abort Transmission
  • ITU-T CRC-16 for Error Checking
  • Multiple files can be sent simultaneously


  • Newer Protocol
  • Combines features of





  • Blocked Asynchronous Transmission
  • More powerful than XMODEM
  • Full Duplex
  • Sliding Window Flow Control
  • Allows transfer of Data and Binary Files


  • Designed at Columbia University
  • Most Widely used Asynchronous Protocol
  • File Transfer protocol is similar in operation to XMODEM, with sender waiting for an NAK before it starts TX
  • Kermit allows the transmission of control characters as Text

Synchronous Protocols

Speed of synchronous TX makes it a better choice over Asynchronous T for LAN, MAN and WAN technology

Synchronous Protocols

Classes of Synchronous Protocols

Synchronous Protocols can be divided into two main classes:

  • Character – Oriented Protocols
  • Bit – Oriented Protocols

Character – Oriented Protocols

  • Also called Byte- Oriented Protocol
  • These protocols interpret a transmission frame or packet as a succession of characters, each usually composed of one byte
  • All control information is in the form of an existing character encoding system

Bit – Oriented Protocols

These protocols interpret a transmission frame or packet as a succession of individual bits, made meaningful by their placement in the frame

Control information can be one or multiple bits depending on the information embodied in the pattern

  • Character –Oriented Protocols are not as efficient as bit – oriented protocols and are seldom used
  • They are easy to comprehend and employ the same logic as bit-oriented protocols
  • Their study will provide the basis for studying the other data link layer protocols
  • In all data link protocols, control information is inserted in the data frame as separate control frames or as addition to existing data frames
  • In character oriented protocols, this info is in the form of code words taken from oexisting character sets such as ASCII
  • IBN’s BSC is the best known character oriented protocol

Binary Synchronous Communication (BSC)

  • Developed by IBM in 1964
  • Usable in both point-to-point and multiple communications
  • It supports half-duplex TX using stop-and-wait ARQ flow control
  • Does not support full duplex TX or sliding window protocol



BSC protocol divides a transmission into frames


  • If a frame is used strictly for control purposes, it is called a Control frame
  • Control frames are used to exchange information b/w communicating devices for example, to establish the connection, to control the flow, to request error correction etc


  • If a frame contains part or all of the message itself, it is called a Data Frame
  • Data frames are used to transmit information, but may also contain control information applicable to that information


  • Figure shows the format of a simple data frame
  • Arrow shows the direct of TX
  • The frame begins with two or more synch. (SYN) characters
  • These characters alert the receiver to the arrival of a new frame and provide a bit pattern used by the receiving device to synch itself with that of the sending device
  • After the two synch characters, comes a start of text (STX) character
  • This character signals to the receiver that the control information is ending and the next byte will be data
  • Data or text can consist of varying number of characters
  • An end of text (ETX) indicates the end of text
  • Finally, the Block Check Count (BCC) are included for error correction
  • A BCC field can be a one-character LRC or a two –character CRC

Data Frame with Header

  • A frame as simple as above is seldom used
  • Usually we need to include the address of the receiving device, the address of the sending device and the identity number of the frame (0 or 1) for stop-and-wait ARQ

Data Frame with Header

  • All the above information is included in a special field called Header
  • Header begins with start of the header (SOH) character
  • The header comes after the SYNs and before the STX character
  • Everything received after the SOH field but before STX character is the Header information

Mltiblock Frame

Mltiblock Frame

  • The probability of an error in the block of text increases with the length of the frame
  • The more bits in a frame, the more are the chances of an error
  • For this reason, text in a message is often divided b/w several blocks
  • Each block starts with STX and ends with ITB, intermediate text block except the last one
  • Last block ends with ETX character
  • After the ETX has been reached, and the last BCC checked, the receiver sends a single ACK for the entire frame
  • The figure includes only 2 blocks but actual frames can have more than two
  • Some messages may be too long to fit in a frame
  • Several frames can carry continuation of a single message
  • To let the Rx know that the end of frame is not the end of transmission, the ETX character in all the frames but the last one is replaced by an End of Transmission block (ETB)
  • The receiver must acknowledge each frame separately

Multi Frames

Control Frames

  • A control frame is used by one device to send commands to or to get information from another device
  • A control frame contains control characters but no data
  • It carries information specific to the functioning of the data link layer itself

Control Frames

Control frames serve 3 purposes:

  • Establishing Connections
  • Maintaining Flow and Error Control during Data Transmission
  • Terminating Connection

Control frames serve 3 purposes


  • Data Link Protocols
  • Asynchronous Protocols
  • Synchronous Protocols

Reading Sections

Section11.1, 11.2

“Data Communications and Networking” 4th Edition by Behrouz A. Forouzan