Character and Bit Oriented Protocols

 [Read FitzGerald p185-187, 198-203 in 5^th Ed. and p148-151 in 6^th Ed.]

Character Oriented Protocols

Character oriented protocols are classified according to the following

• Simplex

• Half Duplex

• Full duplex

Simplex

This is the simplest form of protocol used in the transfer of data.  With this protocol, the transmission takes place in one direction at a time.  Transmission and Reception on the same line is NOT possible.  When simplex is used, the system is set up first to send data, while the other is set up to receive data and vice versa.  This does not mean that there is no communication in the reverse direction.  This Idle RQ system will stop and wait for an acknowledgment after each block, but no data can be sent in the reverse direction.  This is what makes it simplex.

Kermit is an example of this system of data transfer.  With this system, each user initiates the Kermit program and then enters the connect command.  This results in a link between the sending device and the receiving device.  The user at the receiving device enters the receive command and the user in the sending system uses the send command followed by the file(s) name(s).  The Kermit in the sending system then transmits the entire file(s).  This system is used when users wish to transfer files from one DTE to another (eg one PC to another PC) using a point-to-point link up.

Kermit is an asynchronous data link protocol.  This means that it does use start and stop bits and it packages the characters into frames/blocks/packets merely for convenience.

If manual modems are used then one must be set in the originate mode and the other in the answer mode, but both must be set to operate at the same baud rate. As each file segment is transferred, a message is sent to both screens, indicating that a segment is sent and received.  After all segments are transferred, both the sending and the receiving DTEs exit from Kermit and return to the operating system, by giving an exit command. Frames are identified by sending a character which specifies the length of the frame which is about to be sent. An additional control character is also sent after each frame called the carriage return character to indicate that the frame is finished.

With this simplex system the Kermit frame is made up as below:

Half Duplex

Most character oriented protocols operate in half-duplex, stop and wait mode.  These provide data transfer facilities in both directions but not at the same time.  A well known protocol of this type is IBM’s binary synchronous control or Bisync (or BSC) protocol.  It is normally used only with the synchronous mode of operation.  It is a connection oriented (reliable) data link protocol, which means that there is a logical connection between sender and receiver maintained for the duration of the communication.  BSC initiates the connection through a single master (primary) station which controls all data traffic to and from a number of slave (secondary) terminals or DTEs on either a multipoint network (connected via a branching point - a star) or on a multidrop network (- a bus) or both.  Of course, what can be done for many slaves can also be done for just one and therefore point-to-point connections are also possible. 

For shared access to work without contention it is necessary to have control frames as well as data frames.  These control frames allow for a mode known as poll-select to take place in which the master polls a slave to request data and selects the slave before sending data. 

For a successful transfer of data, both character and frame synchronisation must take place. With BSC, these functions are carried out using selected EBCDIC transmission control characters.  (An ISO character oriented protocol known as Basic Mode is based on BSC and uses ASCII control characters.)  Character (or DLE) stuffing is used to provide data transparency.

The  BSC Data Frame (I-Frame) formats look like this:

Full Duplex

A few character oriented protocols operate in full duplex mode.  This allows the transfer of data in both directions simultaneously.  To achieve this, these continuous RQ protocols using sliding window techniques.  The physical link is operated as eight (or sometimes 16) separate logical links. The flow of frames over each link being controlled by its own stop and wait protocol procedure.

Owing to the efficiencies, acceptance and other advantages of the bit oriented duplex protocols, few character oriented duplex protocols are now used.

Bit Oriented Protocols

All new data link protocols are bit oriented protocols.  These protocols use defined bit patterns rather than transmitting control characters to signal the start and end of a frame - known as frame delimiting.  The receiver searches on a bit by bit basis for the known start and end of frame bit pattern - called the flag.  Flags within the data are made transparent by bit stuffing.

IBM’s Synchronous Data Link Control (SDLC) was the predecessor of ISO’s formal bit oriented standard called High-level Data Link Control (HDLC).  Since then many other variants have been formalised.  Refer to your text for further information regarding the operation of these protocols.

Orientation

Many of the modern bit oriented protocols are connectionless (best try) which means that the protocol does not initiate a logical connection from source to destination before sending the data.  It merely drops each frame into the network and hopes that it will eventually arrive at the destination. Frame order cannot be guaranteed in a connectionless environment and the end-to-end layers must be able to cater for this eventuality.

Application Domains for Selected Data Link Protocols