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Part 10: Bit Orientated Protocols
Introduction
This section briefly discusses Bit Orientated Protocols, which uses bit to exchange data.
Bit Orientated Protocols (BOP)
Character orientated protocols are still inefficient. This is because a character is used to convey meaning. As the number of meanings increase, the overhead involved also increases, as a character is used to signal the meaning.
In binary orientated protocols, each bit has significance. The position and value of each bit in the data stream determines its function. Thus, a single character can hold 2.56 different meanings in a bit orientated protocol. This reduces the information needed to convey additional information, thus increasing the efficiency of the protocol. Examples of these types of protocols are,
- X.25 CCITT standard for packet data transmission
- HDLC high level data link control (adopted by ISO in 1970’s)
- SDLC synchronous data link control (developed by IBM)
Links between sender and receiver can be either half duplex, full duplex or both. Information can be sent across the network in two different ways, traveling different routes to the receiver (datagram), or traveling the same route (virtual circuit)
Information is packaged into an envelope, called a FRAME. Each frame has a similar format
- Header containing routing and control information
- Body
- Tail containing checksum data
Frames are responsible for transporting the data to the next point. Consider data that is to be sent from a source to a destination. This involves several intermediate points (called stations). The data is placed into a frame and sent to the next station, where the frame is checked for validity and if valid, the data extracted. The data is now repackaged into a new frame and sent by that station, and the process repeats till the data arrives at the destination.
When a station transmits a frame, it keeps a copy of the frame contents till the frame is acknowledged as correctly received by the next station. When a station receives a frame, it is temporarily stored in a buffer and checked for errors. If the frame has errors, the station will ask
the previous station to resent the frame. Frames that tare received without errors are also acknowledged, at which point the sending station can erase its copy of the frame.
A receiving station has a limited amount of buffer space to store incoming frames. When it runs out of buffer space, it signals other stations that it cannot receive my more frames.
Data is placed into frames for sending across a transmission link. The frame allows intelligent control of the transmission link, as well as supporting multiple stations, error recovery, intelligent (adaptive) routing and other important functions.
For the purposes of sending data on an HDLC link, there are two types of stations,
- Primary station (issues commands)
- Secondary station (responds to commands)
HDLC Primary Station
The primary station is responsible for controlling the data link, initiating error recovery procedures, and handling the flow of transmitting data to and from the primary. In a conversation, there is one primary and one or more secondary stations involved
HDLC Secondary Station
A secondary station responds to requests from a primary station, but may under certain modes of operation, initiate transmission of its own. An example of this is when it runs out of the buffer space, at which point it sends RNR (receiver not ready) to the primary station. When the buffer space is cleared, it sends RR (receiver ready) to the primary station, informing the primary that it is now ready to receiver frames again.
HDLC FRAMES
- Provide flexible format for additional link control
- Do not have sequence numbers
Sliding Windows Because frames are numbered, it is possible for a primary station to transmit a number of frames without receiving an acknowledgement for each frame. The secondary can store the incoming frames and reply using a supervisory frame with the sequence number bits in the control field set so as to acknowledge a group of received frames.
If the secondary runs out of buffer space to store incoming information frames, it can transmit a supervisory frame informing primary stations of its status. Primary stations will thus keep their information frames and wait till the secondary is again able to process Information frames.
When a secondary cannot process information frames, it must still be able to process incoming supervisory and unnumbered frames (because of status requests).
Summary A bit orientated protocol sends information as a sequence of bits. An example of a bit orientated protocol is HDLC. Frames are used as a transport mechanism to transport data from one point to another. A frame contains error checking information which allows data to be sent reliably from a sender to a receiver.
Three frames types are defined, and data is normally send using information frames. At any one time, a number of information frames can be unacknowledged by a secondary station, and this is called the sliding window value, which defaults to 2, but can be negotiated when a call is first established.
