DATA COMMUNICATIONS
© Copyright Brian Brown, 1995-2000. All rights reserved.

Part 13: Bit Orientated Protocols

Bit orientated protocols | HDLC frames | Sliding windows | Summary

Introduction
This section briefly discusses Bit Orientated Protocols, which uses bits to exchange data.

Objectives
At the end of this section you should be able to

• explain how efficiency gains occur with bit-orientated protocols compared to character-orientated protocols
• list three different types of bit-orientated protocols in common use
• draw a diagram of an HDLC frame and label each major section
• state the three different types of HDLC frames and list their purpose
• illustrate the concept of "sliding windows"

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 bit 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 256 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, travelling different routes to the receiver (datagram), or travelling 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 to the next 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 resend the frame. Frames that are 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 any 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 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 receive frames again.

HDLC FRAMES
Data is packaged into frames to be sent across the HDLC link. The typical frame format used is,

Frames are a transport mechanism. Their sole purpose is to transport the data across one link, not end to end. As the frame arrives at the other end of the link, it is checked to errors, and if its okay, the data is stripped out of the frame, a new frame generated for it, the data inserted into the new frame, and then transmitted on the next link and so on until the data reaches its destination.

The various fields of a frame are

• Frame Start/End Flag Each frame begins and ends with a special 8 bit sequence, 7E hexadecimal (binary 01111110). The end flag can also be the start flag of the next frame.
• Address Field In command frames, this contains the address of the secondary (destination) station. In response frames (a reply to a primary station), the address specifies the secondary station sending the response.
• Control Field This field holds commands and responses. Examples are sequence numbers (frames are numbered to ensure delivery), and poll (you must reply) and final (this is the last frame) bits.
• Information Field This field contains the data. It can contain any sequence of bits. It normal practice it is a multiple of 8 bits.
• Frame Check Sequence Field This field contains a 32 bit Cyclic Redundancy Check (CRC) which covers the A, C and I fields.

HDLC FRAME TYPES
There are three types of frames

• information frames, which contain data (the information field)
• supervisory frames, which contain commands and responses
• un-numbered frames, which contain commands and responses and sometimes data

Information frames

• are sequentially numbered
• carry data, message acknowledgements, poll and final bits

Supervisory Frames

• perform link supervisory control
• message acknowledgements
• retransmit requests
• signal temporary hold on receipt on I frames (if secondary is busy)

Unnumbered 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.

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© Copyright B Brown. 1995-2000. All rights reserved.