WHAT ARE DATA PROTOCOLS?
(ISO), OSI and its seven layers. Emphasis will be placed on the first three layers because
they are more directly involved in communication.
Protocols should not be confused with formats. Formats typically show a standard
organization of bits and octets and describe the function of each to achieve a certain
objective. DS1 is a format as are SDH and SONET.
In this section we will familiarize the reader with basic protocol functions. This is
followed by a discussion of the Open System Interconnection (OSI), which has facilitated
a large family of protocols. A brief discussion of HDLC (high-level data-link control) is
provided. This particular protocol was selected because it spawned so many other link
layer protocols. Some specific higher layer protocols are described in Chapter 11.
Basic Protocol Functions
There are a number of basic protocol functions. Typical among these are:
Segmentation and reassembly (SAR)
A short description of each follows.
Segmentation and reassembly. Segmentation refers to breaking up the data message or
file into blocks, packets, or frames with some bounded size. Which term we use depends
on the semantics of the system. There is a new data segment called a cell, used in
asynchronous transfer mode (ATM) and other digital systems. Reassembly is the reverse
of segmentation, because it involves putting the blocks, frames, or packets back into
their original order. The device that carries out segmentation and reassembly in a packet
network is called a PAD (packet assemblerdisassembler).
Encapsulation. Encapsulation is the adding of header and control information in front
of the text or info field and parity information, which is generally carried behind the text
or info fields.
Connection control. There are three stages of connection control:
1. Connection establishment
2. Data transfer
3. Connection termination
Some of the more sophisticated protocols also provide connection interrupt and recovery
capabilities to cope with errors and other sorts of interruptions.
Ordered delivery. Packets, frames, or blocks are often assigned sequence numbers to
ensure ordered delivery of the data at the destination. In a large network with many
nodes and possible routes to a destination, especially when operated in a packet mode,
the packets can arrive at the destination out of order. With a unique segment (packet)
numbering plan using a simple numbering sequence, it is a rather simple task for a long
data file to be reassembled at the destination it its original order.
Flow control. Flow control refers to the management of the data flow from source to
destination such that buffer memories do not overflow, but maintain full capacity of all