The OSI Reference Model
By Stephen Bucaro
In 1984 the International Standards Organization (ISO) released a network
reference called the Open System Interconnect (OSI) model. This model defines a
network operating system as having seven layers, each layer performing a specific task.
No real world network operating systems conform exactly to the OSI model, but it
is useful as a reference when describing existing systems. It is difficult to study
network devices such as routers, switches, and gateways without using the model. It
is also difficult to describe and compare networking protocols such as TCP/IP and
IPX/SPX without using the OSI model as a reference.
Frequently you will see a table of the layers inverted from that shown above, with
Layer 1, the Physical layer on top. This causes confusion in the learning process when
data is described as moving up the OSI model or when a higher layer is
referred to. Be alert to this possibility.
This layer provides the interface to the network for the Network Operating
System (NOS). It provides network services and applications such as HTTP, FTP, TELNET
This layer provides character set conversion and formats the data, It performs
encryption and decryption, compression and decompression.
This layer authenticates security and establishes a connection ID. It
establishes, synchronizes, maintains and ends sessions.
This layer repackages messages that are too long into smaller segments. It adds
segment sequencing numbers, provides message multiplexing and manages flow control. At
the receiving end it provides error detection and recovery, and reassembles the
segments in the proper order.
This layer breaks data into smaller units and assigns logical addresses. It
determines the route from source to destination. On the receiving end it translates
logical addresses into physical addresses and reassembles the units. This layer
manages network traffic and routing.
This layer organizes data into frames and assigns the physical address. It
provides flow control and packages bits from the physical layer into frames and
provides error checking and correction.
This layer describes the physical components of the network, which includes
network interface cards, cables and connectors. It converts digital bits into
electronic signals for sending on the network, and converts received signals into
Donít worry if you donít understand the descriptions of each layer at this time.
Further explanations will follow. If you will need to be able to recall the order of
the layers. You can do that by memorizing the phrase: All People Seem To Need Data Processing.