The Open System Interconnection (OSI) model is a set of protocols
that attempt to define and standardize the data communications
process. The OSI model is set by the International Standards
Organization (ISO). The OSI model has the support of most major
computer and network vendors, many large customers, and most
governments, including the United States.
The OSI model is a concept that describes how data communications
should take place. It divides the process into seven groups, called
layers. Into these layers are fitted the protocol standards
developed by the ISO and other standards bodies, including the
Institute of Electrical and Electronic Engineers (IEEE), American
National Standards Institute (ANSI), and the International
Telecommunications Union (ITU), formerly known as the CCITT (Comite
Consultatif Internationale de Telegraphique et Telephone).
The OSI model is not a single definition of how data communications
actually takes place in the real world. Numerous protocols may exist
at each layer. The OSI model states how the process should be
divided and what protocols should be used at each layer. If a
network vendor implements one of the protocols at each layer, its
network components should work with other vendors’ offerings.
The OSI model is modular. Each successive layer of the OSI model
works with the one above and below it. At least in theory, you may
substitute one protocol for another at the same layer without
affecting the operation of layers above or below. For example, Token
Ring or Ethernet hardware should operate with multiple upper-layer
services, including the transport protocols, network operating
system, internetwork protocols, and applications interfaces.
However, for this interoperability to work, vendors must create
products to meet the OSI model’s specifications.
The OSI model is not a single definition of how data communications
takes place. It states how the processes should be divided and
offers several options. In addition to the OSI protocols, as defined
by ISO, networks can use the TCP/IP protocol suite, the IBM Systems
Network Architecture (SNA) suite, and others. TCP/IP and SNA roughly
follow the OSI structure.
Although each layer of the OSI model provides its own set of
functions, it is possible to group the layers into two distinct
categories. The first four layers— physical, data link, network, and
transport—provide the end-to-end services necessary for the transfer
of data between two systems. These layers provide the protocols
associated with the communications network used to link two
The top three layers—the application, presentation, and session
layers— provide the application services required for the exchange
of information. That is, they allow two applications, each running
on a different node of the network, to interact with each other
through the services provided by their respective operating systems.
.The following is a description of just what each layer does.
1. The Physical layer provides the electrical and mechanical
interface to the network medium (the cable). This layer gives the
data-link layer (layer 2) its ability to transport a stream of
serial data bits between two communicating systems; it conveys the
bits that move along the cable. It is responsible for making sure
that the raw bits get from one place to another, no matter what
shape they are in, and deals with the mechanical and electrical
characteristics of the cable.
2. The Data-Link layer handles the physical transfer, framing (the
assembly of data into a single unit or block), flow control and
error-control functions (and retransmission in the event of an
error) over a single transmission link; it is responsible for
getting the data packaged and onto the network cable. The data link
layer provides the network layer (layer 3) reliable
information-transfer capabilities. The data-link layer is often
subdivided into two parts—Logical Link Control (LLC) and Medium
Access Control (MAC)—depending on the implementation.
3. The Network layer establishes, maintains, and terminates logical
and/or physical connections. The network layer is responsible for
translating logical addresses, or names, into physical addresses. It
provides network routing and flow-control functions across the
4. The Transport layer ensures data is successfully sent and
received between the two computers. If data is sent incorrectly,
this layer has the responsibility to ask for retransmission of the
data. Specifically, it pro-vides a network-independent, reliable
message-independent, reliable message-interchange service to the top
three application-oriented layers. This layer acts as an interface
between the bottom and top three layers. By providing the session
layer (layer 5) with a reliable message-transfer service, it hides
the detailed operation of the underlying network from the session
5. The Session layer decides when to turn communication on and off
between two computers—it provides the mechanisms that control the
data-exchange process and coordinates the interaction between them.
It sets up and clears communication channels between two
communicating components. Unlike the network layer (layer 3), it
deals with the programs running in each machine to establish
conversations between them.
6. The Presentation layer performs code conversion and data
reformatting (syntax translation). It is the translator of the
network, making sure the data is in the correct form for the
receiving application. Of course, both the sending and receiving
applications must be able to use data sub-scribing to one of the
available abstract data syntax forms.
7. The Application layer provides the user interface between the
software running in the computer and the network. It provides
functions to the user’s software, including file transfer access and
management (FTAM) and electronic mail.
Unfortunately, protocols in the real world do not conform precisely
to these neat definitions. Some network products and architectures
combine layers. Others leave layers out. Still others break the
layers apart. But no matter how they do it, all working network
products achieve the same result—getting data from here to there.
The question is, do they do it in a way that is compatible with
networks in the rest of the world?
PROTOCOLS AND THE OSI
MODELA LAN protocol is a set of rules for communicating between
computers. Protocols govern format, timing, sequencing, and error
control. Without these rules, the computer cannot make sense of the
stream of incoming bits.
But there is more than just basic communication. Suppose you plan to
send a file from one computer to another. You could simply send it
all in one single string of data. Unfortunately, that would stop
others from using the LAN for the entire time it takes to send the
message. This would not be appreciated by the other users.
Additionally, if an error occurred during the transmission, the
entire file would have to be sent again. To resolve both of these
problems, the file is broken into small pieces called packets and
the packets are grouped in a certain fashion. This means that
information must be added to tell the receiver where each group
belongs in relation to others, but this is a minor issue. To further
improve transmission reliability, timing information and error
correcting information are added. Because of this complexity,
computer communication is broken down into steps. Each step has its
own rules of operation and, consequently, its own protocol. These
steps must be executed in a certain order, from the top down on
transmission and from the bottom up on reception.
Because of this hierarchical arrangement, the term protocol stack is
often used to describe these steps. A protocol stack, therefore, is
a set of rules for communication, and each step in the sequence has
its own subset of rules.
What is a protocol, really? It is software that resides either in a
computer’s memory or in the memory of a transmission device, like a
network interface card. When data is ready for transmission, this
software is executed. The software prepares data for transmission
and sets the transmission in motion. At the receiving end, the
software takes the data off the wire and prepares it for the
computer by taking off all the information added by the transmitting
There are a lot of protocols, and this often leads to confusion. A
Novell network communicates through its own set of rules (its own
protocol called IPX/SPX), Microsoft does it another way (NetBEUI).
DEC does it a third way (DECnet), and IBM does it yet a fourth
(NetBIOS). Since the transmitter and the receiver have to “speak”
the same protocol, these four systems cannot talk directly to each
other. And even if they could directly communicate, there is no
guarantee the data would be usable once it was communicated.
Anyone who’s ever wanted to transfer data from an IBM-compatible
personal computer to an Apple Macintosh computer realizes that what
should be a simple procedure is anything but. These two popular
computers use widely differing—and incompatible—file systems. That
makes exchanging information between them impossible, unless you
have translation software or a LAN. Even with a network, file
transfer between these two types of computers isn’t always
If two types of personal computers can’t communicate easily, imagine
the problems occurring between PCs and mainframe computers, which
operate in vastly different environments and usually under their own
proprietary operating software and protocols. For example, the
original IBM PC’s peripheral interface—known as a bus—transmits data
eight bits at a time. The newer X86 and Pentium based PCs have
32-bit buses, and mainframes have even wider buses. This means that
peripherals designed to operate with one bus are incompatible with
another bus, and this includes network interface cards (NICs).
Similar incompatibilities also exist with software. For instance,
Unix-based applications (and data generated with them) cannot be
used on PCs operating under MS-DOS. Resolving some of these
incompatibilities is where protocol standards fit in.
A protocol standard is a set of rules for computer communication
that has been widely agreed upon and implemented by many vendors,
users, and standards bodies. Ideally, a protocol standard should
allow computers to talk to each other, even if they are from
different vendors. Computers don’t have to use an industry-standard
protocol to communicate, but if they use a proprietary protocol then
they can only communicate with equipment of their own kind.
There are many standard protocols, none of which could be called
universal, but the successful ones are moving towards full
compliance with some-thing called the OSI model. The standards and
protocols associated with the OSI reference model are the
cornerstone of the open systems concept for linking the literally
dozens of dissimilar computers found in offices throughout the world
WHAT OSI IS AND IS NOT
While discussing the OSI reference model it is important to
understand what the model does not specify as well as what it
actually spells out. The ISO created the OSI reference model solely
to describe the external behavior of electronics systems, not their
The reference model does not determine programming or operating
system functions, nor does it specify an application-programming
interface (API). Neither does it dictate the end-user interface-that
is, the command-line and/or icon-based prompts a user uses to
interact with a computer system.
OSI merely describes what is placed on a network cable and when it
will be placed there. It does not state how vendors must build their
computers, only the kinds of behavior these systems may exhibit
while performing certain communications operations.
The OSI standards can be grouped into pairs-one defines the services
offered by a network component, while the second specifies the
protocol used by that component to provide the defined service. This
concept permits a vendor to develop network elements that are more
or less ignorant of the other components on the network. They are
said to be ignorant in that they may need to know that other network
components exist, but not the specific details about their operating
systems or interface buses. One of the primary benefits of this
concept is that vendors can change the internal design of their
network components without affecting their network functionality, as
long as they maintain the OSI-prescribed external attributes. The
figure on the preceding page shows the protocols in the OSI model.
The OSI model is inherently connection-oriented, but the services
each OSI layer provides can either be connection-oriented, or
connectionless. In the three-step connection-oriented mode operation
(the steps are connection establishment, data transfer, and
connection release), an explicit binding between two systems takes
In connectionless operation, no such explicit link occurs; data
transfer takes place with no specified connection and disconnection
function occurring between the two communicating systems.
Connectionless communication is also known as datagram
AT THE PHYSICAL LAYER
Let's compare some real protocols to the OSI model. The best-known
physical layer standards of the OSI model are those from the IEEE.
That is, the ISO adopted some of the IEEE's physical network
standards as part of its OSI model, including IEEE 802.3 or
Ethernet, IEEE 802.4 or token-passing bus, and IEEE 802.5 or Token
Ring. ISO has changed the numbering scheme, however, so 802.3
networks are referred to as ISO 8802-3, 802.4 networks are ISO
8802-4, and 802.5 networks are ISO 8802-5.
Each physical layer standard defines the network's physical
characteristics and how to get raw data from one place to another.
They also define how multiple computers can simultaneously use the
network without interfering with each other. (Technically, this last
part is a job for the data-link layer, but we'll deal with that
IEEE 802.3 defines a network that can transmit data at 10Mbps and
uses a logical bus (or a straight line) layout. (Physically, the
network can be configured as a bus or a star.) Data is
simultaneously broadcast to all machines on the network and is
non-directional on the cable. All machines receive every broadcast,
but only those meant to receive the data will respond with an
acknowledgment. Network access is determined by a protocol called
Carrier Sense Multiple Access/Collision Detection (CSMA/CD). CSMA/CD
lets all computers send data whenever the cable is free of traffic.
If the data collides with another data packet, both computers "back
off," or wait, then try again to send the data until receipt is
acknowledged. Thus, once there is a high level of traffic, the more
users there are, the more crowded and slower the network will
become. Ethernet has found wide acceptance in office automation
IEEE 802.4 defines a physical network that has a bus layout. Like
802.3, Token Bus is a broadcast network. All machines receive all
data but do not respond unless data is addressed to them. But unlike
802.3, network access is determined by a token that moves around the
network. The token is broadcast to every device but only the device
that is next in line for the token gets it. Once a device has the
token it may transmit data. The Manufacturing Automation Protocol
(MAP) and Technical Office Protocol (TOP) standards use an 802.4
physical layer. Token Bus has had little success outside of factory
IEEE 802.5 defines a network that transmits data at 4Mbps or 16Mbps
and uses a logical ring layout, but is physically configured as a
star. Data moves around the ring from station to station, and each
station regenerates the signal. It is not a broadcast network. The
network access protocol is token passing. The token and data move
about in a ring, rather than over a bus as it does in Token Bus.
Token Ring has found moderate acceptance in office automation
There are other physical and data-link layer standards, some that
conform to the OSI model and others that don't. Arcnet is a well
known one that does not conform to any standard but its own. It uses
a token-passing bus access method, but not the same as does IEEE
802.4. LocalTalk is Apple's proprietary network that transmits data
at 230.4Kbps and uses CSMA/CA (Collision Avoidance). Fiber
Distributed Data Interface (FDDI) is an ANSI and OSI standard for a
fiber-optic LAN that uses a token-passing protocol to transmit data
at 100Mbps on a ring.
WHEN IT BEGAN
The International Standards Organization, based in Geneva,
Switzerland, is a multinational body of representatives from the
standards-setting agencies of about 30 countries. These agencies
include the American National Standards Institute (ANSI) and British
Standards Institute (BSI).
Because of the multinational nature of Europe, and their critical
need for intersystem communication, the market for OSI-based
products is particularly strong there. As a result, the European
Computer Manufacturers' Association (ECMA) has played a major role
in developing the OSI standards. In fact, European networking
vendors and users are generally further advanced in their OSI
implementations than are their American counterparts, who rely
principally on proprietary solutions such as IBM's Systems Network
Architecture (SNA) or the Internet's Transmission Control
Protocol/Internet Protocol (TCP/IP).
Creating the OSI standards has been a long, drawn-out process: The
ISO began work on OSI protocols in the late 1970s, finally releasing
its seven-layer architecture in 1984. It wasn't until 1988 that the
five-step standards-setting process finally resulted in stabilized
protocols for the upper layers of the OSI reference model. As the
OSI protocols continue to stabilize, the marketplace will encourage
vendors to become more compliant. In turn, OSI will continue it's
evolution-incorporating the technological advances that inevitably
occur in the electronics marketplace.
WHERE OSI IS NOW
As noted, the OSI-ratification process progresses slowly; only after
many years have vendors brought OSI-compatible applications to the
market. Among the first of these have been X.400-based electronic
mail packages; Retix (Santa Monica, Calif.) and Touch Communications
(Campbell, Calif.) offer X.400 e-mail products. In real-world
application, these X.400 e-mail packages allow incompatible end-user
e-mail programs, such as Lotus' (Cambridge, Mass.), cc:Mail and
IBM's PROFS, to communicate with each other.
Retix is also at the forefront of providing an OSI-compliant X.500
directory service application. This protocol specifies a global
network-addressing scheme that simplifies sending electronic
messages across large, multi-segment networks. A third OSI
application, File Transfer, Access, and Management (FTAM), is also
in use. This provides the protocols for the exchange of files
between two incompatible systems. In Europe, vendors and users are
implementing what is known as EDIFACT, for Electronic Data
Interchange for Administration, Commerce, and Transport. EDIFACT,
which became ISO international standard 9735 in 1988, provides a
syntax that allows international trading partners to define the
format and structure of business-related documents such as purchase
orders and invoices.
EDIFACT allows one company to create order-entry forms online, then
exchange the data added to those forms with computers in another
company. The receiving company's computers then use the EDIFACT
structural syntax to interpret and process the received document.
When fully implemented, EDIFACT, X.400, and X.500 will allow quick
and easy transmittance of forms-based data across a wide variety of
incompatible computer systems and large, enterprise wide networks,
thus fulfilling the original "open systems communications" promise
The Data-Link layer (the second OSI layer) is often divided into two
sublayers; the Logical Link Control (LLC) and the Medium Access
Control (MAC). The IEEE also defines standards at the data-link
layer. The ISO standards for the MAC, or lower half of the data-link
layer, were taken directly from the IEEE 802.x standards.
Medium Access Control, as its name suggests, is the protocol that
deter-mines which computer gets to use the cable (the transmission
medium) when several computers are trying. For example, 802.3 allows
packets to collide with each other, forcing the computers to retry a
transmission until it is received. 802.4 and 802.5 limit
conversation to the computer with the token. Remember, this is done
in fractions of a second, so even when the network is busy, users
don’t wait very long for access on any of these three network types.
The upper half of the data-link layer, the LLC, provides reliable
data transfer over the physical link. In essence, it manages the
The IEEE splits the data-link layer in half because the layer has
two jobs to do. The first is to coordinate the physical transfer of
data. The second is to manage access to the physical medium.
Dividing the layer allows for more modularity and therefore more
flexibility. The type of medium access control has more to do with
the physical requirements of the network than the actual management
of data transfer. In other words, the MAC layer is closer to the
physical layer than the LLC layer.
References: >> | OSI
Reference Model illustrated - a whatis definition
- International Organization for Standardization |
SYNOPSIS: WHAT HAPPENS WHEN A FILE IS OPENED ON A REMOTE WINDOWS NT
MACHINE ON A TCP/IP NETWORK
When you click on the file. The redirector determines that this
request must go to a remote machine (Application Layer) and passes
the request for data to the presentation layer.
The presentation layer determines that a NT machine will receive
this request and there is no need to format the data to be readable
by the remote machine.
At the session layer, NETBIOS requests a session with the remote
machine. This request makes its way down the layers and the response
comes back to the session layer (This means that TCP establishes a
conversation with the remote machine and receives a 'ready to
receive data' response back and ends the conversation, establishing
the session). NETBIOS hands the file name and remote machine to the
transport layer for delivery.
At the network layer, TCP converts the remote machine name from
NETBIOS name to IP address (If a WINS server is in use, TCP gets the
IP address of the WINS server and passes a request to resolve a name
to IP. IP then sends an ARP request, which finds the MAC address of
the remote machine, and sends the request for name resolution. If a
LMHOSTS file is in use, the IP address of the remote machine is
pulled from that file and resolved to a MAC address. If neither
method works, a broadcast is sent to find the machine.) and
establishes a conversation with the remote machine. It sends the IP
address and the file name (with other data) down to the network
layer for delivery. It then listens for the acknowledgements that
each packet has been received and resends any data that was missed.
At the transport layer, IP determines that the request to open a
file does not need to be fragmented (the data is not large enough to
require multiple packets) and places the data into a packet. It also
converts the addressing request from IP address to MAC address (If
the IP address is on the local network, a broadcast is sent to find
the MAC address of the machine. If routing is required, a broadcast
is sent to the IP address of the default gateway for that route, and
subsequent data is sent to the remote address via the MAC of the
router. Once the router receives the data, it hands the data up to
layer 3 and checks the remote IP address. It then resolves that
address to a destination MAC address using the same procedure a
machine would use) and passes the data with the MAC address to the
Data Link Layer.
The data link layer adds framing information (Ethernet 802.2 most
likely) to the data and sends it to the physical layer.
The physical layer then sends the request to open the file to the
remote machine's network card, where the frame passes up through
every layer and is picked up by the operating system. The operating
system then opens the file as requested and sends the information in
the file back down through all 7 layers to your machine, where it
comes back and is displayed on the screen.
NOTE: Your machine and/or the router will have the name, IP address
and relevant MAC conversions in cache most of the time. The
resolutions are included here for completeness.