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OSI Model - Page 1
During the past two decades there has been a tremendous increase in the
numbers and sizes of networks. Many of the networks, however, were built using
different implementations of hardware and software. As a result, many of the
networks were incompatible and it became difficult for networks using different
specifications to communicate with each other. To address this problem, the
International Organization for Standardization (ISO) researched many network
schemes. The ISO recognized that there was a need to create a network model that
would help network builders implement networks that could communicate and work
together (interoperability) and therefore, released the
Open System InterConnection (OSI)
reference model in 1984.
This Study Guide explains how standards ensure greater compatibility and
interoperability between various types of network technologies. Here
you will learn how the OSI reference model networking scheme supports networking
standards. In addition, you will see how information or data makes its way from
application programs (such as spreadsheets) through a network medium (such as
wires) to other application programs located on other computers on a network. As
you work through this chapter, you will learn about the basic functions that
occur at each layer of the OSI model, which will serve as a foundation as you
begin to design, build and troubleshoot networks.
The most basic level of computer information consists of
binary digits,
or bits (0s and 1s). Computers that send one or two bits of information,
however, would not be very useful, so other groupings -
bytes,
kilobytes,
megabytes, and
gigabytes - are necessary. In
order for computers to send information through a network, all communications on
a network originate at a source, then travel to a destination.
As illustrated in the Figure, the information that travels on a network is
referred to as
data,
packet, or
data packet. A data packet
is a logically grouped unit of information that moves between computer systems.
It includes the source information along with other elements that are necessary
in order to make communication possible and reliable with the destination
device. The source address in a packet specifies the identity of the computer
that sends the packet. The destination address specifies the identity of the
computer that finally receives the packet. If one computer (host A)
wants to send data to another computer (host B), the data must first be
packaged by a process called encapsulation.
In order for data packets to travel from a source to a destination on a
network, it is important that all the devices on the network speak the same
language or
protocol. A
Protocol is a set of rules that make
communication on a network more efficient. Some common examples are as
follows:
-
When you and I want to communicate we use the 26 letters of the alphabet
and the 10 digits (0-9) in our numbering system. With these basic 36 characters
we can communicate. We have agreed to use those 36 characters as our protocol,
or our rules of communication.
-
In Congress, a form of Roberts Rules of Order makes it possible for
hundreds of representatives, who all like to talk, to take turns, and to
communicate their ideas in an orderly manner.
-
While driving a car, other cars (should!) signal when they wish to make a
turn; if they did not, then the roads would be chaos.
-
While flying an airplane, pilots obey very specific rules for communication
with other airplanes and with air traffic control.
-
When answering the telephone, someone says, "Hello," then the person calling says,
"Hello. This is.... "; and so it goes back and forth.
One technical definition of a data communications protocol is: a set of
rules, or an agreement, that determines the format and transmission of data.
Layer n on one computer communicates with
Layer n on another
computer. The rules and conventions used in this communication are collectively
known as the
Layer n protocol.
The early development of LANs, MANs, and WANs was chaotic in many ways. The
early 1980's saw tremendous increases in the numbers and sizes of networks. As
companies realized the money they could save and the productivity they could
gain by using networking technology, they added networks and expanded existing
networks almost as rapidly as new network technologies and products could be
introduced.
By the mid-1980's, these companies began to experience growing pains from all
the expansions they had made. It became harder for networks that used different
specifications and implementations to communicate with each other. They realized
that they needed to move away from
proprietary networking systems.
Proprietary systems are privately developed, owned, and controlled. In the
computer industry, proprietary is the opposite of open. Proprietary means that
one or a small group of companies controls all usage of the technology. Open
means that free usage of the technology is available to the public.
To address the problem of networks being incompatible and unable to
communicate with each other, the International
Organization for
Standardization (ISO) researched network schemes like DECNET,
SNA, and TCP/IP in order to find a set of rules. As a result of this research,
the ISO created a network model that would help vendors create networks that
would be compatible with, and operate with, other networks.
The process of breaking down complex communications into smaller discrete
tasks could be compared to the process of building an automobile. When taken as
a whole, the design, manufacture, and assembly of an automobile is a highly
complex process. It's unlikely that one single person would know how to perform
all the required tasks to build a car from scratch. This is why mechanical
engineers design the car, manufacturing engineers design the molds to make the
parts, and assembly technicians each assemble a part of the car.
The
OSI reference model (Note: Do not confuse with ISO.),
released in 1984, was the descriptive scheme they created. It provided vendors
with a set of standards that ensured greater compatibility and interoperability
between the various types of network technologies that were produced by the many
companies around the world.
The OSI reference model is the primary model for network communications.
Although there are other models in existence, most network vendors, today,
relate their products to the OSI reference model, especially when they want to
educate users on the use of their products. They consider it the best tool
available for teaching people about sending and receiving data on a network.
The OSI reference model allows you to view the network functions that occur
at each layer. More importantly, the OSI reference model is a framework that you
can use to understand how information travels throughout a network. In addition,
you can use the OSI reference model to visualize how information, or data
packets, travels from application programs (e.g. spreadsheets, documents, etc.),
through a network medium (e.g. wires, etc.), to another application program that
is located in another computer on a network, even if the sender and receiver
have different types of network media.
In the OSI reference model, there are seven numbered layers, each of which
illustrates a particular network function. This separation of networking
functions is called
layering. Dividing the network into these seven
layers provides the following advantages:
-
It breaks network communication into smaller, simpler parts.
-
It standardizes network components to allow multiple-vendor development and
support.
-
It allows different types of network hardware and software to communicate
with each other.
-
It prevents changes in one layer from affecting the other layers, so that
they can develop more quickly.
-
It breaks network communication into smaller parts to make learning it
easier to understand.
The problem of moving information between computers is divided into seven
smaller and more manageable problems in the OSI reference model. Each of the
seven smaller problems is represented by its own layer in the model. The seven
layers of the OSI reference model are:
Layer 7: Application Layer
Layer 6: Presentation Layer
Layer 5: Session Layer
Layer 4: Transport Layer
Layer 3: Network Layer
Layer 2: Data Link Layer
Layer 1: Physical Layer
To help remember them in the correct order a common mnemonic is often used from 7 to 1 (top to bottom):
All
People
Seem
To
Need
Data
Processing
For those of you who like the Domino's or Pappa John's pies, try from 1 to 7 (bottom to top):
Please
Do
Not
Throw
Sausage
Pizza
Away
During the course of these Study Guides, you will work your way through the OSI model,
layer by layer. By working through the layers of the OSI reference model, you will understand how data packet
units travel through a network and what devices operate at each layer as data packets travel
through them. As a result, you will understand how to troubleshoot network
problems as they may occur during data packet flow.
Each individual OSI layer has a set of functions that it must perform in
order for data packets to travel from a source to a destination on a network.
Below is a brief description of each layer in the OSI reference model as shown
in the Figures.
Layer 7: The Application Layer
The application layer is the OSI layer that is closest to the user; it provides network
services to the user's applications. It differs from the other layers in that it does not
provide services to any other OSI layer, but rather, only to applications
outside the OSI model. Examples of such applications are spreadsheet programs,
word processing programs, and bank terminal programs. The application layer
establishes the availability of intended communication partners, synchronizes
and establishes agreement on procedures for error recovery and control of data
integrity. If you want to remember Layer 7 in as few words as possible, think of
browsers.
Protocols that operate on this layer include: TELNET, HTTP, FTP, TFTP, SMTP,
NTP, SNMP, EDI.
Layer 6: The Presentation Layer
The presentation layer ensures that the information that the application layer of one
system sends out is readable by the application layer of another system. If necessary, the
presentation layer translates between multiple data formats by using a common
format. If you want to think of Layer 6 in as few words as possible, think of a
common data format. Also think of encryption and compression as types of formats.
Specifications defined at this layer include: GIF, TIFF, JPEG, MPEG, MIME, ZIP, and ASCII.
Layer 5: The Session Layer
As its name implies, the session layer establishes, manages, and terminates
sessions between two communicating hosts. The session layer provides its services
to the presentation layer. It also synchronizes dialogue between the two hosts' presentation
layers and manages their data exchange. In addition to session regulation, the session
layer offers provisions for efficient data transfer, class of service, and
exception reporting of session layer, presentation layer, and application layer
problems. If you want to remember Layer 5 in as few words as possible, think of
dialogues and conversations.
Protocols/API's that operate on this layer include: RPC, SQL, NETBIOS.
Layer 4: The Transport Layer
The transport layer segments data from the sending host's system and reassembles the data into a
data stream on the receiving host's system. The boundary between the transport
layer and the session layer can be thought of as the boundary between
application protocols and data-flow protocols. Whereas the application,
presentation, and session layers are concerned with application issues, the
lower four layers are concerned with data transport issues.
The transport layer attempts to provide a data transport service that shields
the upper layers from transport implementation details. Specifically, issues
such as how reliable transport between two hosts is accomplished is the concern
of the transport layer. In providing communication service, the transport layer
establishes, maintains, and properly terminates virtual circuits. In providing
reliable service, transport error detection-and-recovery and information flow
control are used. If you want to remember Layer 4 in as few words as possible,
think of quality of service, and reliability.
Protocols that operate on this layer: TCP, UDP, NETBEUI, SPX.
These protocols are either
connectionless or
connection-oriented.
Connection-oriented means that a connection (a virtual link)
must be established
before data can be exchanged. This can guarantee that data will
arrive, and in the same order it was sent. It guarantees delivery by sending
acknowledgements back to the source when messages are received. It is a form of
flow control, and is called "Windowing".
TCP is an example of an connection-oriented transport protocol.
A common example of connection-oriented communication
is a telephone call: you call, the 'destination' picks up the phone and
acknowledges and you start talking (sending data). When a message or a piece of
it doesn't arrive, you say: "What!?" and the sender will retransmit the data.
Connectionless is the opposite of
connection-oriented; the sender does not establish a connection before it sends
data, it just sends without guaranteeing delivery. UDP is an example of an
connectionless transport protocol. A common example is regular mail versus registered/return receipt mail.
Regular mail is UDP and registered mail is TCP.
Layer 3: The Network Layer
The network layer is a complex layer that provides connectivity and path selection between two host
systems that may be located on geographically separated networks. If you want to
remember Layer 3 in as few words as possible, think of path selection, routing, and addressing.
Also this layer is responsible for
logical addressing (also known as
network addressing or Layer 3 addressing) - for example, IP addresses.
Examples of protocols defined at this layer: IP, IPX, AppleTalk, ICMP, RIP, OSPF, BGP, IGRP, and EIGRP.
Devices that operate on this layer: Routers, Layer 3 Switches.
Network layer addresses:
Also known as Layer 3 or Logical addresses. These type of addresses are protocol-dependent,
for example if the network protocol is IP, IP addressing will be used which is made up of a network
part and a host part and needs a subnet mask to determine the boundaries of these parts.
An example of an IP address is: 172.16.0.1 and a subnet mask: 255.255.0.0.
Layer 2: The Data Link Layer
The data link layer provides reliable transit of data across a physical link. In so doing,
the data link layer is concerned with physical (as opposed to logical) addressing,
network topology, network access, error notification, ordered delivery of
frames, and flow control. Provides error checking by adding a CRC to the frame, and flow
control. If you want to remember Layer 2 in as few words as
possible, think of frames and media access control.
Devices that operate on this layer: Switches and Bridges.
IEEE 802 Data Link sub layers
Around the same time the OSI model was developed, (February 1980 - hence 802)
the IEEE developed the 802-standards such as 802.5 Token Ring and 802.11 for wireless networks.
Both organizations exchanged information during the development which resulted in two compatible standards.
The IEEE 802 standards define physical network components such as cabling and
network interfaces, and correspond to the Data Link and/or Physical layer of the
OSI model. The IEEE refined the standards and divided the Data Link layer into
two sub layers.
The LLC and the MAC sub layer
LLC sublayer
LLC is short for Logical Link Control. The Logical Link
Control is the upper sublayer of the Data Link layer. LLC masks the underlying
network technology by hiding their differences hence providing a single
interface to the network layer. The LLC sublayer uses Source Service Access
Points (SSAPs) and Destination Service Access Points (DSAPs) to help the lower
layers communicate to the Network layer protocols acting as an intermediate
between the different network protocols (IPX, TCP/IP, etc.) and the different
network types (Ethernet, Token Ring, etc.) This layer is also responsible for
frames sequencing and acknowledgements.
The LLC sublayer is defined in the IEEE standard 802.2.
MAC sublayer
The Media Access Control layer takes care of physical addressing and allows upper layers
access to the physical media, handles frame addressing, error checking. This
layer controls and communicates directly with the physical network media through
the network interface card. It converts the frames into bits to pass them on to
the Physical layer who puts them on the wire (and vice versa).
IEEE LAN standards such as 802.3, 802.4, 802.5 and 802.10 define standards for the MAC
sublayer as well as the Physical layer.
Other standards on this layer include: X.25 and Frame Relay.
Data Link layer addresses
Also known as layer 2 addresses, BIAs (Burned-in Address),
physical address and most commonly referred to as MAC address. This is a fixed
address programmed into a NIC or a router interface for example.
00-10-E3-42-A8-BC is an example of a MAC address. The first 6 hexadecimal
digits (3 bytes) specify the vendor/manufacturer of the NIC, the other 6 digits
(3 bytes) define the host.
The layer 2 broadcast address is FF-FF-FF-FF-FF-FF.
Layer 1: The Physical Layer
The physical layer defines
the electrical, mechanical, procedural, and functional specifications for
activating, maintaining, and deactivating the physical link between end systems.
Such characteristics as voltage levels, timing of voltage changes, physical data
rates, maximum transmission distances, physical connectors, and other, similar,
attributes are defined by physical layer specifications. If you want to remember
Layer 1 in as few words as possible, think of signals and media.
Devices that operate on this layer: HUBs/concentrators, repeaters, NICs, and LAN
and WAN interfaces such as RS-232, OC-3, HDLC, BRI, V.24, V.35, X.25 and Frame Relay.
TCP/IP stack vs. the DoD Model
TCP/IP operation is defined in its own model: the DoD model.
DoD is short for Department of Defense, who designed TCP/IP for ArpaNet.
Although they are similar, in contrary to the 7-layer OSI model the DoD model has 4 layers.
Each DoD layer and its functions corresponds to 1 or more OSI layers and their functions,
which is represented in the image below:
For the CCNA exam you don't need to know the DoD model in
detail, but if you know the OSI model and the related DoD layers you can easily
identify the layer at which a certain protocol or standard is specified, for example:
Process/Application: Telnet, FTP, SMTP, HTTP, SNMP, etc.
Host To Host: TCP UDP.
Internet: IP, ICMP, ARP, RARP, BootP, etc.
Network Access: Ethernet, Fast Ethernet, Token Ring, FDDI, etc.
The Encapsulation Process
Encapsulation wraps data with the necessary protocol information
before network transit. Therefore, as the data packet moves down
through the layers of the OSI model, it receives headers, trailers,
and other information.
NOTE: The word "header" means that address information has been added.
The Application, Presentation and Session layer are known as the
Upper Layer and are implemented in software. The Transport and Network layer are
mainly concerned with protocols for delivery and routing of packets to a
destination and are implemented in software as well. The Data Link is
implemented in hard- and software and the Physical layer is implemented in
hardware only, hence its name. These last two layers define LAN and WAN
specifications.
A more detailed description of each layer follows below,
but here's what basically happens when data passes from Host A to Host B:
1. the Application, Presentation and Session layers take user input and converts it into data,
2. the Transport layer adds a segment header converting the data into segments,
3. the Network layer adds a network header and converts the segments into packets or datagrams,
4. the Data Link layer adds a frame header and trailer converting the packets into frames,
5. the MAC sublayer layer converts the frames into a bits which the Physical layer can put on the wire.
The steps are known as the 5 steps of
data encapsulation.
When the bits stream arrives at the destination, the Physical layer takes it of
the wire and converts it into frames, each layer will remove their corresponding
header while the data flows up the OSI model until it is converted back to data
and presented to the user, this is known as
decapsulation.
To see how encapsulation occurs, lets examine the manner in which
data travels through the layers as illustrated in the Figure .
Once the data is sent from the source, as depicted in the Figure, it travels
through the application layer down through the other layers.
As you can see, the packaging and flow of the data that is exchanged
goes through changes as the networks perform their services for end-users.
As illustrated in the Figures, networks must perform the following five
conversion steps in order to encapsulate data:
Build the data
As a user sends an e-mail message, its alphanumeric characters
are converted to data that can travel across the internetwork.
Package the data for end-to-end transport
The data is packaged for internetwork transport. By using segments,
the transport function ensures that the message hosts at both ends
of the e-mail system can reliably communicate.
Append (add) the network address to the header
The data is put into a packet or datagram that contains a network
header with source and destination logical addresses.
These addresses help network devices send the packets across
the network along a chosen path.
Append (add) the local address to the data link header
Each network device must put the packet into a frame. The frame
allows connection to the next directly-connected network device
on the link. Each device in the chosen network path requires framing
in order for it to connect to the next device.
Convert to bits for transmission
The frame must be converted into a pattern of 1s and 0s (bits) for
transmission on the medium (usually a wire). A clocking function
enables the devices to distinguish these bits as they travel across
the medium. The medium on the physical internetwork can vary along
the path used. For example, the e-mail message can originate on a LAN,
cross a campus backbone, and go out a WAN link until it reaches its
destination on another remote LAN. Headers and trailers are added as
data moves down through the layers of the OSI model.
To help remember them in the correct order, I use this mnemonic:
You
Don't
Dare
Send
Pa
Dy
For
Beer
Taken from my college days when my Irish friend was given our money to go buy beer.
Often he did not return with our drinks. So sad!
This mnemonic represents:
Data at the top 3 Layers
Segments at the Transport/4th Layer
Packets or
Datagrams at the Network/3rd Layer
Frames at the Data Link/2nd Layer, and
Bits at the Phisical/1st Layer
In order for data packets to travel from the source to the destination,
each layer of the OSI model at the source must communicate with its
peer layer at the destination. This form of communication is referred to
as Peer-to-Peer Communications. During this process, each layer's
protocol exchanges information, called
protocol data units (PDUs),
between peer layers. Each layer of communication, on the source
computer, communicates with a layer-specific PDU, and with its peer
layer on the destination computer as illustrated in the Figure.
Data packets on a network originate at a source and then travel to a
destination. Each layer depends on the service function of the OSI
layer below it. To provide this service, the lower layer uses encapsulation
to put the PDU from the upper layer into its data field; then it adds whatever
headers and trailers the layer needs to perform its function. Next, as the
data moves down through the layers of the OSI model, additional headers
and trailers are added. After Layers 7, 6, and 5 have added their information,
Layer 4 adds more information. This grouping of data, the Layer 4 PDU, is called a segment.
The network layer, for example, provides a service to the transport layer,
and the transport layer presents data to the internetwork subsystem. The
network layer has the task of moving the data through the internetwork.
It accomplishes this task by encapsulating the data and attaching a header
creating a packet (the Layer 3 PDU). The header contains information required
to complete the transfer, such as source and destination logical addresses.
The data link layer provides a service to the network layer. It encapsulates
the network layer information in a frame (the Layer 2 PDU); the frame header
contains information (e.g. physical addresses) required to complete the data
link functions. The data link layer provides a service to the network layer by
encapsulating the network layer information in a frame.
The physical layer also provides a service to the data link layer. The physical
layer encodes the data link frame into a pattern of 1s and 0s (bits) for transmission
on the medium (usually a wire) at Layer 1.
Data Encapsulation Summary
What basically happens when data passes from Host A to Host B?
-
Application, Presentation and Session layer take user input and converts it into data,
-
Transport layer adds a segment header converting the data into segments,
-
Network layer adds a network header and converts the segments into packets ,
-
Data Link layer adds a frame header converting the packets into frames,
-
MAC sub layer converts the frames into bits which the Physical layer can put on the wire.
The steps are known as the 5 steps of data encapsulation. When the bits stream arrives at the destination,
the Physical layer takes it of the wire and converts it into frames, each layer will remove their corresponding header
while the dataflow up the OSI model until it is converted back to data and presented to the user, this is known as de-capsulation.
OSI layers reference table:
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