OSI, acronym for:
Open System Interconnection,
• Formed to set rules that would allow different manufacturers to
build products that would seamlessly interact.
• Designed for the development of network protocols.
•Although no protocol has yet been developed using this model, it
has come to be accepted as a standard way of describing and
categorizing existing protocols. This model is called the OSI
Layer Name Function
7 Application Layer Program-to-program communication.
6 Presentation Layer Manages data representation conversions. For
example, the Presentation Layer would be
responsible for converting from EBCDIC to ASCII.
5 Session Layer Responsible for establishing and maintaining
communications channels. In practice, this layer is
often combined with the Transport Layer.
4 Transport Layer Responsible for end-to-end integrity of data
3 Network Layer Routes data from one node to another.
2 Data Link Layer Responsible for physical passing data from one node
1 Physical Layer Manages putting data onto the network media and
taking the data off.
• Physical layer An RF carrier signal that is digitally
modulated to create a bit stream. This bit stream
incorporates forward error correction, interleaving
and other techniques to mitigate the effects of
interference and weak signals that can produce high-
• Link layer Usually a specialized radio protocol that
employs a form of medium access optimized for the
radio environment. Most link protocols involve
interactions between the wireless modem and a base
station, and mobile units do not communicate directly
with each other.
• Network layer Some wireless WANs, such as RAM Mobile Data and ARDIS, use
network-layer protocols designed specifically for that network, however the trend is
toward using IP. This is the case with CDPD, as well as the packet services being
developed for PCS networks (GSM, CDMA, TDMA).
• Transport and higher layers These layers usually are not part of the wireless network,
but implemented as part of the application solution. Some transports have been designed
specifically for wireless networks. But it is also possible to use tried and proven
transports such as TCP, though some optimization of TCP's timing parameters and
algorithms tends to yield better results.
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• This layer is concerned with the encoding and decoding of
digital bits (1s and 0s) between network interfaces. It is
typically a function of the interface card, rather than a
• The Physical Layer manages to put data onto the network
media and taking the data off. This
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Data Link Layer
• The data link layer is concerned with the transmission of packets from one
network interface card to another, based on the physical address of the
interface cards. Typical data link protocols are Token Ring and Ethernet.
These protocols are typically enabled by the device driver that comes with the
network interface card. The device driver will be loaded in a specific order
with the other protocol programs.
• The data link layer is a point-to-point protocol, much like an airline flight. If
you have a direct flight, one plane can get you to your final destination.
However, if you have a connecting flight, the plane gets you to your
connection point, and another will get you from there to your destination, but
its up to you to make the connection yourself. Bridges operate at this layer
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The network layer is concerned with the end-to-end delivery
of messages. It operates on the basis of network addresses,
that are global in nature. Using the airline example, the
network layer makes sure that all the connecting flights are
made, so that you will actually arrive in your final
destination. Network layer protocols include the IPX portion
of the Netware IPX /SPX protocol, and the IP portion of the
TCP/IP protocol stack. Routers operate at this level.
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The transport layer is concerned with issues such as the safe, intact arrival
of messages. It makes the receiver aware that it is going to receive a
message, insures that it does get it, and can control the flow of the
message if the receiver is getting it too fast, or re-transmit portions that
arrive garbled. In our airline analogy, suppose you are flying your children
to Grandmas house unaccompanied. The data link layer planes will make
their flights. A small fee will insure that network layer ground attendants
get your kids from one flight to their connection. The transport layer will
call Grandma to let her know they are coming and what their luggage
looks like, and will expect a call from Grandma when she has them safe
and sound. Typical transport layer protocols are the SPX portion of
Netware SPX /IPX and the TCP portion of TCP/IP
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The session layer is the layer that manages all the activities of
the layers below it. It does this by establishing what is called
a virtual connection. Essentially a virtual connection is
established when a transmitting station exchanges messages
with the receiving station, and tells it to set up and maintain a
communications link. This is similar to what happens when
you log into the network. Once you have logged in, a
connection is maintained throughout the course of your user
session until you log out, even though you may not be
accessing the network continuously
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The presentation layers function is to establish a common
data format between communicating nodes. It is responsible
for formatting the data in a way the receiving node can
understand. It may also perform data translation between
different data formats. Examples of data format differences
include byte ordering (should it be read from left to right, or
vice versa) and character set (ASCII characters or IBMs
EBCDIC character set) as well as differences in numeric
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The application layer provides the user-accessible services of
the network. These services include such things as network
file transfer and management, remote job initiation and
control, virtual terminal sessions with attached hosts,
electronic mail services, and network directory services.
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Most common types of cables used in
networks and other related topics.
•Unshielded Twisted Pair (UTP) Cable
•Shielded Twisted Pair (STP) Cable
•Fiber Optic Cable
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Unshielded Twisted Pair (UTP) Cable
Categories of Unshielded Twisted Pair
•Category 1 Voice Only (Telephone Wire)
•Category 2 Data to 4 Mbps (LocalTalk)
•Category 3 Data to 10 Mbps (Ethernet)
•Category 4 Data to 20 Mbps (16 Mbps Token Ring)
•Category 5 Data to 100 Mbps (Fast Ethernet)
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Shielded Twisted Pair (STP) Cable
A disadvantage of UTP is that it may be susceptible to radio
and electrical frequency interference.
Shielded twisted pair (STP) is suitable for environments with
electrical interference; however, the extra shielding can make
the cables quite bulky.
Shielded twisted pair is often used on networks using Token
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A single copper conductor with a plastic layer insulation
between the center conductor and a braided metal shield. The
metal shield helps to block any outside interference from
fluorescent lights, motors, and other computers.
• Thin coaxial (thinnet 10Base2 carrying Ethernet signals).
• Thick coaxial (thicknet 10Base5 carrying Ethernet signals)
• Common connector for coaxial Bayone-Neill-Concelman
(BNC). Adapters available for BNC: T-connector, barrel
connector, and terminator.
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Fiber Optic Cable
• A center glass core surrounded by several layers of protective materials. Transmits
light, no electrical interference, immunity to the effects of moisture and lighting.
Transmit signals over much longer. Carry information at vastly greater speeds. Cost
of fiber cabling compares to copper cabling; however, more difficult to install and
modify. 10BaseF refers to fiber optic cable carrying Ethernet signals.
• Facts about fiber optic cables:
• Outer insulating jacket is made of Teflon or PVC.
• Kevlar fiber helps to strengthen the cable and prevent breakage.
• A plastic coating is used to cushion the fiber center.
• Center (core) is made of glass or plastic fibers.
• Common connector used is an ST connector, barrel shaped, similar to a BNC. A
newer connector, the SC, has a squared face and easier to connect in confined space.
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Stands at the heart of most networks. Very fast computer with
a large amount of RAM, storage space, and a fast NIC card.
The network operating system software resides on this
computer, along with any software applications and data files
that need to be shared.
The file server controls the communication of information
between the nodes on a network. For example, it may be
asked to send a word processor program to one workstation,
receive a database file from another workstation, and store an
e-mail message during the same time period
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All computers and terminals connected to the file server on a
network are called workstations. A typical workstation is a
computer configured with a network interface card,
networking software, and the appropriate cables.
Workstations do not necessarily need floppy disk drives or
hard drives because files can be saved on the file server.
Almost any computer can serve as a network workstation.
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Network Interface Cards
• The network interface card (NIC) provides the physical connection
between the network and the computer workstation. Most NICs are
internal, with the card fitting into an expansion slot inside the computer.
Some computers, such as Mac Classics, use external boxes which are
attached to a serial port or a SCSI port. Laptop computers generally use
external LAN adapters connected to the parallel port or network cards that
slip into a PCMCIA slot.
• NIC cards are determining factor to speed and performance of a
• The three most common network interface connections are Ethernet
cards, LocalTalk connectors, and Token Ring cards.
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NIC Card Common Types
• Ethernet Cards
• Ethernet cards contain connections for either coaxial( BNC ) or twisted pair cables ( RJ -
45 ). Some Ethernet cards have both BNC and RJ-45 and some also contain an AUI
• LocalTalk Connectors
• LocalTalk is Apple's built-in solution for networking Macintosh computers. It utilizes a
special adapter box and a cable that plugs into the printer port of a Macintosh (See fig. 2). A
major disadvantage of LocalTalk is that it is slow in comparison to Ethernet. Most Ethernet
connections operate at 10 Mbps (Megabits per second). In contrast, LocalTalk operates at
only 230 Kbps (or .23 Mbps).
• Token Ring Cards
• Token Ring network cards look similar to Ethernet cards. One visible difference is the type
of connector on the back end of the card. Token Ring cards generally have a nine pin DIN
type connector to attach the card to the network cable.
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Concentrators / Hubs
• Provide a central connection point for cables from workstations,
servers, and peripherals. In a star topology, twisted-pair wire is run from
each workstation to a central concentrator. Hubs are multislot
concentrators into which can be plugged a number of multi-port cards to
provide additional access as the network grows in size.
• Passive, allow signal to pass from a computer to another without any
• Active, they electrically amplify the signal as it moves from one
device to another. Active concentrators are used like repeaters to extend
the length of a network.
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• Boosts a network's signal as it passes through. It does this by
electrically amplifying the signal it receives and
rebroadcasting it. Repeaters can be separate devices or they
can be incorporated into a concentrator. Typically used when
total length network cable exceeds standards set for the type
of cable being used.
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• Allows to segment a large network into smaller, more efficient networks. Also
allows to add new network to older wiring scheme.
• A bridge monitors the information traffic on both sides of the network so that it
can pass packets of information to the correct location. Most bridges can "listen"
to the network and automatically figure out the address of each computer on both
sides of the bridge. The bridge can inspect each message and, if necessary,
broadcast it on the other side of the network.
• The bridge manages the traffic to maintain optimum performance on both sides
of the network. You might say that the bridge is like a traffic cop at a busy
intersection during rush hour. It keeps information flowing on both sides of the
network, but it does not allow unnecessary traffic through. Bridges can be used to
connect different types of cabling, or physical topologies. They must, however, be
used between networks with the same protocol.
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• Translates information from one network to another; it is similar to a super-
intelligent bridge. Routers select the best path to route a message, based on the
destination address and origin. The router can direct traffic to prevent head-on
collisions, and is smart enough to know when to direct traffic along back roads
• While bridges know the addresses of all computers on each side of the network,
routers know the addresses of computers, bridges, and other routers on the
network. Routers can even "listen" to the entire network to determine which
sections are busiest -- they can then redirect data around those sections until they
• Routers can:
•Direct signal traffic efficiently
•Route messages between any two protocols
•Route messages between linear bus, star, and star-wired ring topologies
•Route messages across fiber optic, coaxial, and twisted-pair cabling
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• local-area networks (LANs)
• wide-area networks (WANs)
Types of networks:
• topology : Common topologies include a bus, star, and ring.
• protocol : Set of rules/signals computers use to communicate.
E.g: One of the most popular protocols for LANs is called Ethernet. Another
popular LAN protocol for PCs is the IBM token-ring network .
• architecture : peer-to-peer or client/server architecture.
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