Computer network

Communication and networking
Computer Network:
A computer network consists of two or more computers that are linked in order to share
resources such as printers and CD-ROMs, exchange files, or allow electronic communications.
The computers on a computer network may be linked through cables, telephone lines, radio
waves, satellites, or infrared light beams.
Purpose and benefits of computer networking:
Linking computers into networks provides benefits in the following areas: information sharing,
resource sharing and application sharing. These benefits help to increase productivity.
 Sharing information: Networks allow users to share information in several different
ways. The most common way of sharing information is to share individual files. For
example, two or more people can work together on a single spreadsheet file or word-processing
document. In most networks, a large hard drive on a central server computer is
set up as a common storage area where users can store files to be shared.
In addition to sharing files, networks allow users to communicate with each other in
various ways. For example, messaging applications let network users exchange messages
with each other using an e-mail application such as Microsoft Outlook. Users can also
hold online meetings over the network. In fact, with inexpensive video cameras and the
right software, users can hold videoconferences over the network.
 Sharing resources: Certain computer resources, such as printers or hard drives, can be
set up so that network users can share them. Sharing these resources can result in
significant cost savings. For example, it’s cheaper to buy a single high-speed printer with
advanced features that can be shared by an entire workgroup than it is to buy separate
printers for each user in the group.
Hard drives can also be shared resources. In fact, providing users with access to a shared
hard drive is the most common method of sharing files on a network. A computer whose
main purpose in life is to host shared hard drives is called a file server.
In actual practice, entire hard drives aren’t usually shared. Instead, individual folders on a
networked hard drive are shared. This way, the network administrator can allow different
network users to have access to different shared folders. For example, a company may set
up shared folders for its sales department and accounting department. Then, sales
personnel can access the sales department’s folder, and accounting personnel can access
the accounting department’s folder.
You can share other resources on a network. For example, a network can be used to share
an Internet connection. In the early days of the Internet, it was common for each user who
required access to the Internet to have his or her own modem connection. Nowadays, it’s
more common for the network to provide a shared, high-speed Internet connection that
everyone on the network can access.
By Er Hari Prasad Ghimire Page 1

 Sharing applications: One of the most common reasons for networking in many
businesses is so that several users can work together on a single business application. For
example, an accounting department may have accounting software that can be used from
several computers at the same time. Or a sales-processing department may have an order-entry
application that runs on several computers to handle a large volume of orders.
Analog signal, digital signal and modulation:
Analog signal: Is a signal that is transmitted as a continuous wave. Analog signals change all the
time.As said earlier, many telephone systems transit sound as analog signals.
Digital signal: Is such signal that is transmitted between computers in which information is
represented by discrete states. For eg: high and low voltage. Digital signal may be represented
either in 0 or 1.
Modulation: Is the process of altering one signal(a carrier) according to a pattern provided by
another signal.There are there types of modulation.they are:
1. Amplitude modulation(AM): Is a method of encoding information in a transmission, such
as radio. Using carrier wave of constant frequency but varying amplitude.
2. Frequency modulation(FM): Is a way of encoding information in an electrical signal by
varying its frequency.The FM radiio band uses frequency modulation.
3. Phase modulation(PM): Is a method of imposing information onto a waveform signal by
shifting the phase of the wave to represent information, Such as the binary digits 0 and 1.
Modes of Channel Operation
Simplex
Data in a simplex channel is always one way. Simplex channels are not often used because it is
not possible to send back error or control signals to the transmit end.

It's like a one way street. An example of simplex is
Television, or Radio.
Half Duplex
A half-duplex channel can send and receive, but not at the same time. It's like a one-lane bridge
where two way traffic must give way in order to cross. Only one end transmits at a time, the
other end receives. In addition, it is possible to perform error detection and request the sender to
retransmit information that arrived corrupted. In some aspects, you can think of Internet surfing
as being half-duplex, as a user issues a request for a web document, then that document is
downloaded and displayed before the user issues another request.
Another example of half-duplex is talk-back radio, and
CB Radio (Citizens Band). You might have seen
movies where truckies (drivers of very big trucks)
communicate to each other, and when they want the
other person to speak they say "over". This is because
only one person can talk at a time.
Full Duplex
Data can travel in both directions simultaneously. There is no need to switch from transmit to
receive mode like in half duplex. Its like a two lane bridge on a two-lane highway. Have you
ever watched these television talk shows where the host has a number of people on the show, and
they all try to talk at once. Well, that's full duplex!
Of course, in the world of data communications, full
duplex allows both way communication
simultaneously. An example can be a consumer which
uses a cable connection to not only receive TV
channels, but also the same cable to support their
phone and Internet surfing. All these activities can
occur simultaneously.
Types of Network:
Network based on services:
Based on how the computers in a network are configured and how they access information,
networks are classified into two types: peer to peer and client-server.
Peer-to-peer network

In a peer-to-peer network a group of computers is connected together so that users can share
resources and information. There is no central location for authenticating users, storing files, or
accessing resources. This means that users must remember which computers in the workgroup
have the shared resource or information that they want to access. It also means that users must
log on to each computer to access the shared resources on that computer.
In most peer-to-peer networks, it is difficult for users to track where information is located
because data is generally stored on multiple computers. This makes it difficult to back up critical
business information, and it often results in small businesses not completing backups. Often,
there are multiple versions of the same file on different computers in the workgroup.
In some peer-to-peer networks, the small business uses one computer that is running a client
operating system, such as Microsoft Windows 98 or Windows XP Professional, as the designated
"server" for the network. Although this helps with saving data in a central location, it does not
provide a robust solution for many of the needs of a small business, such as collaborating on
documents.
Client-Server network
In a server-based network, the server is the central location where users share and access network
resources . This dedicated computer controls the level of access that users have to shared
resources. Shared data is in one location, making it easy to back up critical business information.
Each computer that connects to the network is called a client computer. In a server-based
network, users have one user account and password to log on to the server and to access shared
resources. Server operating systems are designed to handle the load when multiple client
computers access server-based resources.
Windows SBS 2008 is installed and configured as the central server on a server-based network.
Windows SBS 2008 provides the central point for authenticating users, accessing resources, and
storing information.

Network based on geographical area:
Computer networks can be classified based on various parameters . One such classification
parameter is geographical area the networks are spread out. Based on the span and scope the
network types can be :
 LAN – Local Area Network
 WLAN – Wireless Local Area Network
 WAN – Wide Area Network
 MAN – Metropolitan Area Network
 SAN – Storage Area Network
 CAN – Campus Area network
 PAN – Personal Area Network
 DAN – Desktop Area Network.
Of all these, LAN, WLAN, WAN and MAN is mostly used.
LAN (Local Area Network)
1. Local Area Network is a network type which is used to connect computer devices which
are located within small geographical area like home , offices, schools and small group of
buildings.
2. The typical range of LAN is few hundred meters but not more than a mile.
3. Data transfer rates in LAN are very high typically in the range of 0Mbps to 10Gbps
4. Cost of setting up and maintaining LAN network is low as compared to other types of
network.
5. LANs can be either wired or wireless. For wired connection, twisted pair or coaxial cable
or fiber optic can be used.
6. IEEE defines standards for LAN in IEEE 802. Ethernet or IEEE 802.3 is one of those.
7. Nodes in LAN can be organized as Bus, Ring and Star.
Fig: Bus Topology
MAN (Metropolitan Area Network)
 MAN is a computer network type which usually spans a city or large campus. MAN
networks lie in between LAN and WAN
 MAN interconnects a number of LANs using high capacity backbone technology, for
example fiber optic. MAN also provides up-link services to WAN and Internet

WAN (Wide Area Network)
 WAN is a network type which is used to describe network which spans regional, national
or global area. For example, offices located at NewDelhi , Mumbai and New York are
connected by WAN.
 WANs are connected using Leased Line, Dialup connection, Packet Switching and Cell
Relay
 WANs are also used to connect LANs located at different locations.
 Transmission rated in WANs usually range from 1200 bits/sec to 24 Mbps. With
connections like ATM and Leased Lines, the transmission rate can be reached greater
than 156 Mbps
Network Topology:
The physical topology of a network refers to the configuration of cables, computers, and other
peripherals. Physical topology should not be confused with logical topology which is the method
used to pass information between workstations. Logical topology was discussed in the Protocol
chapter.
Main Types of Network Topologies In networking, the term "topology" refers to the layout of
connected devices on a network. This article introduces the standard topologies of computer
networking.
One can think of a topology as a network's virtual shape or structure. This shape does not
necessarily correspond to the actual physical layout of the devices on the network. For example,
the computers on a home LAN may be arranged in a circle in a family room, but it would be
highly unlikely to find an actual ring topology there.
Network topologies are categorized into the following basic types:
 Star Topology
 Ring Topology
 Bus Topology
 Tree Topology
 Mesh Topology
 Hybrid Topology
More complex networks can be built as hybrids of two or more of the above basic topologies.
Star Topology Many home networks use the star topology. A star network features a central
connection point called a "hub" that may be a hub, switch or router. Devices typically connect to
the hub with Unshielded Twisted Pair (UTP) Ethernet.
Compared to the bus topology, a star network generally requires more cable, but a failure in any
star network cable will only take down one computer's network access and not the entire LAN.
(If the hub fails, however, the entire network also fails.)

See the illustration of Star Network Topology.
Advantages of a Star Topology
 Easy to install and wire.
 No disruptions to the network then connecting or removing devices.
 Easy to detect faults and to remove parts.
Disadvantages of a Star Topology
 Requires more cable length than a linear topology.
 If the hub or concentrator fails, nodes attached are disabled.
 More expensive than linear bus topologies because of the cost of the concentrators.
The protocols used with star configurations are usually Ethernet or LocalTalk. Token Ring uses a
similar topology, called the star-wired ring.
Star-Wired Ring
A star-wired ring topology may appear (externally) to be the same as a star topology. Internally,
the MAU of a star-wired ring contains wiring that allows information to pass from one device to
another in a circle or ring (See fig. 3). The Token Ring protocol uses a star-wired ring topology.
Ring Topology In a ring network, every device has exactly two neighbors for communication
purposes. All messages travel through a ring in the same direction (either "clockwise" or
"counterclockwise"). A failure in any cable or device breaks the loop and can take down the
entire network.
To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology.
Ring topologies are found in some office buildings or school campuses.
Advantage:

Because every computer is given equal access to the token.No one computer can monopolize the
network.
Disadvantage:
 Failure of one computer on the ring can affect the entire network.
 It is difficult to troubleshoot a ring network.
 Adding or removing computers disrupts the network.
 More cabling is needed than in a bus network.
See the illustration of Ring Topology.
Bus Topology Bus networks (not to be confused with the system bus of a computer) use a
common backbone to connect all devices. A single cable, the backbone functions as a shared
communication medium that devices attach or tap into with an interface connector. A device
wanting to communicate with another device on the network sends a broadcast message onto the
wire that all other devices see, but only the intended recipient actually accepts and processes the
message.
Ethernet bus topologies are relatively easy to install and don't require much cabling compared to
the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet
cabling options many years ago for bus topologies. However, bus networks work best with a
limited number of devices. If more than a few dozen computers are added to a network bus,
performance problems will likely result. In addition, if the backbone cable fails, the entire
network effectively becomes unusable.
See the illustration of Bus Network Topology.

Advantages of a Linear Bus Topology
 Easy to connect a computer or peripheral to a linear bus.
 Requires less cable length than a star topology.
Disadvantages of a Linear Bus Topology
 Entire network shuts down if there is a break in the main cable.
 Terminators are required at both ends of the backbone cable.
 Difficult to identify the problem if the entire network shuts down.
 Not meant to be used as a stand-alone solution in a large building.
Tree Topology Tree topologies integrate multiple star topologies together onto a bus. In its
simplest form, only hub devices connect directly to the tree bus, and each hub functions as the
"root" of a tree of devices. This bus/star hybrid approach supports future expandability of the
network much better than a bus (limited in the number of devices due to the broadcast traffic it
generates) or a star (limited by the number of hub connection points) alone.
See the illustration of Tree Network Topology.

Advantages of a Tree Topology
 Point-to-point wiring for individual segments.
 Supported by several hardware and software venders.
Disadvantages of a Tree Topology
 Overall length of each segment is limited by the type of cabling used.
 If the backbone line breaks, the entire segment goes down.
 More difficult to configure and wire than other topologies.
Mesh Topology Mesh topologies involve the concept of routes. Unlike each of the previous
topologies, messages sent on a mesh network can take any of several possible paths from source
to destination. (Recall that even in a ring, although two cable paths exist, messages can only
travel in one direction.) Some WANs, most notably the Internet, employ mesh routing.
A mesh network in which every device connects to every other is called a full mesh. As shown in
the illustration below, partial mesh networks also exist in which some devices connect only
indirectly to others.
See the illustration of Mesh Network Topology.
Hybrid Topology
A combination of any two or more network topologies. Note 1: Instances can occur where two
basic network topologies, when connected together, can still retain the basic network character,
and therefore not be a hybrid network. For example, a tree network connected to a tree network
is still a tree network. Therefore, a hybrid network accrues only when two basic networks are
connected and the resulting network topology fails to meet one of the basic topology definitions.
For example, two star networks connected together exhibit hybrid network topologies. Note 2: A
hybrid topology always accrues when two different basic network topologies are connected.
TRANSMISSION MEDIA

The means through which data is transformed from one place to another is called transmission or
communication media. There are two categories of transmission media used in computer
communications.
 BOUNDED/GUIDED MEDIA
 UNBOUNDED/UNGUIDED MEDIA
1. BOUNDED MEDIA:
Bounded media are the physical links through which signals are confined to narrow path. These
are also called guide media. Bounded media are made up o a external conductor (Usually
Copper) bounded by jacket material. Bounded media are great for LABS because they offer high
speed, good security and low cast. However, some time they cannot be used due distance
communication. Three common types of bounded media are used of the data transmission. These
are
 Coaxial Cable
 Twisted Pairs Cable
 Fiber Optics Cable
COAXIAL CABLE:
Coaxial cable is very common & widely used commutation media. For example TV wire is
usually coaxial.
Coaxial cable gets its name because it contains two conductors that are parallel to each other.
The center conductor in the cable is usually copper. The copper can be either a solid wire or
stranded martial.
Outside this central Conductor is a non-conductive material. It is usually white, plastic material
used to separate the inner Conductor form the outer Conductor. The other Conductor is a fine
mesh made from Copper. It is used to help shield the cable form EMI.
Outside the copper mesh is the final protective cover. (as shown in Fig)
The actual data travels through the center conductor in the cable. EMI interference is caught by
outer copper mesh. There are different types of coaxial cable vary by gauge & impedance.
Gauge is the measure of the cable thickness. It is measured by the Radio grade measurement, or
RG number. The high the RG number, the thinner the central conductor core, the lower the
number the thicker the core.
Here the most common coaxial standards.
 50-Ohm RG-7 or RG-11 : used with thick Ethernet.
 50-Ohm RG-58 : used with thin Ethernet

 75-Ohm RG-59 : used with cable television
 93-Ohm RG-62 : used with ARCNET.
CHARACTERISTICS OF COAXIAL CABLE
 Low cost
 Easy to install
 Up to 10Mbps capacity
 Medium immunity form EMI
 Medium of attenuation
ADVANTAGES COAXIAL CABLE
 Inexpensive
 Easy to wire
 Easy to expand
 Moderate level of EMI immunity
DISADVANTAGE COAXIAL CABLE
 Single cable failure can take down an entire network
STP
UTP
Twisted Pair Cable
The most popular network cabling is Twisted pair. It is light weight, easy to install, inexpensive
and support many different types of network. It also supports the speed of 100 mps. Twisted pair
cabling is made of pairs of solid or stranded copper twisted along each other. The twists are done
to reduce vulnerably to EMI and cross talk. The number of pairs in the cable depends on the

type. The copper core is usually 22-AWG or 24-AWG, as measured on the American wire
gauge standard. There are two types of twisted pairs cabling
1. Unshielded twisted pair (UTP)
2. Shielded twisted pair (STP)
1. Unshielded twisted pair (UTP)
UTP is more common. It can be either voice grade or data grade depending on the condition.
UTP cable normally has an impedance of 100 ohm. UTP cost less than STP and easily available
due to its many use. There are five levels of data cabling
Category 1
These are used in telephone lines and low speed data cable.
Category 2
These cables can support up to 4 mps implementation.
Category 3
These cable supports up to 16 mps and are mostly used in 10 mps.
Category 4
These are used for large distance and high speed. It can support 20mps.
Category 5
This is the highest rating for UTP cable and can support up to 100mps.
UTP cables consist of 2 or 4 pairs of twisted cable. Cable with 2 pair use RJ-11 connector and 4
pair cable use RJ-45 connector.
Characteristics of UTP
 low cost
 easy to install
 High speed capacity
 High attenuation
 Effective to EMI
 100 meter limit
Advantages of UTP

 Easy installation
 Capable of high speed for LAN
 Low cost
Disadvantages of UTP
 Short distance due to attenuation
2. Shielded twisted pair (STP)
It is similar to UTP but has a mesh shielding that’s protects it from EMI which allows for higher
transmission rate.
IBM has defined category for STP cable.
Type 1
STP features two pairs of 22-AWG
Type 2
This type include type 1 with 4 telephone pairs
Type 6
This type feature two pairs of standard shielded 26-AWG
Type 7
This type of STP consist of 1 pair of standard shielded 26-AWG
Type 9
This type consist of shielded 26-AWG wire
Characteristics of STP
 Medium cost
 Easy to install
 Higher capacity than UTP
 Higher attenuation, but same as UTP
 Medium immunity from EMI
 100 meter limit
Advantages of STP:

 Shielded
 Faster than UTP and coaxial
Disadvantages of STP:
 More expensive than UTP and coaxial
 More difficult installation
 High attenuation rate
Fiber Optics
Fiber optic cable uses electrical signals to transmit data. It uses light. In fiber optic cable light
only moves in one direction for two way communication to take place a second connection must
be made between the two devices. It is actually two stands of cable. Each stand is responsible for
one direction of communication. A laser at one device sends pulse of light through this cable to
other device. These pulses translated into “1’s” and “0’s” at the other end.
In the center of fiber cable is a glass stand or core. The light from the laser moves through this
glass to the other device around the internal core is a reflective material known as CLADDING.
No light escapes the glass core because of this reflective cladding.
Fiber optic cable has bandwidth more than 2 gbps (Gigabytes per Second)
Characteristics Of Fiber Optic Cable:
 Expensive
 Very hard to install
 Capable of extremely high speed
 Extremely low attenuation
 No EMI interference
Advantages Of Fiber Optic Cable:
 Fast
 Low attenuation
 No EMI interference
Disadvantages Fiber Optics:
 Very costly
 Hard to install
Unguided Media: Transmission media then looking at analysis of using them unguided
transmission media is data signals that flow through the air. They are not guided or bound to a
channel to follow. Following are unguided media used for data communication:

 Radio Transmission
 Microwave
 Satellite Communication

. RF Propagation: There are three types of RF (radio frequency) propagation:
 Ground Wave
 Ionospheric
 Line of Sight (LOS)

Ground wave propagation follows the curvature of the Earth. Ground waves have carrier
frequencies up to 2 MHz. AM radio is an example of ground wave propagation. Ionospheric
propagation bounces off of the Earth’s ionospheric layer in the upper atmosphere.
It is sometimes called double hop propagation. It operates in the frequency range of 30 - 85
MHz. Because it depends on the Earth’s ionosphere, it changes with the weather and time of day.
The signal bounces off of the ionosphere and back to earth. Ham radios operate in this range.
Line of sight propagation transmits exactly in the line of sight. The receive station must be in the
view of the transmit station. It is sometimes called space waves or tropospheric propagation. It is
limited by the curvature of the Earth for ground-based stations (100 km, from horizon to
horizon). Reflected waves can cause problems. Examples of line of sight propagation are: FM
radio, microwave and satellite.
Radio Frequencies : The frequency spectrum operates from 0 Hz (DC) to gamma rays (1019
Hz). Radio frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging
technology called wireless LANs. Some use radio frequencies to connect the workstations
together, some use infrared technology.
Microwave : Microwave transmission is line of sight transmission. The transmit station must be
in visible contact with the receive station. This sets a limit on the distance between stations
depending on the local geography. Typically the line of sight due to the Earth’s curvature is only
50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump
across the country.
Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry
large quantities of data due to their large bandwidth.
Advantages:

(a) They require no right of way acquisition between towers.
(b) They can carry high quantities of information due to their high operating frequencies.
(c) Low cost land purchase: each tower occupies only a small area.
(d) High frequency/short wavelength signals require small antennae.
Disadvantages:
(a) Attenuation by solid objects: birds, rain, snow and fog.
(b) Reflected from flat surfaces like water and metal.
(c) Diffracted (split) around solid objects.
(d) Reflected by atmosphere, thus causing beam to be projected away from receiver.
Satellite: Satellites are transponders (units that receive on one frequency and retransmit on
another) that are set in geostationary orbits directly over the equator. These geostationary orbits
are 36,000 km from the Earth’s surface. At this point, the gravitational pull of the Earth and the
centrifugal force of Earth’s rotation are balanced and cancel each other out. Centrifugal force is
the rotational force placed on the satellite that wants to fling it out into space.
The uplink is the transmitter of data to the satellite. The downlink is the receiver of data. Uplinks
and downlinks are also called Earth stations because they are located on the Earth. The footprint
is the “shadow” that the satellite can transmit to, the shadow being the area that can receive the
satellite’s transmitted signal.
Network Connecting Devices
HUB
Networks using a Star topology require a central point for the devices to connect. Originally this
device was called a concentrator since it consolidated the cable runs from all network devices.
The basic form of concentrator is the hub.
As shown in Figure; the hub is a hardware device that contains multiple, independent ports that
match the cable type of the network. Most common hubs interconnect Category 3 or 5 twisted-pair
cable with RJ-45 ends, although Coax BNC and Fiber Optic BNC hubs also exist. The hub

is considered the least common denominator in device concentrators. Hubs offer an inexpensive
option for transporting data between devices, but hubs don't offer any form of intelligence. Hubs
can be active or passive.
An active hub strengthens and regenerates the incoming signals before sending the data on to its
destination.
Passive hubs do nothing with the signal.
Switches
Switches are a special type of hub that offers an additional layer of intelligence to basic,
physical-layer repeater hubs. A switch must be able to read the MAC address of each frame it
receives. This information allows switches to repeat incoming data frames only to the computer
or computers to which a frame is addressed. This speeds up the network and reduces congestion.
Switches operate at both the physical layer and the data link layer of the OSI Model.
Bridges
A bridge is used to join two network segments together, it allows computers on either segment
to access resources on the other. They can also be used to divide large networks into smaller
segments. Bridges have all the features of repeaters, but can have more nodes, and since the
network is divided, there is fewer computers competing for resources on each segment thus
improving network performance.

Bridges can also connect networks that run at different speeds, different topologies, or different
protocols. But they cannot, join an Ethernet segment with a Token Ring segment, because these
use different networking standards. Bridges operate at both the Physical Layer and the MAC
sublayer of the Data Link layer. Bridges read the MAC header of each frame to determine on
which side of the bridge the destination device is located, the bridge then repeats the
transmission to the segment where the device is located.
Routers
Routers Are networking devices used to extend or segment networks by forwarding packets from
one logical network to another. Routers are most often used in large internetworks that use the
TCP/IP protocol suite and for connecting TCP/IP hosts and local area networks (LANs) to the
Internet using dedicated leased lines.
Routers work at the network layer (layer 3) of the Open Systems Interconnection (OSI) reference
model for networking to move packets between networks using their logical addresses (which, in
the case of TCP/IP, are the IP addresses of destination hosts on the network). Because routers
operate at a higher OSI level than bridges do, they have better packet-routing and filtering
capabilities and greater processing power, which results in routers costing more than bridges.
Gateways
A gateway is a device used to connect networks using different protocols. Gateways operate at
the network layer of the OSI model. In order to communicate with a host on another network, an
IP host must be configured with a route to the destination network. If a configuration route is not

found, the host uses the gateway (default IP router) to transmit the traffic to the destination host.
The default t gateway is where the IP sends packets that are destined for remote networks. If no
default gateway is specified, communication is limited to the local network. Gateways receive
data from a network using one type of protocol stack, removes that protocol stack and
repackages it with the protocol stack that the other network can use.
Examples
 E-mail gateways-for example, a gateway that receives Simple Mail Transfer Protocol
(SMTP) e-mail, translates it into a standard X.400 format, and forwards it to its
destination
 Gateway Service for NetWare (GSNW), which enables a machine running Microsoft
Windows NT Server or Windows Server to be a gateway for Windows clients so that
they can access file and print resources on a NetWare server
 Gateways between a Systems Network Architecture (SNA) host and computers on a
TCP/IP network, such as the one provided by Microsoft SNA Server
 A packet assembler/disassembler (PAD) that provides connectivity between a local area
network (LAN) and an X.25 packet-switching network
NICs (Network Interface Card)
Network Interface Card, or NIC is a hardware card installed in a computer so it can communicate
on a network. The network adapter provides one or more ports for the network cable to connect
to, and it transmits and receives data onto the network cable.
Wireless Lan card
Every networked computer must also have a network adapter driver, which controls the network
adapter. Each network adapter driver is configured to run with a certain type of network adapter.

Network card
Network Interface Adapter Functions
Network interface adapters perform a variety of functions that are crucial to getting data to and
from the computer over the network.
These functions are as follows:
Data encapsulation
The network interface adapter and its driver are responsible for building the frame around the
data generated by the network layer protocol, in preparation for transmission. The network
interface adapter also reads the contents of incoming frames and passes the data to the
appropriate network layer protocol.
Signal encoding and decoding
The network interface adapter implements the physical layer encoding scheme that converts the
binary data generated by the network layer-now encapsulated in the frame-into electrical
voltages, light pulses, or whatever other signal type the network medium uses, and converts
received signals to binary data for use by the network layer.
transmission and reception
The primary function of the network interface adapter is to generate and transmit signals of the
appropriate type over the network and to receive incoming signals. The nature of the signals
depends on the network medium and the data-link layer protocol. On a typical LAN, every
computer receives all of the packets transmitted over the network, and the network interface
adapter examines the destination address in each packet, to see if it is intended for that computer.
If so, the network interface adapter passes the packet to the computer for processing by the next
layer in the protocol stack; if not, the network interface adapter discards the packet.
Data buffering
Network interface adapters transmit and receive data one frame at a time, so they have built-in
buffers that enable them to store data arriving either from the computer or from the network until
a frame is complete and ready for processing.
Serial/parallel conversion
The communication between the computer and the network interface adapter runs in parallel, that
is, either 16 or 32 bits at a time, depending on the bus the adapter uses. Network

communications, however, are serial (running one bit at a time), so the network interface adapter
is responsible for performing the conversion between the two types of transmissions.
Media access control
The network interface adapter also implements the MAC mechanism that the data-link layer
protocol uses to regulate access to the network medium. The nature of the MAC mechanism
depends on the protocol used.
Network protocols
A networked computer must also have one or more protocol drivers (sometimes called a
transport protocol or just a protocol). The protocol driver works between the upper-level network
software and the network adapter to package data to be sent on the network.
In most cases, for two computers to communicate on a network, they must use identical
protocols. Sometimes, a computer is configured to use multiple protocols. In this case, two
computers need only one protocol in common to communicate. For example, a computer running
File and Printer Sharing for Microsoft Networks that uses both NetBEUI and TCP/IP can
communicate with computers using only NetBEUI or TCP/IP.
ISDN (Integrated Services Digital Network) adapters
Integrated Services Digital Network adapters can be used to send voice, data, audio, or video
over standard telephone cabling. ISDN adapters must be connected directly to a digital telephone
network. ISDN adapters are not actually modems, since they neither modulate nor demodulate
the digital ISDN signal.
Like standard modems, ISDN adapters are available both as internal devices that connect directly
to a computer's expansion bus and as external devices that connect to one of a computer's serial
or parallel ports. ISDN can provide data throughput rates from 56 Kbps to 1.544 Mbps (using a
T1 carrier service).

ISDN hardware requires a NT (network termination) device, which converts network data
signals into the signaling protocols used by ISDN. Some times, the NT interface is included, or
integrated, with ISDN adapters and ISDN-compatible routers. In other cases, an NT device
separate from the adapter or router must be implemented. ISDN works at the physical, data link,
network, and transport layers of the OSI Model.
WAPs (Wireless Access Point)
A wireless network adapter card with a transceiver sometimes called an access point, broadcasts
and receives signals to and from the surrounding computers and passes back and forth between
the wireless computers and the cabled network.
Access points act as wireless hubs to link multiple wireless NICs into a single subnet. Access
points also have at least one fixed Ethernet port to allow the wireless network to be bridged to a
traditional wired Ethernet network.
Modems
A modem is a device that makes it possible for computers to communicate over telephone lines.
The word modem comes from Modulate and Demodulate. Because standard telephone lines use
analog signals, and computers digital signals, a sending modem must modulate its digital signals
into analog signals. The computers modem on the receiving end must then demodulate the
analog signals into digital signals.

Modems can be external, connected to the computers serial port by an RS-232 cable or internal
in one of the computers expansion slots. Modems connect to the phone line using standard
telephone RJ-11 connectors.
The OSI Reference Model
The OSI reference model specifies standards for describing "Open Systems Interconnection"
with the term 'open' chosen to emphasise the fact that by using these international standards, a
system may be defined which is open to all other systems obeying the same standards throughout
the world. The definition of a common technical language has been a major catalyst to the
standardisation of communications protocols and the functions of a protocol layer.
The seven layers of the OSI reference model showing a connection between two end systems
communicating using one intermediate system.
The structure of the OSI architecture is given in the figure above, which indicates the protocols
used to exchange data between two users A and B. The figure shows bidirectional (duplex)

information flow; information in either direction passes through all seven layers at the end
points. When the communication is via a network of intermediate systems, only the lower three
layers of the OSI protocols are used in the intermediate systems.
Services provided by each Protocol Layer
The OSI layers may be summarised by:
1. Physical layer: Provides electrical, functional, and procedural characteristics to activate,
maintain, and deactivate physical links that transparently send the bit stream; only
recognises individual bits, not characters or multicharacter frames.
2. Data link layer: Provides functional and procedural means to transfer data between
network entities and (possibly) correct transmission errors; provides for activation,
maintenance, and deactivation of data link connections, grouping of bits into characters
and message frames, character and frame synchronisation, error control, media access
control, and flow control (examples include HDLC and Ethernet)
3. Network layer: Provides independence from data transfer technology and relaying and
routing considerations; masks peculiarities of data transfer medium from higher layers
and provides switching and routing functions to establish, maintain, and terminate
network layer connections and transfer data between users.
4. Transport layer: Provides transparent transfer of data between systems, relieving upper
layers from concern with providing reliable and cost effective data transfer; provides end-to-
end control and information interchange with quality of service needed by the
application program; first true end-to-end layer.
5. Session layer: Provides mechanisms for organising and structuring dialogues between
application processes; mechanisms allow for two-way simultaneous or two-way alternate
operation, establishment of major and minor synchronisation points, and techniques for
structuring data exchanges.
6. Presentation layer: Provides independence to application processes from differences in
data representation, that is, in syntax; syntax selection and conversion provided by
allowing the user to select a "presentation context" with conversion between alternative
contexts.
7. Application layer: Concerned with the requirements of application. All application
processes use the service elements provided by the application layer. The elements
include library routines which perform interprocess communication, provide common
procedures for constructing application protocols and for accessing the services provided
by servers which reside on the network.
The communications engineer is concerned mainly with the protocols operating at the bottom
four layers (physical, data link, network, and transport) in the OSI reference model. These layers
provide the basic communications service. The layers above are primarily the concern of
computer scientists who wish to build distributed applications programs using the services
provided by the network.
Switching: Large internet works may have multiple paths linking sender and receiver devices. In
much the same way that trains are switched on railored tracks. Information may be switched as it

travels through various communication channels. The different types of data switching
techniques are follows.
 Circuit switching
 Message switching
 Packet switching
Circuit switching network establishes a fixed bandwidth circuit (channel) between nodes
before the users may communicate, as if the nodes were physically connected with an electrical
circuit. The bit delay is constant during the connection, as opposed to packet switching, where
packet queues may cause varying delay.
Each circuit cannot be used by other callers until the circuit is released and a new connection is
set up. Even if no communication is taking place in a dedicated circuit then, that channel still
remains unavailable to other users. Channels that are available for new calls to be set up are said
to be idle. Telephone network is example of circuit switching system.
Virtual circuit switching is a packet switching technology that may emulate circuit switching, in
the sense that the connection is established before any packets are transferred, and that packets
are delivered in order.
Message switching was the precursor of packet switching, where messages were routed in their
entirety, one hop at a time. It was first introduced in 1961. Nowadays, message switching
systems are mostly implemented over packet-switched or circuit-switched data networks. E-mail
is example of a message switching system.
Packet switching is a communications paradigm in which packets (discrete blocks of data) are
routed between nodes over data links shared with other traffic. The term "packets" refers to the
fact that the data stream from your computer is broken up into packets of about 200 bytes (on
average), which are then sent out onto the network. Each packet contains a "header" with
information necessary for routing the packet from source to destination. Each packet in a data
stream is independent.
The main advantage of packet-switching is that it permits "statistical multiplexing" on the
communications lines. The packets from many different sources can share a line, allowing for
very efficient use of the fixed capacity. With current technology, packets are generally accepted
onto the network on a first-come, first-served basis. If the network becomes overloaded, packets
are delayed or discarded ("dropped").

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