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 Home > By Career > Computer/IT > Networking
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A computer network, often simply referred to as a network, is a collection of hardware
components and computers interconnected by communication channels that allow sharing
of resources and information.
Networks may be classified according to a wide variety of characteristics such as
the medium used to transport the data, communications protocol used, scale, topology,
and organizational scope.
The rules and data formats for exchanging information in a computer network are
defined by communications protocols. Well-known communications protocols are Ethernet,
a hardware and Link Layer standard that is ubiquitous in local area networks, and
the Internet Protocol Suite, which defines a set of protocols for internetworking,
i.e. for data communication between multiple networks, as well as host-to-host data
transfer, and application-specific data transmission formats.
Computer networking is sometimes considered a sub-discipline of electrical engineering,
telecommunications, computer science, information technology or computer engineering,
since it relies upon the theoretical and practical application of these disciplines.
History
This section requires expansion. Before the advent of computer networks that were
based upon some type of telecommunications system, communication between calculation
machines and early computers was performed by human users by carrying instructions
between them. Many of the social behaviors seen in today's Internet were demonstrably
present in the 19th century and arguably in even earlier networks using visual signals.
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Internet map. The Internet is a global
system of interconnected computer
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• In September 1940, George Stibitz used a Teletype machine to send instructions
for a problem set from his Model at Dartmouth College to his Complex Number Calculator
in New York and received results back by the same means. Linking output systems
like teletypewriters to computers was an interest at the Advanced Research Projects
Agency (ARPA) when, in 1962, J.C.R. Licklider was hired and developed a working
group he called the "Intergalactic Network", a precursor to the ARPANET.
• Early networks of communicating computers included the military radar system Semi-Automatic
Ground Environment (SAGE), started in the late 1950s
• The commercial airline reservation system semi-automatic business research environment
(SABRE) which went online with two connected mainframes in 1960.
• In 1964, researchers at Dartmouth developed the Dartmouth Time Sharing System
for distributed users of large computer systems. The same year, at Massachusetts
Institute of Technology, a research group supported by General Electric and Bell
Labs used a computer to route and manage telephone connections.
• Throughout the 1960s Leonard Kleinrock, Paul Baran and Donald Davies independently
conceptualized and developed network systems which used packets that could be used
in a network between computer systems.
• 1965 Thomas Merrill and Lawrence G. Roberts created the first wide area network
(WAN). • The first widely used telephone switch that used true computer control
was introduced by Western Electric in 1965.
• In 1969 the University of California at Los Angeles, the Stanford Research Institute,
University of California at Santa Barbara, and the University of Utah were connected
as the beginning of the ARPANET network using 50 kbit/s circuits.
• Commercial services using X.25 were deployed in 1972, and later used as an underlying
infrastructure for expanding TCP/IP networks.
Today, computer networks are the core of modern communication. All modern aspects
of the public switched telephone network (PSTN) are computer-controlled, and telephony
increasingly runs over the Internet Protocol, although not necessarily the public
Internet. The scope of communication has increased significantly in the past decade,
and this boom in communications would not have been possible without the progressively
advancing computer network. Computer networks, and the technologies needed to connect
and communicate through and between them, continue to drive computer hardware, software,
and peripherals industries. This expansion is mirrored by growth in the numbers
and types of users of networks from the researcher to the home user.
Properties
Computer networks:
Facilitate communications
Using a network, people can communicate efficiently and easily via email, instant
messaging, chat rooms, telephone, video telephone calls, and video conferencing.
Permit sharing of files, data, and other types of information
In a network environment, authorized users may access data and information stored
on other computers on the network. The capability of providing access to data and
information on shared storage devices is an important feature of many networks.
Share network and computing resources
In a networked environment, each computer on a network may access and use resources
provided by devices on the network, such as printing a document on a shared network
printer. Distributed computing uses computing resources across a network to accomplish
tasks.
May be insecure
A computer network may be used by computer hackers to deploy computer viruses or
computer worms on devices connected to the network, or to prevent these devices
from normally accessing the network (denial of service).
May interfere with other technologies
Power line communication strongly disturbs certain forms of radio communication,
e.g., amateur radio. It may also interfere with last mile access technologies such
as ADSL and VDSL.
May be difficult to set up
A complex computer network may be difficult to set up. It may also be very costly
to set up an effective computer network in a large organization or company.
Communication media
Computer networks can be classified according to the hardware and associated software
technology that is used to interconnect the individual devices in the network, such
as electrical cable (HomePNA, power line communication, G.hn), optical fiber, and
radio waves (wireless LAN). In the OSI model, these are located at levels 1 and
2.
A well-known family of communication media is collectively known as Ethernet. It
is defined by IEEE 802 and utilizes various standards and media that enable communication
between devices. Wireless LAN technology is designed to connect devices without
wiring. These devices use radio waves or infrared signals as a transmission medium.
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Wired technologies
• Twisted pair wire is the most widely used medium for telecommunication. Twisted-pair
cabling consist of copper wires that are twisted into pairs. Ordinary telephone
wires consist of two insulated copper wires twisted into pairs. Computer networking
cabling (wired Ethernet as defined by IEEE 802.3) consists of 4 pairs of copper
cabling that can be utilized for both voice and data transmission. The use of two
wires twisted together helps to reduce crosstalk and electromagnetic induction.
The transmission speed ranges from 2 million bits per second to 10 billion bits
per second. Twisted pair cabling comes in two forms which are Unshielded Twisted
Pair (UTP) and Shielded twisted-pair (STP) which are rated in categories which are
manufactured in different increments for various scenario.
• Coaxial cable is widely used for cable television systems actually, office buildings,
and other work-sites for local area networks. The cables consist of copper or aluminum
wire wrapped with insulating layer typically of a flexible material with a high
dielectric constant, all of which are surrounded by a conductive layer. The layers
of insulation help minimize interference and distortion. Transmission speed range
from 200 million to more than 500 million bits per second.
• ITU-T G.hn technology uses existing home wiring (coaxial cable, phone lines and
power lines) to create a high-speed (up to 1 Gigabit/s) local area network.
• Optical fiber cable consists of one or more filaments of glass fiber wrapped in
protective layers that carries data by means of pulses of light. It transmits light
which can travel over extended distances. Fiber-optic cables are not affected by
electromagnetic radiation. Transmission speed may reach trillions of bits per second.
The transmission speed of fiber optics is hundreds of times faster than for coaxial
cables and thousands of times faster than a twisted-pair wire. This capacity may
be further increased by the use of colored light, i.e., light of multiple wavelengths.
Instead of carrying one message in a stream of monochromatic light impulses, this
technology can carry multiple signals in a single fiber.
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Distributed processing
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Wireless technologies
• Terrestrial microwave – Terrestrial microwaves use Earth-based transmitter and
receiver. The equipment looks similar to satellite dishes. Terrestrial microwaves
use low-gigahertz range, which limits all communications to line-of-sight. Path
between relay stations spaced approx, 48 km (30 mi) apart. Microwave antennas are
usually placed on top of buildings, towers, hills, and mountain peaks.
• Communications satellites – The satellites use microwave radio as their telecommunications
medium which are not deflected by the Earth's atmosphere. The satellites are stationed
in space, typically 35,400 km (22,000 mi) (for geosynchronous satellites) above
the equator. These Earth-orbiting systems are capable of receiving and relaying
voice, data, and TV signals.
• Cellular and PCS systems – Use several radio communications technologies. The
systems are divided to different geographic areas. Each area has a low-power transmitter
or radio relay antenna device to relay calls from one area to the next area.
• Wireless LANs – Wireless local area network use a high-frequency radio technology
similar to digital cellular and a low-frequency radio technology. Wireless LANs
use spread spectrum technology to enable communication between multiple devices
in a limited area. An example of open-standards wireless radio-wave technology is
IEEE 802.11.
• Infrared communication can transmit signals between devices within small distances
of typically no more than 10 meters. In most cases, line-of-sight propagation is
used, which limits the physical positioning of communicating devices.
• A global area network (GAN) is a network used for supporting mobile across an
arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge
in mobile communications is handing off the user communications from one local coverage
area to the next. In IEEE Project 802, this involves a succession of terrestrial
wireless LANs.
Exotic technologies
There have been various attempts at transporting data over more or less exotic media:
• IP over Avian Carriers was a humorous April fool's Request for Comments, issued
as RFC 1149. It was implemented in real life in 2001.
• Extending the Internet to interplanetary dimensions via radio waves.
A practical limit in both cases is the round-trip delay time which constrains useful
communication.
Communications protocol
A communications protocol defines the formats and rules for exchanging information
via a network and typically comprises a complete protocol suite which describes
the protocols used at various usage levels. An interesting feature of communications
protocols is that they may be – and in fact very often are – stacked above each
other, which means that one is used to carry the other. The example for this is
HTTP running over TCP over IP over IEEE 802.11, where the second and third are members
of the Internet Protocol Suite, while the last is a member of the Ethernet protocol
suite. This is the stacking which exists between the wireless router and the home
user's personal computer when surfing the World Wide Web.
Communication protocols have themselves various properties, such as whether they
are connection-oriented versus connectionless, whether they use circuit mode or
packet switching, or whether they use hierarchical or flat addressing.
There exist a multitude of communication protocols, a few of which are described
below.
Ethernet
Ethernet is a family of connectionless protocols used in LANs, described by a set
of standards together called IEEE 802 published by the Institute of Electrical and
Electronics Engineers. It has a flat addressing scheme and is mostly situated at
levels 1 and 2 of the OSI model. For home users today, the most well-known member
of this protocol family is IEEE 802.11, otherwise known as Wireless LAN (WLAN).
However, the complete protocol suite deals with a multitude of networking aspects
not only for home use, but especially when the technology is deployed to support
a diverse range of business needs. MAC bridging (IEEE 802.1D) deals with the routing
of Ethernet packets using a Spanning Tree Protocol, IEEE 802.1Q describes VLANs,
and IEEE 802.1X defines a port-based Network Access Control protocol which forms
the basis for the authentication mechanisms used in VLANs, but also found in WLANs
– it is what the home user sees when they have to enter a "wireless access key".
Internet Protocol Suite
The Internet Protocol Suite, often also called TCP/IP, is the foundation of all
modern internetworking. It offers connection-less as well as connection-oriented
services over an inherently unreliable network traversed by datagram transmission
at the Internet protocol (IP) level. At its core, the protocol suite defines the
addressing, identification, and routing specification in form of the traditional
Internet Protocol Version 4 (IPv4) and IPv6, the next generation of the protocol
with a much enlarged addressing capability.
SONET/SDH
Main article: Synchronous optical networking
Synchronous Optical NETworking (SONET) and Synchronous Digital Hierarchy (SDH) are
standardized multiplexing protocols that transfer multiple digital bit streams over
optical fiber using lasers. They were originally designed to transport circuit mode
communications from a variety of different sources, primarily to support real-time,
uncompressed, circuit-switched voice encoded in PCM format. However, due to its
protocol neutrality and transport-oriented features, SONET/SDH also was the obvious
choice for transporting Asynchronous Transfer Mode (ATM) frames.
Asynchronous Transfer Mode
Main article: Asynchronous transfer mode Asynchronous Transfer Mode (ATM) is a switching
technique for telecommunication networks. It uses asynchronous time-division multiplexing
and encodes data into small, fixed-sized cells. This differs from other protocols
such as the Internet Protocol Suite or Ethernet that use variable sized packets
or frames. ATM has similarity with both circuit and packet switched networking.
This makes it a good choice for a network that must handle both traditional high-throughput
data traffic, and real-time, low-latency content such as voice and video. ATM uses
a connection-oriented model in which a virtual circuit must be established between
two endpoints before the actual data exchange begins.
While the role of ATM is diminishing in favor of next-generation networks, it still
plays a role in the last mile, which is the connection between an Internet service
provider and the home user. For an interesting write-up of the technologies involved,
including the deep stacking of communications protocols used, see.
Scale
Computer network types by geographical scope
• Body (BAN)
• Personal (PAN)
• Near-me (NAN)
• Local (LAN)
o Home (HAN)
o Storage (SAN)
• Campus (CAN)
• Backbone
• Metropolitan (MAN)
• Wide (WAN)
• Internet
• Interplanetary Internet
Networks are often classified by their physical or organizational extent or their
purpose. Usage, trust level, and access rights differ between these types of networks.
Personal area network
A personal area network (PAN) is a computer network used for communication among
computer and different information technological devices close to one person. Some
examples of devices that are used in a PAN are personal computers, printers, fax
machines, telephones, PDAs, scanners, and even video game consoles. A PAN may include
wired and wireless devices. The reach of a PAN typically extends to 10 meters. A
wired PAN is usually constructed with USB and Firewire connections while technologies
such as Bluetooth and infrared communication typically form a wireless PAN.
Local area network
A local area network (LAN) is a network that connects computers and devices in a
limited geographical area such as home, school, computer laboratory, office building,
or closely positioned group of buildings. Each computer or device on the network
is a node. Current wired LANs are most likely to be based on Ethernet technology,
although new standards like ITU-T G.hn also provide a way to create a wired LAN
using existing home wires (coaxial cables, phone lines and power lines).
All interconnected devices must understand the network layer (layer 3), because
they are handling multiple subnets (the different colors). Those inside the library,
which have only 10/100 Mbit/s Ethernet connections to the user device and a Gigabit
Ethernet connection to the central router, could be called "layer 3 switches" because
they only have Ethernet interfaces and must understand IP. It would be more correct
to call them access routers, where the router at the top is a distribution router
that connects to the Internet and academic networks' customer access routers.
The defining characteristics of LANs, in contrast to WANs (Wide Area Networks),
include their higher data transfer rates, smaller geographic range, and no need
for leased telecommunication lines. Current Ethernet or other IEEE 802.3 LAN technologies
operate at speeds up to 10 Gbit/s. This is the data transfer rate. IEEE has projects
investigating the standardization of 40 and 100 Gbit/s. LANs can be connected to
Wide area network by using routers.
Home network
A home network is a residential LAN which is used for communication between digital
devices typically deployed in the home, usually a small number of personal computers
and accessories, such as printers and mobile computing devices. An important function
is the sharing of Internet access, often a broadband service through a cable TV
or Digital Subscriber Line (DSL) provider.
Storage area network
A storage area network (SAN) is a dedicated network that provides access to consolidated,
block level data storage. SANs are primarily used to make storage devices, such
as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so
that the devices appear like locally attached devices to the operating system. A
SAN typically has its own network of storage devices that are generally not accessible
through the local area network by other devices. The cost and complexity of SANs
dropped in the early 2000s to levels allowing wider adoption across both enterprise
and small to medium sized business environments.
Campus network
A campus network is a computer network made up of an interconnection of LANs within
a limited geographical area. The networking equipment (switches, routers) and transmission
media (optical fiber, copper plant, Cat5 cabling etc.) are almost entirely owned
(by the campus tenant / owner: an enterprise, university, government etc.).
In the case of a university campus-based campus network, the network is likely to
link a variety of campus buildings including, for example, academic colleges or
departments, the university library, and student residence halls.
Backbone network
A backbone network is part of a computer network infrastructure that interconnects
various pieces of network, providing a path for the exchange of information between
different LANs or subnetworks. A backbone can tie together diverse networks in the
same building, in different buildings in a campus environment, or over wide areas.
Normally, the backbone's capacity is greater than that of the networks connected
to it.
A large corporation which has many locations may have a backbone network that ties
all of these locations together, for example, if a server cluster needs to be accessed
by different departments of a company which are located at different geographical
locations. The equipment which ties these departments together constitute the network
backbone. Network performance management including network congestion are critical
parameters taken into account when designing a network backbone.
A specific case of a backbone network is the Internet backbone, which is the set
of wide-area network connections and core routers that interconnect all networks
connected to the Internet.
Metropolitan area network
A Metropolitan area network (MAN) is a large computer network that usually spans
a city or a large campus.
Wide area network
A wide area network (WAN) is a computer network that covers a large geographic area
such as a city, country, or spans even intercontinental distances, using a communications
channel that combines many types of media such as telephone lines, cables, and air
waves. A WAN often uses transmission facilities provided by common carriers, such
as telephone companies. WAN technologies generally function at the lower three layers
of the OSI reference model: the physical layer, the data link layer, and the network
layer.
Enterprise private network
An enterprise private network is a network built by an enterprise to interconnect
various company sites, e.g., production sites, head offices, remote offices, shops,
in order to share computer resources.
Virtual private network
A virtual private network (VPN) is a computer network in which some of the links
between nodes are carried by open connections or virtual circuits in some larger
network (e.g., the Internet) instead of by physical wires. The data link layer protocols
of the virtual network are said to be tunneled through the larger network when this
is the case. One common application is secure communications through the public
Internet, but a VPN need not have explicit security features, such as authentication
or content encryption. VPNs, for example, can be used to separate the traffic of
different user communities over an underlying network with strong security features.
VPN may have best-effort performance, or may have a defined service level agreement
(SLA) between the VPN customer and the VPN service provider. Generally, a VPN has
a topology more complex than point-to-point.
Internetwork
An internetwork is the connection of multiple computer networks via a common routing
technology using routers. The Internet is an aggregation of many connected internetworks
spanning the Earth.
Organizational scope
Networks are typically managed by organizations which own them. According to the
owner's point of view, networks are seen as intranets or extranets. A special case
of network is the Internet, which has no single owner but a distinct status when
seen by an organizational entity – that of permitting virtually unlimited global
connectivity for a great multitude of purposes.
Intranets and extranets
Intranets and extranets are parts or extensions of a computer network, usually a
LAN.
An intranet is a set of networks, using the Internet Protocol and IP-based tools
such as web browsers and file transfer applications, that is under the control of
a single administrative entity. That administrative entity closes the intranet to
all but specific, authorized users. Most commonly, an intranet is the internal network
of an organization. A large intranet will typically have at least one web server
to provide users with organizational information.
An extranet is a network that is limited in scope to a single organization or entity
and also has limited connections to the networks of one or more other usually, but
not necessarily, trusted organizations or entities—a company's customers may be
given access to some part of its intranet—while at the same time the customers may
not be considered trusted from a security standpoint. Technically, an extranet may
also be categorized as a CAN, MAN, WAN, or other type of network, although an extranet
cannot consist of a single LAN; it must have at least one connection with an external
network.
Internet
The Internet is a global system of interconnected governmental, academic, corporate,
public, and private computer networks. It is based on the networking technologies
of the Internet Protocol Suite. It is the successor of the Advanced Research Projects
Agency Network (ARPANET) developed by DARPA of the United States Department of Defense.
The Internet is also the communications backbone underlying the World Wide Web (WWW).
Participants in the Internet use a diverse array of methods of several hundred documented,
and often standardized, protocols compatible with the Internet Protocol Suite and
an addressing system (IP addresses) administered by the Internet Assigned Numbers
Authority and address registries. Service providers and large enterprises exchange
information about the reachability of their address spaces through the Border Gateway
Protocol (BGP), forming a redundant worldwide mesh of transmission paths.
Network topology
Common layouts
A network topology is the layout of the interconnections of the nodes of a computer
network. Common layouts are:
• A bus network: all nodes are connected to a common medium along this medium. This
was the layout used in the original Ethernet, called 10BASE5 and 10BASE2.
• A star network: all nodes are connected to a special central node. This is the
typical layout found in in a Wireless LAN, where each wireless client connects to
the central Wireless access point.
• A ring network: each node is connected to its left and right neighbor node, such
that all nodes are connected and that each node can reach each other node by traversing
nodes left- or rightwards. The Fiber Distributed Data Interface (FDDI) made use
of such a topology.
• A mesh network: each node is connected to an arbitrary number of neighbors in
such a way that there is at least one traversal from any node to any other.
• A fully connected network: each node is connected to every other node in the network.
Note that the physical layout of the nodes in a network may not necessarily reflect
the network topology. As an example, with FDDI, the network topology is a ring (actually
two counter-rotating rings), but the physical topology is a star, because all neighboring
connections are routed via a central physical location.
Overlay network
An overlay network is a virtual computer network that is built on top of another
network. Nodes in the overlay are connected by virtual or logical links, each of
which corresponds to a path, perhaps through many physical links, in the underlying
network. The topology of the overlay network may (and often does) differ from that
of the underlying one.
For example, many peer-to-peer networks are overlay networks because they are organized
as nodes of a virtual system of links run on top of the Internet. The Internet was
initially built as an overlay on the telephone network .
The most striking example of an overlay network, however, is the Internet itself:
At the IP layer, each node can reach any other by a direct connection to the desired
IP address, thereby creating a fully connected network; the underlying network,
however, is composed of a mesh-like interconnect of subnetworks of varying topologies
(and, in fact, technologies). Address resolution and routing are the means which
allows the mapping of the fully connected IP overlay network to the underlying ones.
Overlay networks have been around since the invention of networking when computer
systems were connected over telephone lines using modems, before any data network
existed.
Another example of an overlay network is a distributed hash table, which maps keys
to nodes in the network. In this case, the underlying network is an IP network,
and the overlay network is a table (actually a map) indexed by keys.
Overlay networks have also been proposed as a way to improve Internet routing, such
as through quality of service guarantees to achieve higher-quality streaming media.
Previous proposals such as IntServ, DiffServ, and IP Multicast have not seen wide
acceptance largely because they require modification of all routers in the network.[citation
needed] On the other hand, an overlay network can be incrementally deployed on end-hosts
running the overlay protocol software, without cooperation from Internet service
providers. The overlay has no control over how packets are routed in the underlying
network between two overlay nodes, but it can control, for example, the sequence
of overlay nodes a message traverses before reaching its destination.
For example, Akamai Technologies manages an overlay network that provides reliable,
efficient content delivery (a kind of multicast). Academic research includes end
system multicast and overcast for multicast; RON (resilient overlay network) for
resilient routing; and OverQoS for quality of service guarantees, among others.
Basic hardware components
Apart from the physical communications media themselves as described above, networks
comprise additional basic hardware building blocks interconnecting their terminals,
such as network interface cards (NICs), hubs, bridges, switches, and routers.
Network interface cards
A network card, network adapter, or NIC (network interface card) is a piece of computer
hardware designed to allow computers to physically access a networking medium. It
provides a low-level addressing system through the use of MAC addresses.
Each Ethernet network interface has a unique MAC address which is usually stored
in a small memory device on the card, allowing any device to connect to the network
without creating an address conflict. Ethernet MAC addresses are composed of six
octets. Uniqueness is maintained by the IEEE, which manages the Ethernet address
space by assigning 3-octet prefixes to equipment manufacturers. The list of prefixes
is publicly available. Each manufacturer is then obliged to both use only their
assigned prefix(es) and to uniquely set the 3-octet suffix of every Ethernet interface
they produce.
Repeaters and hubs
A repeater is an electronic device that receives a signal, cleans it of unnecessary
noise, regenerates it, and retransmits it at a higher power level, or to the other
side of an obstruction, so that the signal can cover longer distances without degradation.
In most twisted pair Ethernet configurations, repeaters are required for cable that
runs longer than 100 meters. A repeater with multiple ports is known as a hub. Repeaters
work on the Physical Layer of the OSI model. Repeaters require a small amount of
time to regenerate the signal. This can cause a propagation delay which can affect
network communication when there are several repeaters in a row. Many network architectures
limit the number of repeaters that can be used in a row (e.g. Ethernet's 5-4-3 rule).
Today, repeaters and hubs have been made mostly obsolete by switches (see below).
Bridges
A network bridge connects multiple network segments at the data link layer (layer
2) of the OSI model. Bridges broadcast to all ports except the port on which the
broadcast was received. However, bridges do not promiscuously copy traffic to all
ports, as hubs do, but learn which MAC addresses are reachable through specific
ports. Once the bridge associates a port and an address, it will send traffic for
that address to that port only.
Bridges learn the association of ports and addresses by examining the source address
of frames that it sees on various ports. Once a frame arrives through a port, its
source address is stored and the bridge assumes that MAC address is associated with
that port. The first time that a previously unknown destination address is seen,
the bridge will forward the frame to all ports other than the one on which the frame
arrived.
Bridges come in three basic types:
• Local bridges: Directly connect LANs
• Remote bridges: Can be used to create a wide area network (WAN) link between LANs.
Remote bridges, where the connecting link is slower than the end networks, largely
have been replaced with routers.
• Wireless bridges: Can be used to join LANs or connect remote stations to LANs.
Switches
A network switch is a device that forwards and filters OSI layer 2 datagrams (chunks
of data communication) between ports (connected cables) based on the MAC addresses
in the packets. A switch is distinct from a hub in that it only forwards the frames
to the ports involved in the communication rather than all ports connected. A switch
breaks the collision domain but represents itself as a broadcast domain. Switches
make forwarding decisions of frames on the basis of MAC addresses. A switch normally
has numerous ports, facilitating a star topology for devices, and cascading additional
switches.Some switches are capable of routing based on Layer 3 addressing or additional
logical levels; these are called multi-layer switches. The term switch is used loosely
in marketing to encompass devices including routers and bridges, as well as devices
that may distribute traffic on load or by application content (e.g., a Web URL identifier).
Routers
A router is an internetworking device that forwards packets between networks by
processing information found in the datagram or packet (Internet protocol information
from Layer 3 of the OSI Model). In many situations, this information is processed
in conjunction with the routing table (also known as forwarding table). Routers
use routing tables to determine what interface to forward packets (this can include
the "null" also known as the "black hole" interface because data can go into it,
however, no further processing is done for said data).
Firewalls
A firewall is an important aspect of a network with respect to security. It typically
rejects access requests from unsafe sources while allowing actions from recognized
ones. The vital role firewalls play in network security grows in parallel with the
constant increase in 'cyber' attacks for the purpose of stealing/corrupting data,
planting viruses, etc.
Network performance
Network performance refers to the service quality of a telecommunications product
as seen by the customer. It should not be seen merely as an attempt to get "more
through" the network.
The following list gives examples of Network Performance measures for a circuit-switched
network and one type of packet-switched network, viz. ATM: • Circuit-switched networks:
In circuit switched networks, network performance is synonymous with the grade of
service. The number of rejected calls is a measure of how well the network is performing
under heavy traffic loads.Other types of performance measures can include noise,
echo and so on.
• ATM: In an Asynchronous Transfer Mode (ATM) network, performance can be measured
by line rate, quality of service (QoS), data throughput, connect time, stability,
technology, modulation technique and modem enhancements.
There are many different ways to measure the performance of a network, as each network
is different in nature and design. Performance can also be modelled instead of measured;
one example of this is using state transition diagrams to model queuing performance
in a circuit-switched network. These diagrams allow the network planner to analyze
how the network will perform in each state, ensuring that the network will be optimally
designed.
Network security
In the field of networking, the area of network security consists of the provisions
and policies adopted by the network administrator to prevent and monitor unauthorized
access, misuse, modification, or denial of the computer network and network-accessible
resources. Network security is the authorization of access to data in a network,
which is controlled by the network administrator. Users are assigned an ID and password
that allows them access to information and programs within their authority. Network
Security covers a variety of computer networks, both public and private that are
used in everyday jobs conducting transactions and communications among businesses,
government agencies and individuals.
Network resilience
In computer networking: Resilience is the ability to provide and maintain an acceptable
level of service in the face of faults and challenges to normal operation.
Views of networks
Users and network administrators typically have different views of their networks.
Users can share printers and some servers from a workgroup, which usually means
they are in the same geographic location and are on the same LAN, whereas a Network
Administrator is responsible to keep that network up and running. A community of
interest has less of a connection of being in a local area, and should be thought
of as a set of arbitrarily located users who share a set of servers, and possibly
also communicate via peer-to-peer technologies.
Network administrators can see networks from both physical and logical perspectives.
The physical perspective involves geographic locations, physical cabling, and the
network elements (e.g., routers, bridges and application layer gateways) that interconnect
the physical media. Logical networks, called, in the TCP/IP architecture, subnets,
map onto one or more physical media. For example, a common practice in a campus
of buildings is to make a set of LAN cables in each building appear to be a common
subnet, using virtual LAN (VLAN) technology.
Both users and administrators will be aware, to varying extents, of the trust and
scope characteristics of a network. Again using TCP/IP architectural terminology,
an intranet is a community of interest under private administration usually by an
enterprise, and is only accessible by authorized users (e.g. employees).[22] Intranets
do not have to be connected to the Internet, but generally have a limited connection.
An extranet is an extension of an intranet that allows secure communications to
users outside of the intranet (e.g. business partners, customers).
Unofficially, the Internet is the set of users, enterprises, and content providers
that are interconnected by Internet Service Providers (ISP). From an engineering
viewpoint, the Internet is the set of subnets, and aggregates of subnets, which
share the registered IP address space and exchange information about the reachability
of those IP addresses using the Border Gateway Protocol. Typically, the human-readable
names of servers are translated to IP addresses, transparently to users, via the
directory function of the Domain Name System (DNS).
Over the Internet, there can be business-to-business (B2B), business-to-consumer
(B2C) and consumer-to-consumer (C2C) communications. Especially when money or sensitive
information is exchanged, the communications are apt to be secured by some form
of communications security mechanism. Intranets and extranets can be securely superimposed
onto the Internet, without any access by general Internet users and administrators,
using secure Virtual Private Network (VPN) technology.
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