A computer network or data network is a telecommunications network which allows nodes to share resources. In computer networks, networked computing devices exchange data with each other using a data link. The connections between nodes are established using either cable media or wireless media. The best-known computer network is the Internet.
Network computer devices that originate, route and terminate the data are called network nodes. Nodes can include hosts such as personal computers, phones, servers as well as networking hardware. Two such devices can be said to be networked together when one device is able to exchange information with the other device, whether or not they have a direct connection to each other.
Computer networks differ in the transmission medium used to carry their signals, communications protocols to organize network traffic, the network's size, topology and organizational intent.
Computer networks support an enormous number of applications and services such as access to the World Wide Web, digital video, digital audio, shared use of application and storage servers, printers, and fax machines, and use of email and instant messaging applications as well as many others. In most cases, application-specific communications protocols are layered (i.e. carried as payload) over other more general communications protocols.
HISTORY
The chronology of significant computer-network developments includes:
- In the late 1950s, early networks of computers included the military radar system Semi-Automatic Ground Environment (SAGE).
- In 1959, Anatolii Ivanovich Kitov proposed to the Central Committee of the Communist Party of the Soviet Union a detailed plan for the re-organisation of the control of the Soviet armed forces and of the Soviet economy on the basis of a network of computing centres.
- In 1960, the commercial airline reservation system semi-automatic business research environment (SABRE) went online with two connected mainframes.
- In 1962, J.C.R. Licklider developed a working group he called the "Intergalactic Computer Network", a precursor to the ARPANET, at the Advanced Research Projects Agency (ARPA).
- In 1964, researchers at Dartmouth College 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 developed network systems that used packets to transfer information between computers over a network.
- In 1965, Thomas Marill and Lawrence G. Roberts created the first wide area network (WAN). This was an immediate precursor to the ARPANET, of which Roberts became program manager.
- Also in 1965, Western Electric introduced the first widely used telephone switch that implemented true computer control.
- In 1969, the University of California at Los Angeles, the Stanford Research Institute, the University of California at Santa Barbara, and the University of Utah became connected as the beginning of the ARPANET network using 50 kbit/s circuits.
- In 1972, commercial services using X.25 were deployed, and later used as an underlying infrastructure for expanding TCP/IP networks.
- In 1973, Robert Metcalfe wrote a formal memo at Xerox PARC describing Ethernet, a networking system that was based on the Aloha network, developed in the 1960s by Norman Abramson and colleagues at the University of Hawaii. In July 1976, Robert Metcalfe and David Boggs published their paper "Ethernet: Distributed Packet Switching for Local Computer Networks"[4] and collaborated on several patents received in 1977 and 1978. In 1979, Robert Metcalfe pursued making Ethernet an open standard.[5]
- In 1976, John Murphy of Datapoint Corporation created ARCNET, a token-passing network first used to share storage devices.
- In 1995, the transmission speed capacity for Ethernet increased from 10 Mbit/s to 100 Mbit/s. By 1998, Ethernet supported transmission speeds of a Gigabit. Subsequently, higher speeds of up to 100 Gbit/s were added (as of 2016). The ability of Ethernet to scale easily (such as quickly adapting to support new fiber optic cable speeds) is a contributing factor to its continued use.
Properties
Computer networking may be considered a
branch of electrical engineering, telecommunications, computer science, information technology or computer engineering, since it relies upon the
theoretical and practical application of the related disciplines.
A computer network facilitates
interpersonal communications allowing users to communicate efficiently and easily
via various means: email, instant messaging, online chat,
telephone, video telephone calls, and video conferencing. A network allows
sharing of network and computing resources. Users may access and use resources
provided by devices on the network, such as printing a document on a shared
network printer or use of a shared storage device. A network allows sharing of
files, data, and other types of information giving authorized users the ability
to access information stored on other computers on the network. Distributed computing uses computing
resources across a network to accomplish tasks.
A computer network may be used by security hackers to
deploy computer viruses or computer worms on
devices connected to the network, or to prevent these devices from accessing
the network via a denial-of-service attack.
Network packet
Computer communication links that do not
support packets, such as traditional point-to-point telecommunication
links, simply transmit data as a bit stream. However, most information in computer networks is
carried in packets. A network packet is a formatted unit of data (a
list of bits or bytes, usually a few tens of bytes to a few kilobytes long)
carried by a packet-switched network.
In packet networks, the data is
formatted into packets that are sent through the network to their destination.
Once the packets arrive they are reassembled into their original message. With
packets, the bandwidth of
the transmission medium can be better shared among users than if the network
were circuit switched. When one
user is not sending packets, the link can be filled with packets from other
users, and so the cost can be shared, with relatively little interference,
provided the link isn't overused.
Packets consist of two kinds of data:
control information, and user data (payload). The control information provides
data the network needs to deliver the user data, for example: source and
destination network addresses, error detection codes, and sequencing information.
Typically, control information is found in packet headers and trailers, with payload datain between.
Often the route a packet needs to take
through a network is not immediately available. In that case the packet
is queued and waits
until a link is free.
Network
topology
The physical layout of a network is usually less important than
the topology that connects network nodes. Most diagrams that describe a
physical network are therefore topological, rather than geographic. The symbols
on these diagrams usually denote network links and network nodes.
Network links
The transmission media (often referred to in the literature as
the physical media) used to link devices to form a computer network
include electrical cable (Ethernet, HomePNA, power line communication, G.hn), optical fiber (fiber-optic communication), and radio waves (wireless networking). In the OSI model,
these are defined at layers 1 and 2 — the physical layer and the data link
layer.
A
widely adopted family of transmission media used in local area
network (LAN)
technology is collectively known as Ethernet.
The media and protocol standards that enable communication between networked
devices over Ethernet are defined by IEEE 802.3.
Ethernet transmits data over both copper and fiber cables. Wireless LAN
standards (e.g. those defined by IEEE 802.11)
use radio waves,
or others use infrared signals
as a transmission medium. Power line communication uses a building's
power cabling to transmit data.
Wired technologies
are used to
transmit light from one computer/network node to another
The
orders of the following wired technologies are, roughly, from slowest to
fastest transmission speed.
·
Coaxial cable is
widely used for cable television systems, office buildings, and other
work-sites for local area networks. The cables consist of copper or aluminum
wire surrounded by an insulating layer (typically a flexible material with a
high dielectric constant), which itself is surrounded by a conductive layer.
The insulation helps minimize interference and distortion. Transmission speed
ranges from 200 million bits per second 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
·
Twisted pair wire is
the most widely used medium for all 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 network
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: unshielded twisted pair (UTP)
and shielded twisted-pair (STP). Each form comes in several category ratings,
designed for use in various scenarios.
2007
map showing submarine optical fiber telecommunication cables around the world.
·
An optical fiber is
a glass fiber. It carries pulses of light that represent data. Some advantages
of optical fibers over metal wires are very low transmission loss and immunity
from electrical interference. Optical fibers can simultaneously carry multiple
wavelengths of light, which greatly increases the rate that data can be sent,
and helps enable data rates of up to trillions of bits per second. Optic fibers
can be used for long runs of cable carrying very high data rates, and are used
for undersea cables to interconnect
continents.
Price
is a main factor distinguishing wired- and wireless-technology options in a
business. Wireless options command a price premium that can make purchasing wired
computers, printers and other devices a financial benefit. Before making the
decision to purchase hard-wired technology products, a review of the
restrictions and limitations of the selections is necessary. Business and
employee needs may override any cost considerations.[6]
Wireless technologies
Computers
are very often connected to networks using wireless links
Main
article: Wireless network
·
Terrestrial microwave –
Terrestrial microwave communication uses Earth-based transmitters and receivers
resembling satellite dishes. Terrestrial microwaves are in the low gigahertz
range, which limits all communications to line-of-sight. Relay stations are
spaced approximately 48 km (30 mi) apart.
·
Communications satellites –
Satellites communicate via microwave radio waves, which are not deflected by
the Earth's atmosphere. The satellites are stationed in space, typically in
geosynchronous orbit 35,400 km (22,000 mi) 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 divide the region covered into multiple geographic
areas. Each area has a low-power transmitter or radio relay antenna device to
relay calls from one area to the next area.
·
Radio and spread spectrum technologies –
Wireless local area networks 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. IEEE 802.11 defines a common flavor of open-standards
wireless radio-wave technology known as Wifi.
·
Free-space optical communication uses
visible or invisible light for communications. In most cases, line-of-sight propagation is used,
which limits the physical positioning of communicating devices.
Exotic technologies
There
have been various attempts at transporting data over 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.[7]
·
Extending the Internet to
interplanetary dimensions via radio waves, the Interplanetary Internet.[8]
Both
cases have a large round-trip delay time, which gives slow two-way
communication, but doesn't prevent sending large amounts of information.
Network nodes
Apart
from any physical transmission media there may be, networks comprise additional
basic system building blocks, such as network interface controllers (NICs), repeaters, hubs, bridges, switches, routers, modems, and firewalls. Any particular piece of equipment
will frequently contain multiple building blocks and perform multiple
functions.
Network interfaces
An ATM network interface in the form of
an accessory card. A lot of network interfaces are built-in.
A network interface controller (NIC)
is computer hardware that provides a computer
with the ability to access the transmission media, and has the ability to
process low-level network information. For example, the NIC may have a
connector for accepting a cable, or an aerial for wireless transmission and
reception, and the associated circuitry.
The
NIC responds to traffic addressed to a network address for
either the NIC or the computer as a whole.
In Ethernet networks,
each network interface controller has a unique Media Access
Control (MAC) address—usually stored in the controller's
permanent memory. To avoid address conflicts between network devices, the Institute of
Electrical and Electronics Engineers (IEEE) maintains and
administers MAC address uniqueness. The size of an Ethernet MAC address is
six octets. The three most significant octets are
reserved to identify NIC manufacturers. These manufacturers, using only their
assigned prefixes, uniquely assign the three least-significant octets of every
Ethernet interface they produce.
Repeaters and hubs
A repeater is
an electronic device
that receives a network signal, cleans it of unnecessary noise and
regenerates it. The signal is retransmitted 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. With fiber optics, repeaters can be tens or even hundreds of kilometers
apart.
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 that affects network performance and may affect proper
function. As a result, many network architectures limit the number of repeaters
that can be used in a row, e.g., the Ethernet 5-4-3 rule.
Hubs
and repeaters in LANs have been mostly obsoleted by modern switches.
Bridges
A network bridge connects
and filters traffic between two network segments at
the data link layer (layer 2) of the OSI model to
form a single network. This breaks the network's collision domain but maintains
a unified broadcast domain. Network segmentation breaks down a large, congested
network into an aggregation of smaller, more efficient networks.
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 devices to LANs.
Switches
A network switch is
a device that forwards and filters OSI layer 2 datagrams (frames) between ports based on the destination MAC
address in each frame.[9] A
switch is distinct from a hub in that it only forwards the frames to the
physical ports involved in the communication rather than all ports connected.
It can be thought of as a multi-port bridge.[10] It
learns to associate physical ports to MAC addresses by examining the source
addresses of received frames. If an unknown destination is targeted, the switch
broadcasts to all ports but the source. Switches normally have numerous ports,
facilitating a star topology for devices, and cascading additional switches.
Multi-layer switches are capable of
routing based on layer 3 addressing or additional logical levels. The
term switch is often used loosely to include devices such as
routers and bridges, as well as devices that may distribute traffic based on
load or based on application content (e.g., a Web URL identifier).
Routers
A
typical home or small office router showing the ADSL telephone line
and Ethernet network
cable connections
A router is an internetworking device
that forwards packets between networks by
processing the routing information included in the packet or datagram (Internet
protocol information from layer 3). The routing information is often processed
in conjunction with the routing table (or forwarding table). A router uses its
routing table to determine where to forward packets. A destination in a routing
table can include a "null" interface, also known as the "black
hole" interface because data can go into it, however, no further
processing is done for said data, i.e. the packets are dropped.
Modems
Modems (MOdulator-DEModulator)
are used to connect network nodes via wire not originally designed for digital
network traffic, or for wireless. To do this one or more carrier signals are modulated by
the digital signal to produce an analog signal that
can be tailored to give the required properties for transmission. Modems are
commonly used for telephone lines, using a Digital Subscriber Line technology.
Firewalls
A firewall is a network device for
controlling network security and access rules. Firewalls are typically
configured to reject access requests from unrecognized sources while allowing
actions from recognized ones. The vital role firewalls play in network security
grows in parallel with the constant increase in cyber attackss.
Network structure
Network topology is
the layout or organizational hierarchy of interconnected nodes of a computer
network. Different network topologies can affect throughput, but reliability is
often more critical. With many technologies, such as bus networks, a single
failure can cause the network to fail entirely. In general the more
interconnections there are, the more robust the network is; but the more
expensive it is to install.
Common layouts
Common
network topologies
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 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 neighbour 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 neighbours 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.
·
A tree network:
nodes are arranged hierarchically.
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
often a star, because all neighboring connections can be routed via a central
physical location.
Overlay network
A
sample overlay network
An overlay network is
a virtual computer network that is built on top of another network. Nodes in
the overlay network are connected by virtual or logical links. Each link
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. They are organized as nodes of a virtual system of links
that run on top of the Internet.[11]
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.
The
most striking example of an overlay network is the Internet itself. The
Internet itself was initially built as an overlay on the telephone
network.[11] Even
today, each Internet node can communicate with virtually any other through an
underlying mesh of sub-networks of wildly different topologies and
technologies. Address resolution and routing are
the means that allow mapping of a fully connected IP overlay network to its
underlying network.
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
network 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 that a message traverses before it reaches 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,[12] resilient
routing and quality of service studies, among others.
