Computer Network, LAN, MAN, PAN, WAN, Topologies, Internet History PDF Download
Computer network
A computer
network is a digital telecommunications
network for sharing resources between nodes,
which are computing devices that
use a common telecommunications technology. Data
transmission between nodes is supported over data
links consisting of physical cable
media, such as twisted
pair or fiber-optic
cables, or by wireless
methods, such as Wi-Fi, microwave
transmission, or free-space
optical communication.
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Network nodes
are network computer devices that originate, route and
terminate data communication.[1] They
are generally identified by network
addresses, and can include hosts such
as personal computers, phones,
and servers,
as well as networking hardware such
as routers and switches. Two such devices can be said to be networked when one
device is able to exchange information with the other device, whether or not
they have a direct connection to each other. In most cases,
application-specific communications protocols are layered (i.e.
carried as payload)
over other more general communications protocols.
Computer
networks support many 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. Computer networks may be
classified by many criteria, for example, the transmission medium used
to carry their signals, bandwidth, communications protocols to
organize network traffic, the network's size, topology, traffic control mechanism,
and organizational intent. The best-known computer network is the Internet.
History
The chronology
of significant computer-network developments includes:
·
In the late
1950s, early networks of computers included the U.S. military radar
system Semi-Automatic
Ground Environment (SAGE).
·
In 1959, Christopher Strachey filed
a patent application for time-sharing and John McCarthy initiated
the first project to implement time-sharing of user programs at MIT.[2][3][4][5] Stratchey
passed the concept on to J. C. R. Licklider at
the inaugural UNESCO
Information Processing Conference in Paris
that year.[6] McCarthy
was instrumental in the creation of three of the earliest time-sharing systems
(Compatible
Time-Sharing System in 1961, BBN Time-Sharing
System in 1962, and Dartmouth Time
Sharing System in 1963).
·
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, the OGAS.[7]
·
In 1959,
the MOSFET (MOS
transistor) was invented by Mohamed
Atalla and Dawon
Kahng at Bell
Labs.[8] It
later became the basic building block[9][10][11] of
computer network communications
infrastructure[12][13][14] such
as transceivers, base
station modules, routers, RF power amplifiers,[15] microprocessors, memory
chips and telecommunication
circuits.[16]
·
In 1960, the
commercial airline reservation system semi-automatic
business research environment (SABRE)
went online with two connected mainframes.
·
In 1963, J. C. R. Licklider sent
a memorandum to office colleagues discussing the concept of the "Intergalactic
Computer Network", a computer network
intended to allow general communications among computer users.
·
Throughout the
1960s, Paul Baran and Donald
Davies independently developed the concept of packet
switching to transfer information between computers
over a network. Davies pioneered the implementation of the concept with
the NPL network,
a local area network at the National
Physical Laboratory (United Kingdom) using a
line speed of 768 kbit/s.[17][18][19]
·
In 1965, Western
Electric introduced the first widely used telephone
switch that implemented computer control in the
switching fabric.
·
In 1969, the
first four nodes of the ARPANET were
connected using 50 kbit/s circuits between the University of
California at Los Angeles, the Stanford
Research Institute, the University
of California at Santa Barbara, and the University of Utah.[20] In
the 1970s, Leonard Kleinrock carried
out mathematical work to model the performance of packet-switched networks,
which underpinned the development of the ARPANET.[21][22] His
theoretical work on hierarchical routing in
the late 1970s with student Farouk
Kamoun remains critical to the operation of
the Internet today.
·
In 1972,
commercial services using X.25 were
deployed, and later used as an underlying infrastructure for expanding TCP/IP networks.
·
In 1973, the
French CYCLADES network
was the first to make the hosts responsible
for the reliable delivery of data, rather than this being a centralized service
of the network itself.[23]
·
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"[24] and
collaborated on several patents received in 1977 and 1978.
·
In 1974, the
first TCP (Transmission
Control Protocol) specification, RFC 675 (Specification
of Internet Transmission Control Program), was written by Vint
Cerf, Yogen
Dalal and Carl Sunshine. The paper coined the term
"Internet"
as a shorthand for internetworking.[25]
·
In 1976, John
Murphy of Datapoint Corporation created ARCNET,
a token-passing network first used to share storage devices.
·
1977, first
long-distance fiber network deployed by GTE in Long Beach, California.
·
In 1977, Xerox Network Systems (XNS)
was developed by Robert Metcalfe and Yogen Dalal at Xerox.[26]
·
In 1980,
Ethernet was upgraded from the original 2.94 Mbit/s protocol to the
10 Mbit/s protocol, which was developed by Ron Crane,
Bob Garner, Roy Ogus,[28] and
Yogen Dalal.[29]
·
In 1995, NSFNet
was decommissioned and transitioned to the private sector.
·
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 400 Gbit/s were added (as of 2018).
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.[27]
Properties
Computer
networking may be considered a branch of electrical
engineering, electronics
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, the overwhelming majority of computer
networks carry their data 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. Packets are sent through the network to their
destination. A longer message is packetized before it is transferred and once
the packets arrive, they are then reassembled back into their original message.
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 data in
between.
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. 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
Common
network topologies
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 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
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.
This is still a common topology on the data
link layer, although modern physical
layer variants use point-to-point links
instead.
·
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.
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. Physical layout is not completely
irrelevant, however, as common ducting and equipment locations can represent
single points of failure due to issues like fires, power failures and flooding.
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.[30]
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.[30] 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. 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,[31] resilient
routing and quality of service studies, among others.
Network links
The transmission
media (often referred to in the literature as the physical medium)
used to link devices to form a computer network include electrical
cable, optical
fiber, and free space. In the OSI
model, the software to handle the media are defined at
layers 1 and 2 — the physical layer and the data link layer.
A widely
adopted family that uses copper and fiber media 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. Wireless
LAN standards use radio
waves, others use infrared signals
as a transmission medium. Power line communication uses
a building's power cabling to
transmit data.
Wired technologies
Fiber optic cables are
used to transmit light from one computer/network node to another
The following
classes of wired technologies are used in computer networking.
·
Coaxial
cable is widely used for cable television systems,
office buildings, and other work-sites for local area networks. Transmission
speed ranges from 200 million bits per second to more than 500 million bits per
second.[citation needed]
·
ITU-T G.hn technology
uses existing home wiring (coaxial cable,
phone lines and power lines)
to create a high-speed local area network.
·
Twisted
pair cabling is used for wired Ethernet and
other standards. It typically 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 Mbit/s
to 10 Gbit/s. 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 via lasers and optical
amplifiers. Some advantages of optical fibers over metal
wires are very low transmission loss and immunity to electrical interference.
Using dense wave division
multiplexing, optical fibers can simultaneously carry multiple
streams of data on different wavelengths of light, which greatly increases the
rate that data can be sent to 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. There are two basic types of fiber optics, single-mode
optical fiber (SMF) and multi-mode
optical fiber (MMF). Single-mode fiber has the advantage of
being able to sustain a coherent signal for dozens or even a hundred
kilometers. Multimode fiber is cheaper to terminate but is limited to a few
hundred or even only a few dozens of meters, depending on the data rate and
cable grade.[32]
Wireless technologies
Computers
are very often connected to networks using wireless links
Network
connections can be established wirelessly using radio or other electromagnetic
means of communication.
·
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 40 miles (64 km) apart.
·
Communications
satellites – Satellites communicate
also communicate via microwave. 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
networks use several radio communications
technologies. The systems divide the region covered into multiple geographic
areas. Each area is served by a low-power transceiver.
·
Radio and spread
spectrum technologies – Wireless
LANs use a high-frequency radio technology similar
to digital cellular. 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 Wi-Fi.
·
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.[33]
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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, networks are built from 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 so may 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
connects the computer to the network
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.
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.
Repeaters work
on the physical layer of the OSI model but still 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 used in a network, e.g., the Ethernet 5-4-3
rule.
An Ethernet
repeater with multiple ports is known as an Ethernet
hub. Hubs and repeaters in LANs have been largely obsoleted
by modern network switchs.
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.[35] 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.[36] 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.
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
attacks.
Communication protocols
The
TCP/IP model or Internet layering scheme and its relation to common protocols
often layered on top of it.
Message
flows (A-B) in the presence of a router (R), red flows are effective
communication paths, black paths are across the actual network links.
A communication
protocol is a set of rules for exchanging information
over a network. In a protocol
stack (also see the OSI
model), each protocol leverages the services of the
protocol layer below it, until the lowest layer controls the hardware that
sends information across the media. The use of protocol layering is today
ubiquitous across the field of computer networking. An important example of a
protocol stack is HTTP (the World
Wide Web protocol) running over TCP over IP (the Internet
protocols) over IEEE
802.11 (the Wi-Fi protocol). This stack is used
between the wireless router and
the home user's personal computer when the user is surfing the web.
Communication
protocols have various characteristics. They may be connection-oriented or connectionless,
they may use circuit mode or packet
switching, and they may use hierarchical addressing or flat
addressing.
There are many
communication protocols, a few of which are described below.
IEEE 802
IEEE
802 is a family of IEEE standards dealing with local
area networks and metropolitan area networks. The complete IEEE 802 protocol
suite provides a diverse set of networking capabilities. The protocols have a
flat addressing scheme. They operate mostly at levels 1 and 2 of the OSI
model.
For
example, 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 it is also found in WLANs) –
it is what the home user sees when the user has to enter a "wireless
access key".
Ethernet
Ethernet,
sometimes simply called LAN, is a family of protocols used in wired
LANs, described by a set of standards together called IEEE
802.3 published by the Institute
of Electrical and Electronics Engineers.
Wireless LAN
Wireless
LAN, also widely known as WLAN or WiFi, is probably
the most well-known member of the IEEE
802 protocol family for home users today. It is
standardized by IEEE 802.11 and
shares many properties with wired Ethernet.
Internet Protocol Suite
The Internet
Protocol Suite, also called TCP/IP, is the
foundation of all modern networking. 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 specifications for Internet
Protocol Version 4 (IPv4) and for IPv6, the
next generation of the protocol with a much enlarged addressing capability.
SONET/SDH
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 (Pulse-Code
Modulation) 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
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.[37]
Cellular standards
There are a number
of different digital cellular standards, including: Global System
for Mobile Communications (GSM), General Packet
Radio Service (GPRS), cdmaOne, CDMA2000, Evolution-Data
Optimized (EV-DO), Enhanced Data
Rates for GSM Evolution (EDGE), Universal
Mobile Telecommunications System (UMTS), Digital
Enhanced Cordless Telecommunications (DECT), Digital
AMPS (IS-136/TDMA), and Integrated
Digital Enhanced Network (iDEN).[38]
Geographic scale
Computer
network types
by spatial scope |
A network can be
characterized by its physical capacity or its organizational purpose. Use of
the network, including user authorization and access rights, differ
accordingly.
Nanoscale
network
A nanoscale
communication network has key components implemented at the
nanoscale including message carriers and leverages physical principles that
differ from macroscale communication mechanisms. Nanoscale communication
extends communication to very small sensors and actuators such as those found
in biological systems and also tends to operate in environments that would be
too harsh for classical communication.[39]
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.[40] 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 a home, school, office building, or closely positioned group of
buildings. Each computer or device on the network is a node.
Wired LANs are most likely based on Ethernet technology.
Newer standards such as ITU-T G.hn also
provide a way to create a wired LAN using existing wiring, such as coaxial
cables, telephone lines, and power lines.[41]
The defining
characteristics of a LAN, in contrast to a wide
area network (WAN), include higher data transfer rates,
limited geographic range, and lack of reliance on leased
lines to provide connectivity. Current Ethernet or
other IEEE 802.3 LAN
technologies operate at data transfer rates up to 100 Gbit/s,
standardized by IEEE in 2010.[42] Currently, 400 Gbit/s
Ethernet is being developed.
Home
area network
A home
area network (HAN) is a residential LAN 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
area network
A campus area network (CAN)
is 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.).
For example, a
university campus network is likely to link a variety of campus buildings to
connect academic colleges or departments, the library, and student residence
halls.
Backbone
network
A backbone
network is part of a computer network infrastructure
that provides a path for the exchange of information between different LANs or
subnetworks. A backbone can tie together diverse networks within the same
building, across different buildings, or over a wide area.
For example, a
large company might implement a backbone network to connect departments that
are located around the world. The equipment that ties together the departmental
networks constitutes the network backbone. When designing a network
backbone, network
performance and network congestion are
critical factors to take into account. Normally, the backbone network's
capacity is greater than that of the individual networks connected to it.
Another example
of a backbone network is the Internet
backbone, which is a massive, global system of fiber-optic
cable and optical networking that carry the bulk of data between wide
area networks (WANs), metro, regional, national and
transoceanic networks.
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. A WAN uses a communications channel that combines many types of
media such as telephone lines, cables, and air waves. A WAN often makes use of
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 that a
single organization builds to interconnect its office locations (e.g.,
production sites, head offices, remote offices, shops) so they can share
computer resources.
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Virtual
private network
A virtual private
network (VPN) is an overlay 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.
Global
area network
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 user communications from one local coverage area to the next. In
IEEE Project 802, this involves a succession of terrestrial wireless
LANs.[43]
Organizational scope
Networks are
typically managed by the organizations that own them. Private enterprise
networks may use a combination of intranets and extranets. They may also
provide network access to the Internet,
which has no single owner and permits virtually unlimited global connectivity.
Intranet[
An intranet is
a set of networks that are under the control of a single administrative entity.
The intranet uses the IP protocol
and IP-based tools such as web browsers and file transfer applications. The
administrative entity limits use of the intranet to its authorized users. Most
commonly, an intranet is the internal LAN of an organization. A large intranet
typically has at least one web server to provide users with organizational
information. An intranet is also anything behind the router on a local area
network.
Extranet
An extranet is
a network that is also under the administrative control of a single
organization, but supports a limited connection to a specific external network.
For example, an organization may provide access to some aspects of its intranet
to share data with its business partners or customers. These other entities are
not necessarily trusted from a security standpoint. Network connection to an
extranet is often, but not always, implemented via WAN technology.
An internetwork is
the connection of multiple computer networks via a common routing technology
using routers.
Internet
Partial
map of the Internet, based on the January 15, 2005 data found on opte.org.
Each line is drawn between two nodes, representing two IP
addresses. The length of the lines are indicative of the
delay between those two nodes. This graph represents less than 30% of the Class
C networks reachable.
The Internet is
the largest example of an internetwork. It 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
utilizes copper communications and the optical networking backbone
to enable the World Wide Web (WWW),
the Internet of Things,
video transfer and a broad range of information services.
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.
Darknet
A darknet is
an overlay network, typically running on the Internet, that is only accessible through
specialized software. A darknet is an anonymizing network where connections are
made only between trusted peers — sometimes called "friends" (F2F)[44] —
using non-standard protocols and ports.
Darknets are
distinct from other distributed peer-to-peer networks
as sharing is
anonymous (that is, IP
addresses are not publicly shared), and therefore users
can communicate with little fear of governmental or corporate interference.[45]
Routing
Routing
calculates good paths through a network for information to take. For example,
from node 1 to node 6 the best routes are likely to be 1-8-7-6 or 1-8-10-6, as
this has the thickest routes.
Routing is
the process of selecting network paths to carry network traffic. Routing is
performed for many kinds of networks, including circuit
switching networks and packet switched
networks.
In
packet-switched networks, routing directs packet
forwarding (the transit of logically addressed network
packets from their source toward their ultimate
destination) through intermediate nodes.
Intermediate nodes are typically network hardware devices such as routers, bridges, gateways, firewalls,
or switches.
General-purpose computers can also
forward packets and perform routing, though they are not specialized hardware
and may suffer from limited performance. The routing process usually directs
forwarding on the basis of routing
tables, which maintain a record of the routes to various
network destinations. Thus, constructing routing tables, which are held in the
router's memory,
is very important for efficient routing.
There are
usually multiple routes that can be taken, and to choose between them,
different elements can be considered to decide which routes get installed into
the routing table, such as (sorted by priority):
1. Prefix-Length:
where longer subnet masks are preferred (independent if it is within a routing
protocol or over different routing protocol)
2. Metric:
where a lower metric/cost is preferred (only valid within one and the same
routing protocol)
3. Administrative
distance: where a lower distance is preferred (only valid
between different routing protocols)
Most routing
algorithms use only one network path at a time. Multipath
routing techniques enable the use of multiple
alternative paths.
Routing, in a
more narrow sense of the term, is often contrasted with bridging in
its assumption that network
addresses are structured and that similar addresses
imply proximity within the network. Structured addresses allow a single routing
table entry to represent the route to a group of devices. In large networks,
structured addressing (routing, in the narrow sense) outperforms unstructured
addressing (bridging). Routing has become the dominant form of addressing on
the Internet. Bridging is still widely used within localized environments.
Network service
Network
services are applications hosted by servers on
a computer network, to provide some
functionality for members or users of the network, or to
help the network itself to operate.
The World
Wide Web, E-mail,[46] printing and network file
sharing are examples of well-known network services.
Network services such as DNS (Domain Name System)
give names for IP and MAC
addresses (people remember names like “nm.lan” better
than numbers like “210.121.67.18”),[47] and DHCP to
ensure that the equipment on the network has a valid IP address.[48]
Services are
usually based on a service protocol that
defines the format and sequencing of messages between clients and servers of
that network service.
Network performance
Bandwidth
Bandwidth in
bit/s may refer to consumed bandwidth, corresponding to achieved throughput or goodput,
i.e., the average rate of successful data transfer through a communication
path. The throughput is affected by technologies such as bandwidth
shaping, bandwidth management, bandwidth throttling, bandwidth
cap, bandwidth allocation (for
example bandwidth
allocation protocol and dynamic
bandwidth allocation), etc. A bit
stream's bandwidth is proportional to the average consumed signal bandwidth in
hertz (the average spectral bandwidth of the analog signal representing the bit
stream) during a studied time interval.
Network delay
Any data sent
across a network requires time to travel from source to destination. Depending
on the application, the one-way delay or the round-trip
time can have a significant impact on performance.
Quality of service
Depending on the
installation requirements, network performance is
usually measured by the quality of service of
a telecommunications product. The parameters that affect this typically can
include throughput, jitter, bit
error rate and latency.
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.[49] Other
types of performance measures can include the level of noise and echo.
·
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
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. For
example, state transition diagrams are
often used to model queuing performance in a circuit-switched network. The
network planner uses these diagrams to analyze how the network performs in each
state, ensuring that the network is optimally designed.[51]
Network congestion
Network congestion occurs
when a link or node is subjected to a greater data load than it is rated for,
resulting in a deterioration of its quality of service.
Typical effects include queueing
delay, packet
loss or the blocking of
new connections. A consequence of these latter two is that incremental
increases in offered load lead
either to only a small increase in network throughput,
or to a reduction in network throughput.
Network
protocols that use aggressive retransmissions to
compensate for packet loss tend to keep systems in a state of network
congestion—even after the initial load is reduced to a level that would not
normally induce network congestion. Thus, networks using these protocols can
exhibit two stable states under the same level of load. The stable state with
low throughput is known as congestive collapse.
Modern networks
use congestion control, congestion avoidance and traffic control techniques
to try to avoid congestion collapse. These include: exponential backoff in
protocols such as 802.11's CSMA/CA and
the original Ethernet, window reduction
in TCP,
and fair queueing in
devices such as routers.
Another method to avoid the negative effects of network congestion is
implementing priority schemes, so that some packets are transmitted with higher
priority than others. Priority schemes do not solve network congestion by
themselves, but they help to alleviate the effects of congestion for some services.
An example of this is 802.1p. A third method
to avoid network congestion is the explicit allocation of network resources to
specific flows. One example of this is the use of Contention-Free Transmission
Opportunities (CFTXOPs) in the ITU-T G.hn standard,
which provides high-speed (up to 1 Gbit/s) Local area networking over
existing home wires (power lines, phone lines and coaxial cables).
Network resilience
Network resilience is
"the ability to provide and maintain an acceptable level of service in
the face of faults and
challenges to normal operation.”[52]
Security
Network security
Network
security consists of provisions and policies adopted
by the network administrator to
prevent and monitor unauthorized access,
misuse, modification, or denial of the computer network and its
network-accessible resources.[53] 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 is used on a variety of computer networks, both public and
private, to secure daily transactions and communications among businesses,
government agencies and individuals.
Network surveillance
Network surveillance is
the monitoring of data being transferred over computer networks such as
the Internet. The monitoring
is often done surreptitiously and may be done by or at the behest of
governments, by corporations, criminal organizations, or individuals. It may or
may not be legal and may or may not require authorization from a court or other
independent agency.
Computer and
network surveillance programs are widespread today, and almost all Internet
traffic is or could potentially be monitored for clues to illegal activity.
Surveillance is
very useful to governments and law
enforcement to maintain social
control, recognize and monitor threats, and
prevent/investigate criminal activity.
With the advent of programs such as the Total
Information Awareness program,
technologies such as high-speed
surveillance computers and biometrics software,
and laws such as the Communications
Assistance For Law Enforcement Act, governments
now possess an unprecedented ability to monitor the activities of citizens.[54]
However,
many civil rights and privacy groups—such
as Reporters
Without Borders, the Electronic
Frontier Foundation, and the American Civil
Liberties Union—have expressed concern that
increasing surveillance of citizens may lead to a mass
surveillance society, with limited political and personal
freedoms. Fears such as this have led to numerous lawsuits such as Hepting
v. AT&T.[54][55] The hacktivist group Anonymous has
hacked into government websites in protest of what it considers "draconian
surveillance".[56][57]
End to end encryption
End-to-end encryption (E2EE)
is a digital
communications paradigm of
uninterrupted protection of data traveling between two communicating parties.
It involves the originating party encrypting data
so only the intended recipient can decrypt it, with no dependency on third
parties. End-to-end encryption prevents intermediaries, such as Internet providers or application
service providers, from discovering or
tampering with communications. End-to-end encryption generally protects
both confidentiality and integrity.
Examples of
end-to-end encryption include HTTPS for
web traffic, PGP for email, OTR for instant
messaging, ZRTP for telephony,
and TETRA for
radio.
Typical server-based
communications systems do not include end-to-end encryption. These systems can
only guarantee protection of communications between clients and servers,
not between the communicating parties themselves. Examples of non-E2EE systems
are Google Talk, Yahoo
Messenger, Facebook,
and Dropbox.
Some such systems, for example LavaBit and SecretInk, have even described
themselves as offering "end-to-end" encryption when they do not. Some
systems that normally offer end-to-end encryption have turned out to contain
a back door that
subverts negotiation of the encryption
key between the communicating parties, for
example Skype or Hushmail.
The end-to-end
encryption paradigm does not directly address risks at the communications
endpoints themselves, such as the technical
exploitation of clients,
poor quality random number
generators, or key
escrow. E2EE also does not address traffic
analysis, which relates to things such as the identities of
the endpoints and the times and quantities of messages that are sent.
SSL/TLS
The introduction
and rapid growth of e-commerce on the World Wide Web in the mid-1990s made it
obvious that some form of authentication and encryption was needed. Netscape took
the first shot at a new standard. At the time, the dominant web browser
was Netscape Navigator.
Netscape created a standard called secure socket layer (SSL). SSL requires a
server with a certificate. When a client requests access to an SSL-secured
server, the server sends a copy of the certificate to the client. The SSL
client checks this certificate (all web browsers come with an exhaustive list
of CA root certificates preloaded), and if the certificate checks out, the
server is authenticated and the client negotiates a symmetric-key cipher for
use in the session. The session is now in a very secure encrypted tunnel
between the SSL server and the SSL client.[32]
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 via the
transmission media. Logical networks, called, in the TCP/IP architecture, subnets,
map onto one or more transmission 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 are 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). 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, that 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).
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Over the
Internet, there can be business-to-business (B2B), business-to-consumer (B2C) and consumer-to-consumer
(C2C) communications. When money or sensitive
information is exchanged, the communications are apt to be protected 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.
