A
computer network or
data network is a
telecommunications network which allows
computers to exchange
data. 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.
[1] 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, the
communications protocols to organize network traffic, the network's size,
topology and organizational intent.
Computer networks support an enormous number of
applications such as access to the
World Wide Web,
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.
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, chat rooms, telephone, video telephone calls, and
video conferencing. Providing access to information on shared storage
devices is an important feature of many networks. 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. 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.
Distributed computing uses computing resources across a network to accomplish tasks. A computer network may be used by
computer crackers 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
Main article:
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 others 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 data in 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
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
- 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:
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 medium there may be, networks comprise additional basic
system building blocks, such as
network interface controller (NICs),
repeaters,
hubs,
bridges,
switches,
routers,
modems, and
firewalls.
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. 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 have been mostly obsoleted by modern
switches; but repeaters are used for long distance links, notably undersea cabling.
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 attacks.
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.
For further information on Communication protocols, the IEEE802 and others click on the link below
https://en.wikipedia.org/wiki/Computer_network
https://www.youtube.com/watch?v=7q-UW9XT4BM&list=PLV7zFBc6xjSA72uKWnyDAGDYyp2BkoRLC&index=4
do well to follow the link...
