Computer network: Difference between revisions

From Citizendium
Jump to navigation Jump to search
imported>Pat Palmer
imported>Pat Palmer
(deleting article for STARTING OVER, old article is archived off the Talk page)
Line 1: Line 1:
A '''computer network''' is a collection of [[computer]]s or similar digital devices connected by a [[communication]] network. The network allows the computers to cooperate by sharing [[resources]], workload or [[information]] but also provides the possibility of hostile behavior by one or several participants. The communication on the network follows a [[Protocol (computer)|protocol]] understood by all participants which determines how [[data encoding|data is encoded]], how [[medium access control|access to the network]] is handled and how errors are handled and [[acknowledgment schemes|reported]].


The International Standards Organization's [[OSI model|OSI]] (Open Systems Interconnect) model is often used for teaching network [[protocol]] theory to [[computer science]] students.  However, the majority of networks today adhere, not to the OSI standard, but rather to the more light-weight TCP/IP standard.


The participants in a (computer) network are [[computers]], but in network protocol documents, the networked computers are called "devices" or "nodes".  Today, computer networks also contain devices such as intelligent sensors or actuators, which are very small, specialized computers.  Furthermore, the network itself does not always require direct physical connections between participating devices, due to growing availability of [[wireless networking|wireless]] networks.
==References==
 
<references />
A computer network distinguishes itself from a [[power network]] in that it primarily transmits information rather than electricity, and from a [[digital network]] in that the information is transported using a high-level protocol. As evidenced by [[distributed computing]] and techniques like [[AMD]]'s [[Hypertransport]] the distinction between [[parallel computing systems]] and computers connected by a computer network (also called a [[distributed computing system|distributed system]]) is gradual.
 
 
<!-- A note on the organization of the article starting with “applications”, which may appear strange. The idea is to come from applications and their requirements to the protocols as those requirements can then be used to distinguish the protocols. -->
== Applications ==
Computers are connected in a computer network to provide certain [[service]]s to users which would be unavailable or available only with less [[performance]] on a stand-alone machine. Such services include access to [[peripheral]]s such as [[printer]]s or [[sensor]]s connected to other computers, access to resources of the [[remote]] computer itself such as [[computing power]] or [[file storage]], and finally access to the output of applications running on the remote computer such as a [[web server]] or a [[database]]. Since all remote access needs some kind of software running on the remote computer, the boundary between the latter usage and the two former is a bit fuzzy.
 
=== Application architectures ===
The interoperation of computers in a network often follows some general '''[[network architecture]]''' describing roles of and interactions between the participating devices. The most used classes of architectures are:
 
* [[Client-Server architecture]]: One particular computer is denoted as “[[server]]” and provides the services the [[clients]] access. Variations of this architecture provide several [[multi-tier architecture|layers]] of servers or activate [[fall-back]] servers should the master server fail. Examples are the [[World-Wide Web]], [[network file system]] like [[NFS]] or [[SMB]], commercial [[database]] applications such as booking services, as well as [[thin client]] models.
 
* [[Peer-to-Peer architecture]]: All computers provide the same basic services to each other and are thus all denoted “[[peer]]s”. The most prominent example for this architecture are many [[file sharing]] services but also [[VoIP|Voice over IP (VoIP)]] can be considered basically peer-to-peer although current implementations often are mixed set-ups with servers for [[connection]] establishment and bypassing [[firewall]]s.
 
* [[Mobile code architecture]]: Participants do not (only) send data over the network but also software to be run on remote computers for data processing. The most common goal is to avoid network load and thus bring the processing to the data or to the interaction. The most well known example for this architecture within computer science are probably [[mobile software agents|mobile agents]], the most well known example outside of it are Java [[Applet]]s and the [[Web 2.0]].
 
A common name for many of the above systems is “'''[[Distributed System]]'''”, that means a computer system whose constituting computers are locally distributed. Not necessary all computers connected via a network form a distributed system as for example some definitions of the term require [[transparency]] of distribution. <!-- e.g. the definition on http://dict.die.net/distributed%20system/ which I found a bit surprising in that regard -->
 
== Attributes of computer networks ==
Computer networks and the protocols employed on them are judged based on different features they provide to the applications. In general three kinds of such attributes can be distinguished:
 
* [[Network performance|Performance]]: How 'quick' is data transmitted over the network. This is often measured in [[bandwidth]] and [[latency]]. Bandwidth is the amount of data which can be transmitted within a certain time period (typically [[bit]]s/[[second]]). [[Latency]] is the amount of time it takes from transmitting a [[data|datum]] to its arrival at its destination. Depending on whether an application transmits its data in larger or smaller [[chunk]]s, bandwidth and latency have different importance.
 
* [[Reliability]]: How 'undisturbed' is the data transmitted over the network. The main measure for this is the [[bit error rate]] which describes the number of [[bit]]s that arrive [[bit flip|flipped]] at the receiver in relation to the total amount sent. A somewhat more subtle disturbance is measured with “[[jitter]]”. Jitter is the variation in [[latency]] of the network and poses a problem to [[data streaming|streaming]] applications such as [[VoIP]] which require the client to receive data periodically. Note that the effects of more drastic failures such as loss of complete nodes or connections are typically not included in those measures although [[dynamic routing|dynamically routing]] networks can cope with them.
 
* [[Quality of service]]: How 'guaranteed' are the above attributes. Often the measures defined above are given as an average value. But certain, time-critical applications have fixed requirements concerning e.g. the amount of data they need to transmit or the maximum delay allowed. The [[best effort]] approach included in many network protocols is insufficient for this and thus certain protocols are able to provide guarantees which are summarized as Quality of Service (QoS).
 
[[Image:Mesh_Network.png|right|thumb|80px]]
* [[Network topology]], both physical and logical. "Topology" refers to how the network is "laid out." A "bus network" is hooked together like stops on a Public Bus route, one after the other. A "ring network" is literally in the shape of a ring; devices reach each other by passing data around in a ring. There are several different network topologies, and this list is by no means exhaustive.
 
Other attributes not explained here in detail are the [[security]] measures of networks and their protocols and their [[scalability]] with a growing number of participants.
 
== Categorization ==
Computer networks can be classified according to various criteria based on different characteristics.
 
=== Wired versus Wireless ===
The most prominent distinction is between '''[[wired communication|wired networks]]''' and '''[[wireless communication|wireless ones]]'''. The former uses either electrical or [[optical communication|optical]] signals which are transmitted via any kind of cable. The most well known example for this is [[Ethernet]]. The latter typically uses [[electro-magnetic radiation|electro-magnetic radio waves]] transmitted 'through the air' but also schemes using infra-red, sound, or directed laser-light for communication have been proposed. The standard example for a wireless computer network is [[IEEE 802.11]] commonly often called [[Wi-Fi]].
 
As the cable of wired approaches provides a well-confined environment for transmission, wired networks typically perform better in all the attributes mentioned above.  The advantage of wireless networks therefore consists in their much lower dependency on [[infrastructure]] which makes their deployment cheaper and their usage less restricted.
 
The wired versus wireless classification is not restricted to computer networks. As demonstrated by [[fixed-line phone|fixed-line]] and [[mobile phone]]s it exists with any communication network.
 
=== Protocols Employed ===
Computer networks are often named by the protocols they use. '''[[Protocol (computer)|Protocols]]''' define the way individual computers on the network must interact to communicate. This of course needs to be standardized so the participants understand each other. Protocols must handle issues from [[physical layer|how the data is encoded in electrical signals]], over [[data link layer|how transmission errors are found]], [[network layer|how the data finds its 'way' through the network]], and [[transport layer|how an application can determine whom it wants to talk to]] to [[presentation layer|how is the data encrypted against eavesdropping]]. These issues have been divided into layers in the so called '''[[OSI model]]'''. This model defines a stack of communication services where each layer uses the services of the next lower one. Different protocol address different layers in the stack. The [[Internet]] for example is based on various [[physical layer]]s which serve the [[IP|IP layer]] responsible for [[routing]] data through the network, which in turn serves the [[TCP|TCP layer]] that provides the abstraction for the applications, which in turn again use protocols such as [[HTTP]].
 
Among the most often used protocols are low-level protocols such as [[Ethernet]], [[Token-Ring]], [[ATM]], [[CAN]] and [[FlexRay]] in the wired arena and [[IEEE 802.11]], [[Bluetooth]] and [[ZigBee]] for wireless communication, with the latter two also defining higher level services. Higher-level communicaion protocols encompass among others [[IP]], [[TCP]] and [[UDP]] from the TCP/IP world as well as e.g. [[NetBEUI]] or [[iSCSI]]. <!-- what other non-TCP/IP protocols exist? -->  Further up in the area [[application-layer]] protocols an overwhelming multitude exists with [[HTTP]], [[SMTP]], [[IRC]] and [[FTP]] just naming a few.
 
For most of the attributes of the network the lower-level protocols play the major role although some, for example scalability and reliability in the face of partial network failure, build on services provided by the network or transport layer.
 
=== Spanned area ===
There is still another way to classify networks: the area they are designed to cover. This starts small from one person's body and ends with the whole world.
 
* [[BAN]]: The '''B'''ody '''A'''rea '''N'''etwork is a fairly new term sometimes applied to applications of [[nanotechnology]] in [[medicine]] where devices introduced to the human body communicate via wireless links.
 
* [[PAN]]: '''P'''ersonal '''A'''rea '''N'''etworks connect devices at or near to a person with each other. Examples are the connection from [[medical sensor]]s to [[bed-side monitor]]s or the connection from a [[mp3-player]] carried in a pocket to the car radio.
 
* [[LAN]]: '''L'''ocal '''A'''rea '''N'''etworks typically extend throughout a building and consist of one or few [[subnetwork]]s. The primary example here is the connection of computers and [[peripheral device]]s like [[printer]]s or [[PVR]]s in an office or home.
 
* [[MAN]]: '''M'''etropolitan '''A'''rea '''N'''etworks are designed to cover the whole area of a city for example by connecting individual LANs by [[ATM]] or [[microwave radio]] but are nowadays seldom heard of . <!-- At least I  seldom hear of them. Needs confirmation --> The combination of several networking techniques to aggregate and connect citizens via [[DSL]] to the internet could also be seen as a MAN.
 
* [[WAN]]: '''W'''ide '''A'''rea '''N'''etworks connect devices with a significant distance and where transmission often requires several [[hop]]s through [[router]]s. The primary example here is of course the [[Internet]].
 
This list has been extended several times in the past following the progress of technology (mainly in the direction of smaller networks) so expect new types to be introduced once [[nanotechnology]] or [[space exploration]] progresses further. <!-- Actually I remember having read a report on ideas to provide solar-system wide connectivity and its problems (delay) but I can't find it anymore. -->
 
=== Other categories ===
Sometimes the application area of a network is used to coin a phrase for it, especially if the application imposes special requirements on the network. Thus there are [[In-home digitial network|In-home digitial networks (IHDN)]] for transmission of streaming data within the home, [[In-vehicular network|In-vehicular networks (IVN)]] for real-time control of electronic devices within the car and [[Fieldbus]]es for real-time digital communication in industrial applications such as plants.
 
== Challenges ==
As applications and technology progresses, computer networks face new challenges of which a few are listed here.
 
What is called 'the [[spectrum scarcity|scarcity of the spectrum]]' - the problem that the available bandwidth for wireless transmission is limited if transmissions [[interference|interfere]] with each other - has triggered several developments to circumvent this interference. The prominent developments are [[Ultra-wideband|Ultra-wideband (UWB)]] which sends a very low power signal in a large 'band' (=part) of the spectrum and other [[spread-spectrum]] technologies. Also directed transmission and reception using e.g. [[MIMO|multiple antennas]] helps to avoid interference by others.
 
The growing mobility of devices causes problems for protocols not designed with that mobility in mind such as TCP/IP. This has lead to proposals how devices can switch their connection point to the network without changing the identification they are known by (e.g. [[Mobile IP]]). The mobility problem is further worsened with short range radio technology such as [[Bluetooth]] or [[Wi-Fi]] where devices often form networks with their neighbours in an [[ad-hoc networks|ad-hoc]] way or form [[multi-hop network]]s where devices themselves forward data send by their neighbours to its destination.
 
Also [[network security]] becomes a topic again if the network must resist attacks which employ individual devices stolen from the network, a scenario not impossible for [[wireless sensor network]]s.
<!-- And now everyone can add his favourite research topic here -->
 
==Related Topics==
* The [[Internet]] is an example of a global "network of networks"
 
==Subtopics==
* [[Network topology]] describes the different configurations possible for computers participating in a network.


[[Category:CZ Live]]
[[Category:CZ Live]]
[[Category:Computers Workgroup]]
[[Category:Computers Workgroup]]

Revision as of 12:33, 15 May 2007


References

Retrieved from "https://citizendium.org/wiki/index.php?title=Computer_network&oldid=163351"