Computer networks can be classified according to the hardware and
associated software technology that is used to interconnect the
individual devices in the network, such as electrical cable (HomePNA, power line communication, G.hn), optical fiber, and radio waves (wireless LAN). In the OSI model, these are located at levels 1 and 2.
A well-known family of communication media is collectively known as Ethernet. It is defined by IEEE 802 and utilizes various standards and media that enable communication between devices. Wireless LAN technology is designed to connect devices without wiring. These devices use radio waves or infrared signals as a transmission medium.
Communication protocols have themselves various properties, such as whether they are connection-oriented versus connectionless, whether they use circuit mode or packet switching, or whether they use hierarchical or flat addressing.
There exist a multitude of communication protocols, a few of which are described below.
While the role of ATM is diminishing in favor of next-generation networks, it still plays a role in the last mile, which is the connection between an Internet service provider and the home user. For an interesting write-up of the technologies involved, including the deep stacking of communications protocols used, see.[10]
Networks are often classified by their physical or organizational
extent or their purpose. Usage, trust level, and access rights differ
between these types of networks.
Typical library network, in a branching tree topology and controlled access to resources
All interconnected devices must understand the network layer (layer
3), because they are handling multiple subnets (the different colors).
Those inside the library, which have only 10/100 Mbit/s Ethernet
connections to the user device and a Gigabit Ethernet connection to the
central router, could be called "layer 3 switches" because they only
have Ethernet interfaces and must understand IP.
It would be more correct to call them access routers, where the router
at the top is a distribution router that connects to the Internet and
academic networks' customer access routers.
The defining characteristics of LANs, in contrast to WANs (Wide Area Networks), include their higher data transfer rates, smaller geographic range, and no need for leased telecommunication lines. Current Ethernet or other IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s. This is the data transfer rate. IEEE has projects investigating the standardization of 40 and 100 Gbit/s.[13] LANs can be connected to Wide area network by using routers.
A well-known family of communication media is collectively known as Ethernet. It is defined by IEEE 802 and utilizes various standards and media that enable communication between devices. Wireless LAN technology is designed to connect devices without wiring. These devices use radio waves or infrared signals as a transmission medium.
[edit] Wired technologies
- Twisted pair wire is the most widely used medium for telecommunication. Twisted-pair cabling consist of copper wires that are twisted into pairs. Ordinary telephone wires consist of two insulated copper wires twisted into pairs. Computer networking cabling (wired Ethernet as defined by IEEE 802.3) consists of 4 pairs of copper cabling that can be utilized for both voice and data transmission. The use of two wires twisted together helps to reduce crosstalk and electromagnetic induction. The transmission speed ranges from 2 million bits per second to 10 billion bits per second. Twisted pair cabling comes in two forms which are Unshielded Twisted Pair (UTP) and Shielded twisted-pair (STP) which are rated in categories which are manufactured in different increments for various scenarios.
- 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 wrapped with insulating layer typically of a flexible material with a high dielectric constant, all of which are surrounded by a conductive layer. The layers of insulation help minimize interference and distortion. Transmission speed range from 200 million to more than 500 million bits per second.
- ITU-T G.hn technology uses existing home wiring (coaxial cable, phone lines and power lines) to create a high-speed (up to 1 Gigabit/s) local area network.
- Optical fiber cable consists of one or more filaments of glass fiber wrapped in protective layers that carries data by means of pulses of light. It transmits light which can travel over extended distances. Fiber-optic cables are not affected by electromagnetic radiation. Transmission speed may reach trillions of bits per second. The transmission speed of fiber optics is hundreds of times faster than for coaxial cables and thousands of times faster than a twisted-pair wire. This capacity may be further increased by the use of colored light, i.e., light of multiple wavelengths. Instead of carrying one message in a stream of monochromatic light impulses, this technology can carry multiple signals in a single fiber.
[edit] Wireless technologies
- Terrestrial microwave – Terrestrial microwaves use Earth-based transmitter and receiver. The equipment looks similar to satellite dishes. Terrestrial microwaves use low-gigahertz range, which limits all communications to line-of-sight. Path between relay stations spaced approx, 48 km (30 miles) apart. Microwave antennas are usually placed on top of buildings, towers, hills, and mountain peaks.
- Communications satellites – The satellites use microwave radio as their telecommunications medium which are not deflected by the Earth's atmosphere. The satellites are stationed in space, typically 35,400 km (22,200 miles) (for geosynchronous satellites) above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.
- Cellular and PCS systems – Use several radio communications technologies. The systems are divided to different geographic areas. Each area has a low-power transmitter or radio relay antenna device to relay calls from one area to the next area.
- Wireless LANs – Wireless local area network use a high-frequency radio technology similar to digital cellular and a low-frequency radio technology. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. An example of open-standards wireless radio-wave technology is IEEE 802.11.
- Infrared communication can transmit signals between devices within small distances of typically no more than 10 meters. In most cases, line-of-sight propagation is used, which limits the physical positioning of communicating devices.
- A global area network (GAN) is a network used for supporting mobile communications across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is handing off the user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial wireless LANs.[7]
[edit] Exotic technologies
There have been various attempts at transporting data over more or less exotic media:- IP over Avian Carriers was a humorous April fool's Request for Comments, issued as RFC 1149. It was implemented in real life in 2001.[8]
- Extending the Internet to interplanetary dimensions via radio waves.[9]
[edit] Communications protocol
Main article: Communications protocol
A communications protocol defines the formats and rules for
exchanging information via a network and typically comprises a complete protocol suite which describes the protocols used at various usage levels.
An interesting feature of communications protocols is that they may be –
and in fact very often are – stacked above each other, which means that
one is used to carry the other. The example for this is HTTP running over TCP over IP over IEEE 802.11, where the second and third are members of the Internet Protocol Suite, while the last is a member of the Ethernet protocol suite. This is the stacking which exists between the wireless router and the home user's personal computer when surfing the World Wide Web.Communication protocols have themselves various properties, such as whether they are connection-oriented versus connectionless, whether they use circuit mode or packet switching, or whether they use hierarchical or flat addressing.
There exist a multitude of communication protocols, a few of which are described below.
[edit] Ethernet
Main article: Ethernet
Ethernet is a family of connectionless protocols used in LANs, described by a set of standards together called IEEE 802 published by the Institute of Electrical and Electronics Engineers. It has a flat addressing scheme and is mostly situated at levels 1 and 2 of the OSI model. For home users today, the most well-known member of this protocol family is IEEE 802.11, otherwise known as Wireless LAN
(WLAN). However, the complete protocol suite deals with a multitude of
networking aspects not only for home use, but especially when the
technology is deployed to support a diverse range of business needs. MAC bridging (IEEE 802.1D) deals with the routing of Ethernet packets using a Spanning Tree Protocol, IEEE 802.1Q describes VLANs, and IEEE 802.1X defines a port-based Network Access Control
protocol which forms the basis for the authentication mechanisms used
in VLANs, but also found in WLANs – it is what the home user sees when
they have to enter a "wireless access key".[edit] Internet Protocol Suite
Main article: internet protocol suite
The Internet Protocol Suite is used not only in the eponymous Internet, but today nearly ubiquitously in any computer network. While at the Internet protocol (IP) level it operates connectionless, it also offers a connection-oriented service layered on top of IP, the Transmission Control Protocol (TCP). Together, TCP/IP offers a semi-hierarchical addressing scheme (IP address plus port number).[edit] SONET/SDH
Main article: Synchronous optical networking
Synchronous Optical NETworking (SONET) and Synchronous Digital
Hierarchy (SDH) are standardized multiplexing protocols that transfer
multiple digital bit streams over optical fiber using lasers. They were
originally designed to transport circuit mode communications from a
variety of different sources, primarily to support real-time,
uncompressed, circuit-switched voice encoded in PCM
format. However, due to its protocol neutrality and transport-oriented
features, SONET/SDH also was the obvious choice for transporting Asynchronous Transfer Mode (ATM) frames.[edit] Asynchronous Transfer Mode
Main article: Asynchronous transfer mode
Asynchronous Transfer Mode (ATM) is a switching technique for telecommunication networks. It uses asynchronous time-division multiplexing and encodes data into small, fixed-sized cells. This differs from other protocols such as the Internet Protocol Suite or Ethernet that use variable sized packets or frames. ATM has similarity with both circuit and packet
switched networking. This makes it a good choice for a network that
must handle both traditional high-throughput data traffic, and
real-time, low-latency content such as voice and video. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins.While the role of ATM is diminishing in favor of next-generation networks, it still plays a role in the last mile, which is the connection between an Internet service provider and the home user. For an interesting write-up of the technologies involved, including the deep stacking of communications protocols used, see.[10]
[edit] Scale
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[edit] 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.[11] A wired PAN is usually constructed with USB and Firewire connections while technologies such as Bluetooth and infrared communication typically form a wireless PAN.[edit] Local area network
A local area network (LAN) is a network that connects computers and devices in a limited geographical area such as home, school, computer laboratory, office building, or closely positioned group of buildings. Each computer or device on the network is a node. Current wired LANs are most likely to be based on Ethernet technology, although new standards like ITU-T G.hn also provide a way to create a wired LAN using existing home wires (coaxial cables, phone lines and power lines).[12]
The defining characteristics of LANs, in contrast to WANs (Wide Area Networks), include their higher data transfer rates, smaller geographic range, and no need for leased telecommunication lines. Current Ethernet or other IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s. This is the data transfer rate. IEEE has projects investigating the standardization of 40 and 100 Gbit/s.[13] LANs can be connected to Wide area network by using routers.
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