Cisco 1800 Router
Cisco 1800 Router
A router is a device that determines the proper path for data to travel between different networks, and forwards data packets to the next device along this path.[1] They connect networks together; a LAN to a WAN for example, to access the Internet. Some units, like the Cisco 1800 (pictured), are available in both wired and wireless models.
Function
Nortel ERS 8600
Nortel ERS 8600
A more precise definition of a router is a computer networking device that interconnects separate logical subnets. Routers are now available in many types, though all are fundamentally doing the same job. A router is a computer whose software and hardware are usually tailored to the tasks of routing and forwarding, generally containing a specialized operating system (e.g. Cisco's IOS or Juniper Networks JunOS or Extreme Networks XOS), RAM, NVRAM, flash memory, and one or more processors. High-end routers contain many processors and specialized Application-specific integrated circuits (ASIC) and do a great deal of parallel processing. However, with the proper software (such as XORP or Quagga), even commodity PCs can act as routers.
Routers connect with two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router.[2]
The term switch or layer 3 switch or network switch often is used interchangeably with router, but switch is really a marketing term without a rigorous technical definition (though a switch is commonly understood as a network hub with switched ports, which might or might not also perform additional routing functions).
Chassis systems like the Nortel MERS-8600 or ERS-8600 routing switch, allow for a wide variety of LAN, MAN, METRO, and WAN port technologies or other connections that are customizable.
Routers operate in two different planes [3]:
* Control Plane, in which the router learns the outgoing interface that is most appropriate for forwarding specific packets to specific destinations,
* Forwarding Plane, which is responsible for the actual process of sending a packet received on a logical interface to an outbound logical interface.
Routers are like intersections whereas subnets are like streets and hosts like houses
Routers are like intersections whereas subnets are like streets and hosts like houses
Control Plane processing leads to the construction of what is variously called a routing table or routing information base (RIB). The RIB may be used by the Forwarding Plane to look up the outbound interface for a given packet, or, depending on the router implementation, the Control Plane may populate a separate Forwarding Information Base (FIB) with destination information. RIBs are optimized for efficient updating with control mechanisms such as routing protocols, while FIBs are optimized for the fastest possible lookup of the information needed to select the outbound interface.
The Control Plane constructs the routing table from knowledge of the up/down status of its local interfaces, from hard-coded static routes, and from exchanging routing protocol information with other routers. It is not compulsory for a router to use routing protocols to function, if for example it was configured solely with static routes. The routing table stores the best routes to certain network destinations, the "routing metrics" associated with those routes, and the path to the next hop router.
Routers do maintain state on the routes in the RIB/routing table, but this is quite distinct from not maintaining state on individual packets that have been forwarded.
For the pure Internet Protocol (IP) forwarding function, router design tries to minimize the state information kept on individual packets. Once a packet is forwarded, the router should retain no more than statistical information about it. It is the sending and receiving endpoint that keeps information on such things as errored or missing packets.
Forwarding decisions can involve decisions at layers other than the IP internetwork layer or OSI layer 3. Again, the marketing term switch can be applied to devices that have these capabilities. A function that forwards based on data link layer, or OSI layer 2, information, is properly called a bridge, or layer 2 switch. A physical device called a router may also have the capability to forward based on information at other layers, if it has software that can make decisions at these other layers.
[[Image:Cisco-rs1.jpg|thumb|right|Cisco CRS-1 Carrier Routing System
Cisco 7600 Enterprise Routers
Cisco 7600 Enterprise Routers
Routers may provide connectivity inside enterprises, between enterprises and the Internet, and inside Internet Service Providers (ISP). The largest routers (for example the Cisco CRS-1 or Juniper T1600) interconnect ISPs, are used inside ISPs, or may be used in very large enterprise networks. An example of an enterprise router would be the Cisco 7600 (pictured above). The smallest routers provide connectivity for small and home offices (for example the Linksys BEFSR41).
Routers intended for ISP and major enterprise connectivity will almost invariably exchange routing information with the Border Gateway Protocol. RFC 4098[4] defines several types of BGP-speaking routers:
* Provider Edge Router: Placed at the edge of an ISP network, it speaks external BGP (eBGP) to a BGP speaker in another provider or large enterprise Autonomous System (AS).
* Subscriber Edge Router: Located at the edge of the subscriber's network, it speaks eBGP to its provider's AS(s). It belongs to an end user (enterprise) organization.
* Inter-provider Border Router: Interconnecting ISPs, this is a BGP speaking router that maintains BGP sessions with other BGP speaking routers in other providers' ASes.
* Core router: A router that resides within the middle or backbone of the network rather than at its periphery.
Within an ISP: Internal to the provider's AS, such a router speaks internal BGP (iBGP) to that provider's edge routers, other intra-provider core routers, or the provider's inter-provider border routers.
"Internet backbone:" The Internet does not have a clearly identifiable backbone, as did its predecessors. See default-free zone (DFZ). Nevertheless, it is the major ISPs' routers that make up what many would consider the core. These ISPs operate all four types of the BGP-speaking routers described here. In ISP usage, a "core" router is internal to an ISP, and used to interconnect its edge and border routers. Core routers may also have specialized functions in virtual private networks based on a combination of BGP and Multi-Protocol Label Switching (MPLS)[5].
History
The very first device that had fundamentally the same functionality as a router does today, i.e a packet switch, was the Interface Message Processor (IMP); IMPs were the devices that made up the ARPANET, the first packet switching network. The idea for a router (although they were called "gateways" at the time) initially came about through an international group of computer networking researchers called the International Network Working Group (INWG). Set up in 1972 as an informal group to consider the technical issues involved in connecting different networks, later that year it became a subcommittee of the International Federation for Information Processing. [7]
These devices were different from most previous packet switches in two ways. First, they connected dissimilar kinds of networks, such as serial lines and local area networks. Second, they were connectionless devices, which had no role in assuring that traffic was delivered reliably, leaving that entirely to the hosts (although this particular idea had been previously pioneered in the CYCLADES network).
The idea was explored in more detail, with the intention to produce an actual prototype system, as part of two contemporaneous programs. One was the initial DARPA-initiated program, which created the TCP/IP architecture of today. [8] The other was a program at Xerox PARC to explore new networking technologies, which produced the PARC Universal Packet system, although due to corporate intellectual property concerns it received little attention outside Xerox until years later. [9]
The earliest Xerox routers came into operation sometime after early 1974. The first true IP router was developed by Virginia Strazisar at BBN, as part of that DARPA-initiated effort, during 1975-1976. By the end of 1976, three PDP-11-based routers were in service in the experimental prototype Internet. [10]
The first multiprotocol routers were independently created by staff researchers at MIT and Stanford in 1981; the Stanford router was done by William Yeager, and the MIT one by Noel Chiappa; both were also based on PDP-11s. [11] [12] [13] [14]
As virtually all networking now uses IP at the network layer, multiprotocol routers are largely obsolete, although they were important in the early stages of the growth of computer networking, when several protocols other than TCP/IP were in widespread use. Routers that handle both IPv4 and IPv6 arguably are multiprotocol, but in a far less variable sense than a router that processed AppleTalk, DECnet, IP, and Xerox protocols.
In the original era of routing (from the mid-1970s through the 1980s), general-purpose mini-computers served as routers. Although general-purpose computers can perform routing, modern high-speed routers are highly specialized computers, generally with extra hardware added to accelerate both common routing functions such as packet forwarding and specialised functions such as IPsec encryption.
Still, there is substantial use of Linux and Unix machines, running open source routing code, for routing research and selected other applications. While Cisco's operating system was independently designed, other major router operating systems, such as those from Juniper Networks and Extreme Networks, are extensively modified but still have Unix ancestry.
Other changes also improve reliability, such as redundant control processors with stateful failover, and using storage having no moving parts for program loading. As much reliability comes from operational techniques for running critical routers as it does to the router design itself. It is the best common practice, for example, to use redundant uninterruptible power supplies for all critical network elements, with generator backup for the batteries or flywheels of those power supplies.
Úterý, 25. září 2007
Space Base
Class: Manned. Type: Space Station. Destination: Space Station Orbit. Nation: USA. Agency: NASA. Manufacturer: North American, McDonnell.
Growth of Space Station into a 50 man Space Base was a required capability in the Phase B NASA Space Station studies of 1969-1970. The original core Station Module was to be used in the Base build-up several years later or was to be used as a prototype of one of the Space Base modules. Space Base would serve as a major international facility for research, applications and for the support of other space operations such as servicing unmanned satellites. The crew would be large enough to include specialists of many kinds and to minimize even more the need for astronaut-type training. The Space Base would provide nearly equal zero and artificial gravity volumes and would be configured with permanent dedicated laboratories and observatories in addition to general purpose facilities.
Both Phase B teams developed very similar concepts. Living and operation functions were centered in the rotating arms while research laboratories and observatories and applications facilities were found primarily in zero gravity hub modules or in hub sections rotating to local vertical. The most significant difference in the concepts was that one had counter-rotating artificial gravity arms while the other had a single rotating arm. The North American Rockwell single rotation arm concept had a relatively high angular momentum which provided a very stable platform. The McDonnell Douglas counter-rotating arm had zero gyroscopic stability and, therefore, would require little torque to change its attitude. No conclusive arguments favored one approach over the other.
Both concepts employed dual 50 kW nuclear reactor Brayton power generating systems deployed in a "Y" arrangement on the end of the non-rotating hub. This configuration would minimize radiation effects and would simplify maintenance and replacement operations.
The 50-man space base would have used nine of the basic 12-man space station modules. A much larger 400-man 'space hotel' could eventually be assembled from 18 modules. The space base would have been launched around 1980 to support large scale manned space activities in low Earth orbit. Smaller space stations would have been launched into geostationary and lunar orbit during the mid-/late 1970s to support manned lunar exploration. The smaller 6-crew space station module would also have housed Mars-bound astronauts in 1982-86.
An alternate space base design would have moved most of the equipment from the non-rotating weightless core segment to the rotating arms, where artificial gravity would be generated.
Crew Size: 50. Design Life: 10 years. Typical orbit: 456 km x 456 km at 55 degrees inclination. Maximum Diameter: 150.00 m (490.00 ft). Span: 10.00 m (32.00 ft). Habitable Volume: 7 000.00 m3. Mass: 1 000 000 kg (2 200 000 lb). Electrical System: Nuclear. Electric System: 50.00 average kW. Associated Launch Vehicle: Saturn V.
Space Shuttle
NASA's Space Shuttle, officially called Space Transportation System (STS), is the spacecraft currently used by the United States government for its human spaceflight missions. At launch, it consists of a rust-colored external tank (ET), two white, slender solid rocket boosters (SRBs), and a winged orbiter (the space shuttle in the narrow sense). The orbiter carries astronauts and payload such as satellites or space station parts into low earth orbit. Usually, five to seven astronauts ride in the orbiter. The payload capacity is 50,000 lb (22,700 kg). When the orbiter's mission is complete, it fires its orbital maneuvering thrusters to drop out of orbit and re-enters the Earth's atmosphere. During the descent and landing, the shuttle orbiter acts as a glider and makes a completely unpowered ("dead stick") landing. Five spaceworthy orbiters were built, of which three remain.
The Shuttle is the first orbital spacecraft designed for partial reusability. It carries payloads to low Earth orbit, provides crew rotation for the International Space Station (ISS), and performs servicing missions. The orbiter can also recover satellites and other payloads from orbit and return them to Earth, but this capacity has not been used often. However, it has been used to return large payloads from the ISS to Earth, as the Russian Soyuz spacecraft has limited capacity for return payloads. Each Shuttle was designed for a projected lifespan of 100 launches or 10 years' operational life. The man responsible for the design of the STS was Maxime Faget, who had also overseen the Mercury, Gemini and Apollo spacecraft designs. The crucial factor in the size and shape of the Shuttle Orbiter was the requirement that it be able to accommodate the largest planned spy satellites, and have the cross-range recovery range to meet classified USAF missions requirement for a one-around abort for a polar launch. Factors involved in opting for 'reusable' solid rockets and an expendable fuel tank included the desire of the Pentagon to obtain a high-capacity payload vehicle for satellite deployment, and the desire of the Nixon administration to reduce the costs of space exploration by developing a spacecraft with reusable components.
Six shuttles have been built, five of which were spaceworthy. The first orbiter, Enterprise, was not built for actual space flight, and was used only for testing purposes. Enterprise was followed by four operational space shuttles: Columbia, Challenger, Discovery and Atlantis. Challenger was destroyed on launch in 1986, and Endeavour was built as a replacement. Columbia was destroyed on re-entry in 2003.
NASA announced in 2004 that the Space Shuttle will be retired in 2010 and replaced by the Orion, a new vehicle that is designed to take humans to the Moon and beyond.
Universe
The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. Based on observations of the portion of the Universe that is observable, physicists attempt to describe the whole of space-time, including all matter and energy and events which occur, as a single system corresponding to a mathematical model.
The generally accepted scientific theory which describes the origin and evolution of the Universe is Big Bang cosmology, which describes the expansion of space from an extremely hot and dense state of unknown characteristics. The Universe underwent a rapid period of cosmic inflation that flattened out nearly all initial irregularities in the energy density; thereafter the universe expanded and became steadily cooler and less dense. Minor variations in the distribution of mass resulted in hierarchical segregation of the features that are found in the current universe; such as clusters and superclusters of galaxies. There are more than one hundred billion (1011) galaxies in the Universe,[1] each containing hundreds of billions of stars, with each star containing about 1057 atoms of hydrogen.
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