Router bridge


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Router bridge

  1. 1. Router Bridge Switch & Co. Franz Dosch 2001 DOSCH & AMAND Research GmbH & Co KG V1.1RouterOn the Internet, a router is a device or, in some cases, software in a computer, that determines thenext network point to which a packet should be forwarded toward its destination. The router isconnected to at least two networks and decides which way to send each information packet based onits current understanding of the state of the networks it is connected to. A router is located at anygateway (where one network meets another), including each Internet point-of-presence. A router isoften included as part of a network switch.A router may create or maintain a table of the available routes and their conditions and use thisinformation along with distance and cost algorithms to determine the best route for a given packet.Typically, a packet may travel through a number of network points with routers before arriving at itsdestination. Routing is a function associated with the network layer (Layer 3) in the standard model ofnetwork programming, the Open Systems Interconnection (OSI) model. A layer-3 switch is a switchthat can perform routing functions.An edge router is a router that interfaces with an asynchronous transfer mode network. A brouter is anetwork bridge combined with a router.BridgeIn telecommunication networks, a bridge is a product that connects a local area network (LAN) toanother local area network that uses the same protocol (for example, Ethernet or Token Ring). Youcan envision a bridge as being a device that decides whether a message from you to someone else isgoing to the local area network in your building or to someone on the local area network in the buildingacross the street. A bridge examines each message on a LAN, "passing" those known to be within thesame LAN, and forwarding those known to be on the other interconnected LAN (or LANs).In bridging networks, computer or node addresses have no specific relationship to location. For thisreason, messages are sent out to every address on the network and accepted only by the intendeddestination node. Bridges learn which addresses are on which network and develop a learning tableso that subsequent messages can be forwarded to the right network.Bridging networks are generally always interconnected local area networks since broadcasting everymessage to all possible destinations would flood a larger network with unnecessary traffic. For thisreason, router networks such as the Internet use a scheme that assigns addresses to nodes so that amessage or packet can be forwarded only in one general direction rather than forwarded in alldirections.A bridge works at the data-link (physical network) level of a network, copying a data frame from onenetwork to the next network along the communications path.© 2005-2006 DOSCH&AMAND Research GmbH & Co.KG all datas subject to change Moosacherstr. 56a D-80809 Munich GERMANY offer on request Fax +49.89.3589.8519 Email
  2. 2. GatewayA gateway is a network point that acts as an entrance to another network. On the Internet, a node orstopping point can be either a gateway node or a host (end-point) node. Both the computers ofInternet users and the computers that serve pages to users are host nodes. The computers thatcontrol traffic within your companys network or at your local Internet service provider (ISP) aregateway nodes.In the network for an enterprise, a computer server acting as a gateway node is often also acting as aproxy server and a firewall server. A gateway is often associated with both a router, which knowswhere to direct a given packet of data that arrives at the gateway, and a switch, which furnishes theactual path in and out of the gateway for a given packet.HubIn general, a hub is the central part of a wheel where the spokes come together. The term is familiar tofrequent fliers who travel through airport "hubs" to make connecting flights from one point to another.In data communications, a hub is a place of convergence where data arrives from one or moredirections and is forwarded out in one or more other directions. A hub usually includes a switch ofsome kind. (And a product that is called a "switch" could usually be considered a hub as well.) Thedistinction seems to be that the hub is the place where data comes together and the switch is whatdetermines how and where data is forwarded from the place where data comes together. Regarded inits switching aspects, a hub can also include a router.1) In describing network topologies, a hub topology consists of a backbone (main circuit) to which anumber of outgoing lines can be attached ("dropped"), each providing one or more connection port fordevice to attach to. For Internet users not connected to a local area network, this is the generaltopology used by your access provider. Other common network topologies are the bus network andthe ring network. (Either of these could possibly feed into a hub network, using a bridge.)2) As a network product, a hub may include a group of modem cards for dial-in users, a gateway cardfor connections to a local area network (for example, an Ethernet or a Token Ring), and a connectionto a line (the main line in this example).SwitchIn a telecommunications network, a switch is a device that channels incoming data from any ofmultiple input ports to the specific output port that will take the data toward its intended destination. Inthe traditional circuit-switched telephone network, one or more switches are used to set up a dedicatedthough temporary connection or circuit for an exchange between two or more parties. On an Ethernetlocal area network (LAN), a switch determines from the physical device (Media Access Control orMAC) address in each incoming message frame which output port to forward it to and out of. In a widearea packet-switched network such as the Internet, a switch determines from the IP address in eachpacket which output port to use for the next part of its trip to the intended destination.In the Open Systems Interconnection (OSI) communications model, a switch performs the layer 2 orData-Link layer function. That is, it simply looks at each packet or data unit and determines from aphysical address (the "MAC address") which device a data unit is intended for and switches it outtoward that device. However, in wide area networks such as the Internet, the destination addressrequires a look-up in a routing table by a device known as a router. Some newer switches also performrouting functions (layer 3 or the Network layer functions in OSI) and are sometimes called IP switches.On larger networks, the trip from one switch point to another in the network is called a hop.© 2005-2006 DOSCH&AMAND Research GmbH & Co.KG all datas subject to change Moosacherstr. 56a D-80809 Munich GERMANY offer on request Fax +49.89.3589.8519 Email
  3. 3. The time a switch takes to figure out where to forward a data unit is called its latency. The price paidfor having the flexibility that switches provide in a network is this latency. Switches are found at thebackbone and gateway levels of a network where one network connects with another and at thesubnetwork level where data is being forwarded close to its destination or origin. The former are oftenknown as core switches and the latter as desktop switches.In the simplest networks, a switch is not required for messages that are sent and received within thenetwork. For example, a local area network may be organized in a Token Ring or bus arrangement inwhich each possible destination inspects each message and reads any message with its address.Circuit-Switching version Packet-SwitchingA networks paths can be used exclusively for a certain duration by two or more parties and thenswitched for use to another set of parties. This type of "switching" is known as circuit-switching and isreally a dedicated and continuously connected path for its duration. Today, an ordinary voice phonecall generally uses circuit-switching.Most data today is sent, using digital signals, over networks that use packet-switching. Using packet-switching, all network users can share the same paths at the same time and the particular route a dataunit travels can be varied as conditions change. In packet-switching, a message is divided intopackets, which are units of a certain number of bytes. The network addresses of the sender and of thedestination are added to the packet. Each network point looks at the packet to see where to send itnext. Packets in the same message may travel different routes and may not arrive in the same orderthat they were sent. At the destination, the packets in a message are collected and reassembled intothe original message.ATMATM (asynchronous transfer mode) is a dedicated-connection switching technology that organizesdigital data into 53-byte cell units and transmits them over a physical medium using digital signaltechnology. Individually, a cell is processed asynchronously relative to other related cells and isqueued before being multiplexed over the transmission path.Because ATM is designed to be easily implemented by hardware (rather than software), fasterprocessing and switch speeds are possible. The prespecified bit rates are either 155.520 Mbps or622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous OpticalNetwork (SONET) and several other technologies, ATM is a key component of broadband ISDN(BISDN).© 2005-2006 DOSCH&AMAND Research GmbH & Co.KG all datas subject to change Moosacherstr. 56a D-80809 Munich GERMANY offer on request Fax +49.89.3589.8519 Email