1. Router Bridge Switch & Co.
Franz Dosch 2001
DOSCH & AMAND Research GmbH & Co KG
V1.1
Router
On the Internet, a router is a device or, in some cases, software in a computer, that determines the
next network point to which a packet should be forwarded toward its destination. The router is
connected to at least two networks and decides which way to send each information packet based on
its current understanding of the state of the networks it is connected to. A router is located at any
gateway (where one network meets another), including each Internet point-of-presence. A router is
often included as part of a network switch.
A router may create or maintain a table of the available routes and their conditions and use this
information 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 its
destination. Routing is a function associated with the network layer (Layer 3) in the standard model of
network programming, the Open Systems Interconnection (OSI) model. A layer-3 switch is a switch
that can perform routing functions.
An edge router is a router that interfaces with an asynchronous transfer mode network. A brouter is a
network bridge combined with a router.
Bridge
In telecommunication networks, a bridge is a product that connects a local area network (LAN) to
another local area network that uses the same protocol (for example, Ethernet or Token Ring). You
can envision a bridge as being a device that decides whether a message from you to someone else is
going to the local area network in your building or to someone on the local area network in the building
across the street. A bridge examines each message on a LAN, "passing" those known to be within the
same 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 this
reason, messages are sent out to every address on the network and accepted only by the intended
destination node. Bridges learn which addresses are on which network and develop a learning table
so that subsequent messages can be forwarded to the right network.
Bridging networks are generally always interconnected local area networks since broadcasting every
message to all possible destinations would flood a larger network with unnecessary traffic. For this
reason, router networks such as the Internet use a scheme that assigns addresses to nodes so that a
message or packet can be forwarded only in one general direction rather than forwarded in all
directions.
A bridge works at the data-link (physical network) level of a network, copying a data frame from one
network to the next network along the communications path.
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2. Gateway
A gateway is a network point that acts as an entrance to another network. On the Internet, a node or
stopping point can be either a gateway node or a host (end-point) node. Both the computers of
Internet users and the computers that serve pages to users are host nodes. The computers that
control traffic within your company's network or at your local Internet service provider (ISP) are
gateway nodes.
In the network for an enterprise, a computer server acting as a gateway node is often also acting as a
proxy server and a firewall server. A gateway is often associated with both a router, which knows
where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the
actual path in and out of the gateway for a given packet.
Hub
In general, a hub is the central part of a wheel where the spokes come together. The term is familiar to
frequent 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 more
directions and is forwarded out in one or more other directions. A hub usually includes a switch of
some kind. (And a product that is called a "switch" could usually be considered a hub as well.) The
distinction seems to be that the hub is the place where data comes together and the switch is what
determines how and where data is forwarded from the place where data comes together. Regarded in
its 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 a
number of outgoing lines can be attached ("dropped"), each providing one or more connection port for
device to attach to. For Internet users not connected to a local area network, this is the general
topology used by your access provider. Other common network topologies are the bus network and
the 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 card
for connections to a local area network (for example, an Ethernet or a Token Ring), and a connection
to a line (the main line in this example).
Switch
In a telecommunications network, a switch is a device that channels incoming data from any of
multiple input ports to the specific output port that will take the data toward its intended destination. In
the traditional circuit-switched telephone network, one or more switches are used to set up a dedicated
though temporary connection or circuit for an exchange between two or more parties. On an Ethernet
local area network (LAN), a switch determines from the physical device (Media Access Control or
MAC) address in each incoming message frame which output port to forward it to and out of. In a wide
area packet-switched network such as the Internet, a switch determines from the IP address in each
packet 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 or
Data-Link layer function. That is, it simply looks at each packet or data unit and determines from a
physical address (the "MAC address") which device a data unit is intended for and switches it out
toward that device. However, in wide area networks such as the Internet, the destination address
requires a look-up in a routing table by a device known as a router. Some newer switches also perform
routing 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.
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3. The time a switch takes to figure out where to forward a data unit is called its latency. The price paid
for having the flexibility that switches provide in a network is this latency. Switches are found at the
backbone and gateway levels of a network where one network connects with another and at the
subnetwork level where data is being forwarded close to its destination or origin. The former are often
known 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 the
network. For example, a local area network may be organized in a Token Ring or bus arrangement in
which each possible destination inspects each message and reads any message with its address.
Circuit-Switching version Packet-Switching
A network's paths can be used exclusively for a certain duration by two or more parties and then
switched for use to another set of parties. This type of "switching" is known as circuit-switching and is
really a dedicated and continuously connected path for its duration. Today, an ordinary voice phone
call 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 data
unit travels can be varied as conditions change. In packet-switching, a message is divided into
packets, which are units of a certain number of bytes. The network addresses of the sender and of the
destination are added to the packet. Each network point looks at the packet to see where to send it
next. Packets in the same message may travel different routes and may not arrive in the same order
that they were sent. At the destination, the packets in a message are collected and reassembled into
the original message.
ATM
ATM (asynchronous transfer mode) is a dedicated-connection switching technology that organizes
digital data into 53-byte cell units and transmits them over a physical medium using digital signal
technology. Individually, a cell is processed asynchronously relative to other related cells and is
queued before being multiplexed over the transmission path.
Because ATM is designed to be easily implemented by hardware (rather than software), faster
processing and switch speeds are possible. The prespecified bit rates are either 155.520 Mbps or
622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical
Network (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 info@da-research.de