Summary of Network Intermediate Systems

Intermediate Systems; Repeaters, Bridges, Routers, and Gateways, are networking devices that facilitate the transmission of packets between the source computer and the final destination computer. Although the gateway involves all seven levels of the OSI model, these devices are generally thought of as functioning at the lowest three levels; Physical, Data Link, and Network.

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Repeaters strengthen the physical transmission signal. In analog electronic networks, such as Ethernet and Token Ring, the signal is also improved or cleaned up by regenerating the square wave that has degenerated as it traveled down the wire.

Bridges connect multiple subnets, networks, or rings into one large logical network. A bridge maintains a table of node addresses and based on this, forwards packets to a specific subnet, reducing traffic on other subnets. In a bridged network, there can be only one path to any destination. Otherwise packets would circle the network causing network storms.

Routers are more intelligent and use the network address in the frame to decide how to best send a packet to its final destination. Some protocol suites provide a dynamic client protocol that routers send between themselves to exchange information regarding their status and optimal paths between nodes. IPX and its client protocol, RIP (Routing Information Protocol) is an example of this.

Gateways are the most intelligent of the intermediate systems. They allow network nodes using different protocols to communicate with each other. The gateway understands both protocols and receives and disassembles a packet of one protocol and then reassembles and transmits it using other protocol.

Definitions

Gateway:   (All OSI layers)

   Translator. Allows communication between two dissimilar protocols (eg:
   IPX and NetBEUI) or physical networks (eg: Ethernet and Token Ring).

   NOTE: In TCP/IP terminology, a gateway = router.

Router:   (Network Layer.)

   Uses the Network address. Protocol independent. Can change between
   topologies even in the frame size changes between the segments or
   networks. Breaks-up the network into separate physical segments,
   filtering out traffic and increasing each segments bandwidth or
   capacity.

Bridge:   (Data Link level.)

   Frame-level, Understands the network adapter address in the packet.
   Protocol independent. Can change between topologies if the frame size
   between the networks is the same.

   A Bridge with some of the features of a Router.

Repeater:   (Physical level)