Network components

Hubs, Repeaters

A hub a multiple-port repeater. A repeater receives a digital signal and re-amplifies regenerates that signal and then forwards the digital signal out all active ports without looking at any data. An active hub does the same thing. Any digital signal received from a segment on a hub port is regenerated or reamplified and transmitted out all ports on the hub. This means all devices plugged into a hub are in the same collision domain as well as in the same broadcast domain.


Hubs, like repeaters, don't actually examine any of the traffic as it enters, and is then transmitted out to the other parts of the physical media.



By default, switches break up collision domains but are one large broadcast domain.


Switches aren’t used to create internetworks, they’re employed to add functionality to an internetwork LAN. The main purpose of a switch is to make a LAN work better, to optimize its performance, providing more bandwidth for the LAN’s users. And switches don't forward packets to other networks as router do. Instead, they only “ switch”  frames from one port to another within the switched network. A  switch is basically just a multiple-port bridge.




The Bridge provides a means of interconnecting similar LANs.  Bridges function at the data link layer, layer 2  of the OSI reference model. The bridge is designed for use between Local area networks(LAN5) that use identical protocols for the physical and link layers (e.g.,  all conforming to IEEE 802.3). Because the devices all use the same protocols, the amount of processing required bridge is minimal. More sophisticated Bridges are capable of mapping from one MAC format to another(e.g., internet connect an Ethernet and the token ring LAN).


Because the bridge is used in the situation in which all the LANs have the same characteristics, the reader may ask, why not simply have one large LAN? Depending on the circumstance, there are several reasons for the use of multiple LANs connected by bridges.


  • Reliability, The danger in connecting all data processing devices in the organization to one network is that the fault of the network may disable communication for all devices By using Bridges, the network can be partitioned into self-contained units.


  • Performance: In general, performance on a LANs declines with an increase in the number of devices or the length of the wire. A number of the smaller LANs will often give improved performance.


  • security: The establishment of multiple LANs may improve the security of communications. It is desirable to keep different types of traffic (e.g.,  accounting,  personnel, strategic planning) that have different security needs on physically separate media. At the same time, the different types of users with different levels of security need to communicate through controlled and monitored mechanisms.


  • Geography:  Clearly, two separate LANs are needed to support devices clustered in two geographically distant locations. Even in case of two building separated by a highway, it may be far easier to use a microwave bridge link than to attempt to string a coaxial cable between the two buildings.


Functions of a Bridge

The diagram illustrates the action of a bridge connecting two LANs, A and B, using the same MAC protocol. In this example, a single Bridge attaches to both LANs; frequently, the bridge function is performed by two “half-bridges”, one of each LAN. The functions of the bridge are few and simple.


  • Read all frames transmitted on A and accept those addressed to any station on B.
  • Using the medium access control protocol for B, retransmit each frame on B.
  • Do the same for B-to-A traffic.


The diagram illustrates a bridge operation:




Routers are used to connect networks together and route packets of data from one network to another. Routers, by default, break up a broadcast domain.


A router creates & maintains a table of the available routes and their conditions and use the 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.



Often used as a connection to a mainframe or the Internet. Gateways enable communications between different protocols, data types, and environments. This is achieved via protocol conversion, whereby the gateway strips the protocol stack off of the packet and adds the appropriate stack another side. Gateways operate at all layers of the OSI model without making any forwarding decisions.



What is Next?

The next section explains about Internetworking Models. 

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