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Active network elements
 

The active network elements are used for signal transferring and processing.

The most common are:


Bridge

Bridge is a two-port device that separates traffic on two segments of the network by virtue of learning (MAC) addresses of nodes on both ports. Based on these addresses the bridge either transmits the data to the other side or not. It works within the second layer of the OSI model (data link layer) and is therefore protocol-independent. It is, however, dependent on the network technology (transfer method) used.

Bridge


Gateway

Gateway serves for negotiating the communication between two networks of different type.

Speaking of this active network element, it is necessary to point out that the interpretation of this term often varies. Sometimes it is used as a more general term for all the other: e.g. bridge is a kind of gateway that works on the data link layer while router is a gateway working within the network layer. Sometimes the gateway is identified as the router only. A significant example is the terminology used in connection with networks on the basis of TCP/IP protocol family: here, the term gateway commonly means the router. Nowadays, however, even here the much more correct term IP router is beginning to be used, instead of wrong designation gateway. In a narrower sense of the word, the gateway is considered to be a device working on an even higher layer than the network (i.e. on transport or application) layer. To understand the purpose of gateways it is good to note one significant fact: not all computer networks are the same. In fact, they can often differ notably. The extent of their difference influences the possibilities of their interconnection on particular layers. Generally, the bigger is their distinction, the higher layer must be used for their interconnection. From this point of view, the gateway is the device we connect the most mutually different networks with. Very often the gateways act on the highest i.e. application layer where they ensure data transfer between particular applications. Various kinds of mail gateways that provide e-mail transfer between networks can serve as example. Various networks may use different format of messages (particularly the header), other means of addressing or another encoding of characters in both the message body and header.

Try on How the gateway works in network here.

Gateway


Repeater

This active element is used for interconnection of two or more network segments by repeating the signal received on one of the ports to the other ports while retiming the signal, i.e. refreshing the sharp rising and falling edges.

Repeater


Switch

Switch allows for interconnection of particular network segments, having small to greater number of ports (up to several hundred) which network devices or segments are connected to. The term switch is being used for many different devices of various technologies. The common property of switches is that they analyze the incoming packets and decide where to send them according to information they contain (addresses, identifiers, etc.). Most often you can see switches in Ethernet twisted-pair networks. Here, it has replaced the previously used hubs which simply copied the signal into all other interfaces. It works within the second layer of the OSI model.

Switch


Hub

Hub is used to branch the network and as such it is the basic elements of networks of star topology. It works as a repeater which means it copies all the data that come to whichever of its ports to the rest of the ports, no matter which port (computer and IP address) are they indented for. That means all the computers in the network can see all of the network data. This results in worthless overloading of the segments that are not the intended recipients of the data, especially in case of larger networks. Switches are the successors of hubs as they can route the traffic intelligently. Nowadays the hubs are more of a cheaper but less effective and less safe alternative to switches. Mostly they are used in smaller home or office networks.

Try on the difference between hub and switch here.

Hub


Network Interface Controller

This is the interface for mutual communication of computers within the network. In home computers it has a form of a card that is inserted into a slot (ISA, PCI, PCI-e) on the motherboard or (today much more often) is already integrated on the board. The situation is similar in case of notebooks with the integrated version being the most common one. For external connection the PCMCIA interface is used. Every network interface controller has a unique 48-bit identifier stored in its ROM that is called the MAC address (also known as physical or hardware address). There is also firmware stored in the memory that controls the logical circuit and the access to the media within the data link layer of the OSI model.

Network card


Router

Router is a network device that implements a process called routing for retransferring datagrams towards their destination. The routing takes place on the third (network) layer of seven layers of the ISO/OSI model. The router thus connects together two networks and transfers data between them. There is a big difference between a router and a switch connecting the computers within the local network. The distinct functions of routers and switches can be depicted as switches being roads connecting all the cities in a country and routers as border crossing points between countries. Routing is usually connected with the IP protocol however other, less popular protocols are still being used.


Router