CONNECTING DEVICES & ADAPTER CARDS

Connecting Devices

There are five kinds of connecting devices; Repeaters, Hubs, Bridges and two and three-layer Switches.

Repeaters and Hubs operate in the first layer of the internet model.

Bridges and two-layer Switches worked on the first two layer; Routers and three- layer Switches worked on the first three layers.

Repeaters

A Repeater is a device that operates only in the physical layer. Signals within a network can travel a fixed distance before Attenuation endangers the integrity of the data.

A Repeater receives a signal and, before it becomes too weak or corrupted, regenerates the original bit pattern.

A Repeater can extend the physical length of a LAN as shown in figure.

It does not actually connect two LANs; it only connects two segments of the same LAN.

The segments connected are still part of one single LAN. A Repeater is not a device that can connect two LANs of different protocols.

As we know that in different LAN Architectures length of the cable is limited. To extend this length we divide the cable into segments and install Repeaters between segments.

The whole network is still considered one LAN, but the portions of the network separated by Repeaters are called segments.

It is tempting to compare a Repeater to an amplifier, but the comparison is inaccurate.

An amplifier cannot discriminate between the intended signal and noise; it amplifies equal every thing fed into it.

A Repeater does not amplify the signal; it regenerates the signal. When it receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original strength.

The location of a Repeater on a link is very important. A Repeater must be placed so that a signal reaches it before any noise changes the meaning of any of its bits.

A little noise can alter the precision of a bit’s voltage without destroying its identity.

If the corrupted bit travels much farther, however, accumulated noise can change its meaning completely. At that point the original voltage is not recoverable and the error needs to be corrected.

A Repeater placed on the signal becomes list can still read the signal well enough to determine the intended voltages and replicate them in their original forms.

Hubs

Although, in a general sense, the word Hub can refer to any connecting device, it does have a specific meaning, a Hub is actually a multiport Repeater.

It is normally used to create connections between stations in a physical star topology.

However, Hubs can also be used to create multiple levels of hierarchy, as shown in figure.

The hierarchical use of Hubs removes the length limitation of 9 Base-T (90 m).

Bridges

A Bridge works on both the physical and the data link layers.

As a physical layer device, it regenerates the signal it receives. As a data link layer device, the Bridge can check the physical (MAC) addressed (source and destination) contained to the frame.

One may ask, what is the difference in functionality between a Bridge and a Repeater? A Bridge has filtering capability.

It can check the destination address of a frame and decide if the frame should be forwarded or dropped.

If the frame is to be forwarded, the decision must specify the port. A Bridge has a table that maps addresses to ports.

In the given figure destined for station 513B1365452 arrives at port 1. The Bridge queries its table to find the departing port.

According to the table frames for 513B1365452 leave through port 1; therefore, there is no need for forwarding; the frame is dropped.

On the other hand, if a frame for 513B1365452 arrives at port 2, the departing port is port 1 and the frame is forwarded.

In the first case, LAN2 remain free of traffic; in the second case, both LANs have traffic. In our example, we show a two port Bridge; in reality a Bridge usually has more ports.

Switches

When we use the term switch, we should be careful because a switch can mean two different things. We must clarify the term by adding the level at which the device operates.

We can have a two layer switch or a three- layer switch. Let us briefly discuss each.

Two-Layer Switch

A two layer switch is a bridge with many interfaces and a design that allows better (Faster) performance.

A bridge with a few interfaces can connect a few segments of a LAN together.

A bridge with many interfaces may be able to allocate a unique interface to each station, with each station on its own independent segment. This means no competing traffic.

Three-Layer Switch

A three-layer switch is a router with an improved design to allow better performance.

A three-layer switch can receive, process, and dispatch a packet much faster than a traditional router even though the functionality is the same.

Router

A Hardware device designed to take incoming Packets, analyzing the packets and then directing them to the appropriate locations, moving the packets to another network, converting the packets to be moved across a different type of network interface, dropping the packets, or performing any other number of other types of actions.

A router has a lot more capabilities than other network devices such as a hub or a switch which are only able to perform basic network functions.

For example, a hub is often used to transfer data between computers or network devices, but does not analyze or do anything with the data it is transferring.

A network router, however, may take the data being sent over a network, change how it is packaged, and send it to another network or over a different type of network.

For example, routers are commonly used in home networks to share a single internet connection with multiple computers.

Brouter

Short for Bridge Router, a "Brouter" is a networking device that serves as both a Bridge and a router.

Core router

A core router is a router in a computer network that routes data within a network but not between networks.

Edge router

An edge Router is a router in a computer network that routes data between one or more networks.

Virtual router

A Virtual Router is a backup router used in a VRRP setup.

Route Forwarding Routing tables hold the data for making forwarding decisions. Although this is a simple example, routing tables become very complex. Static routing uses fixed tables, but dynamic routing uses routing protocols that let routers exchange data with each other.