Modem:

Modem, modulation, and signal are integrated to each others. The concept of modem is incomplete without the concept of modulation and signals.

Modem: MODEM is an electronic device used to convert digital signal into analog and vice versa. It was developed during 1950 to 1960 to connect mainframe computers.

Modem has two parts- (a) Modulator and (b) Demodulator. Modulator converts digital signal into analog and demodulator converts analog signal into digital signal. The digital signal (square wave) is not suitable for long distance communication through bounded media (cable) because signal suffer from strong attenuation, delay distortion and noises, So, modem is used to convert digital signal into analog signal.

There are three categories of modem available: Internal, External, and PC Card Modem. The PC card modem is a combined form of internal and external modem used in portable categories of computers. It is plugged directly into an external slot available for it.

Network Interface Cards:

The network interface card (NIC) provides the physical connection between the network and the computer workstation. Most NICs are internal, with the card fitting into an expansion slot inside the computer. Some computers, such as Mac Classics, use external boxes which are attached to a serial port or a SCSI port. Laptop computers can now be purchased with a network interface card built-in or with network cards that slip into a PCMCIA slot.

Network interface cards are a major factor in determining the speed and performance of a network. It is a good idea to use the fastest network card available for. The type of workstation you are using.

Switch/Hub:

A switch is a device that provides a central connection point for cables from workstations, servers, and peripherals. In a star topology, twisted-pair wire is run from each workstation to a central switch/hub. Most switches are

active, that is they electrically amplify the signal as it moves from one device to another. Switches no longer broadcast network packets as hubs did in the past, they memorize addressing of computers and send the information to the correct location directly. Switches are:

• Usually configured with 8, 12, or 24 RJ-45 ports
• Often used in a star or star-wired ring topology
• Sold with specialized software for port management
• Also called hubs

Routers:

A router translates information from one network to another; it is similar to a super intelligent bridge. Routers select the best path to route a message, based on the destination address and origin. The router can direct traffic to prevent head-on collisions, and is smart enough to know when to direct traffic along back roads and shortcuts. While bridges know the addresses of all computers on each side of the network, routers know the addresses of computers, bridges, and other routers on the network.

 Routers can even "listen" to the entire network to determine which sections are busiest they can then redirect data around those sections until they clear up. If you want to connect to the Internet in your network, you will need to purchase a router. In this case, the router serves as the translator between the information on your LAN and the Internet. It also determines the best route to send the data over the Internet. Routers can:

• Direct signal traffic efficiently
• Route messages between any two protocols
• Route messages between different topologies
• Route messages across fiber optic, coaxial, and twisted-pair cabling

Gateway:

A gateway is a node or a device that servers as an entrance to another network. The gateway routes the traffic from a workstation to the outside network that is in the network or in the websites. The gateway is used to connect the users to the internet. The gateway also acts as a firewall in the network.

Repeaters:

Since a signal loses strength as it passes along a cable, it is often necessary to boost the signal with a device called a repeater. The repeater electrically amplifies the signal it receives and rebroadcasts it. Repeaters can be separate devices or they can be incorporated into a concentrator. They are used when the total length of your network cable exceeds the standards set for the type of cable being used. A good example of the use of repeaters would be in a local area network using a star topology with unshielded twisted-pair cabling. The length limit for unshielded twisted- pair cable is 100 meters. The most common configuration is for each workstation to be connected by twisted-pair cable to a multi-port active concentrator. The concentrator amplifies all the signals that pass through it allowing for the total length of cable on the network to exceed the 100-meter limit.