Modem
Modem stands for Modulator and Demodulator. The modem (dialup modem) is an electronic device that can convert the digital signals into analog signals and analog signals into digital signals. It is used on both ends of the computers for data communication between, computers through telephone line. We knew that data is communicated through telephone line in the form of analog signals. So data in the form of digital signal from one computer must be converted into analog signals before to transmit it to another computer through telephone line. Similarly, to receive the data from another computer through telephone line in the form of analog signals must be converted to digital before to store into the computer. The modem can be external or internal.
The external modem is a separate device that is attached to the computer through RS-232 serial port. It can be easily moved from, one computer to another. It has also many functions but it is very costly.
An internal modem is an expansion card that is installed into the expansion slot on the computer’s motherboard. Most of the computers at home and offices use the internal modem because it is less expensive than external modem. The Fax machine also uses the internal card; it is because internal modem is also known as Fax Card.
Digital Modem
The modem that is used to send and to receive data to and from a digital telephone line is called digital modem. The examples of digital modems are ISDN and DSL. These modems send and receive the digital signals to and from the digital line. This modem does not convert the digital signals into analog because digital lines already use the digital signals. Therefore, in this case he definition of modem confuses the users, however manufacturers still use the term modem for devices used in digital line for data communication.
Network interface card (NIC)
The most important PC device is the network interface card (NIC). Each computer on the network, including the servers, is required to have one installed. It is the NIC that provides connectivity between the PC and the network’s physical medium, the copper or fiber-optic cable.
Most of the new motherboards available today for PCs and servers have the network interface card integrated with the motherboard. Older computers and some newer computers do not provide onboard network interfaces which will equire a NIC to be added.
NICs provide computers with a connection to the network, but they also handle an important data-conversion function. Data travels in parallel on the PCI’s bus system, but the network medium demands a serial transmission. The transceiver, a transmitter and receiver, on the NIC has the ability to move data from parallel to serial and vice versa. This isn’t any different than an automobiles travelling down a multi-lane superhighway where all lanes must merge into one lane.
Network interface cards also have the ability of supplying a basic addressing system that can be used to get data from one computer to another on the network. The hardware or MAC address is burned into a ROM chip on the NIC. This is referred to as the MAC address because the Media Access Control (MAC) layer is acutally a sublayer of the OSI model’s Data Link layer.
Bridge
A network bridge, also known as a layer 2 switch, is a hardware device used to create a connection between two separate computer networks or to divide one network into two. Both networks usually use the same protocol; Ethernet is an example of a protocol. Network devices include, but are not limited to, Personal Computers (PCs), printers, routers, switches and hubs. Devices connected to a network via an Ethernet adapter card have what is known as a Media Access Control (MAC) address, also called a physical or hardware address. It is this address that uniquely identifies a device to a bridge that can then determine to which network the device is connected.
The principal function of a network bridge is to forward data based on the MAC address of the sending and receiving devices. This operation helps to eliminate what are known as collision domains. One way of defining a collision domain is a network in which one device, also called a node, forces every other device to listen when it is transmitting data packets. Another definition states that a collision domain exists when two or more devices attempt to transmit information at the exact same time. Networks running Carrier Sense Multiple Access/Collision Detection (CSMA/CD) should, in theory, be protected from collisions occurring, but CSMA/CD can fail.
Whenever collisions occur, the efficient transmission of data packets is greatly compromised. The more devices that are on a network trying to transmit data, the greater the chance for a collision to occur. A network bridge can be used to segment one network into two, thereby reducing the number of devices competing for transmission privileges. For example, if network A has 20 devices, there is the likelihood that two or more of them will attempt to transmit data at the same time and cause a collision. If a network bridge is added, it can split network A into networks A and B of 10 devices each.
Once the network bridge is incorporated, it will begin to “listen” to the transmission of data performed by devices on the two networks. It accomplishes this by recording the MAC address of the devices in a table that it automatically generates without being programmed to do so. When the first device transmits data, the network bridge will add its MAC address to what is known as a forwarding table for future reference. The bridge also looks at the MAC address of the destination or receiving device. If it does not appear in its table, the bridge will broadcast the data packet to all devices on both networks to locate the intended destination.
Forwarding tables are not instantly built, rather the network bridge has to wait until it receives a transmission from a device before it can learn its MAC address. MAC addresses of receiving devices also have to be learned via broadcast, a search for the location of the destination. Once the destination responds, its address is also added to the forwarding table of the network bridge. Eventually, all MAC addresses will be captured and data packets will be efficiently routed straight to their destination. This will happen without all devices having to listen to one transmitting device.
Router
Network router is a device or a piece of software in a computer that forwards and routes data packets along networks. A network router connects at least two networks, commonly two LANs or WANs or a LAN and its ISP network. A router is often included as part of a network switch. A router is located at any where one network meets another, including each point-of-presence on the Internet. A router has two key jobs:
* The router ensures that information doesn’t go where it’s not needed. This is crucial for keeping large volumes of data from clogging the network.
* The router makes sure that information does make it to the intended destination.
In performing these two jobs, a router joins the two networks, passing information from one to the other and, in some cases, performing translations of various protocols between the two networks. It also protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other. This process is known as routing.
Routing is a function associated with the Network layer (layer 3) in the Open Systems Interconnection (OSI) model. Routers use network layer protocol headers, such as IP header where the source and destination addresses are included, and routing tables to determine the best path to forward the packets. For the communication among routers and decide the best route between any two hosts, routing protocols such as ICMP are used.
Actually, routers are specialized computers that send messages speeding to their destinations along thousands of possible pathways. One of the tools a router uses to decide which path a packet should go is a routing table. A routing table contains a collection of information, including:
* Information on which connections lead to particular groups of addresses
* Priorities for connections to be used
* Rules for handling both routine and special cases of traffic
Information in the routing tables can be static (with routes manually entered by the network administrator) or dynamic (where routers communicate to exchange connection and route information using various routing protocols). A routing table can be as simple as a few lines in the smallest routers, but can grow to massive size and complexity in the very large routers that handle the bulk of Internet messages. As the number of networks attached to one another grows, the routing table for handling traffic among them grows, and the processing power of the router is increased.
Gateway
A gateway is a node that allows you to gain entrance into a network and vice versa. On the Internet the node which is the stopping point can be a gateway or a host node. A computer that controls the traffic your network or your ISP (Internet Service Provider) receives is a node. In most homes a gateway is the device provided by the Internet Service Provider that connects users to the internet.
When a computer server serves as a Gateway node, the gateway node also operates as a firewall and a proxy server. A firewall is a system created to prevent unauthorized admission into a private network. A proxy server is located right between a client application such as a web browser and the real server. The proxy server sees if the client applications requests can be carried out by the real server.
Usually a gateway is associated with a router. A router is a device or sometimes computer software that lets you know the next network data should be sent to next. A router can be connected to two or more networks at a time, it decides the next destination of the data based on it’s comprehension of the condition of the networks. A gateway is associated with a router because a router which uses headers and forwarding tables to figure out where packets or data is sent provides the path through which information is sent in and out a gateway.
So a gateway is one of the many ways we can communicate over the World Wide Web. The gateway allows us to enter different networks on the internet so we can transmit data back and forth. There’s also software available where you can be able to run several networks on a computer. As you have come to see by now having a gateway on your computer has it’s many benefits.
Repeater
A device, which is used to join same type of networks to extend the length of the communication medium, is called repeater. It also amplifies the signals received from one network and sends the amplified signals to another network.
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Communication Devices in Computer Network