The Network Interface Card (NIC)

The Network Interface Card (NIC) provides interface between computer and cabling. It prepares data, sends data and controls the flow of data. It can also receive and translate data into bytes for the CPU to understand. It communicates at the Physical Layer of the OSI model and comes in many shapes and sizes.

REPEATERS

A repeater is a network device that repeats a signal from one port onto the other ports to which it is connected. Repeaters operate at the OSI Physical layer. A repeater does not filter or interpret—it merely repeats (regenerates) a signal, passing all network traffic in all directions.

A repeater doesn’t require any addressing information from the data frame because a repeater merely repeats bits of data. This means that if data is corrupt, a repeater will regenerate the signal anyway. A repeater will even repeat a broadcast storm caused by a malfunctioning adapter.

Some repeaters simply amplify signals. Although this increases the strength of the data signal, it also amplifies any noise on the network. In addition, if the original signal has been distorted in any way, an amplifying repeater cannot clean up the distortion.

Certainly, it would be nice if repeaters could be used to extend networks indefinitely, but all network designs limit the size of the network. The most important reason for this limitation is signal propagation. Networks must work with reasonable expectations about the maximum time a signal might be in transit.

HUBS

Hubs, also called wiring concentrators, provide a central attachment point for network cabling. Hubs come in three types

1. Passive Hubs

Passive hubs do not contain any electronic components and do not process the data signal in any way. The only purpose of a passive hub is to combine the signals from several network cable segments. All devices attached to a passive hub receive all the packets that pass through the hub. Because the hub doesn’t clean up or amplify the signals (in fact, the hub absorbs a small part of the signal), the distance between a computer and the hub can be no more than half the maximum permissible distance between two computers on the network.

2. Active Hubs

Active hubs incorporate electronic components that can amplify and clean up the electronic signals that flow between devices on the network. This process of cleaning up the signals is called signal regeneration. Signal regeneration has the following benefits:
• The network is more robust (less sensitive to errors).
• Distances between devices can be increased.
These advantages generally outweigh the fact that active hubs cost considerably more than passive hubs. Active hubs function in part as repeaters; they occasionally are called multiport repeaters.

3. Intelligent Hubs or Switch

Intelligent hubs are enhanced active hubs. Several functions can add intelligence to a hub:
• Hub management: Hubs now support network management protocols that enable the hub to send packets to a central network console. These protocols also enable the console to control the hub; for example, a network administrator can order the hub to shut down a connection that is generating network errors.
• Switching: The latest development in hubs is the switching hub, which includes circuitry that very quickly routes signals between ports on the hub. Instead of repeating a packet to all ports on the hub, a switching hub repeats a packet only to the port that connects to the destination computer for the packet. Many switching hubs have the capability of switching packets to the fastest of several alternative paths. Switching hubs are replacing bridges and routers on many networks.

BRIDGES

Bridges, on the other hand, can extend the maximum size of a network. Although the bridged network looks much like the repeater, the bridge is a much more flexible device. Bridges operate at the MAC sub-layer of the OSI Data Link layer. A repeater passes on all signals that it receives. A bridge, on the other hand, is more selective and passes only those signals targeted for a computer on the other side. A bridge can make this determination because each device on the network is identified by a unique physical address.

Bridges are a simpler technology than routers. They operate at the Media Access Control (MAC) sub-layer of the Data Link layer of the OSI model (one layer below routers). Bridges pass frames from one segment of the LAN to another based on the MAC addresses of the sender and receiver network interface cards. All addresses, even across networks, must be unique and of the same format because bridges do not convert addresses. There can be only one route to each station because a bridge does not make routing decisions.

Bridges do not have to be configured. The forwarding database of a bridge is built and maintained automatically.

Routers

An internet-work consists of two or more physically connected independent networks that are able to communicate. The networks that make up an internet-work can be of very different types. For example, an internet-work can include Ethernet and token-ring networks. Because each network in an internet-work is assigned an address, each network can be considered logically separate; that is, each network functions independently of other networks on the internet-work. Internet-work connectivity devices, such as routers, can use network address information to assist in the efficient delivery of messages. Delivering packets according to logical network address information is called routing.

Routers work much like bridges, but they pay attention to the upper network layer protocols (OSI layer 3) rather than physical and data link layer (OSI layer 1 & 2) protocols. A router will decide whether to forward a packet by looking at the protocol level addresses (for instance, TCP/IP addresses) rather than the MAC address. Because routers work at layer 3 of the OSI stack, it is possible for them to transfer packets between different media types (i.e., leased lines, Ethernet, token ring, X.25, Frame Relay and FDDI). Many routers can also function as bridges.

GATEWAYS

The term “gateway” originally was used in the Internet protocol suite to refer to a router. Today, the term “gateway” more commonly refers to a system functioning at the top levels of the OSI model that enables communication between dissimilar protocol systems. A gateway generally is dedicated to a specific conversion, and the exact functioning of the gateway depends on the protocol translations it must perform. Gateways commonly function at the OSI Application layer, but actually can operate at any level of the OSI model. Gateways connect dissimilar environments by removing the layered protocol information of incoming packets and replacing it with the packet information necessary for the dissimilar environment (see Gateways can be implemented as software, hardware, or a combination of both. An example of a gateway is often seen in email systems. When you send email, say from Microsoft Exchange to someone on the Internet, a gateway is responsible for converting the Microsoft Exchange message contents and addressing, to one that is compatible with the SMTP (Internet) message format and addressing.

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