2.2 NETWORK DEVICES

 Let us learn about the basic communication devices which are used to form a network. Network Interface cards, Hubs, bridges, repeaters, and routers are all devices that let you connect one or more computers to other computers, networked devices, or to other networks. Each has two or more connectors called ports into which you plug in the cables to make the connection. Let us understand each of these communication devices in detail.

2.2.1 Network Interface Cards

The network interface card (NIC) provides the physical connection between the network and the computer. Most NICs are internal, with the card fitting into an expansion slot inside the computer. 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 fast transfer of data. The most common network interface connection today is Ethernet cards.

2.2.2 Ethernet Cards

Ethernet cards are usually purchased separately from a computer, although many latest computers now include an option for a preinstalled Ethernet card. Ethernet cards contain connections for either coaxial or twisted pair cables. If it is designed for coaxial cable, the connection will be BNC. If it is designed for twisted pair, it will have a RJ45 connection. Some Ethernet cards also contain an AUI connector. This can be used to attach coaxial, twisted pair, or fiber optics cable to an Ethernet card, we would be studying different type of cabling in later part of this unit.

2.2.3 Repeaters

When a signal travels along a cable, it tends to lose strength. As signals travel along a network cable (or any other medium of transmission), they degrade and become distorted in a process that is called attenuation A repeater is a device that boosts a network's signal as it passes through. The repeater does this by electrically amplifying the signal it receives and rebroadcasting it. 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 twistedpair cabling. If a cable is long enough, the attenuation will finally make a signal unrecognizable by the receiver.

A repeater enables signals to travel longer distances over a network. A repeater regenerates the received signals and then retransmits the regenerated (or conditioned) signals on other segments as shown in Figure 1.

2.2.4 Bridge

Like a repeater, a bridge can join several LANs (Local Area Network). However, a bridge can also divide a network to isolate traffic or problems. For example, if the volume of traffic from one or two computers or a single department is flooding the network with data and slowing down entire operation, a bridge can isolate those computers or that department.

A bridge (Figure 2) is used to connect two LAN segments i.e., segment1 (LAN1) and segment2 (LAN2). Each segment can have several computer attached to it.

2.2.5 Hub

A hub is the simplest of these devices. Any data packet coming from one port is sent to all other ports. It is then up to the receiving computer to decide if the packet is for it. You can imagine packets going through a hub as messages going into a mailing list. The mail is sent out to everyone and it is up to the receiving party to decide if it is of interest.

The biggest problem with hubs is their simplicity. Since every packet is sent out to every computer on the network, there is a lot of wasted transmission. This means that the network can easily become bogged down.

Hubs are typically used on small networks where the amount of data going across the network is never very high. A hub is typically the least expensive, least intelligent, and least complicated of the three. Every computer connected to the hub "sees" everything that every other computer on the hub sees. The hub itself is usually ignorant of the data being transmitted.

2.2.6 Switches

A switch does essentially what a hub does but more efficiently. By paying attention to the traffic that comes across it, it can "learn" where particular addresses are. For example, if it sees traffic from machine A coming in on port 2, it now knows that machine A is connected to that port and that traffic to machine A needs to only be sent to that port and not any of the others. The net result of using a switch over a hub is that most of the network traffic only goes where it needs to rather than to every port. On busy networks this can make the network significantly faster.

A switch steps up on a bridge in that it has multiple ports. When a packet comes through a switch it is read to determine which computer to send the data to which leads to increased efficiency in that packets are not going to computers that do not require them.

Let us take an example of email analogy where multiple people able to send email to multiple users. The switch can decide where to send the mail based on the address. Most large networks use switches rather than hubs to connect computers within the same subnet.

A Switch (or Switching Hub) is a device that that can segment a larger local area network to reduce the traffic load. One should implement a switch when you have a network with 20 or more users that have bogged down the network by excess traffic. It splits the network into two or more segments with devices that normally talk with each other.

Conceptually – switching takes data from one interface and delivers it to another interface.

2.2.7 Router

Let us study about another important communication device, router. A router translates information from one network to another; it is similar to an 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 headon collisions, and is smart enough to know when to direct traffic along 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 are removed.

If you have a school LAN that you want to connect to the Internet, 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. A simple way to think of a router is like a computer that can be programmed to understand, possibly manipulate, and route the data it’s being asked to handle. All routers include some kind of user interface for configuring how the router will treat traffic. The really large routers include the equivalent of a good amount of programming language to describe how they should operate as well as the ability to communicate with other routers to describe or determine the best way to get network traffic from point A to point B.

Routers can switch and route packets across multiple networks. They do this by exchanging protocolspecific information between separate networks. Routers have access to more information in packets than bridges, and use this information to improve packet deliveries. Routers are usually used in a complex network situation because they provide better traffic management than bridges and do not pass broadcast traffic.

The router must understand the addressing structure associated with the networking protocols it supports and take decisions on whether, or how, to forward packets. Routers are able to select the best transmission paths and optimal packet sizes.

2.2.8 Gateways

Let us study about another most important communication device, Gateway. If you are connected to the internet, you have to enter through a gateway. Your machine is connected to a small network and there is a gateway that connects your smaller network to the internet. A gateway passes information from one network to another network as your information travels across the internet.

Gateway Interconnects networks at higher layers than bridges or routers. A gateway  usually supports address mapping from one network to another, and may also provide transformation of the data between the environments to support endtoend application connectivity. Gateways typically limit the interconnectivity of two networks to a subset of the application protocols supported on either one.

Routers exemplify special cases of gateways. Gateways, also called protocol converters, can operate at any layer of the networking model. The job of a gateway is much more complex than that of a router or a switch. Typically, a gateway must convert one protocol into another.

Closely related to routers and gateways is the concept of a firewall or gateway, which is used to restrict access from the Internet to a network or a group of networks, controlled by an organization for security reasons.

Gateways make communication possible between different architectures and environments. They repackage and convert data going from one environment to another so that each environment can understand the other's environment data. 

Gateways can change the format of a message so that it will conform to the application program at the receiving end of the transfer.

The main functionality of a gateway is to convert protocols among communications networks. A router by itself transfers, accepts and relays packets only across networks using similar protocols. A gateway on the other hand can accept data formatted for one protocol and convert it to data formatted for another protocol before forwarding it. 

A gateway can be implemented in hardware, software or both, but they are usually implemented by software installed within a router. A gateway must understand the protocols used by each network linked into the router. Gateways are slower than bridges, switches and (nongateway) routers.

A gateway is a network point that acts as an entrance to another network. On the Internet, a node or stopping point can be either a gateway node or a host (endpoint) node. Both the computers of Internet users and the computers that serve pages to users are host nodes, while the nodes that connect the networks in between are gateways. 

For example, the computers that control traffic between company networks or the computers used by internet service providers (ISPs) to connect users to the internet are gateway nodes.

In the network for an enterprise, a computer server acting as a gateway node is often also acting as a proxy server and a firewall server. A gateway is often associated with both a router, which knows where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of the gateway for a given packet.

A gateway is an essential feature of most routers, although other devices (such as any PC or server) can function as a gateway. A computer running Microsoft Windows however describes this standard networking feature as Internet Connection Sharing; which will act as a gateway, offering a connection between the Internet and an internal network. Such a system might also act as a DHCP server. Dynamic Host Configuration Protocol (DHCP) is a protocol used by networked devices (clients) to obtain various parameters necessary for the clients to operate in an internet environment. By using this protocol, system administration workload greatly decreases, and devices can be added to the network with minimal or no manual configurations.