If we have to be familiar with the various types of network media and connections, we should learn about some devices commonly found on today’s networks. Because these devices connect network entities, they are known as connectivity devices. They are
The network interface card (NIC), as its name suggests, is the expansion card you install in your computer to connect, or interface, your computer to the network. This device provides the physical, electrical, and electronic connections to the network media. A NIC is either an expansion card (the most popular implementation) or built in to the motherboard of the computer. In most cases, a NIC connects to the computer through expansion slots, which are special slots located on a computer’s motherboard that allow peripherals to be plugged directly into it. In some notebook, NIC adapters can be connected to the printer port or through a PC card slot.
NIC cards generally all have one or two light emitting diodes (LEDs) that help in diagnosing problems with their functionality. If there are two separate LEDs, one of them may be the Link LED, which illuminates when proper connectivity to an active network is detected. This often means that the NIC is receiving a proper signal from the hub/MAU or switch, but it could indicate connectivity to and detection of a carrier on a coax segment or connectivity with a router or other end device using a crossover cable. The other most popular LED is the Activity LED.
The Activity LED will tend to flicker, indicating the intermittent transmission or receipt of frames to or from the network.
In a star topology Ethernet network, a hub is the device that connects all the segments of the network together. Every device in the network connects directly to the hub through a single cable. Any transmission received on one port will be sent out all the other ports in the hub, including the receiving pair for the transmitting device, so that CSMA/CD on the transmitter can monitor for collisions. So, if one station sends it, all the others receive it; but based on addressing in the frame, only the intended recipient listens to it. This is to simulate the physical bus that the CSMA/CD standard was based on. It’s why we call the use of a hub in an Ethernet environment a physical star/logical bus topology. It is important to note that hubs are nothing more than glorified repeaters, which are incapable of recognizing frame boundaries and data structures; that’s why they act with such a lack of intelligence. A broadcast sent out by any device on the hub will be propagated to all devices connected to the hub. Any two or more devices connected to the hub have the capability of causing a collision with each other, just as in the case of a physical bus.
Like a hub, a switch connects multiple segments of a network together, with one important difference. Whereas a hub sends out anything it receives on one port to all the others, a switch recognizes frame boundaries and pays attention to the destination MAC address of the incoming frame as well as the port on which it was received. If the destination is known to be on a different port than the port over which the frame was received, the switch will forward the frame out over only the port on which the destination exists.
Otherwise, the frame is silently discarded. If the location of the destination is unknown, then the switch acts much like a hub in that it floods the frame out every port, except for the port over which it was received, unlike a hub. The only way any party not involved in that communication will receive the transmission is if it shares a port with the transmitter or receiver of the frame. This can occur if a hub is attached to the switch port, instead of in a 1:1 relationship of end devices and switch ports. The benefit of a switch over a hub is that the switch increases performance because it is able to support full wire speed on each and every port with a non blocking backplane, meaning the electronics inside the switch are at least equivalent in speed to the sum of the speeds of all ports.
A bridge, specifically a transparent bridge, is a network device that connects two similar network segments together. The primary function of a bridge is to keep traffic separated on both sides of the bridge. Traffic is allowed to pass through the bridge only if the transmission is intended for a station on the opposite side. The main reasons for putting a bridge in a network are to connect two segments together and to divide a busy network into two segments. A switch can be thought of as a hardware-based multiport bridge.
A router is a network device that connects multiple, often dissimilar, network segments into an internetwork. The router, once connected, can make intelligent decisions about how best to get network data to its destination based on network performance data that it gathers from the network itself. Routers are very complex devices. Often, routers are computers unto themselves with their own complex operating systems to manage the routing functions (Cisco’s IOS, for example) and CPUs dedicated to the functions of routing packets. Because of their complexity, it is actually possible to configure routers to perform the functions of other types of network devices (like gateways, firewalls, etc.) by simply implementing the feature within the router’s software.
A gateway is any hardware and software combination that connects dissimilar network environments. Gateways are the most complex of network devices because they perform translations at multiple layers of the OSI model.
For example, a gateway is the device that connects a LAN environment to a mainframe environment. The two environments are completely different. LAN environments use distributed processing, baseband communications, and the ASCII character set. Mainframe environments use centralized processing, broadband and baseband communications, and the EBCDIC character set. Each of the LAN protocols is translated to its mainframe counterpart by the gateway software.
Another popular example is the e-mail gateway. Most LAN-based e-mail software, such as Novell’s GroupWise and Microsoft’s Exchange, can’t communicate directly with Internet mail servers without the use of a gateway. This gateway translates LAN-based mail messages into the SMTP format that Internet mail uses.
In addition to other network connectivity devices, there are several devices that, while maybe not directly connected to a network, participate in moving network data. They are
A modem is a device that modulates digital data onto an analog carrier for transmission over an analog medium and then demodulates from the analog carrier to a digital signal again at the receiving end. The term modem is actually an acronym that stands for MOdulator / DEModulator.
When we hear the term modem, three different types should come to mind:
Most modems you find in computers today fall into the category of traditional modems. These modems convert the signals from your computer into signals that travel over the plain old telephone service (POTS) lines. The majority of modems that exist today are POTS modems, mainly because PC manufacturers include one with a computer.
Digital subscriber line (DSL) is quickly replacing traditional modem access because it offers higher data rates for a reasonable cost. In addition, you can make regular phone calls while online. DSL uses higher frequencies (above 3200Hz) than regular voice phone calls use, which provides greater bandwidth (up to several megabits per second) than regular POTS modems provide while still allowing the standard voice frequency range to travel at its normal frequency to remain compatible with traditional POTS phones and devices, an advantage over ISDN. DSL “modems” are the devices that allow the network signals to pass over phone lines at these higher frequencies.
Most often, when you sign up for DSL service, the company you sign up with will send you a DSL modem for free or for a very low cost. This modem is usually an external modem (although internal DSL modems are available), and it usually has both a phone line and an Ethernet connection. You must connect the phone line to a wall jack and the Ethernet connection to your computer (you must have an Ethernet NIC in your computer in order to connect to the DSL modem). Alternatively, a router, hub, or switch may be connected to the Ethernet port of the DSL modem, increasing the options available for the Ethernet network.
Another high-speed Internet access technology that is seeing widespread use is cable modem access. Cable modems connect an individual PC or network to the Internet using your cable television cable. The cable TV companies use their existing cable infrastructure to deliver data services on unused frequency bands.
The cable modem itself is a fairly simple device. It has a standard coax connector on the back as well as an Ethernet port. You can connect one PC to a cable modem (the PC will need to have an Ethernet NIC installed), or you can connect the modem to multiple PCs on a network (using a hub or switch). A router may also be used to enhance the Ethernet network’s capabilities.
2.ISDN Terminal Adapters
Integrated Services Digital Network (ISDN) is another form of high-speed Internet access. It delivers digital services (over 64Kbps channels) over conditioned telephone copper pairs. The device you must hook up to your computer to access ISDN services is properly known as an ISDN Terminal Adapter. It’s not a modem in the truest sense of the word because a modem changes from digital to analog for transmission. An ISDN TA doesn’t change from digital to analog. It just changes between digital transmission formats.
The box itself is about the size of a modem and looks similar to one. But, as with DSL modems, there is a phone jack and an Ethernet jack. You connect a phone cord from the phone jack to the wall jack where your ISDN services are being delivered. Then you connect an Ethernet cable from your PC to the ISDN TA’s Ethernet jack. Older, less-capable TAs used an EIA/ TIA-232 serial port for PC connectivity.
3.Wireless Access Points (WAPs)
A wireless access point (WAP) allows mobile users to connect to a wired network wirelessly via radio frequency technologies. WAPs also allow wired networks to connect to each other via wireless technologies. Essentially, they are the wireless equivalent of a hub or a switch in that they can connect multiple wireless (and often wired) devices together to form a network.
One of the most popular use for wireless access points is to provide Internet access in public areas, like libraries, coffee shops, hotels, and airports. WAPs are easy to set up; most often, you just need to plug them in to a wired network and power them up to get them to work. Plus, without the clutter or added expense of cables to hook them up, they make ideal foundations for small business networks.
The Channel Service Unit/Data Service Unit (CSU/DSU) is a common device found in equipment rooms when the network is connected via a T-series data connection or other digital serial technology (e.g., a T1 or Digital Data Server [DDS]). It is essentially two devices in one that are used to connect a digital carrier (the T-series or DDS line) to your network equipment (usually to a router). The Channel Service Unit (CSU) terminates the line at the customer’s premises. It also provides diagnostics and remote testing, if necessary. The Data Service Unit (DSU) does the actual transmission of the signal through the CSU. It can also provide buffering and data flow control.
Both components are required if you are going to connect to a digital transmission medium, such as a T1 line. Sometimes, however, one or both of these components may be built into a router. If both components are built into a router, you only have to plug the T1 line directly into the router. Otherwise, some Physical Layer specification, like V.35 or HSSI, will have to be used to cable the interface on the router to the external CSU/DSU.
5.Transceivers (Media Converters)
Another small device that is commonly seen on a network is the external transceiver (also known as a media converter). These are relatively simple devices that allow a NIC or other networking device to connect to a different type of media than it was designed for. Many NICs have special connectors that will allow this, as do hubs and switches.
For example, if you have a 100Base-TX switch and would like to connect it to another switch using fiber-optic cabling, you would connect a fiber transceiver to each switch’s transceiver port and then connect the two transceivers together with the appropriate fiber-optic cabling.
With early Ethernet-style DB-15 female Digital-Intel-Xerox (DIX, or more commonly Attachment Unit Interface [AUI]) NIC interfaces, which are still available as medium-independent connectors on more advanced NICs and other networking devices, an external transceiver has to be used to convert the electrical signal from the device to one that is compatible with the cabling medium. Every other popular type of Ethernet technology, such as the xBase-T standards, has a built-in transceiver on the NIC card or device interface. An external transceiver is necessary with these technologies only to act as a media converter.
A firewall is probably the most important device on a network if that network is connected to the Internet. Its job is to protect LAN resources from attackers on the Internet. Similarly, it can prevent computers on the network from accessing various services on the Internet. It can be used to filter packets based on rules that the network administrator sets. These rules state what kinds of information can flow into and out of a network’s connection to the Internet.
Firewalls can be either stand-alone “black boxes,” or can be set up in software on a server or router. Either way, the firewall will have at least two network connections: one to the Internet (known as the “public” side), and one to the network (known as the “private” side). Sometimes, there is a third network port on a firewall. This port is used to connect servers and equipment that can be considered both public and private (like web and e-mail servers). This intermediary network is known as a demilitarized zone, or DMZ.
Firewalls are the first line of defense for an Internet-connected network. If a network was directly connected to the Internet without a firewall, an attacker could theoretically gain direct access to the computers and servers on that network with little effort.
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