NETWORK DEVICES

NETWORK DEVICES

Networks require devices to provide connectivity and functionality. Understanding how these networking devices operate and identifying the functions they perform are essential skills for any network administrator.

Hubs

Hubs are used in networks that use twisted-pair cabling to connect devices. Hubs can also be joined together to create larger networks. Hubs are simple devices that direct data packets to all devices connected to the hub, regardless of whether the data package is destined for the device. This makes them inefficient devices and can create a performance bottleneck on busy networks. In its most basic form, a hub does nothing except provide a pathway for the electrical signals to travel along. Such a device is called a passive hub. Far more common nowadays is an active hub, which, as well as providing a path for the data signals, regenerates the signal before it forwards it to all of the connected devices. A hub does not perform any processing on the data that it forwards, nor does it perform any error checking. Hubs come in a variety of shapes and sizes. Small hubs with five or eight connection ports are commonly referred to as work group hubs. Others can accommodate larger numbers of devices (normally up to 32). These are referred to as high-density devices. Because hubs don’t perform any processing, they do little except enable communication between connected devices. For today’s high-demand network applications, something with a little more intelligence is required. That’s where switches come in.

Switches

Like hubs, switches are the connectivity points of an Ethernet network. Devices connect to switches via twisted-pair cabling, one cable for each device. The difference between hubs and switches is in how the devices deal with the data that they receive. Whereas a hub forwards the data it receives to all of the ports on the device, a switch forwards it only to the port that connects to the destination device. It does this by learning the MAC address of the devices attached to it, and then by matching the destination MAC address in the data it receives.  Data sent by one node Data is forwarded only to the destination node Switch. By forwarding data only to the connection that should receive it, the switch can improve network performance in two ways.

First, by creating a direct path between two devices and controlling their communication, it can greatly reduce the number of collisions on the network. As you might recall, collisions occur on Ethernet networks when two devices attempt to transmit at exactly the same time. In addition, the lack of collisions enables communicates with devices in full-duplex mode. In a full-duplex configuration, devices can send and receive data from the switch at the same time. Contrast this with half-duplex communication, in which communication can occur in only one direction at a time. Full-duplex transmission speeds are double that of a standard, half-duplex, connection. So, a 10Mbps connection becomes 20Mbps, and a 100Mbps connection becomes 200Mbps. The net result of these measures is that switches can offer significant performance improvements over hub-based networks, particularly when network use is high. Irrespective of whether a connection is at full or half duplex, the method of switching dictates how the switch deals with the data it receives. 

Bridges

Bridges are used to divide larger networks into smaller sections. They do this by sitting between two physical network segments and managing the flow of data between the two. By looking at the MAC address of the devices connected to each segment, bridges can elect to forward the data (if they believe that the destination address is on another interface), or block it from crossing (if they can verify that it is on the interface from which it came).

When bridges were introduced, the MAC addresses of the devices on the connected networks had to be entered manually, a time-consuming process that had plenty of opportunity for error. Today, almost all bridges can build a list of the MAC addresses on an interface by watching the traffic on the network. Such devices are called learning bridges because of this functionality.

Types of Bridges

Three types of bridges are used in networks:

➤ Transparent bridge—Derives its name from the fact that the devices on the network are unaware of its existence. A transparent bridge does nothing except block or forward data based on the MAC address.

➤ Source route bridge—Used in Token Ring networks. The source route bridge derives its name from the fact that the entire path that the packet is to take through the network is embedded within the packet.

➤ Translational bridge—Used to convert one networking data format to another; for example, from Token Ring to Ethernet and vice versa. Data not destined for a device on the other network is prevented from passing over the bridge.

Router

It is derives its name from the fact that it can route data it receives from one network onto another. When a router receives a packet of data, it reads the header of the packet to determine the destination address. Once it has determined the address, it looks in its routing table to determine whether it knows how to reach the destination and, if it does, it forwards the packet to the next hop on the route. The next hop might be the final destination, or it might be another router.

Routing tables play a very important role in the routing process. They are the means by which the router makes its decisions. For this reason, a routing table needs to be two things. It must be up-to-date, and it must be complete. There are two ways that the router can get the information for the routing table—through static routing or dynamic routing.

Network Cards

Network cards, also called Network Interface Cards, are devices that enable computers to connect to the network. When specifying or installing a NIC, you must consider the following issues:

 ➤ System bus compatibility—If the network interface you are installing is an internal device, bus compatibility must be verified. The most common bus system in use is the Peripheral Component Interconnect (PCI) bus, but some older systems might still use Industry Standard Architecture (ISA) expansion cards.

 ➤ System resources—Network cards, like other devices, need IRQ and memory I/O addresses. If the network card does not operate correctly after installation, there might be a device conflict.

 ➤ Media compatibility—Today, the assumption is that networks use twisted-pair cabling, so if you need a card for coaxial or fiber-optic connections, you must specify this.

 To install or configure a network interface, you will need drivers of the device, and might need to configure it, although many devices are now plug and play. Most network cards are now software configured. Many of these software configuration utilities also include testing capabilities. The drivers and software configuration utilities supplied with the cards are often not the latest available, so it is best practice to log on to the Internet and download the latest drivers and associated software.

ISDN Adapters

Integrated Services Digital Network (ISDN) is a remote access and WAN technology that can be used in place of a Plain Old Telephone Service (POTS) dial-up link if it is available. The availability of ISDN depends on whether your local telecommunications service provider offers the service, the quality of the line to your premises, and your proximity to the provider’s location.

ISDN offers greater speeds than a modem and can also pick up and drop the line considerably faster. If ISDN is available and you do elect to use it, a special device called an ISDN terminal adapter is needed to connect to the line. ISDN terminal adapters can be add-in expansion cards, external devices that connect to the serial port of the system, or specialized interfaces built in to routers or other networking equipment.

The ISDN terminal adapter is necessary because, although it uses digital signals, the signals are formatted differently from those used on a LAN. In addition, ISDN can create multiple communication channels on a single line. Today, ISDN is not widely deployed and has been replaced by faster and often cheaper technologies.

Wireless Access Points

Wireless access points (WAPs) are a transmitter and receiver (transceiver) device used to create a wireless LAN (WLAN). WAPs are typically a separate network device with a built-in antenna, transmitter, and adapter.

WAPs use the wireless infrastructure network mode to provide a connection point between WLANs and a wired Ethernet LAN. WAPs also typically have several ports allowing a way to expand the network to support additional clients.

 A WAP can operate as a bridge connecting a standard wired network to wireless devices or as a router passing data transmissions from one access point to another. Saying that an WAP is used to extend a wired LAN to wireless clients doesn’t give you the complete picture.

Modems

 A modem, short for modulator/demodulator, is a device that converts the digital signals generated by a computer into analog signals that can travel over conventional phone lines. The modem at the receiving end converts the signal back into a format the computer can understand. Modems can be used as a means to connect to an ISP or as a mechanism for dialing up to a LAN.

Modems can be internal add-in expansion cards, external devices that connect to the serial or USB port of a system, PCMCIA cards designed for use in laptops, or proprietary devices designed for use on other devices such as portables and handhelds. The configuration of a modem depends on whether it is an internal or external device.