1.6. Identifying Network Components

As we said, the components that are used in a network have a tremendous effect on the capabilities of the network and on your ability to control traffic within the network. Network components have evolved with time because of the need to create fast, efficient network designs for many computers. In this section we discuss the major types of network components and their effect on your network design.

1.6.1. Critical Information

You should be able to identify the purpose, features, and functions of each of the main network components. In this section, we discuss each of these components in detail.

1.6.2. Hubs

A hub is a device that has multiple ports into which connections can be made. All connections made to a hub are also connected to each other. A hub does not filter any communication or provide any intelligence in regard to the data stream; it simply lets all of the information flow through it and connects anything and everything that is connected to it. Hubs are generally used to connect network segments of computers that are physically close to each other, such as all of the computers on one floor of a building or in a computer classroom. There are two major types of hubs: active and passive. An active hub is generally plugged into a power source so that it can amplify signals as well as connect them. A passive hub provides connectivity but no power. An example of a passive hub is a patch panel in a network closet.

1.6.3. Switches

A switch (also called a LAN switch) resembles a hub from the outside, but that is where the resemblance stops. Switches are considerably more expensive than hubs because of the advantages they offer. Whereas a hub simply lets traffic flow through it, a switch controls traffic through it to automatically optimize traffic flow on your network. A switch learns the physical address (MAC address) of all of the devices that are connected to it and then uses it to control traffic flow. Rather than forwarding all data to all of the connected ports, a switch can forward data only to the port where the computer with the destination address actually exists. This process automatically segments the network and dramatically decreases the traffic in the segments that are less used. Because of this, switches are often used to connect departments of a company so that communication between departments does not affect the other departments that are not involved in the communication. Also, large files can be transferred within in the same department without affecting the traffic in any of the other departments. In addition, switches can be used to create virtual local area networks (VLANs), which improve the flexibility of a network design. (We will discuss VLANs in Chapter 3, "Domain 3 Network Implementation.")

1.6.4. Bridges

Bridges are similar to switches in that they can provide some intelligence to segment a network. Bridges, like switches, can learn the MAC address of each of the hosts connected to them and use that address to control traffic to each of the host's ports. Bridges, however, are slower than switches, so they have been largely replaced by switches as a device that is used to segment traffic. You should be aware of two main types of Ethernet bridges:

Transparent bridge

A transparent bridge can connect two dissimilar networks but is "invisible" to both networks and does not provide translation of any kind. For example, if your network contained two Ethernet segments with one Token Ring segment between them, a transparent bridge could connect communication from one of the Ethernet segments, through the Token Ring segment, to the other Ethernet segment. The Ethernet traffic would not be interpreted by the Token Ring segment.

Translational bridge

A translational bridge, as you might expect, actually performs a translation between two dissimilar networks. For example, if you wanted to translate data from an Ethernet segment to a Token Ring segment, you could use a translational bridge.

1.6.5. Routers

Routers are devices that forward traffic from one network (or subnet) to another. Routers first determine whether the traffic belongs on their network; then they deliver the traffic that belongs on their network to the appropriate network hosts and forward the traffic that does not belong on their network to another router. Routers determine where to forward traffic by consulting a routing table. The routing table can be manually entered by an administrator, or it can be learned by the router by using routing protocols. Routers are the devices that connect the Internet and make the World Wide Web possible. They use a higher level of intelligence than that of bridges or switches. (We will discuss routing in greater detail in Chapter 2, "Domain 2 Protocols and Standards.")

1.6.6. Gateways

Actually, the term gateway refers more to a network role than a network device. Any device that is used to translate data from one format to another can be called a gateway. For example, a router that can route data from an Internetwork Package Exchange (IPX) network to an Internet Protocol (IP) network so that it can be understood by the IP network can be considered a gateway. Gateways can also be special servers whose main job is to translate protocols between two networks, such as a Systems Network Architecture (SNA) gateway between an IBM mainframe network and a Microsoft Windows network. The gateway is always positioned logically between the two networks that it will translate.

1.6.7. CSU/DSU

Channel Service Unit/Data Service Unit (CSU/DSU) is a converter used between a LAN and a WAN. Conversion of information is necessary because the technologies that are used in the WAN environment are very different from those used in a LAN environment. The CSU/DSU changes signal from one digital format to another. Originally, the CSU/DSU was always a separate component, but now many routers have this function built into them.

1.6.8. NICs

Network interface cards (NICs) are used to connect a computer to the network. A network interface card is like a small computer in itself. Its job is to translate a stream serial data (one bit at a time) into several streams of parallel data that will be used by the computer. The network interface card also examines every packet on the network cable to determine if the packet has a destination MAC address that matches its MAC address. If it does not, then the NIC does nothing more with the packet, but if the address does match then the NIC will forward the packet to the appropriate port of the computer based on the information contained in the packet. (We will discuss ports in greater detail in Chapter 2.)

1.6.9. ISDN Adapters

Integrated Services Digital Network (ISDN) is a service that was developed by telephone companies in the early 1990s. It was originally intended to provide individuals with a type of simul-taneous voice and data communications capability. Since the Internet was in its infancy, and the World Wide Web did not exist, ISDN was ahead of its time. Today's networks use ISDN as a backup line for other communication lines that provide greater bandwidth. The device that connects an organization's equipment to the telephone company's ISDN line and converts the signal so that it can be sent over the line is called an ISDN terminal adapter. These devices are often built into the latest network equipment.

1.6.9.1. Wireless Access Point

A wireless access point (WAP) is a hub that is used by wireless devices. Typically, a WAP looks just like any other hub except that it has an antenna. The WAP is connected to the wired network of an organization so that any devices that can make a wireless connection to it will also be connected to the wired network. Wireless devices typically connect to the WAP using the 2.4GHz radio frequency. Since wireless networks are continuing to grow in popularity, you're likely to see many more WAPs. (We will discuss wireless connectivity in greater detail later in this chapter.)

1.6.9.2. Modems

The term modem stands for modulator/demodulator. Modulation is the process of converting digital data into analog data, usually in the form of sound. Demodulation is the reverse of this process, whereby the transmitted sounds are converted back into digital data on the other side of the connection. Modems are typically used to communicate on normal telephone wires using computers. The most common types of modems used on standard telephone lines can communicate at a maximum speed of 56Kbps. It is also possible to use multiple telephone lines simultaneously to increase the speed of communication. A simple 56K modem is built into most of the computers sold today.

1.6.9.3. Transceivers

The term transceiver stands for transmitter/receiver. The transceiver is more of a role in a network than a specific device. Any device that receives data, converts it, and then sends it to another location can be called a transceiver. For example, an NIC is a type of transceiver since it converts serial data into parallel data and then sends it to a port.

1.6.9.4. Firewalls

A firewall is a hardware or software system that is used to separate one computer or network from another one. The most common type of firewall is used to protect a computer or an entire network from unauthorized access from the Internet. Firewalls can also be used to control the flow of data to and from multiple networks within the same organization. Firewalls can be programmed to filter data packets based on the information that is contained in the packets.

1.6.9.5. Exam Essentials

Know the major network components. The major components in today's networks include hubs, switches, bridges, routers, gateways, CSU/DSUs, NICs, ISDN Adapters, WAPs, modems, transceivers, and firewalls. You should be able to describe the purpose of each network component and its role in the network.

Be able to describe the functionality of each network component. You should know how each of the major network components function. For example, you should know whether a component has any intelligence or makes any decisions about the traffic flow within a network. You should be able to differentiate between the types of network components and their capability to control network traffic.

List the characteristics of a hub. A hub is a device with multiple ports that is used to connect devices on a network. A hub only connects devices and does not provide any intelligence or filtering. Two types of hubs are generally used in networks: active hubs (which also serve as multiport repeaters) and passive hubs (which are generally called patch panels).

Know the characteristics of a switch. Switches (also called LAN switches) resemble hubs on the outside but perform filtering that hubs cannot handle. The filtering that is performed by the switches is based on the MAC address of the devices that are connected through the switch. The switches build a table of the MAC addresses and then use it to segment network traffic. Switches can be used to create VLANs, which can improve the flexibility of network design.

Describe the characteristics of a bridge. Bridges are similar to switches in that they filter network traffic based on the MAC addresses of devices that are connected through them. Bridges can be used to connect dissimilar networks. The two main types of bridges are transparent and translational.

List the characteristics of a router. Routers are devices that forward traffic from one network to another. Routers use a logical address and a routing table to determine whether network traffic belongs on their network or on another network. The routing table can be entered by an administrator or it can be learned by using routing protocols.

Describe the characteristics of a gateway. The term gateway refers to a network role of translating data from one format or protocol to another. The gateway can be only one of many roles on a device, or the device can be dedicated to that one role. One of the most common gateways is the IBM SNA gateway, which connects a mainframe or minimainframe computer to a network environment.

Know the characteristics of a CSU/DSU. A CSU/DSU is a converter used between a LAN and a WAN environment. This conversion is necessary because of the difference in the technologies that are used in the two environments. The CSU/DSU was originally a separate device, but now many routers have this function built into them.

List the characteristics of an NIC. A network interface card (NIC) is used to connect a computer to the network and facilitate communication. The NIC converts serial and parallel data to and from the computer and is also responsible for examining every packet of a data stream to determine whether that packet has a destination address that matches its own address. If the address matches, then the NIC receives the information and sends it to the correct port in the computer. If it does not match, then the NIC simply ignores the packet and does not process it any further.

Know the characteristics of an ISDN adapter. ISDN adapters were originally developed by the telephone company to convert signals so that telephone and computer equipment could use their ISDN lines. Today's networks sometimes use ISDN but the adapters are often built into the network equipment.

Describe the characteristics of a wireless access point. A wireless access point (WAP) is a hub that is used by wireless devices to connect them to a wired network. Wireless devices generally use a wireless NIC to connect to the WAP, which in turn provides a connection to a network or to the Internet. Wireless technology is growing in popularity.

Know the characteristics of a modem. You should know that the term modem stands for modulator/demodulator because the purpose of a modem is to convert digital signals to analog signal in the form of sounds (modulation) and to convert the sounds into the form of digital signals on the other end (demodulation). The 56K modem is the most commonly used modem on regular telephone lines today. A 56K modem in generally built into most desktop and laptop computers that are sold today.

List the characteristics of a transceiver. The term transceiver stands for transmitter/receiver. A transceiver is more of a role for a device than a device itself. Any device that can receive data, convert it, and then transmit it can be considered a transceiver.

Identify the characteristics of a firewall. A firewall is a hardware or software system that is used to separate one computer network from another. Firewalls are often used to protect a computer network from the Internet and can be programmed to filter packets based on the information that is contained in the packets.