This chapter provides a study guide for the Network+ Exam N10–003. Various sections in this chapter are organized to cover the related objectives of the exam. Each section identifies the exam objective, provides an overview of the objective, and then discusses the key details that you should grasp before taking the exam.
An overview of this chapter's sections is as follows:
- Media and Topologies
This section covers the basics of network media, networking standards, and topologies, as well as offers a brief description of networking devices. It also covers both wired and wireless networks.
- Protocols and Standards
This section covers the Open System Interconnect (OSI) networking model, networking protocols, and services. Also included in this section is a description of wireless technologies and Internet access methods.
- Network Implementation
This section includes a study of network operating systems, their interoperability, and methods of implementing security in wired and wireless networks. Remote access, intranets, extranets, fault tolerance, and disaster recovery are also covered in this section.
- Network Support
This section includes a study of concepts related to troubleshooting methods and utilities for different operating systems and topologies. Also discussed in this section are the effects of adding/removing network services and client connectivity problems.
The sections in this chapter are designed to follow the exam objectives as closely as possible. This Study Guide should be used to reinforce your knowledge of key concepts tested in the exam. If you study a topic and do not understand it completely, I recommend that you go over it again and memorize key facts until you feel comfortable with the concepts.
Studying for the Network+ certification requires that you have access to a computer network. Although it is not essential, it is good to have a Windows- or Unix/Linux-based computer network in order to get familiar with the concepts covered in this Study Guide. Identification of network media, cables, and connectors is required as part of your preparation for the exam. A small network with a Windows XP desktop and a Windows or Unix/Linux server would serve the purpose. Needless to say, you will also need an active Internet connection, just in case you need to search for more information on any topic.
Tip
This chapter contains a number of terms, notes, bulleted points, and tables that you will need to review multiple times. Pay special attention to new terms and acronyms—those you are not familiar with—as these may be tested in the exam.
Networking standards are the basis of any network implementation. Every network, small or large, is based on a networking topology and might use one or more types of cables. Each networking standard defines a certain physical layout of the components of the network. These include servers, desktops, printers, network devices, cables, and connectors. Network administrators have to decide on a networking topology and cabling before chalking out a network plan. For network technicians, a thorough understanding of networks, network standards, topologies and media is essential for keeping the network functional. This section covers a brief description of essential components of any network, media, and topologies.
A computer network refers to two or more computers linked together to share files, printers, and other resources. The computers may be linked through cables, telephone lines, satellite, radio frequencies, or Infrared beams. The network may be as small as just two or more computers linked together at home or in an office, or as big as a corporate network at multiple locations spanning across the globe. The following sections describe different types of networks and the concept of centralized and decentralized computing.
A local area network is a network of computers joined together in a local area such as a small office, a home, or a building. The area covered by a LAN is usually restricted to a single location. The function of a LAN is to provide high-speed connectivity to all computers and network devices. The data transfer speed achieved in a LAN is significantly higher than its counter part, the wide area network (WAN). Figure 8-1 shows a local area network.
A wide area network is a network that connects two or more local area networks. A WAN typically connects separate LANs at different geographic locations. A third party such as an Internet service provider (ISP) or a local telephone company is responsible for providing the required dedicated hardware and/or connectivity lines to implement a WAN. These hardware devices include modems or routers that are required to connect the local LANs to the service provider's network. Figure 8-2 shows a wide area network.
Unlike the name suggests, a personal area network may or may not belong to a single person. The term PAN refers to a network of devices located in close proximity of each other. The devices may include such items as computers, PDAs, or mobile phones, that are connected using a wireless or a wired network. A mobile phone connected to a computer, or a few laptops connected to each other in an ad-hoc fashion are examples of personal area networks. Similarly, two or more computers sharing an Internet connection in a home network is another example of a PAN.
A metropolitan area network is a large internetwork connecting local area networks in a campus or inside the boundaries of one city. The MANs are usually connected using high-speed fiber optic cables. Metropolitan Area Networks can further be connected to form wide area networks.
In a centralized computing model, all processing is done on a central computer. This computer provides data storage as well as controls all peripherals including the clients. Clients are called dumb terminals and are attached to the central computer. This model provides greater security since all functions are controlled from one location. The disadvantage is that it can significantly slow processing, and if the central computer breaks down, the entire system breaks down. The client/server networking model is an example of centralized computing.
In a decentralized computing model, all processing and resources are distributed among several computers, thereby increasing performance and minimizing breakdown of the system. All systems can run independent of each other. A peer-to-peer network is an example of a decentralized computing model.
In a peer-to-peer network, every computer is responsible for processing applications, storing data, and controlling access to its resources. A P2P network is also known as a workgroup. These networks are suitable for a small number of computers only. As the network grows, the administration of resources becomes difficult. For this reason, peer-to-peer networks are not suitable for large networks. The following are some characteristics of P2P networks:
These networks are suitable for only about 10 computers.
They are cost-effective compared to the client/server model.
A network operating system (NOS) does not need to be installed on any computer.
An administrator is not required, and each user is responsible to manage resources on her computer.
These networks are not considered secure because each user individually maintains security of resources on her computer.
In a client/server network model, a centralized server usually holds control of all system and network resources located across the network. These include network services, storage, data backup, security management, and access control. The network consists of dedicated servers and desktops (clients). Servers run network operating systems, such as Windows Server 2000/2003 or Unix/Linux, and the desktops run client operating systems, such as Windows XP. Most modern network environments use the client/server computing model. Some characteristics of client/server networks are shown next.
This model is scalable to very large-scale internetworks.
Skilled administrators are required to manage the network.
Dedicated server and network hardware may be required, which increases the cost of ownership.
Security of the resources can be effectively maintained from a centralized point.
A network topology describes the physical and logical layout of the network components. A physical network topology refers to the actual layout of computers, cables, and other networking devices. The network topology is determined by the connections between different components. A logical topology refers to the communication methods used by different components. The Network+ Exam covers the commonly used physical topologies: star, bus, mesh, ring, and wireless, described in the following sections.
In a star topology, computers (also called nodes) connect to each other through a central device, called a hub or a switch. Since each device is connected independently to the central device using a separate cable, the star network can be expanded at any time without affecting the operation of the network. Failure of one or more nodes also does not affect the network operation. The central device becomes the single point of failure because all nodes are connected to it. This topology is easy to implement, and its cost depends on the type of central device as well as the type of cable used to connect nodes. Figure 8-3 shows a star network, and the advantages and disadvantages are described next.
- Advantages
A star network is easy to implement.
It can be easily expanded without affecting the network operation.
Failure of a single node or the connecting cable does not affect the entire network's operation.
It is easy to isolate nodes in order to troubleshoot problems.
- Disadvantages
Failure of the central device (hub or switch) can bring down the entire network.
The length of cable required is much more than ring and bus networks because each node is connected separately.
Cable length from the central device can be a limiting factor, depending on the type of cable used.
In a bus topology, all computers are connected to a shared communication line, called a trunk or a backbone. The computers are connected to the backbone using T-connectors. Both ends of the backbone use terminators in order to prevent reflection of signals. If the terminator is missing or is deliberately removed, the data transmissions are disrupted. There is no central device or any special configuration. Figure 8-4 shows a bus network, and the advantages and disadvantages are described next.
- Advantages
A bus network is the cheapest of all topologies.
No special configuration is required.
It is easy to install, and no special equipment is needed for installation.
It needs less cable length than do other topologies.
- Disadvantages
A break in cable or a missing terminator can bring down the entire network.
It is not possible to add or remove computers without disrupting the network.
It is difficult to troubleshoot and administer.
Addition of more computers degrades performance.
In a mesh topology, all computers in the network are connected to every other computer, forming a mesh of connections. Each computer makes a point-to-point connection to every other computer. This makes the network highly fault tolerant and reliable, as a break in the cable or a faulty computer does not effect network operation. Ad-hoc wireless networks fall into this category, as each connection is independent of the other. Data can travel from one computer to another using a number of paths. With the exception of wireless networks, mesh networks are very expensive in terms of the length of cable required to create multiple redundant connections. Figure 8-5 shows a mesh network, and the advantages and disadvantages are described next.
- Advantages
A mesh network is highly reliable because of redundant multiple paths between computers.
The failure of a single computer or a cable fault does not affect network operations.
Computers can be added or removed without affecting the network.
- Disadvantages
It is difficult to install and troubleshoot.
It is very expensive because of the length of cable required to make multiple redundant connections.
Only a limited number of computers can be connected in a mesh topology.
In a ring topology, each computer is connected to its neighboring computer to form a logical ring. Data travels in the ring in a circular fashion from one computer to another, forming a logical ring. If one of the computers in the ring fails or if the cable is broken, the entire network becomes inaccessible. The addition or removal of computers also disrupts network transmissions. Ring networks are less efficient than star networks because of the fact that data must pass through each computer on the way to the destination. The physical layout of a ring network actually forms a star network. In a Token Ring network, a MultiStation Access Unit (MSAU), or Media Access Unit (MAU) acts as the central device or hub to process circulation of a special data packet called a Token. The MSAU has Ring In (RI) and Ring Out (RO) ports that facilitate connection of one MSAU to another MSAU for expanding the network. The last MSAU is connected to the first MSAU to complete the ring. Figure 8-6 shows a ring network, and the advantages and disadvantages are described next.
- Advantages
A ring network is relatively easy to install.
There are fewer collisions because only one computer transmits at a time.
- Disadvantages
A break-in cable or a faulty computer can bring down the entire network.
It is not as efficient as a star network.
It is difficult to troubleshoot a ring network.
The addition or removal of computers can disrupt network operation.
A wireless network connects two or more computers without using cables. To communicate with each other, these networks use spread spectrum technology, which is based on radio frequencies. Each device in the network is equipped with a wireless network adapter and is called a station. The area of communication is limited and is known as the basic service set. Wireless stations or clients can freely move within the basic service set. A wireless network can further be connected to a wired network with the help of wireless access Points (AP). The IEEE 802.11 standards define two main configurations of wireless communications: Ad-hoc and Infrastructure.
An Ad-hoc wireless network is also known as a peer-to-peer or an unmanaged wireless network. Two or more computers directly communicate to each other without using an access point. There is no central device (or hub), and these networks can be created spontaneously anywhere when two or more network devices fall within the range of each other. It provides the fastest way to temporarily connect computers and share resources. For example, two or more laptop computers can be connected in a conference room or in a cafeteria. Figure 8-7 shows an ad-hoc network.
In an Infrastructure configuration, a central wireless device known as the access point (AP) is used to authenticate and configure wireless clients that fall within its range. Wireless clients communicate to each other through the AP. A special identifier known as a Service Set Identifier (SSID) must be configured on the AP and on each wireless client. All clients in one Infrastructure network use the same SSID. Different Infrastructure networks are identified by their unique SSIDs. The AP can further be connected to the wired local area network so that wireless clients can access the wired LAN also. Figure 8-8 shows an infrastructure wireless network.
The Institution of Electrical and Electronics Engineers (IEEE) has defined standards for local area networks, metropolitan area networks, and wireless LANs as the IEEE 802 standards. The IEEE 802 standards describe the operation of networking protocols, services, devices, and media at the two lowermost layers of the seven-layer OSI reference model: the Data Link and Physical layers. (The OSI model is discussed later in this section.) The Data Link layer is further divided into two layers: the Logical Link Control (LLC) layer and the MAC layer. Table 8-1 lists various standards in the IEEE 802 family.
Table 8-1. The IEEE 802 family of networking standards
Standard | Description |
|---|---|
802.1 | Defines higher-level standards for internetworking. |
802.2 | Defines Logical Link Control (LLC). |
802.3 | Defines Ethernet networks using Carrier Sense Multiple Access/Collision Detection (CSMA/CD). |
802.4 | Defines Token Bus networks. |
802.5 | Defines Token Ring networks. |
802.6 | Defines Metropolitan Area Networks (MANs). |
802.7 | Technical advisory group for broadband LAN using coaxial cabling. This group is now disbanded. |
802.8 | Technical advisory group for fiber optic. This group is now disbanded. |
802.9 | Technical advisory group for integrated services. This group is now disbanded. |
802.10 | Defines interoperable security for LAN/MAN. |
802.11 | Defines wireless networks. |
802.12 | Defines Demand Priority networks using 100 Mbps or more speeds. including the 100BASEVG-AnyLAN (Hewlett-Packard). |
Each of the standards listed in Table 8-1 defines different characteristics of the network, such as network access method, topology, speed, and type of cabling.
The 802.2 standard describes how the upper-layer protocols access the Logical Link Control (LLC), which is the upper layer of the two Data Link layers in the OSI model. This standard defines how different protocols manage the error control and data flow control. Error control refers to detection and retransmission of dropped packets, if requested. Flow control refers to management of data flow between network devices so that they can efficiently handle flow of information.
The IEEE 802.3 standard describes characteristics for Ethernet networks at the Physical layer and at the MAC sublayer of the Data Link layer. This is a whole family of standards that define Ethernet networks with a variety of speeds and cabling. The IEEE 802.3 family of standards is collectively known as 802.3x standards.
- Speed
The original IEEE 802.3 standard defined a speed of 10 Mbps over thin coaxial cable in Ethernet networks. With the Fast Ethernet standard 802.3u, the speed can go up to 100 Mbps. The 802.3z standard defines Gigabit Ethernet with a speed of up to 1000 Mbps.
- Access method
The access method defines the process for network devices to access network media. Ethernet networks use the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) method. Devices on the network continuously monitor the network media. If two devices start the transmission simultaneously, data collision occurs. If a collision occurs, the sending device is required to wait for a specified time before it can retransmit.
- Topology
Original Ethernet networks could be wired using either the star or the bus topology using coaxial or twisted pair cables. IEEE 802.3u and 802.3z use only star topology with twisted pair cables.
- Media
Media refers to the physical cabling of the network. A variety of cables types can be used with IEEE 802.3x standards including coaxial, twisted pair, and fiber optic. The choice of cables mainly depends on the specific standard used in the network.
The IEEE 802.5 standard defines characteristics for Token Ring networks, originally developed by IBM. Token Ring is a LAN protocol that works at the Data Link layer of the OSI model. The Token Ring technology is rarely used these days because of the popularity of Ethernet networks. Even IBM no longer supports networks based on Token Ring technology. The characteristics of the IEEE 802.5 standard are as follows:
- Speed
The transfer speed of IEEE 802.5 Token Ring networks is 4 Mbps and 16 Mbps.
- Access method
Token Ring networks use the Token Passing access method. This uses a special three-byte frame known as a token that travels around the ring. The token keeps looking for a device on the ring that needs to transmit data. The device must acquire the token before it can transmit data on the network. Only one device can possess the token and transmit data at a time. The token travels with the data to the destination device where it is detached from the data and becomes free.
- Topology
The physical setup of a Token Ring network is a star, while the logical setup is in a ring topology. A central device known as Multi-Station Access Unit (MSAU or MAU) is used to create a physical star topology.
- Media
The IEEE 802.5 standard defines the use of unshielded twisted pair (UTP) and shielded twisted pair (STP) cables.
The IEEE 802.11 family of standards defines several protocols used for wireless communications. This standard defines all aspects of wireless communications from the frequency range specifications to physical layouts to authentication mechanisms. The original IEEE 802.11 standard is known as legacy 802.11. The characteristics of the IEEE 802.11 standard are as follows:
- Speed
The data transfer speed defined in the legacy 802.11 standard was limited to 1 or 2 Mbps within the frequency range of 2.4 GHz. Speeds for other 802.11 standards are discussed later in this section.
- Access method
Wireless networks use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA), which is a variation of the CSMA/CD access method. The devices on the wireless network "listen" to the network for "silence" before they start transmission. This helps avoid collisions on the network media.
- Topology
Wireless networks defined in IEEE 802.11 standards can be implemented in either Ad-hoc or Infrastructure topology as discussed earlier in this section.
Wireless networks defined in IEEE 802.11 standards use radio frequencies with spread spectrum technology: frequency-hopping spread spectrum (FHSS) or directsequence spread spectrum (DSSS). Spread spectrum technologies are discussed later in this section. The most popular of the IEEE 802.11 wireless network standards are 802.11b, 802.11a, and 802.11g. Security standards for these protocols are defined in the 802.11i standard.
The IEEE 802.11b standard defines DSSS-based network devices that use a 2.4 GHz frequency range and can communicate at speeds of 1,2, 5.5, or 11 Mbps. This standard is compatible with the legacy 802.11 standard. 802.11b is designed for a point-to-multipoint wireless communication setup. Usually a wireless access point (AP) is used with an omni-directional transmission antenna and can communicate with wireless clients located in the coverage area around the AP. The indoor range of a 802.11b wireless AP is about 100 feet (30 meters) at 11 Mbps speed When used with 1 Mbps speed, the range can be as high as 300 feet (90 meters).
The IEEE 802.11a standard uses a 5GHz frequency range with up to 54 Mbps data transmission speed. This standard defines the use of 52-subcarrier Orthogonal Frequency-Division Multiplexing (OFMD), which is a modulation technique. (Modulation techniques are covered later in this section.) If required, the data speed can be reduced to 48, 36, 24, 18, 16, 12, 9, and 6 Mbps. The IEEE 802.11a standard is not backward-compatible with the 802.11b standard. The range for 802.11a-based devices is also about 100 feet (30 meters) when used indoors.
The IEEE 802.11g standard defines a frequency range of 2.4 GHz (same as 802.11b) but with much higher data transfer speeds of up to 54 Mbps. The data speed can fall back to lower values. IEEE 802.11g is backward-compatible with 802.11b standard devices. The devices normally use the OFDM modulation technique but can switch back to Quadrature Phase-Shift Keying (QPSK) modulation when the data speed falls back to 5.5 or 11 Mbps. Since it operates in the already crowded frequency range of 2.4 GHz, the 802.11g device is also susceptible to interferences such as the 802.11b devices.
Table 8-2 gives a brief comparison of the characteristics of different 802.11 standards.
The FDDI networking standard is based on Token Ring topology and describes the use of dual rings in order to provide fault tolerance to the network. It uses fiber optic cables, and the length of a single cable segment can be more than 200 Km. A variation of FDDI exists that uses copper wires and is called the Copper Distributed Data Interface (CDDI). CDDI uses the same protocols as FDDI. The characteristics of the FDDI standard are as follows:
- Speed
FDDI networks can achieve a maximum data transfer speed of up to 100 Mbps.
- Access method
Since this topology is based on Token Ring, the devices use the token passing method to access network media
- Topology
FDDI is based on dual ring topology that provides fault tolerance.
- Media
As the name suggests, FDDI uses fiber optic cables.
The cables used for computer networks fall into three main categories: Coaxial, Twisted Pair, and Fiber Optic. Each of the cable types has its own merits and demerits in terms of their cost, installation, maintenance, and susceptibility to interferences. Coaxial cables are rarely used these days because of the vast popularity gained by twisted pair cables. The following sections discuss each of the cable types covered in the Network+ exam.
Coaxial cables are mainly used for carrying television signals (for example, CATV), but some older computer networks based on the 10Base2 standard also utilized these cables for connecting workstations and other network devices. Usually the coaxial cables used for different purposes have different characteristics; cables for one purpose cannot be used for another. For example, the cable used for CATV cannot be used for computer networks. Figure 8-9 shows a piece of coaxial cable.
Coaxial cable networks are easy to install and low in cost. The downside is that they are prone to degradation of signals as they travel long distances. This degradation is called attenuation. They can also break easily and cause network downtime. Coaxial cables fall mainly into the following two categories:
- Thin coaxial cable
Also known as Thinnet. The type of thin coaxial cable used for computer networks is RG-58, which has 50-Ohm resistance. Network segments using this cable are used with 50-Ohm terminators and devices are connected using 50-Ohm BNC-T connectors. The RG-6 type coaxial has 75-Ohm resistance and is used for CATV and cable modem.
- Thick coaxial cable
Also known as Thicknet. The type of thick coaxial cable used for computer networks is RG-8. As the name suggests, this cable is about twice as thick in diameter as thin coaxial cable. These cables use vampire taps, which cut through the cable to provide connectivity to network devices. Vampire Taps use transceivers with a 15-pin AUI connector. Thick coaxial cables also use 50-Ohm terminators on both ends of the network segment.
Twisted pair cables have replaced coaxial cables in most computer networks. These cables use twisted pairs of insulated cables bundled inside a plastic sheath. The twists in cables are used to prevent electromagnetic interference, which results in crosstalk, among cables. Twisted pair cables are easy to install, lower in cost than coaxial and fiber optic cables, and can achieve greater data transmission speeds than coaxial cables. These cables are usually identified by their category numbers. The category number indicates the number of cable pairs and the purpose for which they can be used. These category numbers are denoted as CAT-1, CAT-2, CAT-3, CAT-5, etc. Figure 8-10 shows a piece of twisted pair cable.
UTP cables are the most commonly used of the two types of twisted pair cable categories. UTP cables are inexpensive and easy to install and maintain. These cables are vulnerable to electromagnetic interferences (EMI) and radio frequencies interferences (RFI) and hence cannot carry data signals to longer distances. Electric or electronic equipment and high-voltage electric cables in the vicinity of these cables can cause significant disturbances.
STP cable comes with a layer of shielding material between the cables and the sheath. STP cables provide some degree of protection from EMI and RF disturbances and can carry signals to greater distances. But this advantage comes with the extra cost of installation.
Table 8-3 lists some of the popular UTP and STP categories.
Table 8-3. Categories of UTP and STP cables
Category | Description |
|---|---|
CAT-1 | Used for voice transmissions; not suitable for data transmissions. |
CAT-2 | Used for voice and low speed data transmissions up to 4 Mbps. |
CAT-3 | Used for both voice and data transmissions. Used in Ethernet, Fast Ethernet, and Token Ring networks. It is rated at 16 MHz and 10 Mbps speed. |
CAT-4 | Used for both voice and data transmissions. Rated at 20 MHz and 16 Mbps speed. Used in Ethernet, Fast Ethernet, and Token Ring networks. |
CAT-5 | Used for both voice and data transmissions. Rated at 100 MHz. Used in 100 Mbps Ethernet, 1000BaseT Fast Ethernet, Token Ring, and 155 Mbps ATM networks. |
CAT-5e | Used for 100 Mbps and 1000 Mbps Gigabit Ethernet networks. Rated at 125 MHz. |
CAT-6 | Used for both voice and data transmissions. Rated at 250 MHz. Used in Ethernet, Fast Ethernet, Token Ring, and 155 Mbps ATM networks. |
CAT-6 (STP) | Used for data transmissions. Supports up to 600 MHz and used in Ethernet, Fast Ethernet, Gigabit Ethernet, Token Ring, and 155 Mbps ATM. |
Fiber optic (also called Optical Fiber) cable is made up of very thin glass or plastic stretched out and put inside a sheath. The transmission in fiber optic cables is based on the transport of light signals. An optical transmitter is located at one side of the cable and a receiver is at the other side. Fiber optic cables are immune to EMI and RF disturbances because they depend on optical signals unlike electrical signals in UTP/STP cables. They can also carry data signals longer distances than do UTP or STP cables due to minimal attenuation. It is also considered the most secure of all cable types.
Fiber optic cables are very expensive in terms of the cost involved in installation and maintenance. It needs expensive hardware, skilled technicians, and special tools for installation. This is the reason that fiber optic cable is used only in data centers for providing high-end connections to critical servers and other network devices where high-speed data transfers are required. Figure 8-11 shows a piece of fiber optic cable.
The two main types of fiber optic cables are single mode and multimode.
The single mode fiber optic cable is made up of a core glass or plastic fiber surrounded by a cladding. It uses a single beam of light and can thus travel to greater distances than a multimode fiber optic cable. Single mode fiber optic cable uses an 8 to 10 micron core and 125 micron cladding. Figure 8-12 shows a typical single mode fiber optic cable.
The multimode fiber optic cable is made up of a 50 micron or a 62.5 micron core and 125 micron cladding. In this cable, multiple beams of light travel through the core and are reflected by the cladding. Some of the beams even get refracted into the cladding, causing loss of signal. This reduces the distance that data signals can travel in a multimode fiber optic cable.
Ethernet networking and cabling are defined in IEEE 802.3 standards. There are several variations in this standard, depending on speed, length, topology, and cabling used in implementing networks. The following sections provide a brief summary of the standards tested on the Network+ exam.
The 10 Mbps standards include 10Base2, 10BaseT, and 10BaseFL. All these standards define a maximum data transfer speed of 10 Mbps. This speed is now considered obsolete for most networks. It is unlikely that you will encounter any 10 Mbps networks in your career. The following are different variations of 10 Mbps networks:
- 10Base2
The IEEE 802.3a standard defines 10Base2 Ethernet networks. This standard defines the use of RG-58 coaxial cabling with a maximum segment length of 185 meters. The network can achieve a maximum speed of 10 Mbps. The segments are typically wired in physical bus topology using BNC connectors, and each end of the cable must be terminated using 50-Ohm terminators. The 10Base2 standard allows a maximum of five segments, out of which only three segments can be populated. There should be a minimum distance of 0.5 meter between nodes.
- 10BaseT
The 10BaseT Ethernet standard defines use of CAT 3, 4, or 5 UTP cables with a maximum of 100 meters for each cable length. All computers (nodes) are connected in a point-to-point fashion to a central device known as the hub or the switch. These devices can further be cascaded to extend the network. It is typically wired in a physical star topology that makes it easy to add or remove nodes without affecting the network. 10BaseT also allows a maximum of five network segments in each network. The five segments can be connected using four repeaters. Unlike the 10Base2 networks, all five segments can be populated.
- 10BaseFL
The 10BaseFL Ethernet standard uses fiber optic cables in order to increase the cable segment lengths to 2000 meters. It is also wired in a physical star topology using SC or ST connectors. Due to its speed limitations, this technology is hardly used these days.
Table 8-4 gives a summary of 10 Mbps networking standards.
Most of the modern networks support 100 Mbps speeds, which provide better bandwidth for demanding applications. In fact, the 100 Mbps standard has become a minimum requirement these days. The following is a brief description of 100 Mbps standards.
- 100BaseTX
100BaseTX networks use two pairs of UTP CAT 5 cable. The length of cable segments can be up to 100 meters.
- 100BaseT4
100BaseT4 networks use four pairs of CAT 3, 4, or 5 type cables. The length of cable segments can be up to 100 meters.
- 100BaseFX
100BaseFX networks use multimode or single mode fiber optic cables and provide up to 100 Mbps of data transfer rates. The length of cable segment can be up to 412 meters for multimode and 10,000 meters for single mode cable.
Table 8-5 gives a summary of 100 Mbps networking standards.
The 1000 Mbps (1 Gigabit) Ethernet networks are also known as Gigabit Ethernet. These networks use either copper-based or fiber optic cabling. These networks are implemented mainly as a backbone for large networks. The following is a brief description of Gigabit Ethernet standards:
- 1000BaseX
The IEEE 802.3z specifies the 1000BaseX standards that describe three different Gigabit standards: 1000BaseLX, 1000BaseSX and 1000BaseCX. The 1000BaseLX and 1000BaseSX use multimode or single mode fiber optic cables. The 1000BaseLX uses long wavelength laser beams while the 1000BaseSX uses short wavelength laser beams. The 1000BaseCX standard specifies use of shielded twisted pair (STP) cables.
- 1000BaseT
The IEEE 802.3ab specifies the 1000BaseT standard. It uses four pairs of CAT 5 UTP cable. Each pair of the CAT 5 cable can achieve maximum data transfer speeds of up to 250 Mbps, making it an overall 1000 Mbps.
Table 8-6 gives a summary of Gigabit Ethernet networking standards.
The 10 Gigabit Ethernet networks are specified in the IEEE 802.3ae standard. These networks can achieve a maximum data transfer speed of up to 10,000 Mbps (10 Gbps). All these networks are used for baseband transmissions, in which digital or analog signals are carried on a single channel. The following is a brief description of 10 Gigabit Ethernet standards:
- 10GBaseSR
SR stands for Short Range optical technology. These networks use 50 micron or 62.5 micron multimode fiber optic cable. The length of the cable segment varies from 33 meters to 330 meters, depending on the type of cable used.
- 10GBaseLR
LR stands for Long Range optical technology. These networks use single mode fiber optic cable. The length of the cable segment can be up to 10,000 meters (10 Km).
- 10GBaseER
ER stands for Extended Range optical technology. These networks use single mode fiber optic cable. The length of a cable segment can be up to 40,000 meters (40 Km).
Table 8-7 provides a summary of 10 Gigabit Ethernet standards.
Media connectors are used for terminating cables. In other words, they provide an interface to connect the cables to devices. Different types of cables use different types of connectors. It is not possible to connect a cable to a device without first terminating it with a suitable connector. Each connector has two variations: a male connector and a female connector. The following sections provide a summary of connectors used for computer networking.
The RJ-11 connector is mainly used for terminating telephone wires. It has a capacity of three telephone lines (six pins) but only four pins are commonly used. The connector looks similar to the RJ-45 connector used in computer networking. It comes in a plastic casing and is smaller than the RJ-45 connector. Only two pins are used for a single telephone line, but four pins are used for a Digital Subscriber Line (DSL). Most installers provide all four wires in order to connect an extra telephone line, if required.
The RJ-45 connector is little bigger than the RJ-11 connector and is used for terminating twisted cables. It uses eight pins, instead of four or six, in the RJ-11 connector. RJ-45 is the most common type of connector used in computer networks. Cables can be wired in either a straight or crossover fashion using the RJ-45 connectors. Figure 8-13 shows an RJ-45 connector.
The F-Type connector (or simply the F connector) is used to terminate RG/6 and RG/59 coaxial cables used with cable television. They are also used for connecting cable modems and satellite receivers. The connector has a screw that is tightened to secure the physical connection. Figure 8-14 shows an F-Type connector.
BNC connectors (BNC stands for Bayonet Neil-Concelman) are also used for terminating coaxial cables, but unlike the F-Type connectors, they do not use the screw. The connector is twisted to make the connection. These connectors are commonly used with 10Base2 networks that use thin coaxial cable. Since 10Base2 networks are rarely used these days, the BNC connectors are also not much in use. The BNC family of connectors includes T-connectors, Barrel connectors, and terminators. Figure 8-15 shows some BNC T-connectors.
Connectors used for fiber optic cabling come in a variety of shapes. Due to fast developments in this technology, a large number of connectors are not available in the market. These include push-pull, snap-in, and twist type connectors. All connectors are used in pairs to allow full-duplex communications. The Network+ exam expects you to identify only four types of connectors: SC, ST, LC, and MT-RJ. A brief description of these connectors is given in the following paragraphs:
- Subscriber/Standard Connector (SC)
An SC connector uses the push-pull mechanism and is shown in Figure 8-16. This connector provides good protection for the ends of a fiber optic cable and is easier to connect and disconnect in tight spaces than the ST connector, which is listed next.
- Straight Tip (ST)
An ST connector is an older type of fiber optic connector. It uses the "twist-on/twist-off" bayonet mechanism to make the connection. Figure 8-17 shows an ST connector.
- Lucent Connector (LC)
An LC connector has a small flange on top that secures the connection in place. This connector also uses the push-pull mechanism. Figure 8-18 shows an LC connector.
- Mechanical Transfer-Registered Jack (MT-RJ)
An MT-RJ connector resembles an RJ type connector. These connectors always hold two fiber cables to allow full-duplex communications. Figure 8-19 shows two MT-RJ connectors.
The IEEE 1394 interface is also known as Firewire. This interface is mainly used in high-bandwidth applications, such as digital video and portable storage. The IEEE 1394 connectors come in six-pin and four-pin configurations as shown in Figure 8-20.
USB interfaces have become very popular in computers and other digital consumer devices due to their performance and Plug-n-Play-compatibility. USB devices can be connected or disconnected into any device without having to turn off power. These devices do not need any manual configuration. USB connectors are available in a variety of sizes and shapes, but the two popular types are: Type A and Type B. The Type A connector is mainly used on computers and the Type B connectors are mainly used for peripherals. Figure 8-21 shows both Type A and Type B connectors.
As noted earlier in the section "Physical Network Topologies," a small bus network can be built without any active device. This network is difficult to expand due to its limitations. Network devices, as discussed in the following sections, are used to connect multiple systems as well as to connect smaller network segments to form a large internetwork. This section covers a brief description of commonly used networking devices, which include network interface cards (NICs), hubs, switches, bridges, and routers.
An Ethernet hub (or a concentrator) is the central device in a network segment that connects all nodes in the segment. It receives signals on one of its ports and retransmits them to all other ports except the receiving port. It is also known as a multiport repeater. Hubs work at the Physical layer (Layer 1) of the OSI model. Since a hub cannot decide the destination port, it is considered an inefficient device. In a typical implementation, UTP cables are used to connect nodes (computers or printers) to hubs. Hubs can be cascaded (joined together) to extend the network segment. Most ports of the hub use RJ-45 connectors, but AUI and BNC connectors are also provided to extend the segment to legacy 10Base2 and 10Base5 networks. The two types of hubs are described here:
- Active hub
An active hub receives signals at its ports and regenerates them before passing them onto all other ports. However, it does not perform any processing in terms of error checking.
- Passive hub
A passive hub acts as a simple gateway for incoming signals and does not regenerate them before passing them onto other ports.
Ethernet hubs are available in a variety of sizes and costs, depending on the number of ports. Smaller hubs with 4, 8, or 12 ports are known as workgroup hubs, while hubs with 24 or 32 ports are known as high-density hubs.
Like a hub, a switch is also the central device that connects multiple nodes in a network segment using UTP or STP cables. But unlike the hub that sends the received signal to every port, a switch sends the signal only to the destination node. A switch is an intelligent device that learns the MAC address of the destination from the data packet and sends the packet to the intended node only. This results in data direct communication between two nodes, improved network performance, and reduced collisions.
Switches work at the Data Link layer (Layer 2) of the OSI networking model. Switches can work in a full-a mode, which is a mode that enables nodes to transmit and receive data simultaneously. Thus a 100 Mbps switch working in a full-duplex mode can provide 200 Mbps data transfer speed. Switches are preferred in large networks where hubs can become a bottleneck for network performance.
Switches forward data packets using one of the following forwarding techniques:
- Cut Through
The switch reads only the hardware address from the data frame and starts sending it to the destination. It does not perform any error checking. This improves speed. A switch using a cut-through technique may fall back to the store-and-forward technique if it finds that the destination port is busy at the time of transmission.
- Store-and-Forward
The switch stores the entire packet in its memory buffer and performs error checking. This prevents forwarding of errors onto the rest of the network. This method is slower than the cut-though method due to the error-checking process, and affects network performance.
- Fragment Free
The switch takes advantage of both cut-through and store-and-forward techniques. It reads the first 64 bytes of the frame and leaves error checking to the next device working at the upper layers of OSI model.
An MAU—also called Multi-Station Access Unit (MSAU)—is used in Token Ring networks as a central device that connects all nodes in the network segment. This is equivalent to using a hub or a switch in Ethernet networks and results in giving the network a physical star look, though its logical topology remains a ring. Multiple MAUs can be connected using the Ring In (RI) and Ring Out (RO) ports in order to extend the network. The RO port of one MAU is connected to the RI port of the second MAU, and so on. The RO port of the last MAU is connected back to the RI port of the first MAU in the network to complete the ring.
A network bridge is used for two purposes: connecting two LAN segments to form a larger segment and dividing a large network segment into smaller segments. It works at the Data Link layer (Layer 2) of the OSI model. Like network switches, bridges also learn the MAC address of devices and forward data packets based on the destination MAC address. In older bridges, the MAC addresses had to be defined manually, and it took a significant amount of time to configure a bridge. Most of the newer bridges can dynamically build lists of MAC addresses by analyzing data frames. These bridges are called learning bridges, due to this advanced functionality.
Most of the functionality of bridges is now available in switches. Hence, they are rarely used in networks these days. Bridges fall into the following categories:
- Transparent Bridge
This bridge forwards data packets to the destination network segment by reading the destination MAC address. The network devices are unaware of the presence of the bridge. This bridge builds the MAC address table as it receives data packets. If the bridge does not find a destination MAC address in its list, it floods all ports with the data packet except the source port.
- Source Route Bridge
This bridge is used in Token Ring networks. The bridge uses two frame types to find the route for the data: a Source Route (SR) frame and an All-Route (AR) frame.
- Translation Bridge
This bridge is used to connect two network segments that use different protocols at the Data Link layer. For example, a translation bridge can join a Token Ring network to an Ethernet network or an FDDI to a Token Ring.
The problem with bridges is that they cannot be used for large networks. When multiple bridges are used in a large network, they start confusing each other. This results in bridging loops, a term used when one bridge makes the other bridge believe that a device is located in a network segment—while it actually is not. To overcome the bridging loops problem, bridges use the spanning tree protocol. This is defined in the IEEE 802.1d standard. Using this protocol, the bridge interfaces are assigned a value that helps control the way bridges learn MAC addresses and disable inactive or nonexisting links.
Routers are used to connect two or more network segments. These devices work on the Network layer (Layer 3) of the OSI model. Routers use Internet Protocol (IP) addresses to determine the source and destination of the data packet. Typically, routers receive the data packet, determine the destination IP address, and forward the packet to the next hop, which may either be the final destination of the packet or another router on the path. Routers can be implemented as a software service or as a dedicated hardware device. A wired or wireless router in a home network is an example of a small network router that connects the home network to the ISP's network. Microsoft's Routing and Remote Access Service (RRAS) is an example of a software router. A Windows Server 2000/2003 computer with at least two network interface cards can be configured as a router to connect network segments.
Routers communicate to each other using routing protocols. They maintain a list of IP addresses in routing tables. Routing tables can be built statically or dynamically as discussed in the following list:
- Static routing
When static routing is used, administrators manually configure routing tables by entering appropriate routing information. This method works only for very small networks. In large networks, it is very difficult to manually configure routing tables. As the routing tables grow or there is a change in the network, the routing tables must be updated manually. The process is time-consuming and error-prone.
- Dynamic routing
Routers use dynamic routing protocols when working in a dynamic routing environment. Dynamic routing protocols enable routers to get routing information from other routers and advertise their own routing information in order to build and maintain routing tables. Dynamic routing protocols fall into two categories, discussed next.
A distance vector routing protocol assumes that the network is made up of several routers. Routers using this protocol depend on other routers to advertise their routing information periodically. These advertisements (or updates) are typically sent every 30 seconds. Routers can also be configured to send triggered updates when they detect any change in network topology.
RIPv1 (Routing Information Protocol version 1) and RIPv2 (Routing Information Protocol version 2) are distance vector protocols that work on the principle of hop count. RIPv1 works only on TCP/IP networks, while RIPv2 works on both TCP/IP and IPX/SPX networks. The RIP version that supports IPX is sometimes called IPX RIP also. A hop is a value assigned to each router on the way to the final destination. RIP supports a maximum of 15 hops in the network. A destination beyond 15 hops is considered unreachable. The following are the main disadvantages of distance vector routing protocols:
Periodic update is a slow process that affects network performance.
Periodic updates generate considerable network traffic, making the protocol inefficient on large networks.
Routing loops are created when routers advertise incorrect routing information.
There are two methods to get around the routing loops problem in distance vector protocols. The first method is split horizon, which prevents a router from advertising a route to the same router from which it received the route information. The second method is poison reverse, which advertises back the route it learns from a router with a hop count of 16 (unreachable).
Link state routing protocols use Link State Advertisements (LSA) to update routing tables. The LSA is a data packet that contains routing information about the sending router only. This packet is sent to all routers in the network so that other routers can build routing tables. This is in contrast with the distance routing protocols where all routers advertise their entire routing tables to all other routers, thus generating significant amount of network traffic.
Open Shortest Path First (OSPF) and NetWare Link State Protocol (NLSP) are examples of link state routing protocols. The link state routing protocols are best suited for large networks, as there is no limit such as the hop count. They keep update traffic to the minimum and can correct the routing table information quickly if there is a change in the network topology. This characteristic is known as convergence.
In computer networks, a gateway is a device that translates one format of data packets to another format. They are also called protocol translators. A router connecting two different types of network segments or a bridge connecting two network segments using different Layer 2 protocols are examples of gateways. Gateways are necessary to provide interoperability between two distinct network formats. It is notable that gateways only convert (translate) data formats, but the that data itself remains unchanged.
A CSU/DSU is a digital interface device that connects a local area network to a wide area network. Typically, the CSU/DSU is installed between a LAN and the access point provided by the provider of the WAN service. Most of the newer routers now include the functionality of CSU/DSU. For example, a router connecting a LAN to the Internet is also functioning as a CSU/DSU unit.
An NIC, or a network adapter, is a hardware device that connects a computer to the network. It allows computers to communicate over the network using standard networking protocols. It works at the Data Link layer (Layer 2) of the OSI model. Every card has an RJ-45, a BNC, or an AUI socket where the network cable is connected. A light-emitting diode (LED) usually indicates the status of the card whether it is active or not. Older cards supported only 10 Mbps data transfer speeds, but the newer cards support 10/100 Mpbs or even 1000 Mpbs speeds.
Like other devices in the computer, network cards must also be configured to use certain system resources such as I/O Address (Input/Output Address), IRQ (Interrupt Request), and DMA (Direct Memory Access). Most of the newer cards are Plug-n-Play and are automatically configured by the system. However, before a card is purchased or installed, ensure that it supports the type of cabling used in the network. For example, a NIC-supporting fiber optic cable may not work in a network where UTP/STP cables are used.
Every network card comes with a device driver that needs to be installed to configure it properly on a system. In older cards, network technicians had to configure them manually by setting jumpers for the I/O address and IRQ. The driver software also had to be installed manually. As noted earlier, most new cards are automatically configured by software. However, in certain situations, you may need to download a driver from the vendor's web site and install it in order to let the system configure the card appropriately.
An ISDN adapter, or a terminal adapter, refers to a hardware device that connects a computer (terminal) to the Integrated Services Digital Network (ISDN) network. It is also called an ISDN modem. ISDN technology is mainly used for wide area networking using either Basic Rate Interface (BRI) or Primary Rate Interface (PRI). This technology provides higher data transfer speeds as compared to other technologies such as dial-up over ordinary telephone lines.
ISDN adapters can be added to a system on an expansion slot, or they can be standalone external devices connecting to the serial port of the computer. For example, some routers provide ISDN interfaces as a built-in feature. The ISDN technology is not so popular and has now been replaced by faster and more flexible WAN technologies.
A WAP, or simply an Access Point (AP), is a hardware device that is used to connect wireless devices to form a network. In its typical implementation, a WAP is also connected to the wired network and allows wireless clients to communicate to the clients located on the wired local area network (LAN). In a wireless network, all nodes, including the AP, have wireless transmitters and receivers, and communication takes place using radio frequencies. The transmission range of an AP is limited, and a large wireless network may need more than one AP to provide connectivity to all clients located at different places of the building. The range of the AP signals depends on the type of wireless standard used as well as on electromagnetic and radio frequency interferences.
WAPs are available in several different forms and capabilities. For example, a low-cost, small wireless router used to share an Internet connection at home also acts as an AP. This device not only provides connectivity to all computers but also acts as a gateway for Internet connectivity and automatically assigns IP addresses to them.
The term modem is derived from Modulator/Demodulator. A modem is a hardware device that is used to convert digital signals from a computer to analog signals (modulation) in order to transmit them over analog lines. At the receiving end, it converts the analog signals back to digital signals (demodulation) so that a computer can understand them. In their typical usage, modems are connected to a computer in order to provide remote access (or Internet connectivity) using analog telephone lines. It can be built onto the motherboard of the computer, can be installed as an extension card, or can be an external device. External modems can either be connected to one of the serial ports or to the USB port of the computer.
When used as an internal device, modems must be configured to use system resources such as an I/O address or IRQ. Modems use the serial communication (COM) ports in a computer, and resources used by these ports must be available in order to correctly configure the modem. Table 8-8 provides a summary of the COM ports and resources used by them.
Table 8-8. COM ports and system resources
Serial port | IRQ | I/O address |
|---|---|---|
COM1 | 4 | 03F8H |
COM2 | 3 | 02F8H |
COM3 | 4 | 03E8H |
COM4 | 3 | 02E8H |
Modems are available in different sizes, speed capabilities, and costs. The data transmission speed of a modem depends mainly on the type of Universal Asynchronous Receiver/Transmitter (UART) chip used and varies from 9.6 Kbps to over 900 Kbps. Modems with up to 115 Kbps speeds are commonly used for dial-up networking.
As the name indicates, a transceiver is a device that combines the functions of a transmitter and a receiver. It does not refer to any standalone or separate hardware device but is normally built into devices such as network cards, modems, hubs, switches, or routers. Depending on the type of network cabling in use, you may find fiber optic transceivers used in fiber optic networks; RF transceivers used in wireless networks, and Ethernet transceivers.
Media converters are used to enable interconnection of one type of media (usually cabling) to another type. For example, you may want to connect a network segment wired with a fiber optic cable to another segment wired with UTP/STP cables. In another example, you may wish to connect a coaxial cable segment to a UTP/STP network segment.
In a computer network, a firewall is a hardware device or software that is used to prevent undesired traffic. It protects the network from unauthorized external access and thereby protects system and network resources critical for running the business operations. Firewalls work on the basis of rules that dictate which traffic should be allowed and which traffic should be blocked. A firewall usually sits between an internal network and an external public network such as the Internet. They may also be used to separate different departments within an organization.
Software-based firewalls are usually a built-in feature of many network operating systems. Administrators usually configure these firewalls depending on the requirements of an organization. Hardware-based firewalls are either dedicated devices, or the functionality is built into other devices, such as routers.
Wireless networks rely on radio transmissions to communicate instead of the network cabling used for normal computer networks. Radio frequencies create Electromagnetic (EM) fields, which become the medium to transfer signals from one computer to another. As you go away from the hub, or from the main equipment generating the wireless network's radio transmissions, the strength of the EM field reduces and the signal becomes weak. EM fields are also prone to interference, which can be introduced by walls, reflected radio waves, and the presence of other EM fields. The presence of wireless telephones, microwave ovens, television sets, and a number of other devices can potentially interfere and reduce the signal strength of wireless devices.
In order to reduce the effects of interfering frequencies, wireless devices use the spread spectrum technology. This technology helps share available frequency bandwidth common to wireless devices. It also helps prevent jamming of radio signals due to strong interference from another source of radio frequency. Instead of using a fixed frequency, such as that used with radio and television broadcasts, wireless networks use a spectrum of frequencies. The sender uses a number of narrow-band frequencies to communicate with the receiver. Each narrow band of frequencies contains only a part of the signal. The receiver correlates the signals received at different frequencies to retrieve the original information. Spread spectrum technology synchronizes wireless signals using one of the following methods:
- Frequency Hopping Spread Spectrum (FHSS)
FHSS is the method of transmitting RF signals by rapidly switching frequencies according to a pseudorandom pattern, which is known to both the sender and the receiver. FHSS uses a large range of frequency (83.5 MHz) and is highly resistant to noise and interference. The amount of time the signal spends on any frequency is known as dwell time, and the amount of time it takes from switching one frequency to another is known as hop time. FHSS signals are difficult to intercept because the signals usually appear as noise. FHSS works in the unlicensed frequency range of 2.4 GHz and is used in HomeRF and Bluetooth. It has a limited speed of transmission that ranges from 1.6 to 10 Mbps.
- Direct Sequence Spread Spectrum (DSSS)
DSSS is a modulation technique used by wireless networks. It uses a wide band of frequency and it divides the signal into smaller parts and is transmitted simultaneously on as many frequencies as possible within a particular frequency band. DSSS adds redundant bits of data known as chips. The ratio of chips to data is known as spreading ratio. The higher the spreading ratio, the higher the immunity to interference. DSSS is faster than FHSS and ensures data protection, because chips are redundant and simultaneously transmitted. It utilizes a frequency range from 2.4 GHz to 2.4835 GHz and is used in 802.11b networks.
Infrared technology employs electromagnetic radiations that use wavelengths that are longer than the visible light but shorter than radio frequency. This technology is used in night-vision equipment, thermography, digital cameras, and digital communication systems. Common examples of Infrared devices are the remote controls used by TVs and audio systems. The Infrared technology is standardized by the Infrared Data Association (IrDA). The following are some of the key characteristics of IrDA wireless communication technology:
It supports point-to-point wireless communications between two devices.
Infrared transmission uses a direct line of sight suitable for personal area networks.
Infrared waves cannot penetrate walls.
IrDA wireless communication technology supports data transfer speeds ranging from 10 to 16 Mbps.
Infrared devices consume very low power.
Infrared frequencies do not interfere with radio frequencies.
IrDA wireless communication technology provides a secure wireless medium due to the short distance (usually 3 to 12 feet) between devices.
Bluetooth wireless networking technology provides short-range communications between two or more devices. It is a low-cost networking solution widely used in telephones, entertainment systems, and computers. It is designed to overcome the limitations of IrDA technology. The following are some of the key characteristics of Bluetooth-based wireless communication:
It supports transmission speeds from 1 Mbps (Bluetooth 1.0) to 3 Mbps (Bluetooth 2.0).
It works over the unlicensed frequency range of 2.4 GHz.
The devices must be within a short range of less than 10 meters.
It uses FHSS technology.
Unlike the Infrared signals, it does not require a direct line of sight.
Bluetooth devices consume very low power.
Two or more Bluetooth computers form an ad-hoc wireless network.
Wireless services use radio frequencies that travel through the atmosphere. There are several factors that may affect the speed, signal quality, and range of wireless signals. These include interference from other electrical devices, the type of antenna used, and other environmental factors. This section covers a brief discussion of these factors.
Atmospheric interferences to wireless signals cannot be prevented, but they can certainly be reduced to achieve optimum performance. Some of the major causes of interference include the following:
Physical objects such as buildings, trees, concrete and steel walls. These objects can either significantly reduce signals or even completely block them.
Electromagnetic interference (EMI) generated by high-power electric lines, power transformers, heavy electrical machinery, fans, light fixtures, etc.
Radio frequency interference (RFI) generated by other wireless equipment working in the same frequency ranges used by computer wireless devices. Examples of these types of equipment are wireless phones, wireless game controllers, or microwave ovens.
The range of wireless signals depends on the type of antenna used for transmitting radio frequency signals. Selection of an antenna is a critical part of implementing a wireless network. Different shapes and sizes of antennas offer different signal levels. The strength of a wireless antenna (called its gain) is measured in decibels isotropic (denoted as dBi). An isotropic antenna sends signals of equal strength in all directions. A simple rule for calculating effective strength of an antenna is that every 3 dBi of gain almost doubles its output.
Omni-directional antennas send wireless signals in all directions. This type of antenna is useful when the coverage is required equally around the point of transmission. On the other hand, directional antennas transmit signals in one direction only. This helps send the entire output of the transmitting device in one direction, in which case, signals are more effectively transmitted.