Networks allow computer users to connect computers together and share resources such as files and printers. Networking also allows better management of data, security, and resources in large companies where hundreds of employees work on computers. In this section, we will cover some basic fundamentals of computer networks, installation of networks, and common troubleshooting methods.
A computer network refers to two or more computers linked together to share files, printers, and other resources. 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 spanned across the globe.
Tip
Chapter 8 covers a detailed study of computer networks.
The following are main categories of computer networks:
- Local area network (LAN)
A LAN is a network of computers joined together in a local area such as a small office, home, or 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.
- Wide area network (WAN)
A WAN 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 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 providers network.
- Personal area network (PAN)
A PAN refers to a network of devices located in close proximity of each other. The devices may include computers, PDAs, mobile phones, and similar items, connected using a wireless or a wired network.
- Metropolitan area network (MAN)
A MAN is a large internetwork connecting local area networks in a campus or inside the boundaries of one city.
Networking models can be divided into the following two categories:
Centralized computing
Decentralized computing
In a centralized computing network model, all processing is done on a central computer. This computer provides data storage as well as controls to all peripherals including the clients, which are called dumb terminals. A client/server network is based on the centralized computing model. A centralized server 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, Unix/Linux, etc., and the desktops run client operating systems such as Windows XP or Windows 2000. The following are some features of client/server computing:
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 central point.
In a decentralized computing network model, all processing and resources are distributed among several computers, thereby increasing performance. All systems can run independent of each other. A peer-to-peer (P2P) network or a workgroup is based on a decentralized computing model. Every computer is responsible for processing applications, storage of data, and controlling access to its resources. The following are some features of a peer-to-peer networking model:
These networks are suitable for about 10 computers only.
They are cost effective as compared to the client/server model.
A network operating system (NOS) or skilled administrators are not required.
These networks are not as secure as the client/server model because each user individually maintains security of resources on her computer.
A topology refers to the physical layout of a network. It describes how networking devices such as servers, desktops, printers, and network devices are connected together.
- Star topology
In a star topology, computers (or 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.
- Bus topology
In a bus topology, all computers are connected to a single cable called a 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.
- Mesh topology
In a mesh topology, each computer makes a point-to-point connection to every other computer. This makes the network highly fault-tolerant and reliable because a break in the cable or a faulty computer does not effect network operation. Data can travel from one computer to another using a number of paths.
- Ring topology
In a ring topology, each computer is connected to its neighboring computer to form a logical ring. If one of the computers in the ring fails or if the cable is broken, the entire network becomes inaccessible. Addition or removal of computers also disrupts network transmissions. A Multi-Station Access Unit (MSAU) or Media Access Unit (MAU) acts as the central device.
- Wireless topology
In a wireless topology, computers connect to each other using radio frequencies. Wireless networks can be either Ad-hoc or Infrastructure topology-based. In an ad-hoc wireless network, two or more computers directly communicate to each other without using a central device. There is no central device (hub), and these networks can be created anywhere almost spontaneously. In an Infrastructure network, a central wireless device known as the Access Point (AP) is used to authenticate and configure wireless clients that fall within its range. A special identifier known as the Service Set Identifier (SSID) must be configured on the AP and each wireless client. The AP can further be connected to the wired LAN so that wireless clients can access the wired LAN also.
In computer networking, a duplex communication system is the one where data can be transmitted and received simultaneously in both directions. In a half-duplex communication, setup data can flow in both directions, but only in one direction at a time. If one end is transmitting data, it cannot receive at the same time. In a full-duplex communication setup, both ends can transmit and receive data simultaneously.
The cables used for computer networks fall into three main categories: coaxial (thin and thick), twisted pair (unshielded and shielded), and fiber optic (single-mode and multi-mode). 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 not covered in the A+ Essentials exam as these are rarely used these days.
Twisted pair cables use 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 and lower in cost than coaxial and fiber optic cables. These cables are identified by their category numbers, denoted as CAT-1, CAT-2, CAT-3, CAT-5, etc. Figure 2-15 shows a piece of a twisted pair cable.
- Unshielded twisted pair
Unshielded twisted pair (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.
- Shielded twisted pair
Shielded twisted pair (STP) cables come with a layer of shielding material between the cables and the sheath. STP cables do provide some degree of protection from EMI and RF disturbances and can carry signals to greater distances. But this advantage comes with extra cost of installation.
Table 2-11 lists some of the popular UTP and STP categories.
Table 2-11. Categories of UTP and STP cables
Category | Description |
|---|---|
CAT-3 | Used for both voice and data transmissions in Ethernet, Fast Ethernet, and Token Ring networks. |
CAT-4 | Used for both voice and data transmissions in Ethernet, Fast Ethernet, and Token Ring networks. |
CAT-5 | Used for both voice and data transmissions in Ethernet, Fast Ethernet, Token Ring, and ATM networks. |
CAT-5e | Used in Gigabit Ethernet networks. |
CAT-6 | Used for both voice and data transmissions in Ethernet, Fast Ethernet, Token Ring, and ATM networks. |
CAT-6 (STP) | Used for data transmissions in Ethernet, Fast Ethernet, Gigabit Ethernet, Token Ring, and ATM networks. |
The term plenum refers to the space between the main ceiling and the dropped ceiling of a building. This space is used for air circulation in heating and air-conditioning systems. The network cable used in this space is known as plenum cable. A fire-retardant plastic jacket surrounds the plenum cable. The jacket consists of a low smoke polyvinyl chloride (PVC) or fluorinated ethylene polymer (FEP). These cables are highly resistant to fire and should be used in plenum space. UTP and STP cables should not be used in this space because they pose a serious fire hazard in case of a fire.
Fiber optic cable is made up of very thin glass or plastic stretched out and put inside a sheath. The transmission in fiber optic cables is carried by light signals and is immune to EMI and RF disturbances. They can also carry data signals to longer distances than UTP or STP cables and is considered the most secure of all cable types. Fiber optic cables are very expensive in terms of the cost involved in installation and maintenance. They need 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 2-16 shows a fiber optic cable.
Fiber optic cables fall into the following two categories:
- Single-mode fiber optic cable
The single-mode fiber optic cable is made up of 8 to 10 micron core glass or plastic fiber surrounded by 125 micron cladding. It uses a single beam of light and can thus travel to greater distances than multimode fiber optic cable.
- Multimode fiber optic cable
The multimode fiber optic cable is made up of 50 or 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.
Connectors are used for terminating cables and provide an interface to connect the cables to devices. 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 and a female. The following is a brief description of connectors used for computer networking:
- Registered Jack-11 (RJ-11)
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. A single telephone line uses only two pins. but four pins are used for a Digital Subscriber Line (DSL).
- Registered Jack-45 (RJ-45)
The RJ-45 is an 8-pin connector that is used for terminating twisted pair cables. It is the most common type of connector used in computer networks. Cables can be wired in either a straight or crossover fashion.
- Subscriber/Standard Connector (SC)
An SC connector is used to terminate fiber optic cables. It uses the push-pull mechanism to make the connection.
- 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.
- Lucent (LC)
An LC connector is also used for fiber optic cables with a push-pull mechanism. It has a small flange on top that secures the connection in place.
- Mechanical Transfer-Registered Jack (MT-RJ)
An MT-RJ connector resembles RJ type connectors. They always hold two fiber cables to allow full-duplex communications.
- Universal Serial Bus (USB)
USB connectors are available in a variety of sizes and shapes, but the two most popular types are USB Type A and USB Type B. The Type A connector is mainly used on computers, and the Type B connectors are usually used for peripherals.
- IEEE 1394
An IEEE 1394 connector is also known as Firewire connector. These connectors are mainly used for digital video and portable storage devices. The IEEE 1394 connectors come in 6-pin and 4-pin configurations.
Tip
Refer to Chapter 8, which shows figures of different types of network connectors.
This section covers a brief description of commonly used networking devices, which include hubs, switches, MAUs, bridges, and routers.
An Ethernet hub (or a concentrator) is the central device that connects all nodes in the segment. It receives signals on one of its ports and retransmits it to all other ports except the receiving port. In a typical implementation, UTP cables are used to connect nodes (computers or printers) to hubs. An active hub receives signals at its ports and regenerates it before passing it on to all other ports. A passive hub acts as a simple gateway for the incoming signals and does not regenerate the signal before passing it on to 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. Hubs can be cascaded (joined together) to extend the network segment.
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 hardware address or Media Access Control (MAC) address of the destination from the data packet and sends the packet only to the intended node. This results data direct communication between two nodes, improved network performance, and a reduced number of collisions.
Switches can work in a full-duplex mode, 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.
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 to LAN segments to form a larger segment and dividing a large network segment into smaller segments. Like network switches, bridges also learn the MAC address of the devices and forward data packets based on the destination MAC address. 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 included in switches.
Routers are used to connect two or more network segments. Routers use 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.
Routers communicate to each other using routing protocols. Routers maintain a list of IP addresses in routing tables. Routing tables can be built statically or dynamically as discussed in the following sections:
- 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.
- Dynamic routing
In a dynamic routing environment, routers use Routing Information Protocol (RIP) or Open Shortest Path First (OSPF) to build, maintain, and advertise their routing tables. Most networks are based on dynamic routing.
Networking protocols allow computers to communicate to each other through the networking media. Some of these protocols are common to all operating systems while others are platform-dependent. This section covers a brief description of the commonly used protocols TCP/IP, IPX/SPX, and NetBEUI.
TCP/IP is a set of several protocols. It is the most widely used protocol suite in private networks as well as on the Internet. TCP/IP is not proprietary to any organization but is a public protocol suite. It is a fully routable protocol and is supported by all major network and desktop operating systems. Some of the well-known TCP/IP protocols and their functions are listed in Table 2-12.
Table 2-12. TCP/IP protocols and their functions
Protocol | Function |
|---|---|
Internet Protocol (IP) | IP is a connection-less protocol that provides IP addressing and routing functions. |
Transmission Control Protocol (TCP) | TCP is a connection-oriented protocol that guarantees delivery, flow control, error detection, error correction, and packet sequencing. |
User Datagram Protocol (UDP) | UDP is a connection-less transport protocol. It does not provide guaranteed delivery of data. |
File Transfer Protocol (FTP) | FTP is a client/server application used for file transfers between remote computers. |
Trivial File Transfer Protocol (TFTP) | TFTP is also used to transfer files between two remote computers. It is faster but less reliable than FTP. |
Simple Mail Transfer Protocol (SMTP) | SMTP is used to transport messages between remote email servers. |
HyperText Transfer Protocol (HTTP) | HTTP allows text, images, and multimedia to be downloaded from web sites. |
HTTP Secure (HTTPS) | HTTPS is the secure version of the HTTP protocol that authenticates web servers and clients before the communication session starts. |
Post Office Protocol 3 (POP3) | POP3 is used to download or retrieve email messages from mail servers running the SMTP protocol. |
Internet Message Access Protocol 4 (IMAP4) | IMAP4 is also used to securely retrieve email from mail servers. |
Telnet | Telnet allows connections to remote hosts such as network devices for administrative and maintenance purposes. |
Internet Control Message Protocol (ICMP) | ICMP provides error checking and reporting functions. |
Address Resolution Protocol (ARP) | ARP is used to resolve IP addresses to MAC addresses. |
Network News Transfer Protocol (NNTP) | NNTP provides newsgroup services such as posting and retrieving messages on discussion forums. |
Line Printer Remote (LPR) | LPR provides client connectivity to printers in network operating systems such as Unix, Linux, and Windows. |
IPX/SPX is a full protocol suite used in Novell NetWare networks. The IPX/SPX protocol suite is fully routable, but due to the increasing popularity and extended features of the TCP/IP protocol suite, the usage of IPX/SPX has reduced significantly. Both Microsoft and Novell have made TCP/IP their default protocol in recent versions of operating systems. Different protocols in this suite are listed in Table 2-13.
Table 2-13. IPX/SPX protocols and their functions
Protocol | Function |
|---|---|
Netware Core Protocol (NCP) | Allows client/server interactions such as file and print sharing. |
Service Advertising Protocol (SAP) | Allows systems to advertise their services such as file and print services. |
Internet Packet Exchange (IPX) | Provides network addressing and routing services. |
Sequenced Packet Exchange (SPX) | Provides connection-oriented services on top of the IPX protocol. |
Routing Information Protocol (RIP) | This is the default routing protocol for IPX/SPX networks. It uses Distant Vector Routing Algorithm for building routing tables. |
NetWare Link State Protocol (NLSP) | Provides routing services based on Link State Algorithm for routes and building routing tables. |
Open Datalink Interface (ODI) | Allows NetWare systems to work with any network interface card. |
When discussing the IPX/SPX protocol suite, it is important to include the frame types used in NetWare networks. If there is some connectivity problem between two systems using different versions, it is a good idea to check the frame types used on the network. NetWare uses the following types of frames for encapsulating data at the Data Link layer:
NetWare 2.x and NetWare 3.x use IEEE 802.3 as the default frame type.
NetWare 4.x uses IEEE 8.2.2 as the default frame type.
NetBEUI stands for NetBIOS Extended User Interface. It is an old Microsoft networking protocol used in small Windows-based networks. This protocol uses a broadcast method of finding computer names, and it creates significant network traffic. It is not a routable protocol and as such, cannot be used on large routed networks. It is easy to install and the fastest of all protocols covered in the A+ Essentials exam. Due to its severe limitations, it is not even used in Microsoft networks these days.
Network hosts or computers are identified either by their names or their addresses. The term network addressing refers to the method of identifying networks and hosts located in a particular network. Different networking protocols employ different methods for addressing networks and hosts, as described in the following sections.
Hosts in a TCP/IP network follow IP addressing schemes. The IP address consists of 32 bits composed of 4 sets of 8 bytes (octet) each. It is expressed as decimal numbers separated by a period known as dotted decimal notation. 192.168.2.10 is an example of an IP address. IP addresses can further be divided into public (registered) or private (unregistered) addresses. Organizations using public addresses can be connected to the Internet while the private IP addresses can only be used internally.
IP addresses are classified into classes A, B, C, D, and E. Only addresses from the classes A, B and C are assigned to organizations and are known as Classful IP Addresses. The first byte of an IP address identifies the class of IP addresses used in the network. For example, a host with an IP address 92.137.0.10 is using a class A IP address and a host with an IP address 192.170.200.10 is using a class C IP address.
A second 32-bit number, known as subnet mask, is used to identify the network address from the host address. When converted to a binary number, the network part is assigned a binary value of 1 and the host part is assigned a value of 0 in the subnet mask. For example, if the subnet mask is 255.255.0.0, the first 16 bits of the IP address would represent the network address and the last 16 bits would represent the host address.
Table 2-14 summarizes the main classes of IP addresses, the number of networks and hosts in each class, and the default subnet masks.
Table 2-14. Classful IP address ranges
Class | Range of first byte | Number of networks | Hosts per network | Default subnet mask |
|---|---|---|---|---|
A | 1–126 | 126 | 16,777,214 | 255.0.0.0 |
B | 128–191 | 16,384 | 65,534 | 255.255.0.0 |
C | 192–223 | 2,097,150 | 254 | 255.255.255.0 |
D | 224–239 | N/A | N/A | N/A |
E | 240–255 | N/A | N/A | N/A |
Tip
Notice from Table 2-14 that the network ID 127 is not included in any of the classes. This is because the IP address 127.0.0.1 is reserved as a loopback address for troubleshooting TCP/IP configuration of the computer.
Subnetting is the process of creating two or more network segments by using the host portion of the IP address. Subnetting creates multiple broadcast domains to reduce broadcast traffic and allows administrators to more effectively manage the IP address range. It also increases security of the network and helps contain network traffic to local network segments.
Consider a network with a class C IP address of 192.168.2.0. With the default subnet mask of 255.255.255.0, you can have only one large network segment with 254 hosts. If you use some bits from the host portion, you can create two, three, or four segments. But as the number of segments increases, the number of hosts in each segment reduces.
In a NetWare network environment, only the servers are assigned hostnames. These names consist of a maximum of 47 characters. The clients do not have hostnames and use their IPX addresses instead. NetWare networks are assigned a 32-bit hexadecimal address. The servers and workstations use a 48-bit hexadecimal address, which is the MAC address of the network interface card. This address is appended to the network address to create a unique node address. The following is an example of an IPX address:
0AC74E02:02254F89AE48
Note that the first part of the IPX address is the address of the logical network, and the second part is the unique MAC address of the network interface card. If there are any leading zeros, they are not written. Sometimes the IPX address is also written as groups of four hexadecimal numbers separated by colons. The above address can thus be written as:
AC7:4E02:0225:4F89:AE48
The NetBEUI protocol uses NetBIOS naming conventions for the purpose of addressing computers in a network. NetBIOS computer names consist of a maximum of 15 characters such a Server1, Workstation1, etc. NetBEUI uses the three methods that are shown next to resolve NetBIOS computer names to IP addresses.
- Broadcasting
If a host does not have the IP address of a NetBIOS host in its cache, it broadcasts the NetBIOS name in the entire network.
- LMHOSTS file
This is a text file that maps IP addresses to NetBIOS computer names.
- NBNS
This is the NetBIOS Name Server (known as the WINS server) that maps NetBIOS names to IP addresses.
Since NetBIOS name resolution mainly depends on broadcasts, the NetBEUI protocol creates significant network traffic if there are a large number of computers on the network.
The default configuration on most TCP/IP based operating systems is to dynamically obtain an IP address configuration from a Dynamic Host Configuration Protocol (DHCP) server. When the DHCP server is not available for some reason, the computer can assign itself an IP address automatically. The automatically assigned address is from the range 169.254.0.0 to 169.254.255.255 and a subnet mask of 255.255.0.0. With an APIPA address, the computer can connect only to the other computers with an APIPA address on the local network segment but cannot access any other computers on a remote network. If a computer is configured to obtain an IP address from a DHCP server but does not support APIPA, its IP address defaults to 0.0.0.0.
In computer networks, the term bandwidth refers to the speed of data transmissions. It is a measure of the data that can be transmitted from one point to another in a given amount of time. This bandwidth is expressed as bits of data transmitted in one second or bits per second (bps). Since bps is a very small figure for most modern networks, the bandwidth is expressed as megabits per second (Mbps). Sometimes, the bandwidth is also expressed as bytes per second (Bps) or megabytes per second (MBps), where 1 byte is equal to 8 bits.
Tip
It makes sense to mention bandwidth bottlenecks in this section. In a complex network, the complete path from one computer to another is composed of several links. Each link may have its own bandwidth. The bandwidth of the complete path is determined by the slowest link in the path. If the bandwidth is very low, the link becomes a bandwidth bottleneck.
Ethernet networking and cabling technologies 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 A+ exam.
The 10 Mbps standards include 10Base2, 10BaseT, and 10BaseFL. All of 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
This standard defines 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.
- 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 to a central device known as the hub or the switch. It is typically wired in a physical star topology.
- 10BaseFL
The 10BaseFL Ethernet standard uses fiber optic cables in order to increase the cable segment lengths to 2000 meters.
Table 2-15 gives a summary of 10 Mbps networking standards.
Most of the modern networks support 100 Mbps speeds, which provides better bandwidth for demanding applications. The following is a brief description of 100 Mbps standards:
- 100BaseTX
100BaseTX networks use two pairs or 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 up to 10,000 meters for single-mode cable.
Table 2-16 gives a summary of 100 Mbps networking standards.
The 1000 Mbps (equal to 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
Gigabit standards include 1000BaseLX, 1000BaseSX, and 1000BaseCX. The 1000BaseLX and 1000BaseSX use multimode or single-mode fiber optic cables. The 1000BaseCX standard specifies use of shielded twisted pair (STP) cables.
- 1000BaseT
This standard uses four pairs of CAT 5 UTP cable. Each pair of the CAT 5 cable can achieve maximum data transfer speeds of up to 25 Mbps, making it an overall 1000 Mbps.
Table 2-17 gives a summary of Gigabit Ethernet networking standards.
A wide area network (WAN) consists of two or more interconnected connect local area networks (LANs). Usually a third party—a telephone company or an ISP—is involved in providing a connectivity solution to the organization that needs to set up a WAN. A WAN can be set up using a dial-up telephone line for low bandwidth requirements, or it may be set up using a high-bandwidth dedicated line. It is also possible to tunnel the WAN connection through the Internet. The following sections describe various technologies used for WAN connectivity.
The term Internet Service Provider refers to an organization that provides Internet access or wide area networking facilities. ISPs provide low-cost Internet connectivity to home users via dial-up, cable modem, ISDN (BRI), or Digital Subscriber Lines. For large organizations that require high speed and bandwidth, the connectivity is provided through Gigabit Ethernet, ATM, ISDN (PRI), T-carriers, or Sonet.
On the Internet, there is actually a hierarchy of lower-level and higher-level ISPs. Just as customers connect to an ISP, the ISPs themselves are connected to their upstream ISPs. Several ISPs are usually engaged in peering, where all ISPs interconnect with each other at a point known as Internet Exchange (IX). This is done to allow routing of data to other networks. ISPs who do not have upstream ISPs are called Tier 1 ISPs. These ISPs sit at the top of the Internet hierarchy.
ISDN is a packet-switched network that allows transmission of data and voice over telephone lines. This results in better quality and higher data transfer speeds than regular dial-up connections. ISDN requires dedicated telephone lines or leased lines and hence is expensive. When the two ends need to communicate, one dials the specified ISDN number and the connection is set up. When the communication between the two nodes is over, the user hangs up and the ISDN line becomes free. Computers using the ISDN line need a special network interface known as an ISDN adapter or terminal adapter.
ISDN communications use two types of channels: a bearer channel (B channel) used for data (or voice), and a delta channel (D channel) used for control signals. The two main implementations of ISDN as follows:
- Basic Rate Interface (BRI)
BRI ISDN uses 2 B channels of 64 Kbps each for data/voice, and a D channel of 16 Kbps. The total data transfer speed of BRI ISDN using two B channels is 128 Kbps. The two B channels can also be used separately with 64 Kbps speed.
- Primary Rate Interface (PRI)
PRI ISDN uses 23 B channels of 64 Kbps each for data/voice, and a D channel of 64 Kbps. The total data transfer speed of PRI ISDN is up to 1.544 Mbps. The PRI ISDN is usually carried over dedicated (leased) T1 lines.
Table 2-18 summarizes the two ISDN implementations.
DSL is a family of technologies that uses ordinary analog telephone lines to provide digital data transmissions. It uses different frequencies for voice and data signals, and the same telephone line can simultaneously be used for phone and data transfer. It is commonly used for high-speed Internet access from homes and offices. Different DSL technologies are collectively noted as xDSL and support data transfer speeds from 128 Kbps to 24 Mbps, as given in the following list:
- Asymmetrical DSL (ADSL)
ADSL is the most common of all types of DSL variations. The download speed of data is faster than upload speeds. It uses one channel for analog voice (telephone) transmissions; a second channel for data uploads, and a third channel for data downloads.
- Symmetrical DSL (SDSL)
SDSL supports equal speeds for both data uploads and downloads. It cannot be used for voice transmissions and hence is suitable only for Internet access at offices.
- ISDN DSL (IDSL)
IDSL is a variation of symmetric DSL. It does not support analog voice transmissions and is used only in those environments where ADSL and SDSL are not available.
- Rate Adaptive DSL (RADSL)
RADSL is a variation of asymmetric DSL that can vary the transfer speeds depending on line conditions. It supports both data and voice transmissions.
Table 2-19 provides a summary of different DSL variations and their data transfer speeds.
Broadband Internet Access, or simply Broadband, is provided by the cable companies that provide digital cable services. It is a reliable and efficient means of Internet access. The coaxial cable connects to a cable modem that further connects to the computer or other network device (hub, switch, or router) using a UTP cable. The cable connection can be shared among several computers in a home or in small offices using low-cost wired or wireless routers.
With a cable modem, the user does not have to dial the ISP and the connection is always there. This might pose a security risk for computers that are used for critical purposes. Most cable modems support bandwidths from 1.5 Mbps to 3 Mbps for the Internet access. The cable modem usually supports up to 10 Mbps data speeds for the LAN. The actual Internet access speed depends on the utilization of the shared cable signals in the area. The available bandwidth is always shared with other users in the area and may vary from time to time. In the periods of peak usage, the speed may be low compared to the periods when usage is low.
In such areas where DSL or cable is not available, satellite is the only option for high-speed WAN connectivity and Internet access. For this reason, it is commonly used in rural areas. The signals travel from the ISP to a satellite and then from the satellite to the user. The data transmission speeds vary from 512 Kbps (upload) to 2 Mbps (download). Major drawbacks of satellite Internet access are that it is expensive and offers low-transfer speeds compared to DSL and cable.
Satellite Internet access suffers from propagation delays or latency problems. Latency refers to the time taken for the signal to travel from ISP to the satellite, located in the geostationary orbit at 35,000 Km. above earth, and then back to the user. Latency also depends on atmospheric conditions.
Dial-up networking using the Plain Old Telephone System (POTS) and the Public Switched Telephone Network (PSTN) is the traditional method of connecting to the Internet or to remote access servers. The user typically dials the telephone number of the ISP to authenticate and get Internet connectivity or to connect to another remote network. The telephone line is connected to a modem, which is further connected to a serial or USB port of the user's computer. Most computers these days have built-in modems that can be directly connected to the telephone line. POTS/PSTN provides a maximum data transfer speed of 56 Kbps.
Wireless networks rely on radio frequencies to communicate instead of 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 the main equipment generating the radio frequency of the wireless network, the strength of the EM field reduces and the signal becomes weak.
Wireless networks defined in IEEE 802.11 standards use radio frequencies with spread spectrum technology. The two spread spectrum technologies are as follows:
- Frequency-hopping spread spectrum (FHSS)
This 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.
- Direct-sequence spread spectrum (DSSS)
This is a modulation technique used by wireless networks, which uses a wide band of frequency. It divides the signal into smaller parts and then transmits them simultaneously on as many frequencies as possible. DSSS is faster than FHSS and ensures data protection. It utilizes a frequency range from 2.4 GHz to 2.4835 GHz and is used in 802.11b networks.
The most popular of the IEEE 802.11 wireless network standards are 802.11b, 802.11a, and 802.11g. Table 2-20 gives a brief comparison of the characteristics of different 802.11 standards.
Infrared technology uses electromagnetic radiations using wavelengths that are longer than the visible light but shorter than radio frequency. Common examples of Infrared devices are the remote controls used in TVs and audio systems. The following are some of the key characteristics of IrDA wireless communication technology:
It provides point-to-point wireless communications using a direct line of sight.
Infrared waves cannot penetrate through walls.
It supports data transfer speeds ranging from 10 to 16 Mbps.
Infrared devices consume very low power.
Infrared frequencies do not interfere with radio frequencies.
It provides a secure wireless medium due to the short distance (usually 3 to 12 feet).
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 communications:
It supports transmission speeds from 1 Mbps (Bluetooth 1.0) to 3 Mbps (Bluetooth 2.0) over the unlicensed frequency range of 2.4 GHz.
The devices must be within a short range of less than 10 meters.
It offers high resistance to electromagnetic interferences.
Unlike the Infrared signals, it does not require direct line of sight.
It consumes very low power.
Two or more Bluetooth computers form an ad-hoc wireless network.
A cellular wide area network is made up of a large number of radio cells. A separate transmitter located at a fixed site powers each radio cell. This site is known as a base station. The coverage area of a particular cellular network depends on the number of base stations. The most common example of a cellular network is the mobile phone network.
VoIP stands for Voice over Internet Protocol. Other popular names for this technology are Internet Telephony, IP Telephony, and Broadband Phone. VoIP is a mechanism to transmit voice signals over Internet Protocol (IP). The special protocols used to carry voice signals over an IP network are called VoIP protocols. One of the major advantages of VoIP is the ability of a user to make telephone calls from anywhere in the world. Since the VoIP service is heavily dependent on availability and reliability of the Internet connection, this technology is still in the development process.
As a computer technician, you must be able to install and configure a network adapter. Most new computer motherboards have an integrated network interface. In case you are required to install an additional network adapter or install it on a nonintegrated motherboard, you must know how to complete the required tasks such as obtaining the network connection and configuring the properties of networking protocol. This section covers a brief study of network related exam objectives.
Most new desktops come equipped with built-in network adapters. In newer computers, the network interface is integrated with the motherboard. But you might have to install, replace, or upgrade network adapters in some old desktops. For example, you might be asked to replace a 10 Mbps network adapter with a 10/100 Mbps fast network adapter.
When installing a network adapter, you will need to make sure that:
The adapter is compatible with the existing computer hardware.
The adapter driver is meant for the operating system installed on the computer.
The operating system supports the adapter driver.
Whether the adapter is PnP or not.
The adapter driver is available for installation if it is not automatically installed by the operating system.
The following exercise explains the steps involved in installing a network adapter:
Turn off the power to the computer.
Remove the computer case cover. Locate an expansion slot and remove the plastic or metal strip that covers the case opening.
Insert the network adapter into the expansion slot and tighten the screw, if required.
Put the computer case cover back and tighten the screws.
Obtain a network patch cable and connect it to the RJ-45 socket provided on the network adapter and the wall outlet.
Turn on the computer.
Most new network adapter are PnP-compatible. PnP adapters are automatically detected and configured by most operating systems. This configuration includes setting aside system resources such as IRQ, I/O, and DMA for the adapter as well as installation of an appropriate driver.
In case the network adapter is not PnP, you will be required to install the network driver manually. You will need to obtain the driver, which may be available either on the CD-ROM accompanying the network adapter or from the vendor's web site. On Windows XP and Windows 2000 Professional computers, you can use the Add/Remove Hardware applet in the Control Panel to add the network adapter. The Device Manager snap-in can be used to install the network adapter device driver.
The following sections cover some of the common network problems and basic troubleshooting techniques.
One of the easiest methods to troubleshooting network connectivity is to check the visual status indicators on network devices. Almost every network device has some form or other visual indicator that can help find out if the device is working or not. Some network devices have LEDs that change color according to the condition of the device or an interface of the device.
The following list provides guidelines for diagnosing a connectivity problem depending on the status of the LED lights:
- No light or yellow
The device or the port is not operational. It is either not connected or is faulty.
- Solid green
The device or port is connected but there is no activity on the port.
- Flashing green
The device or the port is functioning properly. It is transmitting and receiving data as expected.
- Flashing amber
The network is congested and collisions are occurring on the network media.
Certain devices provide separate LEDs for power, activity, and network collisions. Each of these LEDs can be a good indicator of the connectivity problem.
The cables and connectors used to interconnect network devices are often the cause of a network connectivity problem. Some of the key points to remember while troubleshooting network media are as follows:
Verify that a correct cable type and connectors are used and that they are properly attached.
The total length of a cable used to connect devices must not exceed the specifications.
UTP cables are also prone to interferences generated by crosstalk and electromagnetic interferences. UTP cables should not be run in areas of high EMI (such as near transformers or beside high-voltage electric cables). For ceilings and ducts, a special type of cable known as plenum-rated cable should be used.
The following are some of the common problems with network devices:
If a hub fails, all computers connected to the hub will experience connectivity problems.
A failed switch will also result in connectivity problems to all computers in the network segment.
Routers are used to connect network segments. If a router fails, computers on one of the network segments will not be able to connect to any other network segment.
The following list provides a quick review of the factors that may affect the wireless networks:
Wireless signals degrade as they travel away from a wireless signal-generating device such as the access point. This degradation or attenuation of signals is caused by several environmental factors such as EMI, RFI, walls, etc.
Make sure that the wireless devices such as wireless router, access points, and wireless adapters all support the standard used on the network.
The Service Set Identifier (SSID) enables wireless clients to connect to a wireless access point and access network resources. If a wireless client is reporting connectivity problems, wireless configuration should be checked to make sure that the client is using the correct SSID.
If a user cannot log on to a wireless network, make sure that he has sufficient permissions to log on. Additionally, confirm that the encryption and authentication settings are configured correctly on his computer.