1.0 Media and Topologies: 1.1 Network Elements: The term Computer Network is used when two or more computers connected together to share data files, Internet, printer, applications etc. Network can be very simple, such as a small group of computers that are connected to share the resources using a hub (or switch) or they can be very complex covering large geographical areas that use multiple media (like Frame Relay, leased lines, Ethernet etc.) and protocols (like TCP/IP, Novell, AppleTalk). LAN: LAN is the acronym of Local Area Network, and usually limited to a specific area or location. For example, LAN used in a factory is limited within the boundary of that building. Several small LANs can be combined to make a big LAN using routers, and switches. Users in a small LAN can be grouped into a workgroup. A big LAN may consist of several such small groups of users.
WAN: WAN stands for Wide Area Network. A WAN may be spread across several cities, states, and countries. WANs normally use public switched networks, and telecom lines to communicate across boundaries. WAN vs LAN: o o
o
WAN cover large geographical areas, where as LANs are limited to specific location. The cost per bandwidth of a WAN is much higher than that of a LAN. This is because, WANs use public communications infrastructure like Internet, and leased lines. On the otherhand, the transmission costs on LAN are very negligible since the cabling is laid within a private premises, and normally there is no need to lease any public communication lines. WANs typically use public as well as private network transports; LANs primarily use private network transports.
o
o
The cost, and reliability of transmission of information over LAN and WAN differ significantly. As a result, LANs typically use lower-layer (physical, and datalink layers) protocols suitable for small distance high-bandwidth communications, whereas WANs use protocols that are suitable for high-cost long-distance communications. Some commonly used LAN protocols include Ethernet, TokenRing, and ATM. Commonly used WAN protocols include Frame-Relay, ATM, and X.25.
The Internet is an example of Wide Area Network. Full-Duplex and Half-Duplex Communications: The term Full-Duplex communication mean simultaneous flow of information in both the directions. The term Half-Duplex mean, communication in only one direction at any given point of time. Host, Workstation, and Server: Host, Workstation, and Server are the most common network entities used. Hosts: Host is a generic term representing a network node, and could mean a work station, or a server. It is most commonly used when discussing TCP/IP-related services and functions. Workstations, servers, and any other network device as long as it has one or more IP address it can be considered as hosts. Workstations: The term network normally refers to any computer that is connected to the network; a workstation is a computer that can request resources. Workstations can be clients, but not all clients are workstations. For example, a printer can request resources from the network, but it is a client and not a workstation. Servers: A Server provides required resources to its clients. The Server will have a Network Operating System (NOS) that enables the communication between the Server and Clients. Usually, servers are specialized for single purpose such as file server, proxy server etc. Of course, you can configure a single server to do several jobs like
authentication, file services, proxy services etc., but that depends on how much load the server can take, and provision for future expansion. The following are the some server examples that are dedicated to a single task: File Server: A file server stores, and distributes files over the network to work stations. Print Server: A print server will have one or more printers attached either directly or indirectly (through network), and are managed by the print server. Typically, print requests from work stations go to the print server, and the print server manages the print job for the work station. Now, with the advent of Network Printers, the need for a separate server computer for print services is diminishing. Proxy Server: Performs a function on behalf of other computers. Application Server: Hosts one or more network applications. If an application is residing on a central application server, there is no need to install the same on individual workstations. Individual workstations can request required application from Application Server, and run them transparently. The chief advantage of Application Server is that the applications can be managed centrally. Web Server: A Web Server holds and delivers web pages and other web contents using HTTP, and other web based protocols. Mail Server: A Mail Server is responsible for storing all incoming and outgoing emails. It also receives and delivers individual emails to respective work stations. The work stations normally use email client software to communicate with the Mail Server. Fax Server: Sends and receives faxes for the entire network without the need for paper. Remote Access Server (RAS): Sometimes, it may be necessary to access a corporate network from a remote place. This can be achieved by Remote Access Server. With the help of a RAS, remote users with appropriate client software can connect to the server using dial-up access or leased line. Client/Server Network: The client/server network uses a Network Operating System (NOS) to manage the entire network from a centralized point, that is the server.
The clients make requests to the server and the server responds with the information or access to a resource.
Peer-to-Peer Network: In Peer-to-Peer networks, the connected computers share the resources. If a user of one computer wants to access a resource on another computer, the security check for access rights is the responsibility of the computer holding the resource. With computers becoming more and more powerful in terms of computing resources, peer-to-peer networking is gaining popularity.
Client/Server network vs Peer-to-Peer Network:
The client/server network is more organized and it is easy to find files and resources because they are stored at a centralized location (Server computer). The security of client/server network is high that all the usernames and passwords are stored in the same database and individual users can’t use the server as a workstation. Hence a client/server network gives a better protection against accidental/intentional misuse. On the other hand, in a Peer-to-Peer network, by spreading the resources across individual workstations, it is possible to use the resources of these machines.
Media and Topologies 1.2 Physical Topologies: The term "Physical topology" is used to explain the broad layout of the network cables and workstations, and the location of all network components. It is also indicative of how the information or data flows within the network. The five most common topologies are: o
Bus Topology
o
Star Topology
o
Ring Topology
o
Mesh Topology
Bus Topology: In this topology all the computers are attached to a single continuous cable. The cable is typically a co-axial one, and terminated at both the ends. The cable termination at the ends ensures that the signals are not reflected after reaching the end of the cable. The computers are attached to the cable using wire taps. Using Bus Topology, the information signals are available at every individual workstation. However, the individual work stations discard information that is not meant for them.
Bus topology network is simple to install, inexpensive, and easy to maintain. However, there are several disadvantages of using this topology: 1. Any cut in the cable will result in severe communication disruption, and need to be fixed on priority. 2. It is difficult to affect server, and workstation moves. Star Topology: In a star topology each computer is connected to a centrally placed Hub or a Switch by a separate cable. A typical star topology network is shown below.
Star Topology is the most widely used network topology, and has the following advantages: o
o o
Addition of new work stations or other network devices is easy. You only need a spare port on the central hub, and a drop-down cable to attach a work station. Even if a work station cable is unplugged, or damaged, the network will still be operative. Maintenance of this type of network is more organized, and relatively easy.
Ring Topology: In the ring topology each computer is connected directly to two other computers (one in the up-stream, and another in the down-stream) in the network. A typical network using Ring Topology is shown in the figure below:
The main advantages of Ring Topology include high bandwidth capacity, fault location identification, and ability to cover relatively larger distances. The main disadvantage is that a single cable fault may bring down the network. Mesh Topology: In Mesh topology, a path exists from each node to one or more nodes in the network. In a Full Mesh, every node will have a connection to every other node in the network. A typical diagram shown full mesh connections among five nodes (or hosts) is shown in the figure below:
The main advantage is redundancy. If one connection fails, a node can communicate over another network connection. The chief disadvantages are cost and complexity of the topology. Among the topologies discussed, Star Topology is the most widely used. Ring Topology is used when you need to connect several smaller networks (say, using Star Topology) using high bandwidth backbone.
1.0 Media and Topologies 1.3 Physical Media: The three types of most commonly used physical media are given below: o o o o
Coaxial Twisted Pair Fiber Optic Wireless
Coaxial cable: Coaxial cable is one of the most widely used physical media. As shown in the figure below, it consists of the following:
a. Copper Conductor b. Insulation c. Mesh d. Outside insulation Note that this is for a typical coax (short for co-axial) cable and available in different varieties. Most popular are RG-8, and RG-58 cables.
Co-axial Cable diagram
RG-8 Coax
Thin Etherne (10Base2): The Thin Ethernet (Thinnet or 10Base-2) is one of the widely used form of LAN cabling. Other form of co-axial cabling is called Thick Ethernet. Thin Ethernet uses RG-58A/U cable, whereas Thick Ethernet uses a bit fatter, and more expensive RG-8 cable. Thinnet cables use BNC connectors on either end of RG-58A/U cables. 10Base-2 uses transceivers to transmit and receive signals. The transceiver is normally built into the NIC (Network Interface Card) A transceiver is a device that translates digital signals from a node (or workstation) and translates it baseband signals to communicate on the cabling system. NICs that support Thinnet (10Base2) have built-in transceivers. External transceivers are used for 10Base5 Thicknet. Thick Ethernet (10Base5): The 10Base5 topology uses an external transceiver to attach to the network interface card. The NIC attaches to the external transceiver by an AUI cable to the DIX connector on the
card. Vampire tap, standard BNC, or N-series barrel connectors are used for connecting the transceiver to thicknet cable. Each network segment must be terminated with matching impedence at both ends. Ethernet Type 10Base2
10Base5
Popular Name
RG-Rating
Thinnet
RG-58 A/U
Thicknet
RG-8
Comments 1. 50 Ohm cable 2. Uses stranded center conductor. 1. 50 Ohm cable. 2. Uses solid center conductor.
Table: Coaxial cable Types Twisted-Pair Cable: Twisted-Pair cable consists of multiple, insulated wires that are twisted together in pairs. Sometimes, a metallic shield is placed around the twisted pairs, and it is called shielded twisted-pair (STP). Normally, the cable is used without shielding of individual wire-pairs, and it is called Unshielded Twisted-Pair cable (UTP cable). The twisting of wires reduces electromagnetic interference in the form of cross-talk.
Unshielded Twisted Pair-Figure Fiber-Optic Connector: Two most commonly used fiber-optic connectors are: • •
Straight Tip (ST) Connector, and Subscriber Connector (SC)
The SC Connectors are latched connectors; the latching mechanism holds the connector in securely while in use.
Installation of Fiber Optic Connector/Cable-Figure 12
1.0 Media and Topologies 1.4 Common Network Connectivity Devices: 1.5. WAN Devices The connectivity devices are:
o o o o o o
The Network Interface card (NIC) The hub The switch The bridge Transceivers Wireless access points The router
o
The gateway
o
Network Interface Card (NIC): The Network Interface Card (NIC) used connect the computer to the external network. It will normally have a PCI connector (Edge connector) to connect to one of the PC expansion slots, and an RJ-45 connector to connect to external Ethernet. Note that the interface connectors may differ depending upon the expansion bus being used (for example, PCI, ISA, EISA, USB etc.), and the networking media being used (for example, 10Base2, 10Base5, 10BaseT, etc.). Each of these have their own interface specifications. Almost all NICs have LED indicators showing the network connectivity. A commonly used Network Interface Card is shown in the figure below.
Network Interface Card Model Hub: A Hub connects all the nodes of a network using Twisted Pair (UTP or STP) cables. In a Hub, the signals received on one port are transmitted to all other ports, and vice versa. All nodes (work stations) connected using a Hub can listen to one another all the time. The advantage of using a Hub is low cost, and easy integration. The disadvantage is reduced bandwidth, and data security. The reduction in bandwidth comes due to the fact that all workstations are in the same collision domain. If two or more workstations try to transmit during the same time, it results in collision of signals, and the signals are lost altogether. As a result, the available bandwidth of the Ethernet network is reduced.
A 4-port Hub is shown in the figure.
Switch: A Switch, on the otherhand, do not distribute signals without verifying whether it really needs to propagate to a given port or ports. It decides it based on its internal configuration settings. We can say that a Switch is a Hub with some intelligence.
48-port Switch Bridge: A Bridge functions very similar to a Switch. It segments a given network according to the requirements. Segmentation using a Bridge enables keeping un-intended traffic from entering different segments of a network. Both Bridge, and Switch are OSI layer-2 devices. Bridges filter traffic based on the destination address of the frame. If a frame's destination is a node on the same segment where it originated, it is not forwarded. If it is destined for a node on another LAN, it is connected to corresponding bridge port and forwarded to that port. Transceivers: Transceivers are commonly used with co-axial media using 10Base2 or 10Base5 networking standards. It allows a Network Interface Card to connect to a coax, providing necessary translation of signals. Wireless Access Points (WAP): A wireless access point allows mobile users to connect to a central network node without using any wires . Wireless connectivity is useful for mobile workstations, since there is no wiring involved. The wireless access standards are broadly divided into 802.11a, 802.11b, and 802.11g. 802.11g is most popular among these due to high bandwidth that it provides, and the availability of hardware. A commercially available wireless access point is shown in the figure below.
A WAP device
Back-panel
Router: A Router connects multiple networks, and uses routing to forward packets. It is a OSI Layer-3 device and works on the logical address of a host or a node. Compare this with a Switch which works on the physical address (such as MAC address) of a host or a node. A simple DSL router is shown in the figure below.
Router Gateways:
Gateways are the most complex devices with respect to the functionality. They typically work at the upper most layers of OSI model. A gateway is used to connect two different environments, such as a Frame-Relay network and an X.25 network. 1.5 WAN devices: Other network connectivity devices that may be not directly participating in moving network data are: • • •
Modems ISDN terminal adapters CSU/DSU
Modems: The Term Modem is the acronym of Modulator/DEModulator. There are several types of modems. These include: • •
Dial-up Analog Modem Broadband Modem
Analog modems are widely used to connect to the Internet using normal telephone lines. These modems use the same frequencies used for voice transmission. Therefore, you can not make a call or receive a call (voice call) when using this modem to connect to the Internet. Broadband modems use a different technology to connect to the central office. They do not use the voice frequencies to communicate over the telephone wire. As a result, you can make or receive voice call when using broadband modem. Another advantage using broadband modem is speed. Speeds of several megabits per second are typical using broadband modems, where as it is limited to 56 kbps (or so) when using analog modems. A broadband modem is shown in the figure below.
DSL Modem ISDN terminal adapters: ISDN is short for Integrated Service Digital Network (ISDN). It delivers digital services over conventional telephone wires. You can connect your phone to an ISDN line using a terminal adapter (TA). An ISDN modem provides higher speeds compared to analog modems, but far less speed when compared to broadband modems. CSU/DSU: CSU/DSU stands for Channel Service Unit/Data Service Unit. These are commonly used for leased lines from Telcos. The CSU terminates the line at the customer’s side. The DSU does the transmission of signal through the CSU.
2.0 Protocols and standards: 2.1 Layers of OSI: The Seven layers of OSI model are: • • • • • • •
Application Presentation Session Transport Network Data Link Physical
1 Application layer: This layer provides a means for the user to access information on the network through an application. Many user applications that need to communicate over the network interact with the Application layer protocol directly. The user applications are not part of OSI Application layer, use the networking services offered by the networking protocol suite. Application layer functions typically include identifying communication partners, and determining availability of required resources. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
2 Presentation layer: Presentation layer converts local host computer data representations into a standard network format for transmission on the network. On the receiving side, it changes the network format into the appropriate host computer's format so that data can be utilized independent of the host computer. ASCII and EBCDIC conversions, cryptography, and the like are handled here. Examples of Presentation layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes. Presentation layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding.
3. Session layer: The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session-layer implementations include AppleTalk's Zone Information Protocol (ZIP), and Decent Phase Session Control Protocol (SCP).
4. Transport layer: Transport layer is responsible for providing reliable service between the hosts. Upper layer datagrams are broken down into manageable datagrams and then appropriate header information (such as sequence number, port number, etc.) is added to the datagram before passing it on to the Network layer. Two frequently used transport protocols are the TCP (Transmission Control Protocol) and the UDP (User Datagram Protocol). Important features of Transport layer: •
Transport layer ensures reliable service.
• •
Breaks the message (from sessions layer) into smaller datagrams, and appends appropriate unit header information. Responsible for communicating with the Session layer
Important features of TCP/UDP: • •
•
•
TCP/IP widely used protocol for Transport/Network layers TCP: (Transport Control Protocol) TCP ensures that a packet has reached its intended destination by using an acknowledgement. If not, it retransmits the lost messages. Hence, TCP is called a connection oriented protocol. UDP (Universal Data gram Protocol): UDP simply transmits packets over the internet. It does not wait for an acknowledgement. It is the responsibility of upper layer protocols to ensure that the information had reached the intended partner(s). Hence, UDP is often called connectionless protocol. Application programs that do not need connection-oriented protocol generally use UDP.
5 . Network layer: Network layer is responsible for the routing of packets through the entire network. The layer uses logical addressing for this purpose. Note that the physical address (like MAC address) keeps changing from hop to hop when a packet travels from source to destination. As a result, an address that doesn't change is required to ensure continuity between hops. This is nothing but logical address. For IP networks, IP address is the logical address; and for Novell network, IPX address is the logical address, and so on. This layer also provides for congestion control, and accounting information for the network. IP (Internet Protocol) is an example of a network layer protocol.
6. Data link layer: Data link layer provides delivery of information frames between communicating partners. This layer is responsible for flow regulation, error detection and correction, and framing of bits for transmission. The network data frame is made up of checksum, source address, destination address, and the data itself. The largest frame size that can be sent is known as the maximum transmission Unit (MTU).
Important features of Data link layer: • • •
Assembles bits into frames, making them ready for transmission over the network. Provides error detection, and correction to transmitted frames. If the checksum is not correct, it asks for retransmission. (Send a control message). Consists of two sub layers: 1. Logical Link Control (LLC): Defines how data is transferred over the cable and provides data link service to the higher layers. 2. Medium Access Control (MAC): Controls media access by regulating the communicating nodes using pre-defined set of rules. (i.e. Token passing, Ethernet [CSMA/CD] all have MAC sub-layer protocol).
Different Data link layer protocols define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to logical addressing) defines how devices are addressed at the data link layer. The protocols used in Data link layer are SLIP, PPP, and CSLP.
7. Physical layer: This is the bottom-most layer of the OSI model. The Physical layer handles the bit-level communications across the physical medium. The physical medium could be made up of wired electrical signals, or light, or radio (wireless) signals. Physical layer specifications define characteristics such as media, data rates, maximum transmission distances, and physical connectors. Frequently used Physical layer protocols:
Some of the important standards that deal with physical layer specifications are:
RS-232(for serial communication lines), X.21, EIA 232, and G730. Physical layer and Data link layer implementations can be categorized as either LAN or WAN specifications. 2.1 MAC address: MAC address (Short for Media Access Control) is assigned at the factory to a network device such as NIC. MAC address is protocol independent. Layer 2 devices such as switches, and bridges use MAC address to distinguish between different nodes in a network segment. Compare this with Layer 3 devices such as router that use IP address for transporting packets between different networks (or sub-networks). Layer 3 addresses are logical addresses as they are assigned by the protocols, whereas Layer 1 addresses are physical addresses, and can't be changed without changing, for example, a network card (NIC). 2.1.1 MAC address Format: A MAC address on a network is a 12-digit hexadecimal number (total 48 bits) in the format: MM-MM-MM-SS-SS-SS The first half of a MAC address contains the ID number assigned to the adapter manufacturer. The second half of a MAC address is the serial number assigned to the adapter by the manufacturer. You can find the MAC address on your computer by typing "ipconfig /all" at the command prompt of a Windows OS computer.
In the figure above, the value 00-C1-26-0D-DE-C9 represents the MAC address of the host named "system". Whichever higher layer protocol you use, the MAC address remains the same. In the MAC address given, 00-C1-26 is NIC manufacturer ID, and 0DDE-C9 is the serial number given to the adapter by the manufacturer.
2.3 Subnetting Subnetting is the process of dividing a network into two or more logically addressable smaller networks. Given below are some of the advantages of subnetting a network: •
•
•
•
The IP address space is in ever growing demand. It is important that assigned IP addresses are well utilized. Subnetting enables one to utilize assigned IP addresses optimally. Subnetting minimizes unnecessary traffic. A router is used to route traffic between different subnets. This eliminates unnecessary flooding of network segments with packets destined for some other subnets. Makes the network more manageable. For example, a Company, by allocating separate subnets to each department, can manage the whole network easily. Improve security: When you subnet a larger network, each subnet is a mini network. Typical routing rules apply. This enhances the security of the overall network.
2.4 Public and private networks: Public networks are those that comply with regional government regulations, and protocol standards. These are normally open to Corporates and general public for use. One example of a public network is the Telcos that operate telephone network. Another example of public data network is the Internet. Private networks are normally limited to a compound. Several Corporates implement private networks using public networks. Technologies like leased lines, and virtual private networks enable implementation of private wide area networks (private WANs). Public IP networks should use valid IP address range, whereas private networks may use private/public ip address range. By using a public address range, a Company may seamlessly integrate the Corporate network with public network. However, by using private address range, it is possible to integrate with public network by using proxy servers. A proxy server translates IP addresses according to a given set of rules.
2.5 TCP/IP Protocols A Protocol is a predefined set of rules that dictates how network devices (such as router, computer, or switch) communicate and exchange data on the network. Application Protocols: The Application Protocol are built on the top of TCP/IP protocol suite. The list of protocol include the following: • Simple Network Management Protocol (SNMP) The Simple Network Management Protocol (SNMP) is an applicationlayer protocol designed to manage complex communication networks. SNMP works by sending messages, called protocol data units (PDUs), to different parts of a network. SNMP-compliant devices, called agents, store data about themselves in Management Information Bases (MIBs) and return this data to the SNMP servers. There are two versions of SNMP: Version 1 and Version 2. • File Transfer Protocol (FTP)
FTP is a Client Server protocol, used for copying files between an FTP server and a client computer over a TCP/IP network. FTP is commonly used to communicate with web servers to upload or download files. FTP, the File Transfer Protocol, documented in RFC 959, is one of oldest Internet protocols still in widespread use. FTP uses TCP protocol for
communication, and capable of transferring both binary files and text files. Some popular FTP clients include FileZilla, and cuteFTP. FTP uses port TCP port number 21. • Trivial File Transfer Protocol (TFTP) TFTP stands for Trivial File Transfer Protocol. TFTP is very similar to FTP, but uses UDP protocol for file transfer. UDP, as discusses elsewhere in the tutorial is considered to an unreliable protocol. Hence, TFTP is not frequently used for normal file transfer applications. • Simple Mail Transfer Protocol (SMTP) SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used for sending e-mail messages between servers. SMTP is also used to send email messages from a client machine to a server. An email client such as MS Outlook Express uses SMTP for sending emails and POP3/IMAP for receiving emails from the server to the client machine. In other words, we typically use a program that employs SMTP for sending email, and either POP3 or IMAP for receiving messages from our local (or ISP) server. SMTP is usually implemented to operate over Transmission Control Protocol port 25. • Post Office Protocol (POP3) POP3 stands for Post of Protocol version 3. It is used for fetching messages from an email server. Most commonly used POP3 client programs include Outlook Express, and Mozilla Thunderbird. • Internet Message Access Protocol (IMAP) The Internet Message Access Protocol (commonly known as IMAP or IMAP4) allows a local client to access e-mail on a remote server. The current version, IMAP version 4 is defined by RFC 3501. IMAP4 and POP3 are the two most prevalent Internet standard protocols for e-mail retrieval. • Network File System (NFS) Network File System is a distributed file system which allows a computer to transparently access files over a network. • Telnet The Telnet service provides a remote login capability. This lets a user on one machine log into another machine and act as if they are directly in front of the remote machine. The connection can be anywhere on the local network, or on another network anywhere in the world, as long as the user has permission to log into the remote system. Telnet uses TCP to maintain a connection between two machines. Telnet uses port number 23. • Hypertext Transfer Protocol (HTTP)
A protocol used to transfer hypertext pages across the World Wide Web. HTTP defines how messages are formatted and transmitted, and what actions Web servers and browsers should take in response to various commands. For example, when you enter a URL in your
browser, this actually sends an HTTP command to the Web server directing it to fetch and transmit the requested Web page. Note that HTML deals with how Web pages are formatted and displayed in a browser. HTTP is called a stateless protocol because each command is executed independently, without any knowledge of the commands that came before it. • Network News Transfer Protocol (NNTP) A protocol used to carry USENET posting between News clients and USENET servers. • Line Printer Daemon (LPD), Line Printer Remote (LPR) LPD, LPR are used for serving, and printing using Unix server computes.
2.6 Network Utilities: ARP Utility: The arp utility shows whether an IP address is being resolved to the MAC address. NETSTAT utility: The netstat utility produces TCP/IP statistics, and nbtstat produces NetBIOS over TCP/IP statistics.
A typical output from netstat command is shown in the figuer above. It shows the mapping of local addresses to foreign addresses. TRACERT Utility: The tracert utility finds the route from the computer to any computer on a network. Ping Utility: Ping determines if a particular IP host is responding.
A ping response is shown in the figure above. Note that the local host is always resolved to IP address 127.0.0.1. The example ping command given in the figure ensures that the local host is reachable. FTP utility: The ftp utility allows reliably downloading and uploading files from and to an FTP server across the Internet.
A typical FTP login screen is shown in the figure above. You can login to wayne.edu anonymously, using username "anonymous", and any password. This is possible only because wayne allows anonymous login. Alternatively, you can also use your browser window to ftp to a site. In such case, you need to use the qualified address ftp://ftp.wayne.edu. ipconfig and ifconfig: Ipconfig displays TCP/IP configuration information for Windows NT and later operating systems. The ifconfig utility performs a similar function in UNIX environments, in addition to perform certain interface configuration tasks.
The figure above shows a typical ipconfig command output. It is useful for troubleshooting network connections. winipcfg utility: Winipcfg displays TCP/IP configuration information for Windows 95/98. nslookup and dig utilities: Nslookup and dig displays DNS resolution information.
3.0 Network Implementation:
3.1 Basic Capabilities of Network Operating System (NOS): UNIX/Linux: Unix/Linux is both an Computer Operating System as well as a Network Operating System. It consists of a kernal and a user interface called a shell. Kernel is a main program of Unix system. it controls CPU, memory, hard disk, network card, and other hardware components. Shell is an interface between user and kernel. Shell interprets your input as commands and pass them to kernel.
The following are the important features of UNIX/Linux Operating system: • •
• • • •
It is a multi-user multi tasking: Several users can use the OS/NOS simultaneously for running single or multiple tasks. Data, directory, process, hard disk etc are treated as a file. This is sometimes confusing to those familiar with Windows OS, where devices are treated separately from files and folders. The directories are in a hierarchical structure. The "Top" directory is “/”, which is called slash or root. Every user will have his own directory, called home directory. UNIX/Linux uses standard TCP/IP protocol for communications and network services. Unlike Windows, Unix is case-sensitive. For example, a file named Mydata.txt is different from a file named mydata.txt.
Some commonly used commands in UNIX are listed below: cd: Change directory cp: Copy file or directory ls: Show files in current position mkdir: Create directory mv: Move file or directory pwd: Show current position rm: Remove file or directory rmdir Remove directory man: Display online manual shutdown: Reboot or turn off machine
Netware: Netware is the first NOS to be used for PC LANs. It has multiple client support for multiple client platforms. Netware 4.x and above uses NDS for authentication. NDS is a directory service that keeps track of all of the network entities and performs authentication for them.
Macintosh: The recent of Mac OS version is based on a UNIX kernel, so essentially it is a UNIX server. In addition to Mac clients, a Mac server can support UNIX clients and Windows clients. Authentication is handheld through Kerberos and Open Directory. File and print services are handheld through AppleShare and other open protocols and utilities. Mac OS has limited application support.
Windows: Windows Server uses the same interface and, to some extent, the same software as the desktop Windows versions. Earlier versions of Windows did not support Networking. Now, ofcourse, all Windows OSes support networking. Given below are the Windows OSes that support networking: • • • • • • • •
Windows 95 Windows 98 and Windows 98 Second Edition Windows Millenium (ME)Windows NT Windows 2000 Windows XP Windows 2000 Server Windows 2003 Server
Though the desktop Windows versions like Win9x support networking, it is limited to peer-to-peer networking. Windows 2000, and Windows2003 support client-server computing. Windows 2003 Server is among the most advanced servers offered by Microsoft. The important features offered by Windows 2003 Server include the following: 1. Terminal Services Terminal Services allows desktop computers to run applications on a central server, as if they were running locally. This enables centralized applications deployment, and management. 2. Web and Application Services Windows Server 2003 provides a integrated Web server solution. The Web server can host and serve web sites over a corporate intranet or the Internet. 3. Security Services Windows 2003 Server offers the following security features for organizations deploying the server: • • • •
Kerberos Authentication Public Key Infrastructure (PKI) Network Access Protection Security Configuration Wizard
4. Networking and Access Technologies Windows 2003 Server has the following networking features: • • • • • • •
DHCP (Dynamic Host Configuration Protocol) Authentication Internet Protocol Security (IPSec) Supports Internet Protocol Version 6 (IPv6) Routing and Remote Access Virtual Private Networks (VPNs) Wireless Networking
5. Windows SharePoint Services: Windows SharePoint Services offers file sharing and team collaboration benefits for Windows Server 2003 environments.
3.2 Firewalls A firewall is a device (sometimes it could be a system) that prevents un-authorized access to a network from external sources. For example, any network that is connected directly to the Internet need some kind of firewall to protect the entire network from potential intrusions from the Internet.
A "Firewall" may be implemented using one or more of the following technologies: • • • • •
Sl. No. 1. 2. 3.
Proxy Server Network Address Translator (NAT) Packet filtering Access Control Lists (ACLs) DMZ Firewall technology Proxy service Packet Filtering Stateful inspection
OSI layer at which the firewall operates Layer 7 Layers 3 and 4 Layers 2,3, and 4
Proxy servers: Proxy servers hides network resources behind itself. For example, by using Proxy Server, the internal IP addresses of a Corporate network can be made invisible to the external world. It is usually a software program, that resides as an application on top of the Operating System. A Proxy Server may work at several layers of OSI model, validating the data at each layer.
Network Address Translator (NAT): Network Address Translation enables an internal network to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. A NAT box located where the local network interfaces with the external network. NAT serves three main purposes: • •
•
Hides internal IP addresses from the external network. Conserve public IP address space by enabling he use of more internal IP addresses. Public IP addresses are used only for communication with external world. Provide security to the internal network resources.
There are two types of NAT widely used: • •
Static NAT Dynamic NAT
Static NAT: In a Static NAT, a private IP address is mapped to a fixed public IP address. The public address is always the same IP address for
a given internal private IP address. The advantage of having a fixed mapping of private IP to public IP is that the internal resources such as web server can be reached from external network. The main disadvantage is that it still takes one precious IP address from the public Internet. Dynamic NAT: Dynamic NAT maps a private IP address to a public IP address that is dynamically selected from a pool of one or more public IP addresses. The main advantages of dynamic NAT include the following: • •
Dynamic NAT provides securoty to an internal network as it masks the internal network from external world. It conserves public IP addresses by using private IP addresses on the internal network.
One of the main disadvantages is that if you need to locate a server on the internal network, such as an e-mail server, that has to be accessed from the public Internet, then you can not use dynamic NAT. The internal email server has to be assigned with a static mapping of IP address.
Packet Filtering: Packet Filtering is the ability of a router or a firewall to discard packets that don’t meet certain criteria. A packet filtering router should be able to filter IP packets based on the following four fields: • • • •
Source IP address Destination IP address TCP/UDP source port TCP/UDP destination port
Filtering is used to: • • • •
Allow/block Allow/block Allow/block Allow/block
connections connections connections connections
from specific hosts or networks to specific hosts or networks to specific ports from specific ports
Packet filtering is usually employed by routers, and faster than Proxy servers that operate at higher layers. The main disadvantage of packet filters is that they operate at layer 3/4 of OSI model, and do not have the capability to analyze data or the traffic. As a result, it is possible for malicious applications to enter a protected network.
Access Control List (ACLs): ACL is similar to packet filtering.
The Demilitarized Zone (DMZ): DMZ is used by most of the firewalls, which is a network segment that is neither public nor local, but halfway between. A standard DMZ setup has three network cards in the firewall computer. The first goes the Internet, the second goes to the network segment and the third connects to the intranet.
3.3 VLANs Using traditional LAN segmentation, all the segments will be in the same broadcast domain. This effectively, reduces the efficiency of the network. A traditional LAN segmentation is shown below.
Figure 1: Traditional LAN Segmentation VLANs: Using VLANs, the broadcast domain gets divided into the number of VLANs. If there are three VLANs, as shown in the figure, the broadcast domain will be split into three.
Figure 2: Segmentation using VLANs Logical View: Given below is the logical view of segmentation using VLANs. Note that a router is required to route traffic between VLANs, and each VLAN is in different broadcast domain.
Only a single router is shown for simplicity. One or more routers may be used for routing interVLAN traffic. With VLAN, its logical topology is independent of the physical topology. Each computer can be assigned a VLAN identification number (ID), and computers with the same VLAN ID can communicate freely as if they were on the same physical segment. The communications between VLANs is secure, because the packets meant for one VLAN will not be forwarded to other VLANs.
3.5 Antivirus software Antivirus software is specialized software that detects and removes harmful programs called viruses from your computer. It must be updated regularly to be aware of the most common viruses and their behavior.
3.6 Fault tolerance Fault tolerance and disaster recovery must be implemented at some point and to some level on every network. Fault tolerance provides a means by which a computer or network has redundancy or the ability to recover from small faults and to continue providing services during fault. The following methods provide fault tolerance for hard-disk systems: o
Mirroring
o
Duplexing
o
Data Striping
o
Redundant Array of Independent Disks (RAID)
Disk Mirroring: Mirroring a drive means designating a hard-disk drive in the computer as a duplicate to another specified drive. The two drives are attached to a single disk controller. This fault tolerance feature is provided by most of the network operating systems. When any data is written into the drive, the same data is also written to the drive designed as the mirror. If the drive fails, the mirror drive is already online, and because it has supplicate information, the users won’t realize that a disk drive in the server has failed. The NOS notifies the admin that the failure has occurred. On the other side if the disk controller fails neither drive is available.
Disk Duplexing:
Duplexing also saves data to a mirror drive; the only major difference between duplexing and mirroring is that duplexing uses two separate controllers. Hence duplexing not only provides redundant disk but also redundant controller. Duplexing provides fault tolerance even if a controller fails.
Disk Striping: Disk striping breaks up the data that are to be saved to the disk into small portions and sequentially writes the portions to all disks simultaneously in small areas called strips. These strips maximize performance because all of the read/write heads are working constantly.
Disk Striping
RAID: (Redundant Array of Inexpensive Disks) RAID uses an array of less-expensive hard disks and provides several methods for writing tot hose disks to ensure redundancy. RAID has seven levels; each level is designed for specific purposes. RAID 0: The RAID 0 is the commonly used disk. This method is the fastest because all read/write heads are constantly being used without the burden of parity or duplicate data being written. This RAID level improves the performance; it does not provide fault tolerance.
RAID 1: This is also commonly used disk. This level uses hard disks, one mirrored to the other. RAID 1 is the most basic level of disk fault tolerance. If the first hard disk fails, the second hard disk automatically takes over. The parity or error-checking information is not stored. Rather the drives have duplicate information. If both the drives fail a new drive must be installed and configured. This level provides fault tolerance.
RAID 2: In this level individual bits are stripped across multiple disks. Multiple redundancy drives in this configuration are dedicated to storing errorcorrecting code.
RAID 3: At this level data is striped across multiple hard drives using a parity drive. The data are striped in bytes and not in bits as of RAID 2. This configuration is popular because more data is written and read in one operation that increases overall disk performance.
RAID 4: This level is similar to RAID 2 and 3 expect the data is striped in blocks, which facilitates fast reads from one drive. This is not popular implementation.
RAID 5: This level is commonly used; at this level the data and parity are striped across three or more drives. This allows fast reads and writes. This works well if one disk fails.
RAID 6: RAID 6 is similar to RAID 5. It is less popular. This level uses RAID 5 as a basis but duplicates the parity information, saving the second copy on a different drive from one on which the first copy was saved.
3.6 Disaster recovery Backup Types: Each backup type differs primarily in the amount backed up and whether the items being backed up have changed. There are three backup types: o
Full
o
Differential
o
Incremental
Full backup: Full backups typically back up the entire contents of a server, whether they’ve changed or not. Incremental backup: An incremental backup stores all files that have changed since the last Full, Differential, or Incremental backup. The chief advantage of an incremental backup is that it takes the least time to complete, and requires less storage space. However, during a restore operation, you need to process each incremental back for complete restoration. This could result in a lengthy restoration time compared to Full and/or Differential backups. Differential backup: A differential backup contains all files that have changed since the last full backup. The advantage of a differential backup is that it shortens the backup and restore time compared to a full backup and incremental backup. Tape Rotation Schedule: The rotating backup tapes is the easiest way to manage a tape backup scheme. Some of the rotations are, Weekly Rotation:
Weekly Rotations are the simplest one. A tape is assigned to each weekday and labeled with the name of the day. There will be five tapes and each tapes are overwritten as the day of the week comes again. Monthly Rotation: Monthly rotation will help in restoring data for an entire month. Managing this type of backup scheme is more complicated because more tapes must be kept in track. Yearly Rotation: Yearly backup can be build on top of the monthly system. The tapes needed are 12, one for each month, labeled with the names of the months. Grandfather-Father-Son Rotation: Grandfather-Father-Son method is one of the most commonly used tape backup schemes. With this rotation backups are taken daily, weekly, and montly. The daily incremental backups are known as the Son, because the daily taps are reused after a week. The last full backup of the week is known as the Father, because the weekly tapes stay for a month and are reused during next month. The last full backup of the month is known as the Grandfather. Grandfather tapes are the oldest and are retained for a year before reusing them. Note that the G-F-S is a hierarchical backup mechanism, and it can be used for backing up using CD media as well, in the same way it is used for tape backup. Virus Protection: A virus is a program that causes malicious change in your computer and makes copies of it. Sophisticated viruses encrypt and hide themselves to thwart detection. File viruses A file virus attacks executable application and system program files, such as those ending in .COM, .EXE and .DLL. Most of these types of viruses replace some or all of the program code with their own. Macro viruses A macro is a command script that is used to automatically perform operations without a user’s intervention. A Macro virus, for example,
may use the Visual Basic macro-scripting language to perform malicious or mischievous functions in data files created with MS Office products. Boot Sector viruses Boot sector viruses affect the master boot record (MBR) of a computer. MBR is track one, sector one on your hard disk. Any computer, at bootup, checks MBR to find a pointer for the operating system. A boot sector virus will overwrite the boot sector, thereby making it look as if there is no pointer to your operating system. Scanning for viruses: An anti-virus scan is the process in which an anti-virus program examines the computer suspected of having a virus and eradicates any virus it finds. There are two types of anti-virus scans: • •
On-demand On-access
An on-demand scan searches a file, a directory, a drive, or an entire computer. An on-access scan checks only file you are currently accessing. To maximize the protection both should be used in combination. One most frequently used application for on-access scan is email access. When you are using, say Thurnderbird (a Mozilla email client program) to access email, an on-demand anti-virus program kicks-in, and scans incoming email One such program, free for home use, is AVG Antivirus program. AVG is both an on-demand, and on-access antivirus package.
4.0 Network Support: 4.1 Troubleshooting scenario Troubleshooting is solving problem. It has eight steps, they are • • • • •
Establish symptoms. Identify the affected area. Establish what has changed. Select the most probable cause. Implement a solution.
• • •
Test the result. Recognize the potential effects of the solution. Document the solution.
Step 1: Establish Symptoms: First, find the symptoms that resulted in failure. This could be obtained by questioning related people about what exactly was the problem, how often and when it happened etc. Step 2: Identify the Affected Area: Identifying the affected area narrows down what to troubleshoot. The main goal is to make problems easier to troubleshoot and, thus, get users working again as soon as possible. Step 3: Establish what has changed: The next step is to determine what has changed. In this step also self questions are asked and the changes are determined. Step 4: Select the Most Probable Cause: After the problem is observed and the cause is isolated, the next step is to select the most probable cause for the problem. This gets easier with time and experience. Step 5: Implement a Solution: On the analysis of the problems the solution is suggested and implemented. Step 6: Test the Result: After the changes are made the testing should be done on the solution to find it it solves the problem. The user repeats the operation that previously did not work, if it works the problem is solved if not the steps 4,5 and 6 are to be redone. Step 7: Recognize the Potential Effects of the Solution: Before fully implementing the solution to a problem make sure of the potential effects of the solution and other problems it may cause are completely aware. Step 8: Document the solution:
Once a solution is obtained to a problem it is to be documented so that it may help in future if the same problem occurs.
4.0 Network Support: 4.2 The Troubleshooter’s Resources Log Files: The log files can indicate the general health of a server; log files contain a running list of all errors, their description, the time and date they occurred and other information. NetWare Log files: Three log files solve NetWare server problems, they are: o
The Console Log file (CONSOLE.LOG) This log file keeps a history of all errors that have occurred and information that has been displayed on the server’s console. It is located in the SYS:\ETC directory on the server.
o
The Abend Log file (ABED.LOG) This log file registers all Abends on a NetWare server. An Abend (Abnormal END) is an error condition that can halt the proper operation of the NetWare server.
o
The Server Log file (SYS$LOG.ERR) The server log file lists any errors that occur on the server, including Abends and NDS errors, time and date that has occurred.
Windows 2000 server Log files: Log files in Windows 2000 server Operating system are: o
The System Log
This log file tracks every event that occurs on that computer. It is similar to NetWare’s SYS$LOG.ERR. The system log tracks only three main types of events. They are
o
§
Information
§
Warning
§
Error
The Security Log This log tracks security events specified by the system or domain’s Audit policy. The security log displays two types of events: § §
o
Success Audit (The event passed the security audit) Failure Audit (The event failed the security audit)
The Application Log
This log is similar to other two logs except that it tracks events for network services and applications.
4.0 Network Support: 4.3 Manufacturers Troubleshooting Resources: The troubleshooting resources are used to solve the pesky problems that have no pattern or few recognizable symptoms. Most popular resources are o
README files
o
Telephone support
o
Technical support CD-ROM
o
Technical support website
README Files: This file contains the latest information released about the software. Tips, default settings and installation information are also found in the file. Telephone support: All the software manufacturers have toll-free support numbers and so the customers can call them for support. The Technical support CD-ROM: With the development of CD-ROM technology, it became possible to put volumes of textual information on a readily accessible medium. The Technical Support Website: The Internet proved to be the perfect medium for allowing network support personnel access to the same information that was on the technical support CD-ROMs.
4.0 Network Support: 4.4 Hardware Network Troubleshooting Tools: The four most popular hardware tools are: o o o o
A A A A
crossover cable hardware loopback tone generator tone locator
The Crossover cable: The crossover cable is typically used to connect two hubs or switches. It can also be used to test communications between two workstations directly, bypassing the hub. The cable is used only in Ethernet UTP installations. The standard Ethernet UTP crossover cable used in both situations has its transmit and receive wire pairs crossed so that the transmit set on one side is connected to the receive set on the other. In the following figure four of the wires are crossed pin 2 & 3 connect to pin 4 & 6 and pin 4 & 6 connect to pin 2 & 3.
Standard Ethernet 10Base-T crossover cable Sometimes, when you try to troubleshoot a computer, you may need to connect to it without the help of a Hub or a Switch. Then, you can not hook a diagnostic equipment like a signal analyzer directly using a straight cable. You need a crossover cable to connect directly. The Hardware Loopback: A hardware loopback is a special connector for the Ethernet 10Base-T NICs. It is used by the NIC’s software diagnostics to test transmission and reception capabilities.
Hardware loopback and its connections The NIC manufacturers provide diagnostic routines that could be used to troubleshoot NICs for proper functioning. Such diagnostic routes normally use hardware loopbacks through which an NIC transmits, and receives the same data for further diagnsis. Tone Generator and Tone Locator: The combination of tone generator and tone locator is used in telephone systems to locate cables. The tone generator is a small electronic device that sends an electrical signal down one set of UTP wires. The tone locator is another device that is designed to emit a tone when it detects the signal in a particular set of wires. When a cable is needed to be traced, hook the generator to the copper ends of
the wire pair to be found. Then move the locator over multiple sets of cables until a tone is heared.
4.0 Network Support: 4.5 Software Troubleshooting Tools: The software troubleshooting tools are: o o
Protocol analyzers Performance-monitoring tools
Protocol Analyzer: Protocol analyzer examine packets from protocols that operate at the lower four layers of the OSI model and can display any errors they detect. Performance-Monitoring Tools: Many network operating systems include tools for monitoring network performance and can display statistics such as the number of packets sent and received, server processor utilization, the amount of data going in and out of the server and etc.
4.0 Repeaters, Bridges, Routers, and Gateways: A comparative study: 4.1 Repeaters: As signals travel along a network cable (or any other medium of transmission), they degrade and become distorted in a process that is called attenuation. If a cable is long enough, the attenuation will finally make a signal unrecognizable by the receiver. A Repeater enables signals to travel longer distances over a network. Repeaters work at the OSI's Physical layer. A repeater regenerates the received signals and then retransmits the regenerated (or conditioned) signals on other segments.
To pass data through the repeater in a usable fashion from one segment to the next, the packets and the Logical Link Control (LLC) protocols must be the same on the each segment. This means that a repeater will not enable communication, for example, between an 802.3 segment (Ethernet) and an 802.5 segment (Token Ring). That is, they cannot translate an Ethernet packet into a Token Ring packet. In other words, repeaters do not translate anything.
4.2 Bridges: Like a repeater, a bridge can join segments or workgroup LANs. However, a bridge can also divide a network to isolate traffic or problems. For example, if the volume of traffic from one or two computers or a single department is flooding the network with data and slowing down entire operation, a bridge can isolate those computers or that department. In the following figure, a bridge is used to connect two segment segment 1 and segment 2.
Bridges can be used to: • • •
Expand the distance of a segment. Provide for an increased number of computers on the network. Reduce traffic bottlenecks resulting from an excessive number of attached computers.
Bridges work at the Data Link Layer of the OSI model. Because they work at this layer, all information contained in the higher levels of the OSI model is unavailable to them. Therefore, they do not distinguish between one protocol and another. Bridges simply pass all protocols along the network. Because all protocols pass across the bridges, it is up to the individual computers to determine which protocols they can recognize. A bridge works on the principle that each network node has its own address. A bridge forwards the packets based on the address of the particular destination node. As traffic passes through the bridge, information about the computer addresses is then stored in the bridge's RAM. The bridge will then use this RAM to build a routing table based on source addresses. 4.3 Routers: In an environment consisting of several network segments with different protocols and architecture, a bridge may not be adequate for ensuring fast communication among all of the segments. A complex network needs a device, which not only knows the address of each segment, but also can determine the best path for sending data and filtering broadcast traffic to the local segment. Such device is called a Router. Routers work at the Network layer of the OSI model meaning that the Routers can switc h and route packets across multiple networks. They do this by exchanging protocol-specific information between separate networks. Routers have access to more information in packets than bridges, and use this information to improve packet deliveries. Routers are usually used in a complex network situation because they provide better traffic management than bridges and do not pass broadcast traffic. Routers can share status and routing information with one another and use this information to bypass slow or malfunctioning connections. Routers do not look at the destination node address; they only look at the network address. Routers will only pass the information if the network address is known. This ability to control the data passing through the router reduces the amount of traffic between networks and allows routers to use these links more efficiently than bridges 4.4 Gateways: Gateways make communication possible between different architectures and environments. They repackage and convert data going from one environment to another so that each environment can understand the other's environment data. A gateway repackages information to match the requirements of the destination system. Gateways can change the format of a message so that it will conform to the application program at the receiving end of the transfer. A gateway links two systems that do not use the same: • • •
Communication protocols Data formatting structures Languages
•
Architecture
For example, electronic mail gateways, such as X.400 gateway, receive messages in one format, and then translate it, and forward in X.400 format used by the receiver, and vice versa. To process the data, the gateway: · Decapsulates incoming data through the networks complete protocol stack. Encapsulates the outgoing data in the complete protocol stack of the other network to allow transmission. 4.5 NIC (Network Interface Card): A NIC or Network Interface Card is a circuit board or chip, which allows the computer to communicate to other computers on a Network. This board when connected to a cable or other method of transferring data such as infrared can share resources, information and computer hardware. Local or Wide area networks are generally used for large businesses as well as are beginning to be found in homes as home users begin to have more then one computer. Utilizing network cards to connect to a network allow users to share data such as companies being able to have the capability of having a database that can be accessed all at the same time send and receive e-mail internally within the company or share hardware devices such as printers. 4.6 Connectors: Network cards have three main types of connectors. Below is an example of what a network card may look like.
4.6.1 BNC connector: As illustrated in the above picture the BNC connector is a round connector, which is used for thin net or 10Base-2 Local Area Network. 4.6.2 DB9 (RJ45 JACK): The DB9 connector not to be confused with the Serial Port or sometimes referred to as the RJ45 JACK not to be confused with the RJ45 connection is used with Token Ring networks. 4.6.3 DB15 Connector: The DB15 connector is used for a Thick net or 10Base-5 Local area network. 4.6.4 RJ45 connector: Today one of the most popular types of connections used with computer networks. RJ45 looks similar to a phone connector or RJ11 connector however is slightly larger. LED -The LED's as shown in the above illustration indicates if it detects a network generally by a green light which may flash as it communicates and then a red light which indicates collisions which will generally flash or not flash at all. 4.7 Cables
The following is a few examples of some of the more commonly used types of cables found with networks.
As illustrated in the above picture you can see three of the main types of cable used today. The first two pictures illustrate Unshielded and Shielded twisted pair cables. Unshielded twisted-pair cable is generally found in phone cables today and are used more often then shielded twisted pair today as it has been found that simply twisting the cable provides more efficient means of protection against interference. In addition shielded twisted-pair cable required the one end of the cable to be grounded. If both ends were to be grounded however this would cause a grounding loop causing low voltage and infinite amperage and various other hazards to the network. The third picture in the above illustration shows a coaxial cable, which are the most commonly used and known types of cables. This cable can be found for cable TV and when used with networks utilize the BNC connector. As illustrated in the above picture you can see three of the main types of cable used today. The first two pictures illustrate Unshielded and Shielded twisted pair cables. Unshielded twisted-pair cable is generally found in phone cables today and are used more often then shielded twisted pair today as it has been found that simply twisting the cable provides more efficient means of protection against interference. In addition shielded twisted-pair cable required the one end of the cable to be grounded. If both ends were to be grounded however this would cause a grounding loop causing low voltage and infinite amperage and various other hazards to the network. The third picture in the above illustration shows a coaxial cable, which are the most commonly used and known types of cables. This cable can be found for cable TV and when used with networks utilize the BNC connector.
