Monday, June 21, 2010

Home Networking

Computer networks have existed for more than thirty years, but only relatively recently have they become popular in homes. In 1999, only a few hundred thousand households in the United States possessed a home network, although many more "expressed interest" in having one.

Today, many millions of households in the U.S. and worldwide have adopted home computer networking. Millions more have yet to build their first home network. Even those who've previously taken the plunge are now beginning to revamp their networks for wireless - the current wave of useful technology for home networking.

Depending on your present goals and past experience, varying types of information may be relevant to your situation. Use the outline below as a comprehensive guide to your personal research.

Do I Really Need a Home Computer Network?

Some of you likely share files between your computers using floppy disks or USB keys. A home network allows you to share these files much faster and more conveniently by utilizing the available connections between computers. Home networks allow sharing of other things, too, like a printer and an Internet connection. Finally, home networks create the possibility to use new applications like multi-player online games.

Benefits of Home Computer Network

  • file sharing - Network file sharing between computers gives you more flexibity than using floppy drives or Zip drives. Not only can you share photos, music files, and documents, you can also use a home network to save copies of all of your important data on a different computer. Backups are one of the most critical yet overlooked tasks in home networking.
  • printer / peripheral sharing - Once a home network is in place, it's easy to then set up all of the computers to share a single printer. No longer will you need to bounce from one system or another just to print out an email message. Other computer peripherals can be shared similarly such as network scanners, Web cams, and CD burners.
  • Internet connection sharing - Using a home network, multiple family members can access the Internet simultaneously without having to pay an ISP for multiple accounts. You will notice the Internet connection slows down when several people share it, but broadband Internet can handle the extra load with little trouble. Sharing dial-up Internet connections works, too. Painfully slow sometimes, you will still appreciate having shared dial-up on those occasions you really need it.
  • multi-player games - Many popular home computer games support LAN mode where friends and family can play together, if they have their computers networked.
  • Internet telephone service - So-called Voice over IP (VoIP) services allow you to make and receive phone calls through your home network across the Internet, saving you money.
  • home entertainment - Newer home entertainment products such as digital video recorders (DVRs) and video game consoles now support either wired or wireless home networking. Having these products integrated into your network enables online Internet gaming, video sharing and other advanced features. 

Although you can realize these same benefits with a wired home network, you should carefully consider building a wireless home network instead, for the following reasons:

  1. Computer mobility. Notebook computers and other portable devices are much affordable than they were a few years ago. With a mobile computer and wireless home network, you aren't chained to a network cord and can work on the couch, on your porch, or wherever in the house is most convenient at the moment.
  2. No unsightly wires. Businesses can afford to lay cable under their floors or inside walls. But most of us don't have the time or inclination to fuss with this in our home. Unless you own one of the few newer homes pre-wired with network cable, you'll save substantial time and energy avoiding the cabling mess and going wireless.
  3. Wireless is the future. Wireless technology is clearly the future of networking. In building a wireless home network, you'll learn about the technology and be able to teach your friends and relatives. You'll also be better prepared for future advances in network technology coming in the future.
Before purchasing and installing equipment, determine your needs. The location of your computer(s), printer(s) and other devices you want to connect can affect your network design. Some systems may already contain built-in networking capability while others may not.

Those devices that need network capability added, probably support only certain kinds of network gear. Take care to buy compatible gear. 

If you desire shared Internet access, be sure to factor this into your design. Other important factors in network design include reach and speed.

Finally, consider both present and future needs in your plan. How many computers will your network have in the next year or two? A network with just two computers can involve a very different design than a network with five, for example.

Use our interactive network advisor tool to walk you through the factors to consider when designing your home network:

Wednesday, June 16, 2010

Introduction to TCP/IP

Summary: TCP and IP were developed by a Department of Defence (DOD) research project to connect a number different networks designed by different vendors into a network of networks (the "Internet"). It was initially successful because it delivered a few basic services that everyone needs (file transfer, electronic mail, remote logon) across a very large number of client and server systems. Several computers in a small department can use TCP/IP (along with other protocols) on a single LAN. The IP component provides routing from the department to the enterprise network, then to regional networks, and finally to the global Internet. On the battlefield a communications network will sustain damage, so the DOD designed TCP/IP to be robust and automatically recover from any node or phone line failure. This design allows the construction of very large networks with less central management. However, because of the automatic recovery, network problems can go undiagnosed and uncorrected for long periods of time.
As with all other communications protocol, TCP/IP is composed of layers:
  • IP - is responsible for moving packet of data from node to node. IP forwards each packet based on a four byte destination address (the IP number). The Internet authorities assign ranges of numbers to different organizations. The organizations assign groups of their numbers to departments. IP operates on gateway machines that move data from department to organization to region and then around the world.

  • TCP - is responsible for verifying the correct delivery of data from client to server. Data can be lost in the intermediate network. TCP adds support to detect errors or lost data and to trigger retransmission until the data is correctly and completely received.

  • Sockets - is a name given to the package of subroutines that provide access to TCP/IP on most systems.
Network of Lowest Bidders
The Army puts out a bid on a computer and DEC wins the bid. The Air Force puts out a bid and IBM wins. The Navy bid is won by Unisys. Then the President decides to invade Grenada and the armed forces discover that their computers cannot talk to each other. The DOD must build a "network" out of systems each of which, by law, was delivered by the lowest bidder on a single contract. 

The Internet Protocol was developed to create a Network of Networks (the "Internet"). Individual machines are first connected to a LAN (Ethernet or Token Ring). TCP/IP shares the LAN with other uses (a Novell file server, Windows for Workgroups peer systems). One device provides the TCP/IP connection between the LAN and the rest of the world.

To insure that all types of systems from all vendors can communicate, TCP/IP is absolutely standardized on the LAN. However, larger networks based on long distances and phone lines are more volatile. In the US, many large corporations would wish to reuse large internal networks based on IBM's SNA. In Europe, the national phone companies traditionally standardize on X.25. However, the sudden explosion of high speed microprocessors, fiber optics, and digital phone systems has created a burst of new options: ISDN, frame relay, FDDI, Asynchronous Transfer Mode (ATM). New technologies arise and become obsolete within a few years. With cable TV and phone companies competing to build the National Information Superhighway, no single standard can govern citywide, nationwide, or worldwide communications.

The original design of TCP/IP as a Network of Networks fits nicely within the current technological uncertainty. TCP/IP data can be sent across a LAN, or it can be carried within an internal corporate SNA network, or it can piggyback on the cable TV service. Furthermore, machines connected to any of these networks can communicate to any other network through gateways supplied by the network vendor.

Each technology has its own convention for transmitting messages between two machines within the same network. On a LAN, messages are sent between machines by supplying the six byte unique identifier (the "MAC" address). In an SNA network, every machine has Logical Units with their own network address. DECNET, Appletalk, and Novell IPX all have a scheme for assigning numbers to each local network and to each workstation attached to the network.

On top of these local or vendor specific network addresses, TCP/IP assigns a unique number to every workstation in the world. This "IP number" is a four byte value that, by convention, is expressed by converting each byte into a decimal number (0 to 255) and separating the bytes with a period. For example, the PC Lube and Tune server is

An organization begins by sending electronic mail to Hostmaster@INTERNIC.NET requesting assignment of a network number. It is still possible for almost anyone to get assignment of a number for a small "Class C" network in which the first three bytes identify the network and the last byte identifies the individual computer. The author followed this procedure and was assigned the numbers 192.35.91.* for a network of computers at his house. Larger organizations can get a "Class B" network where the first two bytes identify the network and the last two bytes identify each of up to 64 thousand individual workstations. Yale's Class B network is 130.132, so all computers with IP address 130.132.*.* are connected through Yale.

The organization then connects to the Internet through one of a dozen regional or specialized network suppliers. The network vendor is given the subscriber network number and adds it to the routing configuration in its own machines and those of the other major network suppliers.

There is no mathematical formula that translates the numbers 192.35.91 or 130.132 into "Yale University" or "New Haven, CT." The machines that manage large regional networks or the central Internet routers managed by the National Science Foundation can only locate these networks by looking each network number up in a table. There are potentially thousands of Class B networks, and millions of Class C networks, but computer memory costs are low, so the tables are reasonable. Customers that connect to the Internet, even customers as large as IBM, do not need to maintain any information on other networks. They send all external data to the regional carrier to which they subscribe, and the regional carrier maintains the tables and does the appropriate routing.

New Haven is in a border state, split 50-50 between the Yankees and the Red Sox. In this spirit, Yale recently switched its connection from the Middle Atlantic regional network to the New England carrier. When the switch occurred, tables in the other regional areas and in the national spine had to be updated, so that traffic for 130.132 was routed through Boston instead of New Jersey. The large network carriers handle the paperwork and can perform such a switch given sufficient notice. During a conversion period, the university was connected to both networks so that messages could arrive through either path.

Although the individual subscribers do not need to tabulate network numbers or provide explicit routing, it is convenient for most Class B networks to be internally managed as a much smaller and simpler version of the larger network organizations. It is common to subdivide the two byte available for internal assignment into a one byte department number and a one byte workstation ID. 

The enterprise network is built using commercially available TCP/IP router boxes. Each router has small tables with 255 entries to translate the one byte department number into selection of a destination Ethernet connected to one of the routers. Messages to the PC Lube and Tune server ( are sent through the national and New England regional networks based on the 130.132 part of the number. Arriving at Yale, the 59 department ID selects an Ethernet connector in the C& IS building. The 234 selects a particular workstation on that LAN. The Yale network must be updated as new Ethernets and departments are added, but it is not effected by changes outside the university or the movement of machines within the department.

Need to Know

There are three levels of TCP/IP knowledge. Those who administer a regional or national network must design a system of long distance phone lines, dedicated routing devices, and very large configuration files. They must know the IP numbers and physical locations of thousands of subscriber networks. They must also have a formal network monitor strategy to detect problems and respond quickly.

Each large company or university that subscribes to the Internet must have an intermediate level of network organization and expertise. A half dozen routers might be configured to connect several dozen departmental LANs in several buildings. All traffic outside the organization would typically be routed to a single connection to a regional network provider.

However, the end user can install TCP/IP on a personal computer without any knowledge of either the corporate or regional network. Three pieces of information are required:

  1.  The IP address assigned to this personal computer
  2.  The part of the IP address (the subnet mask) that distinguishes other machines on the same LAN (messages can be sent to them directly) from machines in other departments or elsewhere in the world (which are sent to a router machine)
  3. The IP address of the router machine that connects this LAN to the rest of the world.
In the case of the PCLT server, the IP address is Since the first three bytes designate this department, a "subnet mask" is defined as (255 is the largest byte value and represents the number with all bits turned on). It is a Yale convention (which we recommend to everyone) that the router for each department have station number 1 within the department network. Thus the PCLT router is Thus the PCLT server is configured with the values:

  •  My IP address:
  •  Subnet mask:
  •  Default router:
The subnet mask tells the server that any other machine with an IP address beginning 130.132.59.* is on the same department LAN, so messages are sent to it directly. Any IP address beginning with a different value is accessed indirectly by sending the message through the router at (which is on the departmental LAN).



LAN Devices

Devices commonly used in LANs include repeaters, hubs, LAN extenders, bridges, LAN switches, and routers.

A repeater is a physical layer device used to interconnect the media segments of an extended network. A repeater essentially enables a series of cable segments to be treated as a single cable. Repeaters receive signals from one network segment and amplify, retime, and retransmit those signals to another network segment. These actions prevent signal deterioration caused by long cable lengths and large numbers of connected devices. Repeaters are incapable of performing complex filtering and other traffic processing. In addition, all electrical signals, including electrical disturbances and other errors, are repeated and amplified. The total number of repeaters and network segments that can be connected is limited due to timing and other issues. Figure 6 illustrates a repeater connecting two network segments.

A hub is a physical-layer device that connects multiple user stations, each via a dedicated cable. Electrical interconnections are established inside the hub. Hubs are used to create a physical star network while maintaining the logical bus or ring configuration of the LAN. In some respects, a hub functions as a multiport repeater.

A LAN extender is a remote-access multilayer switch that connects to a host router. LAN extenders forward traffic from all the standard network-layer protocols (such as IP, IPX, and AppleTalk), and filter traffic based on the MAC address or network-layer protocol type. LAN extenders scale well because the host router filters out unwanted broadcasts and multicasts. LAN extenders, however, are not capable of segmenting traffic or creating security firewalls. Figure 7 illustrates multiple LAN extenders connected to the host router through a WAN.

Introduction to TCP/IP

Sunday, June 13, 2010

LAN Cabling

The LAN must have cabling to link the individual workstation with the file server and other peripherals. If were only one type of cabling available, the decision would be simple. Unfortunately, there are many different types of cabling-each with its own vocal supporters. Because there are considerable range in cost and in capability, this is not a trivial issue. This section examine the advantages and disadvantages of twisted-pair, baseband and broadband coaxial cable, as well as fiber-optic cabling.

Twisted-Pair Cable

Twisted-pair cable is by far the least expensive and most common type of network medium. As Figure A illustrates, this cabling consist of two insulated wire twisted together so that each wire receives the same amount of interference from the environment. This "noise" in the environment becomes part of the signal being transmitted. Twisting the wires together reduces (but does not eliminate) this noise. Twisted-pair wire comes in a wide range of pairs and gauges. Wires have an American Wire Gauge number (AWG) based on their diameter. For example, 26-guage wire has a diameter of 0.01594 inch. For networks, 22-and 24-gauge are two most common types of twisted-cabling.

Figure A: Twisted-pair wire (two pair).

Twisted-pair cable is bundled in groups of pairs. The number of twisted pairs per group can range from 2 to 3,000; many LANs use the very same inexpensive, unshielded twisted-pair cable used for telephones, while others require higher data-grade quality. As one option for its token ring network, for example, IBM supports Type 3 unshielded twisted-pair cable (telephone wire) for its token ring network but requires 22 AWG or 24 AWG with a minimum of two twists per linear foot (the more twists, the less interference). It recommends four twisted-pair when new wire is installed, but existing telephone twisted-pair wire must have two spare pairs that can be delicate to the token ring network. On the other hand, AT&T's STARLAN requires higher data-grade quality cabling. AT&T specifies that its network requires 24 gauge shielded two twisted-pair wire ط·آ·ط¢آ¢ط·آ¢ط¢آ¾one pair of wires to transmit data and pair to receive data. Higher grade cabling makes a difference in data transmission quality over longer distances. For example, compare AT&T's Type 3 twisted-pair telephone-wire standard AT&T's workstations can be to 990 feet from wiring closet, while IBM's workstation must be within 330 feet.

The major limitations of twisted-pair wiring are its limited range and its sensitivity to electrical interference. When standards were first proposed for twisted-pair networks, the medium was able to handle transmission speeds of approximately one million bits per second (mbps) over several hundred feet. Today, the industry standard known as 10 base T reflects the technological advances that make it possible to transmit information at 10 mbps over twisted- pair wire and 100 mbps transmission over unshielded twisted-pair wiring is emerging as a new standard. 

Note that the TX (transmitter) pins are connected to corresponding RX (receiver) pins, plus to plus and minus to minus. And that you must use a crossover cable to connect units with identical interfaces. If you use a straight-through cable, one of the two units must, in effect, perform the cross-over function.

Two wire color-code standards apply: EIA/TIA 568A and EIA/TIA 568B. The codes are commonly depicted with RJ-45 jacks as follows (the view is from the front of the jacks):

If we apply the 568A color code and show all eight wires, our pin-out looks like this:

Note that pins 4, 5, 7, and 8 and the blue and brown pairs are not used in either standard. Quite contrary to what you may read elsewhere, these pins and wires are not used or required to implement 100BASE-TX duplexing--they are just plain wasted.

However, the actual cables are not physically that simple. In the diagrams, the orange pair of wires are not adjacent. The blue pair is upside-down. The right ends match RJ-45 jacks and the left ends do not. If, for example, we invert the left side of the 568A "straight"-thru cable to match a 568A jack--put one 180° twist in the entire cable from end-to-end--and twist together and rearrange the appropriate pairs, we get the following can-of-worms:

This further emphasizes, I hope, the importance of the word "twist" in making network cables which will work. You cannot use an flat-untwisted telephone cable for a network cable. Furthermore, you must use a pair of twisted wires to connect a set of transmitter pins to their corresponding receiver pins. You cannot use a wire from one pair and another wire from a different pair.

Keeping the above principles in mind, we can simplify the diagram for a 568A straight-thru cable by untwisting the wires, except the 180° twist in the entire cable, and bending the ends upward. Likewise, if we exchange the green and orange pairs in the 568A diagram we will get a simplified diagram for a 568B straight-thru cable. If we cross the green and orange pairs in the 568A diagram we will arrive at a simplified diagram for a crossover cable. All three are shown below.

There are only two unique cable ends in the preceding diagrams. They correspond to the 568A and 568B RJ-45 jacks and are shown below.

Again, the wires with colored backgrounds may have white stripes and may be denoted that way in diagrams found elsewhere. For example, the green wire may be labeled Green-White--I don't bother. The background color is always specified first.

Now, all you need to remember, to properly configure the cables, are the diagrams for the two cable ends and the following rules:

  • A straight-thru cable has identical ends.
  • A crossover cable has different ends.

It makes no functional difference which standard you use for a straight-thru cable. You can start a crossover cable with either standard as long as the other end is the other standard. It makes no functional difference which end is which. Despite what you may have read elsewhere, a 568A patch cable will work in a network with 568B wiring and 568B patch cable will work in a 568A network. The electrons couldn't care less.

My preference is to use the 568A standard for straight-thru cables and to start crossover cables with a 568A end. That way all I have to remember is the diagram for the 568A end, that a straight-thru cable has two of them, and that the green and orange pairs are swapped at the other end of a crossover cable.

Coaxial Cable

Coaxial cable is almost as easy to install and maintain as twisted-pair is the medium of choice in many major LANs. As figure B illustrates, "coax" is based on a central copper core encased in a plastic sheath that is then surrounded by an outer jacket composed of copper or aluminum acting as a conductor. This also provides protection. The signal is transmitted along the central core with the outer jacket forming a screen from outside electrical interference. This type of cable is commonly found in the home as integral part of cable television. 

Originally, coax was the most common LAN cable due to its high capacity and resistance to interference. Its thickness means it is limited in its ability to be run through small cable ducts and around tight angles. While coax is still widely used, most of the networks that specified this cable type are now able to operate on various other types such as fiber- optic and twisted-pair. The result of which is that coax as a cabling system is beginning to decline.

Baseband Cable

Baseband coaxial cable has one channel that carries a single message at a time at a very high speed. Its carrier wire is surrounded by a copper mesh, and usually the entire cables diameter is approximately 3/8 inch. Digital information is sent one bit at a time across a baseband cable's bandwidth in serial fashion. Depending on the LAN, it is possible for baseband coaxial cable to handle a data rate of 10-80 mbps. Ethernet, which was the first major LAN with nonproprietary communications interfaces and protocols, uses baseband coaxial cable.

Because the Ethernet standard was developed by Xerox Corporation and has been supported by bought Xerox Corporation and Digital Equipment Corporation, baseband cabling is a popular choice for LAN medium. Because of baseband's single channel limitation, it is not possible to send integrated signals composed of voice, data, or video over baseband cable. One advantage of baseband cabling is how easy it is to tap into this cable and connect or disconnect workstation without disturbing network operations. Although the maximum recommended distance for a baseband LAN is approximately 1.8 miles (3 kilometers), 1500 feet (500 meters) might proved to be a more realistic figure if the network is heavily used. While baseband's inability to send integrated signals as well as its distance limitations must be considered while configuring a network, these disadvantages may not be significant if data transmission speed and cause our primary criteria in medium selection.

Broadband Cables

Unlike baseband, broadband coaxial cables have the capacity to carry several different signals broadcast at different frequencies simultaneously. Cable television companies have taken this approach using 75-ohm broadband coaxial cable. Subscribers can select from several different stations its broadcasting on its own designated frequency. All broadband systems can use a single cable with bi-directional amplifiers, as shown in figure C, or a dual cable system, in either case, carrier signals are sent to a central point known as headend (translating and broadcasting device), from which they are re-transmitted to all points in the network. 

The single-cable approach splits a cable by frequency to achieve bi-directional. Transmission of data commercial cable companies use 6 MHz channels for each communication path. Even with some frequencies design as a guard bands between different channels, it is possible to allocate 346 MHz for forward communications (6 MHz/channel x 56 channels) and 25 MHz for the return data path (6 MHz/ channel x 4 channels). The 25 MHz devoted the returning data can be used for several narrow-band channels. Dual-broadband cable uses one cable for inbound data moving toward the headend, and a second cable looped at the headed for the outbound carries. The full-frequency spectrum is available for both inbound and outbound signals. Because of the duplication of cabling, amplifiers, and hardware, dual-broadband cable is much more expensive than the single cable approach, but it makes twice as many usable channels available, and some network might require then. Let's take a closer look at this particular broadband approach. With a Dual-cable configuration, coaxial cable forms a two-way highway composed of two bands. Each of these bands contains several channels. Standard television channels transmit at 6 MHz. Because we have a band with a range of approximately 300 MHz, it is possible to have as 50 channels broadcasting at a data rate of 5 mbps. The inbound band carries data from the LAN's nodes (individual workstations) to the headed; the outbound band carries data to the network nodes. Broadband cable installation requires more planning than baseband. Because the broadband signals are being broadcast, amplifiers need to be installed to maintain the signal strength. In a company with several departments, each department would have drop line with tap lines coming from this line to each node. These taps contain resistors to ensure that all workstations receive signals at the same strength.

Fiber Optic Cable

One of the most exciting advances in media is the use of fiber optics in LANs. This type of data transmission has a number of advantages over twisted-pair and coaxial cable. Besides data transmission rates far higher than either of these older media, fiber-optic cabling is immune to electromagnetic or radio-frequency interference and capable of sending signals several miles without loss. This mode of transmission is also virtually immune to unauthorized reception.

A fiber-optic cable is made of pure glass drawn into very thin fiber forming a core. As Figure D illustrates, these fibers are surrounded by cladding, a layer of glass with a lower refractive index than the glass in the core.

A fiber-optic network uses a laser or LED (light-emitting diode) to send a signal through the core portion of the cable. Optical repeaters are often used along the path to amplify the signal, so it arrives at its destination at full strength. At the receiving end of the cable, the message is translated back into a digital or analog signal by a photodiode. The cabling can consist of a single fiber (monomode), several fibers (multimode), or a variation of multimode (graded index) in which the index of refraction drops slowly from the center of the fiber toward the outside.

Monomode fiber has a very wide bandwidth, but its tiny core makes it extremely difficult to splice without special kits and technical expertise. Also, monomode requires a laser, rather than an LED, as a signalling source, which is more expensive. Multimode fiber has a smaller bandwidth but is much easier to splice. Graded index multimode fiber is the most expensive medium, but it provides the highest transmission rate over the greatest distance.

Multimode fiber optics for network cabling come in groups of 2 to 24 fibers, with groups of 2 to 4 fibers being the norm. each fiber is unidirectional, because a beam of light is transmitted in only one direction. Two-way communication requires another fiber within the cable so that light can also travel in the opposite direction. The American National Standards Institute (ANSI) has established a standard for the physical media-dependent (PMD) layer of the fiber data distributed interface (FDDI) to work in conjunction with data transmission of 100 mbps. It is possible to achieve rates up to 1 gigabit/second (Gbps).

At present, fiber-optic cabling is expensive for most installations, and its sophisticated technology makes it difficult to add new workstations after initial installations. If a company has a serious interference problem, however, or requires absolute network security or the capability of sending signals several miles, fiber optics might be the only solution. Fiber-optic cabling is currently mainly used to connect different LANs together rather than individual machines to a file server. This connection is a high-speed interconnection of computing devices, and fiber optic may also be used as a backbone connecting low speed LANs together. While set at its present costs fiber cannot compete with either coax or twisted-pair. As demand and higher speed optical fiber use increases, the price will undoubtedly drop. 

Continue to LAN DEVICES >>

Local Area Network

This introduces the various media-access methods, transmission methods, topologies, and devices used in a local area network (LAN). Topics addressed focus on the methods and devices used in Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, and Fiber Distributed Data Interface (FDDI). Figure 1 illustrates the basic layout of these three implementations.

What is a LAN?

Local Area Network (LAN) is a communication network used by a single organization over a limited distance; this work enables users to share information and resources. A LAN is a high-speed, fault-tolerant data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.
LAN Operating Systems have three components.
  • Disk Operating System: present in each PC and file server. It manages the device and provides a standard interface to the network hardware.
  • File Server Software: manages the network resources, particularly the server. This is required to control the flow of messages between the workstations and the servers.
  • Redirector: runs in each workstation on the local area network and directs the requests to appropriate network devices, including various servers.
Based on the NOS (Network Operating System)/ File server software location, the LAN may be divided into two types:
  • Server-based LAN: This is a Network which has a dedicated machine/ Computer, that has the file server software located on it. All the workstations on the LAN point towards this computer for accessing the resources.
  • Peer-To-Peer LAN: This network is usually small and has the file server software installed on each of the workstations, hence, acting as both a file server and a workstation.
LAN Protocols and the OSI Reference Model
LAN protocols function at the lowest two layers of the OSI reference model Internetworking Basics, between the physical layer and the data link layer. Figure 2 illustrates how several popular LAN protocols map to the OSI reference model.

LAN Media-Access Methods

LAN protocols typically use one of two methods to access the physical network medium: carrier sense multiple access collision detect (CSMA/CD) and token passing. In the CSMA/CD media-access scheme, network devices contend for use of the physical network medium. CSMA/CD is therefore sometimes called contention access. Examples of LANs that use the CSMA/CD media-access scheme are Ethernet/IEEE 802.3 networks, including 100BaseT. In the token-passing media-access scheme, network devices access the physical medium based on possession of a token. Examples of LANs that use the token-passing media-access scheme are Token Ring/IEEE 802.5 and FDDI.

LAN Transmission Methods

LAN data transmissions fall into three classifications: unicast, multicast, and broadcast. In each type of transmission, a single packet is sent to one or more nodes.
In a unicast transmission, a single packet is sent from the source to a destination on a network. First, the source node addresses the packet by using the address of the destination node. The package is then sent onto the network, and finally, the network passes the packet to its destination.
A multicast transmission consists of a single data packet that is copied and sent to a specific subset of nodes on the network. First, the source node addresses the packet by using a multicast address. The packet is then sent into the network, which makes copies of the packet and sends a copy to each node that is part of the multicast address.
A broadcast transmission consists of a single data packet that is copied and sent to all nodes on the network. In these types of transmissions, the source node addresses the packet by using the broadcast address. The packet is then sent into the network, which makes copies of the packet and sends a copy to every node on the network.

LAN Topologies

LAN topologies define the manner in which network devices are organized. Four common LAN topologies exist: bus, ring, star, and tree. These topologies are logical architectures, but the actual devices need not be physically organized in these configurations. Logical bus and ring topologies, for example, are commonly organized physically as a star. A bus topology is a linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations. Of the three most widely used LAN implementations, Ethernet/IEEE 802.3 networks , including 100BaseT, implement a bus topology, which is illustrated in Figure 3.

A ring topology is a LAN architecture that consists of a series of devices connected to one another by unidirectional transmission links to form a single closed loop. Both Token Ring/IEEE 802.5 and FDDI networks implement a ring topology. Figure 4 depicts a logical ring topology.

A star topology is a LAN architecture in which the endpoints on a network are connected to a common central hub, or switch, by dedicated links. Logical bus and ring topologies are often implemented physically in a star topology, which is illustrated in Figure 5. A tree topology is a LAN architecture that is identical to the bus topology, except that branches with multiple nodes are possible in this case. Figure 5 illustrates a logical tree topology.

Continue to LAN CABLING>>

Computer Network

In the world of computers, networking is the practice of linking two or more computing devices together for the purpose of sharing data. Networks are built with a mix of computer hardware and computer software.

Area Networks

Networks can be categorized in several different ways. One approach defines the type of network according to the geographic area it spans. Local area networks (LANs), for example, typically reach across a single home, metropolitan area network (MANs) a network spanning a physical area larger than a LAN but smaller than a WAN, such as a city. A MAN is typically owned an operated by a single entity such as a government body or large corporation, whereas wide area networks (WANs), reach across cities, states, or even across the world. The Internet is the world's largest public WAN. There are a lot more different types of area network and i will only cover this three commonly use type of area network. 

Select the Area Network below for more detailed info.
MAN(Metropolitan Area Network)
WAN(Wide Area Network)

Monday, June 7, 2010


A modem is a device or program that enables a computer to transmit data over, for example, telephone or cable lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.

Fortunately, there is one standard interface for connecting external modems to computers called RS-232.

Consequently, any external modem can be attached to any computer that has an RS-232 port, which almost all personal computers have. There are also modems that come as an expansion board that you can insert into a vacant expansion slot. These are sometimes called onboard or internal modems.

While the modem interfaces are standardized, a number of different protocols for formatting data to be transmitted over telephone lines exist. Some, like CCITT V.34, are official standards, while others have been developed by private companies. Most modems have built-in support for the more common protocols -- at slow data transmission speeds at least, most modems can communicate with each other. At high transmission speeds, however, the protocols are less standardized.

external modem setup

56k external modem

56k internal fax modem

Aside from the transmission protocols that they support, the following characteristics distinguish one modem from another:

  • bps : How fast the modem can transmit and receive data. At slow rates, modems are measured in terms of baud rates. The slowest rate is 300 baud (about 25 cps). At higher speeds, modems are measured in terms of bits per second (bps). The fastest modems run at 57,600 bps, although they can achieve even higher data transfer rates by compressing the data. Obviously, the faster the transmission rate, the faster you can send and receive data. Note, however, that you cannot receive data any faster than it is being sent. If, for example, the device sending data to your computer is sending it at 2,400 bps, you must receive it at 2,400 bps. It does not always pay, therefore, to have a very fast modem. In addition, some telephone lines are unable to transmit data reliably at very high rates.
  • voice/data: Many modems support a switch to change between voice and data modes. In data mode, the modem acts like a regular modem. In voice mode, the modem acts like a regular telephone. Modems that support a voice/data switch have a built-in loudspeaker and microphone for voice communication.
  • auto-answer : An auto-answer modem enables your computer to receive calls in your absence. This is only necessary if you are offering some type of computer service that people can call in to use.
  • data compression : Some modems perform data compression, which enables them to send data at faster rates. However, the modem at the receiving end must be able to decompress the data using the same compression technique.
  • flash memory : Some modems come with flash memory rather than conventional ROM, which means that the communications protocols can be easily updated if necessary.
  • Fax capability: Most modern modems are fax modems, which means that they can send and receive faxes.
Related Video

CD & DVD Drives

CD-ROM drives
CD-ROM discs are read using CD-ROM drives. A CD-ROM drive may be connected to the computer via an IDE (ATA), SCSI, S-ATA, Firewire, or USB interface or a proprietary interface, such as the Panasonic CD interface. Virtually all modern CD-ROM drives can also play audio CDs as well as Video CDs and other data standards when used in conjunction with the right software.

CD-ROM drive can sometimes be a misnomer for newer drives that are capable for reading and burning DVDs, the CD's successor which is now the standard optical disc drive.

Laser optics
CD-ROM drives employ a near-infrared 780 nm laser diode. The laser beam is directed onto the disc via an opto-electronic tracking module, which then detects whether the beam has been reflected or scattered.

Transfer rates
If a CD-ROM is read at the same rotational speed as an audio CD, the data transfer rate is 150 KiB/s, commonly referred to as "1×". At this data rate, the track moves along under the laser spot at about 1.2 m/s. To maintain this linear velocity as the optical head moves to different positions, the angular velocity is varied from 500 rpm at the inner edge to 200 rpm at the outer edge. By increasing the speed at which the disc is spun, data can be transferred at greater rates. For example, a CD-ROM drive that can read at 8× speed spins the disc at 1600 to 4000 rpm, giving a linear velocity of 9.6 m/s and a transfer rate of 1200 KiB/s. Above 12× speed most drives read at Constant angular velocity (CAV, constant rpm) so that the motor is not made to change from one speed to another as the head seeks from place to place on the disc. In CAV mode the "×" number denotes the transfer rate at the outer edge of the disc, where it is a maximum. 20× was thought to be the maximum speed due to mechanical constraints until Samsung Electronics introduced the SCR-3230, a 32x CD-ROM drive which uses a ball bearing system to balance the spinning disc in the drive to reduce vibration and noise. As of 2004, the fastest transfer rate commonly available is about 52× or 10,400 rpm and 7.62 MiB/s. Higher spin speeds are limited by the strength of the polycarbonate plastic of which the discs are made. At 52×, the linear velocity of the outermost part of the disk is around 65 m/s. However, improvements can still be obtained by the use of multiple laser pickups as demonstrated by the Kenwood TrueX 72× which uses seven laser beams and a rotation speed of approximately 10×.
CD-Recordable drives are often sold with three different speed ratings, one speed for write-once operations, one for re-write operations, and one for read-only operations. The speeds are typically listed in that order; i.e. a 12×/10×/32× CD drive can, CPU and media permitting, write to CD-R discs at 12× speed (1.76 MiB/s), write to CD-RW discs at 10× speed (1.46 MiB/s), and read from CD discs at 32× speed (4.69 MiB/s).

The 1× speed rating for CD-ROM (150 KiB/s) is different than the 1× speed rating for DVDs (1.32 MiB/s).

A view of a CD-ROM drive's disassembled laser system.

The laser system of a CD Drive.

Common data transfer speeds for CD-ROM drives

DVD uses 650 nm wavelength laser diode light as opposed to 780 nm for CD. This permits a smaller pit to be etched on the media surface compared to CDs (0.74 µm for DVD versus 1.6 µm for CD), allowing for a DVD's increased storage capacity.

Writing speeds for DVD were 1×, that is, 1350 kB/s (1,318 KiB/s), in the first drives and media models. More recent models, at 18× or 20×, have 18 or 20 times that speed. Note that for CD drives, 1× means 150 KiB/s (153.6 kB/s), approximately one ninth as fast


Internal mechanism of a DVD-ROM Drive

Internal mechanism of a drive
This mechanism is shown right side up; the disc is above it. The laser and optical system "looks at" the underside of the disc.

Referring to the photo, just to the right of image center is the disc spin motor, a gray cylinder, with its gray centering hub and black resilient drive ring on top. A clamp (not in the photo, retained in the drive's cover), pulled down by a magnet, clamps the disc when this mechanism rises, after the disc tray stops moving inward. 
This motor has an external rotor – every part of it that you can see spins.

The gray metal chassis is shock-mounted at its four corners to reduce sensitivity to external shocks, and to reduce drive noise when running fast. The soft shock mount grommets are just below the brass-colored washers at the four corners (the left one is obscured). Running through those grommets are screws to fasten them to the black plastic frame that's underneath.

Two parallel precision guide rods that run between upper left and lower right in the photo carry the "sled", the moving optical read-write head. As shown, this "sled" is close to, or at the position where it reads or writes at the edge of the disc.
A dark gray disc with two holes on opposite sides has a blue lens surrounded by silver-colored metal. This is the lens that's closest to the disc; it serves to both read and write by focusing the laser light to a very small spot. It's likely that this disc rotates half a turn to position a different set of optics (the other "hole") for CDs vs. DVDs.

Under the disc is an ingenious actuator comprising permanent magnets and coils that move the lens up and down to maintain focus on the data layer. As well, the actuator moves the lens slightly toward and away from the spin-motor spindle to keep the spot on track. Both focus and tracking are relatively quite fast and very precise. The same actuator rotates the lens mount half.a turn as described.

To select tracks (or files) as well as advancing the "sled" during continuous read or write operations, a stepping motor rotates a coarse-pitch leadscrew to move the "sled" throughout its total travel range. The motor, itself, is the gray cylinder just to the left of the most-distant shock mount; its shaft is parallel to the support rods. The leadscrew, itself, is the rod with evenly-spaced darker details; these are the helical groove that engages a pin on the "sled".

The irregular orange material is flexible etched copper foil supported by thin sheet plastic; these are "flexible printed circuits" that connect everything to the electronics (which is not shown).

Repairing CD/DVD Drives

The Mouse

A device that controls the movement of the cursor or pointer on a display screen. A mouse is a small object you can roll along a hard, flat surface. Its name is derived from its shape, which looks a bit like a mouse, its connecting wire that one can imagine to be the mouse's tail, and the fact that one must make it scurry along a surface. As you move the mouse, the pointer on the display screen moves in the same direction. Mice contain at least one button and sometimes as many as three, which have different functions depending on what program is running. Some newer mice also include a scroll wheel for scrolling through long documents.

Invented by Douglas Engelbart of Stanford Research Center in 1963, and pioneered by Xerox in the 1970s, the mouse is one of the great breakthroughs in computer ergonomics because it frees the user to a large extent from using the keyboard. In particular, the mouse is important for graphical user interfaces because you can simply point to options and objects and click a mouse button. Such applications are often called point-and-click programs. The mouse is also useful for graphics programs that allow you to draw pictures by using the mouse like a pen, pencil, or paintbrush.

Popular Keyboard Manufacturers:

There are three basic types of mice:
  1. mechanical: Has a rubber or metal ball on its underside that can roll in all directions. Mechanical sensors within the mouse detect the direction the ball is rolling and move the screen pointer accordingly.
  2. optomechanical: Same as a mechanical mouse, but uses optical sensors to detect motion of the ball.
  3. optical: Uses a laser to detect the mouse's movement. You must move the mouse along a special mat with a grid so that the optical mechanism has a frame of reference. Optical mice have no mechanical moving parts. They respond more quickly and precisely than mechanical and optomechanical mice, but they are also more expensive.
Mice connect to PCs in one of several ways:
  1. Serial mice connect directly to an RS-232C serial port or a PS/2 port. This is the simplest type of connection.
  2. PS/2 mice connect to a PS/2 port.
  3. USB mice.
Cordless mice aren't physically connected at all. Instead they rely on infrared or radio waves to communicate with the computer. Cordless mice are more expensive than both serial and bus mice, but they do eliminate the cord, which can sometimes get in the way.

Troubleshoot/Repair Computer Mouse

The Keyboard

What is a Keyboard?:
The keyboard is an input device designed to enter text, characters and other commands into the computer.

Popular Keyboard Manufacturers:
Microsoft, Logitech, A4tech

Keyboard Description:

Modern computer keyboards were modeled after and are still very similar to classic typewriter keyboards. Many different layouts are available around the world but most keyboards are of the QWERTY type.
Keyboards may be wired or wireless but they always communicate with the computer via PS2 or USB connections, usually located on the motherboard. Even though the keyboard sits outside the main computer housing, it is an essential part of the complete system. 

Repairing the Keyboard:
(note: since keyboard usually is one of the cheapest part of a desktop computer. It is recommended to buy a new one when you encounter keyboard hardware related problems. The video demonstration below only show you how to repair a laptop keyboard, this procedure is also applicable on some desktop keyboard build similarly to laptop.)

Sunday, June 6, 2010

Network Interface Controller (NIC)

A network interface card (NIC) is a hardware device that handles an interface to a computer network and allows a network-capable device to access that network. The NIC has a ROM chip that contains a unique number, the media access control (MAC) Address burned into it. The MAC address identifies the device uniquely on the LAN. The NIC exists on the 'Data Link Layer' (Layer 2) of the OSI model.

The OSI Model

This section serves as an introduction to the International Standards Organization/Open System Interconnection (ISO/OSI) model. You do not need to understand the OSI model in order to build a home network. However, it can be helpful to have a basic understanding of how your network works in order to troubleshoot future problems. We designed this section with more advanced users in mind.

The OSI model defines a networking framework for implementing protocols according to seven layers. Each layer is functionally independent of the others, but provides services to the layer above it and receives services from the layer below it. The seven OSI layers are explained in more detail below.


The Application layer is the layer at which applications access network services. This layer represents the services that directly support applications such as software for file transfers, database access, email, and network games.


The Presentation layer translates data from the Application layer into a network format (and vice-versa). This layer also manages security issues by providing services such as data encryption and compression.


The Session layer allows applications on different computers to establish, use, and end a session/connection. This layer establishes dialog control between the two computers in a session, regulating which side transmits, and when and how long it transmits.


The Transport layer handles error recognition and recovery. It also repackages long messages when necessary into small packets for transmission and at the receiving end, rebuilds packets into the original message. The receiving Transport layer also sends receipt acknowledgments.


The Network layer addresses messages and translates logical addresses and names into physical addresses. It also determines the route from the source to the destination computer and manages traffic problems (flow control), such as switching, routing, and controlling the congestion of data packets.


The Data Link layer packages raw bits from the Physical layer into frames (logical, structures packets for data). This layer is responsible for transferring frames from one computer to another, without errors. After sending a frame, it waits for an acknowledgment from the receiving computer.


The Physical layer transmits bits from one computer to another and regulates the transmission of a stream of bits over a physical medium. This layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable.



A network interface card, network adapter, network interface controller (NIC), or LAN adapter is a computer hardware component designed to allow computers to communicate over a computer network. It is both an OSI layer 1 (physical layer) and layer 2 (data link layer) device, as it provides physical access to a networking medium and provides a low-level addressing system through the use of MAC addresses. It allows users to connect to each other either by using cables or wirelessly.

Although other network technologies exist (e.g. Token Ring), Ethernet has achieved near-ubiquity since the mid-1990s. Every Ethernet network card has a unique 48-bit serial number called a MAC address, which is stored in ROM carried on the card. Every computer on an Ethernet network must have a card with a unique MAC address. Normally it is safe to assume that no two network cards will share the same address, because card vendors purchase blocks of addresses from the Institute of Electrical and Electronics Engineers (IEEE) and assign a unique address to each card at the time of manufacture.

A 1990s Ethernet network interface controller card which connects to the motherboard via the now-obsolete ISA bus. This combination card features both a (now obsolete) bayonet cap BNC connector (left) for use in coaxial-based 10base2 networks and an RJ-45 connector (right) for use in twisted pair-based10baseT networks. (The ports could not be used simultaneously.)

Madge 4/16Mbps TokenRing ISA NIC.

Ethernet 10Base-5/2 ISA NIC.


The card implements the electronic circuitry required to communicate using a specific physical layer and data link layer standard such as Ethernet or token ring. This provides a base for a full network protocol stack, allowing communication among small groups of computers on the same LAN and large-scale network communications through routable protocols, such as IP.

There are four techniques used to transfer data, the NIC may use one or more of these techniques.

  • Polling is where the microprocessor examines the status of the peripheral under program control.
  • Programmed I/O is where the microprocessor alerts the designated peripheral by applying its address to the system's address bus.
  • Interrupt-driven I/O is where the peripheral alerts the microprocessor that it's ready to transfer data.
  • DMA is where an intelligent peripheral assumes control of the system bus to access memory directly. This removes load from the CPU but requires a separate processor on the card.

A network card typically has a RJ45, BNC, or AUI socket where the network cable is connected, and a few LEDs to inform the user of whether the network is active, and whether or not there is data being transmitted on it. Network cards are typically available in 10/100/1000 Mbit/s varieties. This means they can support a notional maximum transfer rate of 10, 100 or 1000 Megabits per second.

Sometimes the words 'controller' and 'card' are used interchangeably when talking about networking because the most common NIC is the network interface card. Although 'card' is more commonly used, it is less encompassing. The 'controller' may take the form of a network card that is installed inside a computer, or it may refer to an embedded component as part of a computer motherboard, a router, expansion card, printer interface or a USB device.

A MAC address is a 48-bit network hardware identifier that is burned into a ROM chip on the NIC to identify that device on the network. The first 24-bit field is called the Organizationally Unique Identifier (OUI) and is largely manufacturer-specific. Each OUI allows for 16,777,216 Unique NIC Addresses. Smaller manufacturers that do not have a need for over 4096 unique NIC addresses may opt to purchase an Individual Address Block (IAB) instead. An IAB consists of the 24-bit OUI plus a 12-bit extension (taken from the 'potential' NIC portion of the MAC address.)

Network Interface Controller(NIC) Installation

More about Computer Network