Friday, November 27, 2009

Holiday glasses from Zenni

As if you are planning holiday somewhere then don't forget to take your's eye partner,yes you understand right,I am talking about glasses.Now Zenni opticals bring you their special collection of Holiday Fun Eyeglasses where you can find a huge collection of these holiday eyeglasses.You can select eyeglasses as per your vacation like if your going at some beach or sea resort then their are some special eyeglasses for you.You can check all these eyeglasses in their New Arrivals section and best thing about these Holiday Fun Eyeglasses is that all these are available at only $8 also more popularly known as $8 Prescription Zenni Glasses.

Saturday, August 22, 2009

Review on GolfKurse

Golf is now the emerging game that is popularized among all the people. Earlier only the rich and educated people played it. Now the popularity of the game is wide spread, so that every one has interest in playing the game. This particular game attracts all the types of crowds regardless of their age. Many people are now interested in learning this game. Mainly students involve themselves in this game so much and are very eager to learn the game. They are interested in learning in all the nuances, rules and regulations of the game. Since many people are interested in learning the game, there are many golf academies newly erecting in and around the city. Golf academies bear the responsibility to teach the young generation about the game. The rules of the golf etiquette are the first and fore most things, the golf academies teach their students. The rules of the game mainly aim at the safety of the golfers and to the pace of the play, which helps in keeping the game enjoyable. The golf etiquette is an essential part of the game. This is something very vital that all the new comers and the new beginners should learn on the course. For any game to be played successfully, the instruments used for the play should be manufactured or prepared with utmost care and you can found all these instruments at their Golfshop. A standard set of a golf club mainly consists of three woods, eight irons and a putter. Actually according to the rules and regulations of the play a golfer is allowed to carry 14 clubs in the bag. The more clubs the player carries the easier is his victory. To know more about golf academies, please visit Golfreisen.
The above site explains you all the details about the game. They are one of the largest German golf shops with a full range of all brand names. You can also visit golfkurse it will be useful for you.

Sunday, June 14, 2009


LocalTalk is a network protocol that was developed by Apple Computer, Inc. for Macintosh computers. The method used by LocalTalk is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to CSMA/CD except that a computer signals its intent to transmit before it actually does so. LocalTalk adapters and special twisted pair cable can be used to connect a series of computers through the serial port. The Macintosh operating system allows the establishment of a peer-to-peer network without the need for additional software. With the addition of the server version of AppleShare software, a client/server network can be established. The LocalTalk protocol allows for linear bus, star, or tree topologies using twisted pair cable. A primary disadvantage of LocalTalk is speed. Its speed of transmission is only 230 Kbps.

Monday, June 8, 2009

Wireless LAN

Not all networks are connected with cabling; some networks are wireless. Wireless LANs use high frequency radio signals, infrared light beams, or lasers to communicate between the workstations and the file server or hubs. Each workstation and file server on a wireless network has some sort of transceiver/antenna to send and receive the data. Information is relayed between transceivers as if they were physically connected. For longer distance, wireless communications can also take place through cellular telephone technology, microwave transmission, or by satellite.Wireless networks are great for allowing laptop computers or remote computers to connect to the LAN. Wireless networks are also beneficial in older buildings where it may be difficult or impossible to install cables.

The two most common types of infrared communications used in schools are line-of-sight and scattered broadcast. Line-of-sight communication means that there must be an unblocked direct line between the workstation and the transceiver. If a person walks within the line-of-sight while there is a transmission, the information would need to be sent again. This kind of obstruction can slow down the wireless network.

Scattered infrared communication is a broadcast of infrared transmissions sent out in multiple directions that bounces off walls and ceilings until it eventually hits the receiver. Networking communications with laser are virtually the same as line-of-sight infrared networks.
Wireless LANs have several disadvantages. They provide poor security, and are susceptible to interference from lights and electronic devices. They are also slower than LANs using cabling.

Wednesday, June 3, 2009

Bluetooth Technology

First of all, I want to told you from where word “Bluetooth” came, it put on the name of a great Denmark king whose name was “Harald Bluetooth” a thousand years of ago. Now the question is, what is bluetooth technology, actually it is a technology which is used for low-power radio communications to wirelessly link phones, computers and other network devices over short distances. Initially it was developed for support of personal wireless devices including cell phones, PDAs, and wireless headsets.The signals that are transmitted through “Bluetooth” devices are short distance in nature, They diminishes after 10m(30 feet).The data transferred through Bluetooth devices is at a rate of less than 1Mbps.As I already told you “Bluetooth” devices works on the principle of radio communication systems, so it is not required that the two Bluetooth devices must be in line of sight of each other, they can be either in different rooms, as long as the received transmission is powerful enough.

Today,there are many bluetooth enabled devices exists such as phones, printers, modems and headsets etc. This technology is useful only when the Bluetooth enabled devices are present in low- bandwidth position. Today Bluetooth is mainly used in cell phones to transfer sound data, or text data with laptops, and from Bluetooth enabled devices.From the security point of view, the data transferred between blue tooth devices is encrypted through some custom algorithms so that it can be safely transmitted. Hence, we can say that data transferred through Bluetooth devices is secure.As, the work is going on the development of Bluetooth so that it can be made more faster,simpler in comparison to other wireless technology.

Saturday, May 30, 2009

Understsnding FDDI topology

FDDI stands for Fiber Distributed Data Interface. The FDDI standard is ANSI X3T9.5 . The FDDI topology is ring with two counter rotating rings for reliability with no hubs. Cable type is fiber-optic. Connectors are specialized. The media access method is token passing. Multiple tokens may be used by the system. The maximum length is 100 kilometers. The maximum number of nodes on the network is 500. Speed is 100 Mbps. FDDI is normally used as a backbone to link other networks. A typical FDDI network can include servers, concentrators, and links to other networks. CDDI is a copper version of FDDI which uses category 5 cable. Obviously the distance is more limited than FDDI.

Devices called concentrators provide functions similar to hubs. Most concentrators use dual attachment station network cards but single attachment concentrators may be used to attach more workstations to the network.

FDDI token passing allows multiple frames to circulate around the ring at the same time. Priority levels of a data frame and token can be set to allow servers to send more data frames. Time sensitive data may also be given higher priority. The second ring in a FDDI network is a method of adjusting when there are breaks in the cable. The primary ring is normally used, but if the nearest downstream neighbor stops responding the data is sent on the secondary ring in attempt to reach the computer. Therefore a break in the cable will result in the secondary ring being used. There are two network cards which are:

1. Dual attachment stations (DAS) used for servers and concentrators are attached to both rings.
2. Single Attachment stations (SAS) attached to one ring and used to attach workstations to concentrators.
A router or switch can link an FDDI network to a local area network (LAN). Normally FDDI is used to link LANs together since it covers long distances.

Wednesday, May 27, 2009

Deal on business loans

Today you see a lot of people whose business is deeply suffered by recession and now they are in great need of financial support from some external sources.For availing these financial support the only option to them is to contact some banks or organization that can provide financial aid to them.But as all we very well known that these banks have a very tedious process of issuing business loans.But now here is the solution of your problem at,because they provide you their financial help without any personal guarantee,that means they can sanctioned you loan upto $400,000 without any guarantee.

If you need amount much larger than this then also you can apply here and with some easy documentation your application is approved.But generally many people are in need of small business loans for recovering their business needs,hence this is the right place for your business loans.If you are worried about your past bad credit personal history then also they can issue easy unsecured loans within 7 days of your application.If you want more proof on how this company helped other people in recovering their business then you can watch the success stories of the people that already taken benefit from them.For availing all these facilities you had to just sign up with them,and once you sign up a prosper and secure future is waiting for you.

Friday, February 27, 2009

Destination and Next Hop Addresses

What is the relationship between the destination address in a datagram header and the address of the next hop to which the datagram is forwarded?The Destination IP address field in a datagram contains the address of the Ultimate destination.When a router receives a datagram,the router extracts the destination address D,and uses it to compute the address of the next router to which datagram should be sent,N.Although the datagram is sent directly to address N,the header in the datagram retains destination address D.All routes are computed using IP address.After computing the address of a next hop,N,IP software uses the address binding to translate N to an equivalent hardware address for transmission.

Sunday, February 22, 2009

Physical Network Connection With Routers

The basic hardware component used to connect heterogeneous networks is a Router.Physically,routers resembles bridges that means each router is a special purpose hardware system dedicated to the task of interconnecting networks.Like a bridge,a contains a processor and memory as well as a separate I/O interface for each network to which it connects.The network treats a connection to a router, the same as the connection to any other computer.

The most important that you have to kept in your mind about router is that it is not only used to connect two similar technology,it can also be used to connect two different types of network technology. Router can connect two LAN's,a LAN and a WAN,or two WAN's.Furthermore,when a router connects two network in the same general category,the networks do not need to use the same technology.For ex:- A router can connect an Ethernet LAN to a frame relay network.Thus,each cloud represents an arbitrary network technology.On summarizing this whole,we can say that a router is a special purpose system dedicated to the task of interconnecting networks.A router can interconnect networks that use different technologies,including different media,physical addressing schemes or frame formats.

You can also visit my other blogs:
Operating System Concepts
Enter into world of Technology

Saturday, February 21, 2009

Goal of Internet

The main goal of internetworking is to provide a universal service across heterogeneous networks.To provide universal service among all computers on an internet,routers must agree to forward information from a source on one network to the specified destination on the other network.The task is complex because as all we know that different frame formats and different addressing schemes are used by different networks.Hence,to overcome this problem protocol software is needed on computers and routers to make universal service possible.The main task of these internet protocols is to overcome the differences in frame formats and physical addresses to make communication possible among networks that use different technologies.
For knowing more about protocols for internetworking you can visit:

Friday, February 20, 2009

Internet Architecture

With the help of routers it is possible for an organization to choose network technologies appropriate for each need,and to use routers to connect all networks into a single network.As all we know that router is used exactly two connections but commercial routers can connect more than two networks.Thus,a single router could connect any number of networks.However,an organization seldom uses single router to connect all of its networks.There are two main reasons behind it:

1.As all we know that router must forwarded each packet,the processor in given router is insufficient to handle the traffic passing among an arbitrary number of networks.

2.The second reason behind this is redundancy,because redundancy improves reliability.To avoid a single point of failure,protocol software continuously monitors internet connection,and instructs the routers to send traffic long alternative paths when a network or router fails.

Thus,when planning an internet,an organization must choose a design that meet's the organization's need for reliability,capacity and cost.In particular,the exact details of internet topology often depends on the bandwidth of the physical networks,the expected traffic,the organization's reliability requirements,and the cost and performance of available router hardware.

Thursday, February 19, 2009

Connection between a NIC and a network

The type of connection used between a NIC and a network depends on that network technology.In some technologies,the NIC contains most of the necessary hardware,and attaches directly to the network medium using a single cable or optical fiber.In many other technologies,the NIC does not contain all the electronic circuitry needed to attach directly to the network.Instead,the cable from a NIC attaches to an additional electronic component that then attaches to the network.Surprisingly,the exact details of the connection between a NIC and a network are not determined by the technology that means a given network technology can support multiple wiring schemes.Let take the example of Ethernet technology in which technology i.e Ethernet remains same while the wiring scheme changes.Wiring scheme can be Thick Ethernet Wiring,Thin Ethernet Wiring and Twisted pair Ethernet Wiring.

Wednesday, February 18, 2009

Thin Ethernet Wiring

A second generation of Ethernet wiring uses a thinner,more flexible cable than the original thick wiring.Formally named as a 10 Base2 and informally named as thin wire Ethernet or Thinnet.This wiring scheme differs from Thicknet wiring scheme in three important ways.First,Thinnet generally costs less to install and operate than Thicknet.Second,because the hardware that performs the transceiver function is built into the NIC,no external transceivers are needed.Third,Thinnet does not use an AUI cable to attach the NIC to a communication medium.Instead,Thinnet attachs directly to the back of each computer using BNC connector.In a Thinnet installation,a coaxial cable stretches between each pair of machines.The cable doesn't need to follow a straight line-it may lie loosely on the tabletop between computers,run under floor,or run in a conduit.Although,wiring for a thin Ethernet appears to be completely different than the wiring for a thick Ethernet,the two schemes share several important properties.Both thick and thin cables are coaxial,meaning that they shield signals from outer interference.Both thick and thin cables requires termination,and both use the bus topology.Most important,because two wiring systems have similar electrical characteristics,that means signals propagate along the cable is in the same way.

Monday, February 16, 2009

Twisted Pair Ethernet

This type of Ethernet wiring is known as Third Generation of Ethernet wiring.This generation is completely differ from its previous two generation that are thick and thin Ethernet because it does not use a coaxial cable.This generation Ethernet typically known as twisted pair Ethernet or TP Ethernet and its first version was formally known as 10BaseT.The wiring for Fast Ethernet is known as 100BaseT,and wiring for Gigabit Ethernet is known as 1000BaseT.Instead of shared medium,the new technology extends the idea used with connection multiplexing in which an electronic device serves as the center of the network.
The basic electronic device is known as an Ethernet Hub.Like the other wiring schemes,twisted pair requires each computer to have a network interface card and a direct connection from NIC to the network.The connection uses unshielded twisted pair wiring with RJ-45 connectors which are the largest versions of the modular connectors used with the telephones.The connector on one end of a twisted pair cable plugs into the network interface on a computer and the connector on the other end plugs into the hub.
Thus,each computer has a dedicated connection to the hub device.Electronic components in hub emulate a physical cable,making the entire system operate like a conventional Ethernet.For ex:A computer attached to a hub must have a physical Ethernet Address that means each computer must use the CSMA/CD to access the network and the standard Ethernet Frame Format.In fact,software does not distinguish between thick Ethernet,thin Ethernet and twisted pair Ethernet because the network interface handles the details and hides any differences.

Sunday, February 8, 2009

The Berkeley Broadcast Address Form

The University of California at Berkeley developed and distributed an early implementation of TCP/IP protocols as a part of BSD UNIX.The BSD implementation contained a non-standard feature that has affected many subsequent implementations.Instead of using a host suffix of all ones to represent a directed broadcast address,the Berkeley implementation used a host suffix that contained all zeros.The address form is informally known as Berkeley Broadcast.Unfortunately,many computer manufacturers derived their early TCP/IP software from the Berkeley implementation,and a few sites still use Berkeley broadcast.Some implementations include a configuration parameter that can select between the TCP/IPTCP/IP standard and Berkeley form;many implementations are built to accept both standard and Berkeley broadcast address forms.

Saturday, February 7, 2009

Directed Broadcast Address

Sometimes,it is convenient to send a copy of a packet to all hosts on a physical network.To make broadcast easy,IP defines a directed broadcast address for each physical network.When a packet is sent to network's broadcast address,a single copy of the packet travels across the internet until it reaches the specified network.The packet is then delivered to all the hosts on the network.The directed broadcast address for a network is formed by adding a suffix that consists of all 1 bits to the network prefix.

To ensure that each network can have directed broadcast address,IP reserves the host address that contains all 1 bits.An administrator must not assign the all ones host address to a specific computer or the software may malfunction.If a network hardware supports broadcast,a directed broadcast will be delivered using the hardware broadcast capability.In such cases,transmission of the packet will reach all computers on the network.When a directed broadcast is sent to a network that does not have hardware support for broadcast,software must send a separate copy of the packet to each host on the network.

Friday, February 6, 2009

Authority for Addresses

Throughout an internet,each network prefix must be unique.For networks connected to the global internet,an organization obtains network numbers from the communication company that supplies internet connections.Such companies are called Internet Service Providers(ISPs).Internet Service Providers coordinate with a central organization,the Internet Assigned Number Authority,to ensure that each network prefix is unique throughout the entire Internet.For a private internet,the choice of the network prefix can be made by organization.To ensure that each prefix is unique, group that builds a private internet must decide how to coordinate network number assignments.Often,a single network administrator assign prefixes to all networks in the company's internet to ensure that numbers are not duplicated.

Thursday, February 5, 2009

Loopback Address

IP defines a loopback address used to test network applications.Programmers often use loopback testing for preliminary debugging after a network application has been created.To perform a loopback test,a programmer must have two application programs that are intended to communicate across a network.Each application includes the code needed to interact with TCP/IP protocol software.Instead of executing each program on a separate computer,the programmer runs both program on a single computer and instructs them to use a loopback IP address when communicating.

When one application sends data to another,data travels from the protocol stack to the IP software,which forwards it back up through the protocol stack to the second program.Thus,the programmer can test the program logic quickly without needing two computers and without sending packets across a network.IP reserves the network prefix 127/8 for use with loopback.The host address used with 127 is irrelevant that means all host addresses are treated the same.By convention.programmers often use host number 1,making the most popular form of loopback.During loopback testing no packets ever leave a computer that means the IP software forwards packets from one application program to another.Consequently,the loopback address never appears in packet traveling across a network.

Wednesday, February 4, 2009

Subnet and Classless Addressing

As the Internet grew,the original classful addressing scheme became a limitation.On one hand,the IP address space was being exhausted.On the other hand,because all networks had to choose one of the three possible sizes,that means many addresses were unused.Two new mechanisms were invented to overcome the limitations,known as subnet addressing and classless addressing.These two addressing schemes are so closely related that they can be considered to be a part of single abstraction.The generalization is straight forward that means instead of having three distinct address classes,allow the division between prefix and suffix to occur on an arbitrary bit boundary.Let understand this situation with the help of an example,Consider a network that contains 9 hosts.Only four bits of host suffix are needed to represent all possible host values.However,a class C address,which has the fewest address hosts possible,devote eight bits to the host suffix.Classless addressing solves the problem by allowing an ISP(Internet Service Provider) to assign a prefix that is 28 bits long that means network can have up to 14 hosts.

Tuesday, February 3, 2009

Protocol for Internetworking

As all we know that may protocols have been adapted for use in an internet,one suite stnds out as the most widely used for internetworking.The suite is formally known as The TCP/IP Internet Protocols.TCP/IP was the first set of protocols develop for use in an internet.Indeed,researchers who devised TCP/IP also developed the internet architecture.Work on TCP/IP began in 1970s,approximately the same time that Local Area Networks were being developed.The U.S military funded much of the research on the TCP/IP and internetworking through the Advanced Research Projects Agency.The military was among the first organizations to have multiple physical networks.Cosequently,the military was among the first to realize the need for universal service.By the mid-1980s,the National Science Foundation and the other U.S government agencies were funding development of TCP/IP and a large internet that was used to test the protocols.

Monday, February 2, 2009

Art of Protocol Design

Although the techniques needed to solve specific problems are well known,protocol design is on-trivial for two reasons.First to make communication efficient,details must be chosen carefully that means small design errors can result in incorrect operation,unnecessary packets or delays.For example:sequence numbers are usually stored in a fixed field in a packet header.The field must be large enough so sequence numbers are not reused frequently,but small enough to avoid wasting unnecessary bandwidth.Similarly,a higher level protocol can cause excessive overhead if the protocol chooses a non-optimal message size.

Second,protocol mechanism can interact in unexpected ways.For example:Consider the interaction between flow control and congestion control mechanism.A sliding window scheme aggressively use more of the underlying network bandwidth to improve throughput.A congestion control mechanism does,the opposite by reducing the number of packets being inserted to prevent the networking from collapsing.

The balance between sliding window and congestion control can be tricky,and a design that does both well is difficult.A protocol that is too aggressive about using bandwidth can adjust the underlying network and a protocol that is too conservative can result in lower throughput then necessary.More important,designs that attempts to switch from aggressive to conservative whenever congestion occurs tend to oscillate - then slowly increase their use of bandwidth until the network begins to experience congestion,decrease use until the network becomes stable and then begin to increase again.

Saturday, January 31, 2009

Understanding Protocol Suites

After knowing the importance of protocol in communication,designers have chosen to divide the communication problem into sub pieces and to design a protocol for each of the sub piece instead of having a single giant protocol that specifies complete details for all possible forms of communications.Doing so makes each protocol easier to design,analyze,implement and test.The main advantage of dividing communication software into multiple protocols increases flexibility because it allows subsets of protocols to be used as added.The division into separate protocols must be chosen carefully to ensure the resulting communication system is efficient and effective.To avoid duplication of effort,each protocol should handle part of the communication problem not handled by other protocols.

To make efficient implementation possible,protocols should be designed so that,they can share data structures and information.Finally,the combination of protocols should handle all possible hardware failures or other exceptional conditions.Now the main question that arises in front of us is that how can one guarantee that protocols will work together.The answer of this question lies in an overall design plan that means instead of developing each protocol in isolation,protocols are designed and developed in complete,cooperative sets called suites or families.Each protocol in a suite solves one part of the communication problem;together they solve the entire communication problem.Furthermore,the entire suite is designed to make interactions among protocol efficient.

Friday, January 30, 2009

Need for Protocols

As all we know that basic communication hardware consists of mechanisms that can transfer bits from one point to another.However,using raw hardware to communicate is analogous to programming by entering 1s and 0s but it is cumbersome and inconvenient.To aid programmers,computers attached to a network use complex software that provides a convenient,high level interface for applications.The software handles most low level communication details and problems automatically,making it possible for applications to communicate easily.Thus,most application programs rely on network software to communicate that means hey do not interact with network hardware directly.All parties involved in a communication must agree on a set of rules to be used when exchanging messages and such type of agreement is known as protocol.This term is also applied to computer communications meaning of which "a set of rules that specify the format of messages and the appropriate action required for each message is known as a network protocol or a computer communication protocol."The software that implement such rules is called as protocol software.

Wednesday, January 28, 2009

Significance of Internetworking and TCP/IP

Internetworking has become one of the most important ideas in modern networking.In fact,Internet Technology has revolutionized computer communication.Most large organizations already use internetworking as the primary computer communication mechanism.Smaller organizations and individuals are beginning to do so well.More important,in addition to private internet,the TCP/IP technology has made possible a global internet that reaches schools,commercial organizations,and government and military sites in all populated countries around the world.The worldwide demand for internetworking products has affected most companies that sell networking technologies.

Competition has increased because new companies have been formed to sell hardware and software need for internetworking.In addition may companies have modified their protocol designs to accommodate internetworking.In particular,most network protocols were originally designed to work with one network technology and one physical network at a time.To provide internetworking capabilities,companies have extended the designs in two ways:the protocols have been adapted to work with many network technologies and new features have been added that allows the protocols to transfer data across internet.

Tuesday, January 27, 2009

Public and Private Networks

When applied to a network,the term public refers to the availability of the service,not to the data transferred.In particular,most public networks provide private communications i.e when a computer sends a message across a public network,only the intended recipient receives the copy.Some public networks permit a group of computers to communicate analogous to a telephone conference hall.However,the public network doesn't use the broadcast technology.That is,the public network doesn't have a broadcast address nor it does forward broadcast packets.

The chief advantage of a private network is that owner has full control over both the technical decisions and policies.In addition to this it also have some disadvantages like a large private network can be expensive to install and maintain.In addition to purchasing the network hardware,a corporation must hire and train a staff to install,manage and operate the network.

On the other hand the chief advantage of a public network are flexibility and the ability to use state of the art networking without maintaining the technical expertise. public network is flexible because an arbitrary subscriber at a arbitrary location can connect to the network at any time.Furthermore,connections between a computer owned by one organization and a computer owned by another can be made broken the same way a public telephone system allows voice connections to be made or broken

Friday, January 23, 2009

Concept of ISDN

Integrated Services Digital Network(ISDN) is launched by telephone companies and used to provide large scale digital services to subscribers.ISDN provides digitized voice and data to subscribers over conventional local wiring.ISDN uses the same type of twisted pair copper wiring as the analog telephone system.If we discuss from the subscribers point of view then ISDN offers three separate digital channels designated B,B and D(usually written 2B+D).The two B channels operates at a speed of 64 Kbps,are intended to carry digitized voice and data or compresses videos, on the other hand the D channel which operates at 16 Kbps,is intended as a control channel.In general,a subscriber uses the D channel to request services which are then supplied over the B channels.The subscriber can also use the D channel to manage a session that is in progress or to terminate a session.Finally,both the B channels can be combined or bonded together to produce a single channel with effective data rate at 128 Kbps.The 2B+D channels are known as ISDN Basic Rate Interface(BRI).Infact,ISDN uses a form of time division multiplexing to provide the illusion of multiple channels of data to travel over a single pair of wires.

Tuesday, January 20, 2009

Understanding Bridges

A bridge is an electronic device that is used to connects two LAN segments.Unlike a repeater,however a bridge is a digital device that uses the same network interface as a conventional computer and handles frames.The bridge listen to traffic on each segment in promiscuous mode.When it receives a frame from one segment,the bridge verifies that the frame arrived intact i.e there was no electrical interference on LAN during transmission and after that it forwards a copy of the frame to the other segment if necessary.Thus,two LAN segments behave connected by a bridge behave like a single LAN.A computer connected to either segment can send a frame to any of the other computers connected to the two segments.Because each segment supports standard network connections and uses the standard frame format that means computers do not know whether they are connected to a LAN or a bridged LAN.

When we compare bridges with LAN it is found that bridges are more useful and popular than repeaters because they help isolate problems.If two segments are connected by a repeater and lightning causes electrical interference on one of them,the repeater will propagate the interference to the other segment.In contrast if interference occurs on one of the two segments connected by a bridge,the bridge will receive an incorrectly formed frame,which the bridges simply discards the same way a conventional computer discards a frame that contains an error.Similarly,a bridge will not forward a collision from one segment to another.Thus,the bridge keeps problems on one segment from affecting the other.

Friday, January 16, 2009

Combining Switches and Hubs

From the last post it is clear that switching provides higher aggregate data rates in comparison to a hub,but the main drawback that occurred with the switched networks is the higher cost in comparison to hub.To overcome all these problems network administrator choose a compromise between the hub and the switched networks that means instead of connecting one computer to each port on a switch,the administrator connects the hub to each of the port of the switch and then connect each computer to one of the hubs.This result is very much closer to the conventional bridged LAN where each hub appears to be a single LAN segment, and the switch makes it appear that bridges connect all segments.The system also performs like a conventional bridged LAN where a computer must share a bandwidth with other computers connected to the same hub,communication can occur in parallel between a pair of computers attached to one hub and a pair of computers attached to another.

Thursday, January 15, 2009

Concept of Switching

In general,a network technology is called switched if the hardware includes an electronic device that connects to one or more computers and allows them to send and receive data from each other.More specifically,a switched LAN consisted of a single electronic device that transfers frames among many computers.As all we know that a switch resembles a hub,like a hub, a switch consists of a single box with multiple ports that each attach to a single computer.The main difference between a switch and a hub arises from the way the devices operates,a hub simulates a single shared medium while in the case of a switch it simulates a bridged LAN with one computer per segment.

If we talk about the chief advantage of using a switched LAN instead of a hub then the reason is same as that of using bridged LAN instead of a single segment.In the case of a hub a single segment is shared by all computers,that means at most two computers can communicate through hub at a time.Thus the maximum possible throughput of a hub system is R,the rate at which single computer can send data across a LAN segment.In the case of a switched LAN,each computer has a simulated LAN segment to itself that means the segment is busy only when a frame is being transferred to or from the computer.As a result,as many as one half of the computers connected to a switch can send the data at the same time(if they send to one of the computers that is not busy in sending).Thus,in this case the maximum throughput of a switch is RN/2 where R is the rate at which data transmits,and N is total number of computers that are connected to the switch.

Tuesday, January 13, 2009

Understanding Baud Rate and Framing Errors

As all we know that sending and receiving hardware are agreed on the length of the time the voltage will be held for each bit.Instead of specifying the time per bit,which is a small fraction of second,communication systems specify the number of bits that can be transferred in a second.For ex:some early RS-232 connections operated at 300 bits per second,19200 bits per second and 33600 bits per second are more common.Technically ,transmission hardware is rated in baud,the number of changes in the signal per second that the hardware generated.For the simple RS-232 scheme presented, the baud rate is exactly equal to the number of bits per second.Thus,9600 baud means 9600 bits per second.

To make RS-232 hardware more general,manufacturers usually design each piece of hardware to operate at variety of baud rates.The baud rate can be configured either manually by physically setting switches on the hardware when it is installed in a computer or automatically by device driver software in a computer.If the sending and receiving hardware are not configured to use the same baud rate,errors will occur because the receiver's timer will not wait an appropriate length of time for each bit.To detect errors,a receiver's measure the voltage for each bit multiple times and compares the measurements.If the voltage do not all agree or if the stop bit does not occur exactly at the time expected,the receiver reports an error.Such errors are known as framing errors.