Web Server

A Web server is a program that, using the client/server model and the World Wide Web's Hypertext Transfer Protocol ( HTTP ), serves the files that form Web pages to Web users (whose computers contain HTTP clients that forward their requests). Every computer on the Internet that contains a Web site must have a Web server program. Two leading Web servers are Apache , the most widely-installed Web server, and Microsoft's Internet Information Server ( IIS ). Other Web servers include Novell's Web Server for users of its NetWare operating system and IBM's family of Lotus Domino servers, primarily for IBM's OS/390 and AS/400 customers.
Web servers often come as part of a larger package of Internet- and intranet-related programs for serving e-mail, downloading requests for File Transfer Protocol ( FTP ) files, and building and publishing Web pages. Considerations in choosing a Web server include how well it works with the operating system and other servers, its ability to handle server-side programming, security characteristics, and publishing, search engine, and site building tools that may come with it.

URL

Definition:                  URL stands for Uniform Resource Locator. A URL is a formatted text string used by Web browsers, email clients and other software to identify a network resource on the Internet. Network resources are files that can be plain Web pages, other text documents, graphics, or programs.

URL strings consist of three parts (substrings):

1.      network protocol

2.       host name or address

3.      file or resource location

These substrings are separated by special characters as follows:

protocol :// host / location

URL Protocol

The 'protocol' substring defines a network protocol to be used to access a resource. These strings are short names followed by the three characters '://' (a simple naming convention to denote a protocol definition). Typical URL protocols include http://, ftp://, and mailto://.

URL Host

The 'host' substring identifies a computer or other network device. Hosts come from standard Internet databases such as DNS and can be names or IP addresses. For example, compnetworking.about.com is the host for this Web page.

URL Location

The 'location' substring contains a path to one specific network resource on the host. Resources are normally located in a host directory or folder. For example, /od/internetaccessbestuses/bldef-url.htm is the location of this Web page including two subdirectories and the file name.

When the location element is omitted such as in http://compnetworking.about.com/, the URL conventionally points to the root directory of the host and often a home page (like 'index.htm').

Absolute vs. Relative URLs

Full URLs featuring all three substrings are called absolute URLs. In some cases such as within Web pages, URLs can contain only the one location element. These are called relative URLs. Relative URLs are used for efficiency by Web servers and a few other programs when they already know the correct URL protocol and host.

Virtual Circuit

Virtual circuit switching is a packet switching methodology whereby a path is established between the source and the final destination through which all the packets will be routed during a call. This path is called a virtual circuit because to the user, the connection appears to be a dedicated physical circuit. However, other communications may also be sharing the parts of the same path.

Before the data transfer begins, the source and destination identify a suitable path for the virtual circuit. All intermediate nodes between the two points put an entry of the routing in their routing table for the call. Additional parameters, such as the maximum packet size, are also exchanged between the source and the destination during call setup. The virtual circuit is cleared after the data transfer is completed.

Virtual circuit packet switching is connection orientated. This is in contrast to datagram switching, which is a connection less packet switching methodology.

Advantages of virtual circuit switching are:

·         Packets are delivered in order, since they all take the same route;

·         The overhead in the packets is smaller, since there is no need for each packet to contain the full address;

• The connection is more reliable, network resources are allocated at call setup so that even during times of congestion, provided that a call has been setup, the subsequent packets should get through;
• Billing is easier, since billing records need only be generated per call and not per packet.

Disadvantages of a virtual circuit switched network are:

• The switching equipment needs to be more powerful, since each switch needs to store details of all the calls that are passing through it and to allocate capacity for any traffic that each call could generate;
• Resilience to the loss of a trunk is more difficult, since if there is a failure all the calls must be dynamically reestablished over a different route.

Examples of virtual circuit switching are X.25 and Frame Relay.

ADSL

Commonly simplified as DSL, Asymmetric Digital Subscriber Line (ADSL) is technology for high-speed Internet access. It uses existing copper telephone lines to send and receive data at speeds that far exceed conventional dial-up modems, while still allowing users to talk on the phone while they surf. By contrast, DSL is typically not as fast as cable Internet access. It is generally well-suited for moderate gaming, computer-aided design, streaming multimedia, and downloading large files.
ADSL Speed:
The fastest dial-up modems are rated at 56 kilobits per second (Kbps), and usually operate at about 53 Kbps under good conditions. By comparison, ADSL allows download speeds from 1.5 to 8 megabits per second (Mbps), depending on the grade of DSL service purchased. Cable Internet is capable of supporting up to 30 Mbps.
How Does ADSL Work?
ADSL uses standard telephone lines to upload and download data on a digital frequency, which sets these datastreams apart from the analog signals that telephones and fax machines use. The telephone can be used at the same time when surfing the Web with DSL service because the signal is operating on a different frequency; this is not true of conventional dial-up Internet access. It may be necessary to install inexpensive filters on each phone or fax line to remove any "white noise" on the line that might be generated from the DSL signals.
A compatible Internet service provider (ISP) is necessary to receive DSL service, as is a DSL modem. The modem may be provided by the ISP, or it may be purchased separately by the end-user. Most US-based ISPs that offer DSL service require subscriber contracts of at least one year. DSL is usually more expensive than dial-up service, but the latter is slowly becoming obsolete as user bandwidth requirements rise, due to things like streaming video.
DSL is an "always on" service, meaning that as long as the user's computer is powered on, it will automatically stay connected to the Internet unless it is manually disconnected via software or hardware. Family members can share DSL accounts, with a basic monthly fee. Unlike dial-up service, which stipulates that only one session be open at a time, multiple members can be using DSL service at the same time on various computers in the house. A router may also be used with this type of ISP to provide wireless access throughout a home.
Asymmetric vs. Symmetric
The "asymmetric" in ADSL refers to the fact that the speed at which data is downloaded, the data coming to the end-user's computer from the Internet, is faster than the rate for uploaded data, the data traveling from the user's computer to the Internet. The speed of uploading data is slower because Web page requests are fairly small data strings that do not require much bandwidth to handle efficiently. Consequently, more speed can be dedicated to downloading more bandwidth-intensive data.
Some businesses may require matching rates for uploading large files. For them, Symmetric Digital Subscriber Line (SDSL) is an option. "Symmetric" indicates that both datastreams are operating at the same speed of 1.5 to 7 Mbps, depending on the grade of service purchased. SDSL service requires a dedicated telephone line, however, because unlike ADSL, telephone and fax services cannot share a line with this service.
Availability
ADSL is not accessible to all communities, and coverage is often especially spotty in rural areas. Dedicated DSL providers, or even the local phone company, can verify if service is available in a specific locale. Speeds will vary depending upon the physical distance from local hubs, as well as the number of people using the service at one time in the same area.
Some customers who live close to an ISP hub may be able to take advantage of newer varieties of ADSL, called ADSL2 and ADSL2+, which have even greater throughput rates, from 12 to 24 Mbps for downloading and 1 to 3.5 Mbps for uploading. In addition, there are other types of DSL that offer customers other benefits. Rate Adaptive DSL (RADSL) uses a special modem that can adapt to changing line conditions, changing the speed as needed. Very high bit-rate DSL (VDSL) offers download speeds of up to 52 Mbps, but is not as widely available and is only able to achieve such high speeds very close to a hub.
Is DSL Ever as Fast as Cable?
Under identical conditions, cable has a strong speed advantage over ADSL; however, identical conditions rarely exist. In a given locale, cable speeds can suffer if they are too far from the nearest hub, or may suffer a bottle-neck if too many users are online at once. Artificial bandwidth caps placed on service at times of heaviest usage are not uncommon either. While the same factors are true of DSL, it does mean that in certain markets, some DSL providers may actually be able to provide faster service than some cable providers — especially if newer technologies like VDSL are available. As a result, whether DSL is as fast as cable is not always obvious, and it is a good idea for people looking for an ISP to research the local market and read customer reviews before committing to a particular provider.

FTP

Definition: FTP allows you to transfer files between two computers on the Internet. FTP is a simple network protocol based on Internet Protocol and also a term used when referring to the process of copying files when using FTP technology.
To transfer files with FTP, you use a program often called the "client." The FTP client program initiates a connection to a remote computer running FTP "server" software. After the connection is established, the client can choose to send and/or receive copies of files, singly or in groups. To connect to an FTP server, a client requires a username and password as set by the administrator of the server. Many public FTP archives follow a special convention for that accepts a username of "anonymous."
Simple FTP clients are included with most network operating systems, but most of these clients (such as FTP.EXE on Windows) support a relatively unfriendly command-line interface. Many alternative freeware / shareware third-party FTP clients have been developed that support graphic user interfaces (GUIs) and additional convenience features. In any FTP interface, clients identify the FTP server either by its IP address (such as 192.168.0.1) or by its host name (such as ftp.about.com).
FTP supports two modes of data transfer: plain text (ASCII), and binary. You set the mode in the FTP client. A common error when using FTP is attempting to transfer a binary file (such as a program or music file) while in text mode, causing the transfered file to be unusable.

E-mail

Short for electronic mail, the transmission of messages over communications networks. The messages can be notes entered from the keyboard or electronic files stored on disk. Most mainframes, minicomputers, and computer networks have an e-mail system. Some electronic-mail systems are confined to a single computer system or network, but others have gateways to other computer systems, enabling users to send electronic mail anywhere in the world. Companies that are fully computerized make extensive use of e-mail because it is fast, flexible, and reliable.
Most e-mail systems include a rudimentary text editor for composing messages, but many allow you to edit your messages using any editor you want. You then send the message to the recipient by specifying the recipient's address. You can also send the same message to several users at once. This is called broadcasting.
Sent messages are stored in electronic mailboxes until the recipient fetches them. To see if you have any mail, you may have to check your electronic mailbox periodically, although many systems alert you when mail is received. After reading your mail, you can store it in a text file, forward it to other users, or delete it. Copies of memos can be printed out on a printer if you want a paper copy.
All online services and Internet Service Providers (ISPs) offer e-mail, and most also support gateways so that you can exchange mail with users of other systems. Usually, it takes only a few seconds or minutes for mail to arrive at its destination. This is a particularly effective way to communicate with a group because you can broadcast a message or document to everyone in the group at once.
Although different e-mail systems use different formats, there are some emerging standards that are making it possible for users on all systems to exchange messages. In the PC world, an important e-mail standard is MAPI. The CCITT standards organization has developed the X.400 standard, which attempts to provide a universal way of addressing messages. To date, though, the de facto addressing standard is the one used by the Internet system because almost all e-mail systems have an Internet gateway.

A.M & F.M Radio

Definition of Radio:
Radio is "wireless telegraphy or telephony: speeches broadcast by radio." However "radio" describes the tool with which we receive this wireless transmission. Radio means anything associated with radio waves.
History of Radio:
In 1906 some folks aboard a ship who where used to hearing the blips and bleeps of Morse code all day and transcribing it down were treated to a special surprise. They heard over their transmissions the sound of a violin--it was Christmas Eve so the violin was playing "Silent Night." It was magical then, but the idea of creating a public medium out of the ingenious invention came a whole decade later with Frank Conrad who began playing records for his friends over the air, thus radio as we know it was born.
How It Works:Radio waves can bounce around all they want, but no one is going to hear or "intercept" them without a receiver. A radio wave is transmitted from a station's giant transmitter, and these waves go out into the air, in all directions. Your radio antenna, whether external or built into your AM/FM radio, picks up pieces of this wave. The waves don't actually transmit sounds--they carry electronic pulses, and your radio translates these pulses into sound, but all before you can blink your eyes a few times.
FM Radio Vs. AM Radio:
AM (amplitude modulation) and FM (frequency modulation) radio are the same kind of radio wave. They originate as the exact same radio wave, and are encrypted or "modulated" differently. FM radio has a strong advantage over AM radio because it is less likely to have static from fluctuations in signal due to changes in amplitude. FM radio ignores these fluctuations, producing less static, which provides for a better listening experience.
The Physics:
When you listen to your radio you are hearing the movement of atoms. When a radio signal is sent it is made up of lots of atoms that have dispersed from the transmitter antenna to the air. These atoms then travel at light speed in all directions until they reach a radio antenna. Radio transmission may seem like sound just bouncing around in the air, but it's actually a transfer of physical matter.

DNS

Definition: The DNS translates Internet domain and host names to IP addresses. DNS automatically converts the names we type in our Web browser address bar to the IP addresses of Web servers hosting those sites.
DNS implements a distributed database to store this name and address information for all public hosts on the Internet. DNS assumes IP addresses do not change (are statically assigned rather than dynamically assigned).
The DNS database resides on a hierarchy of special database servers. When clients like Web browsers issue requests involving Internet host names, a piece of software called the DNS resolver (usually built into the network operating system) first contacts a DNS server to determine the server's IP address. If the DNS server does not contain the needed mapping, it will in turn forward the request to a different DNS server at the next higher level in the hierarchy. After potentially several forwarding and delegation messages are sent within the DNS hierarchy, the IP address for the given host eventually arrives at the resolver, that in turn completes the request over Internet Protocol.
DNS additionally includes support for caching requests and for redundancy. Most network operating systems support configuration of primary, secondary, and tertiary DNS servers, each of which can service initial requests from clients. Internet Service Providers (ISPs) maintain their own DNS servers and use DHCP to automatically configure clients, relieving most home users of the burden of DNS configuration.

Routers

Definition: Routers are small physical devices that join multiple networks together. Technically, a router is a Layer 3 gateway device, meaning that it connects two or more networks and that the router operates at the network layer of the OSI model.
Home networks typically use a wireless or wired Internet Protocol (IP) router, IP being the most common OSI network layer protocol. An IP router such as a DSL or cable modem broadband router joins the home's local area network (LAN) to the wide-area network (WAN) of the Internet.
By maintaining configuration information in a piece of storage called the routing table, wired or wireless routers also have the ability to filter traffic, either incoming or outgoing, based on the IP addresses of senders and receivers. Some routers allow a network administrator to update the routing table from a Web browser interface. Broadband routers combine the functions of a router with those of a network switch and a firewall in a single unit.

VSAT

VSAT

Definition:

A Very Small Aperture Terminal (VSAT), is a two-way satellite ground station with a dish antenna that is smaller than 3 meters (most VSAT antennas range from 75 cm to 1.2 m). VSAT data rates typically range from narrowband up to 4 Mbit/s. VSATs access satellites in geosynchronous orbit to relay data from small remote earth stations (terminals) to other terminals (in mesh configurations) or master earth station "hubs" (in star configurations).

VSATs are most commonly used to transmit narrowband data (point of sale transactions such as credit card, polling or RFID data; or SCADA), or broadband data (for the provision of Satellite Internet access to remote locations, VoIP or video). VSATs are also used for transportable, on-the-move (with phased-array antennas) or mobile maritime (such as Inmarsat or BGAN) communications.

Most VSAT networks are configured in one of these topologies:

·         A star topology, using a central uplink site, such as a network operations center (NOC), to transport data back and forth to each VSAT terminal via satellite,

• A mesh topology, where each VSAT terminal relays data via satellite to another terminal by acting as a hub, minimizing the need for a centralized uplink site,
• A combination of both star and mesh topologies. Some VSAT networks are configured by having several centralized uplink sites (and VSAT terminals stemming from it) connected in a multi-star topology with each star (and each terminal in each star) connected to each other in a mesh topology. Others configured in only a single star topology sometimes will have each terminal connected to each other as well, resulting in each terminal acting as a central hub. These configurations are utilized to minimize the overall cost of the network, and to alleviate the amount of data that has to be relayed through a central uplink site (or sites) of a star or multi-star network.

Star topology services like HughesNet, Spacenet Connexstar/StarBand, WildBlue and others can be used to provide broadband wide area networks, as well as to provide broadband Internet access. Applications of this include intranet networking for front and back office applications, managed store and forward solutions such as digital signage, and interactive distance learning.

Configuration:

VSAT Frequency Spectrum Allocation This table acts as a guide only.
Band	Frequency GHz	Area Foot-print	Delivered Power	Rainfall effect
Band C	3 to 7	Large	Low	Minimum
Band Ku	10 to 18	Medium	Medium	Moderate
Band Ka	18 to 31	Small	High	Severe

Technology:

VSAT was originally intended for sporadic store-and-forward data communications but has evolved into real-time internet services. VSAT uses existing satellite broadcasting technology with higher powered components and antennas manufactured with higher precision than conventional satellite television systems. The satellite antenna at the customer's location includes, in addition to the receiver, a relatively high-powered transmitter that sends a signal back to the originating satellite. A very small portion of a transponder is used for each VSAT return path channel. Each VSAT terminal is assigned a frequency for the return path which it shares with other VSAT terminals using a shared transmission scheme such as time division multiple access.

An innovative feature of VSAT is that the technology has evolved to the point that something that previously could only be done with large, high-powered transmitting satellite dishes can now be done with a much smaller and vastly lower-powered antenna at the customer's premises. In addition, several return-path channels can co-exist on a single satellite transponder, and each of these return-path channels is further subdivided using to serve multiple customers.

In the system used by WildBlue, 31 different spot beams are used to serve the continental United States instead of the one beam used by conventional satellites. Thus, the same Ka-band transponders and frequencies are used for different regions throughout the United States, effectively re-using the same bandwidth in different regions.

The return path is transmitted from the customer's receiver in the L-band to a device called a low-noise block upconverter. There it is converted into the much higher frequency satellite transmission frequency, such as Ku-band and Ka-band, and amplified. Finally the signal is emitted to the dish antenna which focuses the signal into a beam that approximately covers the satellite with its beam. Because the transmission cannot be precise in these smaller dishes there is some effort to use frequencies for the uplink that are not used by adjacent satellites otherwise interference can occur to those other satellites.

Another satellite communications innovation, also used by satellite trucks for video transmission, is that only a small portion of a single satellite transponder is used by each VSAT channel. Previously a single transponder was required for a single customer but now several customers can use one transponder for the return path. This is in addition to time-based subdivision.