Synchronous describes objects or events that are coordinated in time. In information technology, the term has several different usages.
Asynchronous describes objects or events that are not coordinated in time, which run from different clocks, or at a different transition rate. As with synchronous, the term has several different usages in information technology:
These are signals, which are almost, but not quite, in synchronization, with some method used to adjust them.
In telephone communication, handshaking is the exchange of information between two modems and the resulting agreement about which protocol to use that precedes each telephone connection. The handshaking can be heard in those crunching and other sounds when a dial-out call is made from a computer.
Since the modems at each end of the line may have different capabilities, they need to inform each other and settle on the highest transmission speed they can both use. At higher speeds, the modems have to determine the length of line delays so that echo cancellers can be used properly.
A third level can be defined to identify a particular host server at the Internet address. In our example, "www" is the name of the server that handles Internet requests. (A second server might be called "www2".)
Subdomain levels can be used. For example, you could have "www.news.google.com". Together, "www.google.com" constitutes a fully-qualified domain name.
Second-level domain names must be unique on the Internet and registered with one of the ICANN-accredited registrars for the COM, NET, and ORG top-level domains. Where appropriate, a top-level domain name can be geographic. (Currently, most non-U.S. domain names use a top-level domain name based on the country in which the server is located.)
On the Web, the domain name is that part of the Uniform Resource Locator (URL) that tells a domain name server using the domain name system (DNS) whether and where to forward a request for a Web page. The domain name is mapped to an IP address (which represents a physical point on the Internet).
More than one domain name can be mapped to the same Internet address. This allows multiple individuals, businesses, and organizations to have separate Internet identities while sharing the same Internet server.
The domain name system contains an even higher level of domain than the top-level domain. The highest level is the root domain, which would be represented by a single dot (just as in many hierarchical file systems, a root directory is represented by a "/") if it were ever used. If the dot for the root domain were shown in the URL, it would be to the right of the top-level domain name. However, the dot is assumed to be there, but never shown.
Domain Name System (DNS)
The domain name system is the way that Internet domain names are located and translated into Internet protocol addresses. A domain name is a meaningful and easy-to-remember "handle" for an Internet address.
Because maintaining a central list of domain name/IP address correspondences would be impractical, the lists of domain names and IP addresses are distributed throughout the Internet in a hierarchy of authority. There is probably a DNS server within close geographic proximity to the userís access provider that maps the domain names in the userís Internet requests or forwards them to other servers in the Internet.
The calculations involved in producing an ISBN are outlined below:
ISBN 0 6 0 0 3 2 3 8 2 check
Weights 10 9 8 7 6 5 4 3 2 1
Each digit of the ISBN is then multiplied by the weight below it:
The products are added together and the total is divided by 11. The remainder is subtracted from 11 to give the check digit. In the example given, a remainder of 1 gives rise to a check digit of 10. This is written as an X.
122 / 11 = 11 remainder 1
The complete ISBN is 0 600 32382 X
The URL contains the name of the protocol required to access the resource, a domain name that identifies a specific computer on the Internet, and a hierarchical description of a file location on the computer. The domain name is the name of the website, which sometimes specifies the name of the server on which the web resource is held. It is a string of Ďidentifiersí separated by full stops. The suffix of the domain name specifies the type of organisation and often the country in which the server is located. Unique domain names can be purchased from various vendors.
IP addresses to a web siteís homepage consist of a set of four 3-digit numbers. These are separated by full stops. The domain name system maps the domain name onto the IP system.
On the Web (which uses the Hypertext Transfer Protocol), an example of a URL is:
The above URL describes a Web page to be accessed with an HTTP (Web browser) application that is located on a computer named "www.bbc.co.uk". The specific file is in the directory named "/news" and is the default page in that directory.
An HTTP URL can be for any Web page, not just a home page, or any individual file.