Sequential logic is a form of binary circuit design that employs one or more inputs and one or more outputs, whose states are related by defined rules that depend, in part, on previous states. Each of the inputs and output(s) can attain either of two states: logic 0 (low) or logic 1 (high).
A common example of a circuit employing sequential logic is the flip-flop, also called a bistable gate. A simple flip-flop has two stable states. The flip-flop maintains its states indefinitely until an input pulse called a trigger is received. If a trigger is received, the flip-flop outputs change their states according to defined rules, and remain in those states until another trigger is received.
There are several different kinds of flip-flop circuits, with designators such as D, T, J-K, and R-S. Flip-flop circuits are interconnected to form the logic gates that comprise digital integrated circuits (ICs) such as memory chips and microprocessors.
Sequential logic differs from combinatorial logic (also called combinational logic). In the latter scheme, the output states depend only on the input states at a specific moment in time, and not on previous states.
Server:
Serverless Backup:
SLIP:
SMTP:
SNMP:
SONET:
SONET is the American National Standards Institute standard for synchronous data transmission on optical media. The international equivalent of SONET is synchronous digital hierarchy (SDH). Together, they ensure standards so that digital networks can interconnect internationally and that existing conventional transmission systems can take advantage of optical media through tributary attachments.
SONET provides standards for a number of line rates up to the maximum line rate of 9.953 gigabits per second (Gbps ). Actual line rates approaching 20 gigabits per second are possible. SONET is considered to be the foundation for the physical layer of the broadband ISDN (BISDN).
Asynchronous transfer mode runs as a layer on top of SONET as well as on top of other technologies. SONET defines a base rate of 51.84 Mbps and a set of multiples of the base rate known as "Optical Carrier levels (OCx)."
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Spider:
A spider is a program that visits Web sites and reads their pages and other information in order to create entries for a search engine index. The major search engines on the Web all have such a program, which is also known as a "crawler" or a "bot." Spiders are typically programmed to visit sites that have been submitted by their owners as new or updated. Entire sites or specific pages can be selectively visited and indexed. Spiders are called spiders because they usually visit many sites in parallel at the same time, their "legs" spanning a large area of the "web." Spiders can crawl through a site's pages in several ways. One way is to follow all the hypertext links in each page until all the pages have been read.
The spider for the AltaVista search engine and its Web site is called Scooter . Scooter adheres to the rules of politeness for Web spiders that are specified in the Standard for Robot Exclusion (SRE). It asks each server which files should be excluded from being indexed. It does not (or can not) go through firewall. And it uses a special algorithm for waiting between successive server requests so that it doesn't affect response time for other users.
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Storage Area Network - SAN:
A storage area network (SAN) is a high-speed special-purpose network (or subnetwork) that interconnects different kinds of data storage devices with associated data server s on behalf of a larger network of users. Typically, a storage area network is part of the overall network of computing resources for an enterprise. A storage area network is usually clustered in close proximity to other computing resources such as IBM S/390 mainframes but may also extend to remote locations for backup and archival storage, using wide area network carrier technologies such as asynchronous transfer mode or Synchronous Optical Networks.
A storage area network can use existing communication technology such as IBM's optical fiber ESCON or it may use the newer Fibre Channel technology. Some SAN system integrators liken it to the common storage bus (flow of data) in a personal computer that is shared by different kinds of storage devices such as a hard disk or a CD-ROM player.
SANs support disk mirroring, backup and restore, archival and retrieval of archived data, data migration from one storage device to another, and the sharing of data among different servers in a network. SANs can incorporate subnetworks with network-attached storage (NAS) systems.
