Welcome to the Routers section. Here we will analyse routers quite some depth; what
they do and how they work. I point out to you that you should have some knowlege on
the OSI model and understand how data is sent across the network medium. If you find
the information a bit too confusing or don't quite understand it, I would suggest you go
back to the networking section and do some reading on the OSI model and Protocols.
You will find information on Cisco routers at the end of this page.
What are they and what do they do ?
Routers are very common today in every network area, this is mainly because every
network these days connect to some other network, whether it's the Internet or some other
remote site. Routers get their name from what they do.... which is route data from one
network to another.

For example, if you had a company which had an office in Sydney and another one in
Melbourne, then to connect the two you would use a leased line to which you would
connect a router at each end. Any traffic which needs to travel from one site to another
will be routed via the routers, while all the other unecessary traffic is filtered (blocked),
thus saving you valuable bandwidth and money.
There are two type of routers: 1) Hardware routers 2) Software routers.
So what's the difference ?
When people talk about routers, they usually don't use the terms "hardware" or
"software" router but we are, for the purpose of distinguishing between the two.
Hardware routers are small boxes which run special software created by their vendors
to give them the routing capability and the only thing they do is simply route data from
one network to another. Most companies prefer hardware routers because they are faster
and more reliable, even though their cost is considerably more when compared with a
software router.
So what does a hardware router look like? Check the picture below, it displays a Cisco
1600 and 2500 series router along with a Netgear RT338 router. They look like a small
box and run special software as we said.

Software routers do the same job with the above hardware routers (route data), but they
don't come in small flashy boxes. A software router could be an NT server, NetWare
server or Linux server. All network servers have built-in routing capabilities.
Most people use them for Internet gateways and firewalls but there is one big difference
between the hardware and software routers. You cannot (in most cases) simply replace
the hardware router with a software router.Why? Simply because the hardware router has
the necessary hardware built-in to allow it to connect to the special WAN link (frame
relay, ISDN, ATM etc), where your software router (e.g a NT server) would have a few
network cards one of which connects to the LAN and the other goes to the WAN via the
hardware router.
I have seen a few cards in the market which allow you to connect an ISDN line directly
into them. With these special cards, which retail from $5000 to $15000 depending on
their capacity, you don't need the hardware router. But as you can understand, it's a much
cheaper solution to buy a hardware router. Plus, the hardware routers are far more
advanced and faster than the software routers since they don't have to worry about
anything else but routing data, and the special electronic components they have in them
are developed with this in mind.
The Flash image below shows us what a router does when it receives packets from the
LAN or the Internet. Depending on the source and destination, it will pass them to the
other network or send them to the Internet. The router is splitting the below network into
2. Each network has a hub to which all computers on that network connect to. Futher
more, the router has one interface connected to each network and one connected to the
Internet, this allows it to pass the packets to the right destination:

The picture below illustrates a router's place in the Local Area Network (LAN):


In the example shown, the workstations see the router as their "gateway". This means that
any machine on this LAN that wants to send a packet (data) to the Internet or anywhere
outside its Local Area Network (LAN) will send the packet via the gateway. The router
(gateway) will know where it needs to send it from there on so it can arrive at its
destination.
This explains the reason you need to add an Internet Protocol (IP) number for a gateway,
when you have a LAN at home or in the office, in your TCP/IP network properties on
your windows workstation.
The above figure shows only one example of how routers connect so the LAN gets
Internet access. Let's have a look how 2 offices would use routers to connect them.

The routers in the above picture connect using a particular WAN protocol, e.g ISDN.
In reality, there would be a cable (provided by your service provider) which connects to
the "WAN" interface of the router and from there the signal goes straight to your service
provider's network and eventually ends up at the other router's WAN interface.
Depending on the type of router you get, it will support one of the most commonly used
WAN protocols: ISDN, Frame Relay, ATM, HDLC, PPP. These protocols are discused
in the protocols section.
It's important to note down and remember a few of the main features of a router:
• Routers are Layer 3 devices
• Routers will not propagate broadcasts, unless they are programmed to
• Most serious routers have their own operating system
• Routers use special protocols between them to exchange information about each
other (not data)

Database security denotes the system, processes, and procedures that protect a database from unintended activity.

Security is usually enforced through access control, auditing, and encryption.

* Access control ensures and restricts who can connect and what can be done to the database.
* Auditing logs what action or change has been performed, when and by whom.
* Encryption: Since security has become a major issue in recent years, many commercial database vendors provide built-in encryption mechanisms. Data is encoded natively into the tables and deciphered "on the fly" when a query comes in. Connections can also be secured and encrypted if required using DSA, MD5, SSL or legacy encryption standard.

Enforcing security is one of the major tasks of the DBA.

In the United Kingdom, legislation protecting the public from unauthorized disclosure of personal information held on databases falls under the Office of the Information Commissioner. United Kingdom based organizations holding personal data in electronic format (databases for example) are required to register with the Data Commissioner.


LOCKING

Locking is how the database handles multiple concurrent operations. This is how concurrency and some form of basic integrity is managed within the database system. Such locks can be applied on a row level, or on other levels like page (a basic data block), extend (multiple array of pages) or even an entire table. This helps maintain the integrity of the data by ensuring that only one process at a time can modify the same data.

In basic filesystem files or folders, only one lock at a time can be set, restricting the usage to one process only. Databases, on the other hand, can set and hold mutiple locks at the same time on the different level of the physical data structure. How locks are set, last is determined by the database engine locking scheme based on the submitted SQL or transactions by the users. Generally speaking, no activity on the database should be translated by no or very light locking.

For most DBMS systems existing on the market, locks are generally shared or exclusive. Exclusive locks mean that no other lock can acquire the current data object as long as the exclusive lock lasts. Exclusive locks are usually set while the database needs to change data, like during an UPDATE or DELETE operation.

Shared locks can take ownership one from the other of the current data structure. Shared locks are usually used while the database is reading data, during a SELECT operation. The number, nature of locks and time the lock holds a data block can have a huge impact on the database performances. Bad locking can lead to disastrous performance response (usually the result of poor SQL requests, or inadequate database physical structure)

Default locking behavior is enforced by the isolation level of the dataserver. Changing the isolation level will affect how shared or exclusive locks must be set on the data for the entire database system. Default isolation is generally 1, where data can not be read while it is modified, forbidding to return "ghost data" to end user.

At some point intensive or inappropriate exclusive locking, can lead to the "dead lock" situation between two locks. Where none of the locks can be released because they try to acquire resources mutually from each other. The Database has a fail safe mechanism and will automatically "sacrifice" one of the locks releasing the resource. Doing so processes or transactions involved in the "dead lock" will be rolled back.

Databases can also be locked for other reasons, like access restrictions for given levels of user. Some databases are also locked for routine database maintenance, which prevents changes being made during the maintenance. See "Locking tables and databases" (section in some documentation / explanation from IBM) for more detail.) However, many modern databases don't lock the database during routine maintenance.

Databases are used in many applications, spanning virtually the entire range of computer software. Databases are the preferred method of storage for large multiuser applications, where coordination between many users is needed. Even individual users find them convenient, and many electronic mail programs and personal organizers are based on standard database technology. Software database drivers are available for most database platforms so that application software can use a common Application Programming Interface to retrieve the information stored in a database. Two commonly used database APIs are JDBC and ODBC.

Logical and physical view
Traditional View of Data

A database management system provides the ability for many different users to share data and process resources. But as there can be many different users, there are many different database needs. The question now is: How can a single, unified database meet the differing requirement of so many users?

A DBMS minimizes these problems by providing two views of the database data: a logical (external) view and physical (internal) view. The logical view/user’s view, of a database program represents data in a format that is meaningful to a user and to the software programs that process those data. That is, the logical view tells the user, in user terms, what is in the database. The physical view deals with the actual, physical arrangement and location of data in the direct access storage devices(DASDs). Database specialists use the physical view to make efficient use of storage and processing resources. With the logical view users can see data differently from how they are stored, and they do not want to know all the technical details of physical storage. After all, a business user is primarily interested in using the information, not in how it is stored.

One strength of a DBMS is that while there is only one physical view of the data, there can be an endless number of different logical views. This feature allows users to see database information in a more business-related way rather than from a technical, processing viewpoint. Thus the logical view refers to the way user views data, and the physical view to the way the data are physically stored and processed...

DBMS Features and capabilities

Alternatively, and especially in connection with the relational model of database management, the relation between attributes drawn from a specified set of domains can be seen as being primary. For instance, the database might indicate that a car that was originally "red" might fade to "pink" in time, provided it was of some particular "make" with an inferior paint job. Such higher arity relationships provide information on all of the underlying domains at the same time, with none of them being privileged above the others.

Throughout recent history specialized databases have existed for scientific, geospatial, imaging, document storage and like uses. Functionality drawn from such applications has lately begun appearing in mainstream DBMSs as well. However, the main focus there, at least when aimed at the commercial data processing market, is still on descriptive attributes on repetitive record structures.

Thus, the DBMSs of today roll together frequently-needed services or features of attribute management. By externalizing such functionality to the DBMS, applications effectively share code with each other and are relieved of much internal complexity. Features commonly offered by database management systems include:

Query ability
Querying is the process of requesting attribute information from various perspectives and combinations of factors. Example: "How many 2-door cars in Texas are green?" A database query language and report writer allow users to interactively interrogate the database, analyze its data and update it according to the users privileges on data.

Backup and replication
Copies of attributes need to be made regularly in case primary disks or other equipment fails. A periodic copy of attributes may also be created for a distant organization that cannot readily access the original. DBMS usually provide utilities to facilitate the process of extracting and disseminating attribute sets. When data is replicated between database servers, so that the information remains consistent throughout the database system and users cannot tell or even know which server in the DBMS they are using, the system is said to exhibit replication transparency.

Rule enforcement
Often one wants to apply rules to attributes so that the attributes are clean and reliable. For example, we may have a rule that says each car can have only one engine associated with it (identified by Engine Number). If somebody tries to associate a second engine with a given car, we want the DBMS to deny such a request and display an error message. However, with changes in the model specification such as, in this example, hybrid gas-electric cars, rules may need to change. Ideally such rules should be able to be added and removed as needed without significant data layout redesign.

Security
Often it is desirable to limit who can see or change which attributes or groups of attributes. This may be managed directly by individual, or by the assignment of individuals and privileges to groups, or (in the most elaborate models) through the assignment of individuals and groups to roles which are then granted entitlements.

Computation
There are common computations requested on attributes such as counting, summing, averaging, sorting, grouping, cross-referencing, etc. Rather than have each computer application implement these from scratch, they can rely on the DBMS to supply such calculations.

Change and access logging
Often one wants to know who accessed what attributes, what was changed, and when it was changed. Logging services allow this by keeping a record of access occurrences and changes.

Automated optimization
If there are frequently occurring usage patterns or requests, some DBMS can adjust themselves to improve the speed of those interactions. In some cases the DBMS will merely provide tools to monitor performance, allowing a human expert to make the necessary adjustments after reviewing the statistics collected.

Meta-data repository

Metadata is data describing data. For example, a listing that describes what attributes are allowed to be in data sets is called "meta-information". The meta-data is also known as data about data.

Examples of Database Management Systems

* Alpha Five
* DataEase
* Oracle database
* IBM DB2
* Adaptive Server Enterprise
* FileMaker
* Firebird
* Ingres
* Informix
* Mark Logic
* Microsoft Access



* Microsoft SQL Server
* Microsoft Visual FoxPro
* MySQL
* PostgreSQL
* Progress
* SQLite
* Teradata
* CSQL
* OpenLink Virtuoso
* Daffodil DB

A DBMS includes of four main parts: Modeling language, data structure, database query language, and transaction mechanism:

Modeling language

A data modeling language to define the schema of each database hosted in the DBMS, according to the DBMS database model. The four most common types of organizations are the:

* hierarchical model,
* network model,
* relational model, and
* object model.

Inverted lists and other methods are also used. A given database management system may provide one or more of the four models. The optimal structure depends on the natural organization of the application's data, and on the application's requirements (which include transaction rate (speed), reliability, maintainability, scalability, and cost).

The dominant model in use today is the ad hoc one embedded in SQL, despite the objections of purists who believe this model is a corruption of the relational model, since it violates several of its fundamental principles for the sake of practicality and performance. Many DBMSs also support the Open Database Connectivity API that supports a standard way for programmers to access the DBMS.

Data structure

Data structures (fields, records, files and objects) optimized to deal with very large amounts of data stored on a permanent data storage device (which implies relatively slow access compared to volatile main memory).

Database query language

A database query language and report writer to allow users to interactively interrogate the database, analyze its data and update it according to the users privileges on data. It also controls the security of the database. Data security prevents unauthorized users from viewing or updating the database. Using passwords, users are allowed access to the entire database or subsets of it called subschemas. For example, an employee database can contain all the data about an individual employee, but one group of users may be authorized to view only payroll data, while others are allowed access to only work history and medical data.

If the DBMS provides a way to interactively enter and update the database, as well as interrogate it, this capability allows for managing personal databases. However, it may not leave an audit trail of actions or provide the kinds of controls necessary in a multi-user organization. These controls are only available when a set of application programs are customized for each data entry and updating function.

Transaction mechanism

A database transaction mechanism, that ideally would guarantee the ACID properties, in order to ensure data integrity, despite concurrent user accesses (concurrency control), and faults (fault tolerance). It also maintains the integrity of the data in the database. The DBMS can maintain the integrity of the database by not allowing more than one user to update the same record at the same time. The DBMS can help prevent duplicate records via unique index constraints; for example, no two customers with the same customer numbers (key fields) can be entered into the database. See ACID properties for more information (Redundancy avoidance).

A computer is a machine that manipulates data according to a list of instructions.

The first devices that resemble modern computers date to the mid-20th century (1940–1945), although the computer concept and various machines similar to computers existed earlier. Early electronic computers were the size of a large room, consuming as much power as several hundred modern personal computers (PC).[1] Modern computers are based on tiny integrated circuits and are millions to billions of times more capable while occupying a fraction of the space.[2] Today, simple computers may be made small enough to fit into a wristwatch and be powered from a watch battery. Personal computers, in various forms, are icons of the Information Age and are what most people think of as "a computer"; however, the most common form of computer in use today is the embedded computer. Embedded computers are small, simple devices that are used to control other devices — for example, they may be found in machines ranging from fighter aircraft to industrial robots, digital cameras, and children's toys.

The ability to store and execute lists of instructions called programs makes computers extremely versatile and distinguishes them from calculators. The Church–Turing thesis is a mathematical statement of this versatility: any computer with a certain minimum capability is, in principle, capable of performing the same tasks that any other computer can perform. Therefore, computers with capability and complexity ranging from that of a personal digital assistant to a supercomputer are all able to perform the same computational tasks given enough time and storage capacity.
Contents
[hide]

* History of computing
* Stored program architecture
Programs
Example
* How computers work
Control unit
Arithmetic/logic unit (ALU)
Memory
Input/output (I/O)
Multitasking
Multiprocessing
Networking and the Internet
* Further topics
Hardware
Software
Programming languages
Professions and organizations
* See also
* External links
* Notes
* References

In the generic role as a packet processor, a number of optimised features or functions are typically present in a network processor, these include:

* Pattern matching - the ability to find specific patterns of bits or bytes within packets in a packet stream.
* Key lookup for example, address lookup - the ability to quickly undertake a database lookup using a key (typically an address on a packet) to find a result, typically routing information.
* Computation
* Data bitfield manipulation - the ability to change certain data fields contained in the packet as it is being processed.
* Queue management - as packets are received, processed and scheduled to be send onwards, they are stored in queues.
* Control processing - the micro operations of processing a packet are controlled at a macro level which involves communication and orchestration with other nodes in a system.
* Recirculation is done for packets