Wednesday, June 16, 2010



The introduction of the Extensible Markup Language, or XML as it is commonly known, created a buzz in the Web world. It "provides both a standards-based way to identify the information that is of importance in a particular application, and the ability to process information tagged according to highly user-specific requirements with general-purpose software, such as editing tools, composition engines, and electronic browsers" (Usdin & Graham, 1998). In simpler terms, XML allows users to customize a markup language and apply it to an information object that can then be interpreted to determine its contents, whether it is an order form, a newspaper or an advertisement. Given these descriptions, it becomes apparent that XML is a tool, an enabling technology that can be used in conjunction with other tools to provide powerful Web applications. How this tool can be customized and utilized by the Web community is the subject of this tutorial.

XML's roots lie in the Standard Generalized Markup Language, or SGML. SGML was developed 20 years ago as a formal method of annotating documents to describe their meaning and structure, but it's complexity and cost hindered widespread acceptance. However, a subset of SGML called the Hypertext Markup Language, or HTML, is a phenomena that has enabled the rapid growth of the Web over the past decade. Used primarily for stylistic and formatting purposes, HTML has caused anxiety for many of its users who were interested in utilizing its tag set for more complex presentation control, data processing and programming (Treese, 1998). Because of these issues, the World Wide Web Consortium, or W3C, started a working group for a new subset of SGML, XML, in January 1997. The group "proposed a markup language that could work in concert with existing Web technologies, using some of the tools developed for use with HTML, while moving forward with more manageable techniques" (St. Laurent, 1999). A year later, in February 1998, the XML specification was ratified as a W3C standard.

While XML has its foundation in SGML, its philosophy differs and is based on four fundamental principles (Usdin & Graham, 1998).

1. Separation of Content from Format: What a piece of information is should be managed separately from how the information is presented. Information should be identifiable by its appearance, its use in a particular application, its role in the document in which it is contained, and its nature. For example, "knowing that a phrase is in italic is useful; knowing that it is the title of a subsection of a paper is more useful; and knowing that it is a genus and species name is potentially more useful still."
2. Hierarchical Data Structures: In XML, the data is assumed to be hierarchically organized, that is, a piece of information may contain other pieces of information and may be contained by yet another piece of information. Textual documents often exemplify this type of structure. For example, a book contains several chapters, each of which contains sections. Each section may have a heading, paragraphs and subsections, which also contain a heading and paragraphs.
3. Embedded Tags: The data marked up with XML contains tags, words or phrases enclosed in point brackets, which identify where the data structures begin and end. These tags can also have attributes, which provide information about the data enclosed by the tags. Example: < tag attribute="value"> content
4. User-Definable Structures: As mentioned above, XML is a tool, and it defines a method of customized tag creation. "XML assumes that users will create new tags as they create and work with documents, and that software such as browsers will have to display or process the content of these novel tags." As such, XML provides flexibility and extensibility by not providing a standard tag set like HTML.

Components of XML

The basic components of XML are similar to that of HTML: tags, elements and their attributes. A tag is a piece of markup such as an opening tag <P> and a closing tag This text is part of a paragraph element.
It includes the <B>bold element and
the <I>italics element.

The paragraph above has 6 tags comprising 3 elements, 2 of which are contained within the paragraph element. The paragraph element also contains an attribute specifying that the paragraph should be centered on the page. This style of markup is used in the creation of XML documents, which can be of two types: well-formed and valid.

A well-formed document is syntactically correct but does not refer to a Document Type Definition (DTD) that specifies tag requirements and allows the document to be validated. Syntactical correctness includes:

* utilizing a root element
* providing closing tags for all opening tags
* placing quotes around all attribute values
* ensuring the same case is maintained throughout the tags

A valid XML document is well formed and complies with the guidelines of a DTD, which defines a tag set. The DTD can be part of the XML document, or it can be referred to by the XML document. To create a DTD, please refer to the Creating a DTD section of this tutorial.

The convergence of the XML document and the DTD provides content for the browser (in this case, Internet Explorer 5) to interpret and display. The and the make up the prolog of the XML document, or "the glue that binds DTDs to the code that applies to them" (St. Laurent, 1999). The first statement tells the browser the version of XML in use, and the second statement provides the filename of the DTD, whether it is a system or public DTD, and its location/file name on the system. A system DTD is one that has been developed for a particular Web site or business, while a public DTD has been developed for use by types of organizations (e.g. advertising, newspapers etc.). The DTD defines the available elements and attributes which can be incorporated to comprise the logical structure of a single XML document or document groups.

The contents of an XML document must be formatted by using a stylesheet such as XSL (Extensible Style Language). Using a stylesheet adds another layer of complexity to the XML document display process. In the XML document, a line is added below the line that contains a reference to the XSL formatting file such as,
A valid XML document reflects the collaboration of the DTD, XSL stylesheet and the XML document contents. The document contents can be created by marking up text in a text editor or generated through a database. To generate a valid, formatted XML document from an Access database, please refer to the Creating an XML Document section of this tutorial.

Tuesday, June 15, 2010


What is a computer Network?

A network is any collection of independent computers that communicate with one another over a shared network medium.A computer network is a collection of two or more connected computers. When these computers are joined in a network, people can share files and peripherals such as modems, printers, tape backup drives, or CD-ROM drives. When networks at multiple locations are connected using services available from phone companies, people can send e-mail, share links to the global Internet, or conduct video conferences in real time with other remote users. As companies rely on applications like electronic mail and database management for core business operations, computer networking becomes increasingly more important.

Every network includes:

* At least two computers Server or Client workstation.
* Networking Interface Card's (NIC)
* A connection medium, usually a wire or cable, although wireless communication between networked computers and peripherals is also possible.
* Network Operating system software, such as Microsoft Windows NT or 2000, Novell NetWare, Unix and Linux.

Types of Networks:
LANs (Local Area Networks)

A network is any collection of independent computers that communicate with one another over a shared network medium. LANs are networks usually confined to a geographic area, such as a single building or a college campus. LANs can be small, linking as few as three computers, but often link hundreds of computers used by thousands of people. The development of standard networking protocols and media has resulted in worldwide proliferation of LANs throughout business and educational organizations.
WANs (Wide Area Networks)

Wide area networking combines multiple LANs that are geographically separate. This is accomplished by connecting the different LANs using services such as dedicated leased phone lines, dial-up phone lines (both synchronous and asynchronous), satellite links, and data packet carrier services. Wide area networking can be as simple as a modem and remote access server for employees to dial into, or it can be as complex as hundreds of branch offices globally linked using special routing protocols and filters to minimize the expense of sending data sent over vast distances.

The Internet is a system of linked networks that are worldwide in scope and facilitate data communication services such as remote login, file transfer, electronic mail, the World Wide Web and newsgroups.

With the meteoric rise in demand for connectivity, the Internet has become a communications highway for millions of users. The Internet was initially restricted to military and academic institutions, but now it is a full-fledged conduit for any and all forms of information and commerce. Internet websites now provide personal, educational, political and economic resources to every corner of the planet.

With the advancements made in browser-based software for the Internet, many private organizations are implementing intranets. An intranet is a private network utilizing Internet-type tools, but available only within that organization. For large organizations, an intranet provides an easy access mode to corporate information for employees.
MANs (Metropolitan area Networks)

The refers to a network of computers with in a City.
VPN (Virtual Private Network)

VPN uses a technique known as tunneling to transfer data securely on the Internet to a remote access server on your workplace network. Using a VPN helps you save money by using the public Internet instead of making long–distance phone calls to connect securely with your private network. There are two ways to create a VPN connection, by dialing an Internet service provider (ISP), or connecting directly to Internet.
Categories of Network:
Peer to Peer and Server/Client NetworkNetwork can be divided in to two main categories:

* Peer-to-peer.
* Server – based.

In peer-to-peer networking there are no dedicated servers or hierarchy among the computers. All of the computers are equal and therefore known as peers. Normally each computer serves as Client/Server and there is no one assigned to be an administrator responsible for the entire network.

Peer-to-peer networks are good choices for needs of small organizations where the users are allocated in the same general area, security is not an issue and the organization and the network will have limited growth within the foreseeable future.

The term Client/server refers to the concept of sharing the work involved in processing data between the client computer and the most powerful server computer.
The client/server network is the most efficient way to provide:

* Databases and management of applications such as Spreadsheets, Accounting, Communications and Document management.
* Network management.
* Centralized file storage.

The client/server model is basically an implementation of distributed or cooperative processing. At the heart of the model is the concept of splitting application functions between a client and a server processor. The division of labor between the different processors enables the application designer to place an application function on the processor that is most appropriate for that function. This lets the software designer optimize the use of processors--providing the greatest possible return on investment for the hardware.

Client/server application design also lets the application provider mask the actual location of application function. The user often does not know where a specific operation is executing. The entire function may execute in either the PC or server, or the function may be split between them. This masking of application function locations enables system implementers to upgrade portions of a system over time with a minimum disruption of application operations, while protecting the investment in existing hardware and software.
OSI ModelThe OSI Model:

Open System Interconnection (OSI) reference model has become an International standard and serves as a guide for networking. This model is the best known and most widely used guide to describe networking environments. Vendors design network products based on the specifications of the OSI model. It provides a description of how network hardware and software work together in a layered fashion to make communications possible. It also helps with trouble shooting by providing a frame of reference that describes how components are supposed to function.

There are seven to get familiar with and these are the physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and the application layer.

* Physical Layer, is just that the physical parts of the network such as wires, cables, and there media along with the length. Also this layer takes note of the electrical signals that transmit data throughout system.
* Data Link Layer, this layer is where we actually assign meaning to the electrical signals in the network. The layer also determines the size and format of data sent to printers, and other devices. Also I don't want to forget that these are also called nodes in the network. Another thing to consider in this layer is will also allow and define the error detection and correction schemes that insure data was sent and received.
* Network Layer, this layer provides the definition for the connection of two dissimilar networks.
* Transport Layer, this layer allows data to be broken into smaller packages for data to be distributed and addressed to other nodes (workstations).
* Session Layer, this layer helps out with the task to carry information from one node (workstation) to another node (workstation). A session has to be made before we can transport information to another computer.
* Presentation Layer, this layer is responsible to code and decode data sent to the node.
* Application Layer, this layer allows you to use an application that will communicate with say the operation system of a server. A good example would be using your web browser to interact with the operating system on a server such as Windows NT, which in turn gets the data you requested.

Network Architectures:
Ethernet Network ArchitectureEthernet

Ethernet is the most popular physical layer LAN technology in use today. Other LAN types include Token Ring, Fast Ethernet, Fiber Distributed Data Interface (FDDI), Asynchronous Transfer Mode (ATM) and LocalTalk. Ethernet is popular because it strikes a good balance between speed, cost and ease of installation. These benefits, combined with wide acceptance in the computer marketplace and the ability to support virtually all popular network protocols, make Ethernet an ideal networking technology for most computer users today. The Institute for Electrical and Electronic Engineers (IEEE) defines the Ethernet standard as IEEE Standard 802.3. This standard defines rules for configuring an Ethernet network as well as specifying how elements in an Ethernet network interact with one another. By adhering to the IEEE standard, network equipment and network protocols can communicate efficiently.
Fast Ethernet

For Ethernet networks that need higher transmission speeds, the Fast Ethernet standard (IEEE 802.3u) has been established. This standard raises the Ethernet speed limit from 10 Megabits per second (Mbps) to 100 Mbps with only minimal changes to the existing cable structure. There are three types of Fast Ethernet: 100BASE-TX for use with level 5 UTP cable, 100BASE-FX for use with fiber-optic cable, and 100BASE-T4 which utilizes an extra two wires for use with level 3 UTP cable. The 100BASE-TX standard has become the most popular due to its close compatibility with the 10BASE-T Ethernet standard. For the network manager, the incorporation of Fast Ethernet into an existing configuration presents a host of decisions. Managers must determine the number of users in each site on the network that need the higher throughput, decide which segments of the backbone need to be reconfigured specifically for 100BASE-T and then choose the necessary hardware to connect the 100BASE-T segments with existing 10BASE-T segments. Gigabit Ethernet is a future technology that promises a migration path beyond Fast Ethernet so the next generation of networks will support even higher data transfer speeds.
Token Ring ArchitectureToken Ring

Token Ring is another form of network configuration which differs from Ethernet in that all messages are transferred in a unidirectional manner along the ring at all times. Data is transmitted in tokens, which are passed along the ring and viewed by each device. When a device sees a message addressed to it, that device copies the message and then marks that message as being read. As the message makes its way along the ring, it eventually gets back to the sender who now notes that the message was received by the intended device. The sender can then remove the message and free that token for use by others.

Various PC vendors have been proponents of Token Ring networks at different times and thus these types of networks have been implemented in many organizations.
FDDI architectureFDDI

FDDI (Fiber-Distributed Data Interface) is a standard for data transmission on fiber optic lines in a local area network that can extend in range up to 200 km (124 miles). The FDDI protocol is based on the token ring protocol. In addition to being large geographically, an FDDI local area network can support thousands of users.

Network protocols are standards that allow computers to communicate. A protocol defines how computers identify one another on a network, the form that the data should take in transit, and how this information is processed once it reaches its final destination. Protocols also define procedures for handling lost or damaged transmissions or "packets." TCP/IP (for UNIX, Windows NT, Windows 95 and other platforms), IPX (for Novell NetWare), DECnet (for networking Digital Equipment Corp. computers), AppleTalk (for Macintosh computers), and NetBIOS/NetBEUI (for LAN Manager and Windows NT networks) are the main types of network protocols in use today.

Although each network protocol is different, they all share the same physical cabling. This common method of accessing the physical network allows multiple protocols to peacefully coexist over the network media, and allows the builder of a network to use common hardware for a variety of protocols. This concept is known as "protocol independence,"

Some Important Protocols and their job:
Protocol Acronym Its Job
Point-To-Point TCP/IP The backbone protocol of the internet. Popular also for intranets using the internet
Transmission Control Protocol/internet Protocol TCP/IP The backbone protocol of the internet. Popular also for intranets using the internet
Internetwork Package Exchange/Sequenced Packet Exchange IPX/SPX This is a standard protocol for Novell Network Operating System
NetBIOS Extended User Interface NetBEUI This is a Microsoft protocol that doesn't support routing to other networks
File Transfer Protocol FTP Used to send and receive files from a remote host
Hyper Text Transfer Protocol HTTP Used for the web to send documents that are encoded in HTML.
Network File Services NFS Allows network nodes or workstations to access files and drives as if they were their own.
Simple Mail Transfer Protocol SMTP Used to send Email over a network
Telnet Used to connect to a host and emulate a terminal that the remote server can recognize
Introduction to TCP/IP Networks:

TCP/IP-based networks play an increasingly important role in computer networks. Perhaps one reason for their appeal is that they are based on an open specification that is not controlled by any vendor.
What Is TCP/IP?

TCP stands for Transmission Control Protocol and IP stands for Internet Protocol. The term TCP/IP is not limited just to these two protocols, however. Frequently, the term TCP/IP is used to refer to a group of protocols related to the TCP and IP protocols such as the User Datagram Protocol (UDP), File Transfer Protocol (FTP), Terminal Emulation Protocol (TELNET), and so on.
The Origins of TCP/IP

In the late 1960s, DARPA (the Defense Advanced Research Project Agency), in the United States, noticed that there was a rapid proliferation of computers in military communications. Computers, because they can be easily programmed, provide flexibility in achieving network functions that is not available with other types of communications equipment. The computers then used in military communications were manufactured by different vendors and were designed to interoperate with computers from that vendor only. Vendors used proprietary protocols in their communications equipment. The military had a multi vendor network but no common protocol to support the heterogeneous equipment from different vendors
Net work Cables and Stuff:

In the network you will commonly find three types of cables used these are the, coaxial cable, fiber optic and twisted pair.
Thick Coaxial Cable

This type cable is usually yellow in color and used in what is called thicknets, and has two conductors. This coax can be used in 500-meter lengths. The cable itself is made up of a solid center wire with a braided metal shield and plastic sheathing protecting the rest of the wire.
Thin Coaxial Cable

As with the thick coaxial cable is used in thicknets the thin version is used in thinnets. This type cable is also used called or referred to as RG-58. The cable is really just a cheaper version of the thick cable.
Fiber Optic Cable

As we all know fiber optics are pretty darn cool and not cheap. This cable is smaller and can carry a vast amount of information fast and over long distances.
Twisted Pair Cables

These come in two flavors of unshielded and shielded.
Unshielded Twisted Pair and Shielded Twisted Pair
Shielded Twisted Pair (STP)

Is more common in high-speed networks. The biggest difference you will see in the UTP and STP is that the STP use's metallic shield wrapping to protect the wire from interference.

-Something else to note about these cables is that they are defined in numbers also. The bigger the number the better the protection from interference. Most networks should go with no less than a CAT 3 and CAT 5 is most recommended.

-Now you know about cables we need to know about connectors. This is pretty important and you will most likely need the RJ-45 connector. This is the cousin of the phone jack connector and looks real similar with the exception that the RJ-45 is bigger. Most commonly your connector are in two flavors and this is BNC (Bayonet Naur Connector) used in thicknets and the RJ-45 used in smaller networks using UTP/STP.
Unshielded Twisted Pair (UTP)

This is the most popular form of cables in the network and the cheapest form that you can go with. The UTP has four pairs of wires and all inside plastic sheathing. The biggest reason that we call it Twisted Pair is to protect the wires from interference from themselves. Each wire is only protected with a thin plastic sheath.
Ethernet Cabling

Now to familiarize you with more on the Ethernet and it's cabling we need to look at the 10's. 10Base2, is considered the thin Ethernet, thinnet, and thinwire which uses light coaxial cable to create a 10 Mbps network. The cable segments in this network can't be over 185 meters in length. These cables connect with the BNC connector. Also as a note these unused connection must have a terminator, which will be a 50-ohm terminator.

10Base5, this is considered a thicknet and is used with coaxial cable arrangement such as the BNC connector. The good side to the coaxial cable is the high-speed transfer and cable segments can be up to 500 meters between nodes/workstations. You will typically see the same speed as the 10Base2 but larger cable lengths for more versatility.

10BaseT, the “T” stands for twisted as in UTP (Unshielded Twisted Pair) and uses this for 10Mbps of transfer. The down side to this is you can only have cable lengths of 100 meters between nodes/workstations. The good side to this network is they are easy to set up and cheap! This is why they are so common an ideal for small offices or homes.

100BaseT, is considered Fast Ethernet uses STP (Shielded Twisted Pair) reaching data transfer of 100Mbps. This system is a little more expensive but still remains popular as the 10BaseT and cheaper than most other type networks. This on of course would be the cheap fast version.

10BaseF, this little guy has the advantage of fiber optics and the F stands for just that. This arrangement is a little more complicated and uses special connectors and NIC's along with hubs to create its network. Pretty darn neat and not to cheap on the wallet.

An important part of designing and installing an Ethernet is selecting the appropriate Ethernet medium. There are four major types of media in use today: Thickwire for 10BASE5 networks, thin coax for 10BASE2 networks, unshielded twisted pair (UTP) for 10BASE-T networks and fiber optic for 10BASE-FL or Fiber-Optic Inter-Repeater Link (FOIRL) networks. This wide variety of media reflects the evolution of Ethernet and also points to the technology's flexibility. Thickwire was one of the first cabling systems used in Ethernet but was expensive and difficult to use. This evolved to thin coax, which is easier to work with and less expensive.
Network Topologies:
What is a Network topology?

A network topology is the geometric arrangement of nodes and cable links in a LAN,

There are three topology's to think about when you get into networks. These are the star, rind, and the bus.

Star, in a star topology each node has a dedicated set of wires connecting it to a central network hub. Since all traffic passes through the hub, the hub becomes a central point for isolating network problems and gathering network statistics.

Ring, a ring topology features a logically closed loop. Data packets travel in a single direction around the ring from one network device to the next. Each network device acts as a repeater, meaning it regenerates the signal

Bus, the bus topology, each node (computer, server, peripheral etc.) attaches directly to a common cable. This topology most often serves as the backbone for a network. In some instances, such as in classrooms or labs, a bus will connect small workgroups

Ethernet is a shared media, so there are rules for sending packets of data to avoid conflicts and protect data integrity. Nodes determine when the network is available for sending packets. It is possible that two nodes at different locations attempt to send data at the same time. When both PCs are transferring a packet to the network at the same time, a collision will result.

Minimizing collisions is a crucial element in the design and operation of networks. Increased collisions are often the result of too many users on the network, which results in a lot of contention for network bandwidth. This can slow the performance of the network from the user's point of view. Segmenting the network, where a network is divided into different pieces joined together logically with a bridge or switch, is one way of reducing an overcrowded network.
Ethernet Products:

The standards and technology that have just been discussed help define the specific products that network managers use to build Ethernet networks. The following text discusses the key products needed to build an Ethernet LAN.

Transceivers are used to connect nodes to the various Ethernet media. Most computers and network interface cards contain a built-in 10BASE-T or 10BASE2 transceiver, allowing them to be connected directly to Ethernet without requiring an external transceiver. Many Ethernet devices provide an AUI connector to allow the user to connect to any media type via an external transceiver. The AUI connector consists of a 15-pin D-shell type connector, female on the computer side, male on the transceiver side. Thickwire (10BASE5) cables also use transceivers to allow connections.

For Fast Ethernet networks, a new interface called the MII (Media Independent Interface) was developed to offer a flexible way to support 100 Mbps connections. The MII is a popular way to connect 100BASE-FX links to copper-based Fast Ethernet devices.
Network Interface CardsNetwork Interface Cards:

Network interface cards, commonly referred to as NICs, and are used to connect a PC to a network. The NIC provides a physical connection between the networking cable and the computer's internal bus. Different computers have different bus architectures; PCI bus master slots are most commonly found on 486/Pentium PCs and ISA expansion slots are commonly found on 386 and older PCs. NICs come in three basic varieties: 8-bit, 16-bit, and 32-bit. The larger the number of bits that can be transferred to the NIC, the faster the NIC can transfer data to the network cable.

Many NIC adapters comply with Plug-n-Play specifications. On these systems, NICs are automatically configured without user intervention, while on non-Plug-n-Play systems, configuration is done manually through a setup program and/or DIP switches.

Cards are available to support almost all networking standards, including the latest Fast Ethernet environment. Fast Ethernet NICs are often 10/100 capable, and will automatically set to the appropriate speed. Full duplex networking is another option, where a dedicated connection to a switch allows a NIC to operate at twice the speed.

Hubs/repeaters are used to connect together two or more Ethernet segments of any media type. In larger designs, signal quality begins to deteriorate as segments exceed their maximum length. Hubs provide the signal amplification required to allow a segment to be extended a greater distance. A hub takes any incoming signal and repeats it out all ports.

Ethernet hubs are necessary in star topologies such as 10BASE-T. A multi-port twisted pair hub allows several point-to-point segments to be joined into one network. One end of the point-to-point link is attached to the hub and the other is attached to the computer. If the hub is attached to a backbone, then all computers at the end of the twisted pair segments can communicate with all the hosts on the backbone. The number and type of hubs in any one-collision domain is limited by the Ethernet rules. These repeater rules are discussed in more detail later.
Network Type Max Nodes
Per Segment Max Distance
Per Segment
2 100m
Adding Speed:

While repeaters allow LANs to extend beyond normal distance limitations, they still limit the number of nodes that can be supported. Bridges and switches, however, allow LANs to grow significantly larger by virtue of their ability to support full Ethernet segments on each port. Additionally, bridges and switches selectively filter network traffic to only those packets needed on each segment - this significantly increases throughput on each segment and on the overall network. By providing better performance and more flexibility for network topologies, bridges and switches will continue to gain popularity among network managers.

The function of a bridge is to connect separate networks together. Bridges connect different networks types (such as Ethernet and Fast Ethernet) or networks of the same type. Bridges map the Ethernet addresses of the nodes residing on each network segment and allow only necessary traffic to pass through the bridge. When a packet is received by the bridge, the bridge determines the destination and source segments. If the segments are the same, the packet is dropped ("filtered"); if the segments are different, then the packet is "forwarded" to the correct segment. Additionally, bridges do not forward bad or misaligned packets.

Bridges are also called "store-and-forward" devices because they look at the whole Ethernet packet before making filtering or forwarding decisions. Filtering packets, and regenerating forwarded packets enable bridging technology to split a network into separate collision domains. This allows for greater distances and more repeaters to be used in the total network design.
Ethernet Switches
Ethernet Switches:

Ethernet switches are an expansion of the concept in Ethernet bridging. LAN switches can link four, six, ten or more networks together, and have two basic architectures: cut-through and store-and-forward. In the past, cut-through switches were faster because they examined the packet destination address only before forwarding it on to its destination segment. A store-and-forward switch, on the other hand, accepts and analyzes the entire packet before forwarding it to its destination.

It takes more time to examine the entire packet, but it allows the switch to catch certain packet errors and keep them from propagating through the network. Both cut-through and store-and-forward switches separate a network into collision domains, allowing network design rules to be extended. Each of the segments attached to an Ethernet switch has a full 10 Mbps of bandwidth shared by fewer users, which results in better performance (as opposed to hubs that only allow bandwidth sharing from a single Ethernet). Newer switches today offer high-speed links, FDDI, Fast Ethernet or ATM. These are used to link switches together or give added bandwidth to high-traffic servers. A network composed of a number of switches linked together via uplinks is termed a "collapsed backbone" network.

Routers filter out network traffic by specific protocol rather than by packet address. Routers also divide networks logically instead of physically. An IP router can divide a network into various subnets so that only traffic destined for particular IP addresses can pass between segments. Network speed often decreases due to this type of intelligent forwarding. Such filtering takes more time than that exercised in a switch or bridge, which only looks at the Ethernet address. However, in more complex networks, overall efficiency is improved by using routers.
What is a network firewall?

A firewall is a system or group of systems that enforces an access control policy between two networks. The actual means by which this is accomplished varies widely, but in principle, the firewall can be thought of as a pair of mechanisms: one which exists to block traffic, and the other which exists to permit traffic. Some firewalls place a greater emphasis on blocking traffic, while others emphasize permitting traffic. Probably the most important thing to recognize about a firewall is that it implements an access control policy. If you don't have a good idea of what kind of access you want to allow or to deny, a firewall really won't help you. It's also important to recognize that the firewall's configuration, because it is a mechanism for enforcing policy, imposes its policy on everything behind it. Administrators for firewalls managing the connectivity for a large number of hosts therefore have a heavy responsibility.
Network Design Criteria:

Ethernets and Fast Ethernets have design rules that must be followed in order to function correctly. Maximum number of nodes, number of repeaters and maximum segment distances are defined by the electrical and mechanical design properties of each type of Ethernet and Fast Ethernet media.

A network using repeaters, for instance, functions with the timing constraints of Ethernet. Although electrical signals on the Ethernet media travel near the speed of light, it still takes a finite time for the signal to travel from one end of a large Ethernet to another. The Ethernet standard assumes it will take roughly 50 microseconds for a signal to reach its destination.

Ethernet is subject to the "5-4-3" rule of repeater placement: the network can only have five segments connected; it can only use four repeaters; and of the five segments, only three can have users attached to them; the other two must be inter-repeater links.

If the design of the network violates these repeater and placement rules, then timing guidelines will not be met and the sending station will resend that packet. This can lead to lost packets and excessive resent packets, which can slow network performance and create trouble for applications. Fast Ethernet has modified repeater rules, since the minimum packet size takes less time to transmit than regular Ethernet. The length of the network links allows for a fewer number of repeaters. In Fast Ethernet networks, there are two classes of repeaters. Class I repeaters have a latency of 0.7 microseconds or less and are limited to one repeater per network. Class II repeaters have a latency of 0.46 microseconds or less and are limited to two repeaters per network. The following are the distance (diameter) characteristics for these types of Fast Ethernet repeater combinations:
Fast Ethernet Copper Fiber
No Repeaters
One Class I Repeater
One Class II Repeater
Two Class II Repeaters 100m
205m 412m*
* Full Duplex Mode 2 km

When conditions require greater distances or an increase in the number of nodes/repeaters, then a bridge, router or switch can be used to connect multiple networks together. These devices join two or more separate networks, allowing network design criteria to be restored. Switches allow network designers to build large networks that function well. The reduction in costs of bridges and switches reduces the impact of repeater rules on network design.

Each network connected via one of these devices is referred to as a separate collision domain in the overall network.
Types of Servers:
Device Servers

A device server is defined as a specialized, network-based hardware device designed to perform a single or specialized set of server functions. It is characterized by a minimal operating architecture that requires no per seat network operating system license, and client access that is independent of any operating system or proprietary protocol. In addition the device server is a "closed box," delivering extreme ease of installation, minimal maintenance, and can be managed by the client remotely via a Web browser.

Print servers, terminal servers, remote access servers and network time servers are examples of device servers which are specialized for particular functions. Each of these types of servers has unique configuration attributes in hardware or software that help them to perform best in their particular arena.
Print Servers

Print servers allow printers to be shared by other users on the network. Supporting either parallel and/or serial interfaces, a print server accepts print jobs from any person on the network using supported protocols and manages those jobs on each appropriate printer.

Print servers generally do not contain a large amount of memory; printers simply store information in a queue. When the desired printer becomes available, they allow the host to transmit the data to the appropriate printer port on the server. The print server can then simply queue and print each job in the order in which print requests are received, regardless of protocol used or the size of the job.
Multiport Device Servers

Devices that are attached to a network through a multiport device server can be shared between terminals and hosts at both the local site and throughout the network. A single terminal may be connected to several hosts at the same time (in multiple concurrent sessions), and can switch between them. Multiport device servers are also used to network devices that have only serial outputs. A connection between serial ports on different servers is opened, allowing data to move between the two devices.

Given its natural translation ability, a multi-protocol multiport device server can perform conversions between the protocols it knows, like LAT and TCP/IP. While server bandwidth is not adequate for large file transfers, it can easily handle host-to-host inquiry/response applications, electronic mailbox checking, etc. And it is far more economical than the alternatives of acquiring expensive host software and special-purpose converters. Multiport device and print servers give their users greater flexibility in configuring and managing their networks.

Whether it is moving printers and other peripherals from one network to another, expanding the dimensions of interoperability or preparing for growth, multiport device servers can fulfill your needs, all without major rewiring.
Access Servers

While Ethernet is limited to a geographic area, remote users such as traveling sales people need access to network-based resources. Remote LAN access, or remote access, is a popular way to provide this connectivity. Access servers use telephone services to link a user or office with an office network. Dial-up remote access solutions such as ISDN or asynchronous dial introduce more flexibility. Dial-up remote access offers both the remote office and the remote user the economy and flexibility of "pay as you go" telephone services. ISDN is a special telephone service that offers three channels, two 64 Kbps "B" channels for user data and a "D" channel for setting up the connection. With ISDN, the B channels can be combined for double bandwidth or separated for different applications or users. With asynchronous remote access, regular telephone lines are combined with modems and remote access servers to allow users and networks to dial anywhere in the world and have data access. Remote access servers provide connection points for both dial-in and dial-out applications on the network to which they are attached. These hybrid devices route and filter protocols and offer other services such as modem pooling and terminal/printer services. For the remote PC user, one can connect from any available telephone jack (RJ45), including those in a hotel rooms or on most airplanes.
Network Time Servers

A network time server is a server specialized in the handling of timing information from sources such as satellites or radio broadcasts and is capable of providing this timing data to its attached network. Specialized protocols such as NTP or udp/time allow a time server to communicate to other network nodes ensuring that activities that must be coordinated according to their time of execution are synchronized correctly. GPS satellites are one source of information that can allow global installations to achieve constant timing.
IP Addressing:

An IP (Internet Protocol) address is a unique identifier for a node or host connection on an IP network. An IP address is a 32 bit binary number usually represented as 4 decimal values, each representing 8 bits, in the range 0 to 255 (known as octets) separated by decimal points. This is known as "dotted decimal" notation.


It is sometimes useful to view the values in their binary form.

140 .179 .220 .200


Every IP address consists of two parts, one identifying the network and one identifying the node. The Class of the address and the subnet mask determine which part belongs to the network address and which part belongs to the node address.
Address Classes:

There are 5 different address classes. You can determine which class any IP address is in by examining the first 4 bits of the IP address.

Class A addresses begin with 0xxx, or 1 to 126 decimal.

Class B addresses begin with 10xx, or 128 to 191 decimal.

Class C addresses begin with 110x, or 192 to 223 decimal.

Class D addresses begin with 1110, or 224 to 239 decimal.

Class E addresses begin with 1111, or 240 to 254 decimal.

Addresses beginning with 01111111, or 127 decimal, are reserved for loopback and for internal testing on a local machine. [You can test this: you should always be able to ping, which points to yourself] Class D addresses are reserved for multicasting. Class E addresses are reserved for future use. They should not be used for host addresses.

Now we can see how the Class determines, by default, which part of the IP address belongs to the network (N) and which part belongs to the node (n).

Class A -- NNNNNNNN.nnnnnnnn.nnnnnnn.nnnnnnn

Class B -- NNNNNNNN.NNNNNNNN.nnnnnnnn.nnnnnnnn


In the example, is a Class B address so by default the Network part of the address (also known as the Network Address) is defined by the first two octets (140.179.x.x) and the node part is defined by the last 2 octets (x.x.220.200).

In order to specify the network address for a given IP address, the node section is set to all "0"s. In our example, specifies the network address for When the node section is set to all "1"s, it specifies a broadcast that is sent to all hosts on the network. specifies the example broadcast address. Note that this is true regardless of the length of the node section.
Private Subnets:

There are three IP network addresses reserved for private networks. The addresses are,, and They can be used by anyone setting up internal IP networks, such as a lab or home LAN behind a NAT or proxy server or a router. It is always safe to use these because routers on the Internet will never forward packets coming from these addresses

Subnetting an IP Network can be done for a variety of reasons, including organization, use of different physical media (such as Ethernet, FDDI, WAN, etc.), preservation of address space, and security. The most common reason is to control network traffic. In an Ethernet network, all nodes on a segment see all the packets transmitted by all the other nodes on that segment. Performance can be adversely affected under heavy traffic loads, due to collisions and the resulting retransmissions. A router is used to connect IP networks to minimize the amount of traffic each segment must receive.
Subnet Masking

Applying a subnet mask to an IP address allows you to identify the network and node parts of the address. The network bits are represented by the 1s in the mask, and the node bits are represented by the 0s. Performing a bitwise logical AND operation between the IP address and the subnet mask results in the Network Address or Number.

For example, using our test IP address and the default Class B subnet mask, we get:

10001100.10110011.11110000.11001000 Class B IP Address

11111111.11111111.00000000.00000000 Default Class B Subnet Mask

10001100.10110011.00000000.00000000 Network Address
Default subnet masks:

Class A - - 11111111.00000000.00000000.00000000

Class B - - 11111111.11111111.00000000.00000000

Class C - - 11111111.11111111.11111111.00000000

CIDR -- Classless InterDomain Routing.

CIDR was invented several years ago to keep the internet from running out of IP addresses. The "classful" system of allocating IP addresses can be very wasteful; anyone who could reasonably show a need for more that 254 host addresses was given a Class B address block of 65533 host addresses. Even more wasteful were companies and organizations that were allocated Class A address blocks, which contain over 16 Million host addresses! Only a tiny percentage of the allocated Class A and Class B address space has ever been actually assigned to a host computer on the Internet.

People realized that addresses could be conserved if the class system was eliminated. By accurately allocating only the amount of address space that was actually needed, the address space crisis could be avoided for many years. This was first proposed in 1992 as a scheme called Supernetting.

The use of a CIDR notated address is the same as for a Classful address. Classful addresses can easily be written in CIDR notation (Class A = /8, Class B = /16, and Class C = /24)

It is currently almost impossible for an individual or company to be allocated their own IP address blocks. You will simply be told to get them from your ISP. The reason for this is the ever-growing size of the internet routing table. Just 5 years ago, there were less than 5000 network routes in the entire Internet. Today, there are over 90,000. Using CIDR, the biggest ISPs are allocated large chunks of address space (usually with a subnet mask of /19 or even smaller); the ISP's customers (often other, smaller ISPs) are then allocated networks from the big ISP's pool. That way, all the big ISP's customers (and their customers, and so on) are accessible via 1 network route on the Internet.

It is expected that CIDR will keep the Internet happily in IP addresses for the next few years at least. After that, IPv6, with 128 bit addresses, will be needed. Under IPv6, even sloppy address allocation would comfortably allow a billion unique IP addresses for every person on earth
Examining your network with commands:


PING is used to check for a response from another computer on the network. It can tell you a great deal of information about the status of the network and the computers you are communicating with.

Ping returns different responses depending on the computer in question. The responses are similar depending on the options used.

Ping uses IP to request a response from the host. It does not use TCP

.It takes its name from a submarine sonar search - you send a short sound burst and listen for an echo - a ping - coming back.

In an IP network, `ping' sends a short data burst - a single packet - and listens for a single packet in reply. Since this tests the most basic function of an IP network (delivery of single packet), it's easy to see how you can learn a lot from some `pings'.

To stop ping, type control-c. This terminates the program and prints out a nice summary of the number of packets transmitted, the number received, and the percentage of packets lost, plus the minimum, average, and maximum round-trip times of the packets.

Sample ping session

PING localhost ( 56 data bytes
64 bytes from icmp_seq=0 ttl=255 time=2 ms
64 bytes from icmp_seq=1 ttl=255 time=2 ms
64 bytes from icmp_seq=2 ttl=255 time=2 ms
64 bytes from icmp_seq=3 ttl=255 time=2 ms
64 bytes from icmp_seq=4 ttl=255 time=2 ms
64 bytes from icmp_seq=5 ttl=255 time=2 ms
64 bytes from icmp_seq=6 ttl=255 time=2 ms
64 bytes from icmp_seq=7 ttl=255 time=2 ms
64 bytes from icmp_seq=8 ttl=255 time=2 ms
64 bytes from icmp_seq=9 ttl=255 time=2 ms

localhost ping statistics

10 packets transmitted, 10 packets received, 0% packet loss
round-trip min/avg/max = 2/2/2 ms

The Time To Live (TTL) field can be interesting. The main purpose of this is so that a packet doesn't live forever on the network and will eventually die when it is deemed "lost." But for us, it provides additional information. We can use the TTL to determine approximately how many router hops the packet has gone through. In this case it's 255 minus N hops, where N is the TTL of the returning Echo Replies. If the TTL field varies in successive pings, it could indicate that the successive reply packets are going via different routes, which isn't a great thing.

The time field is an indication of the round-trip time to get a packet to the remote host. The reply is measured in milliseconds. In general, it's best if round-trip times are under 200 milliseconds. The time it takes a packet to reach its destination is called latency. If you see a large variance in the round-trip times (which is called "jitter"), you are going to see poor performance talking to the host

NSLOOKUP is an application that facilitates looking up hostnames on the network. It can reveal the IP address of a host or, using the IP address, return the host name.

It is very important when troubleshooting problems on a network that you can verify the components of the networking process. Nslookup allows this by revealing details within the infrastructure.

NETSTAT is used to look up the various active connections within a computer. It is helpful to understand what computers or networks you are connected to. This allows you to further investigate problems. One host may be responding well but another may be less responsive.

This is a Microsoft windows NT, 2000 command. It is very useful in determining what could be wrong with a network.

This command when used with the /all switch, reveal enormous amounts of troubleshooting information within the system.

Windows 2000 IP Configuration

Host Name . . . . . . . . . . . . : cowder
Primary DNS Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Broadcast
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . :
WAN (PPP/SLIP) Interface
Physical Address. . . . . . . . . : 00-53-45-00-00-00
DHCP Enabled. . . . . . . . . . . : No
IP Address. . . . . . . . . . . . :
Subnet Mask . . . . . . . . . . . :
Default Gateway . . . . . . . . . :
DNS Servers . . . . . . . . . . . :

Traceroute on Unix and Linux (or tracert in the Microsoft world) attempts to trace the current network path to a destination. Here is an example of a traceroute run to

$ traceroute

traceroute to amber.Berkeley.EDU (, 30 hops max, 40 byte packets

1 ( 3.135 ms 3.021 ms 3.616 ms

2 ( 1.829 ms 3.886 ms 2.772 ms

3 ( 5.327 ms 4.597 ms 5.729 ms

4 ( 4.842 ms 4.615 ms 3.425 ms

5 ( 7.488 ms 38.804 ms 7.708 ms

6 ( 6.560 ms 6.631 ms 6.565 ms

7 ( 7.638 ms 7.948 ms 8.129 ms

8 ( 9.504 ms 12.684 ms 16.648 ms

9 ( 9.762 ms 10.611 ms 10.403 ms

10 f0-0.inr-107-eva.Berkeley.EDU ( 11.478 ms 10.868 ms 9.367 ms

11 f8-0.inr-100-eva.Berkeley.EDU ( 10.738 ms 11.693 ms 12.520 ms

Saturday, June 12, 2010


Chapter 1 – Introduction to OS

What is an Operating System?

§       Operating System is a Resource Manager.
      Handles multiple computer resources: CPU, Internal/External memory, Processes, Tasks, Applications, Users, etc…
      Manages and allocates resources to multiple users or multiple jobs running at the same time (e.g., processor time, memory space, I/O devices)
      Arranges to use the computer hardware in an efficient manner (maximize throughput, minimize response time) and in a fair manner.

§       It is a Control Program.
      Manages all the components of a complex computer system in an integrated manner. 
      Controls the execution of user programs and I/O devices to prevent errors and improper use of the computer resources.
      Looks over and protects the computer.

§       It is an extended/virtual machine
-      An interface between the user and hardware that hides the details of the hardware (e.g., I/O).
-      Constructs higher-level (virtual) resources out of lower-level (physical) resources (e.g., files).
-      Definition: Is a collection of software enhancements, executed on the bare hardware, culminating in a high-level virtual machine that serves as an advanced programming environment

Why Operating System?

§       Computer hardware is developed to execute user programs and make solving user problems easier.

§       An operating system makes a computer more convenient to use.
-      It acts as an interface between user and computer hardware. Therefore, the end-users are not particularly concerned with the computer’s architecture, and they view the computer system in terms of an application.
-      To programmers, it provides some basic utilities to assist him in creating programs, the management of files, and the control of I/O devices.

Operating System Objectives

§       Convenience
-      Makes the computer more convenient to use
§       Efficiency
-      Allows computer system resources to be used in an efficient manner
§       Ability to evolve
-      Permit effective development, testing, and introduction of new system functions without interfering with service

Services Provided by Operating Systems

§       Facilities for program creation
-      Editors, compilers, linkers, debuggers, etc.

§       Program execution
-      Loading in memory, I/O and file initialization.

§       Access to I/O and files
-      Deals with the specifics of I/O and file formats.

§       System access
-      Resolves conflicts for resource contention.
-      Protection in access to resources and data.

§       Error detection and response
-      internal and external hardware errors
§       memory error
§       device failure
-      software errors
§       arithmetic overflow
§       access forbidden memory locations
-      operating system cannot grant request of application

§       Accounting
-      collect statistics
-      monitor performance
-      used to anticipate future enhancements
-      used for billing users

Computer System Components

§       A computer system can be divided in to four components.

-      The Hardware: Provides basic computing resources (CPU, memory, I/O devices).

-      The Operating System: Controls and coordinates the use of the hardware among the various application programs for the various users.

-      The Application Programs: Define the ways in which the system resources are used to solve the computing problems of the users (compilers, database systems, video games, business programs).

-      The Users: Users (people, machines, other computers).

§       These components can be viewed as layers, where each layer uses the services provided by the layer beneath it.


A Static View of System Components

Dynamic View of System Components
Another view of computer system components

History of Operating Systems

§       Let’s see how operating systems evolve over time.
§       This will help us to identify some common features of operating systems and how and why these systems have been developed as they are.

Evolution of Operating Systems

§       Early Systems (1950)
§       Simple Batch Systems (1960)
§       Multiprogrammed Batch Systems (1970)
§       Time-Sharing and Real-Time Systems (1970)
§       Personal/Desktop Systems (1980)
§       Multiprocessor Systems (1980)
§       Networked/Distributed Systems (1980)
§       Handheld Systems (1990)

Early Systems

§       Structure
-      Single user system.
-      Large machines run from console.
-      Programmer/User as operator.
-      Paper Tape or Punched cards.
-      No tapes/disks in computer.

§       Early software: Assemblers, Libraries of common subroutines, Device Drivers, Compilers, Linkers.
§       Significant amount of setup time.
§       Low CPU utilization.
§       But very secure.

Simple Batch Systems

§       Mainframe machines. Input devices were card readers. Output devices were line printer, tape drives, and card punch.
§       A job (a single program+ associated data + control information) usually on the punch cards submitted to the operator.
§       The output consisted of the results of the program or memory dump in case of error.
§       The operator used to batch together similar programs and run as a group to reduce setup time.
§       No user interaction while the job is executing.
§       Current examples include .bat files under Dos – Windows and shell files under Unix/Linux.
Example of card deck of a job

§       The operating systems (called resident monitor) manages the execution of each program in the batch.
-      Monitor utilities are loaded when needed.
-      Resident monitor is always in main memory and available for execution.
-      The resident monitor usually has the following part.
§       Control card interpreter – responsible for reading and carrying out instructions on the cards.
§       Loader – loads systems programs and applications programs into memory.
§       Device drivers – know special characteristics and properties for each of the system’s I/O devices.

§       In batch systems:
-      Initial control is in monitor.
-      Load next program and transfer control to it.
-      When a job completes, the control transfers back to monitor.
-      Automatically transfer control from one job to another (Automatic job sequencing).


§       Slow Performance – I/O and CPU could not overlap; card reader very slow.
§       CPU was often idle.


1.    Off-line Operation
-      Speed up computation by loading jobs into memory from tapes while card reading and line printing is done off-line using smaller machines.

2.    Use spooling (Simultaneous Peripheral Operation On Line).
-      Cards are read directly from the card reader onto a disk and location of card images are kept in a table by the operating system.
-      The output is sent to the disk and when the job is completed then the output was actually printed.
-      I/O and computations were overlapped. While executing one job, the OS:

-      Reads next job from card reader into a storage area on the disk (job queue).
-        Outputs printout of previous job from disk to printer.

Uniprogramming Until Now

§       I/O operations are exceedingly slow (compared to instruction execution).
§       A program containing even a very small number of I/O operations will spend most of its time waiting for them.
§       Hence: poor CPU usage when only one program is present in memory.

Memory Layout of Uniprogramming

Memory layout of a simple batch processing system


Multiprogrammed Batch Systems

§       Several jobs are kept in main memory at the same time, and the CPU is multiplexed among them.

§       If memory can hold several programs, then CPU can switch to another one whenever a program is waiting for an I/O to complete – This is multiprogramming.


OS Features Needed for Multiprogramming

§       I/O routine supplied by the system.
§       Memory management – the system must allocate the memory to several jobs.
§       CPU scheduling – the system must choose among several jobs ready to run.
§       Allocation of devices.

Time Sharing Systems (Interactive Systems)

§       TSS extends Batch multiprogramming to handle multiple interactive jobs – It’s Interactive Multiprogramming.
§       Multiple users simultaneously access the system through terminals.
§       Processor’s time is shared among multiple users, that is, the CPU is multiplexed among several jobs that are kept in memory and on disk (the CPU is allocated to a job only if the job is in memory).
§       On-line communication between the user and the system is provided; when the operating system finishes the execution of one command, it seeks the next “control statement” from the user’s keyboard.
§       TS system provides each user with her/her own virtual machine.



§       TS eventually supports multitasking.
§       A time share system that supports multiple processes (program in execution) per user is called a multitasking system.

Why Does Time Sharing Work?

§       Because of slow human reaction time, a typical user needs 2 seconds of processing time per minute.
§       Then many users should be able to share the same system without noticeable delay in the computer reaction time.

Batch Multiprogramming

Vs. Time Sharing

Batch Multi-Prog.
Time Sharing
Principle obj.
Max. Processor use
Min. response time
Source of inst. To OS
JCL provided with the job
Commands entered at the terminal

OS Features Needed for Time Sharing Systems

§       On-line file system must be available for users to access data and code.
§       Should do memory management
§       Should do CPU scheduling
§       Should do job synchronization and have communication facilities.
§       Should ensure that dead lock and indefinite waiting does not occur.
§       Should allow sharing of computer resources.

Personal Computer Systems

§       Personal computers – computer system dedicated to a single user.
§       Have a wide variety of I/O devices – keyboards, mice, display screens, small printers.
§       User convenience and responsiveness are of prime importance.
§       Can adopt technology developed for larger operating system.
§       Often individuals have sole use of computer and do not need advanced CPU utilization of protection features.
§       May run several different types of operating systems (Windows, MacOS, UNIX, Linux)
Two Categories of Computer Systems

§       Single Instruction Single Data (SISD)
-      Single processor executes a single instruction sequence to operate on data stored in a single memory.
-      This is a Uniprocessor.

§       Multiple Instruction Multiple Data (MIMD)
-      A set of processors simultaneously execute different instruction sequences on different data sets.
-      This is a Multiprocessor.

Multiprocessor Systems

§       Multiprocessor systems have more than one CPU in close communication.
-      Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory.
§       Advantages of parallel system:
-      Increased throughput
-      Economical
-      Increased reliability
§       Graceful degradation

Multiprocessor architecture

Symmetric Multiprocessing (SMP)

§       Each processor runs an identical copy of the operating system.
§       Each processor can perform the same functions and share same main memory and I/O facilities (symmetric).
§       The OS schedules processes/threads across all the processors (real parallelism).
§       Existence of multiple processors is transparent to the user.
§       Incremental growth: just add another CPU!
§       Robustness: a single CPU failure does not halt the system, only the performance is reduced.
§       Many processes can run at once without performance deterioration.
§       Most modern operating systems support SMP

Asymmetric multiprocessing

§       Each processor is assigned a specific task; master processor schedules and allocated work to slave processors.
§       More common in extremely large systems

Distributed Systems

§       Distribute the computation among several physically separated processors.
-      Loosely coupled system – each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines.
§       Advantages of distributed systems.
-      Resources Sharing
-      Computation speed up – load sharing
-      Reliability and fault tolerance
-      Communications
§       Requires networking infrastructure - Local area networks (LAN) or Wide area networks (WAN)
§       May be either client-server or peer-to-peer systems.


General structure of client-server

Peer-to-peer systems

Network Operating System

§       Provides file sharing
§       Provides communication scheme
§       Runs independently from other computers on the network

Distributed Operating System

§       Less autonomy between computers
§       Gives the impression there is a single operating system controlling the network.

Clustered Systems

§       Clustering allows two or more systems to share external storage and balance CPU load.
§       Asymmetric clustering: one server runs the application while other servers standby.
§       Symmetric clustering: all N hosts are running the application.

Real-Time Systems

§       Note that not all Operating Systems are general-purpose systems.
§       Real-Time (RT) systems are dedicated systems that need to adhere to deadlines, i.e., time constraints.
§       Correctness of the computation depends not only on the logical result but also on the time at which the results are produced.
§       Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems.
§       Real-Time systems may be either hard or soft real-time.

Hard Real-Time System

§       Must meet its deadline.
§       Conflicts with time-sharing systems, not supported by general-purpose operating systems.
§       Often used as a control device in a dedicated application such as industrial control and robotics
§       Secondary storage limited or absent, data stored in short term memory, or read-only memory (ROM).

Soft Real-Time System

§       A critical real-time task gets priority over the other tasks (Deadline desirable but not mandatory).
§       Limited utility in industrial control of robotics
§       Useful in applications (multimedia, virtual reality) requiring advanced operating-system features.

Hand Held Systems

§       Personal Digital Assistants (PDAs)
§       Cellular telephones
§       Issues:
-      Limited memory
-      Slow processors
-      Small display screens.

Migration of Operating-System Concepts and Features