Dialup services offer cost-effective methods for connectivity across WANs. Two popular dialup implementations are dial-on-demand routing (DDR) and dial backup.
DDR is a technique whereby a router can dynamically initiate a call on a switched circuit when it needs to send data. In a DDR setup, the router is configured to initiate the call when certain criteria are met, such as a particular type of network traffic needing to be transmitted. When the connection is made, traffic passes over the line. The router configuration specifies an idle timer that tells the router to drop the connection when the circuit has remained idle for a certain period.
Dial backup is another way of configuring DDR. However, in dial backup, the switched circuit is used to provide backup service for another type of circuit, such as point-to-point or packet switching. The router is configured so that when a failure is detected on the primary circuit, the dial backup line is initiated. The dial backup line then supports the WAN connection until the primary circuit is restored. When this occurs, the dial backup connection is terminated.
Tuesday, December 2, 2008
WAN Virtual Circuits
A virtual circuit is a logical circuit created within a shared network between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).
SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the virtual circuit between the source and destination devices. Data transfer involves transmitting data between the devices over the virtual circuit, and the circuit termination phase involves tearing down the virtual circuit between the source and destination devices. SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but they decrease the cost associated with constant virtual circuit availability.
PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but they increase costs due to constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.
SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the virtual circuit between the source and destination devices. Data transfer involves transmitting data between the devices over the virtual circuit, and the circuit termination phase involves tearing down the virtual circuit between the source and destination devices. SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but they decrease the cost associated with constant virtual circuit availability.
PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but they increase costs due to constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.
Packet Switching
Packet switching is a WAN technology in which users share common carrier resources. Because this allows the carrier to make more efficient use of its infrastructure, the cost to the customer is generally much better than with point-to-point lines. In a packet switching setup, networks have connections into the carrier's network, and many customers share the carrier's network. The carrier can then create virtual circuits between customers' sites by which packets of data are delivered from one to the other through the network. The section of the carrier's network that is shared is often referred to as a cloud.
Some examples of packet-switching networks include Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Services (SMDS), and X.25. Figure
3-4 shows an example packet-switched circuit.
The virtual connections between customer sites are often referred to as a virtual circuit.
Figure 3-4 Packet Switching Transfers Packets Across a Carrier Network
Some examples of packet-switching networks include Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Services (SMDS), and X.25. Figure
3-4 shows an example packet-switched circuit.
The virtual connections between customer sites are often referred to as a virtual circuit.
Figure 3-4 Packet Switching Transfers Packets Across a Carrier Network
Circuit Switching
Switched circuits allow data connections that can be initiated when needed and terminated when communication is complete. This works much like a normal telephone line works for voice communication. Integrated Services Digital Network (ISDN) is a good example of circuit switching. When a router has data for a remote site, the switched circuit is initiated with the circuit number of the remote network. In the case of ISDN circuits, the device actually places a call to the telephone number of the remote ISDN circuit. When the
two networks are connected and authenticated, they can transfer data. When the data transmission is complete, the call can be terminated. Figure 3-3 illustrates an example of this type of circuit.
Figure 3-3 A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone
two networks are connected and authenticated, they can transfer data. When the data transmission is complete, the call can be terminated. Figure 3-3 illustrates an example of this type of circuit.
Figure 3-3 A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone
Point-to-Point Links
A point-to-point link provides a single, pre-established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network. Point-to-point lines are usually leased from a carrier and thus are often called leased lines. For a point-to-point line, the carrier allocates pairs of wire and facility hardware to your line only. These circuits are generally priced based on bandwidth required and distance between the two connected points. Point-to-point links are generally more expensive than shared services such as Frame Relay. Figure 3-2 illustrates a typical point-to-point link through a WAN.
Figure 3-2 A Typical Point-to-Point Link Operates Through a WAN to a Remote Network
Figure 3-2 A Typical Point-to-Point Link Operates Through a WAN to a Remote Network
What Is a WAN?
A WAN is a data communications network that covers a relatively broad geographic area and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer. Figure 3-1 illustrates the relationship between the common WAN technologies and the OSI model.
Figure 3-1 WAN Technologies Operate at the Lowest Levels of the OSI Model
Tuesday, October 28, 2008
Why We Have Computers
Because computers can display and deal with text, they offer “communication with human beings—communication that far surpasses the numeric display of the common desktop calculator. This communication is achieved through input/output, or I/O. I/O provides the basis for all the work the computer does.
Computers work by dealing with data. You put data into the computer, and it spits something useful back out, for example:
Word Processing: You input text and manipulate it by using word processing software. The computerr can save the text on disk or print the text to create a document.
Databases: You input the raw data or design a database into which others input data. The computer manipulates the data and either displays or prints the results.
Spreadsheets: You enter the calculations, the values, and the way the values relate. The computer makes the calculations and evaluations, displaying or printing the results for you.
Presentations
How Windows XP Works
You can open given computer application programs with Windows by simply finding the program you want and clicking it in the Programs menu of your ttaskbar, or by double-clicking a shortcut for the program on your desktop. Windows acts as an operating system or what many call system software. It can only allow you to run other programs. Within Windows, programs come attached as a package. For example, you have the Windows Explorer, which allows you to delete, copy, move, or rename files. Windows is set up in what is called the GUI environment, which means the Graphical User Interface environment, making it an interfacce that is much easier to use that the old DOS systems of yesteryear.
The Hardware-to-Software Interface
Windows XP interfaces the computer’s hardware with all the software you use. It’s the go-between, the manager, the traffic cop. To do the job, Windows “talks” with the computer’s hardware on the most basic, simple level. Then it provides easy methods of access to other software applications that want to use the computer, especially the disk drives.
Other software can use the computer’s hardware directly as well; Windows has no monopoly. Bust most of the time other programs use Windows to put files on disk, read files, and deal with the disk drives in general.
Computers work by dealing with data. You put data into the computer, and it spits something useful back out, for example:
Word Processing: You input text and manipulate it by using word processing software. The computerr can save the text on disk or print the text to create a document.
Databases: You input the raw data or design a database into which others input data. The computer manipulates the data and either displays or prints the results.
Spreadsheets: You enter the calculations, the values, and the way the values relate. The computer makes the calculations and evaluations, displaying or printing the results for you.
Presentations
How Windows XP Works
You can open given computer application programs with Windows by simply finding the program you want and clicking it in the Programs menu of your ttaskbar, or by double-clicking a shortcut for the program on your desktop. Windows acts as an operating system or what many call system software. It can only allow you to run other programs. Within Windows, programs come attached as a package. For example, you have the Windows Explorer, which allows you to delete, copy, move, or rename files. Windows is set up in what is called the GUI environment, which means the Graphical User Interface environment, making it an interfacce that is much easier to use that the old DOS systems of yesteryear.
The Hardware-to-Software Interface
Windows XP interfaces the computer’s hardware with all the software you use. It’s the go-between, the manager, the traffic cop. To do the job, Windows “talks” with the computer’s hardware on the most basic, simple level. Then it provides easy methods of access to other software applications that want to use the computer, especially the disk drives.
Other software can use the computer’s hardware directly as well; Windows has no monopoly. Bust most of the time other programs use Windows to put files on disk, read files, and deal with the disk drives in general.
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