Criterios para promover el desarrollo de las empresas del sector madera en Cali

The hardware for a shared Internet connection, at minimum, consists of a router and a WAN connection. The various types of WAN technologies that Internet connections use are summarized in Table 3-1.

Table 3-1 Internet Access WAN Technologies

Transmission

Connection Type Speed Applications

Dial-up modem Up to 53 Kbps

Integrated Services Up to 128 Kbps Digital Network

(ISDN) Basic Rate Interface (BRI)

Integrated Services Up To 1.544 Mbps Digital Network

(ISDN) Primary Rate Interface (PRI)

Cable television networks (CATV)

Asymmetrical Digi­ tal Subscriber Line (ADSL) Up to 512 Kbps downstream; up to 128 Kbps upstream Up to 640 Kbps downstream; up to 160 Kbps upstream

■ E-mail for up to 10 simultaneous users

■ Web browsing for 2 to 3 simultaneous users

■ Large FTP downloads for 1 or 2 simul­ taneous users

■ E-mail for up to 20 simultaneous users

■ Web browsing for 6 to 8 simultaneous users

■ Large FTP downloads for 3 or 4 simul­ taneous users

■ E-mail for 120 or more simultaneous users

■ Web browsing for 75 to 100 simulta­ neous users

■ Large FTP downloads for 40 to 50 simultaneous users

■ E-mail for 50 or more simultaneous users

■ Web browsing for 25 to 30 simulta­ neous users

■ Large FTP downloads for 12 to 15 simultaneous users

■ E-mail for 60 or more simultaneous users

■ Web browsing for 30 to 35 simulta­ neous users

■ Large FTP downloads for 15 to 18 simultaneous users

Table 3-1 Internet Access WAN Technologies

Transmission

Connection Type Speed Applications

T-1 1.544 Mbps ■ E-mail for 120 or more simultaneous users

■ Web browsing for 75 to 100 simulta- neous users

■ Large FTP downloads for 40 to 50 simultaneous users

Fractional T-1 Up to 1.544 Mbps, in ■ Variable, depending on bandwidth 64 Kbps increments

T-3 44.736 Mbps ■ E-mail for 3,000 or more simultaneous users

■ Web browsing for 2,000 to 3,000 simul- taneous users

■ Large FTP downloads for 1,000 to 1,500 simultaneous users

Frame relay Variable ■ Variable, depending on bandwidth

Dial-Up Modem Connections

It is possible to share dial-up modem connections with a network. However, a single dial- up modem connection reaching a maximum of 53 kilobits per second (Kbps) can satisfy the Internet bandwidth requirements of only a handful of users and only as long as they don’t run high-bandwidth applications and don’t expect stellar performance. If your net- work has users who need only occasional access to the Internet, you can install one or more modems and share them using Internet Connection Sharing (ICS) on a Windows computer or an inexpensive stand-alone router that provides NAT services.

ISDN

Integrated Services Digital Network (ISDN) is a digital dial-up service that uses the stan­ dard cabling provided by the telephone company plus special equipment at your net- work site. There are two ISD services; Basic Rate Interface (BRI, also known as 2B+D) provides up to 128 Kbps of bandwidth, and Primary Rate Interface (PRI) up to 1.544 Mbps, which is the same speed as a T-1. PRI is common overseas but is rarely seen in the U.S. BRI service is available from many telephone companies, but it doesn’t pro- vide much more bandwidth than a standard dial-up modem connection. You can ser­ vice a small network with a BRI connection or use it to provide limited Internet access to a larger network. For example, if you want to provide the users of a medium-sized network with Internet e-mail capabilities only, ISDN might be an effective solution. The main advantage of ISDN is that it is a dial-up service, like a standard telephone connection. To access the Internet and establish a connection, you use your equipment to dial the ISDN number of your ISP. You can disconnect from the ISP’s network when

you don’t need Internet access, such as after business hours, and if you want to change ISPs, all you have to do is dial a different number.

This ability to disconnect is what makes ISDN an economically viable Internet access solution. In most cases, you must pay a per-minute charge to the telephone company providing the service (in addition to a monthly fee, plus whatever your ISP charges for the connection), so remaining connected around the clock can be expensive. In some cases, using dial-on-demand technology to connect to the ISP only when your users need Internet access can keep the cost lower than a dedicated connection using another technology can.

The disadvantages of ISDN are its relatively high cost per kilobit of bandwidth, and the fact that the end-user equipment must be located within a specified distance (usually 18,000 feet) of the telephone company’s nearest point of presence (POP).

CATV and DSL

CATV networks and Digital Subscriber Line (DSL) are Internet access solutions targeted primarily at home users, but they can be effective for a business network as well. Many CATV providers take advantage of their extensive private fiber optic networks by using them to provide Internet access as well as television signals. The Internet bandwidth that CATV networks provide can vary, depending on the number of subscribers in the local area, but is often as high as 512 Kbps. Cable television companies operate what is essen­ tially a metropolitan area network (MAN), in which neighboring subscribers share a net- work medium and the bandwidth it provides. This means that your available bandwidth can be reduced when other subscribers in your area are transferring a lot of data. DSL is a telephone company service that provides high-speed, digital transmissions using standard telephone cables. A DSL connection is a dedicated link between two sites that provides a predetermined amount of bandwidth at all times. There are many different types of DSL connections, all of which have slightly different names, such as ADSL and High-bit-rate Digital Subscriber Line (HDSL). This is why the technology is sometimes called by the generic name xDSL. ADSL is the most popular DSL variant for Internet access and can provide transmission rates of up to 8 Mbps, although transmis­ sion rates between 256 and 512 Kbps are more common. Telephone companies and other large data carriers frequently use HDSL over fiber optic cables.

Both CATV and ADSL Internet connections suffer from what some network designers would consider a fatal flaw: they are asymmetrical. An asymmetrical connection is one that runs at different speeds in each direction. In these cases, the downstream trans- mission rate (that is, from the service provider to the subscriber) is much higher than the upstream rate. CATV networks are designed to carry signals from the provider to the subscriber; there is very little upstream bandwidth available, and the provider typ­ ically sets a maximum upstream transmission rate ranging from 56 to 128 Kbps. Asym­ metrical DSL connections have an upstream transmission rate that is fixed at a lower speed than the downstream rate.

For network access to the Internet, an asymmetrical connection might not be a major drawback, as long as the majority of network users are running Internet client applica­ tions, such as Web browsers. In normal use, client applications send only brief requests upstream and receive larger amounts of data from the downstream connection. Web servers and other Internet server applications, however, frequently send large amounts of traffic upstream, which is why consumer CATV and DSL connections are often unsuitable for shared network Internet connections. There are DSL technologies that are symmetrical, however. You must check with your local telephone carrier to find out what is available in your area. As with ISDN, DSL service is limited in its maximum dis­ tance from the nearest POP, so DSL might not be an alternative for you.

Internet access through a CATV network is generally inexpensive (about $40 per month, in most cases) and is a viable alternative for a small-to-medium-sized network that does not require a huge amount of bandwidth. However, because CATV providers usually target their Internet service to home users, you should check your local pro­ vider’s policy about using the connection for network access. DSL is far more flexible than CATV. Some DSL providers have packages intended for business use that provide sufficient bandwidth for larger networks.

Leased Lines

A leased line (sometimes called a dedicated line) is a permanent, usually digital tele­ phone connection between two points that provides a fixed amount of bandwidth at all times. The most common type of leased line used for connecting a medium or large network to the Internet is the T-1, running at 1.544 Mbps. A T-1 provides a great deal of bandwidth between your network and that of your ISP, in both directions, 24 hours a day. Although you can use a T-1 as a single data pipe, the connection actually con­ sists of 24 separate channels running at 64 Kbps each. Most service providers enable you to purchase one or more of these separate channels. This is called fractional T-1 service, and it enables you to purchase just about any amount of bandwidth you need. The next step up from a T-1 connection is a T-3, which is the equivalent of 672 chan­ nels running at 64 Kbps each, or 28 T-1s, for a total of 44.736 Mbps. The Internet uses T-3 connections for backbones, and ISPs often use them to obtain their own Internet access, but a T-3 is usually far more bandwidth than a single network installation needs and is also too expensive.

To install a leased line, you must make arrangements with a telephone carrier for the installation and then buy or lease a piece of equipment called a Channel ServiceUnit/ Data Service Unit (CSU/DSU) (see Figure 3-1). You must also arrange with an ISP for the installation of the connection’s other end at their site and negotiate a contract for Internet access at the line’s rate of speed. The price of a leased line is typically a monthly fee based on its transmission rate and on the distance between the two connected sites. Using a local ISP can be a major financial benefit for this reason. Some

telephone companies can provide the Internet access as well as the leased line, eliminating the need to involve a third party in the negotiations. These providers often have package deals available that provide an array of Internet access services at an attractive price. Leased line Workstation Hub Workstation Workstation Router CSU/DSU Subscriber Workstation Hub Workstation Workstation Router CSU/DSU Service provider

Figure 3-1 A leased line connection

When creating an Internet access plan, you must understand that leased lines represent a different world from most of the low-end Internet access solutions described earlier in this lesson. Installing a leased line for Internet access represents a significant invest­ ment in time and money. In addition to the monthly fees for the leased line itself and for the ISP’s services, the cost of the installation and the CSU/DSU at both ends is sub­ stantial, and once the line is in place, it is permanent. You cannot change ISPs without having another line installed to the new destination. Before actually installing a leased line, you should discuss your needs with several telephone carriers and ISPs to get a feel for their services and obtain competitive bids.

As an Internet access solution, leased lines are best suited to networks that require large amounts of bandwidth around the clock. When you install a leased line, you are paying for a given amount of bandwidth 24 hours a day. An organization that completely shuts down and requires no Internet access for a portion of the day is wasting a lot of money. However, for some organizations, the money lost in this way more than offsets the cost of obtaining the bandwidth they need during business hours by some other means.

Frame Relay

Frame relay is a WAN technology designed to provide a variable amount of bandwidth and enable you to pay only for the bandwidth you use. To set up a frame relay WAN con-

nection between two points, you install a leased line at each site, running to a service provider. The service provider maintains a frame relay network called a cloud, which provides the connection between the two leased lines, as shown in Figure 3-2. You con- tract with the service provider for a certain amount of bandwidth through the cloud, called a committed information rate (CIR), which is guaranteed at all times. When your bandwidth needs drop below the CIR, you pay a rate based on the bandwidth you use. Your bandwidth can also exceed the CIR during periods of high traffic, called burst peri­ ods, for an additional fee specified in the contract. The service provider is able to do this because it can reclaim any bandwidth you are not using and sell it to other customers.

Leased line Leased line Workstation Hub Workstation Workstation Router CSU/DSU Subscriber Workstation Hub Workstation Workstation Router CSU/DSU Service provider

Figure 3-2 A frame relay connection

To use frame relay for Internet access, you must locate an ISP that provides this service, and you still must install a leased line between your site and that of the ISP. The band- width of the leased line you install should be slightly more than you actually need for Internet access. You then contract for a CIR that is somewhat lower than the bandwidth of the leased line. For example, for a company that shuts down after business hours, you might design an Internet access strategy consisting of a T-1 line connecting the net- work site to an ISP, with a frame relay contract containing a CIR of 1 Mbps. When the company is closed, it won’t pay any ISP fees, and when it’s opened, it has a guaranteed 1 Mbps of bandwidth, plus the ability to exceed the CIR during high-traffic periods. Frame relay in no way diminishes the cost of installing and maintaining the leased line connecting the network to the ISP, but it can reduce the network’s ISP costs by 20 to 40 percent in cases in which the company does not make full use of the T-1’s band- width at all times.