Multitech RF500S Manuale Utente Scaricare il pdf (Pagina 7)

2 Internet Access – Your Friendly Internet Service Provider

The reason most often cited to purchase a PC is for Internet access. The PC has progressed from a hobbyist

plaything to an important item of telecommunication equipment. The most common access method for

residential customers are: dial-up over a regular phone line, Digital Subscriber Line (DSL) a high-speed

service using existing telephone wiring, and Cable Modem over Cable TV distribution facilities.

The ISP provides the following services:

Connection between the end user and ISP network – so called last mile

Routing between customers and one or more Internet backbone(s)

User authentication

User IP address assignment

DNS name resolution – translate host name to IP address

E-mail account(s)

USENET Newsgroup

Web hosting

Billing

Technical Support

The service provider is the bridge between the retail customer, and the interexchange carriers that operate

the Internet backbone.

2.1 Dialup

Dialup access is available to anyone with an analog telephone line. Modems can also be used with cellular

phones, however data rates are significantly lower then wired phones so it is not commonly used at fixed

locations.

Most ISPs support the ITU V.90 modem standard. The International Telecommunications Union V.90

standard replaced previous generation of proprietary 56Flex and X2 modems. ISPs typically connect

directly to phone company digital trunks. This means only a single analog to digital conversion exists, at

the subscriber's connection in the Telco central office. The ISP’s modem is synchronized to the digital

trunk. This enables the ISP to transmit at up to 56kbps. Current FCC power regulations restrict maximum

speed to 54kbps. Transmission from the subscriber to the ISP is limited to 33.6kbps because the subscriber

does not have access to digital carrier. The ITU recently released the V.92 standard. It increased upload

speed slightly to 44Kbps and implements faster auto negotiation to reduce connection setup time. V.92 also

supports improved data compression. Check with your ISP to see if and when they plan to roll out V.92.

At connect time the modems probe the line to determine noise and attenuation levels. This sets the initial

connection speed. During the course of the connection the modems constantly adjust to varying line

conditions. After the modems synchronize the user is authenticated and an IP address issued. As soon as

the computer has an IP address it is able to access the Internet.

2.2 DSL

Digital Subscriber Line (DSL) technology uses the existing twisted pair telephone copper wiring between

the subscriber and the phone company central office to carry high-speed data. This allows the local

exchange carriers to generate additional revenue by leveraging their massive investment. Several types of

DSL have been developed hence the xDSL moniker. The most common types of DSL are Asymmetric DSL

(ADSL) and Symmetric DSL (SDSL).

ADSL offers higher download speed, toward the subscriber, and then upload. It has the advantage that it

coexists with POTS voice service. This reduces cost by allowing a single copper pair to be used for both

voice and data service. A residence with a single phone line can be equipped with both a standard analog

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Multitech RF500S Manuale Utente Pagina 7

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