Specifications

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2 Types of Internet Access – Your Friendly ISP
The PC has progressed from hobbyist plaything to an indispensable telecommunication device. Almost all
PCs are purchased with the intent to connect to the Internet. The most common access methods 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 using the Cable TV distribution facilities. In
each case the ISP is the link between the end user and the Internet backbone.
Typical ISP services:
Connection between the end user and ISP network – so called first mile
Routing between customer and one or more Internet interexchange carrier
User authentication
IP address assignment
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st
level DNS name resolution – translate host name to IP address
E-mail account
USENET account
Web hosting
Billing
Technical Support
2.1 Dialup
Dialup access is available to anyone with telephone service. Modems are normally used with wired phone
service but can also be used with wireless cellular phones. However the data rate is significantly lower over
cellular and cellular service typically change for connection time so it is not optimal for fixed location use.
Almost all ISPs support the ITU V.90 modem standard. The International Telecommunications Union V.90
standard replaced previous generation of proprietary 56Flex and X2 modems. V.90 takes advantage that the
ISP modem pool is directly connected to phone company digital trunk. This means only one analog to
digital conversion exists between the user and the ISP. The ISP modem is synchronized to the digital trunk.
This enables it to transmit at up to 56kbps toward the user. 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 V.92 a minor enhancement to V.90. It increased upload speed slightly to 44Kbps
and implements faster auto negotiation to reduce connection setup time. The standard also includes V.44
improved data compression. V.92 includes a mechanism to ignore call-waiting tones to allow the user to
answer a second call without dropping the modem connection. V.92 modems are available but ISP’s have
not been rushing to upgrade.
At connect time the modem probes the line to determine noise and attenuation levels. This sets the initial
connect speed, during the session the modem 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. The most common protocol between the end user and ISP is the
Point-to-Point Protocol (PPP) defined by the IETF, the same folks that write the Internet specifications.
This allows the Internet protocol to be carried over the serial telephone link between the user and ISP.
Phone lines are digitized at the Telco central office at 64kbps. This means analog modem has reached its
theoretical maximum speed. To obtain higher speed requires use of different technology.
2.2 DSL
Digital Subscriber Line (DSL) technology utilizes the existing telephone copper wiring between the
subscriber and the phone company central office (CO) to carry high-speed data. This allows the local
exchange carrier to generate additional revenue by leveraging the massive investment in cabling. Modems