27-12-2012, 04:54 PM
Modems: An Introduction
Modems.ppt (Size: 1,016.5 KB / Downloads: 34)
INTRODUCTION
The middlemen between the computer and the telephone system
Convert analog data suitable for transmission over a phone line to digital data suitable for a computer (modulator/demodulator)
Can be either an external device or an internal device that is installed in an expansion slot inside a computer
A phone line connects to a modem or telephone using either an RJ-11 or older RJ-12 connector
Modem Types
Telephone modem
Optical Modem (Optical fiber)
Cable modem
DSL Modem
etc
What is a Cable Modem &how does it work?
A Cable Modem is a digital modem that uses a coaxial cable connection for the data transmission.
This data connection is received by a cable modem that decodes the signal into your PC.
Who Makes Cable Modems?
Bay Networks -- LANcity
Motorola
Zenith
General Instruments
Hewlett-Packard
Scientific Atlanta
Phasecom
A Typical Cable Modem ...
…Connects 1 single host computer
Though some can connect dozens of hosts
Has its own IP address
Though some share an IP address with the single connected host
Operates at speeds from 1 megabit / second to 10 megabits / second
Though some claim up to 30 megabits / sec
Is bidirectional
Though some use “telco return”
How Fast is a Cable Modem?
Variable Bit Rate: Bandwidth on Demand
Asymmetric Configuration
Downstream Transmission
Frequency Range: 50-860 MHz
Transmission Speed: up to 36 Mbps (3~10 Mbps realistic)
Upstream Transmission
Frequency Range: 5-42 MHz
Transmission Speed: up to 10 Mbps (200 Kbps ~ 2 Mbps realistic)
How secure is a Cable Modem?
Cable connections are not 100% secure in any instance like many other connections on the Internet. Even though most cable providers block ports 137-139, cable modems are likely to be generated in any case where a user has file and print sharing turned on, or possibly other services like SMTP (Simple mail transfer protocol), Web Servers and Telnet services. A general rule is to keep passwords long and turn off any service that you don't absolutely need running. A firewall type application should be used to keep a network as secure as possible.
Cable Modems vs. ADSL
There is one major advantage that ADSL has over cable modems. Cable modems use a shared networking technology where all the cable modems share a single pipe to the Internet. This pipe speed will fluctuate depending on the number of subscribers on the network.
When ADSL is used, the pipe to the Internet is solely "yours", and is not shared along the way to a central office. This allows for a more consistent speed, and this speed does not typically fluctuate like cable modem networks.
Telephone Modems
Telephone modems employ all three of the modulation techniques:
FSK
PSK
QAM
Because of the limitations of voice-grade telephone lines, these modems are restricted to a bandwidth of about 3 kHz
The trend in modem design has been towards more sophisticated modulation schemes to achieve the maximum bit rate with available bandwidth
PSK Modems
When faster data rates are needed than available with PSK, phase modulation is often used
Most DPSK (delta phase-shift keying) systems use a four-phase system called quadrature phase-shift keying (QPSK)
The bit rate is twice the baud rate and is referred to as a dibit system
The Bell 212A modem is an example of this type of modulation, capable of data rates up to 1200 bits per second
56K Modems
The V.90 standards allow data transmissions up to a theoretical limit of 56 kb/s, but because of FCC requirements, the maximum allowable is 54 kb/s
V.90 modems appear to exceed the Shannon Limit
However, the higher rate is available only in the downstream direction and a maximum of 33.6 kb/s is available in the upstream direction
Upper limits vary greatly according to line noise, distance from a telephone substation, and availability of digital connections throughout the phone system