M O D E M S: M O D E M S
Presented by Pilar Gomez
June 2, 2004 Modem: Modem enables computers to communicate over standard telephone lines
modulation and demodulation
full duplex mode = simultaneous transmission History: History Evolution: Evolution A dumb terminal at an off-site office or store could "dial in" to a large, central computer.
The 1960s were the age of time-shared computers, and the 300-bit-per-second (bps) modems. Evolution: Evolution Late 1970s, the advent of the PCs:
modem use to dial into a remote mainframe computer
files could be transferred and one PC could connect to another via modems.
The 1980s, the rise of BBS
people would dial in to download free software, participate in discussions on various topics, play on-line games, etc
By mid 1990s, the advance of Internet
modems became fast, cheap and widely used
Today: Today Faster non-analog “modems”.
Users connect to the Internet through a:
local-area network (LAN) connection
cable modem, or
digital subscriber line (DSL) connection
Cable Modem: Cable Modem Cable company offers Internet access over the cable
Speed at more than 1 mbps (1 million bits per second), or about 20 times faster that 56k modems
Downstream data into a 6-MHz channel looks just like a TV channel
Cable modem (customer)
(cable provider) Cable Modem: Cable Modem A CMTS enable as many as 1,000 users to connect to the Internet
Performance does not depend on distance from the central cable office
Single channel aspect
entire bandwidth available for your use if you are among the first users to connect
performance degrade as new users, especially heavy-access users, are connected to the channel Digital Subscriber Line (DSL): Digital Subscriber Line (DSL) very high-speed connection
uses the same wires as a regular telephone line
DSL transceiver (customer)
DSL Access Multiplexer (DSLAM) at DSL service provider ADSL: ADSL DSLAM aggregates connections onto a single, high-capacity connection to the Internet
Dedicated connection from each user back to the DSLAM; no performance decrease as new users are added
As the connection's length increases, the signal quality decreases
Looking at a map is no indication of the distance a signal must travel
Limit for ADSL service is 18,000 feet
Maximum downstream speeds of up to 8 Mbps at a distance of about 6,000 feet
Upstream speeds of up to 640 Kbps
DSL Advantages: DSL Advantages Internet connection open and still phone line available
Much higher speed than a regular modem
(1.5 Mbps vs. 56 Kbps)
It doesn't necessarily require new wiring
“modem” usually provided as part of the installation
DSL Disadvantages: DSL Disadvantages Connection works better when closer to the provider's central office.
The connection is faster for receiving data than it is for sending data over the Internet.
Service is not available everywhere.
Other Types of DSL: Other Types of DSL Very high bit-rate DSL (VDSL):
a fast connection, but works only over a short distance
Symmetric DSL (SDSL):
used mainly by small businesses, doesn't allow use of the phone at the same time, but the speed of receiving and sending data is the same
Rate-adaptive DSL (RADSL)
a variation of ADSL, but the modem can adjust the speed of the connection depending on the length and quality of the line