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M O D E M S: 

M O D E M S Presented by Pilar Gomez June 2, 2004


Modem enables computers to communicate over standard telephone lines modulation and demodulation full duplex mode = simultaneous transmission




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 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 Faster non-analog “modems”. Users connect to the Internet through a: regular modem 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 Equipment Cable modem (customer) CMTS (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 ADSL line DSL transceiver (customer) DSL Access Multiplexer (DSLAM) at DSL service provider


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 Distance-sensitive technology 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

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