南京邮电大学DSP课件.ppt

Chapter Three Analog and Digital Signals Modems A typical telephone line can only accommodate an analog signal (a continuous, curving signal) A computer generates a digital signal representing bits Modem: a device that translates data from digital to analog and analog to digital How a Modem Works Cable Network Architecture: Overview Cable Network Architecture: Overview Cable Network Architecture: Overview Cable Network Architecture: Overview Cable Modem Outline Two channels from cable TV provider dedicated to data transfer One in each direction Each channel shared by number of subscribers Scheme needed to allocate capacity Statistical TDM( see ch4.5.3 ) Cable Modem Operation Downstream Cable scheduler delivers data in small packets If more than one subscriber active, each gets fraction of downstream capacity May get 500kbps to 1.5Mbps Also used to allocate upstream time slots to subscribers Upstream User requests timeslots on shared upstream channel Headend scheduler sends back assignment of future time slots to subscriber Digital Subscriber Line (DSL) Digital subscriber line (DSL): a telecommunications technology that delivers high-bandwidth information to homes and small businesses over ordinary copper telephone wires Provides a transmission rate of 512 Kbps to 1.544 Mbps from the central office to the subscriber Provides a transmission rate of about 128 Kbps from the subscriber to the central office Can carry both data and voice signals ADSL Design Asymmetric Greater capacity downstream than upstream Frequency division multiplexing Lowest 25kHz for voice Plain old telephone service (POTS) Use FDM to give two bands Range 5.5km ADSL Channel Configuration xDSL High data rate DSL Symmetric line DSL Very high data rate DSL Residential access: point to point access Dialup via modem up to 56Kbps direct access to router (often less) Can’t surf and phone at same time: can’t be “always on” Exercises: As a consumer, which one is your choice between a DSL and a cable