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1、1,Chapter Three,Analog and Digital Signals,2,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,3,How a Modem Wo
2、rks,4,Cable Network Architecture: Overview,home,cable headend,cable distribution network (simplified),Typically 500 to 5,000 homes,5,Cable Network Architecture: Overview,home,cable headend,cable distribution network (simplified),6,Cable Network Architecture: Overview,home,cable headend,cable distrib
3、ution network,7,Cable Network Architecture: Overview,home,cable headend,cable distribution network,FDM:,8,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 Statistic
4、al TDM( see ch4.5.3 ),9,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 o
5、n shared upstream channel Headend scheduler sends back assignment of future time slots to subscriber,10,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 w
6、ires 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,11,ADSL Design,Asymmetric Greater capacity downstream than upstream Fre
7、quency division multiplexing Lowest 25kHz for voice Plain old telephone service (POTS) Use FDM to give two bands Range 5.5km,12,ADSL Channel Configuration,13,xDSL,High data rate DSL Symmetric line DSL Very high data rate DSL,14,Residential access: point to point access,Dialup via modem up to 56Kbps
8、direct access to router (often less) Cant surf and phone at same time: cant be “always on”,ADSL: Asymmetric Digital Subscriber Line up to 1 Mbps upstream (today typically 256 kbps) up to 8 Mbps downstream (today typically 1 Mbps) FDM: 50 kHz - 1 MHz for downstream 4 kHz - 50 kHz for upstream 0 kHz - 4 kHz for ordinary telephone,15,Exercises: As a consumer, which one is your choice between a DSL and a cable modem connection to Internet? Why? Required Reading Stallings chapter 3.8,Chapter Three - Analog and Digital Signals,