Subj : Modem or ??? To : ALL From : TOM WALKER Date : Fri May 20 2005 10:29 am With a recent discussion of What exacticlt is or is not a Modem as it concernes DSL and Cable Devices I found the following. * DSL Officaly there are several names for the DSL "adapter". The Telephone CEntral Office people call them DSL Transcievers. The Engineers call them ATU-C and ATU-R units. The ATU stands fro ASDL Termination Unit. And the one in the Home is the R unit, Remote and the one for that line in the Central Office is the C unit. They operate very similar but in a very more complex way as the Analog Modems we al are fimilar with so they can Correctly be called a Modem. There are two methods used. One breaks up the DSL line into three sections. the 0 to 4 Hkz area is the Analog Phone service portion. The 25 to 160 Khz portion is for Upstream Transmissions and the 240 Khz to 1.5 Mhz portion is for the Downstream Transmissions. This is called CAP(Carrierless Amplitude/Phase) Teh other method is called DMT(Discrete Multitone). In this method the bandwidth is broken up into 247 seperate 4 Khz wide channels. Eac channel is monitored and if quality goes down o none chennel that data is shifted to another channel Several of the lower 4 Khz channels are Bi Directional and carry the Upstream Data. the rest are for Downstream data. This system is more complex but has more flexibility on lines of differing quality. Becasue of Signal Cross talk and line losses there is a Maximum transmissions distance of 18 Kilo-Feet form the Central Switching Station having yuor ATU-C unit. * Cable The Cable Unit is a REAL Modem as it has an actual internal Modulator and De-Modualtor for placing the Information on an RF carrier for outgoing data and detecteding(De-Modulating) it from the RF carrier for incoming data. Inside the modem there is also other sections including a Tuner, CPU, USB port and or Net Card and a MAC(Media Access Control chip. The cable system uses qalmost al lof the practical usable RF spectrum. For the TV they need to divide it into 6 Mhz Channels just like it is don for Off the Air Television. For Internet uses with the CAble Moden they assign 2 Mhz wide channels for Upstream pueposes and 6Mhz Channels for Down stream purposes. the systems vary but a lot of CAble Systems assign a 2 Mhz channel between 5 Mhz and 42 MKhs for the upstream flow of data and a 6 Mhz channel between 42 Mhz and 850 Mhz fro Downstream Data. Unlike the DSL where you are the sole user of the Bandwidth the cable system has it's area broken down into "Nodes" and you Share the Bandwidth with others in your Area. They try to monotor typical usage on the nodes and reassign as necessary to maintain some reasonable Speed.Althouhg at some times during the day or night you might notice a slow down during high activity periods on your node. Here is short bits o nthe modulator and De-modualtor. ** Inside the Cable Modem: Demodulator The most common demodulators have four functions. A quadrature amplitude modulation (QAM) demodulator takes a radio-frequency signal that has had information encoded in it by varying both the amplitude and phase of the wave, and turns it into a simple signal that can be processed by the analog-to-digital (A/D) converter. The A/D converter takes the signal, which varies in voltage, and turns it into a series of digital 1s and 0s. An error correction module then checks the received information against a known standard, so that problems in transmission can be found and fixed. In most cases, the network frames, or groups of data, are in MPEG format, so an MPEG synchronizer is used to make sure the data groups stay in line and in order. ** Inside the Cable Modem: Modulator In cable modems that use the cable system for upstream traffic, a modulator is used to convert the digital computer network data into radio-frequency signals for transmission. This component is sometimes called a burst modulator, because of the irregular nature of most traffic between a user and the Internet, and consists of three parts: A section to insert information used for error correction on the receiving end A QAM modulator A digital-to-analog (D/A) converter --- þ SLMR 2.1a þ This note from El Cajon California USA * Origin: Try Our Web Based QWK: DOCSPLACE.ORG (1:123/140) .