A Primer On Asynchronous Modem Communication

Boston2

This is extremely technical in some parts so be warned, this, i hope will explain to some people what the likes of hubson and others are talking about half the time.

its a microsoft commitioned report, writen by Computer Service Engineers and not marketing people.

Ps its long but informative
the pdf file

Boston

thats me btw, had to register again to gain webspace

moist

BTW. that link is broken, its got a space and a BR tag in it for some reason...
try... this

<edit> Hot damn it did it to me too </edit>

Boston2

can you access it now

Boston

so anybody read it, what do you think

Labyrinth

A pretty good primer all round.....

Altho, to be fair, it doesnt cover the later standards, or specify what encoding method is used with each modulation. The spatial maps for trellis encoding is a bit like looking at a star chart .

It does cover the flow control quite well to give it credit. Ive worked in the datacomms industry for several years, and the document does give quite a good explanation of how async modems actually work.

I would put a little addendum in about the frequency ranges used... Because the quality of the local loop copper has increased, we have been able to reliably get data over higher frequencies. This single factor has been the main enabling reason for dsl. Dsl uses the higher frequencies for transmission of data, leaving the 300 to 3600hz range free for normal telephony. The reason behind this frequency range it routed back in the days of bell himself. In order to get telephone calls over long distances they needed to build amplifiers. These were valve operated and needed a lot of power. As the human ear can only hear within those ranges, they decided to amplify only those ranges. The higher the frequency to be amplified the more power it needed.

And this is the crux of the 'distance from exchange' part of dsl. As the length of the copper increases, the rate of attenutation increases. Eventually, there is so much attenuation, that the dsl modems ( yes they are in essence synchronous modems ) cant demodulate the frequencies. With DSL, as the distance increases, you need to step back the speed - hence reducing the frequencies to overcome the higher attenuation at hight frequencies.

We have deployed many broadband modem links across unconditioned Eircom Circuits ( typically up to 128k ) for the last 10 years or so, and have deployed well over 100 private dsl connections for our customers in more recent years. Typically these are HDSL ( 2meg symetrical over 4 wires ) or ADSL ( asymmetrical over 2 wires ).

Sorry if that was all a bit boring....

Ace Rimmer.......

( Datacomms and Networking guru ( self professed ) )

GoneShootin

thanks for that boston/«Bo§ton»

wish i had this last xmas for my Com3 project

Boston

Ace Rimmer, im trying to collect these files, and expand my knowleadge a little, id apreciate if you could point me in the directions of some more, broad broadband guides like this one.
it seems to be working as i understood about 80% of what you said:D

Labyrinth

Bo§ton,

Funnily enuf, its kinda hard to come across that sort of stuff nowadays... What you want to get your hands on is the white papers that manufacturers like Cisco, 3Com and Nortel publish...


But a good starting point is www.protocols.com

The selection of protocols is a bit varied to put it mildly, but certainly worth a good look. I tend to use it as a reference if i need to look up a packet structure or frame structure in a hurry....

Ace.

Labyrinth

Bo§ton,

also,

Try the following.....


www.cisco.com/warp/public/779/servpro/promotions/dsldepot/basics.html

www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/dsl.htm

and....

www.adsl.com


Ace.

hudson806

Originally posted by Ace Rimmer

and have deployed well over 100 private dsl connections for our customers in more recent years. Typically these are HDSL ( 2meg symetrical over 4 wires ) or ADSL ( asymmetrical over 2 wires ).

Just curious: using an asymmetric link is very unusual in cases like this (there are about 10,000 reasons why symmetric makes more sense). Why did you select ADSL over SDSL?

Labyrinth

Originally posted by hudson806


Just curious: using an asymmetric link is very unusual in cases like this (there are about 10,000 reasons why symmetric makes more sense). Why did you select ADSL over SDSL?

Simply - Cost

Eircom will rent u a 2 wire unconditioned circuit for about 500 quid a year. The actual cost of SDSL modems are higher than ADSL for two wire operation. Also, the sites in question were almost always downstream of the main site where services were located.

HDSL & now SDSL would be used over 4 wires if symmetric speeds are needed. Most of the deployments I've come across are purely because its heaper to put in xDSL modems over copper, than getting a 64k leased line.

Ace