Hi John,
<< D.N.--Thought I understood this, but your exchange with MikeM has me wanting to get a picture of the physical layout into, in and out of the CO. Is this correct?: >>
Lots of questions here. Perhaps it would be best if I summarize the DSL arrangement to give you a high level view of how this is done. Actual deployments vary between providers, so this is very generic (and has probably been stated on this thread many times, but a review won't hurt).
First, it is important to distinguish between a dial-up modem (what you were referring to as a 28.8) and a DSL modem. A dial-up modem modulates your computer data onto the analog voice frequencies used by your telephone. The data then passes through the voice switch, and voice network, as a voice call. This has severe limitations, due to the fact that your voice is limited to about 3500Hz as it passes through the network. You therefore have a very limited amount of bandwidth to modulate your data.
DSL is totally different. It utilizes the spare frequencies above the ~3500hz frequencies used for your voice. As such, your telephone works as it always has worked. DSL simply rides on higher frequencies above your voice on the same copper pair of wires. At the central office the data is then "stripped off" the copper wire and sent to the internet before it reaches the central office voice switch. The voice frequencies continue on to the switch as always.
More specific, but still generic, the copper pair from your house enters the central office and goes to a main distribution frame - a convenient place to terminate thousand of individual copper pairs. From that point a regular voice line "cross connects" to a pair of wires that terminate in a D4 channel bank on the voice switch. The signal is then digitized and transmitted through the voice network to its' intended destination.
With DSL, the copper cross connects from the MDF to the DSLAM. At that point the signal is split - the lower voice frequencies are passed off to the voice switch, and the upper frequencies utilized by DSL are terminated on a DSL modem on the DSLAM. From that point the signals of all DSL users that terminate in that office are aggregated onto a broadband circuit(s) and sent to the internet via your ISP.
The main point is that the voice channel is passed on to the central office switch and enters the traditional voice network. The DSL channel is passed off to an entirely different infrastructure that goes through your ISP and onto the Internet.
Pretty straight forward!
The physical arrangements in the C.O. are evolving rapidly as more economical deployments are developed, but this is the basics.
Regards, |
