ISDN - Integrated services digital network    (a.k.a  It Still Does Nothing)

Essential concepts... a collection of well defined STDM

Channels:
B - 64kps PCM for voice or data
D - 16kbps for signaling

Services
Basic rate 2B+1D
Primary rate 23B + 1D (almost a T1).

Providing digital bit pipe services
Connection oriented
Bit stream
Unreliable

Call connection => full bandwidth reserved for that connection.

Since each line consisted of purely digital channels codecs were required for voice

ADSL - Asymmetric digital subscriber line
	
Essentially an admission of the failure of ISDN
Requires removing filters from the local loop
Limited in distance from local office (up to 5.5km ... your mileage may vary)
Asymmetric in that more downsteam bandwidth is available than upstream

Characteristics
Low 25Khz of the circuit is reserved for voice (only 4 Khz is still used)
Other 21Khz is a buffer to reduce crosstalk
Remainder of the bandwith is either...
FDM multiplexed between upstream and downstream service 
Upstream 25Khz - 200Khz
Downstream 250Khz - 1Mhz

or Echo cancellation is used to make full bandwidth available both ways


DMT (Discrete Multitone) Signaling

Used on the both upstream and downstream links

Both upstream and downstream bands are further FDM submuxed into a number of 4Khz channels.   

Adaptive QAM (Quadrature Amplitude Modulation) is used to assign bits to subchannels based upon the carrying capacity of the subchannel.
 


BISDN - ATM - CELL relay

A cell is a small (53 byte) fixed length packet 48 + 5 that can contain
48 PCM slots
48 bytes of data

Connection oriented, ATDM cell switching  based technology

Designed for heterogeneous loads
Video
Voice
Image
Data

Very high data rates
100 Mbit + links
10   Gbit + switches

Cell units managed by switches on a store and forward basis
Priority based services possible