ITPRC - Physical Layer

Analog Modems

Cable Modems

Cable Datacom News - News and information on cable modems
Cable Modem/DSL Tuning Guide - Tips and tricks for getting the most out of your high-speed Internet connection
Cable Modem Home Page - Created by Cable Labs to provide support for cable modem certification services
Cable Modem University - Technical and market research resources for cable modem technology
CATV CyberLab - An extensive collection of links to information cable modem technology, service providers, and industry information
PacketCable - A CableLabs-led iniative aimed at developing interoperable interface specifications for developing advanced, real-time multimedia services over two-way cable plant

Cabling

BICSI Library - Collection of information on cabling and cabling standards
Cabling Directory - Interactive buyers guide and technical information on wiring
CableTesting.Com - Information and resources for cable testing
Connectworld - Tons of resources on cabling including cabling diagrams and standards information
Data Communications Cabling FAQ - Frequently asked questions for the comp.dcom.cabling newsgroup
Network Cabling Diagrams - Pacific Custom Cable's guide to wiring components
Pin-Outs.Com - A comprehensive collection of cables, pinouts, and connectors
R&M International - Provides answers to frequently asked questions about cabling standards

Channelized T1/T3/E1

All you wanted to know about T1 but were afraid to ask - Tutorial on channelized T1
EverythingT1 - Information on T1 Networking including links to technical information and service providers

DSL

ADSL Forum - An industry organization for promotion of ADSL and related DSL technologies
DSL in Depth - An overview of DSL from Sun World
DSL Reports - Searchable DSL service locator and information on various technologies and products
EverythingDSL - Large collection of DSL information including a service locator

Miscellaneous

Acterna Tech Resources Library - Tons of resources on serial communications and testing
DataCom For Business Applications Notes - large collection of resources focusing mainly on wide area networking technologies and serial communications
High Speed Networking Pages - Tons of links to high speed networking technologies. By Joe Lindsay
IEEE 802 LAN/MAN Standards - Collection of information on the various 802 standards
Optical Internetworking Forum -organization the development and deployment of interoperable products and services for data switching and routing using optical networking technologies
SpeedGuide.net - Cable and DSL Tips and Tricks. Includes security guide and installation help
Tektronix Communications Test References - Technical references for testing SONET/SDH and DWDM

SONET/SDH/DPT/RPR

Dynamic Packet Transport - Guide to Cisco's DPT resilient packet ring technology
Resilient Packet Ring (RPR) Alliance - Industry association promoting packet ring standards
SONET Home Page - Information on Synchronous Optical NETworking
SONET 101 - An introduction to SONET, by Nortel (PDF)
What is SONET? - From a SONET to PC interface project by Jasbir S. Mahal and Danny Moore
SONET Interoperability Forum - organization to promote the deployment of SONET technology

Standards

International Telecommunication Union - Geneva, Switzerland based organization within which governments and the private sector coordinate global telecom networks and services
American National Standards Institute (ANSI) - Administrator and coordinator of the United States private sector voluntary standardization system
Standards Committee T1 Telecommunications - develops technical standards and reports regarding interconnection and interoperability of telecommunications networks