Telephone System Physical Layer

Overview

The telephone physical layer is a complicated system using wire, fiber and wireless media and a wide variety of protocols. Here we want to understand the parts of that system that particularly impact data communications. On one end of the spectrum there are modern long distance transmission facilities and on the other, simple modem communications.

Objectives

• Know the basics telephonic systems based on PCM.
• Understand a few of the underlying WAN systems in common use.
• Understand the basic of DSL technology.
• Understand the basics of modem technology.

Preparation

Study section 2.3.3 in the book and the notes.

Terms

Digital Subscriber Line (DSL)
Integrated Services Digital Network (ISDN)
Modem
Pulse Code Modulation (PCM)
Quadrature Amplitude Modulation
STS/OC Standards
Synchronous Optical Network (SONET)
T-1/T-3
Trellis Coding

K&C

Explain why the data rate of a T-1 is 1.544 Mbps.

If you can detect 15 degree phase angle changes and 2 levels of amplitude, how many bits can you encode per baud with QAM? With Trellis Coding?

Draw a diagram of the channel allocation scheme used by SDSL (American).

ASE

What is the utilization of a T-1 channel where the full 8-bits are used for data? What if only 16 of the slots are used in each cycle on average?

If you need to send the string "0000 0011 0001 1101" using QAM-16, what does the transmisson look like?

The DS services and SONET use synchronous framing? Why?

What is a CSU/DSU (it's not in the book or notes)?

Why does the SONET protocol scramble the header?

What is the reason for SONET allowing the frames to be "out-of-phase."