Comp 346/488: Intro to Telecommunications

Tuesdays and Thursdays, 6:00-9:15, Lewis Towers 410

Class 5
AICN readings:
    An Overview of Networks
Optional reading (Stallings 7th -- 9th editions)
Chapters 3, 4 and 5

Homework 1 is now on Sakai. It is due Friday, July 17.

Suppose you want to rehearse music with someone at a remote location. Here, 100 ms of one-way delay is generally a significant problem. Sound moves through air at about 1 foot/ms, so that would be like playing in sync while spaced 100 feet apart. Large orchestras do sometimes have issues with this, and professional musicians do sometimes play varying degrees behind the conductor's beat. The audience in the back of the theater might see the conductor appear to be ahead of the musicians by a quarter-second (250 feet).

If the tempo is ♩ = 150, then that quarter-note takes 400 ms, and a sixteenth-note takes 100 ms.

Classic circuit-switching (including digital trunk lines) can achieve store-and-forward delays of well under 10 ms, though there are still the propagation delays. Across 5000 km, the one-way propagation delay in wire is 25 ms (200 km/ms).

Packet switching has a very hard time keeping delay under 100 ms, though sometimes one gets lucky. A more serious problem is the very large jitter buffer of some VoIP-like systems. For music, one would have to keep the jitter buffer at a minimum, in effect trading a larger loss rate for a minimum of latency.

But check out


The second blog post by Whisper Systems, on Client-Side Audio Quality, goes on to say

All VoIP solutions contend with packet switched networks that were not designed to transmit real-time media streams. Packet latency and packet loss are the principal manifestations of this reality, and the jitter buffer is their canonical solution.

They then describe an adaptive-jitter-buffer algorithm, where the buffer can change size. That's a bit of a trick, as that means slowing down or speeding up the audio. In order to do this at all, one must know that sometimes voice can be stretched (eg by pausing), or shortened. For voice, the resultant distortion is generally acceptable; for music, it is likely not.


Go over homework

Räisänen recap
    clock resynchronization

Shannon-Hartley limit revisited

    Digital data / digital signal: B8ZS, 4b/5b: done
    Analog data/ digital signal: PCM (done), µ-law (still to do)
    Analog data/ analog signal: SSB, FM v PM
    Digital data / analog signal: BFSK v MFSK, PSK, QAM, ... OFDM

Telecom terminology: pots.html

Asterisk: configuring a phone
    Ekiga demo with headset
    PBX Terminology: asterisk_basics.html

    FDM and L-carrier
    TDM: T-carrier and SONET