Miniquiz, CIS 451, Fri 10/31/03.
10 minutes only.

1. What are the main purposes of Sliding Window Protocols?

A. Flow Control: Sliding Window protocols ensure that when two computers
   of unequal capability communicate, the more powerful one (or the one
   with less other work to do) can not overload the other one.

   Not asked: The destination can slow down the source by not sending
   an acknowledgement until it has space in its receive buffer for more 
   data.

B. Error Control: Sliding Window protocols make sure that missing data
   and data damaged during transmission is re-transmitted. Sliding 
   Window Protocols also ensure that data arriving more than once,
   and data arriving out of order, are handled correctly.

2. We have Voice over ATM. Every ATM cell contains 48 data bytes. 
   The voice is PCM encoded, with 8000 samples / sec and 1 byte 
   (8 bits) per sample. No fancy coding or data compression.
   What is the ``cellification'' delay?

   Each cell contains 48 Bytes, e.g. 48 samples. At 8000 samples/sec 
   that represents 48/8000 = 6/1000 of a second on voice signal.
   
   The ``cellification delay'' is 6 msec.

   One student said that the ``serialization delay'' is 6 msec. That
   is incorrect.  
   Suppose the ATM cells are shipped over a link with datarate 45
   Mbit/sec. At the source (voice encoder) it takes 6 msec to build
   the ATM cell. Once it is ready, it is shipped out in
   (8*53)/(45,000,000) = .000,009,422 sec, a little bit less than 10
   mu-sec.  That is the serialization delay. 
   (And then there is propagation delay, depends on how far the cell
   must travel).
   The serialization delay depends on the size of the packet or cell
   and the data rate of the transmission link. (The link carrying the
   packet or cell).  The packetization (cellification delay) of a 
   stream of real-time data depends on the intensity of the data stream
   being carried and the size of the packets or cells.

   A few students said the ``cellification delay'' is 8000/48 = 166.27
   seconds.
   If you are paid $8000 per year, and you get $48 per payday, is 
   there 48/8000 year between paydays? Or 8000/48 ?

   A correct argument would have been: there are 8000/48 = 166.67 cells
   per second, so the time between cells is 1/166.67 = .006 sec = 6 msec.
   Therefore, it takes 6 msec to collect enough voice samples to fill
   a cell.

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Aside: Under the USA proposal (64 data bytes per cell) the cellification
delay would have been 8 msec. 
Under the European proposal (32 data bytes per cell) the cellification 
delay would have been 4 msec. 
Not discussed in class: This extra delay might have caused a need for
many more echo-cancellers in the European telephony system, had 
cells of 64 data bytes been accepted. The USA already had those echo 
cancellers installed. 
(Not to be asked on any test!) (At least not in THIS class.)