BBN Notes for Lecture 2 (1/15/97)
SWITCHING
Current Networks use "Routers",
which route packets using complex algorithms implemented in software.
ATM Networks direct cells using "switches",
which are implemented in hardware.
What is a Switch?
An N x N switch has N input lines and N output lines.
 |
A slotted 4x4 switch with arriving cells.
(# indicates output line for that cell.)
 |
What if two cells destined for
output 3 arrive simultaneously?
There is a Multiple Access problem
(see CCN notes); however,
- No large distance involved.
- Very large scale (# of links = 1000 or more).
Internal TDMA channel operates at N times single link speed.
Assign each cell to TDMA slot # of input channel.
This switch has output queuing:
The output links can't remove cells from the internal channel fast enough.
The cell on input line 2 going to output line 3 must wait in the buffer
at output line 3 behind the cell on input line 1 going to line 3.
Alternatively, assign each cell to slot # of output channel.
This switch has input queuing:
The second cell destined for line 3 has to wait at input line 2
for the next TDMA round,
and so do all the cells behind it.
This is called HoL (Head of Line) blocking.
Notes taken by Kyle B. Hollasch, hollak2@rpi.edu,
from a lecture by Prof. K.S. Vastola, vastola@ecse.rpi.edu,
on January 15, 1997.
