🚀 go-pugleaf

When discussing gigabit transmission, one must define where the rate
is measured and over what period.  For example, multi-gigabit digital
transmission lines already carry telephone conversations among major
cities.  However, a file transfer of 1 gigabit that takes one second
from the user's point of view is an entirely different thing.

Sending bits down a fiber at multi-gigabit rates is not the hard part.
The hard part is controlling the gigabit transmission.  The reason
that a telephone network can use gigabit lines is that few devices in
the network must operate at high speed because the system is circuit
switched.  The only control that occurs is when a telephone call is
set up or shut down.  Otherwise, every nth bit in the wire is yours.
Packet switching is much more difficult because each packet of data
must be interpretted in the period that it takes to transmit one
packet.  So why do we use packet switching?  Packet switching is a
more efficient method of resource allocation than circuit switching
for traffic sources that are bursty, such as computers.  Thus it is
cheaper.

What do we have to do besides send the bits?  The efficiency of packet
switching is achieved with dynamic resource allocation.  This means
that we now need congestion control and flow control to assure that
the transmission rate of our data does not exceed the processing rate
in the network and receiver, respectively.  This is a difficult
control problem, particularly on fast and large networks.

Another problem is retransmission.  If our data must be perfectly
replicated at the receiver, then we must retransmit any data that was
corrupted or lost in transit.  Whether a packet has been lost or is
just delayed is a difficult problem in packet-switching network.  Once
again, the network dynamics make it difficult.  If we are slow to
respond to loss, the transfer time increases because we let the
network go idle even though we could be retransmitting data.  If we
retransmit too quickly, then we may congest the network with redundant
packets.  Determining when to retransmit is another tricky control
problem.

Even if we get the control right, there are few computers that exist
today that could use a gigabit network.  Most buses are slower than
this and the buses aren't faster because the CPU doesn't need a faster
bus.  As CPU speed increases, bus speed will increase also.  The point
is that few people need a gigabit data rate now, and so the
appropriate machines are rare.  Another problem is that if protocol
processing is done with a conventional CPU, the CPU may be unable to
process data at a gigabit.

In summary, packet-switching networks are cheaper because they are
more efficient, and they are more efficient because because they
allocate resources dynamically.  The network dynamics make high-speed
transfer difficult because we our control systems are not good enough.
If one were willing to spend the money so that a circuit-switched
network could be used, then these control problems are simplified.
Even if you have a gigabit network, then you will probably need a
supercomputer to accept and process data at that rate.  The other
possibility is to design a system with custom VLSI protocol processors
and a high-speed bus.  This approach is not cheap either.  As has been
noted previously, running an application at a gigabit today is
possible, but expensive.

						-Dave Feldmeier

P.S. I am exploring multimedia transport protocol design (layer 4) for
the AURORA gigabit network.