[RE] Proposal from Jose Morales
All,
Dear friends, I would like to propose the following solution:
In order to obtain an isochronous network, we can use the following strategies:
- Synchronize at physical/802.3 level.
This implies to modify all the network elements (switches), and to use
new equipment in all the network.
- Synchronize at MAC/802.3 level. It can be origin of incompatibilities.
- Synchronize in LLC/802.3 level .
The perfect place to do this, because it is is transparent to all 802.3
network,
and will work also over 802.11, 802.15, 802.16, etc.
For the synchronization of the network, could be used a technique equivalent to
the one employed in 802.5, where all the stations have capacity to do the synch
master, but only the active monitor does it, and all the others synchronize with
him. Also I suppose that you know the mechanism which guarantees that there is
always only one master synch, that would be the station with the lower value in
the MAC address.
The system that I propose is very simple: The synch master station sends
periodically (for example each 20 or 100 milliseconds) a broadcast or multicast
802.3 frame, containing one 802.2 that transports the time stamp (the value of
the real time clock). All the isochronous stations can synchronize their clock,
compensating the very small jitter that it would take place in the transmission
of the consecutive frames.
The frame will look like this:
(The SAP=27 and 28, as far as I know, are not used in any other application)
|DA=Broad/Multicast | SA | LENGTH |
|DSAP=27|SSAP=27|UI(03)| TIMESTAMP|
|PAD|FCS|
Once synchronized all the stations, the isochronous traffic could be sent in
multicast with LLC type 1 protocol or in unicast with LLC type 2 protocol,
including in each frame the time stamp.
LLC2 protocol has many advantages, like the error and flow control (end-to-end),
the identification of streams by means of P/F bit and that it would make
possible the congestion management. Remember that TCP protocol also have the
timestamp option, with 4 octes. It will be very useful to define the same size
here.
The frame LLC2 will look like this:
|DA| SA | LENGTH |
|DSAP=28|SSAP=28| INFO|N(R)|P/F|N(S)|TIMESTAMP (4 octets)|DATA|FCS|
With the system I propose, we have “High quality synchronization services that
provides all stations with a low jitter house clock”. Now it is only necessary a
802.3 switch that guarantees that "isochronous services can use up to 75% of the
link bandwidth, while the remaining is always available to best-effort traffic"
and "assign resources for isochronous services".
This system is exactly the one that I propose in the manuscript "Universal
Ethernet Telecommunications Service: The Convergence of Internet, Broadband and
Telephone networks on the IEEE 802 standards". I have sent it to the IEEE
Communications Magazine and now is "Under Review", reason why I cannot pass it
to the reflector.
Comments?
Jose Morales Barroso, Ph.D.
jmb@ieee.org