----- Original Message -----
Sent: Friday, November 12, 2004 3:17
PM
Subject: Re: [RE] Proposal from Jose
Morales
Jose,
1. Can you describe your definition of synchronization.
Are you referring to time synchronization, frequency synchronization and/or
phase synchronization, or something other form of synchronization?
In an isochronous system, all the
clocks of the stations "runs" at the same speed. It is very different in the
plesiochronous (like PDH) or synchronous (like SDH) systems. The term
"timestamp" means time synch.
2. Can you provide numbers that explain your following
statement: "High quality synchronization services that provides all stations
with a low jitter house clock".
I copied the phrase from the
objectives of RE (for that reason it is between commas).
In RE, the Ethernet domain can be extended
about 2 km (it is a LAN, not a WAN). The speed of light in copper/fiber is
200.000 km/s. If the network is not congested (if it is in congestion
nothing will work) the delay introduced by the switches in the timestamps is
very low. Then, all the stations will have its clocks running isochronously
at the same speed. The variation produced by jitter, can be compensated in
the receivers discarding the timestamps that arrive delayed. Sending 20 per
second, the synchronization can be very precise, depending on the precision
of the station's clock.
3. Can you also share the applications/services for RE and
their performance levels (in terms of required accuracy/stability, ppm/ppb,
etc.) and the network impairments you think that could affect the performance
levels.
In my previous response to Glenn
Algie I have made reference to the applications based on TCP/IP and
LLC/MAC. The last ones allow much greater efficiency, in special in the
servers. The key of the good
operation in the network is to avoid the congestion. The LLC2 protocol will
be controlled from the network nodes to avoid congestion. This can't be done
with TCP, that also collaborates with the SS/CA, but only end to
end.
4. Can you describe the use of the timestamp, how it is
generated, transported and applied.
The master clock copies the 32 bits
of its clock in the timestamp field, then send it via MAC control
or LLC1 frames in broad/multicast. The station adjust its clock to the
value received. As the frames are going to have delay variations, it is
necessary to apply some type of adjustment (PLL) based on the received
values to minimize the jitter.
Thanks
- Michel
-----Original Message-----
From:
owner-stds-802-3-re@IEEE.ORG [mailto:owner-stds-802-3-re@IEEE.ORG]
On Behalf Of Jose Morales
Sent: Friday, November 12,
2004 5:38 AM
To:
STDS-802-3-RE@listserv.ieee.org
Subject: [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