| T.R | Title | User | Personal
Name | Date | Lines |
|---|
| 2131.1 | Well....not quite 0 latency :-) | NETCAD::BATTERSBY | | Wed Mar 22 1995 12:53 | 8 |
| It is mentioned in the Bridge/Switching Products on the DECHUB web
page that Scott Bradner's test equipment accuracy has a tolerance of
+/- 64 microseconds. We measured 35 microseconds in our lab. This is
mentioned too under Performance. Oh and we do not use cut-through
switching. The 35 usecs is all the time we need to store&forward the
packets.
Bob
|
| 2131.2 | Further clarification.... | NETCAD::BATTERSBY | | Wed Mar 22 1995 12:56 | 5 |
| Also the statement in 716.3 *does* in fact mention that there is a
small finite amount of latency, thus alluding to some test accuracy
allowance in Bradner's test.
Bob
|
| 2131.3 | How Scott timed it. | CGOS01::DMARLOWE | Wow! Reality, what a concept! | Wed Mar 22 1995 19:14 | 5 |
| Also Scott's time starts when the last byte of the packet arrives (CRC)
til the first byte starts coming out of the destination port. Thus the
35uS in the lab fits into the +-64uS accuracy of Scott's clock.
dave
|
| 2131.4 | More on Latency Comparisons | DELNI::PIEPER | | Wed Mar 22 1995 21:28 | 33 |
| It is important to note that latency for store and forward devices is
measured from receipt of the LAST bit of the incoming frame to the
transmission of the first bit of the outgoing frame. This is per an
IETF spec for bridges and routers. As stated in the other responses
the 35uS latency was too fine for Bradners equipment to measure so we
ended up with a 0 latency number (as well as most of the other vendors).
Bradner now has better equipment for measuring the latency and has been
retesting equipment so you will probably see the 35uS number showing up
on his latest reports.
Latency for cut-thru devices is typically measured from the receipt
of the FIRST bit of the incoming frame to the transmission of the
first bit of the outgoing frame. Quite different from the IETF
specification!
If you want to try to compare cut-thru switches and store and
forward switches, you would need to use a new definition instead of
latency. Let's call it "delay". So the "delay" for the cut-thru
device is the latency and the "delay" for the store and foward device
is the length of the packet plus the latency.
Don't get all exited about this apparent "delay" advantage though since
cut-thru switches suffer from many other disadvantages (forwarding of
runt packets that are caused by collisions, inability to interconnect
dissimilar networks (FDDI to Ethernet) or dissimilar speeds (10 Mb
Ethernet to 100 Mb Ethernet), lack of filtering options)
Also realize that cut-thru switches can only cut-thru if the
outbound port is NOT busy, otherwise they too have to store and
forward! So if more than two ports try to get to a single outbound
port, the cut-thru switch needs to revert to store and forward. This
scenario will happen quite a bit in a normally busy network. When
this happens, the "delay" advantage evaporates!
|
| 2131.5 | Ethernet cut thru not worth reduced latency | CGOS01::DMARLOWE | Wow! Reality, what a concept! | Thu Mar 23 1995 15:16 | 7 |
| Also discussed in ETHERNET_V2 1023 were some comments about cut thru
switching (bridging). With low traffic cut thru is only marginally
faster than store-and-forward. With medium and high traffic, even
Digital News and Review didn't like cut thru (Kalpana) due to packet
loss because of small buffers.
dave
|