Interesting links... Again though, we get to electrical
characteristics.
Cat5 is only rated for 100MHz of signal whereas Cat5e is 350MHz. I would
expect that "regular" Cat5 would still have problems with the 802.3ab
spec at full length although I haven't actually tested that out! :)
Cat6 is rated for 500MHz which is what makes that possible.
Hmmmm, hmmmm, hmmmm... Ehhhh... Just run fiber, saves the
arguments! (grin)
Scott Morris, CCIEx4 (R&S/ISP-Dial/Security/Service Provider) #4713,
JNCIE-M #153, JNCIS-ER, CISSP, et al.
JNCI-M, JNCI-ER
evil_at_ine.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Outside US: 775-826-4344
Knowledge is power.
Power corrupts.
Study hard and be Eeeeviiiil......
Ivan Walker wrote:
Hi Scott,
Done some more digging around and testing so thought I would share what I
have found
1000Base-T
==========
IEEE 802.3ab
Approved it as a standard on June 28, 1999 by Institute of Electrical and
Electronics Engineers Standards Association
Requires four pairs of CAT-5, CAT-5e or CAT-6
Transmit and receive 250Mbps over each of the 4 pairs = 1Gbps full duplex
1000Base-TX
===========
TIA/EIA-854
Requires two pairs of CAT-6
Two pairs transmit at 500 Mbps, and two receive at 500 Mbps = 1Gbps full
duplex
Good reference here http://www.cablinginstall.com/index/display/article-display/150060/articles/cabling-installation-maintenance/volume-10/issue-8/contents/standards/the-case-for-category-6-as-a-gigabit-ethernet-infrastructure.html
Definitely plugging a cable missing connections on pins 4,5,7,8 will not
get a 1Gbps link but a 100Mbps link.
Using a crossover only crossing 1/3 and 2/6 and disabling auto mdix a
1Gbps link between 2 switches is successful.
Using a crossover crossing 1/3, 2/6, 4/7 and 5/8 and disabling auto mdix a
1Gbps link between 2 switches is successful.
So back to the original question regarding the requirement to cross or not
to cross 4/7 and 5/8 on a 1000Base-T crossover cable....seems both work.
Some Cisco documents say cross only 2 pairs
( http://www.ciscosystems.or.at/en/US/docs/switches/lan/catalyst2950/hardware/installation/guide/hgcable.html#wp1020386 )
and other that say 4 pairs
( http://www.cisco.com/en/US/docs/app_ntwk_services/data_center_app_services/css11500series/v8.20_v8.10/installation/guide/Pinouts.html#wp1009423 )
There is a patent from Texas Instruments here http://www.freshpatents.com/-dt20091203ptan20090296570.php that is to
"detect and resolve connections for cables that are either fully aligned,
fully crossed or semi-crossed" that could explain things....
Ivan
The 1000Base-T4 became 1000Base-T. 1000Base-TX exists as well.
I wouldn't call it a "commercial failure" at all, though I suppose it
depends on who wrote the wiki page and who they worked for!
There was a long pull at the beginning towards using all the pairs so
that people didn't have to purchase Level 6 cable (prior to Cat6
standard) or Level 7 cable... They could get by with Cat5 or Cat5e.
In short runs, it likely doesn't matter all that much what you use...
But at full length, it will make a difference! I haven't really paid
attention all that much, but Cisco switches are listed as TX.... My
macbook is listed as T. And yet it works fine at 1000M with a two-pair
cable. *shrug*
Not sure what to tell you as I really haven't read the specs to see who
is or isn't compatible with the other! Or if there's a downshift in
speed (still above 100?).
Remember that wikis are written by anyone. So there's room for social
commentary and not 100% accurate information 100% of the time!
Scott Morris, CCIEx4 (R&S/ISP-Dial/Security/Service Provider) #4713,
JNCIE-M #153, JNCIS-ER, CISSP, et al.
JNCI-M, JNCI-ER
evil_at_ine.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Outside US: 775-826-4344
Knowledge is power.
Power corrupts.
Study hard and be Eeeeviiiil......
Ivan Walker wrote:
Hi Scott,
Thanks for the quick response. Again looking a wikipedia
( http://en.wikipedia.org/wiki/Gigabit_Ethernet ) I can find no
reference to
a 1000Base-T4 standard.
I am specifically interested in why a *gigabit* link between two
switches
using a crossover with only 2 pairs crossed actually works.
The wikipedia link above states "1000BASE-T requires all four pairs to
be
present." and also mentions a standard 1000BasseTX which only uses 2
pairs
but "has been a commercial failure".
Perhaps the Cisco switches are using 1000Base-TX???
switch#show interfaces capabilities
GigabitEthernet0/1
Model: WS-C3560G-24PS
Type: 10/100/1000BaseTX
Speed: 10,100,1000,auto
Duplex: half,full,auto
Trunk encap. type: 802.1Q,ISL
Trunk mode: on,off,desirable,nonegotiate
Channel: yes
Broadcast suppression: percentage(0-100)
Flowcontrol: rx-(off,on,desired),tx-(none)
Fast Start: yes
QoS scheduling: rx-(not configurable on per port basis),
tx-(4q3t) (3t: Two configurable values and one
fixed.)
CoS rewrite: yes
ToS rewrite: yes
UDLD: yes
Inline power: yes
SPAN: source/destination
PortSecure: yes
Dot1x: yes
Thanks
Ivan
Because there are two different standards. T4 uses all four pair,
it's
designed for use with lower-quality cables. TX only uses two pair
like
all other ethernet variants, but is supposed to be higher level cable.
HTH,
Scott Morris, CCIEx4 (R&S/ISP-Dial/Security/Service Provider) #4713,
JNCIE-M #153, JNCIS-ER, CISSP, et al.
JNCI-M, JNCI-ER
evil_at_ine.com Internetwork Expert, Inc.
http://www.InternetworkExpert.com Toll Free: 877-224-8987
Outside US: 775-826-4344
Knowledge is power.
Power corrupts.
Study hard and be Eeeeviiiil......
Ivan Walker wrote:
Wikipedia has a detailed explanation of crossover cables here http://en.wikipedia.org/wiki/Ethernet_crossover_cable#Crossover_cable_pinouts covering the pins outs for 1000Base-T and 100Base-TX.
1000Base-T
crossover cables cross all four pairs where as 100Base-TX crossover
cables
cross only 2 pairs (can cross four but only 2 pairs are used
anyway).
Looking at some crossover cables I found I did indeed find some with
all
pairs crossed and some with only 2 pairs crossed. When testing
these in
some Cisco switches both worked fine at 1Gps. This was kind of
unexpected
as I anticipated that the crossover with only 2 pairs crossed would
not
work.
I tried disabling mdix and speed/duplex negotiation etc but could
not
break
it. Can anyone explain why a crossover cable with only 2 pairs
crossed
still works for 1000Base-T.
Cheers
Ivan
Blogs and organic groups at http://www.ccie.net
Received on Thu Feb 04 2010 - 23:41:01 ART
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