Gigabit Ethernet - Page 1
Forum section: electronics / projects

author: michaelym
author_rank: Regular Contributor
timestamp: September 15, 2016, 04:37:35 am
content: A few questions about gigabit EthernetIt is my understanding that in Ethernet that the whole network runs by the speed of the weakest link so if one device is device is 10/100 all the gigabit devices on the network will run at 10/100, is this correct?The actual data I need to pass over Ethernet is simple strings that could also theoretically passed on a 115200 RS232 line, if I use an embedded microcontroller such as a cortex-M4 that does not have gigabit Ethernet built in and only has 10/100 and I connect it to a gigabit Ethernet PHY is my device considered enabled for gigabit Ethernet?Thanks in advance for your reply

author: Berni
author_rank: Super Contributor
timestamp: September 15, 2016, 05:34:52 am
content: As long as you don't connect it to a hub you are fine.Routers and switches that everyone uses now days have a separate PHY on each port. So when you plug a 100/10Mbit device in to a gigabit router they do there special mating dance to let the gigabit PHY in the switch know the device on the other side can do 100Mbit max so it goes in to 100Mbit mode(Only for this port that is).Those PHYs in the switch then feed in to a switch fabric where the packets get routed around to the correct ports. This internal switch fabric will tend to have more than 1Gbit of throughput in case you send a ton of traffic between two gigabit ports and still have throughput left over to service the others (Unless its a cheap home router, then even getting close to 1Gbit of traffic might choke it).

author: dom0
author_rank: Super Contributor
timestamp: September 15, 2016, 10:27:20 am
content: Quote from: Berni on September 15, 2016, 05:34:52 amThere are no gigabit hubs. All GbE links are point-to-point. Quite some of the fast ethernet hubs are also actually mislabelled switches.So to get back toQuoteNo, this is never the case in a GbE infrastructure and not the case for almost any fast ethernet installation, including cheap home routers (at least every model I've ever seen) which have a switch, not a hub, in their SoCs.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 15, 2016, 11:29:31 am
content: Yes. Hubs mostly existed in the 90's. When 100M hit the consumer market in early 2000, they were practically all switches already.

author: rstofer
author_rank: Super Contributor
timestamp: September 15, 2016, 04:08:23 pm
content: But if you want to tap into traffic between a host and a switch so that Wireshark can see what's going on, a hub is a handy tool to have around. Yes, it slows things down but it does allow for the tap.

author: helius
author_rank: Super Contributor
timestamp: September 15, 2016, 04:21:29 pm
content: Even a simple and cheap 10/100 hub is implemented as a two-port switch internally, and partitions between 10 Mbit/s and 100 Mbit/s segments. This issue never actually existed for twisted-pair Ethernet networks, although it can apply with passive optical networks (PONs).Quote from: rstofer on September 15, 2016, 04:08:23 pmSwitches have port mirroring features to handle this (unless you use absolutely lowest end whitebox switches).

author: Monkeh
author_rank: Super Contributor
timestamp: September 15, 2016, 04:24:08 pm
content: Quote from: rstofer on September 15, 2016, 04:08:23 pmThat's why you use a real switch which can mirror traffic.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 15, 2016, 05:20:06 pm
content: Quote from: Monkeh on September 15, 2016, 04:24:08 pmOr use a passive tap

author: Howardlong
author_rank: Super Contributor
timestamp: September 15, 2016, 05:42:35 pm
content: Quote from: Monkeh on September 15, 2016, 04:24:08 pmI keep a small crappy old 8 port 10/100 hub with an integrated 10/100 switch on the bench permanently, available for just this kind of work, although it does also feed a few other less demanding bits and pieces. Having to fanny about configuring a port SPAN on a physically larger managed switch just gets a bit in the way when I can just plug and go, only needing to configure the sniffer interface's link speed.While it might take someone who does networking for their day job a minute or two, I've not done such a thing for nearly ten years, these days I'd have to crack open a manual. Not so with a crappy old hub!

author: dom0
author_rank: Super Contributor
timestamp: September 16, 2016, 08:43:43 am
content: Quote from: Howardlong on September 15, 2016, 05:42:35 pmWell the point kinda is that network debugging often happens in the network, not on the bench. There it's much easier and faster to issue a few commands (or a few clicks) from your office and get the traffic of interest literally delivered directly to the (imaginary) doorstep of your cubicle.

author: Howardlong
author_rank: Super Contributor
timestamp: September 16, 2016, 12:19:00 pm
content: Quote from: dom0 on September 16, 2016, 08:43:43 amDepends very much on what you're doing. If you're developing an embedded prototype connected to your corporate LAN, you should STOP now unless you have discussed it with your infrastructure people and have a segregated LAN. The last thing you want is stuff like inadvertent DHCP servers appearing, or the wrong device appearing on the wrong VLAN.If, on the other hand, you were troubleshooting a distributed application already on your corporate LAN then you may find it easier to mess about with SPAN, once you've engaged the right people from your network team. As this is an electronics forum and not an enterprise software troubleshooting forum, I guess we can discount that scenario.QuoteBut it isn't: how is that faster than plugging into a hub/switch on the same bench where your DUT is? If you (a) know the commands and (b) have access to the managed switches then maybe you can launch a putty, login to your managed switch, key in half a dozen commands and away you go, but that would be a rather unlikely scenario in most medium to large sized corporate environments unless it's in a dedicated lab.In addition it's pretty common to want to air gap your embedded dev stuff particularly in the early stages of development, but also later on during testing to create controlled tests.Unless you're in a small startup, I can't imagine a networking team agreeing to having uncontrolled embedded devices in development hooked up to their infrastructure, or allowing developers direct access to reconfigure their managed switches.Personally I'd much rather keep things simple, and if I'm only interested in 10/100, that means my crappy old hub/switch. Sure, for giga, I agree, you'll need to implement port mirroring.

author: Kjelt
author_rank: Super Contributor
timestamp: September 16, 2016, 12:29:15 pm
content: Quote from: mikeselectricstuff on September 15, 2016, 05:20:06 pmYes I used them and found out that which is logic but I never realized till i used it, that you can only tap into the Tx pair of one of the two parties at the same time. So a TLS handshake for instance you will only tap the Tx of the client or on the other side the Tx of the server but not both atst, which you typically would like. Using a cheap Gb switch with port mirroring you will tap both parties traffic, they can be found as cheap as €35.-

author: Berni
author_rank: Super Contributor
timestamp: September 16, 2016, 12:35:10 pm
content: Quote from: Kjelt on September 16, 2016, 12:29:15 pmYou could probably just use two ethernet ports and wire one with a cross cable. This is an old trick of how to snoop on a bidirectional RS232 com port link by using a special cable with 4 plugs on it. Two plugs are just a straight trough cable while the other two plugs only have the RX pin wired to the data line, one for each direction.

author: dom0
author_rank: Super Contributor
timestamp: September 16, 2016, 12:51:58 pm
content: Quote from: Howardlong on September 16, 2016, 12:19:00 pmYes, absolutely. My mind was in the "network operations corner" while I wrote the post.In theses cases a hub or tap on the bench is of course a very handy item to have at hand.

author: Kjelt
author_rank: Super Contributor
timestamp: September 16, 2016, 02:46:24 pm
content: Quote from: Berni on September 16, 2016, 12:35:10 pmIs it possible to sniff and combine with wireshark from two ports simultaneously ?Else you need two versions running.