Process control LAN v.s. business LAN

[Alan Case]

Interesting discussion. I disagree with TJS on the function of IGMP and a managed switch does not necesarily have the ability to select a different path. Regards Alan Case

 

[Tark]

QoS is Quality of Service. It is part of the IP Header and is part of all IP packets. The QoS is used to mark the packet as having a particular priority. It is used to give real-time data a higher priority, thus enabling routers and switches to deduce the delay in transmitting these packets.

IGMP is Internet Group Management Protocol. IGMP deals with multicasting. When a packet is multicast it is transmitted to every device on the network. This can create a lot of unnecessary traffic on the network. IGMP is a multicast group. If a device sends out a IGMP multicast, that packet is transmitted only to those devices belonging to the IGMP group, thus limiting unnecessary traffic.

QoS and IGMP are unrelated to one another. Although then can be used in conjunction to provide better data transmission.

The AB article that Ron posted a link to makes no mention if they are using QoS, I would make the assumption that they are since it is very easy to implement. They are using IGMP, therefore it would be in ones best interest to use a switch that can handle IGMP and QoS.

 

[Operaghost]

Now the use of VLANs is actually a good thing. But of greater concern to me is how this managed switch (VLAN is a feature found in managed switches) is connecting your process network to you business network. You mentioned a firewall in your original post, did you initially have two totally separate systems that were connected to a common firewall? Was that firewall really a router?

Has any hardware changed? Or more correctly has the cabling changed? Can you tell us exactly what switch you are using where the VLAN is being configured. It is unlikely that we are overloading the switch as most (but not all) managed switches switching fabric can handle full-bandwidth on all ports, however it is possible the memory buffer is filling up. Again, this is not really likely, but possible. If we know more about your switch we can review it's specs.

There is a feature mentioned earlier called IGMP snooping that is highly desirable on an EtherNet/IP system. But different switches use this feature differently depending on whether a router is present or not. This is why it is important to know what switch you are using and whether a router is involved.

As this is a managed switch, your IT guys can pull up diagnostics showing the performance of the various ports as well as the switch as a whole.

Lastly, it is also possible the Ethernet cards in the CLX chassis' are overloading. If you open a web browser and type in the IP address of your CLX ENET bridges there is a Diagnostics option where you can view the performance of the CLX ethernet card. Specifically, look for CPU utilization and (trying to remember how it is worded) packets per second. The 1756-ENBT module supports up to 5000 packets per second. You will see that field on the diagnostic screen and you will see what your module is using. Make sure that module isn't maxed out.

OG

 

[niteRider]

IGMP is Internet Group Management Protocol. IGMP deals with multicasting. When a packet is multicast it is transmitted to every device on the network. This can create a lot of unnecessary traffic on the network. IGMP is a multicast group. If a device sends out a IGMP multicast, that packet is transmitted only to those devices belonging to the IGMP group, thus limiting unnecessary traffic.

not to be confused with ICMP, which is the diagnostic and control implementation of the TCP stack and uses commands such as ping and tracert. (I at first thought Tark was talking about ICMP and was very confused...)

I understand RS Ethernet IP is pretty robust utilizing all 7 layers of the protocol to ensure throughput and security. However I also understand that due to traffic its highly recomended to use Managed ethernet switches, wihich will cost 10-20 times as much as an un-managed switch.

Managed switches utilize admin config tools to set paths and prioritization of packets througput from IP to IP( PLC to HMI). The switch uses IGMP to detect the QOS (quality of signal) data within each packet to analyze transfer performance. The managed switch can "heal" a bad path and reroute packet via other ethernet paths to the same destination, if it is aware of a better QOS path. The managed switch can ensure certain packet make it thru ahead of other unimportant packets i.e plc traffic over security camera traffic!.

Ethernet it a MAC-layer protocol and should not be confused with a Transport-layer protocol (like TCP and UDP). It uses CDMA-CD (carrier-sense multiple access w/ collision detection). Switches use ARP to associate not only IP address-to-MAC address relationships, but also MAC-address to switch port relationships. Each packet destination field is inspected and the packet is sent to the appropriate switch port. This mechanism makes the packet switching much more efficient and eliminates collisions, but comes at the price of processing requirements. As traffic increases, it can bog down the switch and cause latency to skyrocket.

The most common QoS implementations utilize what is defined as the ToS (Type of Service) field in the TCP and UDP headers. It is just an 8-bit field that allows the administrator to classify traffic types and give priority to desired types. Applications that support QoS/ToS insert the service identifier into the ToS field (as defined by network admin). The hardware (be it switch or traffic manager or whatever) then examines that field and takes appropriate action.

Also, I remember seeing someone refer earlier to TCP as a 7-layer transport protocol, which is not quite accurate... TCP actually only classifies 4 layers (application, transport, network, and Link) instead of the 7 standard layers as defined in the OSI model (Application, presentation, session, transport, network, data link, and physical). They are essentially the same, with the difference being that TCP groups several of the OSI layers into one for simplicity of implementation.

-nR