Unlike modular matrix switcher that uses HDMI/DVI with FPGA switching technology, IP-based videowall system depend on switch’s switching bandwidth and forwarding rate. Let’s grasp a good understanding of what these two factors when selecting a switch for your IP-based KVM Videowall in order to get the best user experience. This is especially critical when you have many inputs from various sources and display outputs. This will avoid any obvious video lagging, pixelated pictures, delayed response time or poor picture in your videowall implementation.
Switching Bandwidth
Switching bandwidth is the aggregate input and output bandwidth of all ports. So a 48 port gigabit switch would have 48Gbp/s and 48Gbp/s out, that leaves us with only 96Gbps and apparently, 80GBps would likely be the stacking port rate.Forwarding Rate
That’s a measure of how many packets per second the switch can process for certain sized packets. When packet’s size isn’t described, today it’s usually denoted for minimum size Ethernet packets, i.e. each 64 bytes. Minimum size Ethernet, to run at gig rate, is 1.488 Mpps. Unlike fabric, you don’t need to account for duplex as one port’s in is another port’s out. So for 24 gig ports, and for the optional dual 10g ports, we need (24 + 20) * 1.488 = 65.472 Mpps to support full rate.“Laymens” explanation
Think of many traffic lanes (for network links) leading up to (river) bridge (for network switch [or multi-port bridge]). How many vehicles (packets) you can move across the bridge depends on how fast each vehicle can pass through the toll process (a device’s packets per second, PPS, rate) and how many lanes (bandwidth) the bridge has. So a “forwarding rate”, is the count of how many packets you can examine, per second, as they transit your switch (toll gate analogy). Let’s look at another example, using the previously mentioned 24 port 3750-X. It’s bandwidth is 160 Gbps, the forwarding rate is 65.5 Mpps. It means that the smallest packet size can be ~320 B (160 Gbps / 65.5 Mpps) to utilize the maximum available bandwidth. If the forwarding rate is only 20 Mpps, the smallest packet size would be ~1 KB (160 Gbps / 20 Mpps). If you send smaller packets, you cannot utilize the 160 Gbps bandwidth. With 512 B packets, the bandwidth would be 80 Gbps, even if the switching bandwidth in 160 Gbps. For Ethernet switches, full “speed” requires 1,488 Kpps, for minimum (64 byte) packets, or 81.3 Kpps, for maximum (1500) packets, and 2 gig of internal bandwidth per duplex gig port. 160 Gbps is more than the 24 port versions need to support all their gig ports, but not by as much as you might think at first glance. Don’t forget these models support optional uplink modules, which supports up to two 10g ports. Lastly, in the 3750-X, they have stack ring ports, two 16 Gbps. So we have 24 + 20 + 32 = 76 then we double for duplex giving 152 Gbps. For 24 gig and 2 10g, worst case (minimum size packets – if vehicles were motorcycles) Ethernet requires 24 + 20 = 44 * 1.488 = 65.472 Mpps. If you want a bridge to allow vehicles to cross it without delay, it must have enough lanes to support all the lanes leading up to it (bandwidth) and a tolling system (forwarding rate, PPS) to deal with the number of vehicles (packets) crossing it as they arrive. Many times actual switches (or real bridges) can’t sustain full capacity but that’s fine when there’s less loading (like crossing a bridge at 2 AM vs. 5 PM).
