Timing

CobraNet causes a best effort network technology to perform as an isochronous delivery system. In order for this to work, certain timing constraints must be met.

• Every hop a frame takes through a switch introduces forwarding delay.
• This delay is not always consistent.
• CobraNet cannot tolerate large inconsistencies in forwarding delay.
• Total variation of forwarding delay can worsen with each switch hop.
• The maximum forwarding delay that can be tolerated is no more than 3800 uS (3.8 mS) between the network diameter extremes.
• Whatever this delay is, up to the 3800 uS maximum, it must be relatively consistent between any two CobraNet devices and must not vary by more than –0/+250 uS. Occasional violations of this rule can be tolerated; chronic violations will prevent proper operation of the network.
• It has been empirically determined that the forwarding delay variability in typical 100 BASETX switches limits the number of hops to a maximum of six between any two CobraNet devices.
• Use of gigabit links between switches increases the hop limitation due to a reduction in forwarding delay between switches. However, the forwarding delay can still be variable and this variability, as opposed to the average aggregate delay, is the important factor. Therefore, although more hops can be tolerated when they are gigabit hops, it is still wise to limit the number of hops as much as possible. A properly designed network with a balanced hierarchical layout can contain a huge number of end points while still remaining within the six hop guideline. If your network is using more than this number of hops, it would be useful to reexamine and optimize its layout.
• If the contractor chooses to use lower latency CobraNet settings, the number of hops that can be tolerated will be reduced to two or three hops at 2 2/3 mS latency (or 10-15 with gig links) and one or two at 1 1/3 mS latency. Consult with the system contractor on this issue to determine how CobraNet will be configured.

  Note: The above switch hop rules are guidelines only. Different results can be obtained depending on the characteristics of an individual switch and the nature of the traffic on the network.