Network Design: Estimating Factors for the Enterprise.
Use these factors when estimating for your network solution.
When determining requirements for network solutions, we often find key data points missing from discovery data. This can include traffic volumes, user counts, device inventory etc.
Here are several estimating factors I've used to successfully fill some of these gaps. Note, the use of these methods must be documented as solution assumptions and appropriate risk management must be applied.
Switch and User Volumes
Assume 2 to 3 switch interfaces per user.
I usually go with 2.5 interfaces per user. This assumes VoIP phones are daisy-chained, if not, add an extra interface per VoIP phone or per user if the phone count is unknown.
Unless dealing with small sites (<30 users), go with at least 2* 48 port stack-able switches. For large sites, a chassis design may offer better value, in such cases be sure to use dual chassis to avoid single points of failure.
Always assume dual power supplies, unless stakeholders advise otherwise.
Use Cases:
For a given site with 30 users, we can assume an interface requirement of 75 interfaces. If VoIP phones are NOT daisy chained, we can add 30 additional interfaces to support the phones. This puts us at ~105 interfaces, which can be covered by a 3 unit stack of 48 port switches.
For a given site with a switch inventory that totals 1000 interfaces, we can assume ~400 users.
Wireless (WiFi)
Wireless access point (WAP) volumes usually become a compromise between coverage and/or bandwidth requirements. Most new WAPs use the new AC standard, therefore I find coverage is the more stringent constraint.
In modern "cube" style offices, a mid-range WAP should cover ~2000 square feet, with user work-spaces running ~125 square feet per user.
Given these factors of WAP-Coverage:Floorspace:User-Count, we only need 1 data point with which we can then make assumptions on the other 2.
In addition, include extra WAPs needed for security services such as monitoring, rogue WAP detection and "tar-pitting". Use your best guess if ratios aren't available from the vendor.
For small sites use a 2 WAP minimum.
If possible, it's worth adding 5% to 10% uplift, to provide some wiggle room if you run into environmental issues.
One thing to include in your assumptions is that environmental constraints are unknown without a wireless site survey. Therefore your WAP volume estimates should be considered a minimum, with the risk of additional units required due to environmental constraints.
Use Cases:
Given a site user count of 200 users, we can assume floor-space of 25,000 square feet, and a requirement of at least 13 WAPs.
Given a floor-plan of 5000 square feet, we can assume the requirement of at least 3 WAPs, and most likely a user count of 40 users.
Power over Ethernet (PoE)
For power over Ethernet (PoE), the most common draw will be IP phones and wireless access points (WAPs). If needed, use the User count to determine the number of phones and WAPs, then use the PoE requirements of mid-level devices if specific models are not known. PoE requirements can be found in the datasheet of any PoE device.
Always assume PoE is required, unless stakeholders advise otherwise.
Use Case:
Given a site with 30 users, all with VoIP phones, assuming 2* 48 port switches and 2 WAP. Phone model is unknown, so we'll assume a mid-range PoE Class 3 device (15.4W). WAP model is unknown, so again we assume a mid-rangePoE Class 3 device (15.4W). Therefore total PoE budget is 32* 15.4W, =~246W per switch.
Growth
Always add a factor for growth. If the stakeholders can't provide one, use an assumed factor. I usually go with 20% growth over 5 years, unless the stakeholders request an alternate rate. This also includes assumptions of a 5yr hardware life-cycle which typically aligns with a 5yr contract.
Use Case:
Always increase final volumes by the growth factor, this is usually the last adjustment to any volumes prior to designing.
Circuits
See my post "Sizing Circuits" for details.
Here's a quick & dirty estimate to get started.
Internet = 0.25Mb/s per User.
WAN = 0.5Mb/s per User.
Expect challenges to these ratios. If necessary work with you Stakeholders to determine ratios more suitable for their environment.
Use Case:
1. Given a site with 50 Users with combined Internet and WAN over a single circuit, assume 37.5Mb/s required bandwidth.
Summary
Using discovery data and the assumption factors above, we should have enough data on which we can design a network solution fit for purpose.
A key point to note is that the assumption factors above are not fixed and can be adjusted per stakeholder request.
The relationships between the data points and factors will help guide you to reasonable assumptions on which you can base your design, these have served me well in the past.
It is critical that any assumption is accepted by the stakeholders is documented (including the acceptance), as stakeholder "strategic memory loss" is not uncommon.