Engineer's toolbox

VoIP bandwidth calculator

How much upload and download does your phone system need? Pick the codec and number of simultaneous calls.

Concurrent calls Codec Packet interval (ms) Link type Headroom (%)

Assumes RTP over UDP and IP (40 bytes of headers) plus the link overhead above, with no silence suppression. Calls use the same bandwidth in each direction, so size both upload and download. Headroom covers signalling and bursts. Quality of service on the link matters as much as raw speed.

Erlang trunk sizing

How many channels or SIP trunk lines do you need? This uses Erlang B, which assumes a blocked call is dropped rather than queued.

Calls in the busiest hour Average call length (minutes) Acceptable blocking

Add headroom for growth. If callers wait in a queue (a call centre, for example), Erlang C is the right model instead.

Subnet and CIDR calculator

Enter an IPv4 address with a prefix, such as 192.168.1.0/24. Optionally split it into smaller subnets.

Address and prefix Split into prefix (optional)

IPv4 only. A /31 is shown with two usable addresses (point-to-point links) and a /32 with one.

Wi-Fi access point estimator

A quick starting estimate for how many access points a space needs, based on coverage and on how many devices will connect.

Floor area (m²) Construction Wi-Fi devices (people × about 2) Typical use

These are rule-of-thumb planning figures for indoor offices. Real results depend on walls, ceilings, interference and the equipment used, so a proper site survey is the only way to be sure. We can do one for you.

Email authentication

Three DNS records decide whether your email is trusted or lands in spam.

SPFLists the servers allowed to send mail for your domain.

DKIMA digital signature proving a message came from you and was not altered. Your mail provider generates the key and gives you the record to publish, usually at selector._domainkey.yourdomain.

DMARCTells receivers what to do when SPF or DKIM fail, and sends you reports.

SPF record builder

Other includes (one domain per line) Sending IPv4 addresses or ranges (one per line) When a sender is not listed

Publish it as a single TXT record at the root of the domain. A domain may only have one SPF record. Always confirm include values in your provider's own documentation.

Check an SPF record

Paste a record

DMARC record builder

Policy Report mailbox (optional) Apply to (%) DKIM alignment SPF alignment

Publish as a TXT record at _dmarc.yourdomain. A safe rollout: start at none and read the reports, then move to quarantine, then reject once you know every legitimate sender is covered.